Substantial progress has recently been made in the general understanding of the interfacial CO2 reduction reaction (CO2RR) in electro- and photocatalysis, but the influence of the local chemical environment and its effects on the catalytic interface at the molecular level remains largely elusive. Here, we introduce a classification scheme to group different aspects influencing the interfacial CO2RR thermodynamics and kinetics. This categorization allows a systematic survey of the literature focusing on the local chemical environment encompassing surface effects (adsorbates, support), solution interactions (electrolyte constituents) and three-dimensional chemical surroundings (polymers, metal organic frameworks (MOFs), covalent organic frameworks (COFs)). The review concludes with an outlook discussing possible future concepts for next-generation electrocatalytic and photocatalytic CO2RR. The electro- and photo-catalytic reduction of carbon dioxide are important processes in the context of developing a sustainable carbon-neutral economy. In this Review Article, the authors discuss how the local chemical environment in the proximity of the catalytic active site can influence the reactivity and selectivity of the processes and detail different approaches to achieve their modulation.

Towards molecular understanding of local chemical environment effects in electro- and photocatalytic CO2 reduction

Low-temperature CO2 electrolysis represents a potential enabling process in the production of renewable chemicals and fuels, notably carbon monoxide, formic acid, ethylene and ethanol. Because this technology has progressed rapidly in recent years, a systematic techno-economic assessment has become necessary to evaluate its feasibility as a CO2 utilization approach. Here this work provides a comprehensive techno-economic assessment of four major products and prioritizes the technological development with systematic guidelines to facilitate the market deployment of low-temperature CO2 electrolysis. First, we survey state-of-the-art electrolyser performance and parameterize figures of merit. The analysis shows that production costs of carbon monoxide and formic acid (C1 products) are approaching US$0.44 and 0.59 kg–1, respectively, competitive with conventional processes. In comparison, the production of ethylene and ethanol (C2 products) is not immediately feasible due to their substantially higher costs of US$2.50 and 2.06 kg–1, respectively. We then provide a detailed roadmap to making C2 product production economically viable: an improvement in energetic efficiency to ~50% and a reduction in electricity price to US$0.01 kWh–1. We also propose industrially relevant benchmarks: 5-year stability of electrolyser components and the single-pass conversion of 30 and 15% for C1 and C2 products, respectively. Finally we discuss the economic aspects of two potential strategies to address electrolyte neutralization utilizing either an anion exchange membrane or bipolar membrane. Low-temperature CO2 electrolysis is a promising process for producing renewable chemicals and fuels. This work provides a systematic techno-economic assessment of four major products, prioritizing technological development, and proposes guidelines to facilitate market adoption.

Techno-economic assessment of low-temperature carbon dioxide electrolysis

In this work, the carbon balance during high-rate CO2 reduction in flow electrolyzers was rigorously analyzed. The CO2 consumption at gas-diffusion electrodes due to electrochemical conversion and reaction with OH− at the electrode/electrolyte interface leads to a substantial reduction in the volumetric flowrate of gas flow out of the electrolyzer, especially when highly concentrated alkaline electrolytes and elevated current densities are utilized, which is primarily due to an elevated pH at cathode/electrolyte interface. Without considering the CO2 consumption, the faradaic efficiencies for major gas products could be significantly overestimated during high current density CO2 reduction conditions, particularly in the case of high pH electrolyte. In addition, a detailed carbon balance path is elucidated via a two-step procedure of CO2 reaction with OH− at the cathode/electrolyte interface and subsequent CO2 generation at the anode/electrolyte interface caused by a relatively low pH in the vicinity of the anode. Based on the proposed two-step carbon balance path, a systematic exploration of gases released in the anolyte reveals the transformation of a HCO3− or OH− catholyte to a CO32− catholyte, which was further confirmed by pH measurements.

/pdf/insights-into-the-carbon-balance-for-co2-electroreduction-on-44jp24xr8v.pdf

Insights into the carbon balance for CO2 electroreduction on Cu using gas diffusion electrode reactor designs

CO2 emissions can be recycled via low-temperature CO2 electrolysis to generate products such as carbon monoxide, ethanol, ethylene, acetic acid, formic acid and propanol. In recent years, progress has been made towards an industrially relevant performance by leveraging the development of gas diffusion electrodes (GDEs), which enhance the mass transport of reactant gases (for example, CO2) to the active electrocatalyst. Innovations in GDE design have thus set new benchmarks for CO2 conversion activity. In this Review, we discuss GDE-based CO2 electrolysers, in terms of reactor designs, GDE composition and failure modes, to identify the key advances and remaining shortfalls of the technology. This is combined with an overview of the partial current densities, efficiencies and stabilities currently achieved and an outlook on how phenomena such as carbonate formation could influence the future direction of the field. Our aim is to capture insights that can accelerate the development of industrially relevant CO2 electrolysers. Chemicals and fuels can be generated from CO2 via electrolysers that employ gas diffusion electrodes (GDEs). In this Review, the authors consider promising catalysts and reactors—and how these fail—to identify key advances and remaining gaps in the development of industrially relevant GDE-based CO2 electrolysers. 

Gas diffusion electrodes, reactor designs and key metrics of low-temperature CO2 electrolysers

The CO2 reduction reaction (CO2RR) is a potential means of using renewable electricity to synthesize commodity chemicals and fuels. The CO2RR can be performed at industrially relevant product formation rates in an electrolyser, which must simultaneously manage the transport of electrons, water, CO2 and protons at a cathode. Gas diffusion electrodes (GDEs) and polymer electrolyte membranes are used to mediate these critical processes. Consequently, the design and development of GDEs and membranes tailored for the CO2RR is critical. In this Review, we discuss how the properties of GDEs and polymer electrolyte membranes affect CO2RR electrolysis. The CO2 reduction reaction can be used to produce carbon-neutral chemicals using renewable electricity. This Review presents material design strategies for controlling the transport of products and reactants in electrochemical reactors that convert CO2 emissions into chemicals and fuels.

Gas diffusion electrodes and membranes for CO2 reduction electrolysers

Cell designs that integrate membrane-electrode assemblies (MEAs) with highly selective catalysts are a promising route to reduce ohmic losses and achieve high energy efficiency in CO2 reduction at industrially relevant current densities. In this work, porous silver filtration membranes are demonstrated as simple and efficient gas-diffusion electrodes for CO2 reduction to CO at high current densities in an MEA-type device. A partial current density for CO of up to ca. 200 mA cm-2 was achieved at a cell voltage of ca. 3.3 V, in tandem with minimal H2 production. However, the analysis of cathodic and anodic outlet streams revealed that CO2 cross-over across the anion-exchange membranes, mostly in the form of CO32- but partially as HCOO- generated over the cathode, actually exceeds the amount of CO2 converted to the target product, resulting in a poor utilization of the reactant and in the early onset of mass transfer limitations. In addition, CO2 cross-over leads to a nonstoichiometric decrease of the outlet flow rate from the cathodic compartment. This effect can lead to a substantial overestimation of catalytic performance if the inlet flow rate of CO2 is used as reference for calculating partial current densities and Faradaic efficiencies. The results of this work highlight the importance of carrying out a carbon balance, in addition to traditional measurements of activity and selectivity, to adequately assess the performance of CO2 reduction devices at high current densities, and inform future efforts aimed at mitigating membrane cross-over in MEA-type electrolyzers for CO2 reduction.

/pdf/analysis-of-mass-flows-and-membrane-cross-over-in-co2-1ix14c7w87.pdf

Analysis of Mass Flows and Membrane Cross-over in CO2 Reduction at High Current Densities in an MEA-Type Electrolyzer.

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Analysis of Mass Flows and Membrane Cross-over in CO2 Reduction at High Current Densities in an MEA-Type Electrolyzer.