This review article provides the recent progress in the electrochemical CO2 reduction reaction by understanding and tuning catalyst–electrolyte interfaces.

Electrochemical processes coupling carbon dioxide reduction reactions with organic oxidation reactions are promising techniques for producing clean chemicals and utilizing renewable energy. However, assessments of the economics of the coupling technology remain questionable due to diverse product combinations and significant process design variability. Here, we report a technoeconomic analysis of electrochemical carbon dioxide reduction reaction–organic oxidation reaction coproduction via conceptual process design and thereby propose potential economic combinations. We first develop a fully automated process synthesis framework to guide process simulations, which are then employed to predict the levelized costs of chemicals. We then identify the global sensitivity of current density, Faraday efficiency, and overpotential across 295 electrochemical coproduction processes to both understand and predict the levelized costs of chemicals at various technology levels. The analysis highlights the promise that coupling the carbon dioxide reduction reaction with the value-added organic oxidation reaction can secure significant economic feasibility.

The direct conversion of low concentrations of CO2 is an essential approach, considering the expensive gas conditioning process for pure CO2, but has not yet been intensely studied in a membrane electrode assembly (MEA) electrolyzer. Herein, we explored the CO2 reduction with various CO2 concentrations in a zero-gap MEA electrolyzer and found that suppressing the hydrogen evolution reaction (HER) became more critical at low concentrations of CO2. We demonstrate that a Ni single-atom (Ni-N/C) catalyst exhibits a high tolerance toward low CO2 partial pressure (P CO2) because of the intrinsically large activation energy of the HER. Ni-N/C outperformed the CO productivity of Ag nanoparticles, especially at low concentrations of CO2 in the zero-gap MEA. When the P CO2 was lowered from 1.0 to 0.1 atm, Ni-N/C maintained >93% of CO Faradaic efficiency (FECO), but Ag nanoparticles showed a decrease in FECO from 94% to 40%. Furthermore, on the basis of a computational fluid dynamics simulation, we developed extrinsic operating conditions controlling the water transfer from the anolyte to the catalyst layer and improved CO selectivity at low CO2 concentrations in the MEA electrolyzer.

Electrochemical reduction of O2 provides a clean and decentralized pathway to produce H2O2 compared to the current energy-intensive anthraquinone process. As the electrochemical reduction of O2 proceeds via either two-electron or four-electron pathway , it is thus essential to control the selectivity as well as to maximize the catalytic activity. Siahrostami et al. demonstrated a novel approach to control the reaction pathway by optimizing an adsorption ensemble to tune adsorption sites of reaction intermediates, and identified Pt-Hg catalysts from density functional theory (DFT) calculations and experimentally validated this catalyst (Nat. Mater. 2013, 12, 1137). Inspired by this concept, in this work, we apply a state-of-the-art high-throughput screening to develop O2 reduction catalyst for selective H2O2 production. Starting from Materials Project database, we evaluate activity, selectivity and electrochemical stability. To efficiently perform the screening, we introduce an active motif based approach which pre-screens unpromising materials and only performs DFT calculations for promising materials, which significantly reduce the number of the required calculations. We not only provide a list of promising candidates identified by DFT calculations, but also suggest element species to achieve high catalytic activity or H2O2 selectivity for future experimental attempts. Finally, we discuss a strategy for efficient future high-throughput screening using a machine learning pipeline consisting of a non-linear dimension reduction and a density-based clustering. File list (2) download file view on ChemRxiv active_motif_ORR.pdf (4.40 MiB) download file view on ChemRxiv active_motif_ORR_SI.pdf (3.41 MiB)

Electrochemical CO 2 reduction is always accompanied by a competitive hydrogen evolution reaction as water is used as a hydrogen source. In addition to intrinsic activity control, geometrical factors of electrocatalysts such as their porous structure have been demonstrated to affect the reaction selectivity, but understanding its origin is still important. Herein, we demonstrate that reduced graphene oxide layers can effectively control the Faradaic efficiency for CO production of porous zinc nanoparticle electrocatalysts. Simply tuning the coverage of graphene oxide dramatically varies Faradaic efficiency for CO production from 66 to 94% even in the bicarbonate electrolyte at the same biased potential, in which the hydrogen evolution rate was notably suppressed without sacrificing CO 2 reduction to CO production rate unlike many Zn-based electrocatalysts. The graphene oxide layers are revealed to play roles in providing geometric barriers for the mass transport channels of reactants rather than changing the chemical states of the Zn-based electrocatalysts according to in situ X-ray absorption spectroscopic analysis and electrochemical reaction kinetic studies. In addition, computational fluid dynamics simulation studies estimate the Faradaic efficiency dependence on the surface coverage and suggest that the selective suppression of H 2 evolution is associated with the larger increment in local pH compared to that in local pCO 2 at the porous electrocatalyst surfaces. Decoupling between these reactant concentrations is originated from the higher consumption rate and lower bulk concentration of proton compared to those of CO 2 , and the surface coating with graphene oxide can be an effective way to control mass transport channel.