Design Of Transition Metal Hydroxide Based Materials In Electrocatalytic Water Splitting

Electrocatalytic water splitting with cathodic hydrogen evolution reaction(HER)and anodic oxygen evolution reaction(OER)present large overpotentials due to the polarization on the two electrodes,which greatly decrease the efficiency of water electrolysis.Therefore,it is very important to develop electrocatalysts for HER and OER with high efficient performances.As is known to all,Pt based material is one of the most effeicent HER catalysts and RuO2 and IrO2 are the state-of-the-art OER electrocatalysts.Nevertheless,their high price and low reservation limit their wide practice application,so it is of great significance in both theory and reality to develop high-effeciency precious metal electrocatalysts with low mass loading and non-noble metal catalysts with low cost.Transition metal hydroxides,such as nickel hydroxide and cobalt hydroxide,have stimulated their extensive researches on electroctalytic HER and OER due to their great merits of superior electrochemical activity and durability along with facile preparation.Based on these,the major research of this dissertation is controlled synthesis and performance optimization for these transition metal hydroxide based materials.1.Pt based materials are one of the most efficient HER electrocatalysts,especially in alkaline media,but their high cost and low reservation have limited their wide application.Although many non-noble-metal electrocatalysts have motivated intense research on electrocatalytic HER,their poorer HER performances than Pt-based catalysts enormously restricts their practical applications.Consequently,decrease the mass loading of Pt in Pt-based catalysts but with high efficient performace in basic media is of great significant but very challenging.Herein,we used a facile strategy to have prepared a composite material of atomic-scale Pt clusters decorated on a phase Ni(OH)2 nanowires(Ptc/Ni(OH)2)with low Pt mass loading of 4%and small size of below 1 nm.In 0.1 M KOH,the obtained Ptc/Ni(OH)2 shows superior HER performance to the commercial 20 wt%Pt/C.2.We used a facile hydrothermal method to synthesize two type of three dimensional hierarchical flower-like Co(OH)2 composed of nanosheets(Co(OH)2-NSs)and nanobelts(Co(OH)2-NBs),respectively.By modulating the reaction time,we could precisely operate the building blocks from nanosheets to nanobelts to form the hierarchical flower-like Co(OH)2 nanomaterials.And then we systematically investigated the effect of this structural control on their electrocatalytic performances towards oxygen evolution reaction(OER).We found that Co(OH)2-NSs and Co(OH)2-NBs possessed the same size,mophology,and specific surface area,but in 1 M KOH,there was great difference of catalytic OER properties between the Co(OH)2-NSs and Co(OH)2-NBs,and the Co(OH)2-NBs with large edges exhibited superior OER activity.Moreover,we also used cation exchange method to synthesize Fe incorporated in the three dimensional hierarchical Co(OH)2-NSs and Co(OH)2-NBs(called Fe-Co(OH)2-NSs and Fe-Co(OH)2-NBs)for further optimizing their electrocatalytic OER performances.3.It is pointed out by theory and experiment that,in the whole OER process,surface Ni atoms on the Ni-based materials are first partially oxidized into NiOOH,and followed by catalysis reaction.The NiOOH species are the real active site.Therefore,further understanding of the relationship between electrocatalytic activity and chemical valence of Ni component in nickel-based compounds is very of great significance.Herein,for the first time,we synthesized a case of porous nickel sulfides(NixSy)with different proportion of high-valance Ni hybridized with N,S co-doped carbon nanosphere(NixSy-NSC)materials:Ni9Ss-NSC,Ni9S8-NiS1.03-NSC and NiS1.03-NSC and systematically studied the chemical valence-dependent electrocatalytic activities of these NixSy-NSC.It is found that the increasing proportion of high-valance Ni giving rise to enhanced OER performances of these NixSy-NSC materials.4.Clean energy technologies such as water splitting and fuel cells have been intensively pursued in the last decades for their free pollution.However,there is plenty of fossil energy consumed in the preparation of the catalysts,which results in a false cycle in production of heavy pollution.Therefore,it is much desired but challenging to fabricate high-efficiency catalysts without extra energy input.Herein,we used a facile one-pot room-temperature method to synthesize a highly efficient oxygen evolution reaction(OER)electrocatalyst of nickel iron layered double hydroxide grown on Ni foam(NiFe LDH/NF).And the NiFe LDH/NF exhibits excellent OER performance.In conclusion,we investigated the control synthesis and performance optimization of transition metal hydroxide based materials like Ni(OH)2 and Co(OH)2.In HER,we designed and prepared atomic-scale Pt cluster decorated on a-Ni(OH)2 nanowires with excellent HER performance in alkaline media;in OER,we realized precise controll of the structure of hierarchical Co(OH)2 and NixSy to optimize their OER activities;finally,a high efficienct OER catalyst,NiFe LDH/NF was prepared by green synthesis.All these efforts are beneficial to the design of more efficient catalysts in the future.