Advances in Sn-based electrocatalysts for selective reduction of CO2 to formate

Abstract

The selective reduction of carbon dioxide (CO2) into high-value-added chemicals is one of the most effective means to solve the current energy and environmental problems, which could realize the utilization of CO2 and promote the balance of the carbon cycle. Formate is one of the most economical and practical products of all the electrochemical CO2 reduction products. Among the many metal-based electrocatalysts that can convert CO2 into formate, Sn-based catalysts have received a lot of attention because of their low-cost, non-toxic characteristics and high selectivity for formate. In this article, the most recent development of Sn-based electrocatalysts is comprehensively summarized by giving examples, which are mainly divided into monometallic Sn, alloyed Sn, Sn-based compounds and Sn composite catalysts. Finally, the current performance enhancement strategies and future directions of the field are summarized.

1. Introduction

Since the industrial revolution in the nineteenth century, fossil fuels such as coal, oil and natural gas have become the main energy sources to drive human society and economic development. However, the massive burning of fossil fuels has released large amounts of carbon dioxide gas into the air, bringing about environmental problems implicated by the greenhouse effect of CO2 [1, 2]. To reduce this negative impact, the conversion of CO2 into chemicals with added value is considered to be an effective and promising approach, which means not only a reduction of CO2 concentration in the atmosphere but also the production of usable chemicals [3−5]. Among the present explored CO2 elimination technologies, electrochemical reduction of CO2 (CO2ER) to form fuels and chemicals is widely regarded as a very promising and sustainable solution if combined with intermittent solar and wind energy [6−8].

In general, CO2 can be electrochemically converted into various products such as carbon monoxide (CO), formic acid (HCOOH), methane (CH4), methanol (CH3OH), and ethanol (C2H5OH), through different reaction pathways, as shown in Table 1 [9, 10]. Possible mechanisms and reaction pathways for the reduction of CO2 to formate, CO and other products are shown in Figure 1 [11]. Among them, formic acid (under acidic conditions) and formate (under alkaline conditions) are high-value products used in many industrial processes, and the domestic demand for formic acid reached more than 300000 t per year since 2018 [12, 13].

Behaving as an aldehyde, formic acid and formate both have reducing properties [14]. They are basic organic chemical feedstocks that can be used directly as fuel for fuel cells and hydrogen carriers with high energy density [15]. Traditionally, they are synthesized by the oxidation of methanol, which requires high temperature and pressure. In contrast, the electrochemical synthesis has high selectivity, mild reaction conditions and requires the lowest number of electrons to be transferred and consumes the least amount of energy in CO2ER. Therefore, the electrochemical conversion of CO2 into formate has been one of the hottest topics in CO2 elimination techniques. The corresponding electrocatalysts can boost the CO2ER procedure significantly. Until now, numerous effective metal-based electrocatalysts have been explored for applications in the reduction of CO2 to formate, including Sn [16−19], In [20, 21], Bi [22−25], Pb [26−28] and their related alloys, metal oxides, sulfides and compounds. Among them, as demonstrated by Figure 2, Sn-based electrocatalysts have been considered one of the most attractive and promising catalysts for selectively reducing CO2 to formate due to their low-cost, non-toxic characteristics and high selectivity for formate with Faraday efficiencies (FEs) exceeding 80% originated from their su...