Solar Water Splitting Cells

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A quantitative assessment of the competition between water and anion oxidation at WO_3 photoanodes in acidic aqueous electrolytes

The faradaic efficiency for O_2(g) evolution at thin-film WO_3 photoanodes has been evaluated in a series of acidic aqueous electrolytes. In 1.0 M H_2SO_4, persulfate was the predominant photoelectrochemical oxidation product, and no O_2 was detected unless catalytic quantities of Ag^+(aq) were added to the electrolyte. In contact with 1.0 M HClO_4, dissolved O_2 was observed with nearly unity faradaic efficiency, but addition of a hole scavenger, 4-cyanopyridine N-oxide, completely suppressed O_2 formation. In 1.0 M HCl, Cl_2(g) was the primary oxidation product. These results indicate that at WO_3 photoanodes, water oxidation is dominated by oxidation of the acid anions in 1.0 M HCl, H_2SO_4, and HClO_4, respectively

Photoelectrochemical oxidation of anions by WO_3 in aqueous and nonaqueous electrolytes

The behavior of WO_3 photoanodes has been investigated in contact with a combination of four anions (Cl−, CH_3SO_3−, HSO_4−, and ClO_4−) and three solvents (water, acetonitrile, and propylene carbonate), to elucidate the role of the semiconductor surface, the electrolyte, and redox kinetics on the current density vs. potential properties of n-type WO_3. In 1.0 M aqueous strong acids, although the flat-band potential (E_(fb)) of WO_3 was dominated by electrochemical intercalation of protons into WO_3, the nature of the electrolyte influenced the onset potential (E_(on)) of the anodic photocurrent. In aprotic solvents, the electrolyte anion shifted both E_(fb) and E_(on), but did not significantly alter the overall profile of the voltammetric data. For 0.50 M tetra(n-butyl)ammonium perchlorate in propylene carbonate, the internal quantum yield exceeded unity at excitation wavelengths of 300–390 nm, indicative of current doubling. A regenerative photoelectrochemical cell based on the reversible redox couple B_(10)Br_(10)^(˙−/2−) in acetonitrile, with a solution potential of 1.7 V vs. the normal hydrogen electrode, exhibited an open-circuit photovoltage of 1.32 V under 100 mW cm^(−2) of simulated Air Mass 1.5 global illumination

Thermally Stable N_2-Intercalated WO_3 Photoanodes for Water Oxidation

We describe stable intercalation compounds of the composition xN_2•WO_3 (x = 0.034–0.039), formed by trapping N_2 in WO_3. The incorporation of N_2 significantly reduced the absorption threshold of WO_3; notably, 0.039N_2•WO_3 anodes exhibited photocurrent under illumination at wavelengths ≤640 nm with a faradaic efficiency for O_2 evolution in 1.0 M HClO_4(aq) of nearly unity. Spectroscopic and computational results indicated that deformation of the WO3 host lattice, as well as weak electronic interactions between trapped N_2 and the WO_3 matrix, contributed to the observed red shift in optical absorption. Noble-gas-intercalated WO_3 materials similar to xN_2•WO_3 are predicted to function as photoanodes that are responsive to visible light

Order–disorder transition of a rigid cage cation embedded in a cubic perovskite

In hybrid perovskites, the driving forces of an order–disorder transition that arise from the organic cation and inorganic framework cannot be easily untangled. Here, the authors introduce a cage-in-framework structure in which reorientation of the cage cation does not alter the cubic symmetry of the perovskite lattice

Boosting Charge Separation and Transfer by Plasmon-Enhanced MoS₂/BiVO₄ p–n Heterojunction Composite for Efficient Photoelectrochemical Water Splitting

The poor separation and significant recombination of electron–hole pairs and slow transfer mobility of charge carriers limit the performance of BiVO₄ for photoelectrochemical (PEC) water splitting. To ameliorate the above problems, a novel integrated Ag-embedded MoS₂/BiVO₄ p–n heterojunction ternary composite electrode is fabricated and applied. Surface plasmon resonance (SPR) of Ag nanoparticles (NPs) by the near-field electromagnetic enhancement or abundant hot electrons injection and p–n heterojunction of MoS₂/BiVO₄ by the built-in electrical potential synergistically boost the electron–hole pair separation, transfer properties and suppress the recombination of the electron–hole pairs. Consequently, the BiVO₄−Ag−MoS₂ electrode among four of the BiVO₄-based electrodes achieves the largest photocurrent density of 2.72 mA cm^–2 at 0.6 V vs RHE, which is 2.44 times higher than that of pure BiVO₄ electrode (0.79 mA cm^–2), and possesses the largest IPCE of 51% at 420 nm. This work proposes a worthy design strategy for a plasmon enhanced p–n heterojunction for efficient PEC water splitting