Studies On The Water Splitting Electrocatalyzed By Transition Metal Complexes

The traditional energy consumption structure,which is most based on fossil fuels and nuclear power,is going to be replaced by clean and renewable energy due to the limitation of fossil fuels and the potential dangers of nuclear power.However,renewable energy sources like solar,wind and hydropower are unevenly distributed in time and space,which will lead to instability of energy supply.Therefore,the key issue,energy storgy,must be addressed to ensure the flexibility of this new energy supply system.Water is the most abundant and the only wastefree compound in the world.By the way of water electrolysis,the surplus energy can be stored in chemical bonds in the form of oxygen and hydrogen.Water splitting consists of two processes: oxygen evolution by water oxidation?2H₂O ? O₂ + 4H⁺ +4e^-?and hydrogen evolution by water reduction?4H⁺ +4e^-? 2H₂?.There are large reaction barriers need to be overcome during these processes because of the transference of four electrons and four protons.In nature,water oxidation efficiently takes place at the Oxygen Evolution Complex of photosystem II in green plants and algae,while water reduction is accomplished by the active site of hydrogenases.However,these biological catalysts are difficult to adapt for commercial applications because their stability is often limited outside the native environment.Trying to lower the large reaction barriers and drive water splitting at appreciable rates,people have done a lot on the development of complexes with electrocatalytic properties,particularly those that employing inexpensive and earth-abundant first-row transition metals.Until now,only several transition metal complexes have been used as efficient electrocatalysts,and improvements on water solubility,catalytic efficiency and durability need to be addressed to realize their application in energy storage.To look for some rules,we studied nine transition metal complexes for electrocatalytic water splitting from the perspectives of ligand effect,metal effect and steric effect.The results are described as follows:1.The first three complexes are Co?bpbH₂?Cl₂ 1,Ni?bpbH-_--2?Cl₂ 2 and Cu?bpbH₂?Cl₂ 3.Studies reveal that complex 1 and complex 2 are bifunctional catalysts for both water oxidation and water reduction,and interestingly,complex 3 can only electrocatalyze reduction of water.With the same ligand,1 shows better catalytic performance for water oxidation than 2 both on efficiency and overpotential;and with the same overpotential,the catalytic efficiency for water reduction of 1,2 and 3 are in the order of 1>2>3.2.The next three complexes are FeCl[^{t Bu,OMe}BPDA] 4,FeCl[^Cl,Cl BPDA] 5 and CuCl[^Cl,ClBPDA] 6.Steric hindrance of the central metals of these complexes is much larger than that of the other six complexes,which may probably lead to the absence of water oxidation ability.Complex 5,whose ligand is modified by electron-withdrawing functional groups,shows modest overpotential than complex 4,whose ligand is modified by electron-donating functional groups.And with the same ligand,copper complex 6 shows better performance compared with iron complex 5 in electrocatalytic water reduction.3.Complex CuCl[^tBu,MePDA] 7 and CuCl[^tBu,OMePDA] 8 are designed to learn the influence of steric effect on catalysts' electrocatalytic performance.With little steric hindrance on the central metal,complex 7 and complex 8 show electrocatalytic performance both on water oxidation and water reduction.However,their poor water solubility may lead to their poor catalytic activities.4.Complex [Fe Cyclen?Cl?₂]Cl 9 is a water soluble catalyst with a macrocyclic ligand Cyclen.It shows favorable electrocatalytic performance for water oxidation in basic buffer solution.Besides,complex 9 will slowly decompose and the central metal will electrodeposit on the surface of working electrode during bulk electrolysis.This may be the reason why complex 9 cannot catalyze water reduction.