Photoelectrochemical Performances And Photochargeabilities Of TiO₂, SrTiO₃/ Hydrogen Storage Alloys Electrodes

In order to invent a new type of cells, which can be photocharged directly, some semiconductor such as TiO₂ nanoparticles modified with platinum, strontium titanate nanoparticles and thin films were prepared by different methods. Their photoelectrochemical performances, photochargeability to hydrogen storage alloys, and its mechanism were investigated.The 2-8 nm platinum nanoparticles were deposited photoassistedly on titania nanoparticles whose sizes were about 20-50 nm. The photochargeable electrodes (TPM electrodes) were prepared by modifying the hydrogen storage alloy electrode with the as-prepared 0.5 wt %Pt-loaded TiO₂ powders. The TPM electrodes can be charged by light, but their cycle numbers were too small. The TPM electrodes can be photocharged again when the electrodes were activated newly by small current. When the TPM electrodes were charged with a current of 0.2 mA and illuminated simultaneously, the discharge time was about 630 minutes, which is long than the sum (415 minutes) of the discharged time charged only with current (400 minutes) and that (15minutes) charged only with by light. At the same time, the rephotochargeability of the TPM electrode was improved greatly. When an UV photo is absorbed by TiO₂, an electron-hole pair may be generated. The photogenerated hole cannot only oxidize the water to produce O₂ and the absorbed H to form H₂O, but also may oxidize the hydrogen storage alloy to form MO, which hinders the solid diffusion of the H atom on the surface of TPM electrode. When the TPM electrode was irradiated and charged with a current of 0.2 mA simultaneously, the current could charge the TPM electrode. Furthermore, the current could keep the electrode active. So the electrode can be always charged by light in the process. There are two kinds of theory to explain the photocharging process. Each of them can illustrate the fact that happened in the photocharing process.The strontium titanate nanoparticles were prepared by direct precipitation method. The SrTiO₃ nanoparticles was global with the size of about 30 nm. There was conglomeration when the nanoparticles were annealed. The hydrogen alloy electrode was modified with the SrTiO₃ nanopaarticles to form a hydrogen storage alloy/SrTiO₃ (MHS) electrode. The MHS electrode can be charged by light to -0.79 V.The polycrystalline strontium titanate thin films prepared by hydrothermally were of photoelectrochemical performances. With the reactive temperature increasing, the size of its grains increase and its photocurrent and its open-circuit photo voltage increase first then decrease. When the temperature was 170℃, the photocurrent and the open-circuit photo voltage was the largest.The SrTiO₃ thin films prepared by magnetic controlled sputtering method was amorphous. With the annealing temperature increasing, the SrTiO₃ thin films were well crystallized and the sizes of its grains increase with orientation to (110). The anodic photocurrent and open-circuit photovoltage increase first, then decrease with the annealing temperature increasing. When the annealed temperature was 500℃, they were the largest. The influences of SrTiO₃ thin fihns' thickness on the anodic photocurrents of were very complex. When the potential was lower than -0.71 V, the anodic photocurrents increase first with the increasing thickness of the thin films, then decrease. When the potential was larger than, the anodic photocurrent increase with the thickness of the SrTiO₃ films. The anodic photocurrent increased with the niobium densities in the range of our research (0-2%).The factors that influenced the anodic photocurrent and the open-circuit photovoltage may also influence the photochargeability of the SNH electrodes. With the annealing temperature increasing, the final potential and the discharge time of the SNH electrode first increased, and then decreased. When the annealed temperature was 500℃, the potential was -0.893 V, which was the most negative, and the discharge time was 190 minutes, which was the longest. With the niobium densities in SrTiO₃ increasing, the final potential and the discharge time of the SNH electrode first increased. When the density of Nb in SrTiO₃ thin films was 2%, the Potential was -0.9 V and the discharge time was 25 minutes. The Potential and the discharge time increased, and then decreased with the increasing thickness of thin films. When the thickness was 90 nm, the potential was -0.893 V, which was the most negative, and the discharge time was 190 minutes, which was the longest.