Co-based Transition Metal Oxides And Phosphides As Catalysts For Water Oxidation

The exploration and development of green sustainable clean energy is an important topic for scientific research in the whole world.Since solar energy is widely distributed on the earth and it is inexhaustible,solar energy is regarded as the most promising clean energy for solving the current environmental pollution and energy crisis.However,the solar energy is distributed unevenly and discontinuously.Therefore,the successful development and utilization of solar energy requires a reasonable and efficient energy storage method.Inspired by nature’s photosynthesis,scientists envision the use of solar energy to drive water splitting,converting solar energy into chemical energy stored in hydrogen.As the high calorific value of hydrogen combustion and pollution-free water of hydrogen’s combustion product,hydrogen is regarded as an ideal future fuel to replace coal,petroleum,and natural gas.The bottleneck of photocatalytic water splitting is the extremely slow kinetic water oxidation reaction.Therefore,the development of efficient and stable water oxidation catalyst has become the most important task of artificial photosynthesis.Due to the characteristics of low cost,high yield,good activity,low toxicity,the first-row transition metals,such as,Mn,Fe,Co,Ni,and Cu oxides and phosphides have been widely developed as water oxidation catalysts.This article mainly focuses on the development of efficient and stable photocatalytic and electrocatalytic water oxidation catalysts based on cobalt-based metal oxides and phosphides,and studies the catalytic effects and mechanisms contained therein.The main contents are as follows:1.A series of M_nCo_3-nO₄（M=Mn,Fe,Co）catalysts were obtained by doping Mn and Fe into Co₃O₄ using low temperature co-precipitation to efficiently catalyze water oxidation.In dye-sensitized photocatalytic systems and electrocatalytic systems,we investigated the catalytic activity of M_nCo_3-nO₄ catalysts.In visible light-driven photocatalytic reactions,M_nCo_3-nO₄catalysts can effectively catalyze water oxidation under alkaline,acidic and neutral conditions.The performance of the catalyst Fe_1.1Co_1.9O₄ is very good.The oxygen yield under the optimal conditions is as high as 90.4%.The TOF_M（turnover frequency normalized by per mole of transition metal of initial 60s）and apparent quantum yield of Fe_1.1Co_1.9O₄ are 0.46μmol m^-2_cat and28.4%,respectively.In the M_nCo_3-nO₄ catalyst,Fe_1.1Co_1.9O₄ nanorods also have the best electrocatalytic properties.By Motte-Schottky and cyclic voltammetry analysis,it was found that the flat-band potential of Fe_1.1Co_1.9O₄ nanorods located at exactly between the oxidation potential of O₂/H₂O and the half-wave potential of Ru（bpy）₃³⁺/²⁺.This unique electronic structure may be the reason why Fe_1.1Co_1.9O₄ has excellent water oxidation performance.2.The corresponding catalytic mechanism is revealed by the facet effect of the model catalyst.Using spinel Co₃O₄ nanocubes,nanorods and nanosheets as model catalysts,a bridge between microscopic crystal surface structure and macroscopic catalytic activity was established to study the structure-activity relationship and reaction mechanism of Co₃O₄ towards water oxidation.A maximum TOF（initial 60s turnover frequency,normalized by surface area）of 0.50μmol m^-2_cat s^-1is observed towards Co₃O₄ 112 and a similar TOF of 0.46μmol m^-2_cat s^-11 is obtained over Co₃O₄110,while Co₃O₄ 100 shows a minimum TOF of 0.33μmol m^-2_cat s^-1.The apparent quantum yield normalized with BET surface area(AQE_BET)of Co₃O₄ 112,110 and 100 are 34.4%,31.1%and23.0%,respectively.The time-resolved laser photolysis experiments revealed that the{112}and{110}crystal faces of Co₃O₄ participate in the electron transfer rate are faster than the{100}crystal plane during photocatalytic water oxidation.The Co²⁺-Co²⁺active sites with an atomic distance of 3.495?only appear in the{112}and{110}crystal planes but not in the{100}crystal planes,which may explain why the activities of Co₃O₄{112}crystal planes and{110}crystal planes are better than the{100}crystal plane.In addition,we confirmed that Co²⁺in spinel Co₃O₄is more active than Co³⁺under visible light driven water oxidation.3.The effect of phosphorus vacancies in cobalt phosphide on its water oxidation performance is first constructed and studied.Using a reduction method towards the synthesized cobalt phosphide nanorods supported on graphene,a novel cobalt phosphide catalyst R-CoP_x/rGO with rich phosphorus vacancies and hierarchical nanostructures is obtained.The treated cobalt phosphide catalyst shows better catalytic activity and stability at the same time.P vacancies induced in-situ formation ofβ-CoOOH has abundant defects and more lattice distortions,more active electronic structure,and thus has better water oxidation performance.The overpotential（η）of R-CoP_x/rGO（O）is only 268 mV(current density of 10 mA cm^-2,without iR compensated),and its electrocatalytic water oxidation performance far exceeds the precious metal benchmark catalyst RuO₂.Using the"cladding of carbon layer"method,it is confirmed that the surfaceβ-CoOOH riched in defects and lattice distortions are very important for the good performance of R-CoP_x/rGO（O）.