Electrocatalytic Electrodes For Overall Water Splitting Based On Cobalt Compounds/Graphite Foam

The development of eco-friendly electrocatalysts with earth-abundant metal elements for both hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）is important for overall water splitting to generate clean and sustainable chemical energy.Sulfides and nitrides of transition metals show high performances for HER and OER.The three-dimensional macroporous self-supporting electrocatalysts can be used as electrodes directly owing to the good strength and high electrical conductivity.Meantime,the small charge transfer resistance of graphite foam can provide the possibility of fast electrode reactions.The unique interlayer porous structure can supply a large number of in-situ growth sites for catalytic active species and effective pathways for the release of hydrogen（H₂）or oxygen（O₂）bubbles.The catalytic active species can be uniformly dispersed on graphite sheets,exposing the active sites effectively to catalyze HER and OER.A self-standing cobalt disulfide/graphite foam（CoS₂/GF）electrocatalytic electrode was successfully prepared by in-situ growth of CoS₂ nanosheets on GF.CoS₂/GF showed good activities for both HER and OER,achieving a current density of 20 mA cm^-2 at 144 and 226 mV for HER in acid solution and alkaline solution,respectively,and 298 mV for OER in alkaline solution.The CoS₂/GF electrodes were used as both electrocatalytic cathode and anode for overall water splitting,achieving a current density of 20 mA cm^-2 at a low voltage of 1.74 V.This three-dimensional macroporous electrocatalytic electrode is cheap and available in a large area,leading to its wide practical applications.Trace level cobalt nitrides doped graphite foam（Co-N/GF）electrocatalytic electrode was successfully prepared to work as a bifunctional high-performance and self-standing electrode for both HER and OER in alkaline electrolyte.The catalytic activities of Co-N/GFs with different annealing temperatures（600,700,800,900 and 1000oC）were carefully studied.Among them,Co-N/GF-900exhibited the best HER activity and Co-N/GF-700 showed the best OER activity,achieving the current density of 10 mA cm^-2 at low overpotentials of 165 and313 mV,respectively.Co-N/GF electrodes were further constructed as a catalytic cathode and anode couple（Co-N/GF-900||Co-N/GF-700）for overall water splitting,reaching the current density of 10 mA cm^-2 at a low cell voltage of 1.68 V and good long-term stability.Our work reveals that introducing a trace level of Co-N into graphite foam enhances significantly its electrocatalytic activity and stability for both HER and OER,comparable with the high-loading Co-based catalysts reported previously.Moreover,Co-N/GF electrodes improve the atom utilization significantly,hence reducing the consumption of transition metals and being benifical to the environment.