Photo-Electrochemical Glycerol Conversion over a Mie Scattering Effect Enhanced Porous BiVO4 Photoanode

Modulating built-in electric field via Bi-VO4-Fe interfacial bridges to enhance charge separation for efficient photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting on semiconductor electrodes is considered to be one of the important ways to produce clean and sustainable hydrogen fuel, which is a great help in solving energy and environmental problems. Bismuth vanadate (BiVO4) as a promising photoanode for photoelectrochemical water splitting still suffers from poor charge separation efficiency and photo-induced self-corrosion. Herein, we develop heterojunction-rich photoanodes composed of BiVO4 and iron vanadate (FeVO4), coated with nickel iron oxide (NiFeOx/FeVO4/BiVO4). The formation of the interface between BiVO4 and FeVO4 (Bi-VO4-Fe bridges) enhances the interfacial interaction, resulting in improved performance. Meanwhile, high-conductivity FeVO4 and NiFeOx oxygen evolution co-catalysts effectively enhance bulk electron/hole separation, interface water's kinetics and photostability. Concurrently, the optimized NiFeOx/FeVO4/BiVO4 possesses a remarkable photocurrent density of 5.59 mA/cm2 at 1.23 V versus reversible hydrogen electrode (vs RHE) under AM 1.5G (Air Mass 1.5 Global) simulated sunlight, accompanied by superior stability without any decreased of its photocurrent density after 14 h. This work not only reveals the crucial role of built-in electric field in BiVO4-based photoanode during PEC water splitting, but also provides a new guide to the design of efficient photoanode for PEC.

Evaluation on photocatalytic activity of bismuth nitrate derived materials at different calcination temperatures using paper microzones method

Our work reveals for the first time that directly calcined bismuth nitrate derivatives (BNDs) possess significant photocatalytic activity towards rhodamine B (RhB). As the calcination temperature increased, the Bi(NO3)3·5H2O powder gradually ruptured and transformed into different bismuth nitrate products and their mixtures, finally into stable α-Bi2O3 at 500 °C. Among them, BNDs-100 could achieve 100 % photocatalytic degradation of 10 mg/L RhB solution under UV irradiation for 6 min. The ImageJ-led paper microzones (PMZs) method is introduced for the first time into the performance evaluation process of photocatalysts, which can achieve the green chemistry pathway and the rapid evaluation of different catalysts. The accuracy of the results of the PMZs method relative to the spectrophotometric method was up to 91.14 %, which has a better reliability and is suitable for qualitative analysis, and a certain ability when used for quantitative analysis. The results showed that the PMZs method was used to assess the photocatalytic degradation of rhodamine B by bismuth nitrate-derived materials at different calcination temperatures with well reliability, and the preparation of BNDs by direct calcination was a simple and effective strategy.

Solar-driven highly selective conversion of glycerol to dihydroxyacetone using surface atom engineered BiVO4 photoanodes

Dihydroxyacetone is the most desired product in glycerol oxidation reaction because of its highest added value and large market demand among all possible oxidation products. However, selectively oxidative secondary hydroxyl groups of glycerol for highly efficient dihydroxyacetone production still poses a challenge. In this study, we engineer the surface of BiVO4 by introducing bismuth-rich domains and oxygen vacancies (Bi-rich BiVO4-x) to systematically modulate the surface adsorption of secondary hydroxyl groups and enhance photo-induced charge separation for photoelectrochemical glycerol oxidation into dihydroxyacetone conversion. As a result, the Bi-rich BiVO4-x increases the glycerol oxidation photocurrent density of BiVO4 from 1.42 to 4.26 mA cm-2 at 1.23 V vs. reversible hydrogen electrode under AM 1.5 G illumination, as well as the dihydroxyacetone selectivity from 54.0% to 80.3%, finally achieving a dihydroxyacetone production rate of 361.9 mmol m-2 h-1 that outperforms all reported values. The surface atom customization opens a way to regulate the solar-driven organic transformation pathway toward a carbon chain-balanced product.

Advancing BiVO4 Photoanode Activity for Ethylene Glycol Oxidation via Strategic pH Control

The photoelectrochemical (PEC) conversion of organic small molecules offers a dual benefit of synthesizing value-added chemicals and concurrently producing hydrogen (H2). Ethylene glycol, with its dual hydroxyl groups, stands out as a versatile organic substrate capable of yielding various C1 and C2 chemicals. In this study, we demonstrate that pH modulation markedly enhances the photocurrent of BiVO4 photoanodes, thus facilitating the efficient oxidation of ethylene glycol while simultaneously generating H2. Our findings reveal that in a pH = 1 ethylene glycol solution, the photocurrent density at 1.23 V vs. RHE can attain an impressive 7.1 mA cm-2, significantly surpassing the outputs in neutral and highly alkaline environments. The increase in photocurrent is attributed to the augmented adsorption of ethylene glycol on BiVO4 under acidic conditions, which in turn elevates the activity of the oxidation reaction, culminating in the maximal production of formic acid. This investigation sheds light on the pivotal role of electrolyte pH in the PEC oxidation process and underscores the potential of the PEC strategy for biomass valorization into value-added products alongside H2 fuel generation.

BiVO4 Photoanode with NiV2O6 Back Contact Interfacial Layer for Improved Hole-Diffusion Length and Photoelectrochemical Water Oxidation Activity

The short hole diffusion length (HDL) and high interfacial recombination are among the main drawbacks of semiconductor-based solar energy systems. Surface passivation and introducing an interfacial layer are recognized for enhancing HDL and charge carrier separation. Herein, we introduced a facile recipe to design a pinholes-free BiVO4 photoanode with a NiV2O6 back contact interfacial (BCI) layer, marking a significant advancement in the HDL and photoelectrochemical activity. The fabricated BiVO4 photoanode with NiV2O6 BCI layer exhibits a 2-fold increase in the HDL compared to pristine BiVO4. Despite this improvement, we found that the front surface recombination still hinders the water oxidation process, as revealed by photoelectrochemical (PEC) studies employing Na2SO3 electron donors and by intensity-modulated photocurrent spectroscopy measurements. To address this limitation, the surface of the NiV2O6/BiVO4 photoanode was passivated with a cobalt phosphate electrocatalyst, resulting in a dramatic enhancement in the PEC performance. The optimized photoanode achieved a stable photocurrent density of 4.8 mA cm-2 at 1.23 VRHE, which is 12-fold higher than that of the pristine BiVO4 photoanode. Density Functional Theory (DFT) simulations revealed an abrupt electrostatic potential transition at the NiV2O6/BiVO4 interface with BiVO4 being more negative than NiV2O6. A strong built-in electric field is thus generated at the interface and drifts photogenerated electrons toward the NiV2O6 BCI layer and photogenerated holes toward the BiVO4 top layer. As a result, the back-surface recombination is minimized, and ultimately, the HDL is extended in agreement with the experimental findings.

Synergistic effect of Ag@CN with BiVO4 in a unique Z-type heterojunction for enhancing photoelectrochemical water splitting performance

In the realm of photoelectrochemical technology, the enhancement of photogenerated charge carrier separation is pivotal for the advancement of energy conversion performance. Carbon nitride (CN) is established as a photocatalytic material with significant potential and exhibits unique advantages in addressing the issue of rapid recombination of photogenerated carriers. This study utilized an efficient in situ doping method that combined Mo,W-doped BiVO4 (Mo,W:BVO) with silver-loaded CN (Ag@CN), yielding an all-solid-state Mo,W:BVO/Ag@CN heterostructure that effectively augments the separation efficiency of electron-hole pairs. Through the annealing process, Ag@CN was uniformly coated within the Mo,W:BVO thin film, significantly enlarging the interface contact area to enhance visible light absorption and photogenerated carrier movement. The results of the photoelectrochemical tests showed that the Mo,W:BVO/Ag@CN heterostructure had the highest photocurrent and charge transfer efficiency, which were 6.4 times and 3.6 times higher respectively than those of the unmodified Mo,W:BVO. Our research elucidates the interactions within all-solid-state Z-scheme heterojunctions, outlining strategic approaches for crafting innovative and superior photocatalytic systems.

Nanoconfinement Enables Photoelectrochemical Selective Oxidation of Glycerol via the Microscale Fluid Effect

The glycerol oxidation reaction (GOR) run with photoelectrochemical cells (PECs) is one of the most promising ways to upgrade biomass because it is thermodynamically favorable, while irreversible overoxidation leads to unsatisfactory product selectivities. Herein, a tunable one-dimensional nanoconfined environment was introduced into the GOR process, which accelerated mass transfer of glycerol via the microscale fluid effect and changed the main oxidation product from formic acid (FA) to glyceraldehyde (GLD), which led to retention of the heavier multicarbon products. The rate of glycerol diffusion in the nanochannels increased by a factor of 4.92 with decreasing inner diameters. The main product from the PEC-selective oxidation of glycerol changed from the C1 product FA to the C3 product GLD with a great selectivity of 60.7%. This work provides a favorable approach for inhibiting further oxidation of multicarbon products and illustrates the importance of microenvironmental regulation in biomass oxidation.

Heterointerface-Rich Ni3N/WO3 Hierarchical Nanoarrays for Efficient Glycerol Oxidation-Assisted Alkaline Hydrogen Evolution

Glycerol oxidation-assisted water electrolysis has emerged as a cost-effective way of co-producing green hydrogen and HCOOH. Still, preparing highly selective and stable nickel-based metal electrocatalysts remains a challenge. Herein, heterostructure Ni3N/WO3 nanosheet arrays of bifunctional catalysts with large specific surface areas loaded on nickel foam (denoted as Ni3N/WO3/NF) were synthesized. This catalyst was for glycerol oxidation reaction (GOR) and hydrogen evolution reaction (HER) with excellent catalytic performance, a voltage saving of 267 mV compared to oxygen evolution reaction (OER), and a HER overpotential of 104 mV at 100 mA cm-2. The cell voltage in the assembled GOR//HER hybrid electrolysis system reaches 100 mA cm-2 at 1.50 V, 296 mV lower than the potential required for overall water splitting. This work demonstrates that replacing GOR with OER using a cost-effective and highly active Ni-based bifunctional electrocatalyst can make hybrid water electrolysis an energy-efficient, sustainable, and green strategy for hydrogen production.

HClO-Mediated Photoelectrochemical Epoxidation of Alkenes with Near 100 % Conversion Rate and Selectivity by Regulating Lattice Chlorine Cycle

Directional organic transformation via a green, sustainable catalytic reaction has attracted a lot of attention. Herein, we report a photoelectrochemical approach for highly selective epoxidation of alkenes in a salt solution using Co2 (OH)3 Cl (CoOCl) as a bridge of photo-generated charge, where the lattice Cl- of CoOCl can be oxidized to generate HClO by the photo-generated holes of BiVO4 photoanode and be spontaneously recovered by Cl- of a salt solution, which then oxidizes the alkenes into the corresponding epoxides. As a result, a series of water-soluble alkenes, including 4-vinylbenzenesulfonic acid sodium, 2-methyl-2-propene-1-sulfonic acid sodium, and 3-methyl-3-buten-1-ol can be epoxidized with near 100 % conversion rate and selectivity. Through further inserting a MoOx protection layer between BiVO4 and CoOCl, the stability of CoOCl-MoOx /BiVO4 can be maintained for at least 120 hours. This work opens an avenue for solar-driven organic epoxidation with a possibility of on-site reaction around the abundant ocean.

Organic Upgrading through Photoelectrochemical Reactions: Toward Higher Profits

Aqueous photoelectrochemical (PEC) cells have long been considered a promising technology to convert solar energy into hydrogen. However, the solar-to-H2 (STH) efficiency and cost-effectiveness of PEC water splitting are significantly limited by sluggish oxygen evolution reaction (OER) kinetics and the low economic value of the produced O2 , hindering the practical commercialization of PEC cells. Recently, organic upgrading PEC reactions, especially for alternative OERs, have received tremendous attention, which improves not only the STH efficiency but also the economic effectiveness of the overall reaction. In this review, PEC reaction fundamentals and reactant-product cost analysis of organic upgrading reactions are briefly reviewed, recent advances made in organic upgrading reactions, which are categorized by their reactant substrates, such as methanol, ethanol, glycol, glycerol, and complex hydrocarbons, are then summarized and discussed. Finally, the current status, further outlooks, and challenges toward industrial applications are discussed.

Unveiling the Influence of Sulfur Doping on Photoelectrochemical Performance in BiVO4/FeOOH Heterostructures

BiVO4 is a promising photoanode for photoelectrochemical (PEC) water splitting but suffers from high charge carrier recombination and sluggish surface water oxidation kinetics that limit its efficiency. In this work, a model of sulfur-incorporated FeOOH cocatalyst-loaded BiVO4 was constructed. The composite photoanode (BiVO4/S-FeOOH) demonstrates an enhanced photocurrent density of 3.58 mA cm-2, which is 3.7 times higher than that of the pristine BiVO4 photoanode. However, the current explanations for the generation of enhanced photocurrent signals through the incorporation of elements and cocatalyst loading remain unclear and require further in-depth research. In this work, the hole transfer kinetics were investigated by using a scanning photoelectrochemical microscope (SPECM). The results suggest that the incorporation of sulfur can effectively improve the charge transfer capacity of FeOOH. Moreover, the oxygen evolution reaction model provides evidence that S-doping can induce a "fast" surface catalytic reaction at the cocatalyst/solution interface. The work not only presents a promising approach for designing a highly efficient photoanode but also offers valuable insights into the role of element doping in the PEC water-splitting system.

Prompt Hole Extraction Suppresses V5+ Dissolution and Sustains Large-Area BiVO4 Photoanodes for Over 2100 h Water Oxidation

Nanostructured bismuth vanadate (BiVO4) is at the forefront of emerging photoanodes in photoelectrochemical tandem devices for solar water splitting owing to the suitable band edge position and efficient charge separation capability. However, the (photo)chemical corrosion involving V5+ dissolution limits the long-term stability of BiVO4. Herein, guided by DFT calculations, we introduce an ALD-derived NiOx catalyst layer on BiVO4 to stabilize the surface Bi-O bonds, facilitate hole extraction, and thus suppress the V5+ dissolution. At the same time, the ALD NiOx catalyst layer could efficiently suppress the surface recombination and accelerate the surface OER kinetics, boosting the half-cell applied bias photon-to-current efficiency of BiVO4 to 2.05%, as well as a fill factor of 47.1%. By adding trace NaVO3 to the electrolyte, the NiOx/BiVO4 photoanode with an illumination area of 10.5 cm2 shows a record operational stability of more than 2100 h.

Ultrathin Aerogel Micro/Nanofiber Membranes with Hierarchical Cellular Architecture for High-Performance Warmth Retention

A low temperature environment poses significant challenges to the global economy and public health. However, the existing cold-protective materials still struggle with the trade-off between thickness and thermal resistance, resulting in poor thermal-wet comfort and limited personal cold protection performance. Here, a scalable strategy, based on electrospinning and solution casting, is developed to create aerogel micro/nanofiber membranes with a hierarchical cellular architecture by manipulating the phase separation of the charged jets and of the spreading casting solution. The integration of interconnected nanopores (30-60 nm), ultrafine fiber diameter, and high porosity, enables the aerogel micro/nanofiber membranes with lightweight, ultrathin thickness (∼0.5 mm), and superior warmth retention performance with ultralow thermal conductivity of 14.01 mW m-1 K-1. And the resultant membrane with customized semiclosed walls exhibits both striking wind resistance and satisfactory thermal-wet comfort (3.4 °C warmer than the cutting-edge thermal underwear). This work will inspire the design and development of high-performance fibrous materials for thermal management applications.

Probing and Modulation of the Electric Double Layer at the Insulating Oil-Paper Interface

Charge accumulation in the insulating oil-paper system determines the operating safety of the converter transformers in high-voltage direct current (HVDC) transmissions. However, it has been a long-standing challenge to reveal the charge distribution of the electric double layer (EDL) at the insulating oil-paper interface and relate it to charge transport. In particular, the EDL and charging mechanisms at the oil-paper interface have not been fully understood. We herein demonstrate that the charge distribution of EDL at the oil-paper interface is probed through Kelvin probe force microscopy (KPFM). The origin charge distribution of EDL without any additives shows that the negative charge gathers on the insulating paper surface, while the positive charge diffuses in the insulating oil, which is derived from the electron affinity difference between insulating oil and insulating paper and acts as an additional obstacle to charge transportation at the oil-paper interface. Interestingly, the additive 3-amino-2,4-triazole (ATA) can tune the charge distribution of EDL by bringing extra hole traps, which significantly decreases the interface barrier and reduces the charge accumulation at the oil-paper interface. As well as increasing charge mobility in oil-paper insulation, ATA also ensures stabilization of operation under polarity inversion conditions by accelerating the dissipation rate of accumulated charge.

Insight into interface electronic structure of ZnIn2S4/TiO2 heterostructure for enhanced photoelectrochemical glycerol oxidation

Developing a high-efficiency photoelectrochemical (PEC) electrode for the glycerol oxidation reaction (GOR) is important for producing valuable products. The PEC performance could be enhanced by rationally designing heterostructures with inhibited recombination of charge carriers. Nevertheless, the interface electronic structure of heterostructures has not been comprehensively analyzed. In this work, the PEC GOR performance of ZnIn2S4/TiO2 heterostructure photoanode showed 1.7 folds enhancement than that of pure TiO2 photoanode at 1.23 V vs. RHE. The ZnIn2S4/TiO2 heterostructure was simulated by constructing ZnIn2S4 on the TiO2 single crystal, which was beneficial for investigating the interface electronic structure of heterostructure. Single-particle spectroscopy demonstrated a significantly increased lifetime of charge carriers. Combined with the in-situ X-ray photoelectron spectroscopy, Kelvin probe force microscopy, work function, and electron paramagnetic resonance, the interface electronic structure of the ZnIn2S4/TiO2 heterostructure was proposed with a Z-scheme mechanism. This work provides a comprehensive strategy for analyzing the interface electronic structure of heterostructures.

Synergy Effect of the Enhanced Local Electric Field and Built-In Electric Field of CoS/Mo-Doped BiVO4 for Photoelectrochemical Water Oxidation

Bismuth vanadate is a promising material for photoelectrochemical water oxidation. However, it suffers from a low quantum efficiency, poor stability, and slow water oxidation kinetics. Here, we developed a novel photoanode of CoS/Mo-BiVO4 with excellent photoelectrochemical water oxidation performance. It achieved a photocurrent density of 4.5 mA cm-2 at 1.23 V versus the reversible hydrogen electrode, ∼4 times that of BiVO4. The CoS/Mo-BiVO4 photoanode also exhibited good stability, and the photocurrent density generated by the CoS/Mo-BiVO4 photoanode did not significantly decrease after light irradiation for 2 h. Upon replacement of part of the V with Mo doping in BiVO4, the local electric field around the Mo-O bond was enhanced, thus promoting carrier separation in BiVO4. The CoS was deposited on the surface of Mo-BiVO4, forming a built-in electric field at the interface. Under the action of the bias electric field and the built-in electric field, the carriers of CoS/Mo-BiVO4 were efficiently separated in the direction of the inverse type II heterojunction. In addition, CoS improved the light absorption and charge injection efficiency of the CoS/Mo-BiVO4 photoanode.

Nature of Charge Carrier Recombination in CuWO4 Photoanodes for Photoelectrochemical Water Splitting

CuWO4 is a ternary semiconductor oxide with excellent visible light harvesting properties up to 550 nm and stability at high pH values, which make it a suitable material to build photoanodes for solar light conversion to hydrogen via water splitting. In this work, we studied the photoelectrochemical (PEC) performance of transparent CuWO4 electrodes with tunable light absorption and thickness, aiming at identifying the intrinsic bottlenecks of photogenerated charge carriers in this semiconductor. We found that electrodes with optimal CuWO4 thickness exhibit visible light activity due to the absorption of long-wavelength photons and a balanced electron and hole extraction from the oxide. The PEC performance of CuWO4 is light-intensity-dependent, with charge recombination increasing with light intensity and most photogenerated charge carriers recombining in bulk sites, as demonstrated by PEC tests performed in the presence of sacrificial agents or cocatalysts. The best-performing 580 nm thick CuWO4 electrode delivers a photocurrent of 0.37 mA cm-2 at 1.23 VSHE, with a 7% absorbed photon to current efficiency over the CuWO4 absorption spectrum.

Scalable, Patternable Glass-Infiltrated Ceramic Radiative Coolers for Energy-Saving Architectural Applications

A huge concern on global climate/energy crises has triggered intense development of radiative coolers (RCs), which are promising green-cooling technologies. The continuous efforts on RCs have fast-tracked notable energy-savings by minimizing solar absorption and maximizing thermal emission. Recently, in addition to spectral optimization, ceramic-based thermally insulative RCs are reported to improve thermoregulation by suppressing heat gain from the surroundings. However, a high temperature co-firing process of ceramic-based thick film inevitably results in a large mismatch of structural parameters between designed and fabricated components, thereby breaking spectral optimization. Here, this article proposes a scalable, non-shrinkable, patternable, and thermally insulative ceramic RC (SNPT-RC) using a roll-to-roll process, which can fill a vital niche in the field of radiative cooling. A stand-alone SNPT-RC exhibits excellent thermal insulation (≈0.251 W m-1  K-1 ) with flame-resistivity and high solar reflectance/long-wave emissivity (≈96% and 92%, respectively). Alternate stacks of intermediate porous alumina/borosilicate (Al2 O3 -BS) layers not only result in outstanding thermal and spectral characteristics, causing excellent sub-ambient cooling (i.e., 7.05 °C cooling), but also non-shrinkable feature. Moreover, a perforated SNPT-RC demonstrates its versatility as a breathable radiative cooling shade and as a semi-transparent window, making it a highly promising technology for practical deployment in energy-saving architecture.

Imperfect makes perfect: defect engineering of photoelectrodes towards efficient photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting for hydrogen evolution has been considered as a promising technology to solve the energy and environmental issues. However, the solar-to-hydrogen (STH) conversion efficiencies of current PEC systems are far from meeting the commercial demand (10%) due to the lack of efficient photoelectrode materials. The recent rapid development of defect engineering of photoelectrodes has significantly improved the PEC performance, which is expected to break through the bottleneck of low STH efficiency. In this review, the category and the construction methods of different defects in photoelectrode materials are summarized. Based on the in-depth summary and analysis of existing reports, the PEC performance enhancement mechanism of defect engineering is critically discussed in terms of light absorption, carrier separation and transport, and surface redox reactions. Finally, the application prospects and challenges of defect engineering for PEC water splitting are presented, and the future research directions in this field are also proposed.