Nonmetal Doped Enhanced Performance Of Palladium-iridium Nanoalloys For Electrocatalytic Methanol Oxidation

Direct methanol fuel cells（DMFCs）are a class of advanced new energy technology combining convenience,security and cost-effectiveness.As the energy medium in DMFCs,methanol is the basic chemical product with mature preparation technology,environmentally friendly,high energy conversion rate and convenience of transport.The products of the methanol electrooxidation reaction（MOR）are only H₂O and CO₂,which can be converted into methanol by hydrogenation or other processes to achieve carbon-free methanol economy.Therefore,DMFCs can promote the realization of carbon peak and carbon neutral and have certain development prospect.However,catalyst（especially MOR catalyst）is one of the key bottlenecks restricting the commercialization of DMFCs.Palladium-based catalyst is the most cost-effective catalyst system for MOR and the best substitute for commercial platinum-based catalyst.However,the commercialization of palladium-based catalyst still needs to solve the following problems:（ⅰ）accelerate the MOR reaction kinetics and improve the electrocatalytic activity;（ⅱ）enhance the anti-poisoning ability and durability of catalyst;（ⅲ）improve the utilization of precious metal palladium and reduce the cost.In response to these issues of concern,a controllable nonmetal-palladium electrocatalytic reaction interface supported by nitrogen-doped graphene（NG）was constructed based on the element doping strategy,synthesized three types of nanocatalysts named Pd₇Ir Y_x/NG（Y=O,B,P）,and the role of oxygen（O）,boron（B）and phosphorus（P）for MOR and the catalytic mechanism were studied in detail:（1）The target catalyst（Pd₇Ir O_x/NG）was synthesized by introducing oxygen elements into three-dimensional nitrogen-doped graphene supported Pd nanocatalyst through in situ interfacial oxidation strategy.It was found that the effects of oxygen elements were as follows:the oxygen elements alloys optimized the adsorption performance of Pd interface,including reducing the adsorption energy of CO_ads and promoting the formation of reactive oxygen species(such as OH_ads)for oxidation removal of CO_ads.Compared with commercial Pd/C and reference catalysts（Pd₇Ir/NG and Pd/NG）,the prepared Pd₇Ir O_x/NG showed excellent catalytic performance for MOR in alkaline medium.（2）We inserted metal-like element B into Pd Ir alloy through the co-reduction strategy,and synthesized ultrafine nanoalloy supported on the three-dimensional nitrogen-doped graphene catalytic system（Pd₇Ir B_x/NG）.The results of physical and chemical properties tests revealed that the insertion of interstitial B can effectively inhibited the Ostwald ripening of nanoparticles.The Pd₇Ir B_x/NG catalyst showed high reactivity for MOR in alkaline medium,and it still had 58.2%current density retention rate after 500 cycles of cyclic voltammetry test.The mechanism research indicated that the multifunctional effect（electronic effect and strain effect）induced by B greatly enhanced the stability of the material and played an important role in improving the MOR activity.（3）The three-dimensional nitrogen-doped graphene supported ternary Pd-Ir-P nanoalloy catalyst（Pd₇Ir P_x/NG）was prepared by co-reduction strategy.The physical and chemical characterization showed that the interaction between P-containing alloys made the nanoparticles have better dispersion.Tafel slope calculated by linear sweep voltammetry showed that P element can accelerate MOR kinetics in alkaline medium.The study of CO stripping experiment showed that the electron effect and ligand effect induced by P atoms on the hydrophilic and electron-rich surface can effectively reduce the initial oxidation potential and peak potential of CO_ads,and significantly improved the anti-poisoning ability.Meanwhile,the catalytic activity and stability of Pd₇Ir P_x/NG were much higher than that of commercial Pd/C.Activity,durability and cost were the key factors to evaluated the comprehensive performance of three kinds of electrocatalysts.The experimental results showed that the order of catalytic activity was Pd₇Ir O_x/NG(1374.8 m A mg^-1)>Pd₇Ir P_x/NG(1207.6m A mg^-1)>Pd₇Ir B_x/NG(1141.7 m A mg^-1).The long-term stability of Pd₇Ir B_x/NG（58.2%）,Pd₇Ir O_x/NG（44.9%）and Pd₇Ir P_x/NG（36.8%）showed decreased trend along the activity retention rate after 500 cycles cyclic voltammetry test.The CO initial oxidation potential was regarded as the basis to evaluated the poisoning resistance of Pd₇Ir P_x/NG,Pd₇Ir B_x/NG and Pd₇Ir O_x/NG,which decreased successively.With similar synthesis processes and raw materials,oxygen atoms came from air,and borides were more expensive than phosphates.The cost of Pd₇Ir O_x/NG,Pd₇Ir P_x/NG and Pd₇Ir B_x/NG was increases progressively.Therefore,the otherness of performance made these three catalysts had strong competitiveness in the face of different market demands.