Construction And Performance Of Molecular Cobalt And Nickel Catalysts/Silicon Hybrid Photocathodes For Photoelectrocatalytic H₂ Production

In recent years,the production of hydrogen by solar-driven water splitting and the development of hydrogen energy have attracted more and more extensive attention.A photoelectrochemical cell integrates the light-harvasting with the electrolysis of water,allowing these two processes to occur simultaneously at the photoelectrodes,which is one of the most promising technologies for large-scale solar-driven water splitting.To build efficient bias-free PEC cells,the challenging work is to develop highly active,stable,inexpensive and mutually compatible photoanodes and photocathodes.Generally,semiconductors have very low catalytic H2-evolution activity or even no activity.Immobilization of molecular catalysts on the electrode surface can effectively enhance the rate of interfacial charge transfer and the efficiency of catalytic H2 production for semiconductor-based photocathodes.In this context,a series of hybrid photocathodes were built with p-Si as the light-harvestor,a TiO2 film as the protection layer and the loading layer for molecular catalysts,and cobalt and nickel complexes as H2evolution molecular catalysts.The focus of this thesis is to explore the influences of the structures of anchoring groups,linkers,and molecular catalysts on the PEC H2-evolution activity and stability of hybrid photocathodes,to have a better understanding about the relationship of these structural factors to the interfacial charge transfer kinetics and the PEC performance of photocathodes.The thesis maily involves the following aspects:(1)A series of cobaloxime catalysts,Co(dmgH)2Cl(Py-4-X)(dmgH=dimethylglyoximato anion,Py=pyridinyl;A,X=CONH(OH);B,X=PO3H2;C,X=COOH),bearing different anchoring groups as well as the cobaloxime Co(dmgH)2Cl(Py)(D)without anchoring group,were synthesized.The catalysts A-C were covalently immobilized to the surface of the pSi/TiO2 electrodes.Among the tested hybrid photocathodes,the one with a hydroxamate as an anchoring group displayed higher photoelectrocatalytic activity and better stability than the photocathodes with a carboxylate or a phosphonate as the anchoring group.The p-Si/TiO2/A photocathode exhibited a photocurrent density(J(0 V))of-0.32 mA cm-2 at 0 V vs reversible hydrogen electrode(RHE)in 0.1 M borate buffer at pH 9 under visible light illumination,and the catalytic current was attenuated only by 2.9%in the controlled potential photoelectrolysis(CPP)tests at 0 V vs RHE over 6 h.Moreover,electrochemical impedance spectroscopy(EIS)and transient absorption spectroscopy(TAS)revealed that the hydroxamate as an anchoring group was superior to the widely used carboxylate and phosphonate anchoring groups for reducing the electron transfer resistance,speeding up interfacial electron transfer and inhibiting electron-hole recombination.(2)A series of cobaloxime catalysts,Co(dmgH)2Cl(Py-4-Y-CONH(OH))(A,Y=none;E,Y=CH2;F,Y=CH=CH;G,Y=(CH=CH)2),with different linkers between the catalyst and the hydroxamate anchor were synthesized and immobilized to the surface of p-Si/TiO2.The results demonstrated that the p-Si/TiO2/G electrode exhibited the highest PEC activity among the tested photocathodes,despite the longer linker of-(CH=CH)2-compared to other ones.The J(0 V)of-0.68 mA cm-2 for p-Si/TiO2/G in neutral phosphate buffer solution is higher than the J(0 V)values of previously reported planar Si-based photocathodes decorated with molecular catalysts.Moreover,the p-Si/TiO2/G electrode displayed a steady photocurrent density of-0.53 mA cm-2 over 5 h of CPP test.Importantly,TAS studies showed the long conjugate linker,—(CH=CH)2-,only slightly retarded the interfacial electron transfer compared to other linkers,while it could largely suppress the surface electron-hole recombination,which led to transferring more photogenerated electrons to the surface-bound cobalt catalyst and hence boosting the PEC activity of the hybrid photocathode.(3)To further improve the PEC H2-production activity and stability of the Si-based hybrid photocathode,the hydroxamate anchor was connected to an Ns-chelating ligand,and the corresponding molecular nickel and cobalt catalysts,[(H2O)M{L-CONH(OH)}]2+(M=Ni,H;Co,I;L-CONH(OH)=N-(4-(hydroxycarbamoyl)benzyl)-N,N',N'-tris(2-pyridylmethyl)ethylenediamine),were synthesized.Moreover,catalysts H and I were immobilized to the surface of p-Si/TiO2(ALD/DB)electrodes(ALD=atomic layer deposition,DB=doctor blading)through hydroxamate anchors.Studies on the PEC performances showed that the pSi/TiO2(ALD/DB)electrodes modified with catalysts H and I were superior to that modified with cobaloxime catalyst A in terms of the PEC hydrogen production activity and the stability.Compared to previously reported molecular catalyst-modified Ga-free photocathodes,pSi/TiO2(ALD/DB)/H displayed higher photocurrent density(J(0 V)=-1.31 mA cm-2)and superior stability(>24 h)for PEC H2 production at 0 V vs RHE under simulated sunlight illumination.(4)In the final work,a noble metal-free two-electrode PEC cell was assembled by coupling the newly-built p-Si/TiO2(ALD/DB)/H photocathode with a Co4O4-OC4H9(J)cubane-modified BiVO4 photoanode.This biomimetic PEC cell allowed overall water splitting to produce H2 and O2 simultaneously in a approximate molar ratio of 2:1 in 0.1 M borate buffer solution at pH 9 under simulated sunlight illumination at zero bias.The calculated solar-to-hydrogen efficiency is 0.04%,which is the highest efficiency reported so far for molecular catalyst-based water splitting PEC cells.