Efficient Photo(electro)Catalytic Oxygen Reduction Reaction Based On Non-noble Metal Catalysts

The interconversion of chemical and electrical energy by means of renewable resources is one of the most effective way to address the energy and environmental issues.Oxygen reduction reaction（ORR）is the key process in fuel cell devices,and in other energy-related technologies.During the reaction,the reduction of dioxygen（O₂）molecule involves four protons coupled with four electrons that produces water as the final product and concurrently generates electrical power.However,the ORR in the proton-exchange membrane fuel cell（PEMFC）is kinetically sluggish,which greatly hinders the output efficiency of the fuel cells.At the same time,the development of the noble-based Pt catalysts is hindered by its scarcity,high cost and poor stability.Therefore,developing new types of high-efficiency electrocatalysts is of both academic and technological interest.As a competitive reaction to the four-electron pathway,the two-electron ORR process is thermodynamically unfavorable which would significantly lower the output efficiency of fuel cells.Furthermore,the produced H₂O₂ could deactivate the electrocatalysts by interacting with the specific components in the catalysts that would generate strongly oxidative radicals and destroy the Nafion membrane.However,the two-electron reduction of O₂ itself includes a very important application,i.e.,the H₂O₂ production,which is a green and versatile chemical in a variety of applications including environmental protection,chemcial industry,etc.Meanwhile,solar energy is renewable,abundant and cheap.Thus,utilizing solar energy for the photoelectrochemical reduction of O₂ to H₂O₂ offers a sustainable approach of producing H₂O₂to the anthraquinone process.We dedicate our efforts to developing highly efficient ORR electrocatalysts,and to exploring selective semiconductor catalysts for the photoelectrocatalytic reduction of O₂ to H₂O₂.The main results of the dissertation are summarized as follows.（1）Polythiophene-iron（PTh-Fe）coordination polymer was synthesized by in-situ Fe doping with the thiophene monomer.The coordination between Fe and S could weaken the chemical bond on the polymer and create more active sites.After codoping with N source,the catalysts exhibit excellent ORR activity that is superior to commercial Pt/C catalyst.In contrast,the sample with Fe doping after thiophene polymerization exhibits less porous morphology and much inferior activity.The results demonstrate the importance of the wayof Fe doping into the precursor.（2）CuCl₂ as a promoter was introduced into the Fe-N-C matrix to tune its chemical properties and electronic structures.The introduction of Cu induced the generation of bamboo-like nanotubes structure and enhanced the increase of the N content.XPS results demonstrated that the peaks of Fe 2p shift to higher binding energy,as a result of electron delocalization between Fe and the carbon nanotubes.The electron delocalization alters the electronic structure of the metallic species,enabling uniform distribution of Fe and Cu species.The Cu decorated electrocatalyst exhibits excellent activity toward the ORR with half-wave potential 50 mV more positive than the commercial Pt/C in 0.1 M KOH.It also shows comparable ORR activity in0.1 M HClO₄ solution.Furthermore,we also investigate the doping effects of other transition metal and also found the similar positive effect for the ORR activity.（3）We demonstrated a p-type metal-free polymer semiconductor,namely polyterthiophene（PTTh）,as the selective and efficient photoelectrochemical catalyst for O₂ reduction to H₂O₂.The PTTh electrode could be directly used as the photocathode for ORR without loading a cocatalyst and exhibits⁹5%selectivity for peroxide.PTTh was found to exhibit distinct electrochemical behaviors under different pH conditions.In alkaline solutions,it undergoes one electron attachment as the initial step,whereas in acidic conditions PTTh firstly undergoes the one-electron,one-proton process.DFT calculations based on the reductive intermediate reveal that the 2e^-process is kinetically favored over the 4e^-pathway,thus leading to the selective reduction of O₂ to H₂O₂.In alkaline solution,the photocathode efficiently produced H₂O₂without promoting consecutive H₂O₂ decomposition reactions and reached a record high concentration（¹10 mM）.Furthermore,a solar-driven H₂O₂ production without any applied bias is realized by integrating PTTh photocathode for O₂ reduction and NiFeO_x/BiVO₄photoanode for H₂O oxidation.The system achieved over 100 mM of H₂O₂ within 8 h.