Aravind Kumar Chandiran

The stability of the absorber materials in an aqueous medium is the key to developing successful photoelectrochemical (PEC) solar fuel devices. The halide perovskite materials provide an opportunity to tune desired optoelectronic properties and show very high photovoltaic power conversion efficiency. However, their stability is poor as they decompose instantly in an aqueous electrolyte medium. Here the most stable vacancy ordered double perovskites Cs2PtCl6 and Cs2PtBr6, which remain intact in a wide range of pH values between 1 and 13 is reported. These materials also possess excellent absorption properties covering a significant portion of the visible spectrum. Like conventional ABX3 materials, these ultrastable materials offer tunability in optical properties via mixed halide sites. Through anion exchange, the conversion of Cs2PtCl6 to Cs2PtBr6 through core–shell conversion mechanism is shown. The latter led to the formation of type-II heterostructures. The electrochemical properties of these materials are investigated in detail and their ability to carry out solar water oxidation on an unprotected photoanode, with photocurrent density of >0.2 mA cm−2 at 1.23 V (vs. RHE) is demonstrated.

In this work, osmium-based vacancy-ordered double perovskites Cs2OsX6 (X = Cl−, Br−, I−) are reported and the role of halides on stability, optical and photoelectrochemical properties is investigated. All these three materials crystallize in a cubic phase like Cs2SnX6 or Cs2PtX6 and possess extraordinary stability in ambient conditions and remain stable in strong acids and bases (from pH 1 to 11). One of the unique properties of these materials is that they show panchromatic visible and NIR absorption (up to 1200 nm) through ligand-to-metal [X (Cl−/Br−/I−) to Os] charge transfer and another NIR absorption between 1800 and 2500 nm due to Os d-d transition. Their pH stability and panchromatic light absorption properties enabled them to be employed as photoanodes in PEC water-splitting devices.

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 IrOx 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.

Halide perovskites show incredible photovoltaic power conversion efficiency coupled with several hundreds of hours of device stability. However, their stability is poor in aqueous electrolyte media. Reported here is a vacancy ordered halide perovskite, Cs2PtI6, which shows extraordinary stability under ambient conditions (1 year), in aqueous media of extreme acidic (pH 1), basic (pH 13), and under electrochemical reduction conditions. It was employed as an electrocatalyst and photoanode for hydrogen production and water oxidation, respectively. The catalyst remains intact for at least 100 cycles of electrochemical cycling and six hours of hydrogen production at pH 1. Cs2PtI6 was employed as a photoanode for PEC water oxidation, and the material displayed a photocurrent of 0.8 mA cm−2 at 1.23 V (vs. RHE) under simulated AM1.5G sunlight. Using constant voltage measurement, Cs2PtI6 exhibited over 12 hours of PEC stability without loss of performance.