Computational Design of Durable and Selective Double Atom Catalysts Toward the Electrochemical NH3 Production: Role of Carbon Defects

Abstract

Electrochemical NH3 production through nitrogen reduction reaction (N2RR) has received much attention as an energy-efficient way to produce ammonia, which is one of CO2-free hydrogen carriers. Note that electrochemical N2RR is carried out under mild temperature and pressure conditions, unlike the conventional Haber-Bosch process for chemical NH3 production, which requires a lot of energy due to the high temperature and pressure conditions. In recent years, the concept of double atom catalysts (indicated DAC) has been proposed to improve the reactivity of catalyst owing to the maximized utilization and 100% dispersion of active sites. The main problems in the development of DACs are the poor stability [the dissolution of double atoms into electrolyte and agglomeration of double atoms] and the low selectivity [lower onset potential for N2RR than HER (hydrogen evolution reaction)]. In this study, by using density functional theory (DFT) calculations, we examined the role of carbon defects (di, tri, quad vacancies) in graphene in enhancing the stability and selectivity in single and double atom catalysts (Ru, RuRu, and RuFe) and screened the novel double atom catalysts which have high selectivity to NH3 and durability at reaction conditions by ligand and support engineering. First, we found the optimal carbon defect structure to suppress the dissolution and agglomeration of double atoms and unraveled the factors to enhance the selectivity to N2RR over HER. Second, we developed the multiscale model to design the selective and durable double atom catalysts by identifying the descriptors to represent the durability and selectivity of double atoms catalysts. Next, we computationally screened the novel DACs (consisting of transition metals) by using the developed multiscale model. Finally, through the electronic structure calculation, we elucidated the underlying mechanism of defect in determining the durability and selectivity of double atom catalyst. Our study can provide the design factors to efficiently engineer the carbon defect and double atom in the development of NH3 production catalyst.