Study On Non-noble Metal Electrocatalysts For Hydrogen-oxygen Fuel Cell Related Reactions

With the global energy crisis and growing environmental problems,human demand for new clean energy sources is more urgent.Hydrogen is clean and renewable,so is considered a new energy sources to replace the traditional fossil energy carriers with a high energy density.As a clean power generation technology,hydrogen-oxygen fuel cell is still facing two major scientific and technical challenges for its large-scale application.On the one hand,the cathodic oxygen reduction reaction?ORR?kinetics is slow,and the catalyst activity needs to be improved urgently;on the other hand,an efficient and clean way to generate hydrogen on a large-scale is required.Water electrolysis to produce hydrogen is clean and efficient,however,requires both efficient cathodic hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?catalysts.Focusing on the three electrochemical reactions?HER,OER and ORR?i,in this paper a variety of non-Pt catalysts were designed and synthesized,and the structure-activity relationship of electrocatalysts were studied systematically.The main contents of this thesis are as follows:?1?A series of carbon supported CoP nanoparticles with the average particle sizes from 3.3 to 9.2 nm were synthesized,via a solvothermal process followed by the low-temperature topological phosphorization,and for the first time,the size-dependent HER activity of the CoP is investigated and discovered.By virthe of the TEM,XRD,XPS and the electrochemical techniques,it is concluded that the 9.2 nm-CoP particles possessed high intrinsic HER catalytic activity as compared to the 3.3 nm-CoP,although the smaller one displayed a high overall catalytic activity due to the large surface area.Detailed studies manifested that the small CoP particles suffered from serious oxidation once exposing to air.In contrast,the relatively large CoP particles could maintain most cobalt in metallic state,which is beneficial for desorption of Hads,a rate determining step of the HER process.In addition,the low charge transfer resistance during the HER process is also crucial for the excellent HER activity of the relatively large CoP particles.?2?Ni_xFe_y-LDHs?x,y=0-1?electrocatalysts with lower crystallinity were prepared by hydrolysis after co-reduction.The results show that Fe doping causes Ni to change to a higher valence state,at the same time the electrochemical active area increases and the interface charge transfer resistance decreases.The prepared Ni₃Fe₁-LDHs have excellent OER electrocatalytic performance under alkaline conditions.When the current density reaches 20 mA cm^-2,the overpotential is only 254 mV.To reduce the anodic potential of the electrolyzer,the urea oxidation reaction?UOR?was adopted to replace the anodic oxygen evolution reaction;the Ni₉Fe₁-LDHs catalyst showed the excellent catalytic performance,and the anodic oxidation potential decreased significantly(1.37 V@20 mA cm^-2).Combining the UOR?Ni₉Fe₁-LDHs as the catalyst?with the HER?CoP as the catalyst?in a water electrolyzer,the electrolyzer voltage is only 1.41 V at 10 mA cm^-2.?3?Adopting the electrochemical technique combining the micro-cell reaction,a nanoporous Ag electrode was prepared,and the catalytic performance of the electrode for the ORR was studied.It was shown that AgCl was formed spontaneously on Ag surface through the micro-cell reaction in the presence of Cu²⁺and Cl^-ions.Even a low oxidation voltage(0.336 V_RHE)was applied,the formation of AgCl could be promoted.After a reduction potential was applied,the AgCl was reduced to form a self-supporting nanoporous Ag electrode.Compared with a smooth polycrystalline Ag electrode,the self-supporting nanoporous silver electrode exhibits excellent catalytic activity for the ORR?half-wave potential shifts positively106 mV?,together with good stability and tolerance to methanol.?4?By virtue of MOFs and graphene aerogels,a self-supporting three-dimensional carbon-based oxygen electrode?FeNC@GA-MF?with a hierarchical porous structure was prepared using a comprehensive assembly strategy.The experimental results show that the prepared FeNC@GA-MF catalyst has high specific surface area(1184.8 m²g^-1)and excellent ORR catalytic performance.The ORR initial potential and half-wave potential are 1.018 and 0.866 V,respectively.As the oxygen electrode of the Zn-air battery,the peak power density reaches up to 156mW cm^-2,which is about 17%higher than that of the battery with Pt/C as the cathode.This work provides a strategy for producing 3D self-supporting hierarchical porous carbon-based catalysts.