Studies On Carbon-based Non-noble Metal Oxygen Reduction Catalysts For The Cathode Of Fuel Cell

Fuel cell（FC）is a new type of energy conversion device which has been considered to have broad prospects for development due to its high energy density,environmentally friendliness,and high conversion efficiency.But its commercial development is still a huge challenge.One of the main reasons is that the kinetics of the cathode oxygen reduction reaction is relatively slow,and a large amount of precious metal platinum catalyst is needed to make it happen at a lower overpotential.However,platinum resource reserves are limited,and the market price is high,leading to high catalyst costs and severely hindering the large-scale commercialization of FC.Therefore,the development of inexpensive and easily available high-activity non-noble metal ORR catalysts is of great research significance for promoting the development of fuel cell technology.In recent years,non-precious metal carbon-based oxygen reduction catalysts have become a hot spot in the current fuel cell ORR catalyst research field due to their low cost,abundant resources,and excellent catalytic performance.In this paper,we designed and prepared a series of high-performance Fe-Nx/C catalysts by introducing inorganic iron sources,activation treatment of metal organic framework compounds（MOFs）,immobilization strategy of carbon support and nitrogen-containing additives,and self-sacrificial metal strategies in the self-assembly stage.The main research contents include:（1）The Prussian blue coordination structure was formed by self-assembly of transition metal iron inorganic salt and potassium ferricyanide,and zinc chloride was introduced as the activator.The Fe and N co-doped carbon-based catalysts were prepared by high-temperature pyrolysis of these precursors.The effect of decomposition temperature and the addition of activator on the catalytic performance of ORR has been investigated.The introduction of zinc chloride acted as an activator,significantly improves the graphitization degree of catalysts,and increases the proportion of Fe₃C active species.When the pyrolysis temperature is 700℃,the catalyst has the best ORR activity:in0.1M KOH,the half-wave potential is 0.84V,which exceeds the performance of commercial Pt/C catalysts.And the ORR reaction process of the catalyst is dominated by 4 electrons transfer route.At the same time,we applied this strategy to the preparation of other active metal center catalysts and found that the activation of zinc chloride is also applicable to Co and Ni systems.（2）We selected egg yolk as the precursor,added ferric chloride as the inorganic iron source,and prepared N,P,S,Fe co-doped carbon-based non-precious metal catalysts AL-Fe/Yolk-800through high-temperature heat treatment and acid leaching steps.The catalyst exhibits excellent ORR catalytic activity in alkaline condition,with a half-wave potential of 0.85V,which exceeds that of commercial Pt/C catalysts.We found that the process of acid leaching after the introduction of an inorganic Fe source keeps the physical structure of the catalyst basically unchanged.While removing the inert metal Fe nanoparticles,the catalytic activity is significantly improved.The results of M（?）ssbauer spectroscopy and poisoning test further prove that the ORR active sites in the catalyst are mainly highly dispersed Fe-N₄.（3）Using inorganic iron salt as the metal source,organic molecule terephthalic acid（TPA）as the oxygen-containing ligand,and triethylenediamine（DABCO）as the nitrogen-containing ligand,a dual-ligand metal organic framework was prepared（MOFs）and used as atomic dispersion precursors（ADP）.The ADP was anchored by carbon support and nitrogen-containing additives.The Fe-N-C catalysts with high density of active sites were obtained by high-temperature pyrolysis of the anchored ADP.The effects of the type of ligand,the type and amounts of nitrogen-containing additives,and the pyrolysis temperature on the ORR activity have been studied.We found that the introduction of dual ligands significantly improved the ORR activity of the catalyst.The type and feed ratio of nitrogen-containing additives had a significant impact on the catalytic activity.When1,10-phenanthroline is applied as the nitrogen-containing additive and the mixing mass ratio is1:0.5:1,the catalyst Fe DT/0.5Ph-800 has the best catalytic activity.Under acidic conditions,the initial potential and half-wave potential are 0.9V and 0.78V respectively,which is close to the performance of commercial Pt/C catalysts in the same condition.（4）On the basis of the dual-ligand MOFs material Fe-TPA-DABCO,the self-sacrificial metal Zn was applied to prepare the bimetallic center dual-ligand MOFs precursor Fe-Zn-DABCO-TPA,and further modified by the anchoring strategy.After that,the single-atom Fe catalyst was prepared by high-temperature pyrolysis of the modified precursor.The effect of pyrolysis temperature and carbon support on the catalytic activity was investigated.When the pyrolysis temperature reach to950℃,the Fe/C₉₅₀ catalyst has the best activity.Under acidic conditions,the half-wave potential is0.84V,which is one of the best known non-noble metal ORR catalysts.The study found that the anchoring strategy significantly improved the dispersion of metal sites,inhibited the agglomeration of metal active centers during high-temperature pyrolysis,and made the active metal in the catalyst mainly exist in the form of single-atom Fe.At the same time,the immobilization strategy is also applicable to the active metal center Co.The Co/C₉₅₀ catalyst prepared under the same experimental conditions has the best ORR activity under acidic conditions.Compared with the Co₉₅₀ catalyst without carbon support,the half-wave potential is increased by about 100mV.