One-Step Chemical Vapor Deposition Synthesis of Hierarchical Ni and N Co-Doped Carbon Nanosheet/Nanotube Hybrids for Efficient Electrochemical CO2 Reduction at Commercially Viable Current Densities

Carbon catalysts with metal and nitrogen dopants hold significant promises for an electrochemical CO2 reduction reaction (CO2RR). However, the fabrication of these carbon catalysts normally requires an energy-intensive synthesis process. Traditionally, 2D graphene and 1D carbon nanotubes (CNTs) are the most widely used carbon supports, but graphene tends to aggregate and CNTs suffer from low density of active sites on the surface. In this work, we developed a 3D hybrid carbon nanosheet/nanotube catalyst with nickel (Ni) and nitrogen (N) co-doped active sites for the CO2RR by a one-step chemical vapor deposition (CVD) method. Both single atomic sites and nanoparticles of Ni were observed on the hybrids, but the Ni nanoparticles were encapsulated by graphitic carbon layers during the CVD process, and as a result, the competing hydrogen evolution reaction was suppressed and high CO selectivity was achieved. The as-prepared catalyst with 20 min CVD delivered a stable CO Faradaic efficiency of 91% with a partial current density of 28.9 mA/cm(2) at -0.74 V in an H-cell setup. The same catalyst achieved a commercially viable current density of 600 mA/cm(2) in a flow cell with CO selectivity above 85%, at an applied voltage of -2.0 V vs reversible hydrogen electrode without iR compensation. To the best of our knowledge, these results are among the best performances in the literature in terms of both current density and CO selectivity for the CO2RR by carbon-based catalysts. Furthermore, catalysts developed in this work are synthesized at a moderate temperature without any acid/oxidant pretreatment or post-washing. The energy-efficient and environmentally benign synthesis and the significantly high performance of catalysts are essential to future large-scale CO2RR applications.