Investigation of interfacial charge transfer kinetics of photocharged Co-Bi modified BiVO4 using scanning electrochemical microscopy (SECM)

Enhancing the PEC Efficiency in the Perspective of Crystal Facet Engineering and Modulation of Surfaces

Generation of hydrogen is one of the most promising routes to harvest solar energy for its sustainable utilization. Among different routes, the photoelectrochemical (PEC) process to split water using solar light to produce hydrogen is the green method to generate hydrogen. The sluggish kinetics through complicated pathways makes the oxygen evolution reaction the rate limiting step of the overall water splitting process. Therefore, development of an efficient photoanode for the sustainable oxidation of water is most challenging in an efficient overall PEC water splitting process. The low solar to hydrogen conversion efficiency arises from the slow surface kinetics, poor hole diffusion, and fast charge recombination processes. There have been strategies to improve catalytic performances through the removal of such detrimental effects. The generation of engineered surfaces is one of the important strategies recently adopted for the enhancement of the catalytic efficiencies. The present review has been focused on the discussion of engineered surfaces using crystal facet engineering, protective surface layer, passivation using the atomic layer deposition (ALD) technique, and cocatalyst modified surfaces to enhance the catalytic efficiency. Some of the important parameters defining catalyst performance are also discussed at the beginning of the review.

Bismuth Vanadate and 3D Graphene Composite Photoanodes for Enhanced Photoelectrochemical Oxidation of Water

Bismuth vanadate (BiVO4) has been one of the most promising photoanodes for the photoelectrochemical (PEC) water oxidation process. Efforts are still on to overcome the drawbacks of this photoanode to enhance the catalytic efficiency and improve the stability. In the present work, three-dimensional graphene (3D-G) was incorporated inside the BiVO4 matrix, primarily to improve the conductivity of the material. The photoanodes are fabricated with the incorporation of a SnO2 heterojunction and application of cobalt borate (Co-Bi) as a cocatalyst. The incorporation of 3D-G has enhanced the photocurrent from 0.72 o 1.21 mA cm-2 in ITO/SnO2/BiVO4 and ITO/SnO2/3D-G-BiVO4 materials; the photocurrent has been improved from 0.89 to 1.52 mA cm-2 in ITO/SnO2/BiVO4/Co-Bi and ITO/SnO2/3D-G-BiVO4. Semiconductor properties are evaluated from the Mott-Schottky measurements, and the charge transfer and transport kinetics of the PEC process are measured from several photoelectrochemical investigations. Both the charge transport and the charge transfer efficiencies are enhanced upon inclusion of 3D-G into the catalyst system. The lifetime of the charge carrier is observed to be increased. The decrease in the decay kinetics of the holes, enhancement in the open-circuit photovoltage (OCPV), and the resulting modulation of the surface states are responsible for the enhancement in the surface charge transfer process due to the inclusion of 3D-G into the catalytic system. Therefore, the additional role of 3D-G in the modulation of the surface states and release of the Fermi level pinning has made the band alignment between the semiconductor and the analyte better, which resulted in enhanced catalytic performance in the photoelectrochemical oxidation of water.

Efficient BiVO4-Based Photoanode with Chemically Precipitated Hierarchical Cobalt Borate (Co-Bi) Oxygen Evolution Reaction Catalysts

The integration of cobalt borate OER catalysts with electrodeposited BiVO₄-based photoanodes through a simple drop casting technique was shown to provide an improvement of the photoelectrochemical performance of electrodes under simulated solar light. Catalysts were obtained by chemical precipitation mediated by NaBH₄ at room temperature. Scanning electron microscopy (SEM) investigation of precipitates showed a hierarchical structure with globular features covered in nanometric thin sheets providing a large active area, whereas X-ray diffraction (XRD) and Raman spectroscopy highlighted their amorphous structure. The photoelectrochemical behavior of samples was investigated by linear scan voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) techniques. The amount of particles loaded onto BiVO₄ absorbers was optimized by variation of the drop cast volume. The enhancement of photocurrent generation by Co-B_i-decorated electrodes with respect to bare BiVO₄ was observed with an increase from 1.83 to 3.65 mA/cm² at 1.23 V vs RHE under AM 1.5 simulated solar light, corresponding to a charge transfer efficiency of 84.6%. The calculated maximum applied bias photon-to-current efficiency (ABPE) value for optimized samples was 1.5% at 0.5 V applied bias. Under constant illumination at 1.23 V vs RHE, a depletion of photoanode performances was observed within an hour, likely due to the detachment of the catalyst from the electrode surface.

Waste tire derived carbon as potential anode for lithium-ion batteries

The uncontrolled accumulation of end-of-life tires every year leads to serious environmental concerns, rendering setback to the sustainable growth of the society. The most viable solution to overcome this environmental issue is to convert these hazardness waste tires into value added products. In the present investigation, carbonecous based anode materials has been developed by a novel chemical activation strategy involving aqua regia followed by controlled pyrolytic condition in the selective atmospheres. Raman spectroscopic study displayed a graphitic carbon with significant degree of disordered arrangements. The generation of the turbostratic carbon with higher content of broken crystal edges is corroborated using the structural characterization such as X-ray diffraction (XRD). This fact is further corroborated from surface energy results calculated using the contact angles measured by dynamic wicking method. The prepared turbostratic carbon, when used as lithium anode, renders excellent electrochemical performances with reversible specific capacity of 350 mAhg^-1 (at 300 mAg^-1) with 81% capacity retention after 500 cycles. The present research provides new roadmap in recycling the waste tires for energy storage applications.

Transparent Ultramicroelectrodes for Studying Interfacial Charge-Transfer Kinetics of Photoelectrochemical Water Oxidation at TiO2 Nanorods with Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) has been extensively applied to the electrochemical analysis of the surfaces and interfaces of a photoelectrochemical (PEC) system. A semiconductor photoelectrode with a well-defined geometry and active surface area comparable to SECM's tip is highly desired for accurately quantifying interfacial charge-transfer activities and photoelectrochemically generated redox species, where the broadening effects due to the mass transfer gradient and nonlocal electron transfer at a planar semiconductor surface can be minimized. Here, we present a newly developed platform as a SECM substrate for investigating semiconductor PEC activities, which is based on a transparent ultramicroelectrode (UME) fabricated by using two-step photolithographic patterning and ion milling methods. This transparent UME with a 25 μm recessed disk shape is fully characterized with SECM for quantifying the interfacial charge-transfer rates of IrCl₆^2-/IrCl₆^3- by comparing with theoretical results from finite element simulations in COMSOL Multiphysics. When coated with TiO₂ nanorods as a model semiconductor material, the transparent UME can be used to quantify the catalytic PEC water oxidation in a feedback mode of SECM by sampling tip and substrate current signals simultaneously. This transparent UME-SECM study provides insights into the potential-dependent PEC water oxidation reaction mechanism and the quantitative analysis of photocurrent contributions from water oxidation and the SECM tip-generated redox mediator. The transparent UME-SECM method can be potentially expanded to other SECM operation modes such as surface interrogation for understanding the dynamics of the electrode surfaces and interfaces of a PEC system.

Effect of a Cocatalyst on a Photoanode in Water Splitting: A Study of Scanning Electrochemical Microscopy

With a proper band gap of ∼2.4 eV for solar light absorption and suitable valence band edge position for oxygen evolution, scheelite-monoclinic bismuth vanadate (BiVO₄) has become one of the most attractive photocatalysts for efficient visible-light-driven photoelectrochemical (PEC) water splitting. Several studies have indicated that surface modification of BiVO₄ with a cocatalyst such as NiFe layered double hydroxide (LDH) can significantly increase the PEC water splitting performance of the catalyst. Herein, we experimentally investigated the charge transfer dynamics and charge carrier recombination processes by scanning electrochemical microscopy (SECM) with the feedback mode on the surface of BiVO₄ and BiVO₄/NiFe-LDH as model samples. The ratio of rate constants for photogenerated hole (k_h+⁰) to electron (k_e-⁰) via the photocatalyst of BiVO₄/NiFe-LDH reacting with the redox couple is found to be five times larger than that of BiVO₄ under illumination. In this case, the ratio of the rate constants k_h+⁰/k_e-⁰ stands for the interfacial charge recombination process. This implies the cocatalyst NiFe-LDH suppresses the electron back transfer greatly and finally reduces the surface recombination. Control experiments with cocatalysts CoPi and RuO_x onto BiVO₄ further verify this conclusion. Therefore, the SECM characterization allows us to make an overall analysis on the function of cocatalysts in the PEC water splitting system.