Anchored Anions, Mobile Cations: Charge Storage in MOF-based Supercapacitors Studied with Operando Small-Angle X-ray Scattering

Anchored Anions, Mobile Cations: Charge Storage in MOF-based Supercapacitors Studied with Operando Small-Angle X-ray Scattering | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Anchored Anions, Mobile Cations: Charge Storage in MOF-based Supercapacitors Studied with Operando Small-Angle X-ray Scattering Oskar Paris, Malina Seyffertitz, Chloe Balhatchet, Max Rauscher, and 6 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5973632/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 30 Sep, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract Understanding how ions interact with electrodes in electric double-layer capacitors (EDLCs) is key to advancing energy storage, yet many fundamental aspects remain unclear. Here, we employ operando small-angle X-ray scattering (SAXS) to investigate charge storage in a Ni 3 (HITP) 2 metal–organic framework (MOF), with well-defined pores as an electrode model system. Using 1 M NaTFSI aqueous electrolyte, we show that TFSI − anions are immobilized near MOF pore walls via fluorine–hydrogen interactions with N-H functional groups. We quantify the concentration of pinned anions and demonstrate that their immobilization persists across different applied cell voltages, resulting in a cation-dominated charge storage mechanism governed solely by Na + adsorption and desorption. Charge balancing is unaffected by whether voltage is applied stepwise or gradually, with no dynamic differences between in-pore and outer-pore environments. Additionally, we track reversible adsorption induced pore swelling, rule out ion intercalation, and observe minor irreversible structural expansion after prolonged negative bias. Physical sciences/Energy science and technology/Energy storage/Supercapacitors Physical sciences/Physics/Condensed-matter physics/Surfaces, interfaces and thin films Physical sciences/Materials science/Materials for energy and catalysis/Metal–organic frameworks Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction As the demand for efficient electrical energy storage continues to grow, electric double-layer capacitors (EDLCs), or supercapacitors, are emerging as promising devices for bridging energy needs across various sectors. Owing to their high power-density, supercapacitors can be of particular importance for applications in electric vehicles, smart power grids and intermittent energy demands in light of managing the transition from fossil fuels to renewable energy 1 . A comprehensive molecular-scale understanding of electric double layer formation is therefore considered essential for optimizing the performance of supercapacitors and driving their successful integration in a wider range of applications 2 – 7 To date, activated carbons are predominantly used as supercapacitor electrode materials due to their high share of microporosity (i.e., pores smaller than 2 nm), chemical stability, electrical conductivity, low cost and overall sustainability, as they can be derived from waste organic materials 8 . While organic electrolyte solvents are often preferred in technical applications due to their higher electrochemical stability window 9 , water as solvent offers the advantages of easy handling under ambient conditions, high ion mobility, low cost, reversible redox activity, and overall environmental friendliness 10 , 11 . However, gaining a mechanistic understanding of activated carbon-based aqueous supercapacitors is often challenging due to their disordered pore structures and complex electrode-electrolyte interactions, which hinder straightforward data interpretation 12 . To address these challenges, model materials with well-defined and tunable properties are highly valuable for investigating the mechanisms of charge storage. Metal-organic frameworks (MOFs) stand out as a promising class of such materials due to their highly ordered nanoporous structures and chemically tunable properties. Traditionally, MOFs have been limited by poor electrical conductivity, restricting their use in ion storage devices. However, recent advances have introduced a class of electrically conductive MOFs characterized by 2D π-d conjugated layers that stack into honeycomb-like structures 13 , 14 . One prominent example is the Ni 3 (2,3,6,7,10,11-hexaiminotriphenylene) 2 (Ni 3 (HITP) 2 ) MOF, introduced by Sheberla et al. 15 , which showed promising capacitance values on par with or even superior to conventional carbon electrodes 16 – 18 . The ordered pore structures, tunable surface chemistries, and well-defined functional groups of MOFs such as Ni 3 (HITP) 2 provide an exciting opportunity to systematically explore how electrode architecture, pore geometry, and chemical functionality influence charge storage behavior. Insights gained from such studies not only improve our understanding of MOF-based systems but could also help uncover more general principles of charge storage that can inform the development of next-generation supercapacitor materials more broadly. Recent research efforts have explored the influence of particle morphology on electrochemical behavior using conductive MOFs 17 , as well as the effects of cation size 5 on EDL formation, suggesting that cation dynamics primarily govern EDL formation in MOFs 5 , 19 . Despite these advancements, critical knowledge gaps remain. The detailed mechanisms underlying local ion rearrangement in the pore space, the interplay between electrosorption behavior and functional groups, and the structural changes that occur during charge and discharge cycles remain poorly understood. In-situ Small-Angle Scattering of X-rays (SAXS) or neutrons (SANS) together with Wide-Angle X-ray Scattering (WAXS) have proven effective in analyzing complex multi-component ion storage systems such as electric double-layer capacitors (EDLCs) 20 – 22 or batteries 23 , 24 . The scattering signal from EDLCs is generally composed of contributions from the nanoporous solid electrodes, the electrolyte solvent in- and outside the pores, as well as cations and anions with dynamic local and global concentrations, which often requires new ways of data treatment by, e.g., combining experimental results with computational modelling 12 . However, sharp Bragg peaks originating from highly ordered pores, such as in MOFs, allow for a more direct interpretation of changes in the scattering signal towards physical phenomena, such as ion rearrangement and pore swelling, in real-time. Recent work on in-situ ion electrosorption in MOFs 25 and carbons 26 , 27 , as well as previous work on gas adsorption in carbons and silica 28 – 32 highlights small-angle scattering as a powerful method for studying guest-host interactions in ordered nanoporous systems, with the prospect to better understand fundamentals of charge storage behavior in EDLCs. In this study, we employ SAXS/WAXS to investigate Ni 3 (HITP) 3 MOF-based supercapacitors operated with aqueous electrolytes and different charging protocols. We track subtle structural changes of the MOF framework, including wetting- and electrosorption-induced pore swelling, while also assessing whether ion intercalation takes place. Furthermore, the approach allows for real-time, i.e., operando investigation of a full EDLC device. This delivers critical information on local ion rearrangement within pores in response to an applied cell voltage, providing insights into how the immobilization of specific anions drives cation-dominated charge storage. The observed anion-pinning effects may offer a general strategy to tune supercapacitor charging mechanisms and performance. Results and Discussion Characterization of dry Ni 3 (HITP) 2 electrodes The nanoscale origin of the electrochemical behaviour of Ni 3 (2,3,6,7,10,11-hexaiminotriphenylene) 2 Ni 3 (HITP) 2 metal-organic framework (MOF) based supercapacitors was assessed by small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). Before addressing operando changes of the scattering patterns with electrolyte present, it is essential to first establish a comprehensive understanding of the MOF structure and related scattering features in the dry state of the electrode. Figure 1 displays the scattering signal of the neat Ni 3 (HITP) 2 MOF electrode in the SAXS (Fig. 1 a)) and the WAXS region (Fig. 1 b)). The inset in Fig. 1 b)) displays the real space structural model (top view and side view) of an AB stacked Ni 3 (HITP) 2 model as reported in 15 . The most obvious structural features of this system are the layered structure and the regular arrangement of cylindrical pores perpendicular to the layers on a 2D hexagonal lattice. To better understand the structural features of the MOF and to facilitate comparison with literature, we classify the expected X-ray diffraction peaks into four distinct categories: (i) peaks arising from the 2D pore lattice (vertical gray lines indicating in-plane hk0 reflections), (ii) peaks from the layered structure of the MOF, which give rise to stacking peaks 00l (dashed red lines in Fig. 1 b)), (iii) mixed reflections (hkl, with h or k ≠ 0, and l ≠ 0, orange dash-dotted lines in Fig. 1 a)), and (iv) peaks attributed to the carbon structure of the triphenylene linker, which produce two broad reflections labeled c-100 and c-110 (green dotted lines in Fig. 1 b)). To clearly distinguish between these groups, an example from each category - in-plane pore (solid grey line), layer stacking (red dashed line), mixed (orange dash-dotted line), and triphenylene carbon (green dotted line) - is illustrated in the inset of Fig. 1 b). The peak near q = 12.4 nm − 1 in Fig. 1 a) originates from the 5 wt.% PTFE binder in the Ni 3 (HITP) 2 electrode and is not related to the MOF structure, but acts as a useful internal standard. All in-plane peaks (indices hk0), corresponding to the 2D hexagonal (P6mm) arrangement of cylindrical pores (grey solid lines in Fig. 1 a) and 1b), appear at the expected q-positions with a pore-centre to pore-centre distance a = 2.20 nm. The stacking peaks (002 and 004, red dashed lines in Fig. 1 b) correspond to a layer spacing of c = 0.33 nm. Surprisingly, no peaks with mixed indices, i.e. with h or k ≠ 0, and l ≠ 0 are observed in the SAXS pattern (see orange dash-dotted vertical lines in Fig. 1 a). The absence of mixed-index peaks strongly suggests that the layers are not perfectly stacked in a conventional AB or AA pattern to form a 3D crystal. Instead, the scattering signal resembles a so-called ‘turbostratic’ stacking 34 , 35 , where the layer spacing is regular, but the in-plane positional correlations between the layers lack long-range order leading to the absence of peaks with mixed indices. This interpretation is further supported by the asymmetry of the c-100 peak from the carbon structure within the framework, a characteristic feature of turbostratic carbons, as described in Ref. 35 . This finding is consistent with Ref. 36 , which characterizes the stacking of Ni 3 (HITP) 2 as ‘near-eclipsed’. We have undertaken atomistic simulations of the MOF structure, which reveals a very small energy difference between AB and stacked layer configurations (SI Figure S2 and Supplementary Note SN1). Hence, our results strongly indicate a Ni 3 (HITP) 2 structure where the layers exhibit no long-range positional order in stacking direction, reflecting the absence of a well-defined stacking sequence. In addition to the stacking characteristics, the pore diameter of D ≈ 1.39 nm was determined from the SAXS data in Fig. 1 a) by analysing the experimental data with a single-step form-factor model of infinitely long monodisperse cylindrical pores with circular cross-section 28 , 32 , 37 , 38 (dashed black line in Fig. 1 a)). The regular arrangement of identical pores gives rise to sharp Bragg peaks at distinct positions, with their height being proportional to the pore form-factor (see Eq. 4 in Materials and Methods). This model explains particularly the low intensities observed for the (110) and (300) in-plane reflections, as these nearly coincide with the form-factor minima for pores of this diameter. Surprisingly, this simple approach reproduces the intensities of the diffraction peaks in the SAXS regime very well, although it does neither consider the inhomogeneous electron density distribution in the MOF structure nor the fact that the pores are not perfectly cylindrical in shape. We note that a more advanced (numerical) form-factor model which includes the circularly non-symmetric electron density of the MOF pore walls, leads to a very similar radially averaged form-factor (see SI, supplementary note SN4 and Figure S5). The mean pore diameter of 1.39 nm from the form-factor models was found to perfectly align with Ar@87K gas sorption data evaluated using a zeolite non-local density functional theory (NLDFT) kernel for cylindrical pores on the adsorption branch (see SI Figure S3). The sample’s specific surface area (SSA) was assessed using Ar@87K gas sorption analysis and resulted in a Brunauer–Emmett–Teller (BET) area of 390 m 2 /g. Characterization of wetted Ni(HITP) electrodes After establishing the structural model of the dry Ni 3 (HITP) 2 MOF electrode, attention is now directed towards its behaviour upon the addition of (aqueous) electrolytes. The data presented in Fig. 2 were recorded on a flat-plane detector allowing the continuous acquisition of a very wide q-range using a single detector. While the resolution of this experimental set-up using a single detector is limited, the extended q-range is particularly useful for quantitatively considering the scattering contribution of the bulk electrolyte within the wetted electrode. Figure 2 a) compares the scattering signal of the dry electrode (full black line) to its wetted state in the presence of pure water (light blue full line) and of aqueous 1 M NaTFSI (full orange line). For reference, the recorded scattering signals from bulk liquid water and 1 M NaTFSI (aq.) bulk liquid electrolyte at the same temperature are shown with corresponding dotted lines. The characteristic diffraction peaks of the Ni 3 (HITP) 2 MOF remain visible in the wetted state, although strong diffuse scattering contributions from the bulk-liquids are dominant particularly at larger q above 13 nm − 1 (Fig. 2 a)). Figure 2 b) shows the data after the bulk liquid contributions were subtracted from the wetted electrode data. It is evident that the 002 stacking-peak at approximately 18 nm − 1 clearly shifts towards smaller q values for the MOF soaked in H 2 O or 1 M NaTFSI (aq.), indicating an increase in the layer stacking distance by 1.44% in the wetted state. In contrast, the positions of the in-plane (hk0) peaks remain largely unchanged, suggesting that noticeable dimensional changes of the MOF occur predominantly along the stacking direction. Since this is true for both, pure water and aqueous 1 M NaTFSI electrolyte, we attribute this effect to a wetting-induced swelling of the MOF in stacking direction. Aside from the shift in the stacking peak and a slight deviation at low q values (attributed to changes in contrast between MOF particles and their surrounding medium), the bulk-water corrected scattering curve of the H 2 O filled electrode closely follows that of the neat MOF (Fig. 2 b), black and blue lines, respectively). This suggests that the scattering can be interpreted as a combination of bulk H 2 O and the dry MOF, with a homogeneous distribution of H 2 O within the pores. In contrast to pure water wetting, the corrected scattering profile for aqueous 1M NaTFSI does not resemble a simple incoherent sum of the dry MOF signal and the bulk 1M NaTFSI aqueous solution signal, evident by the drastic change in peak heights as compared to the dry MOF. This indicates the presence of an inhomogeneous electrolyte distribution within the pores, leading to a changed electron density distribution, and consequently a changed scattering form-factor, which directly influences the peak intensity. By way of an example, the inset in Fig. 2 b) illustrates an inhomogeneous distribution of electrolyte in the pore with a higher electrolyte concentration near the pore walls. A quantitative treatment explaining the observed changes in the heights of the peaks will be presented further below. Scattering profiles similar to that from 1 M NaTFSI (orange line in Fig. 2 ) were also obtained from electrodes wetted with 1 M KTFSI and 0.1 M NaTFSI (SI Figure S4a)). In contrast, scattering patterns closely resembling those of electrodes wetted with pure H₂O (light blue line in Fig. 2 ) were observed for 1 M RbBr (aq.) and 1 M Na 2 SO 4 (aq.) (SI Figure S4b)). This suggests that the observed Bragg peak intensity changes are specifically related to the presence of TFSI − anions, rather than to the presence of electrolyte ions or solvent in the pores in general. Notably, the similarity between the scattering patterns of 1 M NaTFSI (Fig. 2 b)) and 0.1 M NaTFSI (SI Figure S4a)) indicates that presumably a high concentration of TFSI − is inhomogeneously distributed within the pore space, even without an applied cell voltage. Due to resolution limitations of the measurements in Fig. 2 , it was not possible to accurately fit a form factor model in order to fully characterize the electrolyte distribution. To address this, further high-resolution SAXS measurements were conducted to confirm the electrolyte concentration gradients within the pores and to study the specific ion arrangement in more detail. These experiments were conducted operando during the charging and discharging of a full supercapacitor cell in order to additionally investigate the implications of this inhomogeneous electrolyte distribution for the systems electrochemical behaviour under working conditions. Specific TFSI adsorption at 0 V Figure 3a) presents the SAXS signal of the working electrode in the wetted state with 1 M NaTFSI at 0 V (grey), +0.4 V (red) and -0.4 V (blue). A hole was punched into the counter electrode enabling to collect the scattering signal for just one electrode specifically 20 . For reference, the scattering profile from the dry electrode has been added again as a full black line. The bulk electrolyte was not measured for this specific experimental set-up and could therefore not be subtracted. The high resolution SAXS data in Figure 3a) confirms the result from Figure 2 about noticeably relative Ni 3 (HITP) 2 Bragg peak intensity changes when electrolyte is added. Diffraction peaks near a form factor minimum, such as the 110 and 300 peaks, are particularly sensitive to form factor changes, and thus, to variations of the electron density distribution within the pores. Upon electrolyte addition, the 110 peak completely disappears (i.e., it now perfectly matches the form-factor minimum), while the 300 peak gains relative intensity. Moreover, the intensity ratio of the 200 and 210 peaks is roughly inverted (Figure 3a)). The simplest possible form-factor to explain these changes is a two-step core-shell model according to Ref. 32 consisting of three parameters (Fig. 3 b)). The outer pore diameter remained fixed at D 1 = 1.39 nm (as in the dry state), marking the cylindrical MOF pore wall, while an inner diameter of D 2 = 0.9 nm forms a shell of approximately 0.25 nm thickness with slightly higher electron density as compared to the rest of the electrolyte-filled pore (Fig. 3 b)). Table 1 summarizes the parameters for the unfilled and filled pore states that best fit the experimental data, while the corresponding form factor models are shown in Fig. 3 a) by dashed and dotted lines, respectively. The associated electron densities for individual ions and bulk electrolytes are provided in SI Table ST1. Table 1 Diameters and electron densities in the dry and 1 M NaTFSI (aq.) wetted state as resulted from the two-step cylindrical core-shell form-factor model. D 1 ρ MOF ρ pore D 2 ρ TFSI−rich nm e − /Å 3 e − /Å 3 nm e − /Å 3 dry 1.39 0.781 0 - - wetted 1.39 0.781 0.350 0.90 0.430 Since none of the other Na-containing electrolytes, but all TFSI-containing electrolytes, showed a change in the form factor (SI Figure S4), this layer of higher electron density observed in NaTFSI electrolyte is interpreted as a TFSI-rich region near the pore wall. The layer thickness of 0.25 nm aligns reasonably well with the short dimension of the roughly prolate ellipsoid-shape TFSI − ion, measuring about 0.29 nm 39 . The increased electron density between the bulk pore filling (ρ pore ) and the interface layer (ρ TFSI−rich ) computes roughly to an 1 M increase of the TFSI − concentration in this layer. Assuming a bulk electrolyte concentration of 1 M in the rest of the pore, this translates to a TFSI − concentration of about 2 M in the TFSI − -rich layer, or in other words, about 20 %of the pore surface occupied by TFSI − ions (see Supplementary Note SN3). We acknowledge that while this model provides both a qualitative and quantitative understanding of the observed changes, it oversimplifies the electron density profile. In particular the MOF pore wall is not homogeneous around its circumference in terms of electron density, limiting the ability to pinpoint specific TFSI − adsorption sites within the pore. A numerical simulation of a form factor model including the radial electron density variations in the MOF unfortunately did not provide sufficient accuracy of the radially averaged form-factor, which is the only experimentally observable quantity (SI Figure S5 and Note SN4). Consequently, while the model presented here offers quantitative information about the radial distribution of TFSI − ions, it cannot resolve their exact spatial location along the circumference of the pore, i.e. their specific adsorption sites. Recent work with NMR, however, postulated possible hydrogen-bond like interactions of fluorine atoms in electrolyte anions (including TFSI − and BF 4 − ) with the N-H moiety of the MOF linker 19 . Our experimental observation of strong form-factor changes for TSFI − , but not for fluorine-free electrolytes (RbBr and Na 2 SO 4 , SI Figure S4) supports the hypothesis that fluorine-containing anions form strong hydrogen-bond-like interactions with the N-H group. The authors of Ref. 16 also briefly note in their supplementary information the presence of BF 4 ⁻ anions trapped in Ni 3 (HITP) 2 pores after using TEABF 4 /acetonitrile (ACN) electrolyte. As an additional proof, we observed a homogeneous electrolyte distribution with LiTFSI/propylene carbonate (PC) electrolyte in Cu 3 (HHTP) 2 electrodes (SI Figure S6), a MOF approximately isostructural to Ni 3 (HITP) 2 , which, however, lacks N-H groups in its linker. This further strengthens the conclusion that F···H-N interactions indeed underlie the observed inhomogeneous electrolyte distribution, leading to a higher concentration of immobilized TFSI − ions near the MOF pore walls. TFSI immobilization at applied cell voltage and cation dominated charge balancing Building on the understanding of the system at no applied voltage (0 V), we now investigate the systems operando response to an applied external cell voltage. In our custom electrochemical cell for operando SAXS, cyclic voltammetry (CV) data showed a rectangular shape supporting pure capacitive behaviour of the MOF (Fig. 3 c)). After the CVs, a voltage sequence was then applied with chronoamperometry (CA), i.e. constant voltage at 0 V, + 0.4 V, 0 V, -0.4 V, and 0 V with 1-hour holds, followed by another set CV cycles. The applied voltage sequence is depicted in the top panel of Fig. 3 d). Following the CA holds, a slightly larger specific capacitance was observed (104 F/g) compared to before the holds (94 F/g). These values are comparable to values previously reported for Ni 3 (HITP) 2 with a similar BET specific surface area using organic electrolyte 19 , particularly as no carbon black or other conductivity enhancing additive has been added. We attribute the increased specific capacitance by approximately 10% to enhanced wetting, a characteristic effect of electrowetting induced by the applied cell voltage 40 . While no electrochemical degradation was observed in this study for Ni 3 (HITP) 2 with aqueous 1 M NaTFSI under maximum applied cell voltage of ± 0.4 V, a significant decline in electrochemical performance was observed when the same voltage series is performed between ± 0.6 V (SI Figure S7). Interestingly, only minute changes are observed in the scattering signal as the voltage is varied, with the averaged scattering curves at constant applied cell voltage nearly coinciding (Fig. 3 a, grey, red, and blue full lines). The left zoomed-in inset in Fig. 3 a) shows details of the 220 in-plane peak pointing out subtle differences in peak intensity at contrasting electrode polarisations, which will be discussed further below. The close similarity of the curves indicates only very small changes to the form factor, suggesting that TFSI − ions remain anchored in their position close to the pore wall, being effectively immobilised even against the repelling electrostatic forces at a constant cell voltage of -0.4 V. It was shown in previous work that X-ray transmission data allow to determine the charge-balancing mechanism in electric-double layer capacitors 20 , 22 , 41 , 42 . The absence of systematic changes in the X-ray transmission signal with applied cell voltage (SI Figure S8) indicates that only the light Na + cations migrate and contribute to charge balancing. If the larger, more strongly X-ray absorbing TFSI − anions were involved, a clear change of the transmitted X-ray intensity with changing TFSI − concentrations would be expected (SI Table TS1 and Supplementary Note SN5). The observation of a purely cation-governed charge balancing is supported by recent experimental findings 19 , demonstrating that charge balancing is cation-dominated in Ni 3 (HITP) 2 -based supercapacitors using 1 M Net 4 BF 4 in deuterated acrylonitrile solvent (d 3 ACN). We hypothesize that such cation-dominated charge-balancing mechanism in Ni 3 (HITP) 2 is most likely promoted by the immobilization of TFSI − , probably at the N-H sites as discussed above. This potentially applies to fluorine-containing anions within MOF pores exhibiting N-H linkers in general, thereby favouring cation-driven charge storage. In the present work, this mechanism manifests as pure co-ion expulsion of Na + at positive voltage and pure counter-ion adsorption of Na + at negative voltage, as sketched in Fig. 4 . To better understand the mechanisms of Na + -driven electric double-layer formation with immobilized TFSI − , changes in the scattering signal at applied voltages were examined in detail. Figure 3 d) illustrates the relative changes of scattering intensity at low-q values (second panel from top) and the evolution of in-plane peak intensities for selected Bragg peaks (panels 3–7 from top) as the cell voltage (top panel) is varied. Interestingly, clear systematic variations are observed in the MOF-related intensities with the applied voltage, while the intensity of the PTFE-binder peak, shown in the bottom panel of Fig. 3 d) and the right inset of Fig. 3 a), remains perfectly constant across applied cell voltages. Since this peak is neither related to the MOF nor the electrolyte, it serves as an internal reference, confirming that the observed small intensity changes are highly reliable. Most interestingly, the intensity response to changes in applied cell voltage is consistent between cyclic voltammetry (CV) with gradual cell voltage increases and chronoamperometry (CA) with sudden cell voltage jumps, suggesting no mechanistic difference between slow charging and rapid cell voltage changes in this system 43 , 44 . The absence of any delay in the scattering response following cell voltage variation and the same levels of peak intensity changes reached in CV’s and in CA’s indicates that charge balancing and ion rearrangement occur very rapidly, as would be expected if only the highly mobile Na + ions in the aqueous electrolyte contribute to charge balance through electric double-layer formation 45 , 46 . These finding highlight the fascinating potential for systems with cation driven charge balancing to maintain high performance even at fast charging rates, addressing a key challenge in the development of MOF-based supercapacitors 47 . Ref. 25 used in-situ Small-Angle Neutron Scattering (SANS) to study a similar Ni 3 (HITP) 2 MOF electrode with NaOTf/dimethylformamide (DMF) electrolyte in a supercapacitor set-up, observing small changes at low q, which they interpreted as ions adsorbing onto the outer surface of the MOF particles. Similarly, we observe systematic changes in scattering intensity at low-q as the voltage is varied (second panel in Fig. 3 d). However, we do not discuss this effect further here, as the outer surface of the MOF particles is estimated to contribute less than 10% to the total surface area (Supporting Information Figure S9 and Note SN6). Given this small contribution from the outer surface compared to the inner surface of the pores, we focus here on the in-pore changes, where we assume the majority of charge balancing to take place. We note, however, that the comparison between outer-surface charge balancing, reflected in the low-q intensity changes (second panel in Fig. 3 d), and in-pore charge balancing, indicated by the Bragg peak intensity variations (other panels in Fig. 3 d), suggests no dynamic differences, including ion transport, between the charge balancing mechanisms at the outer surface of MOF particles and those occurring within the pores. There are some additional interesting details in the systematic intensity changes of the different peaks. Some of the peaks increase at positive and decrease at negative cell voltage (e.g. 300 and 220), others show exactly the opposite behaviour (e.g. 200, 210). Unfortunately, in this case our simple form factor model fails when trying to quantify this behaviour as it lacks the required sensitivity. The observed intensity changes of in-plane pore peaks, as shown in Fig. 3 d), can tentatively be attributed to a combination of effects, including distortions in the unit cell (discussed later) and changes in the form-factor resulting from (slight) variations in the electron density within pores as Na + adsorbs and desorbs, introducing an additional step in the radial electron density profile. With the Na + ion being very small, however, the electron density should only change slightly as Na + adsorbs and desorbs. The precise Na + adsorption sites within the pore space can therefore not be unambiguously determined through a form factor fitting of a multi-step cylinder. Alternatively, the Na + ions may be strongly associated with specific adsorption sites which may effectively change the structure factor of the MOF. Although interesting by itself, we abstain however from further attempts to quantify this behaviour since it is beyond the scope of this work. In-situ structural changes of Ni(HITP) electrodes With the detailed mechanisms of charge balancing established, we finally turn our attention to the structural changes in the MOF electrode. Both, the interlayer spacing and the pore centre-to-centre distance exhibited small, yet systematic, contraction and expansion of approximately 0.1% as the electrode polarization is varied (Fig. 5 ). The slight ‘breathing’ observed in the MOF aligns with adsorption-induced pore swelling, as was also reported from ion electrosorption in carbon-based electrodes 48 . Assuming that Na + is the only ion contributing to the charge balance, the observed expansion upon negative polarization (counter-ion adsorption) and contraction upon positive polarization (co-ion expulsion) is therefore consistent with the overall finding for the in-plane peak shifts (Fig. 5 a)). Contrary to the in-plane strain, however, the layer spacing c increases independently of the polarity of the applied cell voltage, showing expansion at both positive and negative cell voltages, though less pronounced at negative cell voltage (Fig. 5 b)). The origin of this auxetic-like out-of-plane expansion remains unclear. However, since potential Na + intercalation between layers would result in a much larger increase of layer spacing c, intercalation or other major structural changes upon charging and discharging can be ruled out. This is also consistent with the absence of an intercalation signature in the cyclic voltammogram in Fig. 3 c). Notably, after holding at -0.4 V for 1 hour, both the in-plane lattice parameter, a, and the layer spacing, c, exhibit a slight, seemingly irreversible increase. The counter electrode in this symmetric set-up, subjected to the opposite polarity, showed similar non-reversible increases in in-plane lattice parameter and layer spacing already after the initial long-term exposure to negative cell voltage (see SI Figure S9). These findings suggest that while for charge balancing the changes in intensity seem fully reversible and are independent of the polarity of the applied voltage (Fig. 3 d)), there are slight irreversible changes after applying a negative bias over prolonged periods of time. We hypothesize that these effects may be associated with degradation in electrochemical performance, potentially impacting the structural integrity and cycling stability of the MOF, which still poses a significant challenge for MOF based devices 47 . However, a detailed investigation of these processes is beyond the scope of the current work. Conclusions This study establishes a comprehensive understanding of the structural and electrochemical behaviour of a Ni 3 (HITP) 2 MOF-based supercapacitor with aqueous electrolyte using in-situ Small Angle X-Ray Scattering (SAXS). In the dry state, the MOF exhibits a well-defined hexagonally arranged cylindrical pore structure with a pore diameter of 1.39 nm, an average pore-centre distance of 2.20 nm, and a turbostratic (i.e. disordered) stacking with an interlayer distance of 0.33 nm. Upon addition of water or aqueous electrolyte, the layer spacing increases by approximately 1.44%, indicating a wetting induced swelling of the layers. We summarize the electrochemical behaviour of the Ni 3 (HITP) 2 -based supercapacitor with aqueous 1 M NaTFSI electrolyte as follows: Already at no applied cell voltage, TFSI − anions become immobilized within the cylindrical pores via F···H-N interactions with the NH moiety of the MOF linker, with a considerably higher TFSI − concentration at the surface layer. Charge balancing upon cell polarization occurs exclusively through mobile Na + cations via co-ion expulsion and counter-ion adsorption, with no evidence of ion intercalation between Ni 3 (HITP) 2 sheets. Notably, no mechanistic differences are observed between voltage steps and gradual voltage ramps. Charge balancing proceeds on comparable timescales on both the internal pore walls - which provide a significantly larger surface area - and the outer surface of larger MOF particles. Structurally, both the in-plane layer spacing and layer distance exhibit small reversible changes of around 0.1% due to adsorption induced swelling or contraction. After holding at negative cell voltage of -0.4 V for 1 hour, a minimal irreversible change is noted. This work provides some fundamental insights into the charge-balancing mechanisms of Ni 3 (HITP) 2 -based supercapacitors, and demonstrates furthermore a powerful experimental platform to investigate the relationship between electrode structure, functional groups, and charge storage behavior, such as i.e. specific ion anion anchoring in systems that require cation dominated charging. Future investigations could aim at a better understanding of the underlying mechanisms of electrode degradation during extended cycling or at higher operating voltages, as well as the precise origins of electrode swelling. Finally, this study underscores the value of MOFs as chemically well-defined model systems for exploring fundamental processes in energy storage, paving the way for insights that can inform the rational design and optimization of supercapacitors more broadly, beyond the specific performance of MOF-based systems. Materials and Methods Materials The investigated electrolyte salts - NaTFSI (97%), KTFSI (97%), Na 2 SO 4 (≥ 99.0%, anhydrous) and RbBr (99.6% trace metal basis) - were acquired from Sigma Aldrich. Electrolyte solutions were prepared by dissolving the appropriate amount of each salt in Milli-Q lab-grade H 2 O to achieve concentrations of 1 M or 0.1 M. For the MOF synthesis, NiCl 2 ・6H 2 O and ethanol were acquired from Sigma Aldrich, aqueous ammonia (NH 4 OH, 35 % NH 3 ) from Fisher Scientific, and 2,3,6,7,10,11-hexaiminotriphenylene hydrate (H 6 HITP・xH 2 O) from Chemextensions. All were used without further modification. Synthesis of Ni 3 (HITP) 2 and electrode preparation The MOF was synthesized following the protocol described in Ref 19 without further modification. Freestanding electrodes were prepared from the synthesized MOF powder and 5 wt.% PTFE binder (60 wt.% solution in water, Sigma-Aldrich) following the protocol described in 20 without the addition of carbon black or other additives. The MOF-PTFE slurry was rolled into sheets with a thickness of 200 ± 10 µm and dried at room temperature. To ensure the removal of residual moisture, the dried electrode sheets were placed in a vacuum tube furnace at 105°C for at least 24 hours prior to the in-situ measurements. X-Ray scattering High-resolution total X-ray scattering data shown in Fig. 1 b) of the dry Ni₃(HITP)₂ MOF were collected at the European Synchrotron Radiation Facility (ESRF) at the ID22 beamline (Grenoble, France) 49 . The electrode material was loaded into a quartz capillary and measured using a 13-channel Si(111) multi-analyzer stage while rotating the sample. The X-ray beam was 1 × 1 mm in size, with an exposure time of 120 seconds and a photon energy of 29 keV. To prevent beam-induced sample damage, the sample was translated by 1.1 mm between exposures, ensuring fresh material was exposed to the beam. A total of 22 scans were combined to obtain the final dataset. Lower resolution SAXS/WAXS measurements shown in Fig. 2 of the dry and water/electrolyte wetted MOF spanning a large q-range between 2.5–250 nm − 1 were also performed at ESRF (ID22 beamline) using a 2D flat-panel PerkinElmer XRD 1611CP3 detector and a photon energy of 60 keV. A beam size of 1 mm × 1 mm used, with an exposure time of 5 seconds. High resolution operando SAXS experiments of the electrolyte wetted EDLCs at different electrical cell voltages, as shown in Fig. 3 , were conducted at the Austrian SAXS beamline at ELETTRA Sincrotrone Trieste (Italy) 50 . This was done using a custom-built operando electrochemical cell, designed to enable simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS) experiments. This cell, adapted from a design previously used and described in Ref. 22 , features slit shaped Kapton foil windows (SI Figure S11), allowing X-rays in transmission geometry to scan the electrode area. Circular electrodes (14 mm diameter) with off-center 3 mm holes were punched from the electrode sheets and stacked in the cell with a glass fiber separator (40 mm diameter, 200 µm thickness, Whatman GF/A). The off-center holes were arranged not to be congruent, enabling X-rays to independently penetrate each electrode, allowing data collection for both electrodes individually (SI Figure S11). Platinum paper (< 200 nm thickness) was used as a current collector covering the entire electrode area. The cells were subjected to a cell voltage sequence using a Gamry Interface 1010B potentiostat. Before measurements, the electrochemical cells were conditioned with five cycles of cyclic voltammetry (CV) between ± 0.4 V at a scan rate of 10 mV/s. The X-ray beam with a photon energy of 16 keV was focused to a size of 0.5 mm × 2 mm. Data were collected using a Pilatus3 1M 2D detector (Dectris Ltd., Baden-Dättwil, Switzerland). Exposure time for each SAXS measurement was 20 seconds, and the sample was moved between two positions to alternately measure the two electrodes individually. All 2D scattering patterns were azimuthally integrated to obtain 1D scattering profiles, showing the intensity versus the length of the scattering vector ( \(\:q=4\pi\:sin\theta\:/\lambda\:\) , \(\:2\theta\:\) being the scattering angle and \(\:\lambda\:\) the wavelength). Standard data normalization and correction procedures at the respective beamline, include corrections for primary beam intensity changes, sample transmission, and exposure time. Data treatment Diffraction peaks in the scattering profiles were fitted using a custom-written script for Python 3 using a Pseudo-Voigt peak shape and a decaying exponential background 51 . The in-plane lattice parameter describes the pore-centre to pore-centre distance and was calculated according to Eq. 1: \(\:a=\:\frac{4\pi\:}{{q}_{hk0}\sqrt{3}}*\sqrt{{h}^{2}+{k}^{2}+hk}\) Eq. (1) Where q hk0 describes the q-position of the in-plane diffraction peak with Miller Indices hk(l = 0). The layer spacing c was calculated from the (002) stacking peak position as in Eq. 2: \(\:c=\:\frac{2\pi\:}{{q}_{002}}\) Eq. (2) The strain for the lattice parameter a and the layer spacing c was calculated according to Eq. 3: \(\:strain=\:\frac{l-{l}_{0}}{{l}_{0}}\) Eq. (3) With l, l 0 being the actual and the reference values for the in-plane lattice parameter a or the layer spacing c, respectively. The total scattering intensity I(q) for infinitively long cylinders arranged on a 2D hexagonal lattice can be written as in Eq. 4: \(\:I\left(q\right)=K*S\left(q\right){*\left|F\left(q\right)\right|}^{2}\) Eq. (4) where K is a constant factor, S(q) is the spherically averaged structure-factor described by sharp diffraction peaks at discrete q-values \(\:{q}_{hk0}\) defined in Eq. 1, and |F(q)| 2 describes the form-factor 32 . According to Eq. 4, the height of each Bragg peak hk0 from the pore lattice is determined by the respective value of \(\:{\left|F\left({q}_{hk0}\right)\right|}^{2}\) . The form-factor for infinitely long monodisperse cylindrical pores |F(q) cyl .| 2 was used here, following the approach first introduced in Ref 38 and adapted by Ref 32 for multistep core-shell cylinders, in which the scattering amplitude F(q) is given by: \(\:F\left(q\right)=\:\frac{{\varSigma\:}_{i=1}^{N}({\rho\:}_{i}-{\rho\:}_{i-1}){R}_{i}^{2}Z\left(q{R}_{i}\right)}{{\varSigma\:}_{i=1}^{N}({\rho\:}_{i}-{\rho\:}_{i-1}){R}_{i}^{2}}\) Eq. (5) Z(qR i ) is given by 2J 1 (qR)/(qR) with J 1 being the Bessel function of first kind and first order, and ρ i and R i are the electron density and radius of the i-th cylindrical shell, starting from the outermost shell. By using a custom written Python Code (Supplementary Notes SN7), the integrated intensities of Bragg peaks in the experimental data were analysed with a GUI based interface, similar to the approach used Ref. 37 . The analysis files including the software code will be made available upon request. The specific capacitance of the symmetrical two-electrode supercapacitor cell was calculated from cyclic voltammetry according to Eq. 5: \(\:C=\:\frac{4*{\int\:}_{t1}^{t2}I\left(t\right)dt}{\varDelta\:U*{m}_{total}}\) Eq. (6) With I(t) describing the current, ΔU the voltage window and m total the combined mass of both electrodes. Declarations Author Contributions M.S.: Conceptualization, Methodology, Software, Formal analysis, Investigation, Data Curation, Writing - Original Draft, Writing - Review & Editing, Visualization, Project administration; C. J. B.: Investigation, Resources, Writing - Review & Editing, M. V. R.: Investigation, Writing - Review & Editing; S.S.: Writing - Review & Editing; G.F.-P.: Software, Writing - Review & Editing; T.L.: Software, Writing - Review & Editing; D.H.: Writing - Review & Editing; H.A.: Data Curation, Writing - Review & Editing; A.C.F.: Writing - Review & Editing; O.P.: Conceptualization, Methodology, Writing - Original Draft, Writing - Review & Editing, Supervision, Project administration. Acknowledgements We acknowledge CERIC-ERIC for access to the Austrian SAXS beamline at ELETTRA, as well as Andrew Fitch and Catherine Dejoie for support at the ID22 Beamline at ESRF for beamtime SC5511. We also acknowledge Jamie Gittins for synthesis of the Cu 3 (HTTP) 2 sample. We would also like to thank Peter Moharitsch at Montanuniversität Leoben for machining of the electrochemical operando cell. M.S. thanks Markus Kratzer for many constructive discussions. References Supercapacitors- Materials, Systems and Applications . 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ACS Nano 12:9733–9741 Breitsprecher K et al (2020) How to speed up ion transport in nanopores. Nat Commun 11:6085 Yu Z et al (2022) Solvation Structure and Dynamics of Mg(TFSI)2 Aqueous Electrolyte. Energy Environ Mater 5:295–304 Hayamizu K, Chiba Y, Haishi T (2021) Dynamic ionic radius of alkali metal ions in aqueous solution: a pulsed-field gradient NMR study. RSC Adv 11:20252–20257 Shin SJ, Gittins JW, Balhatchet CJ, Walsh A, Forse AC (2023) Metal–Organic Framework Supercapacitors: Challenges and Opportunities. Adv Funct Mater 2308497:1–11 Koczwara C et al (2017) In Situ Measurement of Electrosorption-Induced Deformation Reveals the Importance of Micropores in Hierarchical Carbons. ACS Appl Mater Interfaces 9:23319–23324 Fitch A et al (2023) ID22 - the high-resolution powder-diffraction beamline at ESRF. J Synchrotron Radiat 30:1003–1012 Amenitsch H et al (1998) First performance assessment of the small-angle X-ray scattering beamline at ELETTRA. J Synchrotron Radiat 5:506–508 Newville M, Stensitzki T, Allen DB, Ingargiola ALMFIT (2014) Non-Linear Least-Square Minimization and Curve-Fitting for Python. 10.5281/ZENODO.11813 Additional Declarations There is NO Competing Interest. Supplementary Files AnchoredAnionsMobileCationsSI.docx Supplementary Information: Anchored Anions, Mobile Cations: Charge Storage in MOF-based Supercapacitors Studied with Operando Small-Angle X-ray Scattering Cite Share Download PDF Status: Published Journal Publication published 30 Sep, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5973632","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":424067554,"identity":"bb5466a8-812f-4e73-985b-37be6836c62e","order_by":0,"name":"Oskar Paris","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABHUlEQVRIie2RMWvCQBTHnxyY5YVbL1iar3AhENuh9askBDJlKHQJCG2gEJe639B+j45KwCxB1xQdLIFshYKTdOkpQgVPdHS433LH437833sHoNFcJO3dSaC1BGBXAHi+QrhUcE8hJxR5Y7B9f0KhdBKsEljY3YHR9M2PG6TGcFw/wL0NxmykUiwR5VYJjfOWozc3S4bW6zR0BYROiqEyhVdGaqWQtwTB9tzMGPIq9joIxAc4osyKl7VUeoIYzeNG6VVx9xfh2Qdaq5VRNNmkBIKAR7YpLPYIQu4DU6fIWaLblDehbMy13qXCymnYQV44Gav5kY25n2myuBO0+Pr5zp6u6WA4XmHStykNlsqYXXsHlf0PU3KgaDQajeafP5hLUBxGR3OjAAAAAElFTkSuQmCC","orcid":"","institution":"Montanuniversitaet Leoben","correspondingAuthor":true,"prefix":"","firstName":"Oskar","middleName":"","lastName":"Paris","suffix":""},{"id":424067555,"identity":"32476552-7dd8-4bff-ae23-b66f091ee684","order_by":1,"name":"Malina Seyffertitz","email":"","orcid":"https://orcid.org/0009-0001-6014-5995","institution":"Montanuniversität Leoben, Austria","correspondingAuthor":false,"prefix":"","firstName":"Malina","middleName":"","lastName":"Seyffertitz","suffix":""},{"id":424067556,"identity":"18acec1e-2e5a-4466-94fe-9ebaacfb19d8","order_by":2,"name":"Chloe Balhatchet","email":"","orcid":"","institution":"University of Cambridge, United Kingdom","correspondingAuthor":false,"prefix":"","firstName":"Chloe","middleName":"","lastName":"Balhatchet","suffix":""},{"id":424067557,"identity":"c0953428-017c-4eff-b4d1-42e19aae90eb","order_by":3,"name":"Max Rauscher","email":"","orcid":"","institution":"Montanuniversität Leoben, Austria","correspondingAuthor":false,"prefix":"","firstName":"Max","middleName":"","lastName":"Rauscher","suffix":""},{"id":424067558,"identity":"cf3ed749-c83c-4383-8c1c-33134625e803","order_by":4,"name":"Sebastian Stock","email":"","orcid":"","institution":"Montanuniversität Leoben, Austria","correspondingAuthor":false,"prefix":"","firstName":"Sebastian","middleName":"","lastName":"Stock","suffix":""},{"id":424067559,"identity":"a5842905-b887-408e-9195-116a37b10af0","order_by":5,"name":"Gerhard Fritz-Popovski","email":"","orcid":"","institution":"Montanuniversität Leoben","correspondingAuthor":false,"prefix":"","firstName":"Gerhard","middleName":"","lastName":"Fritz-Popovski","suffix":""},{"id":424067560,"identity":"b9b6b638-674e-4d20-b825-bf976735e4e7","order_by":6,"name":"Thomas Leiner","email":"","orcid":"","institution":"Department of Materials Science, Montanuniversität Leoben","correspondingAuthor":false,"prefix":"","firstName":"Thomas","middleName":"","lastName":"Leiner","suffix":""},{"id":424067561,"identity":"a3d4650c-e2fc-438b-b6fb-30c280120fdf","order_by":7,"name":"David Holec","email":"","orcid":"","institution":"Department of Materials Science, Montanuniversität Leoben","correspondingAuthor":false,"prefix":"","firstName":"David","middleName":"","lastName":"Holec","suffix":""},{"id":424067562,"identity":"a0baf269-ca6a-4e7d-b61a-c8e08794d2e2","order_by":8,"name":"Heinz Amenitsch","email":"","orcid":"","institution":"Graz University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Heinz","middleName":"","lastName":"Amenitsch","suffix":""},{"id":424067563,"identity":"506eb229-4315-4c43-985a-5e7ddd5844f5","order_by":9,"name":"Alexander Forse","email":"","orcid":"https://orcid.org/0000-0001-9592-9821","institution":"University of Cambridge","correspondingAuthor":false,"prefix":"","firstName":"Alexander","middleName":"","lastName":"Forse","suffix":""}],"badges":[],"createdAt":"2025-02-06 12:45:08","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5973632/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5973632/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-025-63772-w","type":"published","date":"2025-09-30T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":77755643,"identity":"5712b875-fab7-4b63-9d0e-8f4f9f80f8e3","added_by":"auto","created_at":"2025-03-05 08:26:25","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":268568,"visible":true,"origin":"","legend":"\u003cp\u003eScattering profiles (black lines) of the dry Ni₃(HITP)₂ MOF electrode in a) SAXS and b) WAXS regions represented by a full black line. We define “SAXS” and “WAXS” here pragmatically by the q-ranges covered by the two detectors used to collect the signal, with an overlap between 14 and 16 nm\u003csup\u003e-1\u003c/sup\u003e. The dashed black line in a) shows the single-step form factor model |F(q)\u003csub\u003ecyl.\u003c/sub\u003e|\u003csup\u003e2\u003c/sup\u003e for infinitely long monodisperse cylindrical pores with diameter D = 1.39 nm. Vertical grey lines represent in-plane (hk0) peaks, orange dash-dotted lines indicate out-of-plane (hkl, l ≠ 0) peaks, dashed red lines mark stacking peaks (00l). In-plane peaks from the atomic scale carbon triphenylene structure are marked by dotted green lines (c-hk0). For clarity, the vertical grey lines in the WAXS pattern are only shown up to 18 nm\u003csup\u003e-1\u003c/sup\u003e (see SI, Figure S1 for full picture). The PTFE-binder peak at q = 12.4 nm\u003csup\u003e-1\u003c/sup\u003e is not related to the MOF structure. The inset in b, visualized using Mercury 3.7\u003csup\u003e33\u003c/sup\u003e, shows the top-view and AB-stacked side view of Ni₃(HITP)₂ with some sets of exemplary families of planes. The zoomed-in section of the inset shows the detailed structure with labeled elements.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-5973632/v1/c6970ccf67fafeb0b84ee398.png"},{"id":77755641,"identity":"4d7fe98d-de4b-4bf3-bdcd-6aa329262059","added_by":"auto","created_at":"2025-03-05 08:26:25","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":117880,"visible":true,"origin":"","legend":"\u003cp\u003ea) Comparison of the dry Ni₃(HITP)₂ electrode (solid black line) with its wetted state using pure H₂O (solid light blue line) and 1 M NaTFSI (solid orange line). The corresponding bulk electrolyte signals for H₂O and 1 M NaTFSI (aq.) are shown as blue and orange dotted lines, respectively. b) Scattering profiles from the wetted electrodes with bulk liquid contributions subtracted. The inset illustrates that the shift of the stacking peak indicates an increased layer spacing upon wetting (not to scale), while deviations in the in-plane peak intensities relative to the dry MOF suggest an inhomogeneous electrolyte distribution within the pore, i.e., a higher ion concentration close to the pore walls.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-5973632/v1/2b3aad0cac41d7c80cb7ffea.png"},{"id":77755645,"identity":"baa730f8-1c16-4760-9d24-f68e97262176","added_by":"auto","created_at":"2025-03-05 08:26:25","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":227012,"visible":true,"origin":"","legend":"\u003cp\u003ea) SAXS data of Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e in the dry state (black) and wetted with 1 M NaTFSI at 0 V (grey), +0.4 V (red), and -0.4 V (blue) applied cell voltage. The corresponding best fitting form factors |F(q)\u003csub\u003ecyl.\u003c/sub\u003e|\u003csup\u003e2\u003c/sup\u003e for the dry and filled states are included as dashed and dotted lines, respectively. The inset highlights the 220 and PTFE peaks. b) Schematic representation of the pore filling and electron density profiles for the dry and wetted states, indicating the formation of a core-shell electrolyte distribution with a TFSI\u003csup\u003e-\u003c/sup\u003e-rich layer of about ~0.25 nm thickness along the MOF pore wall. The top right provides a sketch of a TFSI\u003csup\u003e-\u003c/sup\u003e ion in cis-configuration showing approximate dimensions with atoms represented as blue for nitrogen (N), orange for sulphur (S), red for oxygen (O), grey for carbon (C), and yellow-green for fluorine (F). c) Cyclic voltammograms (CVs) recorded at 1 mV/s before (94 F/g, dark green) and after (104 F/g, light green) the chronoamperometry (CA) steps. d) Cell voltage (top panel) showing CV and CA cycles, with time-resolved data for the average relative scattering intensities at low-q between 0.34 and 0.64 nm\u003csup\u003e-1\u003c/sup\u003e and relative peak intensities of selected peaks (200, 210, 300, 220 and PTFE).\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-5973632/v1/7a0c5239fc2b70610e6c1f61.png"},{"id":77755648,"identity":"2e62e40b-00bf-4f7f-8eab-cf5219876337","added_by":"auto","created_at":"2025-03-05 08:26:25","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":200039,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic representation of the charging and discharging behaviour of Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e MOF electrodes with aqueous 1 M NaTFSI electrolyte. The schematic highlights immobilized TFSI\u003csup\u003e-\u003c/sup\u003e ions near the pore walls, even in the absence of an applied external cell voltage, likely due to hydrogen bond interactions between the fluorine atoms (yellow-green) of TFSI\u003csup\u003e-\u003c/sup\u003e and the NH groups (blue and light grey) of the MOF linker. Charge balancing is cation-dominated, with co-ion expulsion of Na\u003csup\u003e+\u003c/sup\u003e at positive cell voltage and counter-ion adsorption of Na\u003csup\u003e+\u003c/sup\u003e at negative cell voltage. The dashed circles in the pore at 0 V indicate the pore diameter (1.39 nm) and the boundary of the TFSI\u003csup\u003e-\u003c/sup\u003e-rich layer (0.9 nm) from the Form Factor.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-5973632/v1/a0870eb83d02e64b0dd4999c.png"},{"id":77755824,"identity":"535edce0-be63-4223-ad5c-f77a2c908303","added_by":"auto","created_at":"2025-03-05 08:34:25","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":122054,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of the a) in-plane lattice parameter a and b) layer spacing c and the associated strain as a function of applied cell voltage (black thin line). The light colour line in the background shows the measured data, while the thicker darker line represents a smoothed moving point average over 8 data points. The zoomed in section on the bottom left of each plot shows the first 50 minutes for a more detailed view of the CVs. Corresponding plots for the counter electrode, under reversed polarity, are provided in SI Figure S10.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-5973632/v1/ebcfdf001d325d9c6011f546.png"},{"id":92568898,"identity":"6748102c-ae07-47cb-9ed5-1e222e693e26","added_by":"auto","created_at":"2025-10-01 07:20:25","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1638785,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5973632/v1/13a3708a-8d7a-41e5-8c53-e8bb5c8ec35d.pdf"},{"id":77755650,"identity":"a47df25e-4b34-4749-b907-88cd2781a1ab","added_by":"auto","created_at":"2025-03-05 08:26:25","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":3095820,"visible":true,"origin":"","legend":"Supplementary Information: Anchored Anions, Mobile Cations: Charge Storage in MOF-based Supercapacitors Studied with Operando Small-Angle X-ray Scattering","description":"","filename":"AnchoredAnionsMobileCationsSI.docx","url":"https://assets-eu.researchsquare.com/files/rs-5973632/v1/aab55d5691bb34bd65517782.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Anchored Anions, Mobile Cations: Charge Storage in MOF-based Supercapacitors Studied with Operando Small-Angle X-ray Scattering","fulltext":[{"header":"Introduction","content":"\u003cp\u003eAs the demand for efficient electrical energy storage continues to grow, electric double-layer capacitors (EDLCs), or supercapacitors, are emerging as promising devices for bridging energy needs across various sectors. Owing to their high power-density, supercapacitors can be of particular importance for applications in electric vehicles, smart power grids and intermittent energy demands in light of managing the transition from fossil fuels to renewable energy\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. A comprehensive molecular-scale understanding of electric double layer formation is therefore considered essential for optimizing the performance of supercapacitors and driving their successful integration in a wider range of applications\u003csup\u003e\u003cspan additionalcitationids=\"CR3 CR4 CR5 CR6\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eTo date, activated carbons are predominantly used as supercapacitor electrode materials due to their high share of microporosity (i.e., pores smaller than 2 nm), chemical stability, electrical conductivity, low cost and overall sustainability, as they can be derived from waste organic materials\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. While organic electrolyte solvents are often preferred in technical applications due to their higher electrochemical stability window\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e, water as solvent offers the advantages of easy handling under ambient conditions, high ion mobility, low cost, reversible redox activity, and overall environmental friendliness\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e. However, gaining a mechanistic understanding of activated carbon-based aqueous supercapacitors is often challenging due to their disordered pore structures and complex electrode-electrolyte interactions, which hinder straightforward data interpretation\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. To address these challenges, model materials with well-defined and tunable properties are highly valuable for investigating the mechanisms of charge storage. Metal-organic frameworks (MOFs) stand out as a promising class of such materials due to their highly ordered nanoporous structures and chemically tunable properties. Traditionally, MOFs have been limited by poor electrical conductivity, restricting their use in ion storage devices. However, recent advances have introduced a class of electrically conductive MOFs characterized by 2D π-d conjugated layers that stack into honeycomb-like structures\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eOne prominent example is the Ni\u003csub\u003e3\u003c/sub\u003e(2,3,6,7,10,11-hexaiminotriphenylene)\u003csub\u003e2\u003c/sub\u003e (Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e) MOF, introduced by Sheberla et al.\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e, which showed promising capacitance values on par with or even superior to conventional carbon electrodes\u003csup\u003e\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. The ordered pore structures, tunable surface chemistries, and well-defined functional groups of MOFs such as Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e provide an exciting opportunity to systematically explore how electrode architecture, pore geometry, and chemical functionality influence charge storage behavior. Insights gained from such studies not only improve our understanding of MOF-based systems but could also help uncover more general principles of charge storage that can inform the development of next-generation supercapacitor materials more broadly. Recent research efforts have explored the influence of particle morphology on electrochemical behavior using conductive MOFs\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e, as well as the effects of cation size\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e on EDL formation, suggesting that cation dynamics primarily govern EDL formation in MOFs \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. Despite these advancements, critical knowledge gaps remain. The detailed mechanisms underlying local ion rearrangement in the pore space, the interplay between electrosorption behavior and functional groups, and the structural changes that occur during charge and discharge cycles remain poorly understood.\u003c/p\u003e \u003cp\u003eIn-situ Small-Angle Scattering of X-rays (SAXS) or neutrons (SANS) together with Wide-Angle X-ray Scattering (WAXS) have proven effective in analyzing complex multi-component ion storage systems such as electric double-layer capacitors (EDLCs)\u003csup\u003e\u003cspan additionalcitationids=\"CR21\" citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e or batteries\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. The scattering signal from EDLCs is generally composed of contributions from the nanoporous solid electrodes, the electrolyte solvent in- and outside the pores, as well as cations and anions with dynamic local and global concentrations, which often requires new ways of data treatment by, e.g., combining experimental results with computational modelling\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. However, sharp Bragg peaks originating from highly ordered pores, such as in MOFs, allow for a more direct interpretation of changes in the scattering signal towards physical phenomena, such as ion rearrangement and pore swelling, in real-time. Recent work on in-situ ion electrosorption in MOFs\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e and carbons\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e,\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e, as well as previous work on gas adsorption in carbons and silica\u003csup\u003e\u003cspan additionalcitationids=\"CR29 CR30 CR31\" citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e highlights small-angle scattering as a powerful method for studying guest-host interactions in ordered nanoporous systems, with the prospect to better understand fundamentals of charge storage behavior in EDLCs.\u003c/p\u003e \u003cp\u003eIn this study, we employ SAXS/WAXS to investigate Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e3\u003c/sub\u003e MOF-based supercapacitors operated with aqueous electrolytes and different charging protocols. We track subtle structural changes of the MOF framework, including wetting- and electrosorption-induced pore swelling, while also assessing whether ion intercalation takes place. Furthermore, the approach allows for real-time, i.e., \u003cem\u003eoperando\u003c/em\u003e investigation of a full EDLC device. This delivers critical information on local ion rearrangement within pores in response to an applied cell voltage, providing insights into how the immobilization of specific anions drives cation-dominated charge storage. The observed anion-pinning effects may offer a general strategy to tune supercapacitor charging mechanisms and performance.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eCharacterization of dry Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e electrodes\u003c/h2\u003e \u003cp\u003eThe nanoscale origin of the electrochemical behaviour of Ni\u003csub\u003e3\u003c/sub\u003e(2,3,6,7,10,11-hexaiminotriphenylene)\u003csub\u003e2\u003c/sub\u003e Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e metal-organic framework (MOF) based supercapacitors was assessed by small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). Before addressing operando changes of the scattering patterns with electrolyte present, it is essential to first establish a comprehensive understanding of the MOF structure and related scattering features in the dry state of the electrode. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e displays the scattering signal of the neat Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e MOF electrode in the SAXS (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea)) and the WAXS region (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb)).\u003c/p\u003e \u003cp\u003eThe inset in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb)) displays the real space structural model (top view and side view) of an AB stacked Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e model as reported in\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. The most obvious structural features of this system are the layered structure and the regular arrangement of cylindrical pores perpendicular to the layers on a 2D hexagonal lattice. To better understand the structural features of the MOF and to facilitate comparison with literature, we classify the expected X-ray diffraction peaks into four distinct categories: (i) peaks arising from the 2D pore lattice (vertical gray lines indicating in-plane hk0 reflections), (ii) peaks from the layered structure of the MOF, which give rise to stacking peaks 00l (dashed red lines in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb)), (iii) mixed reflections (hkl, with h or k\u0026thinsp;\u0026ne;\u0026thinsp;0, and l\u0026thinsp;\u0026ne;\u0026thinsp;0, orange dash-dotted lines in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea)), and (iv) peaks attributed to the carbon structure of the triphenylene linker, which produce two broad reflections labeled c-100 and c-110 (green dotted lines in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb)). To clearly distinguish between these groups, an example from each category - in-plane pore (solid grey line), layer stacking (red dashed line), mixed (orange dash-dotted line), and triphenylene carbon (green dotted line) - is illustrated in the inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). The peak near q\u0026thinsp;=\u0026thinsp;12.4 nm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea) originates from the 5 wt.% PTFE binder in the Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e electrode and is not related to the MOF structure, but acts as a useful internal standard.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAll in-plane peaks (indices hk0), corresponding to the 2D hexagonal (P6mm) arrangement of cylindrical pores (grey solid lines in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea) and 1b), appear at the expected q-positions with a pore-centre to pore-centre distance a\u0026thinsp;=\u0026thinsp;2.20 nm. The stacking peaks (002 and 004, red dashed lines in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) correspond to a layer spacing of c\u0026thinsp;=\u0026thinsp;0.33 nm. Surprisingly, no peaks with mixed indices, i.e. with h or k\u0026thinsp;\u0026ne;\u0026thinsp;0, and l\u0026thinsp;\u0026ne;\u0026thinsp;0 are observed in the SAXS pattern (see orange dash-dotted vertical lines in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). The absence of mixed-index peaks strongly suggests that the layers are not perfectly stacked in a conventional AB or AA pattern to form a 3D crystal. Instead, the scattering signal resembles a so-called \u0026lsquo;turbostratic\u0026rsquo; stacking\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e,\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e, where the layer spacing is regular, but the in-plane positional correlations between the layers lack long-range order leading to the absence of peaks with mixed indices. This interpretation is further supported by the asymmetry of the c-100 peak from the carbon structure within the framework, a characteristic feature of turbostratic carbons, as described in Ref.\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. This finding is consistent with Ref.\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e, which characterizes the stacking of Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e as \u0026lsquo;near-eclipsed\u0026rsquo;. We have undertaken atomistic simulations of the MOF structure, which reveals a very small energy difference between AB and stacked layer configurations (SI Figure S2 and Supplementary Note SN1). Hence, our results strongly indicate a Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e structure where the layers exhibit no long-range positional order in stacking direction, reflecting the absence of a well-defined stacking sequence.\u003c/p\u003e \u003cp\u003eIn addition to the stacking characteristics, the pore diameter of \u003cem\u003eD\u003c/em\u003e \u0026asymp; 1.39 nm was determined from the SAXS data in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea) by analysing the experimental data with a single-step form-factor model of infinitely long monodisperse cylindrical pores with circular cross-section\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e,\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e,\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e,\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e (dashed black line in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea)). The regular arrangement of identical pores gives rise to sharp Bragg peaks at distinct positions, with their height being proportional to the pore form-factor (see Eq.\u0026nbsp;4 in Materials and Methods). This model explains particularly the low intensities observed for the (110) and (300) in-plane reflections, as these nearly coincide with the form-factor minima for pores of this diameter. Surprisingly, this simple approach reproduces the intensities of the diffraction peaks in the SAXS regime very well, although it does neither consider the inhomogeneous electron density distribution in the MOF structure nor the fact that the pores are not perfectly cylindrical in shape. We note that a more advanced (numerical) form-factor model which includes the circularly non-symmetric electron density of the MOF pore walls, leads to a very similar radially averaged form-factor (see SI, supplementary note SN4 and Figure S5). The mean pore diameter of 1.39 nm from the form-factor models was found to perfectly align with Ar@87K gas sorption data evaluated using a zeolite non-local density functional theory (NLDFT) kernel for cylindrical pores on the adsorption branch (see SI Figure S3). The sample\u0026rsquo;s specific surface area (SSA) was assessed using Ar@87K gas sorption analysis and resulted in a Brunauer\u0026ndash;Emmett\u0026ndash;Teller (BET) area of 390 m\u003csup\u003e2\u003c/sup\u003e/g.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eCharacterization of wetted Ni(HITP) electrodes\u003c/h3\u003e\n\u003cp\u003eAfter establishing the structural model of the dry Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e MOF electrode, attention is now directed towards its behaviour upon the addition of (aqueous) electrolytes. The data presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e were recorded on a flat-plane detector allowing the continuous acquisition of a very wide q-range using a single detector. While the resolution of this experimental set-up using a single detector is limited, the extended q-range is particularly useful for quantitatively considering the scattering contribution of the bulk electrolyte within the wetted electrode. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea) compares the scattering signal of the dry electrode (full black line) to its wetted state in the presence of pure water (light blue full line) and of aqueous 1 M NaTFSI (full orange line). For reference, the recorded scattering signals from bulk liquid water and 1 M NaTFSI (aq.) bulk liquid electrolyte at the same temperature are shown with corresponding dotted lines.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe characteristic diffraction peaks of the Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e MOF remain visible in the wetted state, although strong diffuse scattering contributions from the bulk-liquids are dominant particularly at larger q above 13 nm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea)). Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb) shows the data after the bulk liquid contributions were subtracted from the wetted electrode data. It is evident that the 002 stacking-peak at approximately 18 nm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e clearly shifts towards smaller q values for the MOF soaked in H\u003csub\u003e2\u003c/sub\u003eO or 1 M NaTFSI (aq.), indicating an increase in the layer stacking distance by 1.44% in the wetted state. In contrast, the positions of the in-plane (hk0) peaks remain largely unchanged, suggesting that noticeable dimensional changes of the MOF occur predominantly along the stacking direction. Since this is true for both, pure water and aqueous 1 M NaTFSI electrolyte, we attribute this effect to a wetting-induced swelling of the MOF in stacking direction. Aside from the shift in the stacking peak and a slight deviation at low q values (attributed to changes in contrast between MOF particles and their surrounding medium), the bulk-water corrected scattering curve of the H\u003csub\u003e2\u003c/sub\u003eO filled electrode closely follows that of the neat MOF (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), black and blue lines, respectively). This suggests that the scattering can be interpreted as a combination of bulk H\u003csub\u003e2\u003c/sub\u003eO and the dry MOF, with a homogeneous distribution of H\u003csub\u003e2\u003c/sub\u003eO within the pores.\u003c/p\u003e \u003cp\u003eIn contrast to pure water wetting, the corrected scattering profile for aqueous 1M NaTFSI does not resemble a simple incoherent sum of the dry MOF signal and the bulk 1M NaTFSI aqueous solution signal, evident by the drastic change in peak heights as compared to the dry MOF. This indicates the presence of an inhomogeneous electrolyte distribution within the pores, leading to a changed electron density distribution, and consequently a changed scattering form-factor, which directly influences the peak intensity. By way of an example, the inset in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb) illustrates an inhomogeneous distribution of electrolyte in the pore with a higher electrolyte concentration near the pore walls. A quantitative treatment explaining the observed changes in the heights of the peaks will be presented further below.\u003c/p\u003e \u003cp\u003eScattering profiles similar to that from 1 M NaTFSI (orange line in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) were also obtained from electrodes wetted with 1 M KTFSI and 0.1 M NaTFSI (SI Figure S4a)). In contrast, scattering patterns closely resembling those of electrodes wetted with pure H₂O (light blue line in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) were observed for 1 M RbBr (aq.) and 1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e (aq.) (SI Figure S4b)). This suggests that the observed Bragg peak intensity changes are specifically related to the presence of TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e anions, rather than to the presence of electrolyte ions or solvent in the pores in general. Notably, the similarity between the scattering patterns of 1 M NaTFSI (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb)) and 0.1 M NaTFSI (SI Figure S4a)) indicates that presumably a high concentration of TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e is inhomogeneously distributed within the pore space, even without an applied cell voltage.\u003c/p\u003e \u003cp\u003eDue to resolution limitations of the measurements in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, it was not possible to accurately fit a form factor model in order to fully characterize the electrolyte distribution. To address this, further high-resolution SAXS measurements were conducted to confirm the electrolyte concentration gradients within the pores and to study the specific ion arrangement in more detail. These experiments were conducted \u003cem\u003eoperando\u003c/em\u003e during the charging and discharging of a full supercapacitor cell in order to additionally investigate the implications of this inhomogeneous electrolyte distribution for the systems electrochemical behaviour under working conditions.\u003c/p\u003e\n\u003ch3\u003eSpecific TFSI adsorption at 0 V\u003c/h3\u003e\n\u003cp\u003eFigure 3a) presents the SAXS signal of the working electrode in the wetted state with 1 M NaTFSI at 0 V (grey), +0.4 V (red) and -0.4 V (blue). A hole was punched into the counter electrode enabling to collect the scattering signal for just one electrode specifically\u003csup\u003e20\u003c/sup\u003e. For reference, the scattering profile from the dry electrode has been added again as a full black line. The bulk electrolyte was not measured for this specific experimental set-up and could therefore not be subtracted. The high resolution SAXS data in Figure 3a) confirms the result from Figure 2 about noticeably relative Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e Bragg peak intensity changes when electrolyte is added. Diffraction peaks near a form factor minimum, such as the 110 and 300 peaks, are particularly sensitive to form factor changes, and thus, to variations of the electron density distribution within the pores. Upon electrolyte addition, the 110 peak completely disappears (i.e., it now perfectly matches the form-factor minimum), while the 300 peak gains relative intensity. Moreover, the intensity ratio of the 200 and 210 peaks is roughly inverted (Figure 3a)).\u003c/p\u003e \u003cp\u003eThe simplest possible form-factor to explain these changes is a two-step core-shell model according to Ref.\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e consisting of three parameters (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb)). The outer pore diameter remained fixed at D\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1.39 nm (as in the dry state), marking the cylindrical MOF pore wall, while an inner diameter of D\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.9 nm forms a shell of approximately 0.25 nm thickness with slightly higher electron density as compared to the rest of the electrolyte-filled pore (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb)). Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e summarizes the parameters for the unfilled and filled pore states that best fit the experimental data, while the corresponding form factor models are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea) by dashed and dotted lines, respectively. The associated electron densities for individual ions and bulk electrolytes are provided in SI Table ST1.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDiameters and electron densities in the dry and 1 M NaTFSI (aq.) wetted state as resulted from the two-step cylindrical core-shell form-factor model.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eD\u003csub\u003e1\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eρ\u003csub\u003eMOF\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eρ\u003csub\u003epore\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eD\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eρ\u003csub\u003eTFSI\u0026minus;rich\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003enm\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ee\u003csup\u003e\u0026minus;\u003c/sup\u003e/\u0026Aring;\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ee\u003csup\u003e\u0026minus;\u003c/sup\u003e/\u0026Aring;\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003enm\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003ee\u003csup\u003e\u0026minus;\u003c/sup\u003e/\u0026Aring;\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003edry\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.39\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.781\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003ewetted\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.39\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.781\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.350\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.430\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eSince none of the other Na-containing electrolytes, but all TFSI-containing electrolytes, showed a change in the form factor (SI Figure S4), this layer of higher electron density observed in NaTFSI electrolyte is interpreted as a TFSI-rich region near the pore wall. The layer thickness of 0.25 nm aligns reasonably well with the short dimension of the roughly prolate ellipsoid-shape TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e ion, measuring about 0.29 nm\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. The increased electron density between the bulk pore filling (ρ\u003csub\u003epore\u003c/sub\u003e) and the interface layer (ρ\u003csub\u003eTFSI\u0026minus;rich\u003c/sub\u003e) computes roughly to an 1 M increase of the TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e concentration in this layer. Assuming a bulk electrolyte concentration of 1 M in the rest of the pore, this translates to a TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e concentration of about 2 M in the TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e-rich layer, or in other words, about 20 %of the pore surface occupied by TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e ions (see Supplementary Note SN3). We acknowledge that while this model provides both a qualitative and quantitative understanding of the observed changes, it oversimplifies the electron density profile. In particular the MOF pore wall is not homogeneous around its circumference in terms of electron density, limiting the ability to pinpoint specific TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e adsorption sites within the pore. A numerical simulation of a form factor model including the radial electron density variations in the MOF unfortunately did not provide sufficient accuracy of the radially averaged form-factor, which is the only experimentally observable quantity (SI Figure S5 and Note SN4). Consequently, while the model presented here offers quantitative information about the radial distribution of TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e ions, it cannot resolve their exact spatial location along the circumference of the pore, i.e. their specific adsorption sites.\u003c/p\u003e \u003cp\u003eRecent work with NMR, however, postulated possible hydrogen-bond like interactions of fluorine atoms in electrolyte anions (including TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e and BF\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) with the N-H moiety of the MOF linker\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. Our experimental observation of strong form-factor changes for TSFI\u003csup\u003e\u0026minus;\u003c/sup\u003e, but not for fluorine-free electrolytes (RbBr and Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, SI Figure S4) supports the hypothesis that fluorine-containing anions form strong hydrogen-bond-like interactions with the N-H group. The authors of Ref.\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e also briefly note in their supplementary information the presence of BF\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e⁻\u003c/sup\u003e anions trapped in Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e pores after using TEABF\u003csub\u003e4\u003c/sub\u003e/acetonitrile (ACN) electrolyte. As an additional proof, we observed a homogeneous electrolyte distribution with LiTFSI/propylene carbonate (PC) electrolyte in Cu\u003csub\u003e3\u003c/sub\u003e(HHTP)\u003csub\u003e2\u003c/sub\u003e electrodes (SI Figure S6), a MOF approximately isostructural to Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e, which, however, lacks N-H groups in its linker. This further strengthens the conclusion that F\u0026middot;\u0026middot;\u0026middot;H-N interactions indeed underlie the observed inhomogeneous electrolyte distribution, leading to a higher concentration of immobilized TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e ions near the MOF pore walls.\u003c/p\u003e\n\u003ch3\u003eTFSI immobilization at applied cell voltage and cation dominated charge balancing\u003c/h3\u003e\n\u003cp\u003eBuilding on the understanding of the system at no applied voltage (0 V), we now investigate the systems \u003cem\u003eoperando\u003c/em\u003e response to an applied external cell voltage. In our custom electrochemical cell for operando SAXS, cyclic voltammetry (CV) data showed a rectangular shape supporting pure capacitive behaviour of the MOF (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec)). After the CVs, a voltage sequence was then applied with chronoamperometry (CA), i.e. constant voltage at 0 V, +\u0026thinsp;0.4 V, 0 V, -0.4 V, and 0 V with 1-hour holds, followed by another set CV cycles. The applied voltage sequence is depicted in the top panel of Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). Following the CA holds, a slightly larger specific capacitance was observed (104 F/g) compared to before the holds (94 F/g). These values are comparable to values previously reported for Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e with a similar BET specific surface area using organic electrolyte\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e, particularly as no carbon black or other conductivity enhancing additive has been added. We attribute the increased specific capacitance by approximately 10% to enhanced wetting, a characteristic effect of electrowetting induced by the applied cell voltage\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. While no electrochemical degradation was observed in this study for Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e with aqueous 1 M NaTFSI under maximum applied cell voltage of \u0026plusmn;\u0026thinsp;0.4 V, a significant decline in electrochemical performance was observed when the same voltage series is performed between \u0026plusmn;\u0026thinsp;0.6 V (SI Figure S7).\u003c/p\u003e \u003cp\u003eInterestingly, only minute changes are observed in the scattering signal as the voltage is varied, with the averaged scattering curves at constant applied cell voltage nearly coinciding (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, grey, red, and blue full lines). The left zoomed-in inset in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea) shows details of the 220 in-plane peak pointing out subtle differences in peak intensity at contrasting electrode polarisations, which will be discussed further below. The close similarity of the curves indicates only very small changes to the form factor, suggesting that TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e ions remain anchored in their position close to the pore wall, being effectively immobilised even against the repelling electrostatic forces at a constant cell voltage of -0.4 V.\u003c/p\u003e \u003cp\u003eIt was shown in previous work that X-ray transmission data allow to determine the charge-balancing mechanism in electric-double layer capacitors\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e,\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e. The absence of systematic changes in the X-ray transmission signal with applied cell voltage (SI Figure S8) indicates that only the light Na\u003csup\u003e+\u003c/sup\u003e cations migrate and contribute to charge balancing. If the larger, more strongly X-ray absorbing TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e anions were involved, a clear change of the transmitted X-ray intensity with changing TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e concentrations would be expected (SI Table TS1 and Supplementary Note SN5). The observation of a purely cation-governed charge balancing is supported by recent experimental findings\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e, demonstrating that charge balancing is cation-dominated in Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e-based supercapacitors using 1 M Net\u003csub\u003e4\u003c/sub\u003eBF\u003csub\u003e4\u003c/sub\u003e in deuterated acrylonitrile solvent (d\u003csub\u003e3\u003c/sub\u003eACN). We hypothesize that such cation-dominated charge-balancing mechanism in Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e is most likely promoted by the immobilization of TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e, probably at the N-H sites as discussed above. This potentially applies to fluorine-containing anions within MOF pores exhibiting N-H linkers in general, thereby favouring cation-driven charge storage. In the present work, this mechanism manifests as pure co-ion expulsion of Na\u003csup\u003e+\u003c/sup\u003e at positive voltage and pure counter-ion adsorption of Na\u003csup\u003e+\u003c/sup\u003e at negative voltage, as sketched in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eTo better understand the mechanisms of Na\u003csup\u003e+\u003c/sup\u003e-driven electric double-layer formation with immobilized TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e, changes in the scattering signal at applied voltages were examined in detail. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed) illustrates the relative changes of scattering intensity at low-q values (second panel from top) and the evolution of in-plane peak intensities for selected Bragg peaks (panels 3\u0026ndash;7 from top) as the cell voltage (top panel) is varied. Interestingly, clear systematic variations are observed in the MOF-related intensities with the applied voltage, while the intensity of the PTFE-binder peak, shown in the bottom panel of Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed) and the right inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea), remains perfectly constant across applied cell voltages. Since this peak is neither related to the MOF nor the electrolyte, it serves as an internal reference, confirming that the observed small intensity changes are highly reliable.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eMost interestingly, the intensity response to changes in applied cell voltage is consistent between cyclic voltammetry (CV) with gradual cell voltage increases and chronoamperometry (CA) with sudden cell voltage jumps, suggesting no mechanistic difference between slow charging and rapid cell voltage changes in this system\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e,\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. The absence of any delay in the scattering response following cell voltage variation and the same levels of peak intensity changes reached in CV\u0026rsquo;s and in CA\u0026rsquo;s indicates that charge balancing and ion rearrangement occur very rapidly, as would be expected if only the highly mobile Na\u003csup\u003e+\u003c/sup\u003e ions in the aqueous electrolyte contribute to charge balance through electric double-layer formation\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e,\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e. These finding highlight the fascinating potential for systems with cation driven charge balancing to maintain high performance even at fast charging rates, addressing a key challenge in the development of MOF-based supercapacitors\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eRef.\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e used in-situ Small-Angle Neutron Scattering (SANS) to study a similar Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e MOF electrode with NaOTf/dimethylformamide (DMF) electrolyte in a supercapacitor set-up, observing small changes at low q, which they interpreted as ions adsorbing onto the outer surface of the MOF particles. Similarly, we observe systematic changes in scattering intensity at low-q as the voltage is varied (second panel in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). However, we do not discuss this effect further here, as the outer surface of the MOF particles is estimated to contribute less than 10% to the total surface area (Supporting Information Figure S9 and Note SN6). Given this small contribution from the outer surface compared to the inner surface of the pores, we focus here on the in-pore changes, where we assume the majority of charge balancing to take place. We note, however, that the comparison between outer-surface charge balancing, reflected in the low-q intensity changes (second panel in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed), and in-pore charge balancing, indicated by the Bragg peak intensity variations (other panels in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed), suggests no dynamic differences, including ion transport, between the charge balancing mechanisms at the outer surface of MOF particles and those occurring within the pores.\u003c/p\u003e \u003cp\u003eThere are some additional interesting details in the systematic intensity changes of the different peaks. Some of the peaks increase at positive and decrease at negative cell voltage (e.g. 300 and 220), others show exactly the opposite behaviour (e.g. 200, 210). Unfortunately, in this case our simple form factor model fails when trying to quantify this behaviour as it lacks the required sensitivity. The observed intensity changes of in-plane pore peaks, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed), can tentatively be attributed to a combination of effects, including distortions in the unit cell (discussed later) and changes in the form-factor resulting from (slight) variations in the electron density within pores as Na\u003csup\u003e+\u003c/sup\u003e adsorbs and desorbs, introducing an additional step in the radial electron density profile. With the Na\u003csup\u003e+\u003c/sup\u003e ion being very small, however, the electron density should only change slightly as Na\u003csup\u003e+\u003c/sup\u003e adsorbs and desorbs. The precise Na\u003csup\u003e+\u003c/sup\u003e adsorption sites within the pore space can therefore not be unambiguously determined through a form factor fitting of a multi-step cylinder. Alternatively, the Na\u003csup\u003e+\u003c/sup\u003e ions may be strongly associated with specific adsorption sites which may effectively change the structure factor of the MOF. Although interesting by itself, we abstain however from further attempts to quantify this behaviour since it is beyond the scope of this work.\u003c/p\u003e\n\u003ch3\u003eIn-situ structural changes of Ni(HITP) electrodes\u003c/h3\u003e\n\u003cp\u003eWith the detailed mechanisms of charge balancing established, we finally turn our attention to the structural changes in the MOF electrode. Both, the interlayer spacing and the pore centre-to-centre distance exhibited small, yet systematic, contraction and expansion of approximately 0.1% as the electrode polarization is varied (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The slight \u0026lsquo;breathing\u0026rsquo; observed in the MOF aligns with adsorption-induced pore swelling, as was also reported from ion electrosorption in carbon-based electrodes\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e. Assuming that Na\u003csup\u003e+\u003c/sup\u003e is the only ion contributing to the charge balance, the observed expansion upon negative polarization (counter-ion adsorption) and contraction upon positive polarization (co-ion expulsion) is therefore consistent with the overall finding for the in-plane peak shifts (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea)). Contrary to the in-plane strain, however, the layer spacing c increases independently of the polarity of the applied cell voltage, showing expansion at both positive and negative cell voltages, though less pronounced at negative cell voltage (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb)). The origin of this auxetic-like out-of-plane expansion remains unclear. However, since potential Na\u003csup\u003e+\u003c/sup\u003e intercalation between layers would result in a much larger increase of layer spacing c, intercalation or other major structural changes upon charging and discharging can be ruled out. This is also consistent with the absence of an intercalation signature in the cyclic voltammogram in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec).\u003c/p\u003e \u003cp\u003eNotably, after holding at -0.4 V for 1 hour, both the in-plane lattice parameter, a, and the layer spacing, c, exhibit a slight, seemingly irreversible increase. The counter electrode in this symmetric set-up, subjected to the opposite polarity, showed similar non-reversible increases in in-plane lattice parameter and layer spacing already after the initial long-term exposure to negative cell voltage (see SI Figure S9). These findings suggest that while for charge balancing the changes in intensity seem fully reversible and are independent of the polarity of the applied voltage (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed)), there are slight irreversible changes after applying a negative bias over prolonged periods of time. We hypothesize that these effects may be associated with degradation in electrochemical performance, potentially impacting the structural integrity and cycling stability of the MOF, which still poses a significant challenge for MOF based devices \u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e. However, a detailed investigation of these processes is beyond the scope of the current work.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThis study establishes a comprehensive understanding of the structural and electrochemical behaviour of a Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e MOF-based supercapacitor with aqueous electrolyte using in-situ Small Angle X-Ray Scattering (SAXS). In the dry state, the MOF exhibits a well-defined hexagonally arranged cylindrical pore structure with a pore diameter of 1.39 nm, an average pore-centre distance of 2.20 nm, and a turbostratic (i.e. disordered) stacking with an interlayer distance of 0.33 nm. Upon addition of water or aqueous electrolyte, the layer spacing increases by approximately 1.44%, indicating a wetting induced swelling of the layers.\u003c/p\u003e \u003cp\u003eWe summarize the electrochemical behaviour of the Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e-based supercapacitor with aqueous 1 M NaTFSI electrolyte as follows: Already at no applied cell voltage, TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e anions become immobilized within the cylindrical pores via F\u0026middot;\u0026middot;\u0026middot;H-N interactions with the NH moiety of the MOF linker, with a considerably higher TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e concentration at the surface layer. Charge balancing upon cell polarization occurs exclusively through mobile Na\u003csup\u003e+\u003c/sup\u003e cations via co-ion expulsion and counter-ion adsorption, with no evidence of ion intercalation between Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e sheets. Notably, no mechanistic differences are observed between voltage steps and gradual voltage ramps. Charge balancing proceeds on comparable timescales on both the internal pore walls - which provide a significantly larger surface area - and the outer surface of larger MOF particles. Structurally, both the in-plane layer spacing and layer distance exhibit small reversible changes of around 0.1% due to adsorption induced swelling or contraction. After holding at negative cell voltage of -0.4 V for 1 hour, a minimal irreversible change is noted.\u003c/p\u003e \u003cp\u003eThis work provides some fundamental insights into the charge-balancing mechanisms of Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e-based supercapacitors, and demonstrates furthermore a powerful experimental platform to investigate the relationship between electrode structure, functional groups, and charge storage behavior, such as i.e. specific ion anion anchoring in systems that require cation dominated charging. Future investigations could aim at a better understanding of the underlying mechanisms of electrode degradation during extended cycling or at higher operating voltages, as well as the precise origins of electrode swelling. Finally, this study underscores the value of MOFs as chemically well-defined model systems for exploring fundamental processes in energy storage, paving the way for insights that can inform the rational design and optimization of supercapacitors more broadly, beyond the specific performance of MOF-based systems.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eMaterials\u003c/h2\u003e \u003cp\u003eThe investigated electrolyte salts - NaTFSI (97%), KTFSI (97%), Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e (\u0026ge;\u0026thinsp;99.0%, anhydrous) and RbBr (99.6% trace metal basis) - were acquired from Sigma Aldrich. Electrolyte solutions were prepared by dissolving the appropriate amount of each salt in Milli-Q lab-grade H\u003csub\u003e2\u003c/sub\u003eO to achieve concentrations of 1 M or 0.1 M. For the MOF synthesis, NiCl\u003csub\u003e2\u003c/sub\u003e・6H\u003csub\u003e2\u003c/sub\u003eO and ethanol were acquired from Sigma Aldrich, aqueous ammonia (NH\u003csub\u003e4\u003c/sub\u003eOH, 35 % NH\u003csub\u003e3\u003c/sub\u003e) from Fisher Scientific, and 2,3,6,7,10,11-hexaiminotriphenylene hydrate (H\u003csub\u003e6\u003c/sub\u003eHITP・xH\u003csub\u003e2\u003c/sub\u003eO) from Chemextensions. All were used without further modification.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eSynthesis of Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e and electrode preparation\u003c/h2\u003e \u003cp\u003eThe MOF was synthesized following the protocol described in Ref\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e without further modification. Freestanding electrodes were prepared from the synthesized MOF powder and 5 wt.% PTFE binder (60 wt.% solution in water, Sigma-Aldrich) following the protocol described in\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e without the addition of carbon black or other additives. The MOF-PTFE slurry was rolled into sheets with a thickness of 200\u0026thinsp;\u0026plusmn;\u0026thinsp;10 \u0026micro;m and dried at room temperature. To ensure the removal of residual moisture, the dried electrode sheets were placed in a vacuum tube furnace at 105\u0026deg;C for at least 24 hours prior to the in-situ measurements.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eX-Ray scattering\u003c/h2\u003e \u003cp\u003eHigh-resolution total X-ray scattering data shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) of the dry Ni₃(HITP)₂ MOF were collected at the European Synchrotron Radiation Facility (ESRF) at the ID22 beamline (Grenoble, France) \u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. The electrode material was loaded into a quartz capillary and measured using a 13-channel Si(111) multi-analyzer stage while rotating the sample. The X-ray beam was 1 \u0026times; 1 mm in size, with an exposure time of 120 seconds and a photon energy of 29 keV. To prevent beam-induced sample damage, the sample was translated by 1.1 mm between exposures, ensuring fresh material was exposed to the beam. A total of 22 scans were combined to obtain the final dataset. Lower resolution SAXS/WAXS measurements shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e of the dry and water/electrolyte wetted MOF spanning a large q-range between 2.5\u0026ndash;250 nm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were also performed at ESRF (ID22 beamline) using a 2D flat-panel PerkinElmer XRD 1611CP3 detector and a photon energy of 60 keV. A beam size of 1 mm \u0026times; 1 mm used, with an exposure time of 5 seconds.\u003c/p\u003e \u003cp\u003eHigh resolution operando SAXS experiments of the electrolyte wetted EDLCs at different electrical cell voltages, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, were conducted at the Austrian SAXS beamline at ELETTRA Sincrotrone Trieste (Italy)\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e. This was done using a custom-built operando electrochemical cell, designed to enable simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS) experiments. This cell, adapted from a design previously used and described in Ref.\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e, features slit shaped Kapton foil windows (SI Figure S11), allowing X-rays in transmission geometry to scan the electrode area. Circular electrodes (14 mm diameter) with off-center 3 mm holes were punched from the electrode sheets and stacked in the cell with a glass fiber separator (40 mm diameter, 200 \u0026micro;m thickness, Whatman GF/A). The off-center holes were arranged not to be congruent, enabling X-rays to independently penetrate each electrode, allowing data collection for both electrodes individually (SI Figure S11). Platinum paper (\u0026lt;\u0026thinsp;200 nm thickness) was used as a current collector covering the entire electrode area. The cells were subjected to a cell voltage sequence using a Gamry Interface 1010B potentiostat. Before measurements, the electrochemical cells were conditioned with five cycles of cyclic voltammetry (CV) between \u0026plusmn;\u0026thinsp;0.4 V at a scan rate of 10 mV/s. The X-ray beam with a photon energy of 16 keV was focused to a size of 0.5 mm \u0026times; 2 mm. Data were collected using a Pilatus3 1M 2D detector (Dectris Ltd., Baden-D\u0026auml;ttwil, Switzerland). Exposure time for each SAXS measurement was 20 seconds, and the sample was moved between two positions to alternately measure the two electrodes individually.\u003c/p\u003e \u003cp\u003eAll 2D scattering patterns were azimuthally integrated to obtain 1D scattering profiles, showing the intensity versus the length of the scattering vector (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:q=4\\pi\\:sin\\theta\\:/\\lambda\\:\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:2\\theta\\:\\)\u003c/span\u003e\u003c/span\u003e being the scattering angle and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\lambda\\:\\)\u003c/span\u003e\u003c/span\u003e the wavelength). Standard data normalization and correction procedures at the respective beamline, include corrections for primary beam intensity changes, sample transmission, and exposure time.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eData treatment\u003c/h2\u003e \u003cp\u003eDiffraction peaks in the scattering profiles were fitted using a custom-written script for Python 3 using a Pseudo-Voigt peak shape and a decaying exponential background\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e. The in-plane lattice parameter describes the pore-centre to pore-centre distance and was calculated according to Eq.\u0026nbsp;1:\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:a=\\:\\frac{4\\pi\\:}{{q}_{hk0}\\sqrt{3}}*\\sqrt{{h}^{2}+{k}^{2}+hk}\\)\u003c/span\u003e \u003c/span\u003e Eq.\u0026nbsp;(1)\u003c/p\u003e \u003cp\u003eWhere q\u003csub\u003ehk0\u003c/sub\u003e describes the q-position of the in-plane diffraction peak with Miller Indices hk(l\u0026thinsp;=\u0026thinsp;0). The layer spacing c was calculated from the (002) stacking peak position as in Eq.\u0026nbsp;2:\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:c=\\:\\frac{2\\pi\\:}{{q}_{002}}\\)\u003c/span\u003e \u003c/span\u003e Eq.\u0026nbsp;(2)\u003c/p\u003e \u003cp\u003eThe strain for the lattice parameter a and the layer spacing c was calculated according to Eq.\u0026nbsp;3:\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:strain=\\:\\frac{l-{l}_{0}}{{l}_{0}}\\)\u003c/span\u003e \u003c/span\u003e Eq.\u0026nbsp;(3)\u003c/p\u003e \u003cp\u003eWith l, l\u003csub\u003e0\u003c/sub\u003e being the actual and the reference values for the in-plane lattice parameter a or the layer spacing c, respectively.\u003c/p\u003e \u003cp\u003eThe total scattering intensity I(q) for infinitively long cylinders arranged on a 2D hexagonal lattice can be written as in Eq.\u0026nbsp;4:\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:I\\left(q\\right)=K*S\\left(q\\right){*\\left|F\\left(q\\right)\\right|}^{2}\\)\u003c/span\u003e \u003c/span\u003e Eq.\u0026nbsp;(4)\u003c/p\u003e \u003cp\u003ewhere K is a constant factor, S(q) is the spherically averaged structure-factor described by sharp diffraction peaks at discrete q-values \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{q}_{hk0}\\)\u003c/span\u003e\u003c/span\u003e defined in Eq.\u0026nbsp;1, and |F(q)|\u003csup\u003e2\u003c/sup\u003e describes the form-factor\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. According to Eq.\u0026nbsp;4, the height of each Bragg peak hk0 from the pore lattice is determined by the respective value of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\left|F\\left({q}_{hk0}\\right)\\right|}^{2}\\)\u003c/span\u003e\u003c/span\u003e. The form-factor for infinitely long monodisperse cylindrical pores |F(q)\u003csub\u003ecyl\u003c/sub\u003e.|\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e was used here, following the approach first introduced in Ref \u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e and adapted by Ref \u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e for multistep core-shell cylinders, in which the scattering amplitude F(q) is given by:\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:F\\left(q\\right)=\\:\\frac{{\\varSigma\\:}_{i=1}^{N}({\\rho\\:}_{i}-{\\rho\\:}_{i-1}){R}_{i}^{2}Z\\left(q{R}_{i}\\right)}{{\\varSigma\\:}_{i=1}^{N}({\\rho\\:}_{i}-{\\rho\\:}_{i-1}){R}_{i}^{2}}\\)\u003c/span\u003e \u003c/span\u003e Eq.\u0026nbsp;(5)\u003c/p\u003e \u003cp\u003eZ(qR\u003csub\u003ei\u003c/sub\u003e) is given by 2J\u003csub\u003e1\u003c/sub\u003e(qR)/(qR) with J\u003csub\u003e1\u003c/sub\u003e being the Bessel function of first kind and first order, and ρ\u003csub\u003ei\u003c/sub\u003e and R\u003csub\u003ei\u003c/sub\u003e are the electron density and radius of the i-th cylindrical shell, starting from the outermost shell. By using a custom written Python Code (Supplementary Notes SN7), the integrated intensities of Bragg peaks in the experimental data were analysed with a GUI based interface, similar to the approach used Ref.\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. The analysis files including the software code will be made available upon request.\u003c/p\u003e \u003cp\u003eThe specific capacitance of the symmetrical two-electrode supercapacitor cell was calculated from cyclic voltammetry according to Eq.\u0026nbsp;5:\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:C=\\:\\frac{4*{\\int\\:}_{t1}^{t2}I\\left(t\\right)dt}{\\varDelta\\:U*{m}_{total}}\\)\u003c/span\u003e \u003c/span\u003e Eq.\u0026nbsp;(6)\u003c/p\u003e \u003cp\u003eWith I(t) describing the current, ΔU the voltage window and m\u003csub\u003etotal\u003c/sub\u003e the combined mass of both electrodes.\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contributions\u003c/h2\u003e \u003cp\u003eM.S.: Conceptualization, Methodology, Software, Formal analysis, Investigation, Data Curation, Writing - Original Draft, Writing - Review \u0026amp; Editing, Visualization, Project administration; C. J. B.: Investigation, Resources, Writing - Review \u0026amp; Editing, M. V. R.: Investigation, Writing - Review \u0026amp; Editing; S.S.: Writing - Review \u0026amp; Editing; G.F.-P.: Software, Writing - Review \u0026amp; Editing; T.L.: Software, Writing - Review \u0026amp; Editing; D.H.: Writing - Review \u0026amp; Editing; H.A.: Data Curation, Writing - Review \u0026amp; Editing; A.C.F.: Writing - Review \u0026amp; Editing; O.P.: Conceptualization, Methodology, Writing - Original Draft, Writing - Review \u0026amp; Editing, Supervision, Project administration.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eWe acknowledge CERIC-ERIC for access to the Austrian SAXS beamline at ELETTRA, as well as Andrew Fitch and Catherine Dejoie for support at the ID22 Beamline at ESRF for beamtime SC5511. We also acknowledge Jamie Gittins for synthesis of the Cu\u003csub\u003e3\u003c/sub\u003e(HTTP)\u003csub\u003e2\u003c/sub\u003e sample. We would also like to thank Peter Moharitsch at Montanuniversit\u0026auml;t Leoben for machining of the electrochemical operando cell. M.S. thanks Markus Kratzer for many constructive discussions.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003e\u003cem\u003eSupercapacitors- Materials, Systems and Applications\u003c/em\u003e. (Wiley, Weinheim, Germany, (2013) \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1002/9783527646661\u003c/span\u003e\u003cspan address=\"10.1002/9783527646661\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eB\u0026eacute;guin F, Presser V, Balducci A, Frackowiak E (2014) Carbons and electrolytes for advanced supercapacitors. 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J Synchrotron Radiat 5:506\u0026ndash;508\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eNewville M, Stensitzki T, Allen DB, Ingargiola ALMFIT (2014) Non-Linear Least-Square Minimization and Curve-Fitting for Python. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.5281/ZENODO.11813\u003c/span\u003e\u003cspan address=\"10.5281/ZENODO.11813\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5973632/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5973632/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eUnderstanding how ions interact with electrodes in electric double-layer capacitors (EDLCs) is key to advancing energy storage, yet many fundamental aspects remain unclear. Here, we employ operando small-angle X-ray scattering (SAXS) to investigate charge storage in a Ni\u003csub\u003e3\u003c/sub\u003e(HITP)\u003csub\u003e2\u003c/sub\u003e metal\u0026ndash;organic framework (MOF), with well-defined pores as an electrode model system. Using 1 M NaTFSI aqueous electrolyte, we show that TFSI\u003csup\u003e\u0026minus;\u003c/sup\u003e anions are immobilized near MOF pore walls via fluorine\u0026ndash;hydrogen interactions with N-H functional groups. We quantify the concentration of pinned anions and demonstrate that their immobilization persists across different applied cell voltages, resulting in a cation-dominated charge storage mechanism governed solely by Na\u003csup\u003e+\u003c/sup\u003e adsorption and desorption. Charge balancing is unaffected by whether voltage is applied stepwise or gradually, with no dynamic differences between in-pore and outer-pore environments. Additionally, we track reversible adsorption induced pore swelling, rule out ion intercalation, and observe minor irreversible structural expansion after prolonged negative bias.\u003c/p\u003e","manuscriptTitle":"Anchored Anions, Mobile Cations: Charge Storage in MOF-based Supercapacitors Studied with Operando Small-Angle X-ray Scattering","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-03-05 08:26:20","doi":"10.21203/rs.3.rs-5973632/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"0b3c93b4-d0f6-4dac-b5ff-15b4980a7858","owner":[],"postedDate":"March 5th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":45190957,"name":"Physical sciences/Energy science and technology/Energy storage/Supercapacitors"},{"id":45190958,"name":"Physical sciences/Physics/Condensed-matter physics/Surfaces, interfaces and thin films"},{"id":45190959,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Metal\u0026#x2013;organic frameworks"}],"tags":[],"updatedAt":"2025-10-01T07:20:19+00:00","versionOfRecord":{"articleIdentity":"rs-5973632","link":"https://doi.org/10.1038/s41467-025-63772-w","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2025-09-30 04:00:00","publishedOnDateReadable":"September 30th, 2025"},"versionCreatedAt":"2025-03-05 08:26:20","video":"","vorDoi":"10.1038/s41467-025-63772-w","vorDoiUrl":"https://doi.org/10.1038/s41467-025-63772-w","workflowStages":[]},"version":"v1","identity":"rs-5973632","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5973632","identity":"rs-5973632","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}