Study On The Structure And Electrocatalytic Properties Of Transition Metal Oxides

Hydrogen offers an eco-friendly,renewable and clean alternative resource in various applications,such as being a transport fuel in fuel cell or internal combustion engine vehicles.Water electrolysis is one of significant technologies to generate hydrogen both at large scale and with zero carbon emission.Although Pt-based materials are still the state-of-the-art catalysts for hydrogen evolution reaction(HER),the cost and limited supply of Pt make it impossible to be the catalysts for commercial hydrogen evolution.Consequently,non-noble transition metal-based compounds have attached great attention due to their excellent activity and high abundance.The development of new non-precious metal catalysts and understanding the origin of their activity for hydrogen evolution reaction(HER)is essential for rationally designing highly active low-cost catalysts as alternatives to the state-of-the-art precious metal catalysts.First,we demonstrated for the first time that the manganese oxides/hydroxides can be used as highly-active electrocatalysts for HER by fabricating three-dimensional(3D)Cu2O@MnO2 nanowire/nanosheet(NW/NS)core-shell arrays on Cu foam.Such hierarchical nanostructure not only offers plenty of accessible active sites,but also provides a 3D conductive network for fast electron transfer and facilitates gas bubble escaping.As a result,the developed Cu2O@MnO2 NW/NS arrays show high catalytic activity and excellent durability in terms of an very low overpotential of 137 mV vs.RHE at 10 mA cm-2 in 1 M KOH,which is the first demonstration of pure manganese oxide/hydroxide HER electrocatalysts with comparable activity to the state-of-the-art ones.Furthermore,after the careful analysis and characterization,we discovered that the high activity of Cu2O@MnO2 NW/NS should mainly originate from the evolution of birnessite-type MnO2(?-MnO2)into Mn(OH)2 during electrocatalytic activation,which is highly active for HER.These findings will open up great opportunities for the exploration of other Mn-based electrocatalysts and further understanding their catalytic processes.Second,we present a facile method to fabricate highly active hierarchical architecture of NiCo2O4 nanowire@NiCoO2 nano wire arrays on Ni foam for electrocatalytic applications(HER and OER).The obtained electrode exhibits a hierarchical dendritic structure,with NiCo2O4 nanowires as the backbone and NiCoO2 nanowires as the branches.Benefiting from the well-designed 3D hierarchical architecture with conductive NiCo2O4 nanowires as matrix for effective electron transportation,highly active well-defined NiCoO2 nanowires for sufficient catalytic sites,porous structure for easy mass transfer,and hierarchical dendritic nanostructure for the fast separateness of the as-generated hydrogen gas bubbles in hydrogen evolution process from the electrode surface,the prepared NiCo2O4@NiCoO2 NW/NW arrays demonstrate an excellent catalytic performance and long-term stability in terms of an extremely low potential of 1.756 V vs.RHE at 10 mA cm-2 for water splitting,165 mV vs.RHE at 10 mA cm-2 for HER,291 mV vs.RHE at 10 mA cm-2 for OER in alkaline media.The reported approach provides a facile way to explore new nanostructured electro catalysts for applications involving water splitting and energy conversion.