Impact of Alkali Cation Identity on the Conversion of HCO 3− to CO in Bicarbonate Electrolyzers

Aryl ether-free polymer electrolytes for electrochemical and energy devices

Anion exchange polymers (AEPs) play a crucial role in green hydrogen production through anion exchange membrane water electrolysis. The chemical stability of AEPs is paramount for stable system operation in electrolysers and other electrochemical devices. Given the instability of aryl ether-containing AEPs under high pH conditions, recent research has focused on quaternized aryl ether-free variants. The primary goal of this review is to provide a greater depth of knowledge on the synthesis of aryl ether-free AEPs targeted for electrochemical devices. Synthetic pathways that yield polyaromatic AEPs include acid-catalysed polyhydroxyalkylation, metal-promoted coupling reactions, ionene synthesis via nucleophilic substitution, alkylation of polybenzimidazole, and Diels-Alder polymerization. Polyolefinic AEPs are prepared through addition polymerization, ring-opening metathesis, radiation grafting reactions, and anionic polymerization. Discussions cover structure-property-performance relationships of AEPs in fuel cells, redox flow batteries, and water and CO2 electrolysers, along with the current status of scale-up synthesis and commercialization.

CO2 Electrolyzers

CO2 electrolyzers have progressed rapidly in energy efficiency and catalyst selectivity toward valuable chemical feedstocks and fuels, such as syngas, ethylene, ethanol, and methane. However, each component within these complex systems influences the overall performance, and the further advances needed to realize commercialization will require an approach that considers the whole process, with the electrochemical cell at the center. Beyond the cell boundaries, the electrolyzer must integrate with upstream CO2 feeds and downstream separation processes in a way that minimizes overall product energy intensity and presents viable use cases. Here we begin by describing upstream CO2 sources, their energy intensities, and impurities. We then focus on the cell, the most common CO2 electrolyzer system architectures, and each component within these systems. We evaluate the energy savings and the feasibility of alternative approaches including integration with CO2 capture, direct conversion of flue gas and two-step conversion via carbon monoxide. We evaluate pathways that minimize downstream separations and produce concentrated streams compatible with existing sectors. Applying this comprehensive upstream-to-downstream approach, we highlight the most promising routes, and outlook, for electrochemical CO2 reduction.

Reactive Carbon Capture Enables CO2 Electrolysis with Liquid Feedstocks

ConspectusThe electrochemical reduction of carbon dioxide (CO2RR) is a promising strategy for mitigating global CO2 emissions while simultaneously yielding valuable chemicals and fuels, such as CO, HCOO-, and C2H4. This approach becomes especially appealing when integrated with surplus renewable electricity, as the ensuing production of fuels could facilitate the closure of the carbon cycle. Despite these advantages, the realization of industrial-scale electrolyzers fed with CO2 will be challenged by the substantial energy inputs required to isolate, pressurize, and purify CO2 prior to electrolysis.To address these challenges, we devised an electrolyzer capable of directly converting reactive carbon solutions (e.g., a bicarbonate-rich eluent that exits a carbon capture unit) into higher value products. This "reactive carbon electrolyzer" operates by reacting (bi)carbonate with acid generated within the electrolyzer to produce CO2 in situ, thereby facilitating CO2RR at the cathode. This approach eliminates the need for expensive CO2 recovery and compression steps, as the electrolyzer can then then coupled directly to the CO2 capture unit.This Account outlines our endeavors in developing this type of electrolyzer, focusing on the design and implementation of materials for electrocatalytic (bi)carbonate conversion. We highlight the necessity for a permeable cathode that allows the efficient transport of (bi)carbonate ions while maintaining a sufficiently high catalytic surface area. We address the importance of the supporting electrolyte, detailing how (bi)carbonate concentration, counter cations, and ionic impurities impact selectivity for products formed in the electrolyzer. We also catalog state-of-the-art performance metrics for reactive carbon electrolyzers (i.e., Faradaic efficiency, full cell voltage, CO2 utilization efficiency) and outline strategies to bridge the gap between these values and those required for commercial operation Collectively, these findings contribute to the ongoing efforts to realize industrial-scale electrochemical reactors for CO2 conversion, bringing us closer to a sustainable and closed-loop carbon cycle.

MOF-Derived Ni Single-Atom Catalyst with Abundant Mesopores for Efficient Mass Transport in Electrolytic Bicarbonate Conversion

Direct electrolytic CO₂ capture solution (e.g., bicarbonate), which bypasses the energy-intensive processes of CO₂ desorption, offers a unique route for CO₂ conversion to fuels or value-added chemicals. Nonprecious Ni single-atom catalysts (SACs) anchored on metal-organic frameworks (MOFs) possess abundant porous structures and exhibit a high selectivity for CO production. However, these MOF-derived Ni SACs are usually synthesized by a series of complex procedures, and their abundant micropores (<2 nm) also reduce the local reactant transport in the catalysts. Herein, we report a simple one-step pyrolysis method to prepare a MOF-derived Ni SAC that can efficiently boost bicarbonate conversion to CO. The abundant mesopores around 35.4 nm significantly enhance the transport of local reactants in the catalysts. At a high current density of 100 mA/cm², the tailored catalyst shows 67.2% Faradaic efficiency of CO, which, to the best of our knowledge, exceeds the state-of-the-art precious Ag nanoparticle catalysts reported so far. This study highlights the significance of developing nonprecious catalysts for employment in large-scale bicarbonate electrolysis conversion devices.

Bicarbonate Electroreduction to Multicarbon Products Enabled by Cu/Ag Bilayer Electrodes and Tailored Microenviroments

Bicarbonate electrolyzer can achieve the direct conversion of CO₂ capture solutions that bypasses energy-intensive steps of CO₂ regeneration and pressurization. However, only single-carbon chemicals (i. e., CO, formate, CH₄ ) were reported as the major products so far. Herein, bicarbonate conversion to multicarbon (C₂₊ ) products (i. e., acetate, ethylene, ethanol, propanol) was achieved on rationally designed Cu/Ag bilayer electrodes with bilayer cation- and anion-conducting ionomers. The in-situ generated CO₂ was first reduced to CO on the Ag layer, followed by its favorable further reduction to C₂₊ products on the Cu layer, benefiting from the locally high concentration of CO. Through optimizing the bilayer configurations, metal compositions, ionomer types, and local hydrophobicity, a microenvironment was created (high local pH, low water content, etc.) to enhance bicarbonate-to-C₂₊ conversion and suppress the hydrogen evolution reaction. Subsequently, a maximum C₂₊ faradaic efficiency of 41.6±0.39 % was achieved at a considerable current density of 100 mA cm^-2 .

Investigating the role of potassium cations during electrochemical CO2 reduction

The specific identity of electrolyte cations has many implications in various electrochemical reactions. However, the exact mechanism by which cations affect electrochemical reactions is not agreed upon in the literature. In this report, we investigate the role of cations during the electrochemical reduction of CO₂ by chelating the cations with cryptands, to change the interaction of the cations with the components of the electric double layer. As previously reported we do see the apparent suppression of CO₂ reduction in the absence of cations. However, using in situ-SEIRAS we see that CO₂ reduction does indeed take place albeit at very reduced scales. We also observe that cations play a role in tuning the absorption strengths of not only CO₂ as has been speculated, but also that of reaction products such as CO.

Advancing integrated CO2 electrochemical conversion with amine-based CO2 capture: a review

Carbon dioxide (CO₂) electrolysis is a promising route to utilise captured CO₂ as a building block to produce valuable feedstocks and fuels such as carbon monoxide and ethylene. Very recently, CO₂ electrolysis has been proposed as an alternative process to replace the amine recovery unit of the commercially available amine-based CO₂ capture process. This process would replace the most energy-intensive unit operation in amine scrubbing while providing a route for CO₂ conversion. The key enabler for such process integration is to develop an efficient integrated electrolyser that can convert CO₂ and recover the amine simultaneously. Herein, this review provides an overview of the fundamentals and recent progress in advancing integrated CO₂ conversion in amine-based capture media. This review first discusses the mechanisms for both CO₂ absorption in the capture medium and electrochemical conversion of the absorbed CO₂. We then summarise recent advances in improving the efficiency of integrated electrolysis via innovating electrodes, tailoring the local reaction environment, optimising operation conditions (e.g., temperatures and pressures), and modifying cell configurations. This review is concluded with future research directions for understanding and developing integrated CO₂ electrolysers.

Electrolytic conversion of carbon capture solutions containing carbonic anhydrase

The electrolysis of carbon capture solutions bypasses energy-intensive CO₂ recovery steps that are often required to convert CO₂ into value-added products. We report herein an electrochemical flow reactor that converts carbon capture solutions containing carbonic anhydrase enzymes into carbon-based products. Carbonic anhydrase enzymes benefit CO₂ capture by increasing the rate of reaction between CO₂ and weakly alkaline solutions by 20-fold. In this study, we reduced CO₂-enriched bicarbonate solutions containing carbonic anhydrase ("enzymatic CO₂ capture solutions") into CO at current densities of 100 mA cm^-2. This result demonstrated how to electrolyse enzymatic CO₂ capture solutions, but the selectivity for CO production was two-thirds less than bicarbonate solutions without carbonic anhydrase. This reduction in performance occurred because carbonic anhydrase deactivated the catalyst surface. A carbon microporous layer was found to suppress this deactivation.

Engineering Catalyst-Electrolyte Microenvironments to Optimize the Activity and Selectivity for the Electrochemical Reduction of CO2 on Cu and Ag

The electrochemical reduction of carbon dioxide (CO₂R) driven by renewably generated electricity (e.g., solar and wind) offers a promising means for reusing the CO₂ released during the production of cement, steel, and aluminum as well as the production of ammonia and methanol. If CO₂ could be removed from the atmosphere at acceptable costs (i.e., <$100/t of CO₂), then CO₂R could be used to produce carbon-containing chemicals and fuels in a fully sustainable manner. Economic considerations dictate that CO₂R current densities must be in the range of 0.1 to 1 A/cm² and selectivity toward the targeted product must be high in order to minimize separation costs. Industrially relevant operating conditions can be achieved by using gas diffusion electrodes (GDEs) to maximize the transport of species to and from the cathode and combining such electrodes with a solid-electrolyte membrane by eliminating the ohmic losses associated with liquid electrolytes. Additionally, high product selectivity can be attained by careful tuning of the microenvironment near the catalyst surface (e.g., the pH, the concentrations of CO₂ and H₂O, and the identities of the cations in the double layer adjacent to the catalyst surface).We begin this Account with a discussion of our experimental and theoretical work aimed at optimizing catalyst microenvironments for CO₂R. We first examine the effects of catalyst morphology on the production of multicarbon (C₂₊) products over Cu-based catalysts and then explore the role of mass transfer combined with the kinetics of buffer reactions in the local concentration of CO₂ and pH at the catalyst surface. This is followed by a discussion of the dependence of the local CO₂ concentration and pH on the dynamics of CO₂R and the formation of specific products over both Cu and Ag catalysts. Next, we explore the impact of electrolyte cation identity on the rate of CO₂R and the distribution of products. Subsequently, we look at utilizing pulsed electrolysis to tune the local pH and CO₂ concentration at the catalyst surface. The last part of the discussion demonstrates that ionomer-coated catalysts in combination with pulsed electrolysis can enable the attainment of very high (>90%) selectivity to C₂₊ products over Cu in an aqueous electrolyte. This part of the Account is then extended to consider the difference in the catalyst-nanoparticle microenvironment, present in the catalyst layer of a membrane electrode assembly (MEA), with respect to that of a planar electrode immersed in an aqueous electrolyte.

Cation-Driven Increases of CO2 Utilization in a Bipolar Membrane Electrode Assembly for CO2 Electrolysis

Advancing reaction rates for electrochemical CO₂ reduction in membrane electrode assemblies (MEAs) have boosted the promise of the technology while exposing new shortcomings. Among these is the maximum utilization of CO₂, which is capped at 50% (CO as targeted product) due to unwanted homogeneous reactions. Using bipolar membranes in an MEA (BPMEA) has the capability of preventing parasitic CO₂ losses, but their promise is dampened by poor CO₂ activity and selectivity. In this work, we enable a 3-fold increase in the CO₂ reduction selectivity of a BPMEA system by promoting alkali cation (K⁺) concentrations on the catalyst's surface, achieving a CO Faradaic efficiency of 68%. When compared to an anion exchange membrane, the cation-infused bipolar membrane (BPM) system shows a 5-fold reduction in CO₂ loss at similar current densities, while breaking the 50% CO₂ utilization mark. The work provides a combined cation and BPM strategy for overcoming CO₂ utilization issues in CO₂ electrolyzers.