Covalent organic frameworks bearing Ni active sites for free radical-mediated photoelectrochemical organic transformations

Paired Chemical Upgrading in Photoelectrochemical Cells

Photoelectrochemical (PEC) technology has emerged as a promising platform for sustainable energy conversion and chemical synthesis, utilizing solar energy to facilitate redox reactions. While PEC systems have been extensively studied for water splitting, CO2 reduction, nitrogen reduction for value-added compounds synthesis, the sluggish oxygen evolution reaction (OER) on the anode side and the less economic value of O2 limit system efficiency. To address this, researchers have explored paired chemical upgrading strategies, coupling selective anodic organic oxidation reactions (OORs) with cathodic reduction reactions. This approach enabled the simultaneous production of high-value chemicals and fuels, enhancing the PEC system efficiency and economic viability. In this Perspective, we highlight the latest advancements and milestones in coupling anode OORs and cathode reduction reactions within PEC cells. Particular emphasis is placed on the key design principles, catalyst development, reaction mechanisms, and the performance of paired PEC cells. In addition, challenges and perspectives are provided for the future development of this emerging and sustainable technology.

Surface Coverage Tuning for Suppressing Over-Oxidation: A Case of Photoelectrochemical Alcohol-to-Aldehyde/Ketone Conversion

Suppressing over-oxidation is a crucial challenge for various chemical intermediate synthesis in heterogeneous catalysis. The distribution of oxidative species and the substrate coverage, governed by the direction of electron transfer, are believed to influence the oxidation extent. In this study, we presented an experimental realization of surface coverage modulation on a photoelectrode using a photo-induced charge activation method. Through the surface coverage modulation, both pre-oxidized alcohol substrates and surface coverage were increased, which not only improved the reaction kinetics but also suppressed the over-oxidation of the generated aldehydes/ketones. As a demonstration, the Faradaic efficiency for the conversion of glycerol to dihydroxyacetone increased from 31.8 % to 46.8 % (with selectivity rising from 47.6 % to 71.3 %), from 73.4 % to 87.8 % for benzyl alcohol to benzyl aldehyde (selectivity increasing from 76.7 % to 92.4 %) and from 4.2 % to 53.6 % for ethylene glycol to glycolaldehyde (selectivity increasing from 6.2 % to 62.7 %). Our findings offer a promising strategy for the production of high-value carbon products in heterogeneous catalysis.

Microenvironment Engineering of Heterogeneous Catalysts for Liquid-Phase Environmental Catalysis

Environmental catalysis has emerged as a scientific frontier in mitigating water pollution and advancing circular chemistry and reaction microenvironment significantly influences the catalytic performance and efficiency. This review delves into microenvironment engineering within liquid-phase environmental catalysis, categorizing microenvironments into four scales: atom/molecule-level modulation, nano/microscale-confined structures, interface and surface regulation, and external field effects. Each category is analyzed for its unique characteristics and merits, emphasizing its potential to significantly enhance catalytic efficiency and selectivity. Following this overview, we introduced recent advancements in advanced material and system design to promote liquid-phase environmental catalysis (e.g., water purification, transformation to value-added products, and green synthesis), leveraging state-of-the-art microenvironment engineering technologies. These discussions showcase microenvironment engineering was applied in different reactions to fine-tune catalytic regimes and improve the efficiency from both thermodynamics and kinetics perspectives. Lastly, we discussed the challenges and future directions in microenvironment engineering. This review underscores the potential of microenvironment engineering in intelligent materials and system design to drive the development of more effective and sustainable catalytic solutions to environmental decontamination.

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.

Frontiers in Photoelectrochemical Catalysis: A Focus on Valuable Product Synthesis

Photoelectrochemical (PEC) catalysis provides the most promising avenue for producing value-added chemicals and consumables from renewable precursors. Over the last decades, PEC catalysis, including reduction of renewable feedstock, oxidation of organics, and activation and functionalization of C─C and C─H bonds, are extensively investigated, opening new opportunities for employing the technology in upgrading readily available resources. However, several challenges still remain unsolved, hindering the commercialization of the process. This review offers an overview of PEC catalysis targeted at the synthesis of high-value chemicals from sustainable precursors. First, the fundamentals of evaluating PEC reactions in the context of value-added product synthesis at both anode and cathode are recalled. Then, the common photoelectrode fabrication methods that have been employed to produce thin-film photoelectrodes are highlighted. Next, the advancements are systematically reviewed and discussed in the PEC conversion of various feedstocks to produce highly valued chemicals. Finally, the challenges and prospects in the field are presented. This review aims at facilitating further development of PEC technology for upgrading several renewable precursors to value-added products and other pharmaceuticals.

Electrochemical oxidation of concentrated benzyl alcohol to high-purity benzaldehyde via superwetting organic-solid-water interfaces

Organic electrosynthesis in aqueous media is presently hampered by the poor solubility of many organic reactants and thus low purity of liquid products in electrolytes. Using the electrooxidation of benzyl alcohol (BA) as a model reaction, we present a "sandwich-type" organic-solid-water (OSW) system, consisting of BA organic phase, KOH aqueous electrolyte, and porous anodes with Janus-like superwettability. The system allows independent diffusion of BA molecules from the organic phase to electrocatalytic active sites, enabling efficient electrooxidation of high-concentration BA to benzaldehyde (97% Faradaic efficiency at ~180 mA cm-2) with substantially reduced ohmic loss compared to conventional solid-liquid systems. The confined organic-water boundary within the electrode channels suppresses the interdiffusion of molecules and ions into the counterphase, thus preventing the hydration and overoxidation of benzaldehyde during long-term electrocatalysis. As a result, the direct production of high-purity benzaldehyde (91.7%) is achieved in a flow cell, showcasing the effectiveness of electrocatalysis over OSW interfaces for the one-step synthesis of high-purity organic compounds.