Transition Metal Carbides for Electrochemical CO2 Reduction

• Applicant: Professor Dr. Karsten Reuter
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation ; Theoretical Chemistry: Molecules, Materials, Surfaces
• Term: from 2019 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 414298388

• Final Report Year: 2024

Final Report Abstract

The computational screening study conducted on Mo2C and MoC surfaces for the electrochemical CO2 reduction reaction provided valuable insights into the diverse active-site behaviors of transition metal carbides. By analyzing the adsorption strength of key CO2 reduction intermediates, the study revealed the breaking of scaling relations known from transition metal adsorption, indicating a rich reduction chemistry at the carbides beyond what is optimized for Cu. This underscores the complexity of assessing candidate materials like transition metal carbides with conventional materials computational screening approaches focusing on only a few active-site motifs. Despite their diverse adsorption properties, Mo-rich surfaces showed potential for methane production at lower overpotentials but faced challenges with *OH binding. Surprisingly, carbon-rich surfaces facilitated methanol formation. Further engineering Mo2C or exploring MoC could thus in principle enhance methanol production. Overall, carbon-rich Mo carbides show quite some promise for CO2 reduction. Unfortunately, experimental studies in aqueous electrolytes revealed the passivation of Mo2C surfaces by oxide formation, suppressing the CO2 reduction activity. However, switching to non-aqueous electrolytes, such as acetonitrile-based solutions, allowed to efficiently reduce CO2 to dissolved CO. However, this selectivity of CO2 reduction was largely governed by the acetonitrile-based electrolyte, suggesting that the electrodes primarily acted as electron donors rather than catalysts influencing the selectivity. In general, our thermodynamic investigation suggests that oxide formation poses a significant challenge for early transition metal carbides as electrocatalysts for CO2 reduction in aqueous electrolytes. By integrating surface free energies across different facets, we identified only a few promising facets such as (100) facets of CrC and MoC for CO2 reduction. Furthermore, the correlation between electrochemical stability and carbide-oxygen interaction strength suggests exploring later transition metal carbides where oxide formation might be less problematic. Recent reports on CO2 reduction activity at iron and cobalt carbide catalysts, as well as the efficiency of transition metal carbides in thermal CO2 hydrogenation, confirm the potential of these materials in electrocatalysis. In summary, the study provided insights into the challenges and opportunities associated with transition metal carbides as CO2 reduction electrocatalysts, emphasizing the importance of considering material complexities and electrolyte effects in assessing their performance. This project has been very successful in identifying promising carbides and understanding their catalytic function, but also revealed that the stability of these materials in operando conditions is a real challenge.

Publications

• Active-Site Computational Screening: Role of Structural and Compositional Diversity for the Electrochemical CO2 Reduction at Mo Carbide Catalysts. ACS Catalysis, 10(20), 11814-11821.
  Li, Haobo & Reuter, Karsten
  
• Self-activation of copper electrodes during CO electro-oxidation in alkaline electrolyte. Nature Catalysis, 3(10), 797-803.
  Auer, Andrea; Andersen, Mie; Wernig, Eva-Maria; Hörmann, Nicolas G.; Buller, Nico; Reuter, Karsten & Kunze-Liebhäuser, Julia
  
• True Nature of the Transition-Metal Carbide/Liquid Interface Determines Its Reactivity. ACS Catalysis, 11(8), 4920-4928.
  Griesser, Christoph; Li, Haobo; Wernig, Eva-Maria; Winkler, Daniel; Shakibi, Nia Niusha; Mairegger, Thomas; Götsch, Thomas; Schachinger, Thomas; Steiger-Thirsfeld, Andreas; Penner, Simon; Wielend, Dominik; Egger, David; Scheurer, Christoph; Reuter, Karsten & Kunze-Liebhäuser, Julia
  
• Ab Initio Thermodynamic Stability of Carbide Catalysts under Electrochemical Conditions. ACS Catalysis, 12(16), 10506-10513.
  Li, Haobo & Reuter, Karsten
  
• Electroreduction of CO2 in a Non-aqueous Electrolyte─The Generic Role of Acetonitrile. ACS Catalysis, 13(9), 5780-5786.
  Mairegger, Thomas; Li, Haobo; Grießer, Christoph; Winkler, Daniel; Filser, Jakob; Hörmann, Nicolas G.; Reuter, Karsten & Kunze-Liebhäuser, Julia