Molecular electrocatalysis:Metalloporphyrins for hydrogen evolution and oxygen evolution and oxygen reduction reactions
Hydrogen evolution reaction(HER),oxygen evolution reaction(OER),and oxygen reduction reaction(ORR)are involved in many important energy conversions in living systems and artificial catalytic systems.The H-H and O-O bond formation/cleavage are key steps in these reactions.However,intermediates involved in the H-H and O-O bond formation/cleavage exhibit high reactivity and short lifetimes,making their identification and study difficult.Recently,metalloporphyrins have been extensively studied as catalysts for HER,OER,and ORR.Moreover,porphyrin ligands can effectively stabilize these highly active intermediates,and the ligand skeleton can be systematically functionalized.These characteristics of metalloporphyrins are conducive to studying catalytic reaction mechanisms and the relationship between molecular structures and catalytic performances.This article reviews recent works from our group using metalloporphyrins and their derivatives as molecular catalysts for HER,OER,and ORR.Then focus on the catalytic reaction mechanisms and the influence of catalyst molecular structure regulation on catalytic efficiency.For HER,we established the correlation between three H-H bond formation mechanisms and metal hydride electronic structures and clarified the bimetallic homolytic H-H bond formation mechanism.Metal hydrides(H-Mn+)are generated through protonation of metal centers as key intermediates in HER.Depending on the different metal basicity,H-Mn+has three hydricity to evolve H2.For the homolytic H-H bond formation,the coupling of two H-Mn+molecules is a fast process.Thus,it is a challenge to detect the H-Mn+.Using Ni porphyrins bearing different steric effects,strong evidence for the bimolecular homolytic H-H bond formation mechanism can be provided.Furthermore,electrocatalytic HER activities can be enhanced by tuning porphyrin meso-substituents,introducing axial ligands with an electron-donating ability,and improving electron and proton transfers.For OER,we elucidated O-O bonding formation through the water nucleophilic attack(WNA)mechanism and demonstrated a bimolecular coupling mechanism between two metal-hydroxide radicals.High-valent metal-oxo/oxyl species are involved in O-O bond formation during water oxidation.Generally,with increased metal d electrons,metal-oxo units become more reactive and less stable.However,terminal Cu-oxo species cannot exist due to the large d electron number.Mechanism studies suggested that,the resulting CuⅡ-OH* radicals may undergo bimolecular coupling to form the O-O bond.Moreover,water oxidation selectivity could be controlled:Cu porphyrin catalyzed 4e-water oxidation to O2 and 2e-water oxidation to H2O2,respectively in neutral and acidic solutions under controlled potentials.We also enhance the WNA efficiency by using protective axial ligands and regulating metal ion coordination structures.For ORR,we improved activity and selectivity by providing rapid electron transfer,introducing electron-donating axial ligands and hydrogen-bonding interactions,constructing dinuclear cooperation and porphyrin-support domino catalysis.Moreover,Co porphyrin atropisomers were used to realize both 2e-and 4e-ORR.The ORR selectivity could be controlled by tuning only steric effects without modifying molecular and/or electronic structures.Lastly,we grafted metalloporphyrins on various electrode materials through several strategies,which boosted electrocatalytic performance and highlighted promising applications of molecular electrocatalysis.Thus,the benefits of exploring metalloporphyrins for the HER,OER,and ORR were demonstrated in this Review.The conclusions summarized herein are of great value for the development of molecular and material catalysts used for small molecule activation reactions.
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