ORR Performance Study Of Heteroatom-doped Carbon-loaded Pt-Co Alloy Catalysts

Proton Exchange Membrane Fuel Cells（PEMFCs）are attracting a lot of attention as they are a fast-starting and environmentally friendly clean energy device that continues to meet the needs of a growing population and resource requirements.However,the slow oxygen reduction reaction（ORR）of PEMFC at the cathode is considered one of the key challenges for PEMFC.Platinum（Pt）-based materials are considered to be the most excellent catalyst materials to solve the bottleneck of ORR reaction and enhance the slow process of ORR reaction.However,the commercialization of PEMFC is severely hampered by extremely scarce Pt reserves,low natural abundance and high prices.Therefore,the development of catalysts with low cost and high activity is of great importance for the development of PEMFC.Alloying Pt with transition metals and modulating the electronic structure of Pt through ligand effects to optimize the binding energy of oxygen-containing intermediates in ORR reactions and thus enhance ORR activity has received continued attention in recent years.Among them,Pt-Co alloys have attracted a large number of researchers due to their high activity and stability in acidic media.In addition,pure carbon black carriers are susceptible to corrosion in special reaction environments,resulting in a weakening of the interaction between the nanoparticles and the carbon carriers and deactivation of the catalyst particles due to exfoliation.Therefore,modification of the carbon carrier is also essential.Doping of carbon black carriers with non-metallic atoms（e.g.N,P,etc.）can weaken the forces between the adsorbed oxygen molecules and Pt atoms and improve the lifetime of the carriers.Based on this,Pt-Co alloy particles were prepared and loaded onto heteroatomic N and P doped carbon black carriers respectively from the above perspective,and the microstructure of the materials was tuned by varying the mass ratio of heteroatomic doping.The microstructure and electrochemical properties of the catalyst materials were investigated.The main studies are as follows:（1）Pt-Co/C catalysts with low Pt loading（2%）are prepared using chloroplatinic acid and cobalt acetate as precursors and oleylamine and oleic acid as reducing agents by hot injection synthesis of Pt-Co alloy particles and loading on commercial carbon black Cabot Vulcan xc-72.Alloying Pt with Co changed the d-band center of Pt,exposing more active sites,and Pt-Co/C achieved a half-wave potential(E_1/2)of 0.86 V in alkaline electrolytes at 2%Pt loading and0.78 V under acidic conditions,both of which are close to the performance of 20%commercial Pt/C.The mass activity of 1.24 A mg _Pt^-1 and 0.42 A mg _Pt^-1 in acid and alkaline conditions respectively is better than that of Pt/C.The effective Pt-Co alloying allows the catalyst to maintain excellent electrocatalytic performance while reducing the Pt loading.（2）A nitrogen-doped Pt-Co alloy（Pt-Co/TNC）catalyst is prepared by exfoliating a thin layer of C₃N₄ using a bottom-up self-assembly method and thermally fusing it with a commercial carbon black（Cabot Vulcan xc-72）on which the Pt-Co alloy is anchored.In particular,the bonding between pyridine nitrogen and metallic cobalt atoms（Co-N）not only promotes the homogeneous distribution of the alloy on the carrier,but Co-N acts as a promoter to modify the alloy structure and thus enables the in situ adsorption-diffusion process of oxygen-containing species.The E_1/2 at a mass ratio of 0.25 for N elemental doping（Pt-Co/TNC-25）is 0.885 V under alkaline conditions,which is better than the Pt/C（0.88 V）.From a kinetic analysis,the high proportional thermal fusion of 2D TL-C3N4 with C allowed Pt-Co/TNC-25to obtain an excellent porous structure favorable for mass transfer,with a Tafel slope of only 69m V/dec under alkaline conditions.（3）Phosphorus-doped Pt-Co alloy catalysts（Pt-Co/PC）are prepared by thermal fusion of sodium dihydrogen phosphate and commercial carbon black Cabot Vulcan xc-72,anchored to a Pt-Co alloy.The phosphorus doping yielded catalysts with a 50%mesoporous ratio.The ordered mesoporous material not only facilitates rapid transfer and exposure of the active center,but also limits the size of the nanoparticles anchored on it and avoids agglomeration.Thus,the catalyst material（Pt-Co/PC）doped in the right mass ratio has superior E_1/2（0.83 V）and MA(1.38 A mg _Pt^-1)to commercial Pt/C under acidic conditions and has good accelerative stability in 0.5 M H₂SO₄ as a PEMFC cathode ORR catalyst.