Boosting CO Production in Electrocatalytic CO2 Reduction on Highly Porous Zn Catalysts

Heterogeneous Single-Atom Catalysts for Electrochemical CO2 Reduction Reaction

The electrochemical CO₂ reduction reaction (CO₂ RR) is of great importance to tackle the rising CO₂ concentration in the atmosphere. The CO₂ RR can be driven by renewable energy sources, producing precious chemicals and fuels, with the implementation of this process largely relying on the development of low-cost and efficient electrocatalysts. Recently, a range of heterogeneous and potentially low-cost single-atom catalysts (SACs) containing non-precious metals coordinated to earth-abundant elements have emerged as promising candidates for the CO₂ RR. Unfortunately, the real catalytically active centers and the key factors that govern the catalytic performance of these SACs remain ambiguous. Here, this ambiguity is addressed by developing a fundamental understanding of the CO₂ RR-to-CO process on SACs, as CO accounts for the major product from CO₂ RR on SACs. The reaction mechanism, the rate-determining steps, and the key factors that control the activity and selectivity are analyzed from both experimental and theoretical studies. Then, the synthesis, characterization, and the CO₂ RR performance of SACs are discussed. Finally, the challenges and future pathways are highlighted in the hope of guiding the design of the SACs to promote and understand the CO₂ RR on SACs.

Hexagonal Zn Nanoplates Enclosed by Zn(100) and Zn(002) Facets for Highly Selective CO2 Electroreduction to CO

Electrochemical reduction of CO₂ to carbon-neutral fuels is a promising strategy for renewable energy conversion and storage. However, developing earth-abundant and cost-effective electrocatalysts with high catalytic activity and desirable selectivity for the target fuel is still challenging and imperative. Herein, hexagonal Zn nanoplates (H-Zn-NPs) enclosed by Zn(100) and Zn(002) facets were successfully synthesized and studied for their feasibility toward the CO₂ reduction reaction (CO₂RR). Compared with similarly sized Zn nanoparticles (S-Zn-NPs), the H-Zn-NPs exhibit remarkably enhanced current density, together with an improved CO faradaic efficiency (FE) of over 85% in a wide potential window, where a maximum FE of 94.2% is achieved. The enhancement in the CO₂RR performance benefits from the substantial catalytically active sites introduced by the special architecture of H-Zn-NPs. Density functional theory calculations reveal that the exposed Zn(100) facets and edge sites on H-Zn-NPs are energetically favorable for CO₂RR to CO, which directly result in an enhanced CO₂RR performance. This study undoubtedly provides a straightforward approach to controlling the catalytic activity and selectivity of CO₂RR through tuning the shape of Zn-based catalysts so as to maximize the percentage of exposed Zn(100) facets.

Fundamentals of Gas Diffusion Electrodes and Electrolysers for Carbon Dioxide Utilisation: Challenges and Opportunities

Electrocatalysis plays a prominent role in the development of carbon dioxide utilisation technologies. Many new and improved CO2 conversion catalysts have been developed in recent years, progressively achieving better performance. However, within this flourishing field, a disconnect in catalyst performance evaluation has emerged as the Achilles heel of CO2 electrolysis. Too often, catalysts are assessed in electrochemical settings that are far removed from industrially relevant operational conditions, where CO2 mass transport limitations should be minimised. To overcome this issue, gas diffusion electrodes and gas-fed electrolysers need to be developed and applied, presenting new challenges and opportunities to the CO2 electrolysis community. In this review, we introduce the reader to the fundamentals of gas diffusion electrodes and gas-fed electrolysers, highlighting their advantages and disadvantages. We discuss in detail the design of gas diffusion electrodes and their operation within gas-fed electrolysers in both flow-through and flow-by configurations. Then, we correlate the structure and composition of gas diffusion electrodes to the operational performance of electrolysers, indicating options and prospects for improvement. Overall, this study will equip the reader with the fundamental understanding required to enhance and optimise CO2 catalysis beyond the laboratory scale.

Efficient CO2 electroreduction to CO at low overpotentials using a surface-reconstructed and N-coordinated Zn electrocatalyst

In this work, an effective vapor ammonization-electroreduction strategy is developed to fabricate a surface-reconstructed and N-coordinated Zn electrocatalyst (Zn-N), which exhibits high electrocatalytic activity and stability for CO₂ reduction to CO with a faradaic efficiency (FE) of 85.6% and a CO partial current density of 11.2 mA cm^-2 at -0.91 V vs. reversible hydrogen electrode (RHE), greatly outperforming Zn foil and surface-reconstructed Zn (Zn-H). Moreover, Zn-N also maintains good selectivity for CO production during the long-term CO₂ electrolysis.

Electrochemical reduction of CO2 to ethylene on Cu/Cu x O-GO composites in aqueous solution

Here, we present fabrication of Graphene oxide (GO) supported Cu/Cu x O nano-electrodeposits which can efficiently and selectively electroreduce CO2 into ethylene with a faradaic efficiency (F.E) of 34% and a conversion rate of 194 mmol g-1 h-1 at -0.985 V vs. RHE. The effect of catalyst morphology, working electrode fabricational techniques, the extent of metal-GO interaction and the oxide content in Cu/Cu x O, was studied in detail so as to develop a protocol for the fabrication of an active, stable and selective catalyst for efficient electro-production of ethylene from CO2. Moreover, a detailed comparative study about the effect of the GO support, and the nature of the cathodic collection substrate used for the electro-deposition is presented.

Competition between CO2 Reduction and Hydrogen Evolution on a Gold Electrode under Well-Defined Mass Transport Conditions

Gold is one of the most selective catalysts for the electrochemical reduction of CO₂ (CO2RR) to CO. However, the concomitant hydrogen evolution reaction (HER) remains unavoidable under aqueous conditions. In this work, a rotating ring disk electrode (RRDE) setup has been developed to study quantitatively the role of mass transport in the competition between these two reactions on the Au surface in 0.1 M bicarbonate electrolyte. Interestingly, while the faradaic selectivity for CO formation was found to increase with enhanced mass transport (from 67% to 83%), this effect is not due to an enhancement of the CO2RR rate. Remarkably, the inhibition of the competing HER from water reduction with increasing disk rotation rate is responsible for the enhanced CO2RR selectivity. This can be explained by the observation that, on the Au electrode, water reduction improves with more alkaline pH. As a result, the decrease in the local alkalinity near the electrode surface with enhanced mass transport suppresses HER due to the water reduction. Our study shows that controlling the local pH by mass transport conditions can tune the HER rate, in turn regulating the CO2RR and HER competition in the general operating potential window for CO2RR (-0.4 to -1 V vs RHE).

Catalyst-electrolyte interface chemistry for electrochemical CO2 reduction

The electrochemical reduction of CO2 stores intermittent renewable energy in valuable raw materials, such as chemicals and transportation fuels, while minimizing carbon emissions and promoting carbon-neutral cycles. Recent technoeconomic reports suggested economically feasible target products of CO2 electroreduction and the relative influence of key performance parameters such as faradaic efficiency (FE), current density, and overpotential in the practical industrial-scale applications. Furthermore, fundamental factors, such as available reaction pathways, shared intermediates, competing hydrogen evolution reaction, scaling relations of the intermediate binding energies, and CO2 mass transport limitations, should be considered in relation to the electrochemical CO2 reduction performance. Intensive research efforts have been devoted to designing and developing advanced electrocatalysts and improving mechanistic understanding. More recently, the research focus was extended to the catalyst environment, because the interfacial region can delicately modulate the catalytic activity and provide effective solutions to challenges that were not fully addressed in the material development studies. Herein, we discuss the importance of catalyst-electrolyte interfaces in improving key operational parameters based on kinetic equations. Furthermore, we extensively review previous studies on controlling organic modulators, electrolyte ions, electrode structures, as well as the three-phase boundary at the catalyst-electrolyte interface. The interfacial region modulates the electrocatalytic properties via electronic modification, intermediate stabilization, proton delivery regulation, catalyst structure modification, reactant concentration control, and mass transport regulation. We discuss the current understanding of the catalyst-electrolyte interface and its effect on the CO2 electroreduction activity.

Industrial carbon dioxide capture and utilization: state of the art and future challenges

This review covers the sustainable development of advanced improvements in CO₂ capture and utilization.

Steering CO2 electroreduction toward ethanol production by a surface-bound Ru polypyridyl carbene catalyst on N-doped porous carbon

Electrochemical reduction of CO₂ to multicarbon products is a significant challenge, especially for molecular complexes. We report here CO₂ reduction to multicarbon products based on a Ru(II) polypyridyl carbene complex that is immobilized on an N-doped porous carbon (RuPC/NPC) electrode. The catalyst utilizes the synergistic effects of the Ru(II) polypyridyl carbene complex and the NPC interface to steer CO₂ reduction toward C2 production at low overpotentials. In 0.5 M KHCO₃/CO₂ aqueous solutions, Faradaic efficiencies of 31.0 to 38.4% have been obtained for C2 production at -0.87 to -1.07 V (vs. normal hydrogen electrode) with 21.0 to 27.5% for ethanol and 7.1 to 12.5% for acetate. Syngas is also produced with adjustable H₂/CO mole ratios of 2.0 to 2.9. The RuPC/NPC electrocatalyst maintains its activity during 3-h CO₂-reduction periods.