Local ionic transport enables selective PGM-free bipolar membrane electrode assembly

Local ionic transport enables selective PGM-free bipolar membrane electrode assembly | 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 Local ionic transport enables selective PGM-free bipolar membrane electrode assembly Mengran Li This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3954760/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 19 Sep, 2024 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract Bipolar membranes in electrochemical CO 2 conversion cells enable different reaction environments in the CO 2 -reduction and oxygen-evolution compartments. Under ideal conditions, water-splitting in the bipolar membrane allows for platinum-group-metal-free anode materials and high CO 2 utilizations. In practice, however, even minor unwanted ion crossover limits stability to short time periods. Here we report the vital role of managing ionic species to improve CO 2 conversion efficiency while preventing acidification of the anodic compartment. Through transport modelling, we identify that an anion-exchange ionomer in the catalyst layer improves local bicarbonate availability and increasing the proton transference number in the bipolar membranes increases CO 2 regeneration and limits K + concentration in the cathode region. Through experiments, we show that a uniform local distribution of bicarbonate ions increases the accessibility of reverted CO 2 to the catalyst surface, improving Faradaic efficiency and limiting current densities by twofold. Using these insights, we demonstrate a fully PGM-free bipolar membrane electrode assembly CO 2 conversion system exhibiting < 1% CO 2 /cation crossover rates and 80–90% CO 2 -to-CO utilization efficiency over 150 h operation at 100 mA cm − 2 without anolyte replenishment. Physical sciences/Energy science and technology/Carbon capture and storage Physical sciences/Engineering/Chemical engineering Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Carbon dioxide (CO 2 ) electrolysis is a promising technology for converting CO 2 electrochemically into valuable products such as carbon monoxide (CO) and hydrocarbons. Despite tremendous advances in achieving industrially applicable rates (up to and over 1 A cm − 2 ) 1–5 , one of the formidable challenges faced by this technology is fundamentally unstable cell system designs. 6 – 10 The state-of-the-art membrane electrode assemblies (MEAs) for CO 2 electrochemical reduction are primarily based upon monopolar ion-exchange membranes (IEM), such as cation- 11 – 13 or anion-exchange 14 – 16 membranes (CEM or AEM), where the ionic current relies on the transport of either cations or anions. However, monopolar-ion transport causes significant pH deviation from the initial anolyte conditions due to CO 2 acidification and carbon crossover 17 – 19 . Specifically, under high-rate CO 2 electrolysis, monopolar IEM-based systems result in a substantial loss of feed CO 2 (e.g., > 50% for CO production) due to (bi)carbonate (HCO 3 − /CO 3 2− ) formation, transport, and eventual regeneration at the opposite electrode. 6 , 17 , 18 , 20 – 22 The natural tendency for the anodic environment to shift towards neutral pH with monopolar membranes necessitates the use of anode materials based on platinum-group metal (PGM) elements, such as iridium and ruthenium oxides, to maintain efficient and stable kinetics for water oxidation. 9 However, the demand for PGMs needed to scale CO 2 electrolysis up to practical gigawatt levels is prohibitive from both economic and scarcity perspectives unless the PGMs can be limited to 90% recycled. 23 To allow for the use of a PGM-free anode, one must frequently replenish the alkaline anolyte to resist steady anode acidification. 24 Alkaline electrolytes such as KOH, however, are themselves electrochemically manufactured from KCl via chlor-alkali processes 25 , so the required alkaline electrolyte replenishment rate would be equivalent to the CO 2 reduction rates and necessitate substantial electrolyte and water turnover. Salt precipitation also poses a challenge at the cathode. In AEM-based systems, critical stability issues result from excessive cation crossover and (bi)carbonate salt accumulation, which can precipitate at the gas channels and block CO 2 transport to the active sites. 8 , 10 , 26 – 28 This issue can be relieved to different extents by recently proposed techniques, such as periodic water flushing 14 , 29 , pulsed operation 30 , or process optimization (e.g., increased temperature or decreased anolyte concentration), 14 , 31 , 32 but these cannot completely mitigate the phenomenon. Use of a CEM and acidic media might resolve the carbonation issue by supplying protons to convert (bi)carbonate back to CO 2 . 33 , 34 However, the proton flux across the CEM may supply excess protons at the cathode that promote the unwanted hydrogen evolution reaction (HER) that outcompetes the desired CO 2 reduction. 35 , 36 Modulation of the catalyst reaction environment in acidic media may facilitate higher CO 2 selectivity 12 , nevertheless, the acidic anolyte conditions will still necessitate iridium-based anodes for water oxidation. Lastly, pure water-fed systems have been demonstrated. 2 , 37 However, due to the lower electrolyte conductivity, these systems typically show substantially larger cell potentials than those with low concentration electrolytes. A system configuration based on bipolar membranes (BPM), as depicted in Fig. 1 a, provides a promising avenue for maintaining a stable alkaline condition for PGM-free anode and generating a current-dependent proton flux to ameliorate (bi)carbonate formation (Fig. 1 b) at the cathode. 38 – 43 The BPM comprises a cation-exchange layer (CEL) and an anion-exchange layer (AEL), with their interface (bipolar junction) dissociating water into H + and OH − (Fig. 1 c) when operated in reverse bias. 44 As a result, the BPM can supply current-dependent fluxes of protons to the cathode and OH − to the anode, thus inherently maintaining a stable pH difference at both cathode and anode during electrolysis while limiting ion crossover. BPM-based systems, if operated well, can sustain high anodic alkalinity to allow the use of PGM-free anode (e.g., nickel-based anode 45 , see Fig. 1 d) and utilize protons to revert (bi)carbonate to CO 2 , thus preventing salt precipitation (Fig. 1 b) and increasing CO 2 utilization. At present, however, it remains challenging to achieve an efficient and stable PGM-free BPMEA cell, which requires well-controlled local ionic transport and chemical reactions. Importantly, CO 2 crossover through the membrane should be eliminated to avoid anolyte pH neutralization and allow the use of a PGM-free anode, such as nickel. Meanwhile, alkali cation crossover should be controlled within a range sufficient for the activation of CO 2 reduction, but limited enough to prevent excessive salt precipitation at the cathode. 40 Poor management of the local ionic transport and reactions within the catalyst layer usually entails undesired HER and thus low selectivity for CO 2 reduction at the cathode due to the high availability of protons close to the catalyst that appear to be more easily reduced. 40 , 46 Product selectivity can be improved by either introducing a stagnant catholyte layer at the cathode/CEL interface 38 , 41 or applying an acid-tolerant and selective catalyst 39 , 42 . The abundant cations in the catholyte layer can activate CO 2 reduction but may cause salt precipitation. 41 Recent reports 39 , 42 have also shown that acid-tolerant catalysts, such as molecular or metal-nitrogen-carbon catalysts, are more selective than silver catalysts for CO 2 reduction due to their weak binding with protons 47 – 49 but remain far inferior to monopolar IEM-based systems. This work reported here uses a combined theoretical and experimental approach to understand the scientific challenges central to BPMEA systems and presently impede their prospects. The results unveil that the ion transference number of the membrane and local ion transport within the catalyst layers serve a pivotal role in eliminating counterion crossover and maximising accessibility of the catalyst surface to the reverted CO 2 . The insights provided by our work can guide the rational design of BPM-based electrochemical systems. Results To understand the local ion transport in a BPMEA, a 1D isothermal continuum model was developed based on previous work by Weng et al. 50 and Lees et al. 51 The model domain includes an ionomer-immersed porous cathode catalyst layer (CL) and CEL of the BPM. The catalyst layer consists of nickel-nitrogen-carbon catalyst (NiNC-IMI) mixed with either cation- (Nafion, a sulfonated fluoropolymer 52 ) or anion-exchange ionomer (Sustainion, an imidazolium functionalized styrene polymer 31 ). We chose a steady-state model to capture the ionic behaviour at the initial stage of BPMEA operation. The model was fit to the experimental CO Faradaic efficiency data collected from NiNC-IMI catalyst layers with Sustainion ionomers by adjusting the electrochemical parameters such as the transfer coefficients and exchange current densities for CO 2 reduction and the competitive HER (see Fig. 2 a). To validate the model, the same kinetic parameters were then used to predict the CO Faradaic efficiency data collected with the Nafion ionomer. Details of the models (e.g., equations, boundary conditions, and parameters) are described in the Supplementary Information. Since previous work has shown the role of anolyte ion concentration and crossover on cathodic performance and stability in a BPMEA CO 2 electrolyzer, 40 we used the model to more deeply investigate two strategies to control the ionic transport within the cathode. Specifically, (i) the use of ionomer in the CL for selective ionic transport and (ii) promoting an increased proton transference number in the CEL. Including ionomer in the catalyst layer is a common effective approach to modulate local ionic transport. Further, maintaining a high proton flux from the CEL to the cathode is also a prerequisite for a stable and efficient BPMEA system. Near-unity water dissociation efficiencies are desired as they limit ionic interactions between the anode and cathode environments that can impact anolyte pH and salt precipitation at the cathode. Therefore, these two strategies are perceived as practical approaches to managing local ion transport and thus reaction microenvironment in the electrolysis cell. We first examined the role of ionomer choice for the CL in determining local ionic concentrations in the catalyst layer. As presented in Fig. 2 b and c , incorporating anion-selective Sustainion ionomer in the CLs (Sus-CLs) leads to a counterintuitively lower pH and CO 3 2− concentration than the Nafion ionomer in the CL (Naf-CLs). Notably, the Sus-CLs case provided a substantially higher (7 folds) HCO 3 − concentration near the CEL|CL interface, and over the entire catalyst layer an average HCO 3 − concentration that is more than twice that of the Naf-CLs case. This discernible divergence is a result of the different fixed charges of the two ionomers. Anion exchange ionomers promote the transport of generated (bi)carbonates near the gas-liquid interface towards the BPM, while the Nafion rejects this transport and promotes (bi)carbonate accumulation near the generation point. The positive fixed charge of the Sus-CLs case then provides ample HCO 3 − available for acidification and CO 2 regeneration near the CEL|CL interface. In addition to the anion transport, it is important to assess the cation transport (H + and K + ). As further suggested from the calculated K + profiles shown in Fig. S3a, the positively charged quaternary ammonium groups in the ionomer at least partially exclude K + transport from CEL to CL in the Sus-CLs and thus likely contribute to the observed reduced pH in the CLs by lowering the required amount of OH − ions needed to balance the positive charge. By contrast, the concentration profiles predicted for Naf-CL indicate a more uneven ionic distribution as compared to Sus-CL. The negatively charged sulfonic groups in Nafion lead to an excessive amount of K + in the Naf-CL (see Fig. S3a), which then fosters a high content of OH − (or high pH shown in Fig. 2 b) and CO 3 2− to maintain the charge neutrality. Overall, both ionomer cases show the ability for CO 2 converted to (bi)carbonates to be regenerated into CO 2 by the proton flux from the CEL of the BPM. Interestingly, the concentration of HCO 3 − at Naf-CL shown in Fig. 2 c decreases from around 1.0 M at CL|GDL interface down to 0.047 M at the CEL|CL interface. Such a steep decrease in HCO 3 − concentration is a result of increased local pH inside Naf-CL. Due to the high local pH inside Naf-CL (Fig. 2 b), however, the regenerated CO 2 tends to diminish and form back to CO 3 2− within the Naf-CL (Fig. 2 c). As such, the uneven ionic distribution in Naf-CL might not be ideal for an efficient electrochemical conversion of CO 2 because it could cause a low utilisation efficiency of the reverted CO 2 for electrochemical conversion. Similarly, the local CO 2 concentration near the Sus-CL|CEL interface is slightly higher than Naf-CL|CEL interface (Fig. 2 d) due to a higher concentration of HCO 3 − which promotes CO 2 regeneration. However, the CO 2 concentration throughout the bulk of the CL is similar for the Sus-CL and Naf-CL because of the constant excess CO 2 supply provided at the CL|GDL interface. Next, we use the Sus-CL model to investigate the impact of proton transference number on the local ionic transport across the CLs. The ionic conduction across the CEL|CL interface relies on the transport of H + , K + , and (bi)carbonate ions, with H + being the primary charge carrier. The H + transference number quantifies the fraction of the current crossing the CEL in the absence of concentration gradients, which is a result of proton transport from the BPM. The total ionic current is the sum of the H + transport, K + crossover from the anode and anion crossover from the cathode. By sweeping the H + transference number from 0.75 to 0.95, as presented in Fig. 2 e-g and Fig. S3b, we observed a decrease in pH, K + and CO 3 2− concentrations but an increase in HCO 3 − concentrations in the CLs. This trend is as expected because an increase in H + transference number indicates a suppressed current associated with K + cations and promotes carbonate acidification within the CLs. Additionally, as shown in Fig. 2 g, increasing the H + transference number does not notably increase the local CO 2 concentration but reduces the CO 2 concentration within the CEL due to the minimised (bi)carbonate crossover from CL to CEL. The downside of an increased H + flux across CEL|CL interface could be HER out competing CO 2 reduction through direct H + reduction and limited cations available to activate CO 2 electrochemical reduction. The role of ionomer in CL on BPMEA performance To validate the predictions of the model, we compared experimentally NiNC-IMI CLs prepared with Sustainion and Nafion ionomers. A comprehensive study of the NiNC-IMI catalyst has been published elsewhere 53 , 54 , so this study will focus on the role of the ionic transport on the BPMEA performance by using the NiNC-IMI as a model CL. The products from the BPMEA cells are primarily CO and H 2 with minor formate or formic acid (see nuclear magnetic resonance results in Fig. S4). Because the BPM is effective in minimizing formate crossover to the anolyte 55 , it is challenging to calculate the formate FEs accurately by analyzing anolyte compositions. Hence, this work focuses mainly on the CO and H 2 products from the electrolysis. Figure 3 b- 3 d show that the ionomer in the CL defines cathode activity and selectivity in the BPMEA configuration. 15 wt% Sustainion ionomer in the CL can profoundly improve the catalyst CO FE and partial current densities compared to the 15 wt% Nafion. Due to the different equivalent weight of the ionomers (1100 g/mol for Nafion; ~ 225 g/mol for Sustainion 31 ), 15 wt% Sustainion contains approximately five times the concentration of ionic groups as compared to the 15 wt% Nafion. As shown in Fig. 3 b, the CO FE is 69.7 ± 0.5% at 50 mA cm − 2 and 45.4 ± 0.5% at 200 mA cm − 2 for CLs with 15% Sustainion ionomer, much higher than the CLs based with Nafion (53.5 ± 4.2% at 50 mA cm − 2 and 21.4 ± 1.0% at 200 mA cm − 2 ). The Naf-CLs reach a CO limiting current density of ~ 41 mA cm − 2 , more than two-fold lower than the Sus-CLs (> 105 mA cm − 2 ). The notable enhancement in CO 2 -to-CO upon FEs and partial current densities is related to the local mass transport within the CLs that either improves the local concentration of CO 2 or limits the local concentration of protons. Fig. S5 demonstrates no significant suppression of HER by the Sustainion ionomer, implying a similar local proton availability and/or catalytic HER activity in both cases. Hence, we postulate that the observed improvement likely originates from the enhanced bicarbonate local transport within Sustainion-based CLs, as predicted from our model in Fig. 2 c. The increase in local bicarbonate concentration in the CL provides an even CO 2 distribution in the CL and thus promote CO 2 electroreduction. In the BPMEA reported here, there are two sources of CO 2 : CO 2 that dissolves from the gas feed and CO 2 that is regenerated from bicarbonate via acid-base chemistry. The modelled CO 2 concentration profiles (Fig. 2 c and f ) show that the potent proton flux from CEL yields a peak in CO 2 concentration at the CEL|CL interface. The increased CO 2 concentration results in an increase in the predicted CO partial current density near the CEL|CL interface as shown in Fig. S6. This result is consistent with our previous bicarbonate electrolyser model that shows high CO formation rates at the CEL|CL interface. 51 In this BPMEA model, a high rate of CO formation is also observed at the CL|GDL interface. This difference is attributed to the additional CO 2 source from the gas phase, which is absent in the bicarbonate electrolyser. These modelling results confirm that both regenerated and dissolved CO 2 contribute to CO production in a BPMEA. Since bicarbonate anions exhibit more facile transport in the anion-exchange Sustainion ionomer than the cation-exchange Nafion ionomer 18 (see Fig. 2 b), we predict that the Sustainion ionomer should allow a facile transport of bicarbonate anions for CO 2 regeneration, providing ample local CO 2 for electrochemical conversion across the CL structure (see Fig. 3 a). By contrast, in the Naf-CL, the regenerated CO 2 tends to be spatially localized, causing a limited proportion of catalyst surface in the CLs to be accessible by the regenerated CO 2 . Therefore, the even distribution of bicarbonate enables the Sustainion-based CL with a larger catalyst surface area accessible by the regenerated CO 2 than a Nafion-based CL 56 . Thus, while both systems enable reasonable CO 2 utilization efficiencies, Sus-CL case exhibits both higher CO 2 utilization efficiencies, CO FE, and partial current densities at comparable cell voltages (Fig. 3 d). Figure 3 e shows that the CO 2 -to-CO utilisation efficiency increases with current density. Here, the CO 2 -to-CO utilisation efficiency is defined as the ratio of the CO 2 reduced to CO versus the total CO 2 consumed in the cell (i.e., due to crossover and reaction). The observed CO 2 -to-CO utilisation trend as a function of current density could result from an increase in the H + transference number across the CEL of the BPM, because water dissociation dictates the overall ionic current while the co- and counter-ion crossover is mass-transport limited at increased current densities. 57 When one mole of CO is produced, for example, there will be two moles of electrons consumed, two moles of H + /K + transported from the CEL, and two moles of OH − co-produced. The OH − can then be converted into one-mole carbonate or two moles of bicarbonate, which require two moles of protons to remove the (bi)carbonate species. Therefore, the H + transference number determines the availability of the H + to revert the generated (bi)carbonate species back to CO 2 . As the H + transference number is always below 1, it means that some CO 2 converted to (bi)carbonates are never recovered and will either precipitate or crossover to the anode. Consequently, high H + transference numbers across the BPM are beneficial to lowering the local pH and regenerating CO 2 from (bi)carbonate, hence, maximizing CO 2 -to-CO utilisation efficiency. A further increase of Sustainion ionomer loading to 30 wt% in the CL lowered the FEs and limiting current density for CO production. (Fig. 3 b- 3 d) Such decline in performance is likely attributable to the blockage of the CL pores and reactive sites by the ionomer itself, resulting in an increased diffusion length for CO 2 gas from gas channels to the catalyst surface or loss of active surface in the CLs, respectively. However, the CO 2 -to-CO utilisation efficiency is not significantly impacted by the increment of the ionomer loading, implying that in-situ formed (bi)carbonate ions can be reverted to CO 2 easily within the CL matrix. More importantly, as shown in Fig. 3 f, the anolyte shows only a slight decrease in pH after the cell testing, with a total charge of 6300 C passed through the cell during the test. The pH of the anolyte could have been decreased because of both carbon crossover from the cathode to the anode and cation crossover from the anode to the cathode. The minimal change in the anolyte pH is due to the unique BPM function, which supplies protons at the cathode to convert (bi)carbonate anions to CO 2 for CO production and minimises ion crossover rates across the cell. This feature allows BPMEA cells to be operated stably using a PGM-free anode based on nickel and with a high CO 2 -to-product utilisation efficiency, which could not be easily done using monopolar membrane-based electrolysis cells. The role of the H + transference number on BPMEA performance The role of proton transference number is investigated experimentally by examining the effect of K + cation crossover on CO 2 reduction using 0.1 M and 1 M KOH as the anolyte. A concentrated KOH is expected to accelerate the crossover rate of K + due to the large concentration gradient across the membrane (see Fig. S7a). Because cations are essential in activating CO 2 reduction, the results in Fig. S7b show that 1 M KOH anolyte can slightly improve CO FE at current densities > 150 mA cm − 2 . This finding is consistent with our previous report highlighting the cations' vital role in enhancing CO FEs over silver electrodes. 40 Fig. S7c suggests that the BPMEA cell with a dilute anolyte shows a higher CO 2 -to-CO utilisation efficiency than the concentrated anolyte. This trend matches our model predictions of the proton transference number in Fig. 2 d-f, because a reduced K + crossover rate achieved by using a dilute anolyte leads to an increase in proton transference number and acidification of (bi)carbonate within the CL. Therefore, a trade-off exists between product selectivity and CO 2 utilisation efficiency when choosing the anolyte for the BPMEA cell. Local H + enrichment at CEL/CL interface In addition to the distribution of (bi)carbonate ions within the CL, we studied the local reaction environment at the CEL|CL interface by introducing a hydrophilic porous spacer (65 um thick) between the CL and CEL. (Fig. 4 a) This configuration is fundamentally different from the previously reported method 41 that includes concentrated salts within the spacer. In this study, we applied the spacer pre-soaked with ultrapure water before the cell assembly. The ion conduction within the spacer solely depends on the ionic fluxes of protons, (bi)carbonate anions, and K + cations that come from the fixed charge in the CEL and anolyte. As shown in Fig. 4 b, the spacer significantly suppresses the HER down to below 15% and boosts the CO FEs up to 91% at 50 mA cm − 2 and 88% at 100 mA cm − 2 , which is almost comparable to MEA cells based on monopolar membranes. The comparison of the CO partial current densities vs. cell potentials, as shown in Fig. S8, suggests cells with and without spacer achieve similar CO partial current densities under similar cell voltages. Therefore, the observed CO FE enhancement is mainly attributable to the suppression of HER in the CLs. This finding also indicates that the majority of HER in the absence of spacer occurs at the CL|CEL interfaces due to the direct contact of the CL with excess protons; the spacer increases the retention time for protons to reach the catalyst surface. The drawback to including the spacer in the BPMEA cell is the large ohmic loss due to the slow ionic conduction across the spacer, as verified by the electrochemical impedance spectroscopic analyses in Fig. S9. The absence of abundant water at the cathode side of the BPMEA accelerates the dehydration of the spacer, which causes further reduction of the ionic conductivity of the spacer and eventually rapid cell voltage overshoot (See Fig. S10). Nonetheless, the results highlight the importance of the cell configuration in determining the CO 2 -to-CO selectivity of the BPMEA cells. Long-term stability and ion crossover Finally, we evaluated the long-term stability of the PGM-free BPMEA cell, particularly as it relates to the anolyte pH and the stable use of nickel anodes. Figure 5 a demonstrates that the CO Faradaic efficiency rapidly dropped from 73–64% within the first 1.5 h, then decreased linearly to 30% after 150 h at a degradation rate of 0.36% per hour. Meanwhile, the HER increased along with the loss in CO FE. Such discernible selectivity loss likely originates from an increase in contact area between catalyst and H + and possible deactivation of the catalyst in the acidic environment. Both could contribute to the rise in the rate of HER and suppression of the rate of CO 2 reduction. The cell voltage also increased at a degradation rate of 0.086% per hour. By comparing the ohmic resistances obtained from electrochemical impedance (Fig. S11), we found that the degradation of the cell potential should be related to the increase of the polarization resistance over either the cathode or anode rather than the deterioration of the BPM. On the other hand, as shown in Fig. 5 b, the CO 2 -to-CO utilisation efficiency increased rapidly in the first 1.5 h and was subsequently sustained at above 80–90% across the measurement. The rapid rise in the CO 2 utilisation within the first 1.5 h implies that the rapid drop in CO FE is mainly a result of an increase of proton local availability for HER in the CL. The subsequent linear degradation is likely associated with the instability of the catalyst layer in a strongly acidic environment. Without replenishing the anolyte across the entire test, we observed no significant change in the pH value of the anolyte after the test, as shown in Fig. 5 c. The titration results shown in Fig. 5 d and Fig. S12 revealed that about 0.006 mol K + cations have migrated to the cathode side, which is equivalent to 0.21% of the ionic current during the conditioning. Due to the anolyte's high alkalinity, most of the CO 2 crossover should be converted to carbonate ions in the anolyte. Figure 5 d shows that the anolyte after the test is composed of 0.0735 M OH − and 0.011 M CO 3 2− , implying that the (bi)carbonate crossover contributed to < 0.774% of the ionic current. The extremely low K + and CO 2 crossover rates were essential in sustaining the Ni-based anode stability without replenishing the anolyte during the 150-h test. We posit that a proton transference number of ~ 99% was maintained over the course of the experiment. While the 0.21% K + crossover is likely needed to maintain CO 2 reduction at the cathode, the 0.774% carbonate crossover is all that needs to be avoided to maintain anolyte pH indefinitely. After 150 h conditioning within the BPMEA cell, as revealed by the XRD results in Fig. S13, the Ni anode remained as metallic Ni, while additional minor phases related to potassium nickelates 58 formed due to the long-term oxidising treatment. Importantly, no nickel carbonate phase was detected in the conditioned anode, implying that the BPMEA configuration stabilised the Ni-based anode. Ex-situ surface analyses further confirm that both fresh and conditioned anodes are predominantly covered with Ni(OH) 2 species at the surfaces (Fig. S14), and that their structures shown in Fig. S15 and S16 remain intact after 150 h conditioning. The retained structural and chemical stability of the Ni anode is a main result of the alkaline local environment achieved by the BPM, which supplies hydroxide ions and suppresses K + and CO 2 crossover. We also noticed that the inlet pressure of the cell rose after an interval of 12–30 h. The rise of the inlet pressure is mainly a result of the build-up of precipitated (bi)carbonate salts at the entrance of the gas channel in the cell, though there was no discernible salt precipitation elsewhere in the gas channels or the back of the cathode (see Fig. S17). The salt precipitation occurs typically when the concentration of the local (bi)carbonate exceeds the solubility (e.g., 8.03 M for K 2 CO 3 and 3.62 M for KHCO 3 ) 26 , 28 at the cathode structure and the crystal can grow continuously to reach the gas channels, similar to efflorescence process, due to its hygroscopic and porous nature. The inlet pressure rise (due to the salt precipitation) elucidates that there is a continuous supply of K + from the cation crossover of about 0.21% to the cathode for the salt precipitation, which cannot be easily mitigated, especially at a high current density. To circumvent this issue, we applied ~ 1 mL pure water pulse to wash off the precipitated salt from the gas channel during operation when an increase of inlet pressure was observed. Unlike the reported hourly water flush to remove the salts at the cathode for the anion-exchange membrane electrode assembly cell 14 , 29 , the BPMEA cell requires a much less frequent water pulse, thanks to its high CO 2 utilisation and controlled K + concentrations at the cathode. The water pulse operation also showed negligible impact on the cell potential and CO Faradaic efficiency, as shown in Fig. 5 a. Discussions In summary, this combined modelling and experimental work highlights and elucidates the critical role of the local ionic transport and reaction environment in determining the CO 2 -to-CO activity, selectivity, and utilisation efficiency in BPM electrode assemblies constructed free of platinum group metals. The combined theoretical and experimental results reveal that an even distribution of bicarbonate across cathode catalyst layers enhances the accessibility of the reverted CO 2 to the catalyst layer and thus boost CO Faradaic efficiency and limiting current densities more than two-fold. We also found that the proton transference number determines the CO 2 utilisation efficiency: a high proton transference number generally achieves a high CO 2 -to-CO efficiency but may not necessarily lead to a high CO 2 reduction selectivity. Although hydrogen evolution still outcompetes CO 2 reduction at > 150 mA cm − 2 , our results suggest that hydrogen evolution occurs primarily at the membrane/catalyst interfaces and can be suppressed effectively by the inclusion of a spacer at the interface. Lastly, we demonstrate notably low crossover rates for CO 2 and K + (< 1% of total charge) over a PGM-free BPMEA cell operating at 100 mA cm − 2 for 150 h. This cell achieves a 80–90% CO 2 -to-CO utilisation efficiency and stable alkalinity in the anolyte. The findings of this work provide new insights into advancing and assessing BPM electrochemical systems via catalyst material design and modulation of local transport and reaction microenvironment. Declarations Acknowledgment M.L. acknowledge the financial support from Australian Research Council (DE230100637). W.J., T.B., and P.S. acknowledge the financial support from EU project 851441 – SELECTCO2. W.J. and P.S. acknowledge the support from EU project 101006701 – Ecofuel. J.C.B. was supported in part by a fellowship award under contract FA9550-21-F-0003 through the National Defense Science and Engineering Graduate (NDSEG) fellowship program, sponsored by the Army Research Office (ARO) H-P.I.v.M. and S.S acknowledge the financing provided for this project in the context of the e-Refinery Institute by Shell Global Solutions International B.V. and the Top Consortia for Knowledge and Innovation (TKI’s) of the Dutch Ministry of Economic Affairs. A.Z.W., A.T.B., and J.C.B. all acknowledge support under the Liquid Sunlight Alliance, which is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Fuels from Sunlight Hub under award number DE-SC0021266. E.W.L and A.Z.W. also acknowledge support for the modeling under the CO 2 Consortium funded by BETO in EERE under contract DE-AC02-05CH11231. Competing interests The authors declare no competing interests. References Samu, A. A. et al. 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Efficiency and selectivity of CO2 reduction to CO on gold gas diffusion electrodes in acidic media. Nat. Commun. 12 , 4943 (2021). Bondue, C. J., Graf, M., Goyal, A. & Koper, M. T. M. Suppression of Hydrogen Evolution in Acidic Electrolytes by Electrochemical CO2 Reduction. J. Am. Chem. Soc. 143 , 279–285 (2021). She, X. et al. Pure-water-fed, electrocatalytic CO2 reduction to ethylene beyond 1,000 h stability at 10 A. Nat. Energy 1–11 (2024) doi:10.1038/s41560-023-01415-4. Salvatore, D. A. et al. Electrolysis of Gaseous CO2 to CO in a Flow Cell with a Bipolar Membrane. ACS Energy Lett. 3 , 149–154 (2018). Eriksson, B. et al. Mitigation of Carbon Crossover in CO2 Electrolysis by Use of Bipolar Membranes. J. Electrochem. Soc. 169 , 034508 (2022). Yang, K. et al. Cation-Driven Increases of CO2 Utilization in a Bipolar Membrane Electrode Assembly for CO2 Electrolysis. ACS Energy Lett. 6 , 4291–4298 (2021). Xie, K. et al. 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Electrochemical Reduction of CO2 on Metal-Nitrogen-Doped Carbon Catalysts. ACS Catal. 9 , 7270–7284 (2019). Möller, T. et al. Efficient CO2 to CO electrolysis on solid Ni–N–C catalysts at industrial current densities. Energy Environ. Sci. 12 , 640–647 (2019). Ju, W. et al. Understanding activity and selectivity of metal-nitrogen-doped carbon catalysts for electrochemical reduction of CO2. Nat. Commun. 8 , 944 (2017). Weng, L.-C., Bell, A. T. & Weber, A. Z. Towards membrane-electrode assembly systems for CO2 reduction: a modeling study. Energy Environ. Sci. 12 , 1950–1968 (2019). Lees, E. W., Bui, J. C., Song, D., Weber, A. Z. & Berlinguette, C. P. Continuum Model to Define the Chemistry and Mass Transfer in a Bicarbonate Electrolyzer. ACS Energy Lett. 7 , 834–842 (2022). Kusoglu, A. & Weber, A. Z. New Insights into Perfluorinated Sulfonic-Acid Ionomers. Chem. Rev. 117 , 987–1104 (2017). Brückner, S. et al. Design and Diagnosis of high-performance CO2-to-CO electrolyzer cells. Preprint at https://doi.org/10.26434/chemrxiv-2023-z6v6m (2023). Wang, J. et al. Design of NiNC single atom catalyst layers and AEM electrolyzers for stable and efficient CO2-to-CO electrolysis: Correlating ionomer and cell performance. Electrochimica Acta 461 , 142613 (2023). Li, Y. C. et al. Bipolar Membranes Inhibit Product Crossover in CO2 Electrolysis Cells. Adv. Sustain. Syst. 2 , 1700187 (2018). Subramanian, S. et al. Geometric Catalyst Utilization in Zero-Gap CO2 Electrolyzers. ACS Energy Lett. 222–229 (2022) doi:10.1021/acsenergylett.2c02194. Bui, J. C., Digdaya, I., Xiang, C., Bell, A. T. & Weber, A. Z. Understanding Multi-Ion Transport Mechanisms in Bipolar Membranes. ACS Appl. Mater. Interfaces 12 , 52509–52526 (2020). Oliva, P. et al. Review of the structure and the electrochemistry of nickel hydroxides and oxy-hydroxides. J. Power Sources 8 , 229–255 (1982). Additional Declarations There is NO Competing Interest. Supplementary Files SIBPMEA.docx Local ionic transport enables selective PGM-free bipolar membrane electrode assembly Cite Share Download PDF Status: Published Journal Publication published 19 Sep, 2024 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. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-3954760","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":273018697,"identity":"f08c6df8-3553-4d7e-bd0d-25f439a3410a","order_by":0,"name":"Mengran Li","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA80lEQVRIie3PuwrCMBSA4SOFuBx0TVHrK0QCrUXRV7EU6uLgJIKDhQ59BQefo3Olg4viWnCxCE7OgpOmXkCQoG4O+afkwJcLgEr1vzWRFH2AHkCViS39glAsYXwj+DUBg/Zuq8+kHK6z/RBolehHc5cN2miBttgidBxfdvyqz/lMPIxUBhZzIg9tn7gtBFdKIPZIBe/EpE6UIIvRFBNNSuqbw4Poq5xcBCmfxGQqJSx93kIxJ3F+Sz5JpKSRHjSOTBD0RoK4aAeE23O25DJibLzCHsftbj1MIv0cdQyrGGTpcTypSb9/f97rRnubqFQqlerXroOhQhLEq7GAAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0001-7858-0533","institution":"University of Melbourne","correspondingAuthor":true,"prefix":"","firstName":"Mengran","middleName":"","lastName":"Li","suffix":""}],"badges":[],"createdAt":"2024-02-14 00:30:09","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-3954760/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3954760/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-024-52409-z","type":"published","date":"2024-09-19T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":51216719,"identity":"8d3e1e6c-1fe4-4535-b8cb-5faf58e7d698","added_by":"auto","created_at":"2024-02-16 08:06:46","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":600777,"visible":true,"origin":"","legend":"\u003cp\u003eIllustration of an ideal BPM-based membrane electrode assembly for CO\u003csub\u003e2\u003c/sub\u003e-to-CO electrolysis. (a) A schematic illustration of an ideal BPMEA cell configuration. (b) Elucidation of the use of the reverted CO\u003csub\u003e2\u003c/sub\u003e from (bi)carbonate acidification for CO\u003csub\u003e2\u003c/sub\u003e reduction at the interface between the cathode and cation-exchange layer (CEL). (c) Water dissociation at the interface between CEL and anion-exchange layer (AEL). (d) Pourbaix diagram of Ni-(bi)carbonate species, reproduced from Materials Project.\u003csup\u003e45\u003c/sup\u003e\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-3954760/v1/85e5c6991b185c00008fb5fa.jpeg"},{"id":51216720,"identity":"90afa2ff-e869-4e16-ac1a-a73419372bca","added_by":"auto","created_at":"2024-02-16 08:06:46","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":198911,"visible":true,"origin":"","legend":"\u003cp\u003e1D continuum modelling results of the BPMEA. (a) Modelled and experimental Faradaic efficiencies of CO for catalyst layer (CL) incorporated with Sustainion (Sus-CL) or Nafion (Naf-CL) as a function of current densities. \u0026nbsp;Comparison of (b) pH, (c) (bi)carbonate concentrations, and (d) CO\u003csub\u003e2\u003c/sub\u003e local concentration across CLs at 100 mA cm\u003csup\u003e-2\u003c/sup\u003e. Profiles of (e) pH, (f) (bi)carbonate ions, and (g) CO\u003csub\u003e2\u003c/sub\u003e local concentration across CEL and Sus-CL as a function of proton transference numbers. The bipolar junction is located at x = 0 μm, and the CEL|CL is located at x = 75 μm. The concentration profiles of the CEL for (c) and (d) are presented in Fig. S2, and not shown here for clarity.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-3954760/v1/84d1af4aed8bd23d81a335ec.jpeg"},{"id":51217182,"identity":"4f8c2238-33bc-4fa5-8f30-df1672772869","added_by":"auto","created_at":"2024-02-16 08:14:46","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":330112,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of catalyst layers for CO\u003csub\u003e2\u003c/sub\u003e reduction to CO in BPMEA cells. (a) Schematic illustration of the CL on a gas-diffusion layer (GDL), the catalyst materials, and the importance of the migration of (bi)carbonate across the Sustainion-based CL for CO\u003csub\u003e2\u003c/sub\u003e local regeneration. Faradaic efficiency of (b) CO and (c) hydrogen of the CLs versus total current densities. (d) CO partial current density and (e) CO\u003csub\u003e2\u003c/sub\u003e-to-CO utilization efficiency over CLs. (f) pH changes of 100 mL 0.1 M KOH aqueous anolyte as a function of CLs before and after cell tests (6300 C total charge) under 50-300 mA cm\u003csup\u003e-2\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-3954760/v1/f026a7180e946de6032e1baa.jpeg"},{"id":51216725,"identity":"676a1909-b468-4b05-81fc-1406540bcf26","added_by":"auto","created_at":"2024-02-16 08:06:46","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":425061,"visible":true,"origin":"","legend":"\u003cp\u003eThe effects of the cathode/CEL spacer on the performance of the BPMEA cell. (a) A schematic of the spacer at the cathode/CEL interface. Comparison of the (b) CO Faradaic efficiency and (c) CO\u003csub\u003e2\u003c/sub\u003e-to-CO utilisation efficiency versus total current densities of the BPMEA with and without the spacer. The BPMEA cell used cathodes with NiNC-IMI catalyst with 15% Sustainion ionomer as the catalyst layer.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-3954760/v1/f6ec218ee838b19bdd07e267.jpeg"},{"id":51216724,"identity":"476be869-9602-4b87-9123-f3f0c6d5c53c","added_by":"auto","created_at":"2024-02-16 08:06:46","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":413392,"visible":true,"origin":"","legend":"\u003cp\u003eShort-term stability of the PGM-free BPMEA cell. (a) Faradaic efficiency (FE) of gas products and cell potential and (b) CO\u003csub\u003e2\u003c/sub\u003e-to-CO utilisation efficiency and inlet pressure of the BPMEA cell as a function of time for the BPMEA cell. Comparison of the (c) pH and (d) anion concentration of the anolyte before and after the stability test. The BPMEA cell was operated at a constant current density of 100 mA cm\u003csup\u003e-2\u003c/sup\u003e, using 1 L 0.1 M KOH as the anolyte, Ni foam as the anode and NiNC-IMI catalyst with 15% Sustainion ionomer as the cathode CL. The flow rate was 20 sccm for the CO\u003csub\u003e2\u003c/sub\u003e gas inlet and 30 mL min\u003csup\u003e-1\u003c/sup\u003e for the anolyte.\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-3954760/v1/03f90b97619e0279fe8e5fd4.jpeg"},{"id":64896112,"identity":"e16be5ec-e089-4fa7-8751-00b292df4651","added_by":"auto","created_at":"2024-09-20 07:07:00","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2545957,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3954760/v1/aee4f535-500e-4d1f-9cb8-b26f330ce471.pdf"},{"id":51216722,"identity":"52c34fa0-194f-4c0a-a504-1d6fae8cbf9c","added_by":"auto","created_at":"2024-02-16 08:06:46","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":2255626,"visible":true,"origin":"","legend":"\u003cp\u003eLocal ionic transport enables selective PGM-free bipolar membrane electrode assembly\u003c/p\u003e","description":"","filename":"SIBPMEA.docx","url":"https://assets-eu.researchsquare.com/files/rs-3954760/v1/31e9f7fbc39e0a2ffa08c0f7.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Local ionic transport enables selective PGM-free bipolar membrane electrode assembly","fulltext":[{"header":"Introduction","content":"\u003cp\u003eCarbon dioxide (CO\u003csub\u003e2\u003c/sub\u003e) electrolysis is a promising technology for converting CO\u003csub\u003e2\u003c/sub\u003e electrochemically into valuable products such as carbon monoxide (CO) and hydrocarbons. Despite tremendous advances in achieving industrially applicable rates (up to and over 1 A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e)\u003csup\u003e1\u0026ndash;5\u003c/sup\u003e, one of the formidable challenges faced by this technology is fundamentally unstable cell system designs.\u003csup\u003e\u003cspan additionalcitationids=\"CR7 CR8 CR9\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e The state-of-the-art membrane electrode assemblies (MEAs) for CO\u003csub\u003e2\u003c/sub\u003e electrochemical reduction are primarily based upon monopolar ion-exchange membranes (IEM), such as cation-\u003csup\u003e\u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e or anion-exchange\u003csup\u003e\u003cspan additionalcitationids=\"CR15\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e membranes (CEM or AEM), where the ionic current relies on the transport of either cations or anions. However, monopolar-ion transport causes significant pH deviation from the initial anolyte conditions due to CO\u003csub\u003e2\u003c/sub\u003e acidification and carbon crossover\u003csup\u003e\u003cspan additionalcitationids=\"CR18\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. Specifically, under high-rate CO\u003csub\u003e2\u003c/sub\u003e electrolysis, monopolar IEM-based systems result in a substantial loss of feed CO\u003csub\u003e2\u003c/sub\u003e (e.g., \u0026gt; 50% for CO production) due to (bi)carbonate (HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e/CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e) formation, transport, and eventual regeneration at the opposite electrode.\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\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\u003c/p\u003e \u003cp\u003eThe natural tendency for the anodic environment to shift towards neutral pH with monopolar membranes necessitates the use of anode materials based on platinum-group metal (PGM) elements, such as iridium and ruthenium oxides, to maintain efficient and stable kinetics for water oxidation.\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e However, the demand for PGMs needed to scale CO\u003csub\u003e2\u003c/sub\u003e electrolysis up to practical gigawatt levels is prohibitive from both economic and scarcity perspectives unless the PGMs can be limited to \u0026lt;\u0026thinsp;0.1 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e or \u0026gt;\u0026thinsp;90% recycled.\u003csup\u003e23\u003c/sup\u003e To allow for the use of a PGM-free anode, one must frequently replenish the alkaline anolyte to resist steady anode acidification.\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e Alkaline electrolytes such as KOH, however, are themselves electrochemically manufactured from KCl via chlor-alkali processes\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e, so the required alkaline electrolyte replenishment rate would be equivalent to the CO\u003csub\u003e2\u003c/sub\u003e reduction rates and necessitate substantial electrolyte and water turnover.\u003c/p\u003e \u003cp\u003eSalt precipitation also poses a challenge at the cathode. In AEM-based systems, critical stability issues result from excessive cation crossover and (bi)carbonate salt accumulation, which can precipitate at the gas channels and block CO\u003csub\u003e2\u003c/sub\u003e transport to the active sites.\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan additionalcitationids=\"CR27\" citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e This issue can be relieved to different extents by recently proposed techniques, such as periodic water flushing\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e, pulsed operation\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e, or process optimization (e.g., increased temperature or decreased anolyte concentration),\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e,\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e but these cannot completely mitigate the phenomenon. Use of a CEM and acidic media might resolve the carbonation issue by supplying protons to convert (bi)carbonate back to CO\u003csub\u003e2\u003c/sub\u003e.\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e However, the proton flux across the CEM may supply excess protons at the cathode that promote the unwanted hydrogen evolution reaction (HER) that outcompetes the desired CO\u003csub\u003e2\u003c/sub\u003e reduction.\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e,\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e Modulation of the catalyst reaction environment in acidic media may facilitate higher CO\u003csub\u003e2\u003c/sub\u003e selectivity\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e, nevertheless, the acidic anolyte conditions will still necessitate iridium-based anodes for water oxidation. Lastly, pure water-fed systems have been demonstrated. \u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e However, due to the lower electrolyte conductivity, these systems typically show substantially larger cell potentials than those with low concentration electrolytes.\u003c/p\u003e \u003cp\u003eA system configuration based on bipolar membranes (BPM), as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, provides a promising avenue for maintaining a stable alkaline condition for PGM-free anode and generating a current-dependent proton flux to ameliorate (bi)carbonate formation (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) at the cathode.\u003csup\u003e\u003cspan additionalcitationids=\"CR39 CR40 CR41 CR42\" citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e The BPM comprises a cation-exchange layer (CEL) and an anion-exchange layer (AEL), with their interface (bipolar junction) dissociating water into H\u003csup\u003e+\u003c/sup\u003e and OH\u003csup\u003e\u0026minus;\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec) when operated in reverse bias.\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e As a result, the BPM can supply current-dependent fluxes of protons to the cathode and OH\u003csup\u003e\u0026minus;\u003c/sup\u003e to the anode, thus inherently maintaining a stable pH difference at both cathode and anode during electrolysis while limiting ion crossover. BPM-based systems, if operated well, can sustain high anodic alkalinity to allow the use of PGM-free anode (e.g., nickel-based anode\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e, see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed) and utilize protons to revert (bi)carbonate to CO\u003csub\u003e2\u003c/sub\u003e, thus preventing salt precipitation (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) and increasing CO\u003csub\u003e2\u003c/sub\u003e utilization.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAt present, however, it remains challenging to achieve an efficient and stable PGM-free BPMEA cell, which requires well-controlled local ionic transport and chemical reactions. Importantly, CO\u003csub\u003e2\u003c/sub\u003e crossover through the membrane should be eliminated to avoid anolyte pH neutralization and allow the use of a PGM-free anode, such as nickel. Meanwhile, alkali cation crossover should be controlled within a range sufficient for the activation of CO\u003csub\u003e2\u003c/sub\u003e reduction, but limited enough to prevent excessive salt precipitation at the cathode.\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003ePoor management of the local ionic transport and reactions within the catalyst layer usually entails undesired HER and thus low selectivity for CO\u003csub\u003e2\u003c/sub\u003e reduction at the cathode due to the high availability of protons close to the catalyst that appear to be more easily reduced.\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e,\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e Product selectivity can be improved by either introducing a stagnant catholyte layer at the cathode/CEL interface\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e,\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e or applying an acid-tolerant and selective catalyst\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e,\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e. The abundant cations in the catholyte layer can activate CO\u003csub\u003e2\u003c/sub\u003e reduction but may cause salt precipitation.\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e Recent reports\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e,\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e have also shown that acid-tolerant catalysts, such as molecular or metal-nitrogen-carbon catalysts, are more selective than silver catalysts for CO\u003csub\u003e2\u003c/sub\u003e reduction due to their weak binding with protons\u003csup\u003e\u003cspan additionalcitationids=\"CR48\" citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e but remain far inferior to monopolar IEM-based systems.\u003c/p\u003e \u003cp\u003eThis work reported here uses a combined theoretical and experimental approach to understand the scientific challenges central to BPMEA systems and presently impede their prospects. The results unveil that the ion transference number of the membrane and local ion transport within the catalyst layers serve a pivotal role in eliminating counterion crossover and maximising accessibility of the catalyst surface to the reverted CO\u003csub\u003e2\u003c/sub\u003e. The insights provided by our work can guide the rational design of BPM-based electrochemical systems.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eTo understand the local ion transport in a BPMEA, a 1D isothermal continuum model was developed based on previous work by Weng et al.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e and Lees et al.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e The model domain includes an ionomer-immersed porous cathode catalyst layer (CL) and CEL of the BPM. The catalyst layer consists of nickel-nitrogen-carbon catalyst (NiNC-IMI) mixed with either cation- (Nafion, a sulfonated fluoropolymer\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e) or anion-exchange ionomer (Sustainion, an imidazolium functionalized styrene polymer\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e). We chose a steady-state model to capture the ionic behaviour at the initial stage of BPMEA operation. The model was fit to the experimental CO Faradaic efficiency data collected from NiNC-IMI catalyst layers with Sustainion ionomers by adjusting the electrochemical parameters such as the transfer coefficients and exchange current densities for CO\u003csub\u003e2\u003c/sub\u003e reduction and the competitive HER (see Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea). To validate the model, the same kinetic parameters were then used to predict the CO Faradaic efficiency data collected with the Nafion ionomer. Details of the models (e.g., equations, boundary conditions, and parameters) are described in the Supplementary Information.\u003c/p\u003e\n\u003cp\u003eSince previous work has shown the role of anolyte ion concentration and crossover on cathodic performance and stability in a BPMEA CO\u003csub\u003e2\u003c/sub\u003e electrolyzer,\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e we used the model to more deeply investigate two strategies to control the ionic transport within the cathode. Specifically, (i) the use of ionomer in the CL for selective ionic transport and (ii) promoting an increased proton transference number in the CEL. Including ionomer in the catalyst layer is a common effective approach to modulate local ionic transport. Further, maintaining a high proton flux from the CEL to the cathode is also a prerequisite for a stable and efficient BPMEA system. Near-unity water dissociation efficiencies are desired as they limit ionic interactions between the anode and cathode environments that can impact anolyte pH and salt precipitation at the cathode. Therefore, these two strategies are perceived as practical approaches to managing local ion transport and thus reaction microenvironment in the electrolysis cell.\u003c/p\u003e\n\u003cp\u003eWe first examined the role of ionomer choice for the CL in determining local ionic concentrations in the catalyst layer. As presented in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb and \u003cstrong\u003ec\u003c/strong\u003e, incorporating anion-selective Sustainion ionomer in the CLs (Sus-CLs) leads to a counterintuitively lower pH and CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e concentration than the Nafion ionomer in the CL (Naf-CLs). Notably, the Sus-CLs case provided a substantially higher (7 folds) HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e concentration near the CEL|CL interface, and over the entire catalyst layer an average HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e concentration that is more than twice that of the Naf-CLs case. This discernible divergence is a result of the different fixed charges of the two ionomers. Anion exchange ionomers promote the transport of generated (bi)carbonates near the gas-liquid interface towards the BPM, while the Nafion rejects this transport and promotes (bi)carbonate accumulation near the generation point. The positive fixed charge of the Sus-CLs case then provides ample HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e available for acidification and CO\u003csub\u003e2\u003c/sub\u003e regeneration near the CEL|CL interface.\u003c/p\u003e\n\u003cp\u003eIn addition to the anion transport, it is important to assess the cation transport (H\u003csup\u003e+\u003c/sup\u003e and K\u003csup\u003e+\u003c/sup\u003e). As further suggested from the calculated K\u003csup\u003e+\u003c/sup\u003e profiles shown in Fig. S3a, the positively charged quaternary ammonium groups in the ionomer at least partially exclude K\u003csup\u003e+\u003c/sup\u003e transport from CEL to CL in the Sus-CLs and thus likely contribute to the observed reduced pH in the CLs by lowering the required amount of OH\u003csup\u003e\u0026minus;\u003c/sup\u003e ions needed to balance the positive charge. By contrast, the concentration profiles predicted for Naf-CL indicate a more uneven ionic distribution as compared to Sus-CL. The negatively charged sulfonic groups in Nafion lead to an excessive amount of K\u003csup\u003e+\u003c/sup\u003e in the Naf-CL (see Fig. S3a), which then fosters a high content of OH\u003csup\u003e\u0026minus;\u003c/sup\u003e (or high pH shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb) and CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e to maintain the charge neutrality.\u003c/p\u003e\n\u003cp\u003eOverall, both ionomer cases show the ability for CO\u003csub\u003e2\u003c/sub\u003e converted to (bi)carbonates to be regenerated into CO\u003csub\u003e2\u003c/sub\u003e by the proton flux from the CEL of the BPM. Interestingly, the concentration of HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e at Naf-CL shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec decreases from around 1.0 M at CL|GDL interface down to 0.047 M at the CEL|CL interface. Such a steep decrease in HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e concentration is a result of increased local pH inside Naf-CL. Due to the high local pH inside Naf-CL (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb), however, the regenerated CO\u003csub\u003e2\u003c/sub\u003e tends to diminish and form back to CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e within the Naf-CL (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec). As such, the uneven ionic distribution in Naf-CL might not be ideal for an efficient electrochemical conversion of CO\u003csub\u003e2\u003c/sub\u003e because it could cause a low utilisation efficiency of the reverted CO\u003csub\u003e2\u003c/sub\u003e for electrochemical conversion. Similarly, the local CO\u003csub\u003e2\u003c/sub\u003e concentration near the Sus-CL|CEL interface is slightly higher than Naf-CL|CEL interface (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed) due to a higher concentration of HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e which promotes CO\u003csub\u003e2\u003c/sub\u003e regeneration. However, the CO\u003csub\u003e2\u003c/sub\u003e concentration throughout the bulk of the CL is similar for the Sus-CL and Naf-CL because of the constant excess CO\u003csub\u003e2\u003c/sub\u003e supply provided at the CL|GDL interface.\u003c/p\u003e\n\u003cp\u003eNext, we use the Sus-CL model to investigate the impact of proton transference number on the local ionic transport across the CLs. The ionic conduction across the CEL|CL interface relies on the transport of H\u003csup\u003e+\u003c/sup\u003e, K\u003csup\u003e+\u003c/sup\u003e, and (bi)carbonate ions, with H\u003csup\u003e+\u003c/sup\u003e being the primary charge carrier. The H\u003csup\u003e+\u003c/sup\u003e transference number quantifies the fraction of the current crossing the CEL in the absence of concentration gradients, which is a result of proton transport from the BPM. The total ionic current is the sum of the H\u003csup\u003e+\u003c/sup\u003e transport, K\u003csup\u003e+\u003c/sup\u003e crossover from the anode and anion crossover from the cathode. By sweeping the H\u003csup\u003e+\u003c/sup\u003e transference number from 0.75 to 0.95, as presented in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ee-g and Fig. S3b, we observed a decrease in pH, K\u003csup\u003e+\u003c/sup\u003e and CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e concentrations but an increase in HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e concentrations in the CLs. This trend is as expected because an increase in H\u003csup\u003e+\u003c/sup\u003e transference number indicates a suppressed current associated with K\u003csup\u003e+\u003c/sup\u003e cations and promotes carbonate acidification within the CLs. Additionally, as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eg, increasing the H\u003csup\u003e+\u003c/sup\u003e transference number does not notably increase the local CO\u003csub\u003e2\u003c/sub\u003e concentration but reduces the CO\u003csub\u003e2\u003c/sub\u003e concentration within the CEL due to the minimised (bi)carbonate crossover from CL to CEL. The downside of an increased H\u003csup\u003e+\u003c/sup\u003e flux across CEL|CL interface could be HER out competing CO\u003csub\u003e2\u003c/sub\u003e reduction through direct H\u003csup\u003e+\u003c/sup\u003e reduction and limited cations available to activate CO\u003csub\u003e2\u003c/sub\u003e electrochemical reduction.\u003c/p\u003e\n\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n\u003ch2\u003eThe role of ionomer in CL on BPMEA performance\u003c/h2\u003e\n\u003cp\u003eTo validate the predictions of the model, we compared experimentally NiNC-IMI CLs prepared with Sustainion and Nafion ionomers. A comprehensive study of the NiNC-IMI catalyst has been published elsewhere\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e53\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e, so this study will focus on the role of the ionic transport on the BPMEA performance by using the NiNC-IMI as a model CL. The products from the BPMEA cells are primarily CO and H\u003csub\u003e2\u003c/sub\u003e with minor formate or formic acid (see nuclear magnetic resonance results in Fig. S4). Because the BPM is effective in minimizing formate crossover to the anolyte\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e, it is challenging to calculate the formate FEs accurately by analyzing anolyte compositions. Hence, this work focuses mainly on the CO and H\u003csub\u003e2\u003c/sub\u003e products from the electrolysis.\u003c/p\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb-\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed show that the ionomer in the CL defines cathode activity and selectivity in the BPMEA configuration. 15 wt% Sustainion ionomer in the CL can profoundly improve the catalyst CO FE and partial current densities compared to the 15 wt% Nafion. Due to the different equivalent weight of the ionomers (1100 g/mol for Nafion; ~ 225 g/mol for Sustainion\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e), 15 wt% Sustainion contains approximately five times the concentration of ionic groups as compared to the 15 wt% Nafion. As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb, the CO FE is 69.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5% at 50 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e and 45.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5% at 200 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e for CLs with 15% Sustainion ionomer, much higher than the CLs based with Nafion (53.5\u0026thinsp;\u0026plusmn;\u0026thinsp;4.2% at 50 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e and 21.4\u0026thinsp;\u0026plusmn;\u0026thinsp;1.0% at 200 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e). The Naf-CLs reach a CO limiting current density of ~\u0026thinsp;41 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, more than two-fold lower than the Sus-CLs (\u0026gt;\u0026thinsp;105 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e).\u003c/p\u003e\n\u003cp\u003eThe notable enhancement in CO\u003csub\u003e2\u003c/sub\u003e-to-CO upon FEs and partial current densities is related to the local mass transport within the CLs that either improves the local concentration of CO\u003csub\u003e2\u003c/sub\u003e or limits the local concentration of protons. Fig. S5 demonstrates no significant suppression of HER by the Sustainion ionomer, implying a similar local proton availability and/or catalytic HER activity in both cases. Hence, we postulate that the observed improvement likely originates from the enhanced bicarbonate local transport within Sustainion-based CLs, as predicted from our model in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec. The increase in local bicarbonate concentration in the CL provides an even CO\u003csub\u003e2\u003c/sub\u003e distribution in the CL and thus promote CO\u003csub\u003e2\u003c/sub\u003e electroreduction.\u003c/p\u003e\n\u003cp\u003eIn the BPMEA reported here, there are two sources of CO\u003csub\u003e2\u003c/sub\u003e: CO\u003csub\u003e2\u003c/sub\u003e that dissolves from the gas feed and CO\u003csub\u003e2\u003c/sub\u003e that is regenerated from bicarbonate via acid-base chemistry. The modelled CO\u003csub\u003e2\u003c/sub\u003e concentration profiles (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec and \u003cstrong\u003ef\u003c/strong\u003e) show that the potent proton flux from CEL yields a peak in CO\u003csub\u003e2\u003c/sub\u003e concentration at the CEL|CL interface. The increased CO\u003csub\u003e2\u003c/sub\u003e concentration results in an increase in the predicted CO partial current density near the CEL|CL interface as shown in Fig. S6. This result is consistent with our previous bicarbonate electrolyser model that shows high CO formation rates at the CEL|CL interface.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e In this BPMEA model, a high rate of CO formation is also observed at the CL|GDL interface. This difference is attributed to the additional CO\u003csub\u003e2\u003c/sub\u003e source from the gas phase, which is absent in the bicarbonate electrolyser. These modelling results confirm that both regenerated and dissolved CO\u003csub\u003e2\u003c/sub\u003e contribute to CO production in a BPMEA.\u003c/p\u003e\n\u003cp\u003eSince bicarbonate anions exhibit more facile transport in the anion-exchange Sustainion ionomer than the cation-exchange Nafion ionomer\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e (see Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb), we predict that the Sustainion ionomer should allow a facile transport of bicarbonate anions for CO\u003csub\u003e2\u003c/sub\u003e regeneration, providing ample local CO\u003csub\u003e2\u003c/sub\u003e for electrochemical conversion across the CL structure (see Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea). By contrast, in the Naf-CL, the regenerated CO\u003csub\u003e2\u003c/sub\u003e tends to be spatially localized, causing a limited proportion of catalyst surface in the CLs to be accessible by the regenerated CO\u003csub\u003e2\u003c/sub\u003e. Therefore, the even distribution of bicarbonate enables the Sustainion-based CL with a larger catalyst surface area accessible by the regenerated CO\u003csub\u003e2\u003c/sub\u003e than a Nafion-based CL\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e. Thus, while both systems enable reasonable CO\u003csub\u003e2\u003c/sub\u003e utilization efficiencies, Sus-CL case exhibits both higher CO\u003csub\u003e2\u003c/sub\u003e utilization efficiencies, CO FE, and partial current densities at comparable cell voltages (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed).\u003c/p\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ee shows that the CO\u003csub\u003e2\u003c/sub\u003e-to-CO utilisation efficiency increases with current density. Here, the CO\u003csub\u003e2\u003c/sub\u003e-to-CO utilisation efficiency is defined as the ratio of the CO\u003csub\u003e2\u003c/sub\u003e reduced to CO versus the total CO\u003csub\u003e2\u003c/sub\u003e consumed in the cell (i.e., due to crossover and reaction). The observed CO\u003csub\u003e2\u003c/sub\u003e-to-CO utilisation trend as a function of current density could result from an increase in the H\u003csup\u003e+\u003c/sup\u003e transference number across the CEL of the BPM, because water dissociation dictates the overall ionic current while the co- and counter-ion crossover is mass-transport limited at increased current densities.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e57\u003c/span\u003e\u003c/sup\u003e When one mole of CO is produced, for example, there will be two moles of electrons consumed, two moles of H\u003csup\u003e+\u003c/sup\u003e/K\u003csup\u003e+\u003c/sup\u003e transported from the CEL, and two moles of OH\u003csup\u003e\u0026minus;\u003c/sup\u003e co-produced. The OH\u003csup\u003e\u0026minus;\u003c/sup\u003e can then be converted into one-mole carbonate or two moles of bicarbonate, which require two moles of protons to remove the (bi)carbonate species. Therefore, the H\u003csup\u003e+\u003c/sup\u003e transference number determines the availability of the H\u003csup\u003e+\u003c/sup\u003e to revert the generated (bi)carbonate species back to CO\u003csub\u003e2\u003c/sub\u003e. As the H\u003csup\u003e+\u003c/sup\u003e transference number is always below 1, it means that some CO\u003csub\u003e2\u003c/sub\u003e converted to (bi)carbonates are never recovered and will either precipitate or crossover to the anode. Consequently, high H\u003csup\u003e+\u003c/sup\u003e transference numbers across the BPM are beneficial to lowering the local pH and regenerating CO\u003csub\u003e2\u003c/sub\u003e from (bi)carbonate, hence, maximizing CO\u003csub\u003e2\u003c/sub\u003e-to-CO utilisation efficiency.\u003c/p\u003e\n\u003cp\u003eA further increase of Sustainion ionomer loading to 30 wt% in the CL lowered the FEs and limiting current density for CO production. (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb-\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed) Such decline in performance is likely attributable to the blockage of the CL pores and reactive sites by the ionomer itself, resulting in an increased diffusion length for CO\u003csub\u003e2\u003c/sub\u003e gas from gas channels to the catalyst surface or loss of active surface in the CLs, respectively. However, the CO\u003csub\u003e2\u003c/sub\u003e-to-CO utilisation efficiency is not significantly impacted by the increment of the ionomer loading, implying that in-situ formed (bi)carbonate ions can be reverted to CO\u003csub\u003e2\u003c/sub\u003e easily within the CL matrix.\u003c/p\u003e\n\u003cp\u003eMore importantly, as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ef, the anolyte shows only a slight decrease in pH after the cell testing, with a total charge of 6300 C passed through the cell during the test. The pH of the anolyte could have been decreased because of both carbon crossover from the cathode to the anode and cation crossover from the anode to the cathode. The minimal change in the anolyte pH is due to the unique BPM function, which supplies protons at the cathode to convert (bi)carbonate anions to CO\u003csub\u003e2\u003c/sub\u003e for CO production and minimises ion crossover rates across the cell. This feature allows BPMEA cells to be operated stably using a PGM-free anode based on nickel and with a high CO\u003csub\u003e2\u003c/sub\u003e-to-product utilisation efficiency, which could not be easily done using monopolar membrane-based electrolysis cells.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n\u003ch2\u003eThe role of the H\u003csup\u003e+\u003c/sup\u003e transference number on BPMEA performance\u003c/h2\u003e\n\u003cp\u003eThe role of proton transference number is investigated experimentally by examining the effect of K\u003csup\u003e+\u003c/sup\u003e cation crossover on CO\u003csub\u003e2\u003c/sub\u003e reduction using 0.1 M and 1 M KOH as the anolyte. A concentrated KOH is expected to accelerate the crossover rate of K\u003csup\u003e+\u003c/sup\u003e due to the large concentration gradient across the membrane (see Fig. S7a). Because cations are essential in activating CO\u003csub\u003e2\u003c/sub\u003e reduction, the results in Fig. S7b show that 1 M KOH anolyte can slightly improve CO FE at current densities\u0026thinsp;\u0026gt;\u0026thinsp;150 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. This finding is consistent with our previous report highlighting the cations' vital role in enhancing CO FEs over silver electrodes.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e Fig. S7c suggests that the BPMEA cell with a dilute anolyte shows a higher CO\u003csub\u003e2\u003c/sub\u003e-to-CO utilisation efficiency than the concentrated anolyte. This trend matches our model predictions of the proton transference number in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed-f, because a reduced K\u003csup\u003e+\u003c/sup\u003e crossover rate achieved by using a dilute anolyte leads to an increase in proton transference number and acidification of (bi)carbonate within the CL. Therefore, a trade-off exists between product selectivity and CO\u003csub\u003e2\u003c/sub\u003e utilisation efficiency when choosing the anolyte for the BPMEA cell.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n\u003ch2\u003eLocal H\u003csup\u003e+\u003c/sup\u003e enrichment at CEL/CL interface\u003c/h2\u003e\n\u003cp\u003eIn addition to the distribution of (bi)carbonate ions within the CL, we studied the local reaction environment at the CEL|CL interface by introducing a hydrophilic porous spacer (65 um thick) between the CL and CEL. (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea) This configuration is fundamentally different from the previously reported method\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e that includes concentrated salts within the spacer. In this study, we applied the spacer pre-soaked with ultrapure water before the cell assembly. The ion conduction within the spacer solely depends on the ionic fluxes of protons, (bi)carbonate anions, and K\u003csup\u003e+\u003c/sup\u003e cations that come from the fixed charge in the CEL and anolyte.\u003c/p\u003e\n\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb, the spacer significantly suppresses the HER down to below 15% and boosts the CO FEs up to 91% at 50 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e and 88% at 100 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, which is almost comparable to MEA cells based on monopolar membranes. The comparison of the CO partial current densities vs. cell potentials, as shown in Fig. S8, suggests cells with and without spacer achieve similar CO partial current densities under similar cell voltages. Therefore, the observed CO FE enhancement is mainly attributable to the suppression of HER in the CLs. This finding also indicates that the majority of HER in the absence of spacer occurs at the CL|CEL interfaces due to the direct contact of the CL with excess protons; the spacer increases the retention time for protons to reach the catalyst surface.\u003c/p\u003e\n\u003cp\u003eThe drawback to including the spacer in the BPMEA cell is the large ohmic loss due to the slow ionic conduction across the spacer, as verified by the electrochemical impedance spectroscopic analyses in Fig. S9. The absence of abundant water at the cathode side of the BPMEA accelerates the dehydration of the spacer, which causes further reduction of the ionic conductivity of the spacer and eventually rapid cell voltage overshoot (See Fig. S10). Nonetheless, the results highlight the importance of the cell configuration in determining the CO\u003csub\u003e2\u003c/sub\u003e-to-CO selectivity of the BPMEA cells.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n\u003ch2\u003eLong-term stability and ion crossover\u003c/h2\u003e\n\u003cp\u003eFinally, we evaluated the long-term stability of the PGM-free BPMEA cell, particularly as it relates to the anolyte pH and the stable use of nickel anodes. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea demonstrates that the CO Faradaic efficiency rapidly dropped from 73\u0026ndash;64% within the first 1.5 h, then decreased linearly to 30% after 150 h at a degradation rate of 0.36% per hour. Meanwhile, the HER increased along with the loss in CO FE. Such discernible selectivity loss likely originates from an increase in contact area between catalyst and H\u003csup\u003e+\u003c/sup\u003e and possible deactivation of the catalyst in the acidic environment. Both could contribute to the rise in the rate of HER and suppression of the rate of CO\u003csub\u003e2\u003c/sub\u003e reduction.\u003c/p\u003e\n\u003cp\u003eThe cell voltage also increased at a degradation rate of 0.086% per hour. By comparing the ohmic resistances obtained from electrochemical impedance (Fig. S11), we found that the degradation of the cell potential should be related to the increase of the polarization resistance over either the cathode or anode rather than the deterioration of the BPM.\u003c/p\u003e\n\u003cp\u003eOn the other hand, as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eb, the CO\u003csub\u003e2\u003c/sub\u003e-to-CO utilisation efficiency increased rapidly in the first 1.5 h and was subsequently sustained at above 80\u0026ndash;90% across the measurement. The rapid rise in the CO\u003csub\u003e2\u003c/sub\u003e utilisation within the first 1.5 h implies that the rapid drop in CO FE is mainly a result of an increase of proton local availability for HER in the CL. The subsequent linear degradation is likely associated with the instability of the catalyst layer in a strongly acidic environment.\u003c/p\u003e\n\u003cp\u003eWithout replenishing the anolyte across the entire test, we observed no significant change in the pH value of the anolyte after the test, as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ec. The titration results shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ed and Fig. S12 revealed that about 0.006 mol K\u003csup\u003e+\u003c/sup\u003e cations have migrated to the cathode side, which is equivalent to 0.21% of the ionic current during the conditioning. Due to the anolyte's high alkalinity, most of the CO\u003csub\u003e2\u003c/sub\u003e crossover should be converted to carbonate ions in the anolyte. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ed shows that the anolyte after the test is composed of 0.0735 M OH\u003csup\u003e\u0026minus;\u003c/sup\u003e and 0.011 M CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, implying that the (bi)carbonate crossover contributed to \u0026lt;\u0026thinsp;0.774% of the ionic current. The extremely low K\u003csup\u003e+\u003c/sup\u003e and CO\u003csub\u003e2\u003c/sub\u003e crossover rates were essential in sustaining the Ni-based anode stability without replenishing the anolyte during the 150-h test. We posit that a proton transference number of ~\u0026thinsp;99% was maintained over the course of the experiment. While the 0.21% K\u003csup\u003e+\u003c/sup\u003e crossover is likely needed to maintain CO\u003csub\u003e2\u003c/sub\u003e reduction at the cathode, the 0.774% carbonate crossover is all that needs to be avoided to maintain anolyte pH indefinitely.\u003c/p\u003e\n\u003cp\u003eAfter 150 h conditioning within the BPMEA cell, as revealed by the XRD results in Fig. S13, the Ni anode remained as metallic Ni, while additional minor phases related to potassium nickelates\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e58\u003c/span\u003e\u003c/sup\u003e formed due to the long-term oxidising treatment. Importantly, no nickel carbonate phase was detected in the conditioned anode, implying that the BPMEA configuration stabilised the Ni-based anode. Ex-situ surface analyses further confirm that both fresh and conditioned anodes are predominantly covered with Ni(OH)\u003csub\u003e2\u003c/sub\u003e species at the surfaces (Fig. S14), and that their structures shown in Fig. S15 and S16 remain intact after 150 h conditioning. The retained structural and chemical stability of the Ni anode is a main result of the alkaline local environment achieved by the BPM, which supplies hydroxide ions and suppresses K\u003csup\u003e+\u003c/sup\u003e and CO\u003csub\u003e2\u003c/sub\u003e crossover.\u003c/p\u003e\n\u003cp\u003eWe also noticed that the inlet pressure of the cell rose after an interval of 12\u0026ndash;30 h. The rise of the inlet pressure is mainly a result of the build-up of precipitated (bi)carbonate salts at the entrance of the gas channel in the cell, though there was no discernible salt precipitation elsewhere in the gas channels or the back of the cathode (see Fig. S17). The salt precipitation occurs typically when the concentration of the local (bi)carbonate exceeds the solubility (e.g., 8.03 M for K\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e and 3.62 M for KHCO\u003csub\u003e3\u003c/sub\u003e)\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e at the cathode structure and the crystal can grow continuously to reach the gas channels, similar to efflorescence process, due to its hygroscopic and porous nature. The inlet pressure rise (due to the salt precipitation) elucidates that there is a continuous supply of K\u003csup\u003e+\u003c/sup\u003e from the cation crossover of about 0.21% to the cathode for the salt precipitation, which cannot be easily mitigated, especially at a high current density.\u003c/p\u003e\n\u003cp\u003eTo circumvent this issue, we applied\u0026thinsp;~\u0026thinsp;1 mL pure water pulse to wash off the precipitated salt from the gas channel during operation when an increase of inlet pressure was observed. Unlike the reported hourly water flush to remove the salts at the cathode for the anion-exchange membrane electrode assembly cell\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e, the BPMEA cell requires a much less frequent water pulse, thanks to its high CO\u003csub\u003e2\u003c/sub\u003e utilisation and controlled K\u003csup\u003e+\u003c/sup\u003e concentrations at the cathode. The water pulse operation also showed negligible impact on the cell potential and CO Faradaic efficiency, as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"Discussions","content":"\u003cp\u003eIn summary, this combined modelling and experimental work highlights and elucidates the critical role of the local ionic transport and reaction environment in determining the CO\u003csub\u003e2\u003c/sub\u003e-to-CO activity, selectivity, and utilisation efficiency in BPM electrode assemblies constructed free of platinum group metals. The combined theoretical and experimental results reveal that an even distribution of bicarbonate across cathode catalyst layers enhances the accessibility of the reverted CO\u003csub\u003e2\u003c/sub\u003e to the catalyst layer and thus boost CO Faradaic efficiency and limiting current densities more than two-fold. We also found that the proton transference number determines the CO\u003csub\u003e2\u003c/sub\u003e utilisation efficiency: a high proton transference number generally achieves a high CO\u003csub\u003e2\u003c/sub\u003e-to-CO efficiency but may not necessarily lead to a high CO\u003csub\u003e2\u003c/sub\u003e reduction selectivity. Although hydrogen evolution still outcompetes CO\u003csub\u003e2\u003c/sub\u003e reduction at \u0026gt;\u0026thinsp;150 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, our results suggest that hydrogen evolution occurs primarily at the membrane/catalyst interfaces and can be suppressed effectively by the inclusion of a spacer at the interface. Lastly, we demonstrate notably low crossover rates for CO\u003csub\u003e2\u003c/sub\u003e and K\u003csup\u003e+\u003c/sup\u003e (\u0026lt;\u0026thinsp;1% of total charge) over a PGM-free BPMEA cell operating at 100 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e for 150 h. This cell achieves a 80\u0026ndash;90% CO\u003csub\u003e2\u003c/sub\u003e-to-CO utilisation efficiency and stable alkalinity in the anolyte. The findings of this work provide new insights into advancing and assessing BPM electrochemical systems via catalyst material design and modulation of local transport and reaction microenvironment.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgment\u003c/h2\u003e \u003cp\u003eM.L. acknowledge the financial support from Australian Research Council (DE230100637). W.J., T.B., and P.S. acknowledge the financial support from EU project 851441 \u0026ndash; SELECTCO2. W.J. and P.S. acknowledge the support from EU project 101006701 \u0026ndash; Ecofuel. J.C.B. was supported in part by a fellowship award under contract FA9550-21-F-0003 through the National Defense Science and Engineering Graduate (NDSEG) fellowship program, sponsored by the Army Research Office (ARO) H-P.I.v.M. and S.S acknowledge the financing provided for this project in the context of the e-Refinery Institute by Shell Global Solutions International B.V. and the Top Consortia for Knowledge and Innovation (TKI\u0026rsquo;s) of the Dutch Ministry of Economic Affairs. A.Z.W., A.T.B., and J.C.B. all acknowledge support under the Liquid Sunlight Alliance, which is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Fuels from Sunlight Hub under award number DE-SC0021266. E.W.L and A.Z.W. also acknowledge support for the modeling under the CO\u003csub\u003e2\u003c/sub\u003e Consortium funded by BETO in EERE under contract DE-AC02-05CH11231.\u003c/p\u003e\n \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e "},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eSamu, A. A. \u003cem\u003eet al.\u003c/em\u003e Intermittent Operation of CO2 Electrolyzers at Industrially Relevant Current Densities. \u003cem\u003eACS Energy Lett.\u003c/em\u003e \u003cstrong\u003e7\u003c/strong\u003e, 1859\u0026ndash;1861 (2022).\u003c/li\u003e\n\u003cli\u003eLi, W. \u003cem\u003eet al.\u003c/em\u003e Bifunctional ionomers for efficient co-electrolysis of CO2 and pure water towards ethylene production at industrial-scale current densities. \u003cem\u003eNat. 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T. \u0026amp; Weber, A. Z. Understanding Multi-Ion Transport Mechanisms in Bipolar Membranes. \u003cem\u003eACS Appl. Mater. Interfaces\u003c/em\u003e \u003cstrong\u003e12\u003c/strong\u003e, 52509\u0026ndash;52526 (2020).\u003c/li\u003e\n\u003cli\u003eOliva, P. \u003cem\u003eet al.\u003c/em\u003e Review of the structure and the electrochemistry of nickel hydroxides and oxy-hydroxides. \u003cem\u003eJ. Power Sources\u003c/em\u003e \u003cstrong\u003e8\u003c/strong\u003e, 229\u0026ndash;255 (1982).\u003c/li\u003e\n\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-3954760/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3954760/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eBipolar membranes in electrochemical CO\u003csub\u003e2\u003c/sub\u003e conversion cells enable different reaction environments in the CO\u003csub\u003e2\u003c/sub\u003e-reduction and oxygen-evolution compartments. Under ideal conditions, water-splitting in the bipolar membrane allows for platinum-group-metal-free anode materials and high CO\u003csub\u003e2\u003c/sub\u003e utilizations. In practice, however, even minor unwanted ion crossover limits stability to short time periods. Here we report the vital role of managing ionic species to improve CO\u003csub\u003e2\u003c/sub\u003e conversion efficiency while preventing acidification of the anodic compartment. Through transport modelling, we identify that an anion-exchange ionomer in the catalyst layer improves local bicarbonate availability and increasing the proton transference number in the bipolar membranes increases CO\u003csub\u003e2\u003c/sub\u003e regeneration and limits K\u003csup\u003e+\u003c/sup\u003e concentration in the cathode region. Through experiments, we show that a uniform local distribution of bicarbonate ions increases the accessibility of reverted CO\u003csub\u003e2\u003c/sub\u003e to the catalyst surface, improving Faradaic efficiency and limiting current densities by twofold. Using these insights, we demonstrate a fully PGM-free bipolar membrane electrode assembly CO\u003csub\u003e2\u003c/sub\u003e conversion system exhibiting\u0026thinsp;\u0026lt;\u0026thinsp;1% CO\u003csub\u003e2\u003c/sub\u003e/cation crossover rates and 80\u0026ndash;90% CO\u003csub\u003e2\u003c/sub\u003e-to-CO utilization efficiency over 150 h operation at 100 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e without anolyte replenishment.\u003c/p\u003e","manuscriptTitle":"Local ionic transport enables selective PGM-free bipolar membrane electrode assembly","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-02-16 08:06:41","doi":"10.21203/rs.3.rs-3954760/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":"3e45cfb3-b814-4ed2-be69-163366fa34cd","owner":[],"postedDate":"February 16th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":28779873,"name":"Physical sciences/Energy science and technology/Carbon capture and storage"},{"id":28779874,"name":"Physical sciences/Engineering/Chemical engineering"}],"tags":[],"updatedAt":"2024-09-20T07:06:55+00:00","versionOfRecord":{"articleIdentity":"rs-3954760","link":"https://doi.org/10.1038/s41467-024-52409-z","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2024-09-19 04:00:00","publishedOnDateReadable":"September 19th, 2024"},"versionCreatedAt":"2024-02-16 08:06:41","video":"","vorDoi":"10.1038/s41467-024-52409-z","vorDoiUrl":"https://doi.org/10.1038/s41467-024-52409-z","workflowStages":[]},"version":"v1","identity":"rs-3954760","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3954760","identity":"rs-3954760","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}