Nickel And Cobalt Catalyst-based Homogeneous Photocatalytic Systems And Silicon Photocathodes For H₂ Production

Conversion and storage of solar energy to a chemical fuel,that is H2,by sunlight-driven water splitting is an ideal approach to solve the problems of energy shortage and carbon dioxide over-discharge.During the past decades,studies on the homogeneous photocatalytic systems and photocathodes for ligt-driven hydrogen production have achieved substantial progresses.However,there are still many crucial scientific and technique problems to be studied and solved.One of these problems is developing highly active and low-cost hydrogen evolution catalysts and photocathodes.On the basis of such background,this thesis focused on a noble-metal-free homogeneous photocatalytic system with a new nickel complex as catalyst and on two types of Co and Ni catalyst-modified silicon photocat!xodes for hydrogen production.(1)A nickel complex Ni(L)2(H2O)2](BF4)2(1,L = 2-(2-pyriyl)-1,8-naphthyrdine)containing internal bases was prepared and well characterized.With this nickel complex as a catalyst,a noble-metal-free homogeneous photocatalytic system was constructed in combination with fluorescein(Fl2-)as a photosensitizer and triethylamine as a sacrificial electron donor for visible-light-driven water reduction to hydrogen.Under optimal conditions,the turnover number of H2 evolution reached 3230 based on 1.The oxidative quenching of Fl2-emission by 1 was in accordance with the equation that describes the model of the coexistence of dynamic quenching and sphere-of-action static quenching,most possibly due to the electrostatically attractive interaction between the anionic nickel catalyst[1]2+and the cationic photosensitizer Fl2-at the ground state.A plausible mechanism for the photocatalytic H2 generation of Ni was presented based on DFT calculations.(2)The n+p-Si MPs/TiO2/3(3 = CoCl(dmglH)2(py(PO3H2)),dmg H= dimethylglyoxime,py(PO3H2)= pyridyl-4-phosphonate)photocathode was fabricated by grafting the phosphonate-functionalized cobaloxime molecular catalyst 3 to the surface of TiO2 protecting layer of a Si micro-pyramid array electrode(Si MPs).It was found that the photoelectrochemical deposition of 3 on the surface of Si photocathode resulted in the increase of photocatalytic H2-evolution activity.To enhance the loading of cobalt catalyst on the Si electrode surface,hydroxylamine was used as flocculant.The n+p-Si MPs/TiO2/Co NFs(nano-flowers)photocathode with high loading of cobalt catalyst was fabricated by the process of flocculating precipitation-photoelectric deposition Under optimal conditions,n+p-Si MPs/TiO2/Co NFs photocathode displayed the highest activity for photocatalytic water reduction to hydrogen:in 1.0MKOH solution with Jight intensity of 100 MW cm-2,the open-circuit voltage(Voc),short-circuit current density(Jsc),and half-cell photopower conversion efficiency(?hc)were 0.33 V vs RHE,-31.41 mA cm-2,and 2.05%,respectively.The Nyquist impedance and Mott-Schottky plots of n+p-Si MPs/TiO2 and n+p-Si MPs/Ti?2/Co NFs photocathodes indicated that the modification of Si electrode by Co NFs catalyst apparently reduced the charge transfer resistance between photocathode and electrolyte and inproved the band bending of Si photocathode,which led to enhancement of H2-evolution activity of n+p-Si MPS/Ti?2/Co NFs photocathode.(3)Hi'ghly efficient electrocatalysts,M-B(M = Ni,Co),have been grafted on the surface of a Si nanowire array electrode(Si NWs),respectively,by facile and quick electroless plating.Compared with the bare Si NWs photocathode,composite Si NWs/M-B photocathodes displayed evidently enhanced photoelectrochemical performance.The Voc of cathodic photocurrent was positively shifted by 530-540 mV to 0.44-0.45 V vs RHE,and the Jsc was up to-19.5 mA cm-2 in neutral buffer solution under simulated 1 sun illumination.Impressively,the ?hc values of the optimized Si NWs/Co-B(2.53%)and SiNWs/Ni-B(2.45%)photocathodes are comparable to that of Si NWs/Pt(2.46%).The Nyquist impedance and Mott-Schottky plots of Si NWs/M-B(M = Ni,Co)photocathodes indicated the uniformly decorated Ni-B and Co-B nanoparticles on the surface of Si NWs acted as effective mediators,which not only greatly accelerated the charge transfer kinetics between illuminated Si NWs and electrolyte,but also improved the open-circuit voltage of photocathode,leading to fast reduction of H+ to H2.