Sulfophenylated centimeter-size graphene membrane in a direct methanol fuel cell

Sulfophenylated centimeter-size graphene membrane in a direct methanol fuel cell | 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 Sulfophenylated centimeter-size graphene membrane in a direct methanol fuel cell Gregory Schneider, Weizhe Zhang, Max Makurat, Xue Liu, Xiaoting Liu, and 19 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4807293/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 27 Nov, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract An ideal proton exchange membrane should only permeate protons and be leak-tight for fuels. Graphene is impermeable to water and poorly conducting to protons. Next to long-term stability an ideal and optimized proton exchange membrane therefore needs to fulfil two main criteria: proton permeability and selectivity. Within methanol fuel cells, the first ensures a high-power density, while the second prevents fuel cross-over between the electrodes, which deteriorates catalyst performance and, thereby, drastically lowers performance. However, proton conductivity and selectivity are antagonistic in polymer membranes concerning their performance1. Long channel length in state-of-the-art membranes such as Nafion 117 is therefore a prerequisite to obtaining proton selectivity, at the cost of an additional ionic resistance through such long channels. Pristine graphene2 already fulfils these two criteria, partly as the graphene basal plane is impermeable to water and other molecules3, and exhibits a certain degree of proton conductivity4, influenced by nanoscaled ripples5, corrugations6, particularly in monolayer graphene oxide7 and hydrogenated graphene8. Here, we chemically functionalized monolayer graphene to install sulfophenylated sp3 dislocations by diazotization. Selective to protons, transmembrane areal conductances surpass those of polymer membranes, while providing proton selectivity over methanol through such an atomically thin layer. By creating proton-conductive and selective paths through graphene, we unveil a covalent chemical route to rationalize transmembrane proton transport through 2D materials. Physical sciences/Nanoscience and technology/Graphene/Mechanical and structural properties and devices Physical sciences/Energy science and technology/Fuel cells Physical sciences/Chemistry/Electrochemistry/Fuel cells Figures Figure 1 Figure 2 Figure 3 Figure 4 Full Text Protons, as one form of hydrons 9 , can translocate through graphene and other 2D materials such as hexagonal boron nitride ( h -BN) 10 and 2D mica 11 at room temperature, making two-dimensional (2D) materials promising candidates for proton-exchange membrane applications 12 . Beyond monolayer 2D materials, proton transport has also been demonstrated using 2D laminates ( i.e., 2D crystals assembled in a layered structure, made from graphene flakes 13 , and other 2D nanosheets 14,15 , and covalent organic frameworks 16 ), making them suitable for membrane-based applications. Remarkably, functionalizing 2D materials and reducing interlayer distances down to sub-nanometre dimensions yielded membranes with high selectivity, as well as opportunities to observe ion transport under highly confined spaces 17,18 . However, the ion transport distance in these systems is currently, particularly in laminated structures 19 , beyond ‘one atom’, thus yielding additional parallel resistances. Protons were also shown not to permeate through defect-free regions of graphene 20 , which demonstrates the need for proton-selective pathways. Open questions focus on the role of defects and strain 21 , and – we believe with our work – also the role of out-of-plane polar sp 3 dislocations installed onto the graphene basal plane. To open pores enabling transmembrane ion transport in graphene 22 and other 2D materials, such as MoS 2 23 , methods have been established using electron beams, ion bombardment, and plasma exposure 24-30 . Ion bombardment allows obtaining a high pore density and small pores (>10 14 cm -2 ) 25 , while electron beam sculpting, suffers from a lower control over the resulting pore size distribution, and chemistry. Pore and pore-like defects have also been realized via bottom-up approaches including chemical vapor deposition (CVD) growth of amorphous carbon 31,32 , using 2D polymers 33,34 , and graphyne 35 . These bottom-up strategies benefit from remarkable scalability in combination with the possibility of controlling the size, shape, and chemistry of the pore, unfortunately yet with pores that are larger than the diameter of a hydrated proton. The controlled manipulation of electron density and hydrogenation within a double-gated graphene lattice also results in an adjustable proton conductance 8 . Defects in graphene, such as seven- and higher-member rings, together with small lattice disorders such as sp 3 dislocations and polar functional groups are promising approaches for enhancing proton conductivity, while maintaining the impermeability of the basal plane to other substances 36-38 , which are particularly of interests in energy devices such as fuel cells. Here, we introduce sp 3 lattice distortions paired with sulfophenyl groups on the basal plane of single-layer graphene via a diazotization strategy 39 using 4-sulfobenzenediazonium (4-SBD) as a reactant (Fig. 1A, Supplementary material 2.1). The functionalization converts sp 2 carbons from graphene into an active sp 3 site carrying a hard charge (from the sulfonate; pK a ~ -2.8 in water) and a resulting out-of-plane dipole. Chemically, a delocalized electron from the graphene lattice reacts with the 4-SBD cation leading to the covalent grafting of a sulfophenyl radical orthogonally to the graphene lattice with N 2 expelled as a leaving group. The resulting aryl radical reacts with a sp 2 carbon atom on the graphene basal plane, which is converted to sp 3 upon grafting. Meanwhile, side reactions might also occur: 4-SBD can react with an already grafted aryl group (at the ortho position) either via a radical or azo-coupling mechanism, both of which might result in oligomerization 40 of the aryl species on the graphene basal plane. Using circumcoronene models (Fig. 1B), we calculated the extend of lattice expansion upon diazotization, including hydrogenated and phenylated circumcoronene (Table S1, see Supplementary material 1.1) to explore their impact on proton transmembrane conductance through graphene membranes. Fig. 1B shows that larger functional groups induced greater distortion in the hexagonal ring (up to 6.8% for double sulfophenyl functionalization), resulting in reduced potential energy barriers and increased proton flux. Functionalization, on both sides, with phenyl or sulfophenyl groups increased the proton flux by eight to nine orders of magnitude, while hydrogenation only increased the flux by one order of magnitude. Notably, quantum tunnelling was less prominent with larger ring sizes. Additionally, we investigated the shuttle of the proton from the water bulk to the graphene basal plane by performing 1.0 ns of ReaxFF MD plus metadynamics (see Supplementary material 1.2) 41 on a model system with one sulfophenylated sp 3 functionality bonded to the graphene basal plane (Fig. S2). First, the proton is captured from the water bulk by the SO 3 - functionality with a barrier of ~0.15-0.20 eV and then shuttled and released closer to the basal plane (~3 Å) with a lower barrier of ~0.1 eV (Fig. S3). Next, the proton could tunnel the graphene basal plane to the other side of the graphene. 36,42 The energy barrier of the proton quantum tunneling for the case of hydrogenated graphene was estimated by Feng et al. to be <1.0 eV. 36 The distortion produced by the sulfophenylated functionality when compared to a case involving only hydrogenation shows an enhanced proton flux (Fig.1B) with a more efficient tunneling. 36 Therefore, the proton-shuttling effect of the sulfophenylated functionality coupled to the enhanced proton tunneling induced by the distortion of the graphene basal plane where the sulfophenylated functionality is located, foster an effective proton-transport mechanism. Next, we prepared SO 3 - -graphene as a proton-selective membrane in a direct methanol fuel cell (DMFC) (Fig. 2A). The centimeter-size single-crystal CVD graphene films, which were seamlessly stitched into well-aligned domains 43 , were used to minimize the defects and grain boundaries. Fig. 2B shows scanning electron microscopy (SEM) images of >200 μm wide CVD graphene single crystals grown on a copper foil with minimized grain boundaries and intrinsic defects. Multilayer patches were negligibly observed in the samples. We then transferred CVD graphene to a quantifoil TEM grid and incubated graphene with 4-SBD for up to 6 days (1 mg·ml -1 4-SBD in 0.1 M HCl) and characterized SO 3 - -graphene by atomic force microscopy (AFM, Fig. 2C) and high-resolution transmission electron microscopy (HRTEM, Fig. 2D). From the AFM images, the arithmetic average (Ra) and root mean square roughness (RMS) increased by 0.5nm within the first 24 h of incubation (Fig. 2C). After 24 h, only a minimal change in surface roughness occurred (Fig. 2C and Fig. S18). However, from the error image channels (for a spatially better-resolved image) a clear change in morphology occurred for longer treatment times: graphene surface features such as folds resulting from the transfer became decreasingly visible while denser agglomerates appeared upon longer 4-SBD treatments. Pristine graphene is therefore covered by 4-SBD already after 24 h after which the 4-SBD layer becomes thicker and more homogenous. Oligomerization, which is commonly observed during diazonium treatment 39 and here on AFM images, likely leads to the observed additional resistance that protons encounter resulting in the significant decrease in proton conductance after 84h of treatment as shown in Fig. 1C. Importantly, at the nanometer scale, HRTEM images of graphene after 4-day and 6-day of SBD treatment did not show the apparition of large >1 nm pores (Fig. 2E). Instead, the large defect-free graphene area suggests that individual grafting sites could not be observed on the monolayer regions (Fig. 2E). The spontaneous oligomerization of SBD, giving rise to agglomerates visible in AFM, were therefore not distinguishable by HRTEM from common hydrocarbon contaminations resulting from the growth of CVD graphene (supplementary material, Fig. S19-S21). Next, we studied 4-SBD reactivity with graphene covered with a spin-coated Nafion TM layer (Fig. 3Ai). The choice of Nafion TM (instead of PMMA) as a support polymer is because Nafion TM will be used as a support film for graphene in the DMFC setup compromising both the mechanical stability of graphene and being freestanding in water. For both graphene carried by Nafion TM (Fig. 3Aiii) and bare graphene on SiO 2 /Si (Fig. S15), Raman spectroscopy showed an increasing D peak for increasing 4-SBD treatment times and I D /I G reached a plateau indicating a maximum reactivity after 4 days of treatment. The increasing value of the D peak intensity ratio (~1350 cm -1 ) over the G peak (~1580 cm -1 ) ( I D /I G ) indicates the formation of defects upon 4-SBD treatment, primarily the introduction of sp 3 dislocations 44 . Moreover, the intensity ratio of the 2D peak (~2700 cm -1 ) over the G peak (0.6< I 2D /I G < 0.8) indicates no major changes in the quality and number of layers of graphene, although the value is not 2.0 in the case of polymer (Nafion TM ) coated graphene 45 (see Supplementary material Fig. S15, I 2D /I G > 2.0 for untreated graphene, on SiO 2 /Si, and after 5 days treatment I 2D /I G = 0.6). Additionally to Raman, we used X-ray photoelectron spectroscopy (XPS) to study the sulphur content and hybridization of the carbon atoms on the lattice upon 4-SBD functionalization. Fig. 3B shows XPS core level spectra (C1s, N1s, and S2p) of pristine graphene and after 2 days, 4 days, and 6 days of treatment. The fitting of the C1s spectra leads to four distinct species including the characteristic C sp 2 (BE284.5 eV) and C sp 3 (BE285.3 eV) of graphene. Moreover, semi-quantitative analyses were also conducted to determine the chemical composition of the grafted sites (Table S2). The sp 3 C/sp 2 C ratio, initially at 0.2 for pristine graphene, indicates impurities from transfer and manipulation as pristine graphene should be free of sp 3 hybridization. Therefore, the absolute value of the sp 3 C/sp 2 C ratio could not reveal the number of sp 3 sites introduced by the SBD treatment. However, discussing the sp 3 C/sp 2 C ratio evolution reveals the state of sp 3 /sp 2 conversion of graphene upon 4-SBD treatment. Relative ratios of -SO 3 - and -N=N with respect to the total chemical species from XPS, particularly the S2p, C1s and N1s spectrum, indicate the relative increase of sulfophenyl groups grafted on the graphene surface vs reaction time. For ease of comparison, we plotted sp 2 C/sp 3 C and -SO 3 - , -N=N- in Fig. 3C. Incubating graphene with diluted SBD (0.1 mg·ml -1 instead of 1 mg·ml -1 ), resulted in no more changes in the relative sp 3 C/sp 2 C (0.86) and S2p content (1.7) after 2-day of SBD treatment (Table S2), validating that the changes in relative component ratios were raised by SBD. On the contrary, the sp 3 C/sp 2 C ratio, for the treatment with 1 mg·ml -1 SBD, first increased to 1.0 after 2-day treatment and then dropped to 0.3 after 6-day treatment. Indeed, the oligomerization procedure only introduces more sp 2 C in benzene rings instead of more sp 3 sites on graphene. Considering that a benzene ring contains six sp 2 C, we assume that the drop of sp 3 /sp 2 ratio after 4-day and 6-day treatment is due to the addition of 4-SBD groups via an oligomerization mechanism instead of grafting to graphene directly. Accordingly, the relative ratio of -SO 3 - increases from 0 for pristine graphene to 5.2 after 4 days of treatment and further increases to 6.4 after 6-day of treatment. The relative ratio of -N=N- shows a similar trend to SO 3 - , which also supports simultaneous azo coupling of sulfophenyl groups, in agreement with AFM. According to the Raman results, however, the intensity of the D peak plateaued after 4 days of treatment, suggesting that only oligomerization (coupling between 4-SBD and grafted sulfonatophenyl groups) occurred for treatment times longer than 4 days. Next, we tested SO 3 - -graphene as a membrane in a DMFC. We measured three independent batches of SO 3 - -graphene after each day of treatment ( i.e. , after 0, 1, 2, 3, 4, 5, and 6 days; 21 samples in total) and determined maximum power densities, membrane conductance, and methanol crossover. Fig. 4A shows the power-current ( P-I ) and voltage-current ( V-I ) plots for pristine CVD graphene, Nafion TM , and SO 3 - -graphene at 60 °C. Pristine graphene showed a maximum power density of 25.8 ±15.4 mW·cm -2 , at 212 mV, which is about half (~ 45%) compared to Nafion 117 and a proton conductance of 2.4 S·cm -2 . These results can be attributed to defects resulting from the membrane electrode assembly (MEA) including CVD graphene 21 . It is notable that, however, for SO 3 - -graphene after 4 days of reaction time with 4-SBD, the maximum power density increased to 109.8 ±14.8 mW·cm -2 ( i.e. , ~2 times higher than Nafion TM ). The power output did not increase further with a prolonged 4-SBD reaction time. Instead, it decreased to 44.9 ±17.3 mW·cm -2 after a 6-day treatment, again in line with our hypothesis of the oligomerization of the sulfophenyls after 4 days of treatment. Subsequently, we monitored the power density for a range of operating temperatures ranging from room temperature (rt) to 70 °C (Fig. 4B). In general, an increase in temperature leads to an elevated catalytic activity as well as an increased diffusion rate of both protons and methanol across the membrane. As expected, the power density for 4-day treated SO 3 - -graphene increased linearly with the temperature rising from 20 °C to 70 °C (Fig. 4B). In contrast, for pristine graphene and 6-day treated graphene, the maximum power densities did not show a linear increase with temperature, and even decreased for operation temperatures above 50 °C. We attribute this observation to a higher methanol crossover rate at higher temperatures suggesting the importance of an optimal 4-SBD treatment time of 4 days. In fact, in the pristine sample, proton transport occurs primarily through inherent defects, which have poor proton selectivity and we presume therefore more methanol crossover. Similarly, for the oligomerized samples, the proton selectivity sites are proven to be blocked, resulting in relatively low proton conductance. As a result, inherent defects also facilitate the methanol crossover. To establish a correlation between areal proton conductance and DMFC performance ( i.e. , power density; and in the next section also fuel crossover rates), we first investigated how the membrane resistance of SO 3 - -graphene correlates with the operating temperature of the fuel cell. Fig. 4C shows the equivalent circuit and Nyquist plots obtained by electrochemical impedance spectroscopy (EIS), in which the intercept of the x-axis indicates the overall resistance of the membrane. Remarkably, the lowest resistance was also achieved after 4 days of treatment with 4-SBD ( i.e. , R = 0.59 Ω, which is ~ 30% lower than 6-day treated graphene and 2.5-fold lower than Nafion 117). The higher resistance after 6 days of reaction with 4-SBD also suggests a hindrance of the proton transport pathway due to oligomerization. As opposed to the conductance normalized to the electrode area, electrodes cover the entire SO 3 - -graphene membrane while graphene is only free-standing over 16% compared to the electrode area ( i.e. only ~ 16% of the polycarbonate membrane area is open with free-standing graphene). Importantly, pristine CVD graphene shows a proton conductance of 6.9 ±1.1 S·cm -2 , normalized to the exposed CVD graphene area, which originates from defects in the form of pinholes or cracks in CVD graphene 21,46 , at room temperature. For SO 3 - -graphene, similarly, the areal conductance observed after 4 days of reaction time is 30.9 ±2.3 S·cm -2 per electrode area (25 devices measured; respectively 33.8 S·cm -2 for the membrane mounted in the reverse direction with the sulfophenylated groups facing the anode, see the Supplementary materials section 3.2 and Fig. S28). By analyzing the measured conductance and temperature-dependence, the proton conductances of SO 3 - -graphene membranes exhibit an Arrhenius behavior as described by equation (1): ln(σ𝑇) = ln(σ 0 ) − ln(𝑒)𝐸 𝑎 /𝑘 𝐵 ×𝑇 -1 (1) where σ is the measured conductance, T is the temperature, E a is the activation energy and k B is the Boltzmann’s constant. Samples after 1, 2, 3 and 4 days of SBD treatment show a linear relationship of ln(sT) vs. T -1 , with a fitted E a of 6.9 kJ·mol -1 (0.072 eV) after 4 days of SBD treatment (Fig. 4D). Pristine graphene does not show an Arrhenius behavior. The relatively low E a after 4 days of SBD treatment suggests that protons cross the membrane via a Grotthus mechanism (≤0.4 eV) where the proton hops through the membrane at a faster rate than dissociating from the hydronium ion 47 . After 5-day and 6-day, ln(sT) vs. T -1 present a less linear relationship compared to 4-day of treatment (Fig. S45), which could be caused by the polymerization of SBD, therefore hindering proton transport, leaving larger defects/cracks with a lower proton selectivity compare to methanol crossover. Next, we evaluated the methanol crossover by testing the DMFCs with a higher methanol concentration and determining fuel efficiency (details see Supplementary Information section 3.3). After a switch from 1 M to 5 M methanol, the maximum power density with Nafion 117 dropped by ~53% (Fig. 4E), while remaining similar for sulfonated graphene. Additionally, the drop in maximum power density, switching methanol from 1 M to 5 M, was reduced by ~10% after 4 days of reaction time with 4-SBD (Fig. 4F). The fuel efficiency increases after 4-day SBD treatment, indicating that the introduced proton path is selective for proton over methanol (Fig. S29). For the 4-day SO 3 - -graphene at 60 °C, η 200mV reaches ~79% at a current density of 457.5 mA cm -2 . The sulfonation of graphene using 4-SBD, therefore, not only allows an efficient selectivity towards proton transport but also prevents fuels (here methanol) from crossing the membrane in a DMFC. To conclude, the functionalization of centimeter-size CVD graphene with an out-of-plane polar group and an sp 3 dislocation by diazotization with a sulfophenyl radical opened an atomically thin transmembrane proton selective path which yielded highly selective proton conductivity (30.9 ±2.3 S·cm -2 ) and also enabled application in a direct methanol fuel cell delivering a power density more than twice higher than its Nafion TM counterpart according to the electrode area (127 mW·cm - 2 at 60 °C). This finding calls for more extensive views and understandings of the crucial reactions and mechanisms at the origin of this enhancement of power with limited solvent crossover. Particular attention is required to understand the proton transport through sp 3 distorted graphene, including theoretical insights on how the proton tunnelling barrier could be further tuned, and the importance of the geometry and polarity of the functional distortions. Practically, improving the sp 3 density might further increase the proton conductance through functionalized graphene. On a fundamental aspect, polarizing a 2D membrane may become increasingly important in controlling with precision the translocation-transport of ions, also for example in more complex two-dimensional polymer architectures 48,49 . Declarations Acknowledgments. This research was supported by the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. 335879 project acronym “Biographene” and the Netherlands Organization for Scientific Research (NWO) VIDI 723.013.007, awarded to G.F. Schneider. Z.F. Liu, L.Z. Sun and X.T. Liu acknowledge the support from the National Natural Science Foundation of China (NSFC, No. T2188101). C. Maheu acknowledges funding by the German Research Foundation (DFG) under project no. 423746744. The authors acknowledge Hans van den Elst for carrying out the HRMS measurement. I.E., A.K., T.H. thank Deutsche Forschungsgemeinschaft (DFG) – Project-ID 443871192 – GRK 2721 for funding and Dresden ZIH and Paderborn PC 2 supercomputer centers for computer time. The authors thank Prof. Dima Filippov, Prof. Sylvestre Bonnet, and Hugo Schellevis for helpful discussions. Author contributions. W.Z. discovered that diazotization of graphene with 4-SBD yields a giant increase of transmembrane areal proton conductance with selectivity to proton. W.Z. prepared graphene samples for morphological/elemental characterizations, and fuel cell tests, alongside the manuscript preparation. Fuel cell tests were conducted by W.Z.. M.M. synthesized the 4-SBD compound and characterized it by NMR and analyzed the HRMS data. M.M. designed and carried out the AFM experiments as well as analyzed the AFM data. X.L. (Xue) proposed the idea of sulfonating graphene to form a 2D proton exchange membrane with the reported 4-SBD compound. X.L. (Xiaoting), Y.L., L.S., and Z.L. synthesized the single-crystalline CVD graphene for fuel cell tests. T.J.F.K. helped with the sample preparation of graphene on TEM grids and early-stage study with TEM. A. J. designed a reactivity test for 4-SBD. C.L., H.Q., X.F., and U.K. contributed to the HRTEM characterization and discussion. C.M., H.S., and J.P.H. contributed to the XPS characterization and discussion. D.C, F.D., I.E., A.K., and T.H. contributed to the discussions and calculations to support using theoretical models of the areal conductances measured experimentally. This work was under the supervision of G.F.S. who led all the experimental progress, discussions, and the elaboration of the manuscript draft. All co-authors contributed to the writing and editing of this manuscript. Competing interests. The authors declare that they have no competing interests. Data and materials availability. All data are available in the main text or in the supplementary information. 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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-4807293","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":346231143,"identity":"447c9eea-25e1-4ae7-a986-4259c6e2e179","order_by":0,"name":"Gregory 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Dresden-Rossendorf","correspondingAuthor":false,"prefix":"","firstName":"Agnieszka","middleName":"","lastName":"Kuc","suffix":""},{"id":346231162,"identity":"98df11fc-c380-4cc3-923f-1fb5f2198b70","order_by":19,"name":"Jan Hofmann","email":"","orcid":"","institution":"Surface Science Laboratory, Department of Materials and Earth Sciences, Technical University of Darmstadt","correspondingAuthor":false,"prefix":"","firstName":"Jan","middleName":"","lastName":"Hofmann","suffix":""},{"id":346231163,"identity":"a8429aaa-8608-4d20-8fc8-8418c1b2fd38","order_by":20,"name":"Ute Kaiser","email":"","orcid":"https://orcid.org/0000-0003-0582-4044","institution":"University of Ulm","correspondingAuthor":false,"prefix":"","firstName":"Ute","middleName":"","lastName":"Kaiser","suffix":""},{"id":346231164,"identity":"1d983391-398b-44dc-bc8b-24b23d53a659","order_by":21,"name":"Luzhao Sun","email":"","orcid":"https://orcid.org/0000-0003-0518-0744","institution":"Beijing Graphene Institute","correspondingAuthor":false,"prefix":"","firstName":"Luzhao","middleName":"","lastName":"Sun","suffix":""},{"id":346231165,"identity":"ffe1ca99-c4d4-40a2-9c10-983746cabfb4","order_by":22,"name":"Lin Jiang","email":"","orcid":"","institution":"Leiden University","correspondingAuthor":false,"prefix":"","firstName":"Lin","middleName":"","lastName":"Jiang","suffix":""},{"id":346231166,"identity":"f0c043b0-c3f7-4ebe-ba4e-91e7c4263e73","order_by":23,"name":"Zhongfan Liu","email":"","orcid":"https://orcid.org/0000-0001-5554-1902","institution":"Center for Nanochemistry, Beijing National Laboratory for Molecular Science, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China.","correspondingAuthor":false,"prefix":"","firstName":"Zhongfan","middleName":"","lastName":"Liu","suffix":""}],"badges":[],"createdAt":"2024-07-26 10:07:02","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4807293/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4807293/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-025-65507-3","type":"published","date":"2025-11-27T05:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":65040475,"identity":"6f972827-a2a9-40c2-82fc-c1ee310bdbd8","added_by":"auto","created_at":"2024-09-23 03:09:58","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":599782,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eProton flux through six-membered benzenoid ring expansion in graphene, after diazotization with 4-sulfonato-benzene-diazonium tetrafluoroborate (4-SBD). (A) \u003c/strong\u003eIllustration depicting the functionalization of graphene with 4-SBD. Besides the direct coupling of the sulfophenylated radical to the graphene basal plane, which converts sp\u003csup\u003e2\u003c/sup\u003e carbon to sp\u003csup\u003e3\u003c/sup\u003e, 4-SBD could also link to the ortho position of a previously grafted benzene ring via a radical coupling or an azo coupling. \u003cstrong\u003e(B)\u003c/strong\u003e Simulations on the relationship of transmembrane transport proton flux with different functional states of monolayer graphene. The flux was simulated using the Wentzel–Kramers–Brillouin (WKB) tunneling approximation based on the density functional theory (DFT) with PBE0 density functional potential energy barriers. The cropped images of the simulation models near functional groups indicate the correspondence between functional states and proton flux. *The protonated -SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e groups with identical functions are presented in parallel.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-4807293/v1/56c59dce281906b9961cddda.png"},{"id":65040807,"identity":"731d71b3-e2fa-4122-b8ff-5676901d5f08","added_by":"auto","created_at":"2024-09-23 03:17:57","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1028700,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003csup\u003e\u003cstrong\u003e-\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e-graphene in a direct methanol fuel cell (DMFC), growth of single-crystalline chemical vapor deposition (CVD) graphene, and characterization by scanning electron microscopy (SEM), atomic force microscopy (AFM), and high-resolution transmission electron microscopy (HRTEM).\u003c/strong\u003e \u003cstrong\u003e(A)\u003c/strong\u003e Illustration of a DMFC, redox reactions involved in the conversion of methanol to carbon dioxide, and a photograph of CVD graphene transferred on a polycarbonate support. \u003cstrong\u003e(B)\u003c/strong\u003e SEM images of hexagonal graphene single crystals after 30 mins of growth on a copper foil (left) to a fully covered and confluent monolayer after one hour of growth (right). Scale bar: 200 μm. (\u003cstrong\u003eC\u003c/strong\u003e) AFM images of pristine free-standing single-crystalline CVD graphene transferred over a holey quantifoil TEM grid and after 4 and 6 days of 4-SBD treatment. The graphene was transferred to a TEM grid using a polymer-free transfer method by depositing the TEM grid on the graphene side of the copper foil and floating the TEM grid and graphene/copper stack on a 0.5M ammonium persulfate aqueous copper-etching solution. Scale bar: 500 nm. (\u003cstrong\u003eD\u003c/strong\u003e)\u003cstrong\u003e \u003c/strong\u003eArithmetic roughness average (Ra) and root mean square average (RMS) of graphene and 4-SBD treated SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e- \u003c/sup\u003etreated graphene derived from AFM image analysis. (\u003cstrong\u003eE\u003c/strong\u003e) HRTEM image of graphene after 4-day and 6-day 4-SBD treatment. Scale bar: 5 nm.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-4807293/v1/7afde8fb39f69ee2b83b803d.png"},{"id":65040809,"identity":"bcc06296-d556-4308-8771-5c23287d5333","added_by":"auto","created_at":"2024-09-23 03:17:58","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":976104,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eRaman spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis of SO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003csup\u003e\u003cstrong\u003e-\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e-graphene.\u003c/strong\u003e (\u003cstrong\u003eA\u003c/strong\u003e) i) Illustration of graphene coated with Nafion\u003csup\u003eTM\u003c/sup\u003e floating on and reacting with a solution of 4-SBD (1 mg·ml\u003csup\u003e-1\u003c/sup\u003e SBD in aqueous 0.1 M HCl). ii) Raman spectra for graphene on Nafion\u003csup\u003eTM\u003c/sup\u003e transferred on a SiO\u003csub\u003e2\u003c/sub\u003e/Si substrate after 0-6 days of floating incubation with 4-SBD. Raman spectra were recorded over a 10×10 μm area using a 457 nm laser set to 1.5 mW power to avoid any laser-induced damage to the SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-graphene. We normalized the spectra by the intensity of the G peak to facilitate a comparison of the D and 2D peaks. iii) Plot of \u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003eD\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/I\u003c/em\u003e\u003csub\u003e\u003cem\u003eG\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003e2D\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/I\u003c/em\u003e\u003csub\u003e\u003cem\u003eG\u003c/em\u003e\u003c/sub\u003e versus 4-SBD treatment time derived from panel Aii). \u003cstrong\u003e(B)\u003c/strong\u003e C1s, N1s, and S2p XPS core level spectra of pristine graphene and graphene after 2-day, 4-day, and 6-day of 4-SBD treatment on SiO\u003csub\u003e2\u003c/sub\u003e/Si substrate with fitted components. \u003cstrong\u003e(C)\u003c/strong\u003e\u003csup\u003e \u003c/sup\u003eComposition relative ratios as a function of 4-SBD reaction time. The ratios are derived for the specific component from the overall chemical bonds characterized by XPS. sp\u003csup\u003e3\u003c/sup\u003eC/sp\u003csup\u003e2\u003c/sup\u003eC ratio, N=N and SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e were plotted together for comparison with sp\u003csup\u003e3\u003c/sup\u003eC/sp\u003csup\u003e2\u003c/sup\u003eC ratio linked to the left and N=N/SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e linked to the right y-axis.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-4807293/v1/9d55e039232799761886daa8.png"},{"id":65040474,"identity":"634c40eb-a8ab-470f-a8ff-307fd36ff761","added_by":"auto","created_at":"2024-09-23 03:09:58","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":544165,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDMFC performance with CVD graphene and SO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003csup\u003e\u003cstrong\u003e-\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e-graphene\u003c/strong\u003e \u003cstrong\u003emembranes. (A)\u003c/strong\u003e\u003cem\u003e \u003c/em\u003ePower-current density (\u003cem\u003eP-I)\u003c/em\u003e and voltage-current density (\u003cem\u003eV-I)\u003c/em\u003e curves for Nafion 117, pristine graphene, 4-day, and 6-day 4-SBD treated SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-graphene. The cell voltage \u003cem\u003e(V)\u003c/em\u003e and the power density \u003cem\u003e(P)\u003c/em\u003e as a function of current density \u003cem\u003e(I)\u003c/em\u003e were measured in a 1 M methanol/water solution at 60 °C. \u003cstrong\u003e(B)\u003c/strong\u003e Maximum power density as a function of temperature.\u003cstrong\u003e (C)\u003c/strong\u003e Equivalent circuit and Nyquist plots of electrochemical impedance spectroscopy (EIS) measured at the open circuit voltage of the DMFC with graphene and SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-graphene membranes. A Nafion\u003csup\u003eTM\u003c/sup\u003e layer was spin coated on graphene and SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-graphene, and the stack was transferred on a polycarbonate porous support (~16% of PC with \u003cem\u003eca.\u003c/em\u003e 3μm diameter holes) filled with Nafion\u003csup\u003eTM\u003c/sup\u003e. \u003cstrong\u003e(D)\u003c/strong\u003e ln(σ𝑇) as a function of the inverse of temperature for graphene membranes.\u003cstrong\u003e \u003c/strong\u003eThe energy barriers were calculated based on eq. (1) following an Arrhenius-type analysis.\u003cstrong\u003e (E)\u003c/strong\u003e \u003cem\u003eP-I\u003c/em\u003e curves of the DMFC fuel cell operating at 60 °C with aqueous solutions of methanol (respectively 1 and 5 M) for a ~183 μm thick Nafion 117 layer and 4-day SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-graphene. \u003cstrong\u003e(F)\u003c/strong\u003e Decreases of maximum power density upon operating the DMFC in 5 M methanol compared to 1 M methanol. The blue dashed line corresponds to the decrease of the power density of a DMFC with only Nafion 117 when switching from 1 M methanol to 5 M methanol.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-4807293/v1/b8bd0eb47cf8d341ba4ec914.png"},{"id":97418707,"identity":"07a2ca80-ec47-4070-b522-653a8f7b1e8c","added_by":"auto","created_at":"2025-12-04 08:07:11","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4223770,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4807293/v1/3d97a2e6-b98f-4714-9c48-93565fd6500b.pdf"},{"id":65041204,"identity":"32c5307f-c1c2-4470-bb54-fc039d9561bb","added_by":"auto","created_at":"2024-09-23 03:25:57","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":12467824,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementaryinformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-4807293/v1/e80243ba3a9d1de1f0441612.docx"},{"id":65040471,"identity":"1260b313-3e32-4847-b56f-6739b7d99f8f","added_by":"auto","created_at":"2024-09-23 03:09:57","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":12175493,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"R1reviewreportandResponsetoreviewers.docx","url":"https://assets-eu.researchsquare.com/files/rs-4807293/v1/75f1be678bdc01a82d04670e.docx"},{"id":65040806,"identity":"b3d1a372-16aa-4863-9e28-8f9a10a4fd21","added_by":"auto","created_at":"2024-09-23 03:17:57","extension":"pdf","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":13352,"visible":true,"origin":"","legend":"","description":"","filename":"R2ReviewReport.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4807293/v1/d67b8cccb6c77b872842276f.pdf"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Sulfophenylated centimeter-size graphene membrane in a direct methanol fuel cell","fulltext":[{"header":"Full Text","content":"\u003cp\u003eProtons, as one form of hydrons\u003csup\u003e9\u003c/sup\u003e, can translocate through graphene and other 2D materials such as hexagonal boron nitride (\u003cem\u003eh\u003c/em\u003e-BN)\u003csup\u003e10\u003c/sup\u003e and 2D mica\u003csup\u003e11\u003c/sup\u003e at room temperature, making two-dimensional (2D) materials promising candidates for proton-exchange membrane applications\u003csup\u003e12\u003c/sup\u003e. Beyond monolayer 2D materials, proton transport has also been demonstrated using 2D laminates (\u003cem\u003ei.e.,\u003c/em\u003e 2D crystals assembled in a layered structure, made from graphene flakes\u003csup\u003e13\u003c/sup\u003e, and other 2D nanosheets\u003csup\u003e14,15\u003c/sup\u003e, and covalent organic frameworks\u003csup\u003e16\u003c/sup\u003e), making them suitable for membrane-based applications. Remarkably, functionalizing 2D materials and reducing interlayer distances down to sub-nanometre dimensions yielded membranes with high selectivity, as well as opportunities to observe ion transport under highly confined spaces\u003csup\u003e17,18\u003c/sup\u003e. However, the ion transport distance in these systems is currently, particularly in laminated structures\u003csup\u003e19\u003c/sup\u003e, beyond \u0026lsquo;one atom\u0026rsquo;, thus yielding additional parallel resistances. Protons were also shown not to permeate through defect-free regions of graphene\u003csup\u003e20\u003c/sup\u003e, which demonstrates the need for proton-selective pathways. Open questions focus on the role of defects and strain\u003csup\u003e21\u003c/sup\u003e, and \u0026ndash; we believe with our work \u0026ndash; also the role of out-of-plane polar sp\u003csup\u003e3\u003c/sup\u003e dislocations installed onto the graphene basal plane.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTo open pores enabling transmembrane ion transport in graphene\u003csup\u003e22\u003c/sup\u003e and other 2D materials, such as MoS\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e23\u003c/sup\u003e, methods have been established using electron beams, ion bombardment, and plasma exposure\u003csup\u003e24-30\u003c/sup\u003e. Ion bombardment allows obtaining a high pore density and small pores (\u0026gt;10\u003csup\u003e14\u003c/sup\u003e cm\u003csup\u003e-2\u003c/sup\u003e)\u003csup\u003e25\u003c/sup\u003e, while electron beam sculpting, suffers from a lower control over the resulting pore size distribution, and chemistry. Pore and pore-like defects have also been realized via bottom-up approaches including chemical vapor deposition (CVD) growth of amorphous carbon\u003csup\u003e31,32\u003c/sup\u003e, using 2D polymers\u003csup\u003e33,34\u003c/sup\u003e, and graphyne\u003csup\u003e35\u003c/sup\u003e. These bottom-up strategies benefit from remarkable scalability in combination with the possibility of controlling the size, shape, and chemistry of the pore,\u0026nbsp;unfortunately yet with pores that are larger than the diameter of a hydrated proton.\u0026nbsp; The controlled manipulation of electron density and hydrogenation within a double-gated graphene lattice also results in an adjustable proton conductance\u003csup\u003e8\u003c/sup\u003e. Defects in graphene, such as seven- and higher-member rings, together with small lattice disorders such as sp\u003csup\u003e3\u003c/sup\u003e dislocations and polar functional groups are promising approaches for enhancing proton conductivity, while maintaining the impermeability of the basal plane to other substances\u003csup\u003e36-38\u003c/sup\u003e, which are particularly of interests in energy devices such as fuel cells.\u003c/p\u003e\n\u003cp\u003eHere, we introduce sp\u003csup\u003e3\u003c/sup\u003e lattice distortions paired with sulfophenyl groups on the basal plane of single-layer graphene via a diazotization strategy\u003csup\u003e39\u003c/sup\u003e using 4-sulfobenzenediazonium (4-SBD) as a reactant (Fig. 1A, Supplementary material 2.1). The functionalization converts sp\u003csup\u003e2\u003c/sup\u003e carbons from graphene into an active sp\u003csup\u003e3\u003c/sup\u003e site carrying a hard charge (from the sulfonate; pK\u003csub\u003ea\u003c/sub\u003e ~ -2.8 in water) and a resulting out-of-plane dipole. Chemically, a delocalized electron from the graphene lattice reacts with the 4-SBD cation leading to the covalent grafting of a sulfophenyl radical orthogonally to the graphene lattice with N\u003csub\u003e2\u003c/sub\u003e expelled as a leaving group. The resulting aryl radical reacts with a sp\u003csup\u003e2\u003c/sup\u003e carbon atom on the graphene basal plane, which is converted to sp\u003csup\u003e3\u003c/sup\u003e upon grafting. Meanwhile, side reactions might also occur: 4-SBD can react with an already grafted aryl group (at the ortho position) either via a radical or azo-coupling mechanism, both of which might result in oligomerization\u003csup\u003e40\u003c/sup\u003e of the aryl species on the graphene basal plane.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eUsing circumcoronene models (Fig. 1B), we calculated the extend of lattice expansion upon diazotization, including hydrogenated and phenylated circumcoronene (Table S1, see Supplementary material 1.1) to explore their impact on proton transmembrane conductance through graphene membranes. Fig. 1B shows that larger functional groups induced greater distortion in the hexagonal ring (up to 6.8% for double sulfophenyl functionalization), resulting in reduced potential energy barriers and increased proton flux. Functionalization, on both sides, with phenyl or sulfophenyl groups increased the proton flux by eight to nine orders of magnitude, while hydrogenation only increased the flux by one order of magnitude. Notably, quantum tunnelling was less prominent with larger ring sizes.\u0026nbsp;Additionally, we investigated the shuttle of the proton from the water bulk to the graphene basal plane by performing 1.0 ns of ReaxFF MD plus metadynamics (see\u0026nbsp;Supplementary material 1.2)\u003csup\u003e41\u003c/sup\u003e on a model system with one sulfophenylated sp\u003csup\u003e3\u0026nbsp;\u003c/sup\u003efunctionality bonded to the graphene basal plane (Fig. S2). First, the proton is captured from the water bulk by the SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e\u003csub\u003e\u0026nbsp;\u003c/sub\u003efunctionality with a barrier of ~0.15-0.20 eV and then shuttled and released closer to the basal plane (~3 \u0026Aring;) with a lower barrier of ~0.1 eV (Fig. S3). \u0026nbsp;Next, the proton could tunnel the graphene basal plane to the other side of the graphene.\u003csup\u003e36,42\u003c/sup\u003e\u0026nbsp; The energy barrier of the proton quantum tunneling for the case of hydrogenated graphene was estimated by Feng \u003cem\u003eet al.\u003c/em\u003e to be \u0026lt;1.0 eV. \u003csup\u003e36\u003c/sup\u003e The distortion produced by the sulfophenylated functionality when compared to a case involving only hydrogenation shows an enhanced proton flux (Fig.1B) with a more efficient tunneling.\u003csup\u003e36\u003c/sup\u003e Therefore, the proton-shuttling effect of the sulfophenylated functionality coupled to the enhanced proton tunneling induced by the distortion of the graphene basal plane where the sulfophenylated functionality is located, foster an effective proton-transport mechanism.\u003c/p\u003e\n\u003cp\u003eNext, we prepared SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-graphene as a proton-selective membrane in a direct methanol fuel cell (DMFC) (Fig. 2A). The centimeter-size single-crystal CVD graphene films, which were seamlessly stitched into well-aligned domains\u003csup\u003e43\u003c/sup\u003e, were used to minimize the defects and grain boundaries. Fig. 2B shows scanning electron microscopy (SEM) images of \u0026gt;200 \u0026mu;m wide CVD graphene single crystals grown on a copper foil with minimized grain boundaries and intrinsic defects.\u0026nbsp;Multilayer patches were negligibly observed in the samples.\u0026nbsp;We then transferred CVD graphene to a quantifoil TEM grid and incubated graphene with 4-SBD for up to 6 days\u0026nbsp;(1\u0026nbsp;mg\u0026middot;ml\u003csup\u003e-1\u003c/sup\u003e 4-SBD in 0.1 M HCl)\u0026nbsp;and characterized SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-graphene\u0026nbsp;by atomic force microscopy (AFM, Fig. 2C) and high-resolution transmission electron microscopy (HRTEM, Fig. 2D). From the AFM images, the arithmetic\u0026nbsp;average (Ra) and root mean square roughness (RMS) increased by 0.5nm within the first 24 h of incubation (Fig. 2C). After 24 h, only a minimal change in surface roughness occurred (Fig. 2C and Fig. S18). However, from the error image channels (for a spatially better-resolved image) a clear change in morphology occurred for longer treatment times: graphene surface features such as folds resulting from the transfer became decreasingly visible while denser agglomerates appeared upon longer 4-SBD treatments. Pristine graphene is therefore covered by 4-SBD already after 24 h after which the 4-SBD layer becomes thicker and more homogenous. Oligomerization, which is commonly observed during diazonium treatment\u003csup\u003e39\u003c/sup\u003e and here on AFM images,\u0026nbsp;likely leads to the observed additional resistance that protons encounter resulting in the significant decrease in proton conductance after 84h of treatment as shown in Fig. 1C.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eImportantly, at the nanometer scale, HRTEM images of graphene after 4-day and 6-day of SBD treatment did not show the apparition of large \u0026gt;1 nm pores (Fig. 2E). Instead, the large defect-free graphene area suggests that individual grafting sites could not be observed on the monolayer regions (Fig. 2E). The spontaneous oligomerization of SBD, giving rise to agglomerates visible in AFM, were therefore not distinguishable by HRTEM from common hydrocarbon contaminations resulting from the growth of CVD graphene (supplementary material, Fig. S19-S21).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eNext, we studied 4-SBD reactivity with graphene covered with a spin-coated Nafion\u003csup\u003eTM\u003c/sup\u003e layer (Fig. 3Ai). The choice of Nafion\u003csup\u003eTM\u003c/sup\u003e (instead of PMMA) as a support polymer is because Nafion\u003csup\u003eTM\u003c/sup\u003e will be used as a support film for graphene in the DMFC setup compromising both the mechanical stability of graphene and being freestanding in water. For both graphene carried by Nafion\u003csup\u003eTM\u003c/sup\u003e (Fig. 3Aiii) and bare graphene on SiO\u003csub\u003e2\u003c/sub\u003e/Si (Fig. S15), Raman spectroscopy showed an increasing D peak for increasing 4-SBD treatment times and\u0026nbsp;I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e reached a plateau indicating a maximum reactivity after 4 days of treatment. The increasing value of the D peak intensity ratio (~1350 cm\u003csup\u003e-1\u003c/sup\u003e) over the G peak (~1580 cm\u003csup\u003e-1\u003c/sup\u003e) (\u003cem\u003eI\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e\u003c/em\u003e) indicates the formation of defects upon 4-SBD treatment, primarily the introduction of sp\u003csup\u003e3\u003c/sup\u003e dislocations\u003csup\u003e44\u003c/sup\u003e. Moreover, the intensity ratio of the 2D peak (~2700 cm\u003csup\u003e-1\u003c/sup\u003e) over the G peak (0.6\u0026lt;\u003cem\u003eI\u003csub\u003e2D\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e\u0026lt;\u003c/em\u003e0.8) indicates no major changes in the quality and number of layers of graphene, although the value is not 2.0 in the case of polymer (Nafion\u003csup\u003eTM\u003c/sup\u003e) coated graphene\u003csup\u003e45\u003c/sup\u003e(see Supplementary material Fig. S15, \u003cem\u003eI\u003csub\u003e2D\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e\u0026nbsp;\u003c/em\u003e\u0026gt; 2.0 for untreated graphene, on SiO\u003csub\u003e2\u003c/sub\u003e/Si, and after 5 days treatment \u003cem\u003eI\u003csub\u003e2D\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e\u0026nbsp;\u003c/em\u003e= 0.6).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAdditionally to Raman, we used X-ray photoelectron spectroscopy (XPS) to study the sulphur content and hybridization of the carbon atoms on the lattice upon 4-SBD functionalization. Fig. 3B shows XPS core level spectra (C1s, N1s, and S2p) of pristine graphene and after 2 days, 4 days, and 6 days of treatment. The fitting of the C1s spectra leads to four distinct species including the characteristic C sp\u003csup\u003e2\u003c/sup\u003e (BE284.5 eV) and C sp\u003csup\u003e3\u003c/sup\u003e (BE285.3 eV) of graphene. Moreover, semi-quantitative analyses were also conducted to determine the chemical composition of the grafted sites (Table S2).\u0026nbsp;The sp\u003csup\u003e3\u003c/sup\u003eC/sp\u003csup\u003e2\u003c/sup\u003eC ratio, initially at 0.2 for pristine graphene, indicates impurities from transfer and manipulation as pristine graphene should be free of sp\u003csup\u003e3\u003c/sup\u003e hybridization. Therefore, the absolute value of the sp\u003csup\u003e3\u003c/sup\u003eC/sp\u003csup\u003e2\u003c/sup\u003eC ratio could not reveal the number of sp\u003csup\u003e3\u003c/sup\u003e sites introduced by the SBD treatment. However, discussing the\u0026nbsp;sp\u003csup\u003e3\u003c/sup\u003eC/sp\u003csup\u003e2\u003c/sup\u003eC ratio evolution reveals the state of sp\u003csup\u003e3\u003c/sup\u003e/sp\u003csup\u003e2\u003c/sup\u003e conversion of graphene upon 4-SBD treatment. Relative ratios of -SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e and -N=N with respect\u0026nbsp;to the total chemical species from XPS, particularly the S2p, C1s and N1s spectrum,\u003cu\u003e\u0026nbsp;\u003c/u\u003eindicate the\u003cu\u003e\u0026nbsp;\u003c/u\u003erelative increase of\u003cu\u003e\u0026nbsp;\u003c/u\u003esulfophenyl groups grafted on the graphene surface \u003cem\u003evs\u003c/em\u003e reaction time. For ease of comparison, we plotted sp\u003csup\u003e2\u003c/sup\u003eC/sp\u003csup\u003e3\u003c/sup\u003eC and -SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, -N=N- in Fig. 3C.\u0026nbsp;Incubating graphene with diluted SBD (0.1 mg\u0026middot;ml\u003csup\u003e-1\u003c/sup\u003e instead of 1 mg\u0026middot;ml\u003csup\u003e-1\u003c/sup\u003e), resulted in no more changes in the relative sp\u003csup\u003e3\u003c/sup\u003eC/sp\u003csup\u003e2\u003c/sup\u003eC (0.86) and S2p content (1.7) after 2-day of SBD treatment (Table S2), validating that the changes in relative component ratios were raised by SBD. On the contrary, the sp\u003csup\u003e3\u003c/sup\u003eC/sp\u003csup\u003e2\u003c/sup\u003eC ratio, for the treatment with 1 mg\u0026middot;ml\u003csup\u003e-1\u003c/sup\u003e SBD,\u0026nbsp;first increased to 1.0 after 2-day treatment and then dropped to 0.3 after 6-day treatment. Indeed, the oligomerization procedure only introduces more sp\u003csup\u003e2\u003c/sup\u003eC in benzene rings instead of more sp\u003csup\u003e3\u003c/sup\u003e sites on graphene.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eConsidering that a benzene ring contains six sp\u003csup\u003e2\u003c/sup\u003eC, we assume that the drop of sp\u003csup\u003e3\u003c/sup\u003e/sp\u003csup\u003e2\u003c/sup\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003eratio after 4-day and 6-day treatment is due to the addition of 4-SBD groups via an oligomerization mechanism instead of grafting to graphene directly. Accordingly, the relative ratio of -SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eincreases from 0 for pristine graphene to 5.2 after 4 days of treatment and further increases to 6.4 after 6-day of treatment. The relative ratio of -N=N- shows a similar trend to SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, which also supports simultaneous azo coupling of sulfophenyl groups, in agreement with AFM. According to the Raman results, however, the intensity of the D peak plateaued after 4 days of treatment, suggesting that only oligomerization (coupling between 4-SBD and grafted sulfonatophenyl groups) occurred for treatment times longer than 4 days.\u003c/p\u003e\n\u003cp\u003eNext, we tested\u0026nbsp;SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-graphene\u0026nbsp;as a membrane in a DMFC. We measured three independent batches of\u0026nbsp;SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-graphene\u0026nbsp;after each day of treatment (\u003cem\u003ei.e.\u003c/em\u003e, after 0, 1, 2, 3, 4, 5, and 6 days; 21 samples in total) and determined maximum power densities, membrane conductance, and methanol crossover. \u0026nbsp;Fig. 4A shows the power-current (\u003cem\u003eP-I\u003c/em\u003e) and voltage-current (\u003cem\u003eV-I\u003c/em\u003e) plots for pristine CVD graphene, Nafion\u003csup\u003eTM\u003c/sup\u003e, and\u0026nbsp;SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-graphene\u0026nbsp;at 60\u0026nbsp;\u0026deg;C. Pristine graphene showed a maximum power density of 25.8 \u0026plusmn;15.4 mW\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e, at 212 mV, which is about half (~ 45%) compared to Nafion 117 and a proton conductance of 2.4 S\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e. These results can be attributed to defects resulting from the membrane electrode assembly (MEA) including CVD graphene\u003csup\u003e21\u003c/sup\u003e. It is notable that, however, for SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-graphene after 4 days of reaction time with 4-SBD, the maximum power density increased to 109.8 \u0026plusmn;14.8 mW\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e (\u003cem\u003ei.e.\u003c/em\u003e, ~2 times higher than Nafion\u003csup\u003eTM\u003c/sup\u003e). The power output did not increase further with a prolonged 4-SBD reaction time. Instead, it decreased to 44.9 \u0026plusmn;17.3 mW\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e after a 6-day treatment, again in line with our hypothesis of the oligomerization of the sulfophenyls after 4 days of treatment. Subsequently, we monitored the power density for a range of operating temperatures ranging from room temperature (rt) to 70\u0026nbsp;\u0026deg;C\u0026nbsp;(Fig. 4B). In general, an increase in temperature leads to an elevated catalytic activity as well as an increased diffusion rate of both protons and methanol across the membrane. As expected, the power density for 4-day treated SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-graphene increased linearly with the temperature rising from 20\u0026nbsp;\u0026deg;C\u0026nbsp;to 70\u0026nbsp;\u0026deg;C\u0026nbsp;(Fig. 4B). In contrast, for pristine graphene and 6-day treated graphene, the maximum power densities did not show a linear increase with temperature, and even decreased for operation temperatures above 50\u0026nbsp;\u0026deg;C. We attribute this observation to a higher methanol crossover rate at higher temperatures suggesting the importance of an optimal 4-SBD treatment time of 4 days. In fact, in the pristine sample, proton transport occurs primarily through inherent defects, which have poor proton selectivity and we presume therefore more methanol crossover. Similarly, for the oligomerized samples, the proton selectivity sites are proven to be blocked, resulting in relatively low proton conductance. As a result, inherent defects also facilitate the methanol crossover.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTo establish a correlation between areal proton conductance and DMFC performance (\u003cem\u003ei.e.\u003c/em\u003e, power density; and in the next section also fuel crossover rates), we first investigated how the membrane resistance of SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-graphene correlates with the operating temperature of the fuel cell. Fig. 4C shows the equivalent circuit and Nyquist plots obtained by electrochemical impedance spectroscopy (EIS), in which the intercept of the x-axis indicates the overall resistance of the membrane. Remarkably, the lowest resistance was also achieved after 4 days of treatment with 4-SBD (\u003cem\u003ei.e.\u003c/em\u003e, R = 0.59 \u0026Omega;, which is ~ 30% lower than 6-day treated graphene and 2.5-fold lower than Nafion 117).\u0026nbsp;The higher resistance after 6 days of reaction with 4-SBD also suggests a hindrance of the proton transport pathway due to oligomerization.\u003cu\u003e\u0026nbsp;\u003c/u\u003e\u003c/p\u003e\n\u003cp\u003eAs opposed to the conductance normalized to the electrode area, electrodes cover the entire SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-graphene membrane while graphene is only free-standing over 16% compared to the electrode area (\u003cem\u003ei.e.\u003c/em\u003e only ~ 16% of the polycarbonate membrane area is open with free-standing graphene). Importantly, pristine CVD graphene shows a proton conductance of 6.9 \u0026plusmn;1.1 S\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e, normalized to the exposed CVD graphene area, which originates from defects in the form of pinholes or cracks in CVD graphene\u003csup\u003e21,46\u003c/sup\u003e, at room temperature. For SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-graphene, similarly, the areal conductance observed after 4 days of reaction time is 30.9 \u0026plusmn;2.3 S\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e per electrode area (25 devices measured; respectively 33.8 S\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e for the membrane mounted in the reverse direction with the sulfophenylated groups facing the anode, see the Supplementary materials section 3.2 and Fig. S28).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eBy analyzing the measured conductance and temperature-dependence, the proton conductances of SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-graphene membranes exhibit an Arrhenius behavior as described by equation (1):\u003c/p\u003e\n\u003cp\u003eln(\u0026sigma;𝑇) = ln(\u0026sigma;\u003csub\u003e0\u003c/sub\u003e) \u0026minus; ln(𝑒)𝐸\u003csub\u003e𝑎\u003c/sub\u003e/𝑘\u003csub\u003e𝐵\u003c/sub\u003e\u0026times;𝑇\u003csup\u003e-1\u003c/sup\u003e\u0026nbsp; \u0026nbsp;(1)\u003c/p\u003e\n\u003cp\u003ewhere\u0026nbsp;\u0026sigma; is the measured conductance, T is the temperature, E\u003csub\u003ea\u003c/sub\u003e is the activation energy and k\u003csub\u003eB\u003c/sub\u003e is the Boltzmann\u0026rsquo;s constant.\u0026nbsp;Samples after 1, 2, 3 and 4 days of SBD treatment show a linear relationship of ln(sT) \u003cem\u003evs.\u003c/em\u003e T\u003csup\u003e-1\u003c/sup\u003e, with a\u0026nbsp;fitted E\u003csub\u003ea\u003c/sub\u003e of\u0026nbsp;6.9 kJ\u0026middot;mol\u003csup\u003e-1\u003c/sup\u003e (0.072 eV)\u0026nbsp;after\u0026nbsp;4 days of SBD treatment (Fig. 4D). Pristine graphene does not show an Arrhenius behavior. The relatively low\u0026nbsp;E\u003csub\u003ea\u003c/sub\u003e after 4 days of SBD treatment suggests that protons cross the membrane via\u0026nbsp;a Grotthus mechanism (\u0026le;0.4 eV) where the proton hops through the membrane at a faster rate than dissociating from the hydronium ion\u003csup\u003e47\u003c/sup\u003e. After 5-day and 6-day, ln(sT) \u003cem\u003evs.\u003c/em\u003e T\u003csup\u003e-1\u0026nbsp;\u003c/sup\u003epresent a less linear relationship compared to 4-day of treatment (Fig. S45), which could be caused by the polymerization of SBD, therefore hindering proton transport, leaving larger defects/cracks with a lower proton selectivity compare to methanol crossover.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eNext, we evaluated the methanol crossover by testing the DMFCs with a higher methanol concentration and determining fuel efficiency (details see Supplementary Information section 3.3).\u0026nbsp;After a switch from 1 M to 5 M methanol, the maximum power density with Nafion 117 dropped by ~53% (Fig. 4E), while remaining similar for sulfonated graphene. Additionally, the drop in maximum power density, switching methanol from 1 M to 5 M, was reduced by ~10% after 4 days of reaction time with 4-SBD (Fig. 4F). The fuel efficiency increases after 4-day SBD treatment, indicating that the introduced proton path is selective for proton over methanol (Fig. S29).\u0026nbsp;For the 4-day SO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-graphene at 60 \u0026deg;C, \u0026eta;\u003csub\u003e200mV\u003c/sub\u003e reaches ~79% at a current density of 457.5 mA cm\u003csup\u003e-2\u003c/sup\u003e. The sulfonation of graphene using 4-SBD, therefore, not only allows an efficient selectivity towards proton transport but also prevents fuels (here methanol) from crossing the membrane in a DMFC.\u003c/p\u003e\n\u003cp\u003eTo conclude, the functionalization of centimeter-size CVD graphene with an out-of-plane polar group and an sp\u003csup\u003e3\u003c/sup\u003e dislocation by diazotization with a sulfophenyl radical opened an atomically thin transmembrane proton selective path which yielded highly selective proton conductivity (30.9 \u0026plusmn;2.3 S\u0026middot;cm\u003csup\u003e-2\u003c/sup\u003e) and also enabled application in a direct methanol fuel cell delivering a power density more than twice higher than its Nafion\u003csup\u003eTM\u003c/sup\u003e counterpart according to the electrode area (127 mW\u0026middot;cm\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e2\u003c/sup\u003e at 60\u0026nbsp;\u0026deg;C).\u0026nbsp;This finding calls for more extensive views and understandings of the crucial reactions and mechanisms at the origin of this enhancement of power with limited solvent crossover.\u0026nbsp;Particular attention is required to understand the proton transport through sp\u003csup\u003e3\u003c/sup\u003e distorted graphene, including theoretical insights on how the proton tunnelling barrier could be further tuned, and the importance of the geometry and polarity of the functional distortions. Practically, improving the sp\u003csup\u003e3\u0026nbsp;\u003c/sup\u003edensity might further increase the proton conductance through functionalized graphene. On a fundamental aspect, polarizing a 2D membrane may become increasingly important in controlling with precision the translocation-transport of ions, also for example in more complex two-dimensional polymer architectures\u003csup\u003e48,49\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments.\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was supported by the European Research Council under the European Union\u0026rsquo;s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. 335879 project acronym \u0026ldquo;Biographene\u0026rdquo; and the Netherlands Organization for Scientific Research (NWO) VIDI 723.013.007, awarded to G.F. Schneider. Z.F. Liu, L.Z. Sun and X.T. Liu acknowledge the support from the National Natural Science Foundation of China (NSFC, No. T2188101). C. Maheu acknowledges funding by the German Research Foundation (DFG) under project no. 423746744. The authors acknowledge Hans van den Elst for carrying out the HRMS measurement.\u0026nbsp;I.E., A.K., T.H. thank Deutsche Forschungsgemeinschaft (DFG) \u0026ndash; Project-ID 443871192 \u0026ndash; GRK 2721 for funding and Dresden ZIH and Paderborn PC\u003csup\u003e2\u003c/sup\u003e supercomputer centers for computer time.\u0026nbsp;The authors thank Prof. Dima Filippov, \u0026nbsp;Prof. Sylvestre Bonnet, and Hugo Schellevis for helpful discussions.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eW.Z. discovered that diazotization of graphene with 4-SBD yields a giant increase of transmembrane areal proton conductance with selectivity to proton. W.Z. prepared graphene samples for morphological/elemental characterizations, and fuel cell tests, alongside the manuscript preparation. Fuel cell tests were conducted by W.Z.. M.M. synthesized the 4-SBD compound and characterized it by NMR and analyzed the HRMS data. M.M. designed and carried out the AFM experiments as well as analyzed the AFM data. X.L. (Xue) proposed the idea of sulfonating graphene to form a 2D proton exchange membrane with the reported 4-SBD compound. X.L. (Xiaoting), Y.L., L.S., and Z.L. synthesized the single-crystalline CVD graphene for fuel cell tests. T.J.F.K. helped with the sample preparation of graphene on TEM grids and early-stage study with TEM. A. J. designed a reactivity test for 4-SBD. C.L., H.Q., X.F., and U.K. contributed to the HRTEM characterization and discussion. C.M., H.S., and J.P.H. contributed to the XPS characterization and discussion. D.C, F.D., I.E., A.K., and T.H. contributed to the discussions and calculations to support using theoretical models of the areal conductances measured experimentally. This work was under the supervision of G.F.S. who led all the experimental progress, discussions, and the elaboration of the manuscript draft. All co-authors contributed to the writing and editing of this manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests.\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData and materials availability.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data are available in the main text or in the supplementary information.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003ePark HB, Kamcev J, Robeson LM, Elimelech M, Freeman BD (2017) Maximizing the right stuff: The trade-off between membrane permeability and selectivity. 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Chem Rev 122:442\u0026ndash;564. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1021/acs.chemrev.0c01184\u003c/span\u003e\u003cspan address=\"10.1021/acs.chemrev.0c01184\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eEnd notes\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-4807293/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4807293/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"An ideal proton exchange membrane should only permeate protons and be leak-tight for fuels. Graphene is impermeable to water and poorly conducting to protons. Next to long-term stability an ideal and optimized proton exchange membrane therefore needs to fulfil two main criteria: proton permeability and selectivity. Within methanol fuel cells, the first ensures a high-power density, while the second prevents fuel cross-over between the electrodes, which deteriorates catalyst performance and, thereby, drastically lowers performance. However, proton conductivity and selectivity are antagonistic in polymer membranes concerning their performance1. Long channel length in state-of-the-art membranes such as Nafion 117 is therefore a prerequisite to obtaining proton selectivity, at the cost of an additional ionic resistance through such long channels. Pristine graphene2 already fulfils these two criteria, partly as the graphene basal plane is impermeable to water and other molecules3, and exhibits a certain degree of proton conductivity4, influenced by nanoscaled ripples5, corrugations6, particularly in monolayer graphene oxide7 and hydrogenated graphene8. Here, we chemically functionalized monolayer graphene to install sulfophenylated sp3 dislocations by diazotization. Selective to protons, transmembrane areal conductances surpass those of polymer membranes, while providing proton selectivity over methanol through such an atomically thin layer. By creating proton-conductive and selective paths through graphene, we unveil a covalent chemical route to rationalize transmembrane proton transport through 2D materials.","manuscriptTitle":"Sulfophenylated centimeter-size graphene membrane in a direct methanol fuel cell","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-09-23 03:09:53","doi":"10.21203/rs.3.rs-4807293/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":"3de54bdd-a7e1-4f95-9577-bd7478adadd4","owner":[],"postedDate":"September 23rd, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":36709734,"name":"Physical sciences/Nanoscience and technology/Graphene/Mechanical and structural properties and devices"},{"id":36709735,"name":"Physical sciences/Energy science and technology/Fuel cells"},{"id":36709736,"name":"Physical sciences/Chemistry/Electrochemistry/Fuel cells"}],"tags":[],"updatedAt":"2025-12-04T08:07:04+00:00","versionOfRecord":{"articleIdentity":"rs-4807293","link":"https://doi.org/10.1038/s41467-025-65507-3","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2025-11-27 05:00:00","publishedOnDateReadable":"November 27th, 2025"},"versionCreatedAt":"2024-09-23 03:09:53","video":"","vorDoi":"10.1038/s41467-025-65507-3","vorDoiUrl":"https://doi.org/10.1038/s41467-025-65507-3","workflowStages":[]},"version":"v1","identity":"rs-4807293","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4807293","identity":"rs-4807293","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}