An in-Depth Exploration of the Electrocatalytic Stability of Sn-Based Electrocatalysts for the Electrochemical CO2 Reduction Towards Formate

Abstract

Over the past decades, the electrochemical CO2 reduction (eCO2R) into industrially valuable products has become one of the most promising technologies to valorize anthropogenic CO2 emission, while also providing a means of energy storage for intermittent renewable sources. A wide variety of products, such as formic acid/formate (HCOOH/HCOO-), carbon monoxide (CO), methane (CH4), methanol (CH3OH), ethylene (C2H4) etc., can be obtained. The eCO2R towards formic acid/formate (FA), a 2-electron transfer process, has the potential to generate the highest revenue per mole of consumed electrons, which originates from the fact that FA can be produced with high Faradaic efficiencies on cheap and earth abundant metals, such as Sn, Bi, Pb, Hg, In and Cd. State-of-the-art Sn-based electrocatalysts have been reported to reach selectivities approaching 100% at industrially relevant current densities. Looking at the long-term performance of these state-of-the-art Sn-based electrocatalysts, only seven have a minimum reported stability of 72 hours, with the maximum being 100 days (2400 h) of stable operation. Even though Bi-based catalysts currently outperform Sn-based electrocatalysts in terms of stability, Sn-based catalysts are still believed to be viable alternatives if an extended stability of over 80 000 hours can be achieved. Sn-based electrocatalyst stability thus remains inadequate and appears to be a crucial piece to the puzzle. 1 In this research, we elucidate the major degradation pathways that impair long-term electrocatalytic performance, by determining the chemical and physical phenomena that occur during the electrochemical reduction reaction on the surface and in the bulk of Sn-based catalysts. Simultaneously, the possibilities of a variety of mitigation strategies are explored, and insight into stability issues related to Sn-based electrocatalysts and CO2 electrolysers is gathered.Firstly, Sn was incorporated into a more open, carbon based supporting material (N-doped ordered mesoporous carbon) in an attempt to significantly increase the stability by inhibiting agglomeration and nanoparticle detachment. An N-doped carbon supporting material was chosen for its high surface area and in order to enhance the initial adsorption of CO2, which is commonly referred to as the rate-limiting step, for the eCO2R towards formate, in literature. These novel electrocatalysts provide us with valuable insights into the influence of the supporting material on their electrochemical performance and highlighted the potential of the particle confinement strategy to increase the morphological electrocatalyst stability during electrolysis.In a next step, we demonstrated the successful application of the recently developed pomegranate-structured SnO2 (Pom. SnO2) and SnO2@C (Pom. SnO2@C) nanocomposite electrocatalysts for the efficient electrochemical conversion of CO2 to formate. With an initial selectivity of 83 and 86% towards formate and an operating potential of -0.72 V and -0.64 V vs. RHE, respectively, these pomegranate SnO2 electrocatalysts are able to compete with most of the current state-of-the-art Sn-based electrocatalysts in terms of activity and selectivity. Given the importance of electrocatalyst stability, long-term experiments (24 h) were performed and a temporary loss in selectivity was noticed for the Pom. SnO2@C electrocatalyst. Ex situ XRD and XPS were used to link this temporary selectivity loss of the Pom. SnO2@C electrocatalyst to the in situ SnO2 reduction to metallic Sn. While this electrochemical degradation occurs in both electrocatalysts, it is more pronounced in the Pom. SnO2@C electrocatalyst since it isn’t offset by the morphological electrocatalyst degradation revealing new and selective SnOx active sites, as suspected for the Pom. SnO2. Furthermore, we were able to largely restore its selectivity upon drying and exposure to air. Of all the used (24 h) electrocatalysts, the pomegranate SnO2@C had the highest selectivity over a time period of one hour, reaching an average recovered FE of 85%, while the commercial SnO2 and bare pomegranate SnO2 electrocatalysts reached an average of 79 and 80% FE towards formate, respectively. Finally, the pomegranate structure of Pom. SnO2@C was largely preserved due to the presence of the heterogeneous carbon shell, which acts as a protective layer, physically inhibiting particle segregation/pulverisation and agglomeration.Utilizing the particle confinement strategy, we increased the morphological stability of pomegranate structured SnO2 electrocatalysts and demonstrated the reversible nature of the in situ SnO2 reduction. Van Daele, K. et al. Sn-Based Electrocatalyst Stability: A Crucial Piece to the Puzzle for the Electrochemical CO2Reduction toward Formic Acid. ACS Energy Lett. 6, 4317–4327 (2021). Figure 1