Solar Hydrogen Production Under Low Applied Bias Using Oxide Semiconductor Photocatalysts and Photoanodes

It is important to carry out research strategically in order to put new solar energy conversion technologies into practical use and to realize a society based on renewable solar energy. In this presentation, we mainly showed the effectiveness of the photocatalysis-electrolysis hybrid system using redox mediator, as a promising candidate technology for low cost solar hydrogen production.

  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 (η_sun) 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 η_sun 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 (η_sun) 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 η_sun.  The ideal η_sun 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 E_g of BiVO₄ (E_g = 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 E_CB 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.

References

[1] K. Sayama, et al.: Jpn. Patent 3198298 (2001); U.S. Patent 09/028495 (1998).

[2] Y. Miseki, K. Sayama, et al., J. Phys. Chem. Lett., 1, 1196, 2010; RSC Adv. , 4 8308 (2014).

[3] K. Sayama, Y. Miseki, Synthesiology, 7, 79 (2014).

[4] K. Sayama, et al., Chem. Commun., 2908 (2003).

[5] T. Kim and K. Choi, Science, 343, 990 (2014); X. Shi, et al., Nature Commun., 5, 5775 (2014).