Carbon-based materials as highly efficient catalysts for the hydrogen evolution reaction in microbial electrolysis cells: Mechanisms, methods, and perspectives

Developing sustainable hydrogen production by efficient conversion technologies is vital to address environmental and energy challenges. Microbial electrolysis cells (MECs) can produce hydrogen from wastewater and solid wastes, which is considered a promising technology. The design and fabrication of cathodic catalysts for the hydrogen evolution reaction (HER) is a key factor in the future development of MECs. Carbon materials have the advantages of large specific surface area, high electrical conductivity, good stability, and low price. However, the catalytic activity of pristine carbon materials is usually low. Previous reviews mainly focused on the classification of different catalyst materials, but little about mechanisms. This review comprehensively addresses the mechanism related to enhancing HER performance and the regulation of the characteristics of composites based on non-noble metals and carbon materials via different interfacial/defect engineering methods for achieving the ultimate goals: (i) increasing the number of HER active sites; (ii) reducing the Gibbs free-energy of hydrogen binding (& UDelta;GH*); (iii) enhancing the electrical conductivity. It is noteworthy that when the electrochemical desorption, i.e., the Heyrovsky step or Tafel step, is the rate-determining step of HER, adjusting the d-band center of the transition metal site to optimize the hydrogen adsorption energy may further enhance catalytic activity. In addition, the effects of cathodic biofilms on HER performance are discussed. Finally, we present several potential perspectives for boosting HER to promote the application of MECs.