Photoelectrolysis of water in cells with TiO2 anodes

The photoelectrolysis of water has been investigated by experiments on cells consisting of an illuminated nTiO₂ (rutile) anode, an aqueous electrolyte, and a platinized-Pt cathode. It has been found that such cells operate either in the photogalvanic mode (no H₂ evolved) or in the photoelectrolytic mode (H₂ evolved at the cathode by decomposition of water), depending on whether or not the electrolyte surrounding the cathode contains dissolved oxygen. In both cases, current flows through the external circuit and O₂ is evolved at the anode. For operation in the photogalvanic mode, maximum values of 80–85% for the external quantum efficiency (η) for current production have been measured at hv≈4 eV with both single-crystal and polycrystalline TiO₂ anodes. Similar results have been obtained in preliminary experiments with SrTiO₃ anodes. The internal quantum efficiencies, corrected for reflection and absorption losses, are close to 100%, indicating that the band bending in TiO₂ under photogalvanic conditions is sufficient to separate the electron-hole pairs generated by photon absorption and also that the oxygen over-voltage for charge transfer at the semiconductor-electrolyte interface is negligible for illuminated anodes. For operation in the photoelectrolytic mode, η is only 1–2% if the anode and cathode are shorted together, but the efficiency can be greatly increased by applying a bias voltage. By using a photogalvanic TiO₂-Pt cell to supply this voltage, it has been possible to obtain η values of ～20%, computed on the basis of the total number of photons incident on the anodes of both cells. All the observations can be given a straightforward explanation in terms of the energy levels of the electrodes and the electrolyte.     

A metallized channel guide antenna for use over a cylindrical ground screen

A low profile metallized channel guide antenna mounted over a cylindrical ground plane is shown to have excellent characteristics for sector elevation coverage. Proper selection of the spacing between antenna and ground plane allows optimum gain at the horizon and relatively constant gain over a wide elevation sector. Metallizing the sides and bottom of the channel guide improves the elevation coverage by suppressing the image of the antenna below the cylindrical ground plane surface.     

Experimental evaluation of an array technique for zenith to horizon coverage

An experimental study of a new array concept with application to providing low cost efficient antennas with hemispherical coverage for aircraft to satellite communication links is described. The combined array-surface wave antenna consists of 64 waveguide elements scanned conventionally except at endfire. At endfire the array is shorted to become a corrugated surface-wave antenna and excited by an eight-element feed to provide a directional beam near the horizon. The array is rotated to give hemispherical coverage.     

Electrode materials for the photoelectrolysis of water

There has recently been a renewed interest in photoelectrochemical processes at semiconductor-electrolyte interfaces for possible applications in solar conversion. In particular, the photoelectrolysis of water by sunlight in electrochemical cells to produce the fuel H₂ has resulted in considerable research. In this paper, the concepts of photoelectrolysis are used as a guide for deriving the physical and chemical properties necessary for materials to be efficient electrodes in such cells. Presently available materials are considered in this context along with prospects for the future.