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

doi:10.21203/rs.3.rs-3954760/v1

Download PDF

Article

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

https://doi.org/10.21203/rs.3.rs-3954760/v1

This work is licensed under a CC BY 4.0 License

Journal Publication

published 19 Sep, 2024

Read the published version in Nature Communications →

Version 1

posted

You are reading this latest preprint version

Bipolar membranes in electrochemical CO₂ conversion cells enable different reaction environments in the CO₂-reduction and oxygen-evolution compartments. Under ideal conditions, water-splitting in the bipolar membrane allows for platinum-group-metal-free anode materials and high CO₂ 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 CO₂ 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 CO₂ 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 CO₂ 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 CO₂ conversion system exhibiting < 1% CO₂/cation crossover rates and 80–90% CO₂-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

Carbon dioxide (CO₂) electrolysis is a promising technology for converting CO₂ 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 CO₂ 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 CO₂ acidification and carbon crossover^17–19. Specifically, under high-rate CO₂ electrolysis, monopolar IEM-based systems result in a substantial loss of feed CO₂ (e.g., > 50% for CO production) due to (bi)carbonate (HCO₃⁻/CO₃²⁻) 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 CO₂ electrolysis up to practical gigawatt levels is prohibitive from both economic and scarcity perspectives unless the PGMs can be limited to < 0.1 mg cm^− 2 or > 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 processes²⁵, so the required alkaline electrolyte replenishment rate would be equivalent to the CO₂ 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 CO₂ 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 operation³⁰, 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 CO₂.^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 CO₂ reduction.^35,36 Modulation of the catalyst reaction environment in acidic media may facilitate higher CO₂ selectivity¹², 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. 1a, 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. 1b) 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. 1c) 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 anode⁴⁵, see Fig. 1d) and utilize protons to revert (bi)carbonate to CO₂, thus preventing salt precipitation (Fig. 1b) and increasing CO₂ 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, CO₂ 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 CO₂ 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 CO₂ 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 CO₂ 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 CO₂ 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 CO₂. The insights provided by our work can guide the rational design of BPM-based electrochemical systems.

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 fluoropolymer⁵²) or anion-exchange ionomer (Sustainion, an imidazolium functionalized styrene polymer³¹). 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 CO₂ reduction and the competitive HER (see Fig. 2a). 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 CO₂ 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. 2b and c, incorporating anion-selective Sustainion ionomer in the CLs (Sus-CLs) leads to a counterintuitively lower pH and CO₃²⁻ concentration than the Nafion ionomer in the CL (Naf-CLs). Notably, the Sus-CLs case provided a substantially higher (7 folds) HCO₃⁻ concentration near the CEL|CL interface, and over the entire catalyst layer an average HCO₃⁻ 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 HCO₃⁻ available for acidification and CO₂ 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. 2b) and CO₃²⁻ to maintain the charge neutrality.

Overall, both ionomer cases show the ability for CO₂ converted to (bi)carbonates to be regenerated into CO₂ by the proton flux from the CEL of the BPM. Interestingly, the concentration of HCO₃⁻ at Naf-CL shown in Fig. 2c 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 HCO₃⁻ concentration is a result of increased local pH inside Naf-CL. Due to the high local pH inside Naf-CL (Fig. 2b), however, the regenerated CO₂ tends to diminish and form back to CO₃²⁻ within the Naf-CL (Fig. 2c). As such, the uneven ionic distribution in Naf-CL might not be ideal for an efficient electrochemical conversion of CO₂ because it could cause a low utilisation efficiency of the reverted CO₂ for electrochemical conversion. Similarly, the local CO₂ concentration near the Sus-CL|CEL interface is slightly higher than Naf-CL|CEL interface (Fig. 2d) due to a higher concentration of HCO₃⁻ which promotes CO₂ regeneration. However, the CO₂ concentration throughout the bulk of the CL is similar for the Sus-CL and Naf-CL because of the constant excess CO₂ 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. 2e-g and Fig. S3b, we observed a decrease in pH, K⁺ and CO₃²⁻ concentrations but an increase in HCO₃⁻ 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. 2g, increasing the H⁺ transference number does not notably increase the local CO₂ concentration but reduces the CO₂ 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 CO₂ reduction through direct H⁺ reduction and limited cations available to activate CO₂ 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 H₂ 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 anolyte⁵⁵, it is challenging to calculate the formate FEs accurately by analyzing anolyte compositions. Hence, this work focuses mainly on the CO and H₂ products from the electrolysis.

Figure 3b-3d 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 Sustainion³¹), 15 wt% Sustainion contains approximately five times the concentration of ionic groups as compared to the 15 wt% Nafion. As shown in Fig. 3b, 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 CO₂-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 CO₂ 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. 2c. The increase in local bicarbonate concentration in the CL provides an even CO₂ distribution in the CL and thus promote CO₂ electroreduction.

In the BPMEA reported here, there are two sources of CO₂: CO₂ that dissolves from the gas feed and CO₂ that is regenerated from bicarbonate via acid-base chemistry. The modelled CO₂ concentration profiles (Fig. 2c and f) show that the potent proton flux from CEL yields a peak in CO₂ concentration at the CEL|CL interface. The increased CO₂ 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 CO₂ source from the gas phase, which is absent in the bicarbonate electrolyser. These modelling results confirm that both regenerated and dissolved CO₂ 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 ionomer¹⁸ (see Fig. 2b), we predict that the Sustainion ionomer should allow a facile transport of bicarbonate anions for CO₂ regeneration, providing ample local CO₂ for electrochemical conversion across the CL structure (see Fig. 3a). By contrast, in the Naf-CL, the regenerated CO₂ tends to be spatially localized, causing a limited proportion of catalyst surface in the CLs to be accessible by the regenerated CO₂. Therefore, the even distribution of bicarbonate enables the Sustainion-based CL with a larger catalyst surface area accessible by the regenerated CO₂ than a Nafion-based CL⁵⁶. Thus, while both systems enable reasonable CO₂ utilization efficiencies, Sus-CL case exhibits both higher CO₂ utilization efficiencies, CO FE, and partial current densities at comparable cell voltages (Fig. 3d).

Figure 3e shows that the CO₂-to-CO utilisation efficiency increases with current density. Here, the CO₂-to-CO utilisation efficiency is defined as the ratio of the CO₂ reduced to CO versus the total CO₂ consumed in the cell (i.e., due to crossover and reaction). The observed CO₂-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 CO₂. As the H⁺ transference number is always below 1, it means that some CO₂ 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 CO₂ from (bi)carbonate, hence, maximizing CO₂-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. 3b-3d) 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 CO₂ gas from gas channels to the catalyst surface or loss of active surface in the CLs, respectively. However, the CO₂-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 CO₂ easily within the CL matrix.

More importantly, as shown in Fig. 3f, 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 CO₂ 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 CO₂-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 CO₂ 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 CO₂ 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 CO₂-to-CO utilisation efficiency than the concentrated anolyte. This trend matches our model predictions of the proton transference number in Fig. 2d-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 CO₂ 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. 4a) This configuration is fundamentally different from the previously reported method⁴¹ 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. 4b, 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 CO₂-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 5a 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 CO₂ 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. 5b, the CO₂-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 CO₂ 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. 5c. The titration results shown in Fig. 5d 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 CO₂ crossover should be converted to carbonate ions in the anolyte. Figure 5d shows that the anolyte after the test is composed of 0.0735 M OH⁻ and 0.011 M CO₃²⁻, implying that the (bi)carbonate crossover contributed to < 0.774% of the ionic current. The extremely low K⁺ and CO₂ 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 CO₂ 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 nickelates⁵⁸ 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 CO₂ 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 K₂CO₃ and 3.62 M for KHCO₃)^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 CO₂ 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. 5a.

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 CO₂-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 CO₂ 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 CO₂ utilisation efficiency: a high proton transference number generally achieves a high CO₂-to-CO efficiency but may not necessarily lead to a high CO₂ reduction selectivity. Although hydrogen evolution still outcompetes CO₂ 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 CO₂ 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% CO₂-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.

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 CO₂ Consortium funded by BETO in EERE under contract DE-AC02-05CH11231.

Competing interests

The authors declare no competing interests.

Samu, A. A. et al. Intermittent Operation of CO2 Electrolyzers at Industrially Relevant Current Densities. ACS Energy Lett. 7, 1859–1861 (2022).
Li, W. et al. Bifunctional ionomers for efficient co-electrolysis of CO2 and pure water towards ethylene production at industrial-scale current densities. Nat. Energy (2022) doi:10.1038/s41560-022-01092-9.
Wen, G. et al. Continuous CO2 electrolysis using a CO2 exsolution-induced flow cell. Nat. Energy 7, 978–988 (2022).
Hansen, K. U., Cherniack, L. H. & Jiao, F. Voltage Loss Diagnosis in CO2 Electrolyzers Using Five-Electrode Technique. ACS Energy Lett. 4504–4511 (2022) doi:10.1021/acsenergylett.2c02096.
García de Arquer, F. P. et al. CO2 electrolysis to multicarbon products at activities greater than 1 A cm−2. Science 367, 661–666 (2020).
Rabinowitz, J. A. & Kanan, M. W. The future of low-temperature carbon dioxide electrolysis depends on solving one basic problem. Nat. Commun. 11, 5231 (2020).
Wu, Y. et al. Mitigating Electrolyte Flooding for Electrochemical CO ₂ Reduction via Infiltration of Hydrophobic Particles in a Gas Diffusion Layer. ACS Energy Lett. 2884–2892 (2022) doi:10.1021/acsenergylett.2c01555.
Disch, J. et al. High-resolution neutron imaging of salt precipitation and water transport in zero-gap CO2 electrolysis. Nat. Commun. 13, 6099 (2022).
Vass, Á., Kormányos, A., Kószó, Z., Endrődi, B. & Janáky, C. Anode Catalysts in CO2 Electrolysis: Challenges and Untapped Opportunities. ACS Catal. 12, 1037–1051 (2022).
Garg, S. et al. Urea-Functionalized Silver Catalyst toward Efficient and Robust CO2 Electrolysis with Relieved Reliance on Alkali Cations. ACS Appl. Mater. Interfaces 14, 35504–35512 (2022).
Pan, B. et al. Close to 90% Single-Pass Conversion Efficiency for CO2 Electroreduction in an Acid-Fed Membrane Electrode Assembly. ACS Energy Lett. 4224–4231 (2022) doi:10.1021/acsenergylett.2c02292.
Huang, J. E. et al. CO2 electrolysis to multicarbon products in strong acid. Science 372, 1074–1078 (2021).
O’Brien, C. P. et al. Single Pass CO2 Conversion Exceeding 85% in the Electrosynthesis of Multicarbon Products via Local CO2 Regeneration. ACS Energy Lett. 6, 2952–2959 (2021).
Endrődi, B. et al. Operando cathode activation with alkali metal cations for high current density operation of water-fed zero-gap carbon dioxide electrolysers. Nat. Energy 6, 439–448 (2021).
Garg, S., Rodriguez, C. A. G., Rufford, T. E., Varcoe, J. R. & Seger, B. How membrane characteristics influence the performance of CO2 and CO electrolysis. Energy Environ. Sci. 15, 4440–4469 (2022).
Ren, S. et al. Molecular electrocatalysts can mediate fast, selective CO2 reduction in a flow cell. Science 365, 367–369 (2019).
Larrazábal, G. O. et al. Analysis of Mass Flows and Membrane Cross-over in CO2 Reduction at High Current Densities in an MEA-Type Electrolyzer. ACS Appl. Mater. Interfaces 11, 41281–41288 (2019).
Ma, M., Kim, S., Chorkendorff, I. & Seger, B. Role of ion-selective membranes in the carbon balance for CO2 electroreduction via gas diffusion electrode reactor designs. Chem. Sci. 11, 8854–8861 (2020).
Salvatore, D. A. et al. Designing anion exchange membranes for CO2 electrolysers. Nat. Energy 6, 339–348 (2021).
Vass, Á. et al. Local Chemical Environment Governs Anode Processes in CO2 Electrolyzers. ACS Energy Lett. 6, 3801–3808 (2021).
Ma, M. et al. Insights into the carbon balance for CO2 electroreduction on Cu using gas diffusion electrode reactor designs. Energy Environ. Sci. 13, 977–985 (2020).
Ge, L. et al. Electrochemical CO2 reduction in membrane-electrode assemblies. Chem 8, 663–692 (2022).
Hubert, M. A., King, L. A. & Jaramillo, T. F. Evaluating the Case for Reduced Precious Metal Catalysts in Proton Exchange Membrane Electrolyzers. ACS Energy Lett. 7, 17–23 (2022).
Dinh, C.-T., García de Arquer, F. P., Sinton, D. & Sargent, E. H. High Rate, Selective, and Stable Electroreduction of CO2 to CO in Basic and Neutral Media. ACS Energy Lett. 3, 2835–2840 (2018).
Schultz, H., Bauer, G., Schachl, E., Hagedorn, F. & Schmittinger, P. Potassium Compounds. in Ullmann’s Encyclopedia of Industrial Chemistry (John Wiley & Sons, Ltd, 2000). doi:10.1002/14356007.a22_039.
Sassenburg, M., Kelly, M., Subramanian, S., Smith, W. A. & Burdyny, T. Zero-Gap Electrochemical CO2 Reduction Cells: Challenges and Operational Strategies for Prevention of Salt Precipitation. ACS Energy Lett. 321–331 (2022) doi:10.1021/acsenergylett.2c01885.
Cofell, E. R., Nwabara, U. O., Bhargava, S. S., Henckel, D. E. & Kenis, P. J. A. Investigation of Electrolyte-Dependent Carbonate Formation on Gas Diffusion Electrodes for CO2 Electrolysis. ACS Appl. Mater. Interfaces 13, 15132–15142 (2021).
Li, M. et al. The role of electrode wettability in electrochemical reduction of carbon dioxide. J. Mater. Chem. A 9, 19369–19409 (2021).
Endrődi, B. et al. Multilayer Electrolyzer Stack Converts Carbon Dioxide to Gas Products at High Pressure with High Efficiency. ACS Energy Lett. 4, 1770–1777 (2019).
Xu, Y. et al. Self-Cleaning CO2 Reduction Systems: Unsteady Electrochemical Forcing Enables Stability. ACS Energy Lett. 6, 809–815 (2021).
Kutz, R. B. et al. Sustainion Imidazolium-Functionalized Polymers for Carbon Dioxide Electrolysis. Energy Technol. 5, 929–936 (2017).
Liu, Z. et al. CO2 Electrolysis to CO and O2 at High Selectivity, Stability and Efficiency Using Sustainion Membranes. J. Electrochem. Soc. 165, J3371 (2018).
Kim, J. Y. ‘Timothy’ et al. Recovering carbon losses in CO2 electrolysis using a solid electrolyte reactor. Nat. Catal. 5, 288–299 (2022).
Xie, Y. et al. High carbon utilization in CO2 reduction to multi-carbon products in acidic media. Nat. Catal. 5, 564–570 (2022).
Monteiro, M. C. O., Philips, M. F., Schouten, K. J. P. & Koper, M. T. M. 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. Bipolar membrane electrolyzers enable high single-pass CO2 electroreduction to multicarbon products. Nat. Commun. 13, 3609 (2022).
Siritanaratkul, B. et al. Zero-Gap Bipolar Membrane Electrolyzer for Carbon Dioxide Reduction Using Acid-Tolerant Molecular Electrocatalysts. J. Am. Chem. Soc. 144, 7551–7556 (2022).
Bui, J. C. et al. Multi-scale physics of bipolar membranes in electrochemical processes. Nat. Chem. Eng. 1, 45–60 (2024).
Pärnamäe, R. et al. Bipolar membranes: A review on principles, latest developments, and applications. J. Membr. Sci. 617, 118538 (2021).
Patel, A. M., Nørskov, J. K., Persson, K. A. & Montoya, J. H. Efficient Pourbaix diagrams of many-element compounds. Phys. Chem. Chem. Phys. 21, 25323–25327 (2019).
Yue, P. et al. Microenvironment Regulation Strategies Facilitating High-Efficiency CO2 Electrolysis in a Zero-Gap Bipolar Membrane Electrolyzer. ACS Appl. Mater. Interfaces 15, 53429–53435 (2023).
Varela, A. S. et al. 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).

There is NO Competing Interest.

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

Download PDF

Journal Publication

published 19 Sep, 2024

Read the published version in Nature Communications →

Version 1

posted

You are reading this latest preprint version

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

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Results

The role of ionomer in CL on BPMEA performance

The role of the H⁺ transference number on BPMEA performance

Local H⁺ enrichment at CEL/CL interface

Long-term stability and ion crossover

Discussions

Declarations

Acknowledgment

Competing interests

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1

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

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Results

The role of ionomer in CL on BPMEA performance

The role of the H+ transference number on BPMEA performance

Local H+ enrichment at CEL/CL interface

Long-term stability and ion crossover

Discussions

Declarations

Acknowledgment

Competing interests

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1

The role of the H⁺ transference number on BPMEA performance

Local H⁺ enrichment at CEL/CL interface