A Study Of Anode Catalysts For Alkaline Polymer Electrolyte Fuel Cells

The proton exchange membrane fuel cell（PEMFC）has dramatically progressed in the last few years because of its advantages of compact structure,high efficiency of energy conversion and zero emission.The further development of this technology,however,has been greatly hindered due to the high-cost components it utilizes,such as Nafion membrane and Pt-based catalysts.By contrast,the alkaline polymer electrolyte fuel cell（APEFC）works in the alkaline media,therefore its key components are able to use low-cost materials,providing the feasibility of reducing the total costs.Recently,APEFC has confronted enormous challenges and one of the most demanding problems is the development of high-performance non-noble catalysts on both anode and cathode.For the cathodic oxygen reduction reaction（ORR）,several non-noble catalysts have shown the catalytic activity comparable to Pt and the power density of APEFC using non-noble cathodic catalysts has also dramatically increased.However,as for the anodic hydrogen oxidation reaction（HOR）,even on Pt surface,the exchange current density of the HOR in alkaline media decreases by two orders of magnitude than in acidic media,hindering the search for high-performance non-noble catalysts.Up to now,the debate over this phenomenon has not been settled down.Therefore,at present it is of paramount importance to clarify the reason for sluggish HOR kinetics in alkaline media.In order to study the mechanism of the alkeline HOR,we have systematically deposited ten types of metal oxides/hygroxides clusters with even particle size and defined structure on planar Pt model electrodes.Electrochemical tests showed that there exists the proton-coupled electron transfer（PCET）at the interface between Pt and deposited clusters.We observed that the HOR exchange current density increases monotonically with higher electrode’s oxophilicity,indicating the existence of bi-functional mechanism.However,the Ru modification exhibits a clear deviation from the monotonic relation and it demonstrates a superior catalytic activity towards HOR,relative to other metal modifications.This unexpected result is ascribed to the reduction in hydrogen binding energy（HBE）owing to the modification of metallic Ru rather than its corresponding hydroxide or oxide.The present study indicates that the electronic effect and the oxophilic effect co-exist in the HOR on Pt-based catalysts in alkaline media,but the former one is of more significance.After reconsidering the research history of the HOR,we find that most studies only focused on the descriptors associated with adsorption,such as the HBE and OHBE,but ignored the chemical environment over electrodes,which could have great influences on the reaction kinetics.In this work,we investigated the impact of surface hydrophilicity on the HOR kinetics.The same Pt/C catalysts modified with seventeen types of small molecules were utilized to achieve a delicate control of hydrophilicity.A physical property of molecule,the difference between electrostatic potential extremums,was defined to characterize interaction between molecule and water and demonstrated to correlate well with the surface hydrophilicity.Moreover,a volcano relationship was established between the HOR catalytic activity and surface hydrophilicity,suggesting that the surface with middle hydrophilicity possesses the highest activity.The attained highest activity by tuning surface hydrophilicity is two times higher than that of bare Pt/C,quite close to that on Pt Ru/C.In situ electrochemical impedance spectroscopy shows that the enhancement of HOR rates on hydrophilic surfaces is attributed to the accelerated PCET.In situ surface-enhanced spectroscopic investigations further confirm that the hydrophilicity increase originates from the strengthened interaction between modified molecules and water,consistent with the results of molecular dynamic calculations.And this finding also verifies the feasibility of using the difference between electrostatic potential extremums as the descriptor of hydrophilicity.This work reveals the dual effect of the adsorbed water intensity on the alkaline HOR,that is the enhanced water adsorption draws the water closer to the electrode and thus benefit the HOR process,while the excessively reinforced water adsorption would impedes the H₂ diffusion and decreases the solubility of H₂,lowering the HOR kinetics.Frow the perspective of applications,it is an effective way to reduce the cost of APEFC by replacing Pt catalysts with non-platinum noble metals.The Pd-based catalyst is a good choice.To combine the mechanism study and screening of high-performance catalysts,we synthesized 31 kinds of Pd-based alloys with even distribution of particle size and defined bulk structure through the molten salt method.21 of these catalysts were washed with acid to produce the surface exposed only with Pd atoms.We found that the HOR catalytic activities of 52 types of Pd-based catalysts show a volcano relationship against the HBE and OHBE of these surfaces,with the surface of moderate HBE and OHBE exhibiting the highest HOR kinetics.This result not only manifests the existence of bi-functional mechanism on the surface of Pd-based catalysts,but also predicts the Pd Ni Co ternary alloy possessing the best performance.We then synthesized the Pd Ni Co/C catalyst of high metal loadings and well-defined structure via the improved impregnation method.This catalyst not only exhibited good performance in the RDE test,but also reached the peak power density of 1.6 W/cm² in the APEFC,which is much higher than the Pd/C.The primary advantage of APEFC compared to PEMFC is the total utilization of non-noble catalysts on both anode and cathode.Here,we prepared the non-noble Ni₅Mo catalyst with well-defined structure after the optimization of the element ratio and synthesis methods.Ni₅Mo shows both much higher specific and mass activities in the RDE test over the Ni catalyst,which also outperforms the reported non-noble Ni-based catalysts.Utilizing direct surface characterization methods,we attributed the origin of ehhanced activity of Ni₅Mo to the weakened HBE and strengthened OHBE due to the alloying of Mo.Moreover,the peak power density of APEFC using Ni₅Mo as the anode achieved～600 m W/cm².This work further comfirms that non-surported catalysts are able to be used as the anode in the APEFC.