In recent years, bismuth-based nanomaterials have drawn considerable interest as potential candidates for photoelectrochemical (PEC) water splitting owing to their narrow band gaps, nontoxicity, and low costs. The unique electronic structure of bismuth-based materials with a well-dispersed valence band comprising Bi 6s and O 2p orbitals offers a suitable band gap to harvest visible light. This Review presents significant advancements in exploiting bismuth-based nanomaterials for solar water splitting. An overview of the different strategies employed and the new ideas adopted to improve the PEC performance of bismuth-based nanomaterials are discussed. Morphology control, the construction of heterojunctions, doping, and co-catalyst loading are several approaches that are implemented to improve the efficiency of solar water splitting. Key issues are identified and guidelines are suggested to rationalize the design of efficient bismuth-based materials for sunlight-driven water splitting.
Tungsten oxide (WO 3 ) and bismuth vanadate (BiVO 4 ) are one of the most attractive combinations to construct an efficient heterojunction for photoelectrochemical (PEC) applications. Here, we report an all-solution-processed WO 3 /BiVO 4 heteronanostructure photoanode with highly enhanced photoactivity and stability for sustainable energy production. The vertically aligned WO 3 nanorods were synthesized on a fluorine-doped tin oxide/glass substrate by the hydrothermal method without a seed layer and BiVO 4 was deposited by pulsed electrodeposition for conformal coating. Owing to the long diffusion lengths of charge carriers in the WO 3 nanorods, the ability to absorb the wider range of wavelengths, and appropriate band-edge positions of the WO 3 /BiVO 4 heterojunction for spontaneous PEC reaction, the optimum WO 3 /BiVO 4 photoanode has a photocurrent density of 4.15 mA/cm 2 at 1.23 V versus RHE and an incident-photonto-current efficiency of 75.9% at 430 nm under front illumination, which are a double and quadruple those of pristine WO 3 nanorod arrays, respectively. Our work suggests an environment-friendly and low-cost all-solution process route to synthesize high-quality photoelectrodes.
Herein, we report a new Na-insertion electrode material, NaTiO, as a potential candidate for Na-ion hybrid capacitors. We study the structural properties of nanostructured NaTiO, synthesized by a hydrothermal technique, upon electrochemical cycling vs Na. Average and local structures of NaTiO are elucidated from neutron Rietveld refinement and pair distribution function (PDF), respectively, to investigate the initial discharge and charge events. Rietveld refinement reveals electrochemical cycling of NaTiO is driven by single-phase solid solution reaction during (de)sodiation without any major structural deterioration, keeping the average structure intact. Unit cell volume and lattice evolution on discharge process is inherently related to TiO distortion and Na ion perturbations, while the PDF reveals the deviation in the local structure after sodiation. Raman spectroscopy and X-ray photoelectron spectroscopy studies further corroborate the average and local structural behavior derived from neutron diffraction measurements. Also, NaTiO shows excellent Na-ion kinetics with a capacitve nature of 86% at 1.0 mV s, indicating that the material is a good anode candidate for a sodium-ion hybrid capacitor. A full cell hybrid Na-ion capacitor is fabricated by using NaTiO as anode and activated porous carbon as cathode, which exhibits excellent electrochemical properties, with a maximum energy density of 54 Wh kg and a maximum power density of 5 kW kg. Both structural insights and electrochemical investigation suggest that NaTiO is a promising negative electrode for sodium-ion batteries and hybrid capacitors.
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