Conductive Metal-Organic Frameworks for Electrocatalysis：Achievements, Challenges, and Opportunities

Abstract

To fulfill the demands of green and sustainable energy, the production of novel catalysts for different energy conversion processes is critical. Owing to the intriguing advantages of the intrinsic active species, tunable crystal structure, remarkable chemical and physical properties, and good stability, metal-organic frameworks (MOFs) have been extensively investigated in various electrochemical energy conversions, such as the CO2 reduction reaction, N2 reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, and oxygen reduction reaction. More importantly, it is feasible to change the chemical environments, pore sizes, and porosity of MOFs, which will theoretically facilitate the diffusion of reactants across the open porous networks, thereby improving the electrocatalytic performance. However, owing to the high energy barriers of charge transfer and limited free charge carriers, most MOFs show poor electrical conductivity, thus limiting their diverse applications. As reported previously, MOFs were used as a porous substrate to confine the growth of nanoparticles or co-doped electrocatalysts after annealing. The conductive MOFs can combine the advantages of conventional MOFs with electronic conductivity, which significantly enhance the electrocatalytic performance. In addition, conductive MOFs can achieve conductivity via electronic or ionic routes without post-annealing treatment, thereby extending their potential applications. Different synthesis strategies have recently been developed to endow MOFs with electrical conductivity, such as post-synthesis modification, guest molecule introduction, and composite formatting. The performance of conductive MOFs can even outperform those of commercial RuO2 catalysts or Pt-group catalysts. However, it is difficult to endow most MOFs with high conductivity. This review summarizes the mechanisms of constructing conductive MOFs, such as redox hopping, through-bond pathways, through-space pathways, extended conjugation, and guest-promoted transport. Synthetic methods, including hydro/solvothermal synthesis and interface-assisted synthesis, are introduced. Recent advances in the use of conductive MOFs as heterogeneous catalysts in electrocatalysis have been comprehensively elucidated. It has been reported that conductive MOFs can demonstrate considerable catalytic activity, selectivity, and stability in different electrochemical reactions, revealing the immense potential for future displacement of Pt-group catalysts. Finally, the challenges and opportunities of conductive MOFs in electrocatalysis are discussed. Based on systematic synthesis strategies, more conductive MOFs can be constructed for electrocatalytic reactions. In addition, the morphology and structure of conductive MOFs, which can change the electrochemical accessibility between substrates and MOFs, are also crucial for catalysis, and thus, they should be extensively studied in the future. It is believed that a breakthrough for high-performance conductive MOF-based electrocatalysts could be achieved.