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Review

Challenges and Opportunities of Choosing a Membrane for Electrochemical CO2 Reduction

by
Helene Rehberger
,
Mohammad Rezaei
* and
Abdalaziz Aljabour
GIG Karasek GmbH, Neusiedlerstraße 15-19, A-2640 Gloggnitz, Austria
*
Author to whom correspondence should be addressed.
Membranes 2025, 15(2), 55; https://doi.org/10.3390/membranes15020055
Submission received: 19 December 2024 / Revised: 23 January 2025 / Accepted: 3 February 2025 / Published: 8 February 2025
(This article belongs to the Special Issue Innovative Membrane-Based Approaches to CO2 Electroreduction: From Fundamentals to Applications)
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Abstract

:
The urgent need to reduce greenhouse gas emissions, particularly carbon dioxide (CO2), has led to intensive research into novel techniques for synthesizing valuable chemicals that address climate change. One technique that is becoming increasingly important is the electrochemical reduction of CO2 to produce carbon monoxide (CO), an important feedstock for various industrial processes. This comprehensive review examines the latest developments in CO2 electroreduction, focusing on mechanisms, catalysts, reaction pathways, and optimization strategies to enhance CO production efficiency. A particular emphasis is placed on the role of ion exchange membranes, including cation exchange membranes (CEMs), anion exchange membranes (AEMs), and bipolar membranes (BPMs). The review explores their advantages, disadvantages, and the current challenges associated with their implementation in CO2 electroreduction systems. Through careful analysis of the current literature, this report aims to provide a comprehensive understanding of state-of-the-art methods and their potential impact on sustainable CO production, with a special focus on membrane technologies.

1. Introduction

The 21st century presents humanity with unprecedented environmental challenges, foremost among which is the urgent need to reduce greenhouse gas emissions in order to combat climate change. Among these emissions, carbon dioxide (CO2) stands out as one of the main contributors to global warming. The continued reliance on fossil fuels for energy production, transportation, and industrial processes has led to a steady increase in CO2 concentrations in the atmosphere and has exacerbated the climate crisis [1].
In Figure 1, the CO2 emissions in Austria from 1990 to 2022 are illustrated. To address this amount of emitted CO2, researchers have been exploring innovative strategies to reduce CO2 emissions while meeting energy and industrial needs. Carbon capture utilization (CCU) has proven to be a promising approach to reduce CO2 emissions by converting captured CO2 into valuable products. In the context of CCU, electrochemical reduction of CO2 is particularly promising as it can convert CO2 into useful chemicals and fuels, thus, closing the carbon cycle and contributing to a circular economy [2].
In this review, the concept of carbon electroreduction is introduced as a key strategy within the broader CCU framework. We outline the motivations driving research in this area, including the need to decarbonize the economy, diversify energy sources, and promote sustainable development. By examining the role of CO2 electricity reduction in achieving climate change goals, we set the stage for a comprehensive exploration of the latest advances in this area [2].

1.1. Carbon Capture Utilization (CCU)

CCU is a promising approach to reducing CO2 emissions while producing valuable products. CCU represents a paradigm shift in the approach to reducing CO2 emissions by focusing on converting captured CO2 into useful products instead of simply storing it underground. By utilizing CO2 as a feedstock for chemical synthesis and industrial processes, CCU offers a double benefit: it reduces greenhouse gas emissions while creating economic value [2,3].
In the field of CCU, electrochemical CO2 reduction has gained significant attention due to its versatility and efficiency in converting CO2 into a range of valuable chemicals and fuels. Unlike traditional carbon capture and storage (CCS) methods, which capture and sequester CO2 emissions, electrochemical CO2 reduction enables the conversion of CO2 into products such as carbon monoxide (CO), methane (CH4), ethylene (C2H4) and formic acid (HCOOH) [4].
Potential applications of CO as a feedstock for various industrial processes including metal production, hydrogen generation, and chemical synthesis. Carbon monoxide (CO) serves as a versatile building block for numerous industrial processes, ranging from metal production to chemical synthesis and hydrogen generation. As a feedstock, CO finds applications in a wide range of sectors, including petrochemicals, pharmaceuticals, and materials science. The electrochemical reduction of carbon dioxide (CO2) offers a sustainable pathway for CO production, enabling the synthesis of CO from renewable electricity and captured CO2 emissions [5].
In metal production, CO serves as a reducing agent in processes such as iron smelting and steelmaking, where it reacts with metal oxides to yield pure metals. In chemical synthesis, CO is used as a precursor to produce a variety of organic compounds, including alcohols, acids, and esters. In hydrogen generation, CO can be converted into hydrogen gas via the water–gas shift reaction, providing a clean and renewable source of hydrogen for fuel cells and industrial applications [5].
Furthermore, CO finds applications as fuel and chemical feedstock in the synthesis of methanol, ammonia, and other valuable chemicals. By harnessing CO as a platform molecule, it is possible to create a wide range of products with diverse applications in industry, transportation, and energy storage. The sustainable production of CO via electrochemical CO2 reduction offers a promising pathway for meeting industrial demands while reducing greenhouse gas emissions and promoting a circular economy [5].

1.2. Conventional Strategies for CO Production

The production of carbon monoxide (CO) is an essential part of various industrial processes, including the synthesis of chemicals, fuels, and materials. In the past, CO was mainly produced by conventional processes such as coal gasification, steam reforming of natural gas, and partial oxidation of hydrocarbons. These processes are based on high-temperature processes for the thermal decomposition of carbonaceous feedstocks, producing CO together with other by-products [6].
In coal gasification, one of the oldest methods of CO production, coal is converted into a gas mixture of CO, hydrogen (H2), and other gasses through high-temperature reactions in the presence of steam or oxygen. Similarly, steam reforming of natural gas uses steam and a catalyst to react with methane (CH4) to produce CO and H2. In partial oxidation, hydrocarbons are burned with a limited supply of oxygen, producing CO and H2 [6].
While these conventional methods must fulfill the industry’s CO production needs, they have several drawbacks, such as high energy consumption, greenhouse gas emissions, and dependence on fossil fuels. As the global transition to sustainable energy and production intensifies, there is a growing need to explore alternative ways to produce CO that are more efficient, environmentally friendly, and economically viable [6].

2. Novel Green Conversion of CO2 into CO

With the growing urgency to mitigate climate change, researchers are increasingly turning to new technologies to convert carbon dioxide (CO2) into valuable chemicals and fuels. Among these technologies, electrochemical CO2 reduction holds promise because of its ability to selectively convert CO2 into carbon monoxide (CO) using renewable electricity as the driving force. By harnessing the power of electrons, electrochemical CO2 reduction offers a sustainable way to produce CO.
The general technical scheme of the CO2 electroreduction cell is illustrated by Figure 2 [7]. The electrochemical cell is a membrane electrode assembly. The Ag-based cathode represents the catalyst, which can be varied in order to produce CO. In this setup, an aqueous solution is used as the electrolyte. In the setup, the aqueous solution is 10 mM KHCO3.
By improving the electrochemical CO2 reduction to CO, a syngas production route is established, which is independent on fossil fuels [8]. In Table 1, a comparison based on current research and literature in the field of CO2 reduction technologies is summarized. In general, the products are generated at the cathode side via the electrochemical CO2 reduction and the H2O is oxidized at the anode side.

3. Electrochemical CO2 Reduction Mechanisms

In-depth analysis of electrochemical reduction pathways for the conversion of CO2 to CO is needed.
The electrochemical reduction of carbon dioxide (CO2) offers a promising route for the conversion to carbon monoxide (CO), a valuable industrial raw material, while reducing CO2 emissions. Central to this process is the electrochemical reactions that take place at the electrode surface and drive the conversion of CO2 to CO via a series of intermediate steps. Understanding the mechanisms underlying these reactions is crucial for optimizing the efficiency and selectivity of CO production [11].
The electrochemical reduction of CO2 usually proceeds via several pathways, each comprising a sequence of elementary steps that determine the formation of specific products. These pathways are influenced by factors such as the electrode material, the reaction conditions, and the composition of the electrolyte, which determine the kinetics and thermodynamics of the electrochemical process. The main intermediates of the CO2 reduction mechanism include adsorbed CO2 species, CO2 ions, and various surface-bound species formed during the reduction process (see Figure 3) [11].
  • Adsorption strength for the key intermediate (∗COOH);
  • proper CO adsorption energy;
  • inert CO reduction ability;
  • abundant and inexpensive raw materials;
  • efficient production of H2 and CO simultaneously.

4. Catalysts for Electrochemical CO2 Reduction

There are different types of catalysts, including metals, metal oxides, chalcogenides, metal complexes, monatomic catalysts, and metal-free catalysts, which is illustrated in Figure 4 [13].
Catalysts play a crucial role in the electrochemical reduction of carbon dioxide (CO2) by facilitating the conversion of CO2 into valuable products such as carbon monoxide (CO), methane (CH4) and ethylene (C2H4). A wide range of catalysts has been explored for CO2 electroreduction, including metals, metal oxides, chalcogenides, metal complexes, single-atom catalysts and metal-free materials. Each type of catalyst offers unique advantages and challenges in terms of activity, selectivity, stability, and cost effectiveness [13].
In addition to inorganic catalysts, organic molecules and complexes have also been explored as catalysts for CO2 electroreduction. Metal complexes containing transition metals such as iron (Fe), cobalt (Co) and nickel (Ni) show catalytic activity in CO2 reduction, while single-atom catalysts provide precise control over active sites and reaction pathways. According to the literature [14], the total reaction barrier for an Fe surface suggests a high activity on the Fe surface. The reaction barrier matches with the reaction energies as Fe < Co < Ni < Cu. Additionally, metal-free catalysts based on carbon materials, nitrogen-doped graphene, and other carbonaceous substrates are promising for CO2 electroreduction in aqueous and non-aqueous electrolytes [15].
As an example, copper is a useful non-noble metal catalyst, which is able to produce hydrocarbons as well as alcohols through the reduction of CO2 [16]. It is important to mention that by using copper a high overpotential is needed and, therefore, it influences energy efficiency negatively. The performance of Cu can be improved by changing the size and morphology design, alloying, as well as surface oxidation–reduction [16,17,18,19,20,21].
In order to improve the performance, nanoparticles (NP) provide a large surface area as well as active sites with a low coordination number such as surface planes, edges, and corners. For instance, by using diameters of around 7 nm, a faradaic efficiency of 76% of methane is obtained, which is higher than compared with the result of polycrystalline Cu, which is only 44% of faradaic efficiency. The trend shows that a smaller Cu-NP size enables a larger total current density. However, it generates lower selectivity in terms of hydrocarbon formation [19].
Table 2 shows a summary of Cu-based catalysts, tested to determine the faradaic efficiency as well as the current density [20].
In order to produce CO, there are many inorganic-based catalysts. By changing the potential different faradaic efficiencies are reported in the literature. The conditions for the electrochemical reduction of carbon dioxide (CO2RR) are summarized in Figure 5 [18,22,23,24,25,26,27,28,29,30,31,32,33].
According to the literature, catalysts such as CdS, MoS2, and Ag2S show a lower overpotential to award a high faradaic efficiency for producing CO. We refer this to the crystal structure of the catalyst and the conductive properties.

Strategies to Improve CO Production Efficiency

The efficiency of carbon monoxide (CO) production by electrochemical reduction of carbon dioxide (CO2) is influenced by several factors, including catalyst activity and selectivity, electrode morphology, and electrolyte composition. To improve the efficiency of CO production, researchers have developed strategies to optimize these parameters through catalyst engineering, electrode design, and electrolyte optimization. By adjusting the properties of catalysts, electrodes, and electrolytes, it is possible to improve CO selectivity, Faradaic efficiency, and overall process performance [15].
Catalyst engineering is about the development and synthesis of catalyst materials with tailored properties to optimize CO2 reduction kinetics and selectivity. Strategies such as doping, alloying, and nanostructuring can increase catalyst activity and stability and, thus, improve the efficiency of CO production. Similarly, electrode design plays a crucial role in optimizing mass transfer, charge transfer, and surface area, which are essential for efficient CO2 electroreduction. By controlling the electrode morphology, composition, and architecture, it is possible to increase CO selectivity and minimize competing side reactions [15].
Optimizing electrolyte composition and operating conditions is another key strategy to improve CO production efficiency. Electrolytes play a critical role in facilitating CO2 reduction reactions by providing ions for charge transfer and stabilizing reaction intermediates. By adjusting the pH, concentration and composition of the electrolyte, reaction kinetics and product selectivity can be modulated. In addition, the optimization of operating parameters such as temperature, pressure, and flow rate can further improve the efficiency of CO2 electroreduction [3]. The reason behind this is that temperature increases reaction rates by providing the necessary thermal energy, which accelerates CO2 conversion. However, it must be carefully regulated to avoid overheating and damage to the system’s components. Pressure affects the CO2 concentration at the electrode by increasing its partial pressure, which enhances the reaction rate. Higher pressure improves CO2 availability but requires robust equipment to manage the added mechanical stresses. Flow rate ensures a steady supply of CO2 to the electrode and efficient removal of reaction products. Proper flow management helps maintain a high reactant concentration and prevents product buildup that could inhibit the reaction.

5. Role of Membranes in CO2 Electrolyzers

In CO2 electrolyzers, membranes are integral to the system’s ability to convert CO2 into valuable chemicals and fuels. They serve as selective barriers that separate the anode and cathode compartments, allowing ion transport while preventing the undesired crossover of gasses like CO2, H2, or O2. This selectivity is essential for high Faradaic efficiency (i.e., the efficiency of charge transfer in producing desired products) and for minimizing side reactions that can degrade the electrolyzer’s performance over time [34,35,36].
Three common classes of membranes are used in a CO2 flow reactions. Based on the ionic functional groups attached to polymer chains, ion exchange membranes (IEM) are divided into anion exchange membranes (AEM), cation exchange membranes (CEM), and bipolar membranes (BPM). The type of membrane has an influence on the pH of both sides of the membrane. Based on the pH impact, the reactant availability and reaction potential of cathodic as well as anodic reactions changes. CEM is used to transport cations from an acidic anode to the cathode. In contrast to the CEM, AEM transport anions from the basic cathode to the anode. BPM leads to dissociation of H2O in order to enable the transportation of H+ to the cathode and the OH- to the anode or transportation of H+ from the anode and OH- from the cathode and the formation of water at the center of membrane [37].

5.1. Proton Exchange Membranes (PEM)

Proton exchange membranes are commonly used in acidic electrolysis environments. They facilitate the movement of protons (H+) from the anode to the cathode while preventing other species, such as CO2, from crossing. Nafion® is a widely used PEM due to its high ionic conductivity and chemical stability under acidic conditions. However, environments in PEM cells can limit the choice of catalysts and present challenges for CO2 reduction due to possible catalyst degradation. PEMs also show CO2 crossover, which can affect reaction selectivity and reduce overall efficiency [38,39].
A cation exchange membrane is suitable for a zero-gap CO2 electrolyzer system. A big advantage is that carbonation can be avoided, and carbon efficiency is improved. Different cation exchange membranes are based on Nafion® membranes (111, 112, 115, 211, and 212) with different thicknesses. Higher FECO, as well as energy efficiency, is achieved by thinner Nafion® membranes. Non-acidic anolyte promotes the formation of carbonates and bicarbonates in the gas diffusion electrode, which would lead to clogging of the flow field channel as well as blocking of the catalytic surfaces. On the other hand, acidic anolyte remains an issue due to high hydrogen production [40].

5.1.1. Structure and Application of CEMs in CO2RR

CEMs are designed to selectively transport cations, such as protons (H+), while blocking anions. They are commonly employed in batch-type H-cell reactors for initial catalyst development, where CO2-saturated aqueous electrolytes are used. CEMs enable efficient conversion of CO2 into valuable products such as carbon monoxide (CO), formic acid (HCOOH), methane (CH4), and alcohols under near-neutral to alkaline conditions. Nafion™, a widely used PFSA-based CEM, has shown remarkable Faradaic efficiencies (FE) for these reactions, with up to 98% for formic acid and 97% for CO. While flow reactors have begun adopting CEMs, their application is still less prevalent compared to batch-type setups [36,41,42].
In flow electrolyzers, CEMs inhibit anionic product crossover, making them particularly suitable for producing formate (HCOO−). Novel cell designs leveraging the high proton conductivity of CEMs have achieved significant reductions in ohmic losses. For instance, using Nafion™ in a zero-gap configuration allows for high CO partial current densities with remarkably low cell voltages. Moreover, hybrid cell structures with buffer layers between the CEM and cathode provide opportunities to suppress undesired hydrogen evolution reactions (HER), albeit at the cost of increased energy losses due to additional resistance [43,44,45].

5.1.2. Proton Conductivity

The core functionality of CEMs lies in their ability to transport protons efficiently. Proton conductivity in these membranes is governed by mechanisms such as the Grotthuss mechanism, where protons “hop” between hydrolyzed anionic sites through the formation of hydronium ions. This mechanism is faster than vehicular transport, making it ideal for CEM-based systems. Factors influencing proton conductivity include the ion exchange capacity (IEC), water uptake, and the morphology of ionic channels. For example, perfluorosulfonic acid (PFSA)-based CEMs like Nafion™ exhibit high proton conductivities (>100 mS/cm) due to their phase-separated structure, which forms a highly interconnected network of hydrated ion channels [46].
Improving proton conductivity involves increasing the IEC, which enhances the density of ionic charges within the membrane. However, this must be balanced against potential drawbacks, such as excessive swelling and mechanical instability [36].

5.1.3. Cation Crossover and CO2RR Selectivity

In CO2RR systems, cation crossover is a critical factor influencing product selectivity. The transport of cations such as K+, Na+, and Cs+ through CEMs can affect CO2 reduction kinetics by altering the local electric field and enhancing CO2 adsorption. Research has demonstrated that using CEMs saturated with specific alkali cations can improve selectivity for certain products, such as CO, by suppressing proton transport and reducing competing reactions like HER [46,47,48,49].
However, cation crossover also poses challenges. For example, the leaching of substituted cations to the cathode can impact long-term stability and necessitate post-electrolysis purification. Achieving a balance between optimizing cation transport for selectivity, and minimizing undesired crossover remains an area of active research [47,48,49,50].

5.1.4. Product Crossover

Product crossover is a significant limitation in scaling CO2RR systems for industrial applications. Neutral products, such as alcohols, can diffuse through CEMs, contaminating the anolyte and increasing the complexity of downstream separation processes. Unlike anion exchange membranes (AEMs), CEMs exhibit electroosmotic drag (EOD) of water toward the cathode, mitigating anolyte contamination. However, this advantage is offset by challenges associated with high alcohol permeability and acidic product crossover [51,52,53,54,55,56].
Strategies to minimize product crossover include modifying PFSA membranes with inorganic materials, such as silica or zirconium phosphate, or employing nonfluorinated polymers with reduced permeability. Surface functionalization and polymer crosslinking are additional approaches to enhance selectivity and reduce crossover [36,57,58,59].

5.1.5. Stability and Durability

The long-term stability of CEMs is crucial for their viability in commercial CO2RR systems. Membrane degradation, often caused by radical-induced chain scission or excessive swelling, leads to a decline in ionic conductivity and mechanical integrity. In PEM water electrolyzers, strategies such as incorporating radical scavengers or using hydrocarbon-based membranes have shown promise in mitigating degradation. These approaches can be adapted for CEMs in CO2RR systems.
Hydrocarbon-based CEMs offer lower gas crossover rates but are more susceptible to free radical attacks than PFSA membranes. Advances in polymer chemistry, such as the development of sulfonated polyphenylenes, aim to address this trade-off by improving both chemical stability and ionic conductivity. However, achieving high stability while maintaining performance remains a key challenge [60,61].

5.1.6. Summary and Future Directions for CEM

CEMs have significant potential for enabling efficient and scalable CO2 electrolysis systems. Their high proton conductivity, low anionic crossover, and compatibility with various electrolyzer designs make them an attractive choice for CO2RR applications. However, challenges such as excessive swelling, neutral product crossover, and susceptibility to degradation under acidic conditions must be addressed [62,63].
Future research should focus on the following:
  • Material Innovations: Developing novel CEM materials with tailored properties, such as improved ionic selectivity, reduced permeability, and enhanced durability;
  • Cell Design Optimization: Integrating buffer layers or hybrid structures to suppress HER while minimizing energy losses;
  • Scalability and Cost Reduction: Reducing the reliance on expensive PFSA-based membranes by exploring nonfluorinated alternatives and scalable fabrication methods;
  • Stability Enhancements: Incorporating radical scavengers, surface modifications, and crosslinking to improve long-term performance.
In conclusion, while CEMs offer promising advantages for CO2RR, addressing their limitations through material science and engineering innovations will be critical for realizing their full potential in sustainable carbon utilization technologies [64,65].

5.2. Anion Exchange Membranes (AEM)

AEMs are typically employed in alkaline CO2 electrolyzers, where they allow hydroxide ions (OH−) to transport from the cathode to the anode. This alkaline environment has advantages for CO2 reduction reactions as it can improve reaction rates and enhance product selectivity, especially for multi-carbon products. However, AEMs face issues with stability in alkaline solutions, as they can degrade over time, particularly when exposed to high pH and carbonate ions that form when CO2 dissolves in the electrolyte. Efforts are underway to discover AEMs with improved chemical and mechanical stability by using novel polymer backbones and functional groups that resist degradation [36,66].
Membranes contain layers that conduct both H+ and OH− ions, allowing them to create a locally acidic environment at the cathode and an alkaline environment at the anode. This dual environment can help balance the pH gradients across the cell, which is beneficial for sustaining high activity in both half reactions. BPMs offer flexibility in reaction tuning, leading to higher selectivity for products like formate and ethylene, depending on catalyst choice and operating conditions. However, these membranes also face challenges related to water dissociation and require optimization to minimize energy losses associated with ion transport [36,67].
In Table 3 the comparison of different membrane types are described. By comparing CEMs, AEMs, and BPMs, CEMs has high conductivity but an excessive swelling as well as low faradaic efficiency. AEMs has a high faradaic efficiency, but the main concern is the product and the carbonate ion crossover. The BPM indicates minimal crossover but has a higher resistance as well as delamination [36].
The recent status of the IEM-based CO2RR reactors show cation exchange membranes as being mostly used in batch-type reactors. This process is used to test new developed electrocatalysts. In order to test the design and tuning of selectivity, H-cells also utilize CEM. The recent trend is that the CEM is also used in flow reactors to investigate and overcome the challenges associated with CEMs covering flux of protons toward the cathode during a continuous electrolysis producing hydrogen over the CO2RR [36].
In order to overcome the challenges related to CEMs, AEMs are also investigated, especially when alkaline conditions are chosen. Therefore, different alkaline stable and conductive AEM materials are developed. AEMs are more popularly chosen than CEMs by usage in flow CO2 electrolyzers [36]. In Table 4 the state-of-the-art performance metrics of flow CO2RR electrolyzers are summarized.
A new approach is to use an IEM as a BPM. This membrane has a laminating cation exchange layer as well as an anion exchange layer. Therefore, cationic and anionic mobile counterions are transported through their respective segments either to form water or transport from the interface where a rapid dissociation of water happens. BPMs are applied in reactors using liquid bicarbonate as feedstock [36].

5.2.1. Anion Exchange Membranes (AEMs)

Anion Exchange Membranes (AEMs) are key components in electrochemical systems such as fuel cells, electrolyzers, and CO2 reduction cells. Their efficiency and stability are critical in determining the overall performance of these systems, especially in applications that operate under high pH conditions. A thorough understanding of the behavior of AEMs in these environments reveals several important aspects, which include selectivity in CO2 reduction reactions (CO2RR), rapid carbonation reactions, ionic conductivity, product crossover, and system stability [36].

5.2.2. CO2RR Selectivity Under High-pH Conditions

One of the central concerns in using AEMs for CO2RR is maintaining high selectivity for the desired products under high pH conditions. At an elevated pH, the solubility and the reactivity of CO2 changes significantly. AEMs, which are typically used to transport anions such as hydroxide ions (OH−), facilitate CO2RR by enabling the transport of bicarbonate (HCO3−) and carbonate (CO32−) ions within the electrolyte. However, in high-pH environments, the increased presence of OH− ions can compete with CO2, potentially leading to undesirable side reactions. Optimizing AEMs involves adjusting their composition and structure to reduce such competition, thereby increasing the efficiency and selectivity for CO2RR products such as carbon monoxide (CO), methane (CH4), or other carbon-based chemicals [77,78,79,80,81].

5.2.3. Rapid Carbonation Reactions

AEMs are susceptible to carbonation reactions due to the affinity of the membrane material for CO2. These carbonation reactions can significantly affect the membrane’s performance by altering its ionic conductivity and mechanical integrity. Under high-pH conditions, where CO2 is readily available, the formation of bicarbonate and carbonate ions is enhanced. The reaction between CO2 and the membrane material often leads to the formation of ionic clusters that can increase resistance and reduce the overall ionic conductivity of the AEM. Thus, controlling the rate of carbonation is critical to maintain the efficiency of the electrochemical process. Strategies to mitigate carbonation include modifying the chemical structure of the membrane or employing additives that hinder CO2 absorption without compromising ionic conductivity [50,82].

5.2.4. Ionic Conductivity

Ionic conductivity is a key performance indicator for AEMs, as it directly impacts the efficiency of the electrochemical reactions. AEMs rely on the mobility of anions (typically hydroxide or carbonate ions) for ion transport within the system. High ionic conductivity is crucial for minimizing ohmic losses and ensuring efficient electron transfer across the system. However, the ionic conductivity of AEMs can be significantly impacted by high-pH conditions due to the formation of insoluble carbonate species or by changes in membrane structure resulting from carbonation. The development of AEMs with enhanced ionic conductivity under such harsh conditions is a primary goal of ongoing research. This can be achieved by modifying the polymer backbone, incorporating highly conductive ionic groups, or introducing fillers that promote ion transport without introducing detrimental effects on the membrane’s mechanical properties [50,83,84,85].

5.2.5. Product Crossover

In electrochemical systems, product crossover refers to the undesired transport of reaction products across the membrane from one side of the electrochemical cell to the other. In AEM-based systems, the main concern is the crossover of products such as CO, H2, or other gasses, which can lead to efficiency losses and degradation of the system. High-pH conditions can exacerbate this issue by increasing the solubility of certain products or by altering the properties of the AEM. To minimize product crossover, it is essential to design membranes with appropriate ion-exchange capacities, porosities, and surface chemistries that can selectively allow the passage of ions without permitting gasses or other products to diffuse through the membrane. Enhancing the selectivity of AEMs is critical for ensuring high product yields and maintaining the overall system’s stability and efficiency [86,87].

5.2.6. System Stability

The long-term stability of AEMs under operating conditions is another important factor. High-pH environments, especially when combined with the presence of CO2, can degrade the membrane materials over time. This degradation can manifest as mechanical breakdown, reduced ionic conductivity, or increased susceptibility to carbonation. In CO2RR applications, maintaining membrane integrity over long periods is crucial for ensuring stable and reliable performance. Strategies to enhance system stability include optimizing the chemical composition of the membrane, utilizing protective coatings, or incorporating stabilizing agents that prevent the formation of harmful ionic species. Additionally, improving the robustness of the membrane under high-pH conditions helps to extend the lifespan of the electrochemical system and reduce maintenance costs [36,88].

5.2.7. Summary and Future Directions for AEMs

In summary, AEMs play a critical role in electrochemical applications, especially in CO2 reduction reactions, by facilitating the transport of anions like hydroxide and carbonate. Their performance is influenced by a variety of factors, including the selectivity for desired products, the occurrence of carbonation reactions, their ionic conductivity, the potential for product crossover, and their overall stability under high-pH conditions. Addressing these challenges requires continuous advancements in membrane design, material science, and process optimization to improve selectivity, minimize side reactions, and enhance long-term stability. As research progresses, it is expected that AEMs will continue to evolve, offering more efficient and sustainable solutions for CO2RR and other electrochemical applications [36].

5.3. Bipolar Membranes (BPMs)

Bipolar membranes (BPMs) are an emerging class of ion exchange membranes (IEMs) used in electrochemical systems, particularly for CO2 reduction reactions (CO2RR). BPMs are composed of a cation exchange layer (CEL) and an anion exchange layer (AEL), which together form a unique structure capable of supporting different pH environments on either side of the membrane, crucial for CO2RR processes. This pH difference is established through the dissociation of water at the interface between the CEL and AEL, driven by an electric field. Depending on the direction of the applied voltage, BPMs can operate with reverse or forward bias, each mode having specific advantages and challenges for CO2RR [89,90,91,92].
Water Dissociation and Overpotentials: The reverse bias mode of BPMs facilitates water dissociation at the CEL-AEL interface, generating protons and hydroxide ions that are transported across the respective layers. This electrochemical process is essential for maintaining ionic conductivity, especially under conditions where water splitting is necessary, such as in CO2RR or water electrolysis. However, the overpotentials associated with water dissociation remain significant, particularly at current densities above 20 mA cm−2. For optimal performance, efficient water dissociation catalysts are needed at the interface to lower the overpotentials, as these can exceed 100 mV even at moderate current densities. Studies have shown that employing specific metal oxide or polymer-based catalysts at the CEL-AEL interface can improve the efficiency of water dissociation, enabling better performance under high current densities [93,94].
Anionic and Neutral Product Crossover: A significant advantage of BPMs in CO2RR reactors is their ability to limit the crossover of anionic species such as HCO3− and CO32−, which can negatively affect reactor performance. In reverse bias, the protonation of bicarbonate and carbonate species helps prevent their crossover to the anode, facilitating CO2 capture. This selective exclusion is essential in ensuring the efficiency of CO2RR, where the presence of unwanted ions can decrease the Faradaic efficiency (FE). However, under forward bias, neutral products and CO2 produced at the interface can lead to accumulation issues if not efficiently removed, causing potential membrane degradation [36,85,95,96].
Mechanical Degradation and Delamination: The mechanical stability of BPMs is a crucial consideration in their development, particularly under harsh electrochemical conditions. The different chemical and physical properties of the CEL and AEL can lead to mechanical degradation, such as delamination of the two layers. This can occur due to internal stressors like temperature fluctuations, pressure differentials, and water accumulation at the interface, particularly during forward bias operation. Delamination is further exacerbated by the formation of water and CO2 at the interface in the forward bias, which can cause structural breakdown if not properly managed. Research efforts have focused on improving the adhesion between the layers, using strategies such as electrospinning to enhance the interfacial area and prevent delamination. Additionally, the differential thickness of the layers has been explored to mitigate water transport limitations and reduce mechanical degradation [89].
Recent Developments and Applications: Recent studies have explored novel approaches to improve BPM performance. For instance, electrospinning has been used to create three-dimensional (3D) BPM structures that offer improved mechanical integrity and catalytic performance. The 3D structure increases the interfacial area, which is crucial for effective water dissociation and for preventing localized dehydration. These advanced BPMs are designed to accommodate high-current density operations, which are necessary for industrial-scale CO2RR reactors. Moreover, modifications to the membrane structure, such as roughening the interfaces between the CEL and AEL, have shown promise in reducing mechanical failures and enhancing performance under high-stress conditions [36].
Challenges and Future Directions: Despite the potential of BPMs, several challenges remain in their application for CO2RR. The high cell voltages required in reverse bias configurations, especially under conditions of low pH at the cathode, limit their efficiency. Additionally, the low Faradaic efficiencies observed in CO2RR reactors using BPMs, due to parasitic hydrogen evolution reactions (HER) at the cathode, must be addressed. However, the use of liquid CO2 feedstocks and strategies to improve the local pH environment at the cathode can help mitigate some of these challenges. Further development of water dissociation catalysts, membrane design improvements, and reactor configurations will be necessary to make BPMs a viable option for CO2RR at industrial scales [36].

Summary of the Ion Exchange Membranes

BPMs represent a promising alternative to traditional anion exchange membranes (AEMs) and cation exchange membranes (CEMs) for CO2RR applications. Their ability to control pH at both the anode and cathode, minimize product crossover, and provide a mechanism for water dissociation at the CEL-AEL interface makes them particularly suitable for CO2RR systems. However, challenges related to overpotentials, mechanical degradation, and the need for efficient water dissociation catalysts remain significant hurdles to their commercial application. Future research focusing on improving BPM performance, including the development of more robust materials, better catalysts, and optimized reactor designs, will be essential to realizing their full potential in CO2 reduction and other electrochemical applications.

5.4. Performance and Challenges

The performance of electrolyzers depends heavily on membrane characteristics such as ionic conductivity, selectivity, chemical stability, and gas separation properties [36].
Ion Conductivity: High conductivity reduces resistive losses, allowing more efficient transport of ions without needing high operating voltages, which can otherwise lead to undesirable side reactions [36].
Chemical Stability: The acidic and alkaline environments in PEM and AEM electrolyzers, respectively, present harsh conditions that can degrade membrane materials over time. Improvements in stability are crucial for reducing maintenance costs and enhancing long-term durability [36,97].
Gas Crossover: Preventing the crossover of CO2 and H2 for maintaining product purity and minimizing side reactions. Developing membranes with low gas permeability but high ionic conductivity is an ongoing research focus [36,98].
Membrane degradation from high current densities and carbonate buildup (especially in alkaline environments with AEMs and BPMs) can affect system stability. For example, carbonate formation in AEMs reduces CO2 availability for the reduction reaction, leading to lower overall efficiency [36].

6. Challenges and Future Directions

Identifying the key challenges hindering the widespread adoption of electrochemical CO2 reduction technologies is important.
Despite the considerable progress that has been made in electrochemical carbon dioxide (CO2) reduction, there are a number of challenges that hinder the adoption of this technology for CO production on an industrial scale. These challenges include technical, economic, and regulatory barriers that need to be addressed in order to realize the full potential of electrochemical CO2 reduction as a sustainable route to CO production. Regulatory barriers that hinder the adoption of electrochemical CO2 reduction technology include the lack of specific financial incentives, inconsistent regulations across regions, and stringent environmental standards. Additionally, complex carbon emission reporting requirements, restrictions on CO2 storage, lengthy approval processes, and intellectual property issues further complicate the technology’s widespread implementation. These obstacles collectively impact the technology’s feasibility and delay its industrial adoption.
Key research challenges include the development of efficient and selective catalysts, optimization of reactor design and operation, integration with renewable energy sources, and scalability of CO2 electroreduction processes.
One of the biggest challenges in CO2 electroreduction is the development of catalysts that exhibit high activity, selectivity, and stability under realistic operating conditions. Despite advances in catalyst synthesis and characterization, precise control of reaction pathways and product distributions remain a challenging task. In addition, the integration of CO2 electroreduction with renewable energy sources such as solar and wind energy poses logistical and technical challenges related to intermittency, grid integration, and energy storage.
Economic considerations also play an important role in the commercial viability of CO2 electroreduction technologies. The high capital costs associated with electrochemical reactors, catalyst materials, and infrastructure pose financial barriers to widespread adoption. In addition, uncertainties related to market demand, regulatory frameworks, and political incentives complicate investment decisions and technology deployment strategies.
The economic assessment of electrochemical CO2 reduction methods in comparison with non-electrochemical CO production technologies is a crucial area of study. Such analyses provide valuable insights into the cost competitiveness and scalability of these emerging technologies. By evaluating the potential economic advantages of electrochemical methods, researchers can better understand their viability and the factors driving their adoption. Moving forward, it is essential to prioritize this aspect to comprehensively highlight the practical implications and transformative potential of electrochemical CO2 reduction in addressing global CO production demands.

7. Conclusions

In summary, the electrochemical reduction of carbon dioxide (CO2) represents a promising pathway for sustainable carbon monoxide (CO) production with implications for climate change mitigation, industrial decarbonization, and renewable energy integration. By utilizing renewable electricity and captured CO2 emissions, electrochemical CO2 reduction enables the synthesis of CO with high efficiency and selectivity, reducing dependence on fossil fuels and contributing to a circular carbon economy.
In this review, we have examined the mechanisms, catalysts, strategies, and applications of electrochemical CO2 reduction, highlighting its potential to address pressing environmental issues while creating economic value. A particular focus was placed on the critical role of ion exchange membranes (IEMs), including cation exchange membranes (CEMs), anion exchange membranes (AEMs), and bipolar membranes (BPMs). We discussed their advantages, such as improved ion transport and system stability, as well as their drawbacks, including material degradation and ion crossover. The current challenges associated with integrating these membranes into CO2 electroreduction systems, such as cost, scalability, and long-term performance, were also highlighted.
In the future, continued investment in research, development, and commercialization is essential to overcome the remaining challenges and realize the full potential of electrochemical CO2 reduction technologies. This includes advancing the design, functionality, and durability of ion exchange membranes, which are key to optimizing reactor performance and CO production efficiency. By fostering collaboration between academia, industry, and government, it is possible to accelerate innovation, scale up production, and create new markets for CO-derived products. Ultimately, the transition towards sustainable CO production requires concerted efforts from all stakeholders to build a resilient, resource-efficient, and low-carbon economy for future generations.

Author Contributions

Conceptualization, M.R., H.R. and A.A.; methodology, M.R. and H.R.; validation, A.A.; formal analysis, H.R. and A.A.; investigation, H.R. and A.A., data curation, H.R. and A.A.; writing—original draft preparation, H.R.; writing—review and editing, A.A. and M.R.; visualization, H.R. and A.A.; supervision, M.R.; project administration, M.R.; funding acquisition, M.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Österreichische Forschungsförderungsgesellschaft (FFG) (eCall number 48671894, Förderungsansuchen: Frontrunner: Electrochemical Conversion of CO2 into Valuable Basic Chemicals and Fuels FFG, Projektnummer: FO999909936, Ausschreibung: Frontrunner 2023).

Acknowledgments

We would like to acknowledge the generous funding provided by Österreichische Forschungsförderungsgesellschaft (FFG), which made this research possible (Förderungsansuchen: Frontrunner: Electrochemical Conversion of CO2 into Valuable Basic Chemicals and Fuels FFG, Projektnummer: FO999909936, eCall Antragsnummer: 48671894, Ausschreibung: Frontrunner 2023).

Conflicts of Interest

The all authors are employed by GIG Karasek GmbH.

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Figure 1. CO2 Emissions in Austria from 1990 to 2022 [1].
Figure 1. CO2 Emissions in Austria from 1990 to 2022 [1].
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Figure 2. Principle of CO2 electrolyzer. Reprinted with permission from Ref. [7]. Copyright The Author(s) 2018. Published by ECS.
Figure 2. Principle of CO2 electrolyzer. Reprinted with permission from Ref. [7]. Copyright The Author(s) 2018. Published by ECS.
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Figure 3. The proposed mechanism on electrochemical CO2 reduction. Reprinted with permission from Ref. [12]. Copyright 2022 Lin, J., Yan, S., Zhang, C., Hu, Q., & Cheng, Z.
Figure 3. The proposed mechanism on electrochemical CO2 reduction. Reprinted with permission from Ref. [12]. Copyright 2022 Lin, J., Yan, S., Zhang, C., Hu, Q., & Cheng, Z.
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Figure 4. Types of electrocatalysts. Reprinted with permission from Ref. [13]. Copyright© 2020 The Author(s).
Figure 4. Types of electrocatalysts. Reprinted with permission from Ref. [13]. Copyright© 2020 The Author(s).
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Figure 5. Summary of CO2RR results reported in the literature [18,22,23,24,25,26,27,28,29,30,31,32,33].
Figure 5. Summary of CO2RR results reported in the literature [18,22,23,24,25,26,27,28,29,30,31,32,33].
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Table 1. Comparison of electrochemical and photochemical CO2 reduction technologies.
Table 1. Comparison of electrochemical and photochemical CO2 reduction technologies.
AspectElectrochemical CO2 ReductionPhotochemical CO2 Reduction
Advantages
Selectivity and EfficiencyHigh selectivity towards specific products like CO or ethylene, especially with advanced catalysts (e.g., copper-based) [9,10].Utilizes solar energy directly, offering a potentially more sustainable approach if efficient catalysts and systems are developed [9].
ScalabilityEasier to scale and integrate with renewable energy sources such as solar and wind, making it suitable for large-scale applications [10].Potential for simpler system design, particularly in regions with abundant sunlight [9].
Energy Input ControlPrecise control over applied voltage allows optimization of the reduction process and management of energy input, enhancing efficiency [9].Direct harnessing of sunlight can make the process more cost-effective and sustainable if material and system efficiency is improved [9].
Disadvantages
Energy DemandHigh energy requirements, especially for producing multi-carbon products, which necessitate significant energy input [10].Lower efficiency and selectivity compared to electrochemical methods, limiting the production of high-value products at scale [9].
System ComplexityRequires complex system designs with precise control over reaction conditions, adding to the operational complexity and cost [9].Highly dependent on sunlight availability and intensity, making it less reliable in regions with variable sunlight exposure [9].
Material DegradationElectrodes can degrade over time in harsh conditions, leading to increased costs and reduced system efficiency [9].Photocatalysts may degrade under prolonged sunlight exposure, reducing system lifespan and increasing maintenance needs [9].
Environmental ImpactDepending on the energy source, the process may have a high environmental impact if non-renewable electricity is used [9].Potentially lower environmental impact if stable and efficient photocatalysts that are environmentally benign are developed and utilized [9].
Table 2. Summary of Cu-based catalysts [20].
Table 2. Summary of Cu-based catalysts [20].
CatalystsProductsPotential/
V vs. RHE		Current Density/
mA cm−2		FE/%Electrolyte
Cu-NPsCH4−1.35~9760.1 M NaHCO3
CuPcC2H4−1.382.8250.5 M KCl
CuDAT-wire samplesC2H4
C2H5OH		−0.89040% C2H4, 20% C2H5OH1 M KHCO3
Cu-N-substituted arylpyridiniumC2H4
C2H5OH		−1.11.0278.2 (C > 1)0.1 M KHCO3 + 10 mM additive
Cu(II)-NU-1000Formate
CO		−0.8 to −1.01.2300.1 M NaClO4
Cu/SnO2CO−0.74.6930.5 M KHCO3
AuCu alloy nanoparticlesCO−0.771.39800.1 M KHCO3
Cu dendriteC2H4, C2H6−0.85–7551 M Na2SO4
N-C/CuC2+−0.86300931 M KOH
Oxygen-bearing CuC2H4−1.044.7450.5 M KHCO3
Sn-Cu/SnOxFormate and CO−0.7243981 M KOH
Cu2O@CuHHTPCH4−1.411730.1 M KCl
0.1 M KHCO3
Cu2SHCOOH−0.919870.1 M NaHCO3
Cu2O/Cu2SHCOOH−0.915.367.60.1 M KHCO3
Table 3. Comparision of different membrane types including properties [34,35,36,37,38,39,40].
Table 3. Comparision of different membrane types including properties [34,35,36,37,38,39,40].
Membrane TypeTransported IonOperating pHConductivityThicknessAdvantagesChallenges
PEMH+Acidic50–100 mS/cm at 80 °C20–50 µmHigh ionic conductivity; effective in suppressing side reactions; suitable for compact designs.Limited stability in acidic conditions; catalyst options limited by acidic environment; CO2 crossover.
AEMOH−Alkaline20–80 mS/cm at room temperature25–100 µmFavorable for CO2 reduction rates and multi-carbon product selectivity; reduced catalyst degradation in alkaline environment.Vulnerable to carbonate buildup; long-term stability issues in high-pH environments.
BPMH+ and OH−Acidic/Alkaline (bipolar)5–20 mS/cm per layer (H+ and OH− layers)50–200 µm (combined thickness)Allows local pH tuning (acidic at one electrode, alkaline at the other); improved selectivity for target products.Water dissociation increases energy loss; sensitivity to ion imbalance and membrane swelling.
Table 4. State-of-the-art performance metrics of flow CO2RR electrolyzers using different types of IEMs.
Table 4. State-of-the-art performance metrics of flow CO2RR electrolyzers using different types of IEMs.
IEMIEM TypeCathode FeedAnode FeedBase Metal of CO2RR CatalystEcell/VMain Product (FE/%)Total Current Density /(mA cm−2)StabilityRef.
Nafion™ 117CEMCO2 + 2 M KHCO32 M KOHAg3.9–4.9CO (90)22010 h, r. t. b(room temperature)[68]
NafionTM 117CEMCO2 + 2 M KCl2 M KOHCu3.30C2+ (80)15030 h, r. t.[69]
NafionTM 212CEMDry CO2Humidified H2Sn2.47HCOOH (72)3851 h, r. t.[69]
PiperION TP-85AEMHumidified CO20.1 M CsOHAg3.20CO (85)>1000100 h (60 °C)[70]
AemionTM AF1-CNN8-60-XAEMHumidified CO20.1 M KHCO3Cu3.75CH3OH + C3H7OH (–)150100 h (40 °C)[71]
Sustainion® X37-50AEMHumidified CO20.01 M KHCO3Ag2.89CO (99)100>3000 h (hours)[72]
SelemionTM DSVNAEMCO2 + 1 M KHCO31 M KHCO3Ni–CO (98)40030 h[73]
QAPPTAEMCO2 + 10 mM CsOH10 mM CsOHAg3.3CO (90)550100 h[74]
Fumasep® FBMBPM3 M KHCO31 M KOHAg3.5CO (82)1008 h, r. t.[75]
Fumasep® FBM-PKBPMCO2 (40 bar) + 0.5 M KHCO31 M KOHSn3.5HCOOH (90)301/3 h, r. t.[76]
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Rehberger, H.; Rezaei, M.; Aljabour, A. Challenges and Opportunities of Choosing a Membrane for Electrochemical CO2 Reduction. Membranes 2025, 15, 55. https://doi.org/10.3390/membranes15020055

AMA Style

Rehberger H, Rezaei M, Aljabour A. Challenges and Opportunities of Choosing a Membrane for Electrochemical CO2 Reduction. Membranes. 2025; 15(2):55. https://doi.org/10.3390/membranes15020055

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Rehberger, Helene, Mohammad Rezaei, and Abdalaziz Aljabour. 2025. "Challenges and Opportunities of Choosing a Membrane for Electrochemical CO2 Reduction" Membranes 15, no. 2: 55. https://doi.org/10.3390/membranes15020055

APA Style

Rehberger, H., Rezaei, M., & Aljabour, A. (2025). Challenges and Opportunities of Choosing a Membrane for Electrochemical CO2 Reduction. Membranes, 15(2), 55. https://doi.org/10.3390/membranes15020055

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