Investigation Of Early-Transition-Metal Nitrides Based Fuel Cell Electrocatalysts

Proton exchange membrane fuel cells（PEMFCs）are widely regarded as one of the most promising clean energy technologies due to their high energy conversion efficiency,ease of operation and zero/low emissions.Unfortunately,the commercialization of fuel cells has been hindered by several factors,such as the sluggish oxygen reduction reaction（ORR）rate,high cost—caused mainly by the use of Pt-based catalysts—and insufficient stability.Many researchers have dedicated to address these issues and great progress has been made.Researches on low-Pt catalysts,such as Pt-based core-shell structured catalysts,has not only lowered the Pt loading but also improved the ORR activity of Pt.In the mean time,Fe-N_x/C-based non-Pt catalysts have shown even better ORR activity in alkaline solution than the commercial Pt/C catalyst.At the aspect of stability,calix[4]arene-modified Pt electrode as oxygen-tolerant anode catalyst provides a new solution to solve the stability issues of fuel cells in the stage of startup and shutdown.Nevertheless,the high cost and scarcity of Pt still hinders the large-scale application of low-Pt catalysts,while the ORR activity and stability of non-Pt catalysts in acidic solution are still unqualified for practical application.Therefore,efforts to exploit new and cheap catalysts with high activity and stability remain greatly needed.Early-transition-metal nitrides are known for their tremendous physical properties,including high hardness,high melting points and high corrosion resistance.They combine the characteristic properties of covalent solids,ionic crystals and transition metals and have good stability and conductivity.These merits enable them as promising catalysts and supports.Herein,in this thesis,we conducted a series of researches on early-transition-metal nitrides-based fuel cell catalysts.（1）Many early-transition-metal nitrides have been investigated as ORR catalysts,and their ORR activities were found to be poor,but the reason for this has barely been discussed.Herein,we took TiN,VN,and CrN as example and explored the possible limitations preventing early transition-metal nitrides from being competitive catalysts towards the ORR.Based on experimental results and theoretical analysis,we suggest that insufficient d electrons is the main limitation for O₂ dissociation on TiN and VN,whereas an unsuitable surface geometric structure that is hard to form a bridge adsorption is the main limitation for O₂ dissociation on CrN.（2）We prepared a series of transition-metal doped VN via a complexation-nitridation method and investigated the effects of doping on the ORR activity of VN.The results showed that the Co-doped VN showed greatly enhanced ORR activity over pure VN in both alkaline solution and acidic solution.The ORR activity of Co-doped VN in alkaline solution was close to that of JM 20 wt%Pt/C,with the half-wave potential gap less than 100 mV.It was found that Co doping facilitated the four-electron-transfer ORR pathway.X-ray photoelectron spectroscopy（XPS）clearly reveals that Co doping significantly increased the d electrons on V atoms.O₂ temperature-programmed desorption（O₂-TPD）measurements indicate that Co doping created new sites for oxygen adsorption.Both the enhanced d electrons on V atoms and the new formed adsorption sites induced by Co doping facilitated O₂ dissociation therefore remarkably improved the ORR activity of VN.（3）We prepared a series of transition-metal-doped CrN and carbon-supported CrN via the same complexation-nitridation method and a hydrothermal-nitridation method,respectively.The effects of doping and conductivity on the ORR activity of CrN were investigated.The results indicated that the transition-metal-doped CrN only showed slightly enhanced ORR activity.Contrarily,the carbon-supported CrN exhibited remarkably enhanced ORR activity in acidic solution,its ORR onset potential reached up to 0.81 V（vs.RHE）.It was found that the ORR in acidic solution on the carbon-supported CrN was dominated by the four-electron-transfer mechanism.According to the results of X-ray photoelectron spectroscopy（XPS）,O₂ temperature-programmed desorption（O₂-TPD）,electrochemical impedance spectroscopy（EIS）,and four-point probe method,it was confirmed that the significantly enhanced ORR activity of the carbon-support CrN was not derived from the new formed active sites or enhanced oxygen adsorption,but derived from the much enhanced electron transfer rate（or conductivity）.The research on transition-metal doped CrN and the carbon-supported CrN suggests that the insufficient electron transfer rate（or conductivity）is the key factor that limits the ORR activity of CrN in acidic solution.（4）According to the distinct structure-sensitivity between theσ-type bond in H₂ and theπ-type bond in O₂,we designed and prepared a HD-Pt/TiN material by highly dispersing Pt on the TiN surface via an incipient wetness impregnation method.The results showed that HD-Pt/TiN exhibited excellent selectivity toward the hydrogen oxidation reaction（HOR）and the ORR.Compared with commercial Pt/C catalyst,our HD-Pt/TiN catalyst retained the excellent HOR activity of Pt,significantly inhibited the ORR activity of Pt,and remarkably improved the Pt utilization ratio.These merits make our catalysts a promising and cost-effective oxygen-tolerant anode catalyst to improve the stability of PEMFCs during startup and shutdown.