A p/n photoelectrochemical (PEC) system composed of series-connected photocathode and photoanode is highly desirable for solar-energy conversion to storable and conveyable hydrogen via overall water splitting.  To achieve a solar-to-hydrogen conversion efficiency (STH) as high as or higher than 10%, it is needed to use photocatalytic materials permitting visible light absorption up to >650 nm.

　　We have recently reported that a p/n PEC system composed of a La₅Ti₂(Cu,Ag)S₅O₇ (LTCA) photocathode^[1] and a BaTaO₂N (BTON) photoanode^[2], both of which have photoactivity under irradiation up to 650 nm, is capable of spontaneous overall water splitting under visible light irradiation without external bias voltage^[3].  The STH of the p/n PEC system of LTCA and BTON was approximately 0.1 % in the initial stage^[3].  In the p/n PEC system of LTCA-BTON, the photocurrent and the working-potential (E_work) are determined by the intersection of the current-potential curves of the respective LTCA photocathode and the BTON photoanode used.  Therefore, an increment of the photocurrent and a high stability in aqueous electrolyte solutions at E_work are important.

　　Intriguingly, both the water reduction reaction on LTCA photocathodes and the water oxidation reaction on BTON photoanodes are strongly affected by ionic species existing in the electrolyte solution.  We will demonstrate in this paper the effect of coexistent ions in aqueous solutions and surface modification of the LTCA and BTON photoelectrodes upon the performance of the p/n PEC systems in the overall water splitting reaction.

Experimental.

　　LTCA photocathodes and BTON photoanodes were prepared using powder materials through the particle transfer method^[4].  For LTCA photocathodes, Pt catalyst promoting hydrogen evolution reaction was deposited by potentiostatic photodeposition under irradiation of simulated sunlight (AM1.5G) ^[3].  For BTON photoanodes, CoP_i catalysts for oxygen evolution reaction was deposited on the surface by electrodeposition^[2].  The PEC measurement was carried out using an Ag/AgCl/sat’d KCl reference electrode and a coiled Pt wire counter electrode in an Ar gas atmosphere at room temperature.  To examine the effect of coexistent ions in aqueous solution, M₂SO₄, MClO₄, and M₂HPO₄ (M = Li, Na, K) were used as supporting electrolytes.  The pH of aqueous solutions was adjusted to 11–13 by an aqueous MOH (M = Li, Na, K) solution.  A p/n PEC system was prepared by connecting a LTCA photocathode and a BTON photoanode fabricated individually.  The p/n PEC system was irradiated with visible light from a 300 W Xe lamp through a cut-off filter (l > 420 nm).  The amounts of evolved H₂ and O₂ were quantified with a gas chromatograph.

Results and Discussion.

　　The photocurrents of LTCA photocathodes at 0.7 V_RHE, typical E_work for p/n PEC systems of LTCA-BTON, increased in the order of Li⁺ > Na⁺ > K⁺ and ClO₄^- > SO₄^2- >> PO₄^3- when coexistent cations and anions were changed, respectively.  The photocurrents of BTON photoanodes at 0.7 V_RHE increased in the order of K⁺ > Na⁺ > Li⁺ and PO₄^3- > (ClO₄^- ~ SO₄^2-) for variation of cationic and anionic species, respectively.  It is likely that the dependence of the photocurrent of the respective photoelectrodes on the coexistent ions is attributable in part to the specific adsorption of ions.  For the p/n PEC systems of LTCA-BTON, LiClO₄ was employed as a supporting electrolyte because the LTCA photocathode in the presence of Li⁺ and ClO₄^- generated the high photocathodic current.  A p/n PEC system composed of a LTCA photocathode and a BTON photoanode in 0.1 M LiClO₄ + LiOH (pH = 12) was active in overall water splitting without external voltage.  Hydrogen and oxygen were generated at the stoichiometric ratio of 2:1 over 20 h.  In this presentation, the effect of surface modification for respective photoelectrodes on the performance of the p/n PEC system will be discussed in detail.

References.

[1] J. Liu et al., Energy Environ. Sci., 7, 2239-2242 (2014).  [2] K. Ueda et al., J. Am. Chem. Soc., 137, 2227-2230 (2015).  [3] T. Hisatomi et al., Energy Environ. Sci., 8, 3354-3362, (2015).  [4] T. Minegishi et al., Chem. Sci., 4, 1120-1124 (2013).