| Hydrogen production from water electrolysis is one of the most promising pathways in the field of green hydrogen preparation due to its green,flexible production and high purity.However,the sluggish chemical reaction process of oxygen evolution reaction(OER)results in a high overpotential.The construction of high performance and low-cost OER electrocatalysts is the key to break the bottleneck of the overall efficiency of water electrolysis.Transition metals(Co,Ni,Fe,etc.)and their phosphides are efficient non-precious metal catalysts for OER.However,the conventional solid structure of catalysts limited their OER electrocatalytic performance due to the problems of few active sites and high resistance of mass transfer.To address these problems,transition metals/metal phosphides/nitrogen-doped carbon composites with hierarchical hollow structure were prepared.By designing and optimizing the microstructure of the materials and using the synergistic effects of transition metals,transition metal phosphides,and nitrogen-doped carbon,the electrocatalytic performance was improved effectively.1.Using cetyltrimethyl ammonium bromide(CTAB)as a soft template,ZIF-67 hollow spherical structure material was prepared.ZIF-8 was further coated to ensure the maintenance of structural skeleton after the carbonization at high temperature.The material was carbonized in inert atmosphere at different temperatures to explore the influence of carbonization temperatures on the OER properties.At the appropriate carbonization temperature,carbon nanowhiskers were formed on the surface of hollow carbon spheres by the catalytic effect of Co nanoparticles at high temperature.Co nanoparticles/nitrogen doped carbon composites with hierarchical hollow structure(Co/HNC)were successfully prepared.The overpotential of the synthesized catalyst(Co/HNCT-800)was 320 mV at 10 mA cm-2 in 1 M KOH.2.Solid cubic ZIF-67 was used as a precursor and was transformed into hollow CoFe PBA by anion exchange reaction with[Fe(CN)6]3-ions.Phenolic resin was further coated to ensure the maintenance of the structural skeleton of the material after high temperature carbonization.Hollow CoFe/nitrogen-doped carbon(CoFe/HNC)materials were prepared to investigate the OER electrocatalyst performance of hollow bimetallic carbon-based materials.The effect of different carbon shell thickness on the performance of OER electrocatalysts after carbonization was investigated by regulating the thickness of the coated phenolic resin.The material with the best coating thickness(CoFe PBA@PR-2)was selected for carbonization at different temperatures to investigate the effect of different carbonization temperatures on the electrocatalytic performance.The overpotential of the synthesized CoFe/HNC-2-700 catalyst was 331 mV at 10 mA cm-2 in 1 M KOH.3.Hollow transition metal/nitrogen-doped carbon materials were further phosphated to transform the metal in the materials into metal phosphides,thus investigating the effect of the synergistic interaction between the transition metals,metal phosphides,and heteroatom-doped carbon-based materials on the OER electrocatalytic activity.The Kirkendall effect was also used to transform the material into a more complex hierarchical hollow structure to expose more active sites and reduce the mass transfer resistance,thus enhancing the performance of OER electrocatalysts.For Co-P/HNCT-800-300,solid Co nanocrystalline particles were transformed to hollow CoP and Co2P particles during the phosphating process due to the Kirkendall effect.The overpotential of the synthesized Co-P/HNCT-800-300 was 287 mV in 1 M KOH at 10 mA cm-2.For CoFe/HNC-2-700,solid CoFe nanocrystalline particles were transformed to hollow cobalt-iron phosphide particles during the phosphating process due to the Kirkendall effect.The synthesized CoFe-P/HNC-2-700-300 exhibited an overpotential of 304 mV at 10 mA cm-2 in 1 M KOH. |
