Hydrogen Fuel Cells Anode Electrocatalysis:Mechanism Study,Catalyst Synthesis And Membrane Electrode Assemblyapplication

In recent years,the increased demand for energy along with growing environmental concerns have made the research and application of new energy a pressing need.Hydrogen energy,known for its high energy density,abundant sources,and pollution-free nature,has garnered widespread attention as a critical component of future energy structures.The efficient use of hydrogen relies on hydrogen fuel cells.However,the widely used proton exchange membrane fuel cells heavily rely on costly platinum group metal（PGM）catalysts,which hinders the widespread commercialization of fuel cells.The emerging hydroxide exchange membrane fuel cells（HEMFC）offer the potential to reduce reliance on PGMs,which can effectively reduce the demand for PGMs at the cathode.However,the anode hydrogen oxidation reaction（HOR）kinetics faces a significant slowdown by approximately2-3 orders of magnitude when changing the electrolyte from acid to alkaline,resulting in high PGMs demand in anode of HEMFC.In addition,the unsatisfied alkaline HOR stability of traditional Pt-based catalysts further increase the PGM loading,thereby significantly raising the cost.Therefore,it is crucial to develop highly active and stable alkaline HOR catalysts for the further advancement of HEMFCs.A deep understanding of the fundamental reasons on p H effect of HOR is key to develop high-performance alkaline HOR catalysts.However,the HOR micro-mechanism is still debated.In this paper,we first systematically studied the HOR process of the model system by using electrochemical in-situ spectroscopy.The core issues such as the role of adsorbed hydroxide(OH_ad)in HOR have been well discussed.And we revealed the essence of the p H effect in HOR and added a description of the electric double layer（EDL）effect to amend the kinetic equations.Subsequently,based on the conclusions obtained from the mechanistic studies,we developed new types of alkaline HOR catalysts with excellent activity and stability.The efficient,stble and low-cost HEMFCs were achieved by using these catalysts.The specific contents of this paper include the following four aspects.（1）The role of OH_adin enhancing the kinetics of the HOR in alkaline media is a topic of debate.In this paper,we clarified the specific function of OH_adby studying the acidic HOR electrocatalysis,where the OH_adcannot perform as an intermediate.We show that the OH_adpromotes the acidic HOR kinetics over three platinum-group metals（Ru,Pd,and Rh）.In-situ spectroscopies verify that OH_adpersists on pre-oxidized metals,thereby weakening the hydrogen binding but strengthening the interfacial water binding through the hydrogen bond.Instead of serving as a reactant or product under alkaline conditions,the OH_adplays a catalytic role for acidic HER/HOR by facilitating proton transfer across the electrified double layer.Inspired by the fundamental understandings,a Ru-Ru O_x（OH）_y/C nanocatalyst is further developed and shows comparable activity to Pt in both half-cell RDE test and practical PEMFC.（2）The non-Nernst p H effect of HOR kinetics on PGMs is perplexing.Unlike other PGMs,Ru electrode exhibited a unique"V"shaped p H-dependent HOR activity,which helps unraveling various influencing factors.In this paper,we systematically investigated the intermediates of Ru-catalyzed HOR in different p H electrolytes by using in-situ electrochemical infrared spectroscopy.The position of the interfacial water peak displayed a similar"V"shaped p H dependency,while the shift in the Ru-H peak correlated monotonically with p H changes,clearly indicating that the structure of interfacial water is the dominant descriptor of HOR kinetics.The presence of OH_adon the Ru surface in high p H media improved the interfacial hydrogen-bond network,resulting in the observed"V"shaped p H dependency of interfacial water.Based on these findings,we further modified the Ru surface with different types of hydroxides to optimize the ratio of effective interfacial water and enhance the HOR activity.A linear correlation between the percentage of O-down water and the apparent activation energy of HOR is finding,which reveals the role of interfacial water during HOR process.Ultimately,we formulated a new kinetic equation by considering the effective interfacial water to describe the EDL effect.（3）Guided by the mechanism research,we developed a Rh Cr cluster catalyst supported on single atom sites modified carbon（named as Rh Cr/C-SA）as an excellent alkaline HOR catalyst,by simultaneously optimizing the hydrogen binding energy and EDL structure.The electronic tuning from Cr to Rh and the redistributed interfacial water structure induced by Cr single atom sites on supports contributed to the ultrahigh activity.The activity of this catalyst exhibited more than two order of magnitude enhancement compared with Pt/C and comparable to the activity of Pt/C in acid.With this catalyst as anode catalyst and high-performace Ag Mn/C as cathode catalyst,high performance HEMFCs with peak power density of 1.81 W cm^-2was achieved with total PGM loading of only 0.011 mg cm^-2.An ultra-high mass activity of 165 W mg_PGM^-1was achieved,surpassing the DOE 2025 target by an order of magnitude.（4）In this paper,we developed an atomically dispersed Ir catalysts on carbon supported Mo₂C substrate(named as Ir _SA-Mo₂C/C)through the strong interaction between Mo₂C and PGM.Benefited form the unique electronic structure of Mo₂C and Ir single atom site,the high stability of the Mo₂C substrate and the effective separation between the carbon support and PGM,the obtained catalyst exhibited high activity and stability for alkaline HOR.The specific exchange current density and mass activity at 0.05 V versus reversible hydrogen electrode（RHE）of Ir _SA-Mo₂C/C are respectively 4.1 m A cm_ECSA^-2and18.0 A mg_Ir^-1,which are 8.9 and 13 times that of Ir/C catalyst.Moreover,negligible decay was observed for Ir _SA-Mo₂C/C after continuously operating for 120 hours and 30,000-cycle accelerated stability test.This work illustrates that atomically dispersed precious metal on carbides may be a promising strategy for high performance HEMFCs.