Photoelectrochemical water oxidation
is a challenging reaction
in solar water splitting due to the parasitic recombination process,
sluggish catalytic activity, and electrode stability. Oxide semiconductors
are stable in an aqueous medium but show huge charge carrier recombination.
Creation of a heterojunction is found to be effective for extracting
the photogenerated electrons/holes before they recombine to the ground
state. In this work, we created a heterojunction of BiVO4 with vacancy-ordered halide perovskite Cs2PtI6 and used it as a photoanode in PEC water oxidation. Cs2PtI6 is the only halide perovskite that is found to be
extremely stable even in strong acids and bases. We utilized the stability
of this material and its panchromatic visible light absorption property
and made the first unprotected heterojunction dual-absorber photoanode
for PEC water oxidation. At 1.23 V (vs RHE), bare BiVO4 gave 0.6 mA cm–2 photocurrent density,
whereas the BiVO4/Cs2PtI6 heterojunction
shows 0.92 mA cm–2. With the addition of
IrO
x
cocatalyst, at 1.23 V (vs RHE), the
heterojunction gave ∼2 mA cm–2. To
obtain 2 mA cm–2 photocurrent, pure BiVO4 requires 560 mV overpotential, whereas the heterojunction
requires 250 mV. The increase in the photocurrent arises from the
increase in the efficiency of charge separation from BiVO4 to Cs2PtI6 and the complementary absorption
offered by the latter.
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