Unraveling in-situ electrochemical reconstruction of indium oxide catalysts with oxygen vacancy for enhanced electrocatalytic CO2-to-formate conversion

Abstract

Indium-based materials have emerged as promising alternative catalysts for the selective electroreduction of CO2 to formate, yet the optimal catalytic configuration remains elusive. Herein, theoretical calculation reveals that metallic indium over oxygen vacancy-containing In2O3 support (In/In2O3-VO) possesses the lowest energy barriers (0.99 eV) for CO2 reduction to formate. A rational air-annealing strategy applied to In3+-adsorbed resin is developed to synthesize indium oxide catalysts containing oxygen vacancy (R-In2O3). In-situ spectroscopy techniques confirm in-situ electrochemical reconstruction of the In/In2O3 configuration and the effective stabilization of the key reaction intermediate (HCOO*). Consequently, the catalyst delivers excellent CO2-to-formate conversion performance, maintaining a current efficiency above 92% over 56 h of galvanostatic electrolysis at −250 mA·cm−2. These insights provide an effective strategy for the rational design of high-performance and durable indium-based electrocatalysts for sustainable formate production.

1. Introduction

Climate change has emerged as a paramount environmental challenge, primarily driven by the escalating concentrations of greenhouse gases released into the atmosphere [1–2]. Electrochemical CO₂ reduction reaction (CO₂RR) enables the conversion of the greenhouse gas CO₂ into high-value chemicals using intermittent renewable energy sources, providing a promising route to a sustainable carbon-neutral economy [3–6]. Among the broad possible products of electrochemical CO₂ reduction, formate is one of the target products, receiving widespread attention for its versatility and economic viability [7]. However, the C=O bond in the CO₂ molecule is robust with difficult adsorption and activation, while the complex CO₂ electroreduction reaction process involves multiple proton-coupled electron transfer and inevitably compromised by competition from the hydrogen evolution reaction (HER). Consequently, the development of high-performance catalysts for formate production in CO₂RR has become a research focus [8–10].

Recently indium-based electrocatalysts have been investigated broadly owing to their low toxicity and excellent catalytic selectivity in CO₂-to-formate conversion, positioning them as high-performance candidates [11–13]. However, these catalysts still suffer from the challenges of limited current density (j) and inadequate long-term stability for CO₂RR [14–19]. For this, various indium-based materials have been developed, including metallic indium [12], indium oxide [20], indium sulfide [21], indium phosphide [22], indium-organic framework [23], indium cyanamide [24], single-atom indium [25], and gallium indium liquid metal [26]. Nonetheless, there are still controversies in the rational material design of indium-based catalysts. Generally, the excellent CO₂RR performance of indium-based catalysts is fundamentally governed by the oxidation state of the metal sites. For instance, maintaining a high oxidation state of indium oxide is favorable to reduce the activation energy of CO₂, whereas metallic indium can effectively suppress the competing reaction of HER [8,27–29]. Moreover, modulating the concentration of oxygen vacancies in indium oxide catalysts regulates the coordination environment of indium site, thereby enhancing the adsorption process of key intermediates for formate generation [30–31]. Consequently, engineering a hybrid structure comprising indium oxide, metallic indium, and oxygen vacancy could constitute an optimal configuration for electrochemical CO₂-to-formate conversion.

Considering the cathodic potential requisite for CO₂ reduction, a rational coupling of the electrochemical reconstruction of In/In₂O₃ configuration from indium oxide electrocatalysts with formate generation becomes possible. Hence, comprehensive theoretical studies and in-situ characterizations on the electrochemical reconstruction process in oxygen vacancy-containing In₂O₃ catalysts in CO₂RR are imperative. This will contribute to the revealing of the in-situ reconstruction evolution of In₂O₃ and uncover the mechanistic pathways of formate production over In/In₂O₃ configuration, thereby providing pivotal guidance for deciphering catalytic mechanisms and designing high-performance catalysts. We herein report the in-situ reconstruction of In₂O₃ catalyst with oxygen vacancy to form an In/In₂O₃ interfacial configuration, thereby enhancing CO₂-to-formate conversion performance. As illustrated schematically in Fig. 1, considering the theoretical facts of the thermodynamic standard generation potentials of indium metal and formate as well as the lowest energy barriers for the rate-determining step (RDS) in formate production for metallic indium over oxygen vacancy-containing In₂O₃ support (In/In₂O₃-V_O) configuration, an air annealing process with adsorbed resins in In³⁺ cation solution (Resin–In) as the precursor has been rationally designed for the preparation of indium oxides containing oxygen vacancies (R-In₂O₃).