Research On Transition Metal Compounds As Catalysts For Water Splitting

Developing clean energy to substitute the traditional petrochemical energy is a potential way to meet the expansion of the energy demand of humanity.Hydrogen?H₂?is a typical clean energy which has high energy density and zero-emission.Hydrogen evolution reaction?HER?from electrolysis is a green way to get H₂.The electrolysis can be realized anywhere if water and electricity are supplied.However,the HER is limited by the half-reaction of oxygen evolution reaction?OER?,which also limits the practical application.Currently,the catalysts for HER and OER are noble metal-based materials.For instance,the commercial Pt/C is usually used as catalysts for HER.The RuO₂/IrO₂ are used for OER.The low crustal reserves and high-cost of those noble metals hamper the practical applications of electrolysis.Thus,it is essential to develop new catalysts with high efficiency for water electrolysis.In this work,three fundamental ways is used to achieve highly active OER catalysts.One is to synthesis new materials for OER.The second is systematic characterization on catalysts.Last but not least,the mechanism study can make the new materials design easier.These methods ensure the higher possibility of achieving highly active OER catalyst.Here,we use transmission electron microscopy?TEM?and X-ray absorption spectroscopy?XAS?to character the materials.The OER activity is also measured using electrochemical method.Here we found the correlation between the structure of materials and catalytic activity.The major study is displayed below.1.The Ir sing atoms are loading onto the ultrathin CoO_x amorphous nanosheets?ANSs?using co-precipitation method.The ANSs have complicate structure with high surface area of 213.3 m²/g.The electrochemical results show that the ANSs have high OER activity,which is 160-fold of commercial IrO₂.Meanwhile the ANSs have high stability during OER.The in-situ XAS and TEM are used to investigate the origin of high catalytic activity and stability.The Ir-O-Co sites are in-situ formed during OER.The O in Ir-O-Co is preferentially oxidized into O₂ during OER.The Ir-O-Co can further promote the valence of Co ions,and thus promote the negative-charged species adsorption efficiency.The Ir-O bonding length and average coordination numbers shrinks during OER,and then rebound to its initial state.During this process,the Ir atoms are trapped into CoO_x ANSs,ensuring the high catalytic activity stability.2.Using struvite-precipitation method synthesized amorphous N-Co₃Ni₁-P microplates?MPs?.The morphology and structure of N-Co₃Ni₁-P MPs can be controlled by changing surfactant and ageing temperature in reaction.This method can modify the catalytic activity of N-Co₃Ni₁-P MPs.The different stoichiometric ratio in struvite results the activity variation.The N-Co₃Ni₁-P MPs show best OER/HER activity among the synthesized struvite.The OER activity of N-Co₃Ni₁-P MPs is far better than commercial IrO₂.Meanwhile the HER activity of N-Co₃Ni₁-P MPs is near to commercial Pt/C.N-Co₃Ni₁-P MPs is mesoporous,and the pore in struvite is<10 nm.This structure ensuring high surface of N-Co₃Ni₁-P MPs.The N-Co₃Ni₁-P MPs also show high catalytic activity in overall water splitting.Moreover,the NH₄⁺and water molecules can be evaporated by thermal decomposition method.The electrochemical results show that the NH₄⁺can enhance the adsorption efficiency of OH^-on struvite.Besides,the crystallized water molecules in struvite is preferentially oxidized into O₂during OER.3.The ultrathin Co?OH?₂ nanosheets?NSs?are prepared via hydrothermal method.The morphology and crystallization of these Co-based NSs can be realized by calcination,electrostatic adherence and electrochemical oxidation.Once the Au loading onto the Co?OH?₂ NSs,the localized electron spin state would be changed,and thus lead to the improvement of adsorption and desorption efficiency on surface.Moreover,the Au loading can further improve the characteristic activity of CoOOH and the electrochemical surface area.Besides,the Au loading will increase the probability of low-bonded oxygen formation.These results can promote the OER.4.The cube-like Nickel hexacyanoferrates?Ni-HCFs?are prepared by co-precipitation method.The cube-like Ni-HCFs are used to synthesize Ni-HCFs@Ni-S core-shell nanostructures?CSNs?.The amorphous Ni-S shell modifies the sluggish surface of Ni-HCFs,and thus lead to the chemically active surface formation,ensuring the highly active OER catalysts formation.The CSNs have high morphology stability during OER.The in-situ Raman and in-situ fourier transform infrared spectroscopy?FTIR?results demonstrate that the organic metal partial can be oxidized in 20 min once the OER process started.This corrosion process leads to the FeNiO_x formation.During this process,the Ni-rich surface of HCFs@Ni-S CSNs transformed into Fe-rich surface,leading to the formation of NiO_x@FeO_x?OH?_y.