Study On Hydrogen Production By Photoelectrochemically Splitting Solutions Of Organic Compounds

With the social development, demand for energy continues to grow. As a result, energy crisis and environmental problems are the most pressing problems we are facing now. With its clean, high energy density and good thermal conductivity as advantages, hydrogen energy will be the most important part of future energy development. So far, hydrogen is still mainly produced from steam reforming of fossil fuel or gasification of coal while hydrogen formed in sustainable manner, such as electrolysis, takes only small part of that. Using solar energy as a renewable source for hydrogen producing is conducted through pyrolysis, electrolysis, bio-fermentation method. Photoelectrochemical (PEC) way is a method that using solar energy to split the substrate for producing hydrogen accompanied with photocatalysts modified electrodes through which hydrogen energy can be combined with solar energy and electricity. Hydrogen and electricity are inter-convertible that hydrogen can be produced by electricity extracted from renewable source while hydrogen can be converted to electricity by means of a fuel cell and can be as complementary of each.PEC cell is a special electrochemical cell in which at least one electrode is modified with photocatalyst working as photoanode or photocathode. Most research are focused on materials preparation for photoanode, enhancing the PEC performance in order to improve the energy conversion efficiency. Oxide semiconductor is the most studied photocatalyst. Water was always used as the substrate to be split in a PEC hydrogen producing course. The theoretical splitting voltage of water is 1.23 V, while the practical voltage is about 2.0 V taking the over-potential generated from oxygen and hydrogen redox into account. Water splitting takes serious requirements to the property of semiconductors. Decomposing of some alcohols and organic acid needs less energy than that of water, which means lower the splitting voltage can be obtained. To electrolysis, photocatalytic reaction can used to lower the decomposing voltage, giving enhancement to efficiency. These organic compounds widely exist in waste water and products from biomass conversion. Research work aiming at decomposing these organic compounds for hydrogen production utilizing PEC system is attacting great attention. The formed CO2 during this process can be easily captured, separated, and used to form useful organic compounds through photosynthesis or artificial way. PEC splitting organic compounds for hydrogen production can not only provide clean energy but also solve the biomass reforming and protect the environment.This dissertation aimed at finding out proper photocatalyst modified photoanode to split organic compounds for hydrogen, and enhancing the solar energy conversion efficiency. Research work was started from using the most studied TiO2, then the visible light driven photocatalyst CdS modified electrode as photoanode, formic acid (FA) and methanol (MA) as splitting substrate. The PEC behaviors of these photoanode in the organic aqueous solution were studied and reaction mechanisms were discussed. Hydrogen evolution and energy conversion efficiency were also tested and evaluated. Then, in order to improve the stability and PEC performance of the electrode, the composite electrode of CdS and TiO2 was prepared. The PEC behaviors of CdS/TiO2 composite electrode in the solution of FA were also studied.The main research contents and results are as follows:1. Using F doped Tin oxide (FTO) glass as substrate, TiO2/FTO electrode was prepared by dip-coating method in sol solution. The sample was mixed-phase of anatase and rutile while annealing temperature exceeded 500℃. The impact of annealing temperature on PEC performance was also tested,500℃was chosen as annealing temperature. The surface of TiO2/FTO electrode is compact and uniform, covered with assemblies of TiO2 nanoparticles with a diameter of approximately 20 nm. The thickness of the film is about 140 nm. Under Xe lamp irradiation, FA splitting behavior was tested at different applied current density employing TiO2/FTO as photoanode and Pt foil as cathode. Compared to splitting water or utilizing FTO and Pt foil as the anode, the splitting voltage is much lower and can be as low as -0.27 V. The experiment results show that the splitting voltage has a relationship with the concentration of free formate group and charge properties of photocatalyst surface. The evolution rate of hydrogen measured by GC is 130μmol h-1 at the current density of 20 mA cm-2 and energy conversion efficiency can be 1.79%. Photoelectrolysis of formic acid has the potential to be an efficient way for hydrogen production with high energy conversion efficiency.PEC behaviors of TiO2/FTO as photoanode in solution of 1 mol L-1 MA with 1 mol L-1 NaOH and 0.5 mol L-1 H2SO4 were studied while Pt as cathode. In H2SO4 solution, the splitting voltage is close to that of NaOH. However, gas evolution can be rarely observed which is suppressed by dissolved O2. The cyclic voltammetry experiment result shows that alkaline condition favors hydroxyl to radical OH transformation and PEC oxidation of MA. The evolution of hydrogen was measured by GC. At 20 mAcm-2, energy conversion efficiency can be 1.69%, which is close to that of FA. From the utilization view, only one mole hydrogen is can be obtained from one mole FA decomposition while three mole hydrogen from one mole MA. MA has higher ratio than that of FA, seems more suitable using as source.2. CdS/FTO electrode was prepared by chemical bath deposition method. From data of UV-vis spectra of CdS/FTO, the band gap is calculated as 2.4 eV. The surface of the electrode is uniform and flat, and is made of small grains with diameter about 100 nm. Small particles about 20 nm can be observed on those grains which may favor the charge separation. The thickness of the film is about 120 nm. FA splitting behavior was tested at different applied current density employing CdS/FTO as photoanode and Pt foil as cathode. No matter irradiated with visible light or whole spectra light, CdS/FTO shows good PEC performance. The splitting voltage can be as low as -0.4 V. while applied with higher current density, no obvious voltage fall was observed which is caused by CdS electrochemical corrosion. PEC behavior of CdS/FTO in solution of different concentration FA and NaOH was tested. Best photocatalytic performance was obtained in solution of 4 mol L-1 FA and 2 mol L-1 NaOH with pH value about 3.5 which is related to surface charging state. The evolution of hydrogen was measured by GC. At 0.4 mAcm-2, energy conversion efficiency can be 1.2% under visible light irradiation. Further improvement of the PEC performance and stability should be done by mean of semiconductor combination or other measures.3. Using FTO glass as substrate, TiO2/FTO electrode was firstly prepared by dip-coating method. Then CdS/TiO2/FTO composite electrode was fabricated by coating CdS on the surface of TiO2 utilizing sequential chemical bath deposition method. The electrode is made of anatase. Deposition of CdS over electrode is confirmed by EDX data. The band gap of the composite electrode is about 2.62 eV which means visible light absorption. The surface of the electrode is uniform and compact, and the thickness of the film is about 280 nm. Utilizing CdS/TiO2/FTO as photoanode and Pt foil as cathode, the PEC behavior of CdS/TiO2/FTO was studied through liner sweep voltammetry, constant current electrolysis and stable photocurrent measurement. The results show that CdS/TiO2/FTO has higher stability and PEC performance than single content electrode which benefit from enhanced charge separation efficiency and visible light absorption and which is good for FA splitting and has the potential to improve the energy conversion efficiency.