Enhanced Stability and CO/Formate Selectivity of Plasma-Treated SnO x/AgO x Catalysts during CO2 Electroreduction

Progress and Perspective for In Situ Studies of CO2 Reduction

CO₂ conversion to chemical fuels through photoreduction, electroreduction, or thermoreduction is considered as one of the most effective methods to solve environmental pollution and energy shortage problems. However, recent studies show that the involved catalysts may undergo continuous reconstruction under realistic working conditions, which unfortunately causes controversial results concerning the active sites and reaction mechanism of CO₂ reduction. Thus, it is necessary, while challenging, to monitor in real time the dynamic evolution of the catalysts and reaction intermediates by in situ techniques under experimental conditions. In this Perspective, we start with the working principle and detection modes of various in situ characterization techniques. Subsequently, we systematically summarize the recent developments of in situ studies on probing the catalyst evolution during the CO₂ reduction process. We further focus on the progress of in situ studies in monitoring the reaction intermediates and catalytic products, in which we also highlight how the theoretical calculations are combined to reveal the reaction mechanism in detail. Finally, based on the achievements in the representative studies, we present some prospects and suggestions for in situ studies of CO₂ reduction in the future.

Anodic SnO2 porous nanostructures with rich grain boundaries for efficient CO2 electroreduction to formate

Formic acid (HCOOH), the acidic form of formate, is an important hydrogen carrier which can be directly used in fuel cells. Development of earth-abundant element-based catalysts to convert carbon dioxide (CO₂) into HCOOH or formate with high selectivity and high efficiency has been a vigorous research activity in recent years but remains an unsolved challenge. In this contribution, using one-step anodization, we prepare nanotubular SnO₂ porous nanostructures with high surface area (90.1 m² g⁻¹), large porosity (0.74 cm³ g⁻¹), and rich grain boundaries for electrochemical CO₂ reduction (CO₂RR). They exhibit stable 95% faradaic efficiency (FE) towards CO₂RR and 73% FE for formate at −0.8 V_RHE. The notable performance of such SnO₂ nanostructures can be attributed to their unique structural and chemical properties, which provide active sites for CO₂ adsorption and conversion, and easy access for CO₂ to the active sites. The insights gained from the structure/property relationships might be beneficial for designing superior electrocatalysts for CO₂ electroreduction into formate.

The formate is electrochemical synthesized from a porous anodic SnO₂ with the faradic efficiency over 70% under low potential.

Operando Insight into the Correlation between the Structure and Composition of CuZn Nanoparticles and Their Selectivity for the Electrochemical CO2 Reduction

Bimetallic CuZn catalysts have been recently proposed as alternatives in order to achieve selectivity control during the electrochemical reduction of CO₂ (CO₂RR). However, fundamental understanding of the underlying reaction mechanism and parameters determining the CO₂RR performance is still missing. In this study, we have employed size-controlled (∼5 nm) Cu_100–xZn_x nanoparticles (NPs) supported on carbon to investigate the correlation between their structure and composition and catalytic performance. By tuning the concentration of Zn, a drastic increase in CH₄ selectivity [∼70% Faradaic efficiency (F.E.)] could be achieved for Zn contents from 10 to 50, which was accompanied by a suppression of the H₂ production. Samples containing a higher Zn concentration displayed significantly lower CH₄ production and an abrupt switch in the selectivity to CO. Lack of metal leaching was observed based on quasi in situ X-ray photoelectron spectroscopy (XPS). Operando X-ray absorption fine structure (XAFS) spectroscopy measurements revealed that the alloying of Cu atoms with Zn atoms takes place under reaction conditions and plays a determining role in the product selectivity. Time-dependent XAFS analysis showed that the local structure and chemical environment around the Cu atoms continuously evolve during CO₂RR for several hours. In particular, cationic Zn species initially present were found to get reduced as the reaction proceeded, leading to the formation of a CuZn alloy (brass). The evolution of the Cu–Zn interaction with time during CO₂RR was found to be responsible for the change in the selectivity from CH₄ over Cu-ZnO NPs to CO over CuZn alloy NPs. This study highlights the importance of having access to in depth information on the interplay between the different atomic species in bimetallic NP electrocatalysts under operando reaction conditions in order to understand and ultimately tune their reactivity.

Selective CO2 Electroreduction to Ethylene and Multicarbon Alcohols via Electrolyte-Driven Nanostructuring

Production of multicarbon products (C₂₊) from CO₂ electroreduction reaction (CO₂RR) is highly desirable for storing renewable energy and reducing carbon emission. The electrochemical synthesis of CO₂RR catalysts that are highly selective for C₂₊ products via electrolyte‐driven nanostructuring is presented. Nanostructured Cu catalysts synthesized in the presence of specific anions selectively convert CO₂ into ethylene and multicarbon alcohols in aqueous 0.1 m KHCO₃ solution, with the iodine‐modified catalyst displaying the highest Faradaic efficiency of 80 % and a partial geometric current density of ca. 31.2 mA cm⁻² for C₂₊ products at −0.9 V vs. RHE. Operando X‐ray absorption spectroscopy and quasi in situ X‐ray photoelectron spectroscopy measurements revealed that the high C₂₊ selectivity of these nanostructured Cu catalysts can be attributed to the highly roughened surface morphology induced by the synthesis, presence of subsurface oxygen and Cu⁺ species, and the adsorbed halides.

Modulating Activity through Defect Engineering of Tin Oxides for Electrochemical CO2 Reduction

The large-scale application of electrochemical reduction of CO2, as a viable strategy to mitigate the effects of anthropogenic climate change, is hindered by the lack of active and cost-effective electrocatalysts that can be generated in bulk. To this end, SnO2 nanoparticles that are prepared using the industrially adopted flame spray pyrolysis (FSP) technique as active catalysts are reported for the conversion of CO2 to formate (HCOO-), exhibiting a FEHCOO - of 85% with a current density of -23.7 mA cm-2 at an applied potential of -1.1 V versus reversible hydrogen electrode. Through tuning of the flame synthesis conditions, the amount of oxygen hole center (OHC; Sn≡O●) is synthetically manipulated, which plays a vital role in CO2 activation and thereby governing the high activity displayed by the FSP-SnO2 catalysts for formate production. The controlled generation of defects through a simple, scalable fabrication technique presents an ideal approach for rationally designing active CO2 reduction reactions catalysts.

Advances in Sn-Based Catalysts for Electrochemical CO2 Reduction

The increasing concentration of CO₂ in the atmosphere has led to the greenhouse effect, which greatly affects the climate and the ecological balance of nature. Therefore, converting CO₂ into renewable fuels via clean and economical chemical processes has become a great concern for scientists. Electrocatalytic CO₂ conversion is a prospective path toward carbon cycling. Among the different electrocatalysts, Sn-based electrocatalysts have been demonstrated as promising catalysts for CO₂ electroreduction, producing formate and CO, which are important industrial chemicals. In this review, various Sn-based electrocatalysts are comprehensively summarized in terms of synthesis, catalytic performance, and reaction mechanisms for CO₂ electroreduction. Finally, we concisely discuss the current challenges and opportunities of Sn-based electrocatalysts.