Preparation of High-Efficiency Transitional Metal Electrocatalysts for Water Splitting and Nitrogen Reduction

Author:Liu Guo Qiang

Supervisor:zhang hai min


Degree Year:2018





The huge consumption of fossil fuels and corresponding environmental pollution problems have attracted great attention of people.To meet the demand of increasing energy for human beings,it is critically important to develop and utilize renewable energy technologies.In the field of clean energy technology,electrochemical energy storage and conversion technologies have been received widly attention because of its simple operation and high energy conversion efficiency.An effective way to develop green energy technologies is to convert natural and inexpensive small molecule compounds(such as H2O,N2,CO2,etc.)into high value-added chemicals(such as H2,NH3,CH3CH2OH,etc.)through electrocatalytic technique.As we all know,precious metal catalysts have exhibited excellent catalytic performances for the hydrogen evolution reaction(HER),oxygen evolution reaction(OER)and oxygen reduction reaction(ORR),however,the high cost and source scarity of precious metals limit their large-scale commercial applications.Therefore,development of low-content precious metal based catalysts,especially non-precious metal catalysts,has been attracting wide attention.The aim of this thesis is to develop several high-efficiency transition metal based electrocatalysts to promote the efficient conversion of energy in electrocatalytic water splitting to generate hydrogen and electrocatalytic synthesis of ammonia at ambient conditions.The findings obtained in this thesis have important guiding significance of designing and fabricating high-efficiency transition-metal electrocatalysts for electrocatalytic water splitting and N2 reduction reaction.The main research results are summarized as follows:1.Commercial activated carbon(AC)was used as the substrate to first pre-adsorb the Co2+/Ni2+ source,followed by thermal treatment in N2 atmosphere to obtain the CO0.83Ni0.17/AC electrocatalyst.As a multifunctional electrocatalyst,the as-prepared Co0.83Ni0.17/AC demonstrated bifunctional electrocatalytic activities toward the HER and OER in 1.0 M KOH electrolyte,delivering the overpotentials of 193 and 324 mV at 10 mA cm-2,respectively.Moreover,the presence of 10 mM benzyl alcohol in 1.0 M KOH,it was found that the electrocatalytic oxidation of benzyl alcohol to benzoic acid on Co0.83Ni0.17/AC is more favorable than the OER process,showing almost 224 mV less overpotential at 10 mA cm-2 with an oxidation potential of 1.33 V vs.RHE.As a high efficiency reaction,in a two-electrode system,the benzyl alcohol oxidation reaction is applied to replace the OER process as the oxidation half-reaction of overall water splitting,significantly improving the generation rate of H2 with 40%.Furthermore,the obtained Co0.83Ni0.17/AC as air cathode material is assembled into a rechargeable Zn-air battery,exhibiting lower charging voltage in the presence of 100 mM benzyl alcohol compared to the battery without benzyl alcohol in electrolyte.2.Commercial carbon fabric cloth with high conductivity and specific surface area was used as the matrix for the in-situ growth of NiMn LDH/CFC electrocatalyst by a facile solvolthermal method.The nanosheets are vertically grown on the CFC surface,favourable for the electron transport.Simultaneously,the introduction of Mn in catalyst can effectively mediate the electronic structure of NiMn LDH/CFC,thus enhancing its electrocatalytic activity.As an electrocatalyst,NiMn LDH/CFC displays superior performance of UOR(urea oxidation reaction)with an oxidation potential of 1.34 V vs.RHE in 1.0 M KOH containing 0.5 M urea.Furthermore,in a two-electrode system,NiMn LDH/CFC as both anobe and cathode catalyst,UOR is applied to replace OER process as the oxidation half-reaction of overall water splitting,and the generation rate of H2 is 3.7 times of the OER-introduced overall water splitting.3.Ultrathin nanosheet array Ni3S2 with 9~14 nm in thickness was directly grown on commercial Ni foam substrate(Ni3S2/NF)by in-situ vapor-phase hydrothermal(VPH)transformation of the nanosheet array structure Ni(OH)2/NF precursor.The ultrathin nanosheet structure of Ni3S2/NF-2(VPH time of 2 h)is favorable for exposing high ratio of catalytic active sites.As a multifunctional electrocatalyst,the as-fabricated Ni3S2/NF-2 displays excellent electrocatalytic activities toward the hydrazine oxidation reaction(HzOR)with a potential of 0.415 V(vs.RHE)to deliver a current density of 100 mA cm-2,OER with an overpotential of 425 mV obtaining the same current density.Owing to the superior HER and HzOR activities,Ni3S2/NF-2 is concurrently used as the cathode and anode material to construct a two-electrode system for the overall water splitting,delivering a high current density of 100 mA cm-2 at only 0.867 V in 1.0 M KOH with 0.2 M hydrazine.Comparatively,the applied voltage is 2.01 V for such two-electrode system to achieve a current density of 100 mA cm-2 in 1.0 M KOH without hydrazine in electrolyte.Density functional theory(DFT)calculations results reveal that the adsorption of N2H4 molecules on the Ni3S2(110)is more thermodynamically favourable than H2O,thus contributing to its high HzOR activity.4.Core-shell structure Au@CeO2 electrocatalyst was synthesized by a room-temperature spontaneous redox approach using Ce3+ and Au3+ as reaction precursors.The obtained Au@CeO2 composite displays irregular spherical morphology,and the porous structure of Au@CeO2 was beneficial for mass transport during NRR(nitrogen reduction reaction).Significantly,the characterization results demonstrate that the core layer Au nanoparticles possess small particle sizes(<10 nm)and the loading amount on CeO2 is 10.2 wt%,simultaneously,there are rich oxygen vacancies in shell layer CeO2 nanoparticles in the Au@CeO2.As the electrocatalyst,the Au@CeO2 is confirmed to be electrochemically active for the NRR with a NH3 yield rate of 28.2 μg h-1 cm’2(10.6 μg h-1 mg-1cat,293.8μg h-1 mg-1Au)and a Faradaic efficiency(FE)of 9.50%at-0.4 V(vs.RHE)in 0.01 M H2SO4 electrolyte under ambient conditions.The DFT calculations reveal that the CeO2(111)surface has high catalytic activity for NRR,together with high active Au nanoparticles,synergistically enhancing the NRR performance.5.Commercial carbon fabric cloth(CFC)was used as the substrate for the growth of ribbon-like MoO3 nanostructure.The MoN/CFC electrode was subsequently achieved through nitrogenization using MoO3/CFC as the precursor in Ar/NH3 atmosphere.As the NRR electrocatalyst,the MoN/CFC electrode exhibited superior NRR activity with a NH3 yield rate of 6.31 μmoL h-1 cm-2(107.3μg h-1 cm-2)and a Faradaic efficiency of 12.84%at-0.45 V vs.RHE in 0.1 M KOH electrolyte.The NRR active mechniasm can follow the Mars-van Krevelen process,namely,the surface N atom on MoN was firstly reduced to ammonia and the catalyst later regenerated with N2.