Solar Hydrogen Production Under Low Applied Bias Using Oxide Semiconductor Photocatalysts and Photoanodes
In the photocatalysis-electrolysis hybrid systemusing Fe3+/Fe2+ redox mediator, Fe2+ is produced from Fe3+ with producing O2 by oxidizing water over photocatalysts, and then H2 is produced in low-voltage electrolysis (around +0.77 V) with re-oxidizing Fe3+ to Fe2+.1,2 In this hybrid system, the potential restriction of the semiconductor is loosened, and several visible light responsive materials can be used. Moreover, since H2 is not produced in the photocatalysis, there is no need to depend on a precious metal cocatalyst, and H2 trapping is very simple. Therefore, the photocatalysis-electrolysis hybrid system is a breakthrough system that solves almost all the negative issues of conventional photocatalysis reaction.
It was found that the WO3 photocatalyst that was surface treated with cesium salt solution showed extremely high oxygen production activity.2 The QE at 420 nm reached 31%, and this was the highest value in the visible light range. It is thought that there is a mechanism that ion exchange sites are newly formed on the WO3 surface by cesium treatment and that the adherence and reaction of Fe3+ and H3O+ are facilitated. When the solar energy conversion efficiency (ηsun) is calculated whereby the energy of sunlight is converted to the chemical energy of Fe2+ ion, it reaches 0.38%. This value is half level of ηsun for corn.
In the discussion of practical use, it is necessary to estimate the cost of the whole system and compare the costs of H2 production. As the water electrolysis device to be compared, a large-scale solid polymer hydrolysis device (32,000 Nm3/h) is assumed. For the electricity cost, the least expensive power during the time zone is selected, and 8 yen/kWh ($0.08/kWh) at 40% operation is assumed. For the photocatalysis part, iron ion redox is used with solar energy conversion efficiency (ηsun) of the photocatalyst at 3%. From the above assumption, the H2 production cost of the photocatalysis-electrolysis hybrid system was estimated to be about 25 yen/Nm3 ($2.7/kg-H2).3 While, under the same conditions, the H2 production cost by the usual large-scale water electrolysis would be ca. 41 yen/Nm3 ($4.5/kg-H2). It can be said that 30 yen/Nm3 ($3.3/kg-H2) or less of the target cost in Japan can be achieved in the photocatalysis-electrolysis hybrid system with moderate ηsun. The ideal ηsun is > 24%, therefore, the H2 cost can be further reduced. In the presentation, we will show the progress of photocatalysis for photocatalysis-electrolysis hybrid system.
As an another solar hydrogen production under low allied bias, the photoelectrodes system have been widely investigated.Porous n-type semiconductor photoelectrodes, especially, have been vigorously investigated using visible-light responsive metal oxides, as a practical water splitting method using solar light. This is because oxide semiconductor photoelectrodes are superior in terms of the facility of oxygen evolution, as well as in the simplicity of preparation by a wet-coating and calcining process, and easy hydrogen collection at the counter electrode. Since our first report on BiVO4 photoanode,4 BiVO4 is attracting attention as an effective candidate material for water splitting. The Eg of BiVO4 (Eg = 2.4 eV) is smaller than that of WO3, which means it can utilize visible light below approximately 520 nm. It is noted that the ECB of BiVO4 is more negative than those of WO3 and Fe2O3. The applied bias photon-tocurrent efficiency (ABPE) of BiVO4 is more than 1.7% now.5 In the presentation, we will also show the progress of BiVO4 photoanodes for solar hydrogen production with high ABPE and Solar-to-hydrogen efficiency.
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