Proton-donating cations enable efficient and stable acidic CO₂ reduction in membrane electrode assemblies

ABSTRACT

Electrochemical CO₂ reduction (CO₂R) in acidic membrane electrode assemblies (MEAs) represents a promising pathway for sustainable chemical production, but achieving high selectivity, low cell voltage and long-term stability remains challenging. Current approaches using alkali cations can promote selectivity through cationic effects, but relying on H₂O as a weak proton donor results in high overpotential and severe precipitation, causing elevated cell voltage and poor operational stability. Here, we introduce NH₄⁺ as a proton-donating cation that simultaneously addresses these challenges in acidic MEAs. As a cation, it electromigrates to the catalyst surface, stabilizing *CO₂ intermediates and reducing localized H⁺ concentration for high selectivity. As a proton donor, it provides superior proton-donating ability compared to H₂O when H⁺ mass transport is limited, which decreases the protonation barrier and reduces CO₂R overpotential on CoPc@CNT, resulting in a lower cell voltage. Furthermore, NH₄⁺ effectively donates protons to bicarbonate, promoting its decomposition at significantly lower temperatures compared to KHCO₃, thereby enabling easy removal of precipitates through mild heating and maintaining an NH₃/NH₄⁺ recirculation system for operational stability. As a result, this approach achieves an average CO₂-to-CO selectivity of 86% in acidic MEAs at 100 mA cm⁻² and 60°C using CoPc@CNT–NH₂ catalyst, with stable performance over 110 h at an average cell voltage of 2.84 V, corresponding to a 40.6% energy efficiency. This strategy advances acidic MEA-based CO₂R toward practical implementation by simultaneously achieving high selectivity, low overpotential and stable operation.

Beyond enhancing selectivity via cation effects, this work employs the proton-donating ability of NH4⁺ to lower both the protonation barrier of CO₂ reduction and the bicarbonate decomposition barrier, achieving high energy efficiency and stability in acidic membrane electrode assemblies.

INTRODUCTION

Electrocatalytic CO₂ reduction (CO₂R) powered by renewable energy represents a promising strategy for converting CO₂ into valuable chemical products, offering a sustainable pathway toward carbon-neutral chemical industry [1–3]. While both alkaline and acidic electrolyzers have been extensively studied for CO₂ conversion [4,5], acidic electrolyzers offer unique advantages by preventing CO₂ carbonation and eliminating CO₂ crossover [6–8], thereby enabling higher single-pass CO₂ conversion efficiency and avoiding the purification costs associated with CO₂ and O₂ mixing [9–11]. However, the implementation of acidic CO₂R systems faces critical challenges in achieving both high energy efficiency and long-term stability. High energy efficiency requires the maintenance of high product selectivity while operating at low cell voltage. Addressing these challenges is crucial for realizing the full potential of acidic electrolyzers in industrial-scale sustainable chemical production.

The industrial implementation of CO₂R requires electrolyzers that can deliver both high energy efficiency and scalability [12]. Among various configurations, membrane electrode assemblies (MEAs) represent the most promising pathway toward commercialization [13]. By eliminating liquid catholyte and enabling zero-gap operation, MEAs significantly reduce ohmic losses and system complexity, providing a practical route to industrial-scale production. While both cation exchange membranes (CEMs) and anion exchange membranes can be utilized in MEA systems [14,15], CEMs offer a distinctive acid-environment advantage in preventing CO₂ carbonation and crossover [16]. However, this acidic environment poses a significant challenge for CO₂R selectivity, with a limited selectivity of 37% in pure acidic MEAs [17].

To address the selectivity challenge in acidic MEAs, two main strategies have been developed to modify the local reaction environment. The first strategy employs alkali cations to enhance product selectivity by stabilizing reaction intermediates and reducing local H⁺ concentration [6,9,11,18,19]. However, the fundamental challenge of bicarbonate precipitation emerges during extended operation [20,21], affecting CO₂ transport and electrolyte stability, which limits continuous operation to around 20 h [10,22]. The second strategy utilizes polymer cations to circumvent precipitation issues [16,23–25]; however, the limited density of fixed cations poses an intrinsic ionic conductivity challenge in acidic MEAs. To suppress the hydrogen evolution reaction (HER) and achieve high CO₂R selectivity, this strategy requires operation in weak acid or pure water environments, where the significant ohmic losses result in prohibitively high cell voltages (3.6–4.0 V) and low energy efficiency (22%–26%) even at moderate current densities (100 mA cm⁻²) [16,25], presenting a major barrier to practical implementation.

The progress achieved through alkali cations and polymer cations demonstrates that the cationic effects, including stabilizing reaction intermediates and regulating local pH, are essential for high CO₂R selectivity. To make acidic CO₂ electrolysis practically viable, we need to maintain this high selectivity while addressing the remaining challenges in cell voltage and stability. The decomposition of bicarbonate requires proton participation, and a more efficient proton source could facilitate bicarbonate decomposition, preventing precipitation and enhancing operational stability. Moreover, our previous mechanistic studies on CoPc@CNT catalyst reveal that protonation is the potential-determining step in the CO₂R process [17]. The reliance on water as a proton donor, with its inherently weak proton-donating ability (i.e. a high thermodynamic barrier for proton donation), contributes to a high overpotential. A stronger proton donor could potentially lower the barrier, thereby reducing cell voltage and improving energy efficiency. Therefore, designing a system that can provide efficient ability of proton donating while maintaining the beneficial effects of cationic species becomes crucial for achieving high selectivity, low cell voltage and long-term stability in acidic CO₂ electrolysis.

To address these challenges, we propose that a proton-donating cation could provide an ideal solution by combining the beneficial properties of cations with enhanced proton-donating ability (Fig. 1). Our computational and experimental studies identify NH₄⁺ as an optimal candidate that simultaneously fulfills the dual requirements of our design strategy. First, like alkali cations and polymer cations, NH₄⁺ maintains high CO₂-to-CO selectivity by stabilizing reaction intermediates and reducing local H⁺ concentration (Fig. 1a). Second, and uniquely, NH₄⁺ acts as a superior proton donor compared to water under mass transport-limited conditions for H⁺ (Fig. 1b), effectively lowering the protonation barrier and reducing the CO₂R overpotential on CoPc@CNT by 280 mV at 100 mA cm⁻². This reduction in cathodic overpotential contributes to lowering of the cell voltage of the MEA system, thereby enhancing energy efficiency. Furthermore, NH₄⁺ proves to be a more efficient proton donor than HCO₃⁻, leading to a significantly lower decomposition temperature for NH₄HCO₃ relative to KHCO₃ (Fig. 1b). This property facilitates the easy removal of NH₄HCO₃ precipitates via mild heating, allowing NH₄⁺ to function within an NH₃/NH₄⁺ recirculation system (Fig. 1c). Consequently, the facile thermal removal of precipitated salts enables continuous NH₄⁺ supply, fundamentally resolving the long-standing salt precipitation issue in acidic MEAs and maintaining consistent performance throughout the reaction. As a result, CoPc@CNT–NH₂ catalyst achieves an average CO₂-to-CO selectivity of 86% at a current density of 100 mA cm⁻² and 60°C in acidic MEAs, with stable performance sustained over 110 h with an average cell voltage of 2.84 V, corresponding to an energy efficiency of 40.6%. To the best of our knowledge, this represents the most stable system among acidic MEAs while maintaining high energy efficiency throughout the CO₂R operation. With salt precipitation issues resolved, the system's long-term stability now depends primarily on catalyst stability, suggesting that more stable catalysts would further enhance operational longevity. These findings present a novel strategy for overcoming the inherent challenges of acidic CO₂R, offering a sustainable pathway toward carbon-neutral chemical production.

Figure 1.

Schematic of NH₄⁺-mediated CO₂R in acidic MEAs. NH₄⁺ enables efficient and stable CO₂R with cationic, proton-donating effects, supported by an NH₃/NH₄⁺ recirculation system. (a) Cationic effects: NH₄⁺ migrates and accumulates at the electrode surface, stabilizing *CO₂ intermediates through dipole–electric field interactions, and suppressing H⁺ migration due to the competitive migration of NH₄⁺. (b) Proton-donating effects: NH₄⁺ donates protons more readily than water, facilitating the protonation of *CO₂ and lowering the CO₂R overpotential. Furthermore, NH₄⁺ effectively donates protons to bicarbonate, promoting bicarbonate decomposition. (c) NH₃/NH₄⁺ recirculation system: NH₄HCO₃ precipitates decompose at low temperatures into NH₃, which is protonated by H⁺ in an ‘NH₃ adsorption container’ to form NH₄⁺. These NH₄⁺ ions diffuse back to an ‘NH₄⁺ supply container’, ensuring a steady NH₄⁺ supply.

RESULTS AND DISCUSSION

The cationic effects of NH₄⁺ on acidic CO₂R

Unlike conventional alkali cations, NH₄⁺ possesses dual functionality as both a cation and a proton donor, making it uniquely suited for acidic CO₂R. To understand how NH₄⁺ enhances CO₂R selectivity through its cationic properties, we first investigate its effects on the reaction interface using computational calculations.

COMSOL simulations are conducted to assess the effects of NH₄⁺ on localized H⁺ concentration under reaction conditions. Given that cations significantly accumulate at the electrode surface under the influence of electric field, both spatial constraints and ion size must be considered in the simulations. Therefore, we employ the generalized modified Poisson-Nernst-Planck (GMPNP) model [20,26–28], which accounts for electric field effects, diffusion and reactions to precisely calculate species concentration distributions in the reaction region (Fig. 2a). The simulation details are provided in the Methods and Table S1 of the Supplementary data.

Figure 2.

The cationic effects of NH₄⁺ on acidic CO₂R. (a) Schematic of ion transport near the cathode based on the GMPNP model, showing the Stern layer, diffuse layer and diffusion layer, with electromigration and diffusion processes for solvated ions (H⁺, NH₄⁺ and K⁺). (b–d) Ion concentration profiles as a function of distance from the cathode for different electrolytes: (b) 0.1 M H₂SO₄, (c) 0.1 M H₂SO₄ and 0.5 M (NH₄)₂SO₄, and (d) 0.1 M H₂SO₄ and 0.5 M K₂SO₄, simulated under varying current densities. (e) Atomic structure of NH₄⁺-containing electrolyte with CO₂ adsorption on CoPc@CNT. (f) Averaged electric field distribution along the z-axis near the catalyst surface under different ion conditions (H₂O, K⁺ and NH₄⁺). (g) Adsorption energy of CO₂ on CoPc@CNT in the presence of H₂O, K⁺ and NH₄⁺.

Both the CO₂-to-CO conversion and HER similarly affect the electrode interface, as they involve proton consumption at the catalyst surface. The corresponding electrode reactions are as follows:

(1)

(2)

Here, refers to the active protons involved in the HER or CO₂R processes (Fig. S1). In the case of a pure acidic system, the sources of are H⁺ and H₂O. For the NH₄⁺ system, originates from H⁺, NH₄⁺ and H₂O. It is important to note that NH₄⁺ and water supply for HER or CO₂R, and the resulting NH₃ and OH⁻ quickly react with H⁺ when H⁺ is sufficient (Fig. S2), converting back to NH₄⁺ and H₂O:

(3)

(4)

(5)

(6)

From an overall reaction perspective, the protons consumed in the electrode reactions all originate from H⁺. These reactions establish the framework for analyzing species distribution during CO₂R. We compare the species distributions in 0.1 M H₂SO₄ alone and with the addition of 0.5 M (NH₄)₂SO₄, at current densities ranging from 50 to 200 mA cm⁻² (Fig. 2b and c). In the pure acidic system (0.1 M H₂SO₄), a significant accumulation of H⁺ at the surface occurs under the influence of the electric field, approaching a saturation concentration of 9.50 mol L⁻¹ (Fig. 2b). In the NH₄⁺-containing system, under the influence of the electric field, NH₄⁺ competes with H⁺ for accumulation at the surface. As the current density increases, the concentration of H⁺ gradually decreases, while the concentration of NH₄⁺ continues to rise. At the current density of 100 mA cm⁻², the surface H⁺ concentration in the NH₄⁺-containing system is 0.58 M, 6% of that in the pure acidic system. At 200 mA cm⁻², it even decreases to 0.12 M, 1.3% of the pure acidic system's concentration.

When comparing the effects of different cations, we found that NH₄⁺ produces species distributions similar to those observed with K⁺ (Fig. 2d). This can be attributed to two main factors. First, both NH₄⁺ and K⁺ are cations that compete with H⁺ for electromigration, which effectively reduces the localized concentration of H⁺. Second, the comparable hydrated radii and migration rates of NH₄⁺ and K⁺ result in similar electromigration behaviors. As shown above, NH₄⁺ effectively reduces the localized H⁺ concentration, thereby inhibiting the conversion of H⁺ to H₂. These findings demonstrate that NH₄⁺ effectively modulates the reaction interface by reducing local H⁺ concentration, which is crucial for suppressing the competing HER.

Beyond controlling H⁺ concentration, the accumulated NH₄⁺ at the surface may create a localized electric field that stabilizes key reaction intermediates, particularly those with large dipole moments such as *CO₂ [18]. To investigate this effect, we perform density functional theory (DFT) calculations using CoPc@CNT as a model catalyst system. This well-defined single-atom catalyst not only enables accurate computational modeling [29] but also demonstrates high activity under acidic conditions in our previous study [17], making it ideal for studying the role of NH₄⁺ in acidic CO₂R. Based on this, we examine the electric field distribution at the interface under various electrolyte conditions (Fig. 2e and f, and Fig. S3). Pure water without cations is used as the blank control, while K⁺, a well-known cation capable of creating a localized electric field [30,31], serves as the reference. In the presence of pure water, the interfacial electric field remains negligible due to the lack of significant charge separation between water molecules and the CoPc@CNT catalyst. However, upon introducing NH₄⁺ into the electrolyte, a pronounced electric field develops at the interface, with the field strength peaking at z = 15 Å, corresponding to the position of adsorbed *CO₂. It is important to note that the electric field strength shown here is z-averaged, and the local cation-induced electric field is significantly stronger in the vicinity of the cation. Thus, we anticipate a much stronger field strength near the active Co site when the cation is close to the Co. Interestingly, we found that the electric field induced by NH₄⁺ closely resembles that generated by K⁺, suggesting that NH₄⁺ and K⁺ can induce similar field–dipole interactions. As shown in Fig. 2g, both K⁺ and NH₄⁺ significantly stabilize the *CO₂ intermediate, increasing its binding energy by approximately 0.3 eV. This stabilization is attributed to the large dipole moment of *CO₂, which is highly sensitive to the local electric field. The negative field stabilizes the dipole, thus favoring the activation of CO₂. Since *H has a minimal dipole moment and is therefore less sensitive to the electric field, our previous work indicates that *H and *CO₂ compete with each other in the absence of cations [17]. Consequently, the presence of cations is expected to have a more pronounced promotional effect on CO₂R. Integrating the results from COMSOL simulations and DFT calculations, NH₄⁺ electromigrates and accumulates at the catalyst surface, leading to a reduction in localized H⁺ concentration and stabilization of *CO₂ intermediates, which ensures the selectivity for CO₂-to-CO conversion.

Collectively, our computational calculations reveal that NH₄⁺ serves two essential functions as a cation: reducing local H⁺ concentration through competitive electromigration, and stabilizing reaction intermediates via field–dipole interactions. These effects work synergistically to promote CO₂R selectivity while suppressing the competing HER, addressing a key challenge in acidic CO₂R systems.

The proton-donating effects of NH₄⁺ on acidic CO₂R

Having established the role of NH₄⁺ as a cation in modulating the reaction interface, we next investigate its function as a proton donor. While CO₂R involves protonation as a key elementary step, the conventional proton source H₂O (pKa = 14) presents a high kinetic barrier [32]. NH₄⁺, with a significantly lower pKa of 9.25, could potentially serve as a more efficient proton donor. For our model catalyst CoPc@CNT, where protonation is the potential-determining step [17], this property of NH₄⁺ may substantially reduce the overpotential required for CO₂R.

To systematically investigate the proton-donating abilities of NH₄⁺, we first employ rotating disk electrode (RDE) measurements to compare different proton sources under conditions where the mass transfer of H⁺ is limiting. Linear sweep voltammetry (LSV) measurements during HER are used to analyze the onset potentials at which NH₄⁺ and H₂O act as proton sources. Figure 3a shows the LSV results for HER at a rotation speed of 900 r/min in various acidic electrolytes. For the 0.1 M H₂SO₄ electrolyte, the HER current density increases as the potential becomes more negative, and the LSV curve follows the typical e-exponential behavior described by the Butler–Volmer equation (Fig. S4a) [33]. In contrast, for the electrolyte system with the addition of NH₄⁺ and K⁺, the LSV curve exhibits distinct differences, with the curve clearly divided into three regions. For the NH₄⁺-containing electrolyte, in the first region (−0.6 to −1.0 V), H⁺ reduction occurs, but the current density is significantly lower than in the pure acidic system (0.1 M H₂SO₄). This reduction is attributed to NH₄⁺ lowering the localized H⁺ concentration, which leads to an elevated local pH, as confirmed by the RDE measurements (Fig. S5). In the second region (−1.0 to −1.2 V), the current density remains nearly constant despite increasingly negative potentials, which is characteristic of mass transport limitation [20]. In this case, the reactant is H⁺. This phenomenon is not observed under pure acidic conditions (Fig. S4a), indicating that the mass transport limitation of H⁺ is caused by cations. Higher plateau current densities observed with increased rotation speeds from 900 to 2500 r/min confirm that this potential range is H⁺ diffusion-limited (Fig. S4b). In the third region (potentials more negative than −1.2 V), the increased current density indicates the involvement of a new proton source in HER. Similarly, LSV curves obtained in an acidic solution containing K⁺ also display three distinct regions (Fig. 3a and Fig. S4c). The key difference lies in the onset potential of the third region, which corresponds to the potential at which the new proton source begins to participate in HER. In the K⁺-containing system, water acts as the proton source at −1.5 V (Fig. 3b), whereas in the NH₄⁺ system, HER begins at −1.2 V. The different onset potentials confirm that NH₄⁺, rather than water, serves as the proton source in the NH₄⁺ system (Fig. 3c). The more positive onset potential in the NH₄⁺ system demonstrates that NH₄⁺ is a more effective proton donor than H₂O. This characteristic is expected to lower the protonation barrier in the CO₂R process, thereby reducing the overpotential and facilitating CO₂R.

Figure 3.

The proton-donating effects of NH₄⁺ on acidic CO₂R. (a) LSV curves for 0.1 M H₂SO₄ with 0.5 M (NH₄)₂SO₄ or 0.5 M K₂SO₄, and for 0.1 M H₂SO₄ alone, measured using a rotating disk electrode at 900 r/min in an Ar-saturated environment. (b and c) Schematic illustration of ion distribution at the cathode in the presence of (b) K⁺, where H₂O acts as the proton donor, and (c) NH₄⁺, where NH₄⁺ serves as the proton donor. Both K⁺ and NH₄⁺ hinder the migration of H⁺ to the electrode surface. (d) Free energy diagrams for CO₂R pathways on CoPc@CNT at −1 V vs SHE, comparing H₂O and NH₄⁺ as proton donors. (e) Calculated energy span as a function of potential for H₂O and NH₄⁺. (f) CO partial current density as a function of potential in 0.1 M H₂SO₄ electrolytes containing NH₄⁺ or K⁺. (g) Gibbs free energy diagrams comparing the proton-donating abilities of NH₄⁺ and HCO₃⁻. (h) Thermogravimetric analysis of NH₄HCO₃ and KHCO₃, showing the decomposition temperatures of bicarbonate species. (i) Weight loss profiles of NH₄HCO₃ at different temperatures.

These experimental results clearly demonstrate the superior proton-donating ability of NH₄⁺ compared to H₂O. To understand the thermodynamic basis for this enhancement, we turn to DFT calculations. Our recent work demonstrates that the pH level in proton-coupled electron transfer (PCET) processes can be correlated with the pKa of the proton source [34]. The chemical potential of a coupled proton and electron is determined by the equilibrium with H₂ gas at the hydrogen electrode as shown:

(7)

where is the Gibbs free energy of a coupled proton–electron pair, is the Gibbs free energy of H₂ in the gas phase at standard state, U is the electrode potential vs standard hydrogen electrode (SHE), is the Boltzmann constant, T is the absolute temperature, and pK_a is the negative log₁₀ of the acid dissociation constant of the relevant acid.

As protons are always reactants in these systems, lowering the pKa of the proton source decreases the overall reaction energy, thereby affecting both the equilibrium and onset potentials. When the H⁺ mass transfer is limited, water (with a pKa of 14) typically acts as the proton source. However, NH₄⁺ (with a lower pKa of 9.25) can facilitate PCET processes more efficiently. The CO₂ conversion to CO on CoPc@CNT proceeds through two key PCET steps: CO₂(g) to *COOH, followed by *COOH to CO(g). As shown in Fig. 3d, the first step (CO₂ to *COOH) is the potential-determining step, with *COOH being the intermediate with the highest energy level along the reaction pathway. When NH₄⁺ is used as the proton source, there is a significant reduction in the overall energy span (Δ), as follows:

(8)

where and are pKa values of H₂O and NH₄⁺. This is independent of the electrode potential, as depicted in Fig. 3e. Given that the CO₂-to-CO reaction involves two PCET steps, the equilibrium potential when NH₄⁺ is used as the proton source is reduced by 2Δ, approximately 0.56 V (Fig. S6). Thus, the pKa of the proton source significantly impacts both the energy span and equilibrium potential for CO₂R to CO. A lower pKa results in a reduced energy span, implying slower kinetics according to the Bell–Evans–Polanyi (BEP) relationship when compared to that when water is used as the proton source. Meanwhile, the decrease in equilibrium potential provides a stronger thermodynamic driving force for CO₂ conversion to CO when NH₄⁺ is present. Therefore, compared to water, NH₄⁺ offers superior kinetic and thermodynamic advantages, which can effectively reduce the overpotential in CO₂R.

To confirm that NH₄⁺ more effectively promotes the protonation process in CO₂R compared to H₂O, CO₂R activity tests are operated by using a three-electrode flow cell. Unlike two-electrode MEAs, the three-electrode system can accurately evaluate the effect of cations on the cathode potential, minimizing interference from anode-related factors such as gas bubbles and device assembly during testing. Figure 3f and Fig. S7 shows that NH₄⁺-mediated CO₂R operates at a more positive reaction potential than K⁺-mediated CO₂R, resulting in a lower overpotential. At a partial current density of 100 mA cm⁻² for CO production, the overpotential with NH₄⁺ is up to 280 mV lower than that with K⁺. In addition, as shown in Fig. S8, the overpotentials for CO₂R mediated by Li⁺ and Na⁺ are also higher than that of NH₄⁺. These confirm that NH₄⁺, with its superior proton-donating ability compared to H₂O, more effectively facilitates the protonation process on CoPc@CNT, thereby significantly reducing the overpotential.

The strong proton-donating ability of NH₄⁺ plays dual beneficial roles in acidic CO₂R. First, as demonstrated above, it effectively reduces the reaction overpotential. Second, this same property proves advantageous in addressing another critical challenge in CO₂R systems—the formation and removal of bicarbonate precipitates. In both NH₄⁺ and conventional cation systems, the reaction of CO₂ with alkaline species (OH⁻ and NH₃) leads to bicarbonate precipitation (KHCO₃ and NH₄HCO₃), which can obstruct CO₂ transport channels and compromise system stability. The key to removing these precipitates lies in their decomposition mechanism, which fundamentally involves proton transfer processes. Here, the superior proton-donating ability of NH₄⁺ plays a crucial role in facilitating this decomposition. Given that thermal treatment can decompose bicarbonate precipitates, understanding the decomposition mechanisms of KHCO₃ and NH₄HCO₃ is crucial for developing effective mitigation strategies. The decomposition of bicarbonates requires proton transfer processes, as the formation of final products (CO₂ and H₂O) necessitates the combination of protons with bicarbonate ions. The key difference between KHCO₃ and NH₄HCO₃ lies in their proton transfer pathways during decomposition. The decomposition of KHCO₃ requires intermolecular proton transfer between adjacent molecules, while NH₄HCO₃ can decompose through intramolecular proton transfer:

(9)

(10)

(11)

(12)

Here, a and b represent two adjacent KHCO₃ molecules. The energy difference between these decomposition pathways primarily stems from the distinct proton donation barriers of HCO₃⁻ and NH₄⁺. In KHCO₃, protons must transfer between neighboring molecules, creating a higher energy barrier. In contrast, NH₄HCO₃ contains its own proton source (NH₄⁺), enabling more efficient intramolecular proton transfer.

DFT calculations indicate that the proton-donation barrier for NH₄⁺ is lower than that for HCO₃⁻ (Fig. 3g). This implies that the decomposition temperature of NH₄HCO₃ is substantially lower than that of KHCO₃. Thermogravimetric (TG) analysis further corroborates this finding (Fig. 3h and Fig. S9), with decomposition temperatures of NH₄HCO₃ and KHCO₃ observed at 45°C and 133°C, respectively. Moreover, the decomposition of NH₄HCO₃ does not yield solid residues, allowing for its complete decomposition. As the temperature increases, the decomposition rate of NH₄HCO₃ accelerates (Fig. 3i). In contrast, KHCO₃ decomposes to form K₂CO₃, which is more thermally stable and resists further decomposition. This makes it challenging to fully remove KHCO₃ through mild heating.

These findings comprehensively demonstrate the dual advantages of NH₄⁺ as a proton donor in acidic CO₂R. First, its lower pKa compared to H₂O enables more efficient proton transfer, leading to a significant reduction in overpotential (up to 280 mV lower than K⁺-mediated systems) during CO₂R. Second, while both NH₄⁺ and K⁺ systems face challenges from bicarbonate precipitation, the intramolecular proton transfer ability of NH₄HCO₃ enables its complete decomposition at mild temperatures (45°C vs 133°C for KHCO₃). These unique properties suggest that NH₄⁺-mediated systems can achieve cell voltage through enhanced proton donation and good stability through facile precipitate removal, addressing two key challenges in acidic CO₂R.

NH₃/NH₄⁺ recirculation system enables stable CO₂R in acidic MEA

Based on our understanding of the functions of NH₄⁺ and the thermal decomposition behavior of NH₄HCO₃, we design an NH₃/NH₄⁺ recirculation system to address a critical challenge in MEA operation: maintaining stable NH₄⁺ supply (Fig. 1c and Fig. S10). In MEA devices, NH₄⁺ at the cathode interface is sourced from an ‘NH₄⁺ supply container’, while bicarbonate precipitated at the cathode consumes and decreases its concentration in the supply container. Insufficient NH₄⁺ at the cathode would weaken its crucial roles in stabilizing reaction intermediates, moderating local acidity and providing protons for CO₂R. Our system incorporates a mild heating strategy to achieve dynamic equilibrium of NH₄⁺ concentration. The anolyte contains both NH₄⁺ and H⁺ ions, with NH₄⁺ migrating to the cathode under the electric field to fulfill its multiple functions. When NH₄HCO₃ precipitation occurs, mild heating facilitates its decomposition into NH₃, CO₂ and H₂O. The gaseous NH₃ can be captured by an ‘NH₄⁺ adsorption container’, where it reacts with H⁺ to regenerate NH₄⁺. These regenerated NH₄⁺ ions can then diffuse through the gas–liquid separation membrane to the ‘NH₄⁺ supply container’. This NH₄⁺/NH₃ cycling system maintains sufficient NH₄⁺ concentration at the cathode to sustain both its cationic and proton-donating effects. In contrast, K⁺-mediated MEAs lack such a self-regulating mechanism. Once K⁺ is lost to KHCO₃ precipitation, it cannot be readily recovered through thermal decomposition and gas-phase transport, leading to continuous depletion of K⁺ supply and eventual system failure. Therefore, this NH₃/NH₄⁺ recirculation system provides a robust foundation for achieving stable CO₂R in acidic MEAs.

Before experimentally verifying the stability of NH₄⁺-mediated acidic MEAs, anolyte composition should be first determined, especially for H⁺ concentration. The H⁺ concentration in the anolyte plays a critical role in both reaction selectivity and system stability through two mechanisms [10,22]. First, H⁺ depletion at the cathode interface is necessary for achieving high CO₂R selectivity by suppressing the competing HER. Second, H⁺ helps neutralize the in situ-formed HCO₃⁻ in the diffusion layer, preventing undesired salt precipitation. These dual roles of H⁺ create competing requirements for its concentration. At low H⁺ concentrations (e.g. 0.1 M), while H⁺ depletion is readily achieved at low current densities favoring CO₂R selectivity, the insufficient H⁺ cannot effectively neutralize the HCO₃⁻ formed from the reaction between NH₃ (produced by NH₄⁺ proton donation), CO₂ and H₂O, leading to salt precipitation. Conversely, higher H⁺ concentrations (e.g. 0.6 M) better prevent salt precipitation but require impractically high current densities (>150 mA cm⁻²) to achieve the H⁺ depletion necessary for high selectivity. To address this trade-off, we fixed the NH₄⁺ concentration at 0.2 M while varying H⁺ concentrations. SO₄²⁻ was chosen as the counter anion for its chemical inertness at both electrodes. Selectivity analysis (Fig. 4a and Fig. S11) reveals that intermediate H⁺ concentrations (0.2 and 0.4 M) achieve the best balance. While both concentrations show comparable CO selectivity at 100 mA cm⁻², the higher H⁺ concentration (0.4 M) demonstrates better stability by more effectively preventing salt precipitation (Fig. 4b). Therefore, we determine the anolyte composition to be 0.1 M (NH₄)₂SO₄ and 0.2 M H₂SO₄, which balances high selectivity with good stability at our target current density of 100 mA cm⁻². Notably, the NH₄⁺/H⁺ ratio used in the MEA system is significantly lower than that in flow cells, as the absence of flowing catholyte in MEAs minimizes the convective impact on the local pH.

Figure 4.

NH₃/NH₄⁺ recirculation system enables stable CO₂R in acidic MEA. (a) CO-FE tests at various H⁺ concentrations with 0.2 M NH₄⁺ in the anolyte on CoPc@CNT. (b) Stability tests with anolytes containing 0.2 M NH₄⁺ and either 0.2 M or 0.4 M H⁺. (c) CO-FE tests under temperatures from 25 to 60°C, using 0.2 M NH₄⁺ and 0.4 M H⁺ in the anolyte at 100 mA cm⁻². (d) CO-FE tests under CO₂ pressures of 1–3 atm at 60°C under identical conditions. (e) Stability tests at 100 mA cm⁻², 3 atm and 60°C, comparing catalyst loadings of 1 and 2 mg cm⁻². (f) Polarization curves recorded before and after stability testing at 1 mg cm⁻² catalyst loading.

The temperature is another key parameter for system performance, as it directly influences the NH₄HCO₃ decomposition. Figure 4c illustrates that as the temperature increases, stability improves gradually, with stability at 60°C remaining essentially unchanged over a period of 3 h. However, this enhancement in stability comes at the expense of CO selectivity, which decreases from 90% to 72%. This decrease is primarily attributed to the greater influence of temperature on the kinetics of the HER compared to CO₂R, with elevated temperatures significantly enhancing HER kinetic activity. To enhance selectivity, increasing the CO₂ partial pressure effectively reduces the CO₂ adsorption barrier and improves the competitiveness of CO₂R relative to HER. Considering that CO₂ is typically stored and transported under pressurized conditions (50–100 atm), the use of CO₂ at pressures below 10 atm does not require additional energy input. Figure 4d (details in Fig. S12) shows that increasing the pressure from 1 to 3 atm leads to an improvement in CO₂R selectivity, increasing from 73% to 80% at a current density of 100 mA cm⁻². However, further pressure increases are not pursued, as they have a minimal effect on selectivity while significantly increasing the demands on reactor gas tightness.

Under these conditions, we evaluate the stability of NH₄⁺-mediated MEAs. The system demonstrates stable operation at 100 mA cm⁻² for 10 h (Fig. 4e). Interestingly, we observe an increase in CO₂R selectivity from 80% to 90% during the initial 10 h, which we attribute to gradual catalyst deactivation. As the number of active catalytic sites decreases, the increased current density per site further reduces local H⁺ concentration, suppressing HER and enhancing CO₂R selectivity. This catalyst deactivation is confirmed by the decreased current density in post-stability cyclic voltammetry measurements (Fig. 4f). Notably, increasing the catalyst loading from 1 to 2 mg cm⁻² extends the stable operation period from 10 to 20 h (Fig. 4e and Fig. S13). These results demonstrate that our NH₃/NH₄⁺ recirculation strategy effectively solves the salt precipitation challenge in acidic MEAs, with system stability now primarily limited by catalyst deactivation rather than precipitate-related issues. This finding motivates us to focus on catalyst engineering to further improve the stability of acidic CO₂R systems.

NH₄⁺-mediated stable CO₂R on CoPc@CNT–NH₂

Having established an effective NH₃/NH₄⁺ recirculation strategy, we next focus on improving catalyst stability to achieve stable operation. The deactivation of CoPc@CNT primarily stems from Co atoms detaching from the phthalocyanine ring during the reaction [35]. Previous studies have shown that CoPc supported on amino-functionalized CNT (CNT–NH₂) exhibits enhanced electrochemical stability [36], as the lone-pair electrons from –NH₂ groups can coordinate axially with Co atoms, preventing their detachment. Based on this insight, we synthesize the molecularly dispersed catalyst CoPc@CNT–NH₂ (Figs S14–S16), and evaluate its performance in acidic MEAs with a catalyst loading of 2 mg cm⁻².

To further mitigate the effects of salt precipitation on device performance during prolonged stability testing, we implement an operational protocol consisting of 10-hour reaction cycles followed by 1-hour intermittent pauses. These strategic interruptions serve dual purposes: first, they allow for the maintenance of optimal CO₂ humidification conditions through the replenishment of water in the humidification bottle, otherwise dry CO₂ would remove water from the cathode region, accelerating salt precipitation; second, they provide sufficient time for the complete decomposition of accumulated NH₄HCO₃, preventing its precipitation within the system. Based on this, the CoPc@CNT–NH₂ catalyst maintains an average CO₂-to-CO selectivity of 86% at 100 mA cm⁻² for 110 h at an average cell voltage of 2.84 V, corresponding to a 40.6% energy efficiency (Fig. 5a and b, and Fig. S17). To the best of our knowledge, this represents the most stable system among acidic MEAs while maintaining high energy efficiency throughout the CO₂R operation (Fig. 5c and Table S2). Furthermore, compared to the most stable neutral MEA systems reported recently [37] (Table S3), our system exhibits clear advantages in both single-pass conversion efficiency (Fig. S18) and cell voltage (Tables S3 and S4), which translate into significantly reduced energy consumption (Fig. S19) and operational costs (Fig. S20).

Figure 5.

NH₄⁺-mediated stable CO₂R on CoPc@CNT–NH₂. (a and b) Comparison of stability tests for NH₄⁺- and K⁺-mediated CO₂R systems at 0.2 M NH₄⁺/K⁺ and 0.4 M H⁺, using CoPc@CNT–NH₂ catalyst under conditions of 60°C and 3 atm CO₂ at 100 mA cm⁻². (c) Comparison of energy efficiency and stability with literature reports. (d) Cross-sectional SEM image and EDS mapping of gas diffusion electrode from the K⁺-mediated system, showing extensive salt precipitation throughout the electrode structure. (e) Cross-sectional SEM image and EDS mapping of gas diffusion electrode from the NH₄⁺-mediated system, with N signals confined to the catalyst layer, confirming stable operation without salt precipitation.

It should be noted that the system's stability is currently constrained by catalyst degradation rather than salt participation issues. Inductively coupled plasma analysis reveals a significant decrease in Co content in CoPc@CNT–NH₂ after electrolysis (Fig. S21 and Table S5). This is further supported by transmission electron microscopy observations showing reduced Co signal intensity (Figs S15 and S22), and X-ray photoelectron spectroscopy results indicating a decline in the Co–N ratio (Fig. S23). Meanwhile, X-ray diffraction analysis shows no evidence of new phase formation (Fig. S24). Together, these characterizations confirm that the primary degradation pathway of CoPc@CNT–NH₂ involves the leaching of Co from the phthalocyanine structure. Notably, the Co leaching rate in CoPc@CNT–NH₂ is one-fifth of that in CoPc@CNT (Table S5), which correlates well with the slower rate of cell voltage increase observed during operation (Fig. S25). Moreover, the NH₄⁺ concentration remains stable in both the NH₃ adsorption container and the NH₄⁺ supply container, and no NH₃ leakage is detected (Fig. S26), ruling out the possibility of instability arising from the NH₃/NH₄⁺ recirculation system. These findings suggest that further development of more stable catalysts could directly lead to extended device lifetime.

To validate the effectiveness of our NH₄⁺-mediated system, we conduct comparative tests with conventional K⁺-mediated MEAs under identical conditions (Fig. 5a). Due to the weaker proton-donating ability of H₂O compared to NH₄⁺, K⁺-mediated MEAs exhibit higher CO-Faradaic efficiency (CO-FE), which is consistent with results obtained in flow cell configurations (Fig. S7c). However, its weaker proton-donating ability also leads to a significantly higher cell voltage in the K⁺-mediated MEA compared to the NH₄⁺-mediated MEAs (Fig. 5b). Moreover, the K⁺-mediated system shows significant CO-FE fluctuations within the first 4 h (Fig. 5b and Fig. S27), followed by rapid deactivation. This instability manifests through several observable phenomena: a notable decrease in product outlet flow rate, declining flowmeter readings (Fig. S28 and Table S6), and visible salt deposits blocking the gas channel outlet upon post-reaction inspection (Fig. S29). These observations indicate that salt precipitation severely compromises CO₂ transport by blocking gas channels. Therefore, while the NH₄⁺ system sacrifices a small degree of selectivity due to its stronger proton-donating ability, it significantly lowers the cell voltage and effectively eliminates the issue of salt precipitation, leading to dramatically improved operational stability. This trade-off is well justified for long-term electrolysis performance.

The contrasting stability of these systems is further evidenced by post-reaction scanning electron microscopy energy-dispersive spectroscopy (SEM-EDS) analysis of the gas diffusion electrodes. In K⁺-mediated MEAs (Fig. 5d), K⁺ is detected throughout the entire electrode structure, including the catalyst layer, microporous layer and fiber layer, confirming extensive salt precipitation. In contrast, NH₄⁺-mediated MEAs (Fig. 5e) show nitrogen signals confined to the catalyst layer, likely originating from the catalyst components (CoPc structure and CNT–NH₂) and residual nitrogen species. This localized distribution confirms the absence of salt precipitation in our NH₄⁺-mediated system, validating its effectiveness in maintaining stable operation.

These comprehensive results demonstrate the successful development of a stable acidic CO₂R MEAs through two key innovations: the NH₃/NH₄⁺ recirculation strategy that ensures the stable supply of NH₄⁺, and the modified CoPc@CNT–NH₂ catalyst that enables extended operation. The combination achieves stable CO₂R performance with over 80% CO selectivity at 100 mA cm⁻² for 110 h, representing a significant advancement in acidic MEA systems.

Compared with organic amines, NH₄⁺ salts are among the most readily available and cost-effective proton-donating cations, making them an ideal and practical candidate for fundamental investigation. Therefore, NH₄⁺ is chosen as the primary focus in this work. Meanwhile, common organic amines such as CH₃NH₃⁺ and C₂H₅NH₃⁺ introduce longer carbon chains, which tend to decrease the solubility of bicarbonate salts and increase their decomposition temperature. These properties exacerbate salt precipitation issues and complicate the stability. Addressing such challenges would require precise molecular design and structural tuning, which lie well beyond the scope of the present study. Nevertheless, organic amines offer a unique advantage in their structural tunability. In future studies, we aim to exploit this tunability to improve bicarbonate solubility and lower decomposition temperatures, thereby enabling further reductions in operating temperature.

CONCLUSION

In this work, we demonstrate an effective strategy to achieve both high energy efficiency and stability in acidic CO₂R by utilizing NH₄⁺ as a proton-donating cation. Our systematic investigations reveal two distinct roles of NH₄⁺ in promoting CO₂R performance. As a cation, NH₄⁺ reduces surface H⁺ concentration and stabilizes *CO₂ intermediates, leading to enhanced CO selectivity, as supported by DFT calculations and COMSOL simulations. As an efficient proton donor, NH₄⁺ offers the advantages over both H₂O and HCO₃⁻: it reduces the CO₂R overpotential on CoPc@CNT by 280 mV at 100 mA cm⁻² through lowered protonation barriers, and enables facile removal of salt precipitation due to the significantly lower decomposition temperature of NH₄HCO₃ compared to KHCO₃. Based on this low-temperature decomposition behavior, an NH₃/NH₄⁺ recirculation system is further established to ensure a stable supply of NH₄⁺ during electrolysis. As a result, the NH₄⁺-mediated CoPc@CNT–NH₂ system achieves an average CO selectivity of 86% at 100 mA cm⁻² with stable performance for 110 h at an average cell voltage of 2.84 V, corresponding to a 40.6% energy efficiency. To the best of our knowledge, this represents the first acidic MEA system that simultaneously achieves high energy efficiency (40.6%) over 110 h, addressing a long-standing challenge in CO₂R technology. Notably, the stability of the system is now determined by catalyst stability rather than salt precipitation, indicating that advances in catalyst robustness would extend system lifetime. Beyond the demonstrated high performance of the NH₄⁺-mediated CoPc@CNT–NH₂ system achieving stable CO production at acidic MEA, this strategy provides insights for designing next-generation acidic CO₂R systems. The dual functionality principle established here could guide the development of other proton-donating species, particularly organic amines with tunable substituents, where the proton-donating ability can be systematically modulated through electronic and steric effects to improve CO₂R performance.

FUNDING

Jia Zhu, Xiaojun Wang, Shijia Feng and Hongzhi Zheng acknowledge the support from the National Natural Science Foundation of China (92262305, 92462301, 52525202, 52302225, 52202251 and 52461160296), the ‘GeoX’ Interdisciplinary Project of Frontiers Science Center for Critical Earth Material Cycling (20250106), the New Cornerstone Science Foundation through the XPLORER PRIZE, and the Natural Science Foundation of Jiangsu Province (BK20233001 and BK20243009). Philippe Sautet and Dongfang Cheng have been solely supported by the US National Science Foundation CBET Grant 2103116 and the Audi CO₂ Cy Pres Award.

AUTHOR CONTRIBUTIONS

S.F. performed the majority of the synthesis, characterization, electrochemical tests and COMSOL simulations. Z.L. optimized the CO₂R stability, performed the energy and cost analysis, and contributed to manuscript revision. D.C. performed DFT calculations, under the guidance of P.S., X.Z., and T.W. optimized the electrochemical performance. J.L. and X.D. were involved in the COMSOL simulations and DFT calculations, respectively. Z.W. conducted the SEM-EDS experiments. Y.W. performed the RDE tests. Y.Y. conducted the TG tests. S.C. and Y.H. contributed to the optimization of the figures presented in the manuscript. S.F., D.C., H.Z., X.W. and J.Z. analyzed the data. S.F. and D.C. drafted the manuscript, while X.W. and J.Z. provided overall direction and oversight for the research.

Conflict of interest statement. None declared.