High-Performance Non-Precious Metal Electrocatalysts For Oxygen Reduction Reaction

The oxygen reduction reaction(ORR),which played critical roles in the application of fuel cell,metal-air batteries and electrochemical H2O2 generation,is an important reaction in electrocatalysis area.However,owing to the sluggish nature of the reaction,it needs electrocatalysts to speed up reaction kinetics.In mechanism,the oxygen reduction includes the four-electron reduction to H2O and two-electron reduction to H2O2.Therefore,ORR catalysts also could be categorized into four-electron process electrocatalysts and two-electron process electrocatalysts for H2O2 generation.Presently,the best four-electron and two-electron ORR catalysts are precious metal-based catalysts.However,high cost of precious metals and the limited abundance hinder the large-scale commercialization.Therefore,it's imperative to explore highly efficient and inexpensive non-precious electrocatalysts to accelerate the oxygen reduction reaction kinetics to motivate the commercialization of fuel cells and metal-air batteries.This paper contains two main studies that the author explored the highly efficient non-noble electrocatalysts for the four-electron ORR process and two-electron process.In the first study,a novel type of organic-carbon hybrids was developed from in-situ polymerization of iron phthalocyanine on conductive multiwalled carbon nanotube scaffolds using a low-temperature microwave heating method.At an optimal polymer to carbon ratio,the FePPc/CNT-1.5 hybrid electrocatalyst exhibits excellent ORR activity with a positive half-wave potential of 0.80 V vs.RHE(reversible hydrogen electrode),a large mass activity up to 18.3 A/g at 0.80 V and a low peroxide yield of<3%in acid.In 0.1 M KOH,the FePPc/CNT-1.5 exhibits further improved ORR activity featuring a positive half-wave potential of 0.93 V vs.RHE,even outperforming that of Pt/C by 40?50 mV.In addition,the strong electronic coupling between polymer and carbon nanotubes is believed to suppress the demetallization of the macrocycles,affording significantly improved cycling stability in acid.Our study represents one of few examples of non-precious metal based electrocatalysts prepared in the absence of high temperature pyrolysis while having an ORR activity in acidic media meaningful toward practical applications.In the second study,liquid phase exfoliated(LPE)molybdenum ditelluride(MoTe2)nanosheets based on graphene was first found that it is a highly active and selective electrocatalyst for H2O2 electrochemical synthesis from two-electron O2 reduction.At an optimal ratio of MoTe2+40%G exhibits excellent two-electron ORR performance with an onset potential of 0.57 V vs.RHE and a highly electrocatalytic H2O2 production selectivity up to nearly 80%.Furthermore,in the investigation of the stability of MoTe2+40%G for electrochemical H2O2 production from O2 in 0.5 M H2SO4,the percentage of electrochemical H2O2 keeps increasing and arrived to 90%.The remarkable two-electron ORR performance in experimental measurements make the MoTe2+40%G a promising electrocatalyst.The DFT computations were further performed to obtain a fundamental understanding of this unexpected high-efficient electrocatalytic performance.According to our calculations,the OOH*binding free energy(?GOOH*=4.17 eV)of MoTe2 with a small overpotential of 0.05 V is an indeed competitive value which emphasizes a fact that MoTe2 is rather active for the catalysis of two-electron ORR.Moreover,MoTe2 has a quite good two-electron selectivity as it has a rather weak O*binding free energy(?GO*= 2.56 eV),consistent with the experimentally observed considerable selectivity toward H2O2 production.