Predictive control of selective secondary alcohol oxidation of glycerol on NiOOH

Photoelectrochemical glycerol oxidation to high value-added products over BiVO4/CuWO4 heterojunction photoanodes

Developing high-efficiency, long-term stable photoanodes to harness solar energy for photoelectrocatalytic (PEC) glycerol oxidation into value-added chemicals is pivotal for advancing green chemistry. However, controlling selectivity while achieving high efficiency and stability poses a significant challenge in photoelectrode design. Here, a BiVO4/CuWO4 (abbreviated as BVO/CWO) heterojunction photoanode was synthesized via an electrodeposition method followed by spin-coating. Under AM 1.5G illumination, the BVO/CWO photoanode achieves a glycerol oxidation current density of 1.55 mA∙cm-2 at 1.23 VRHE, maintaining approximately 99 % stability after a 10-hour stability test. The production rate of 1,3-dihydroxyacetone (DHA) for BVO/CWO photoanode reaches 140.3 mmol m-2 h-1 at 1.4 VRHE, representing a nearly 12-fold increase compared to CuWO4 (12.1 mmol m-2 h-1), while the DHA selectivity improves to 50.1 % from 12.5 %. This enhanced performance is attributed to the heterojunction with strong interface interaction within BVO/CWO, which generates a greater photovoltage driving force and improves bulk charge transport efficiency. Introducing the BiVO4 layer on the CuWO4 accelerates interfacial reaction rates and inhibits the photo-corrosion of CuWO4 in strongly acidic conditions. Furthermore, a self-powered system, PEC coupled with a photovoltaic cell (PV-PEC), is developed using a dual-electrode configuration. A PV-PEC device with the BVO/CWO photoanode achieves a DHA productivity of 186.9 mmol m-2 and an H2 productivity of 416.4 mmol m-2 at an operating voltage of 1.5 V over 2.5 h, indicating the application potential of the BVO/CWO photoanode under more realistic conditions.

Foreign cation-promoted active species formation enables efficient electrochemical glycerol valorization

The foreign cation substitution approach to Ni-Co spinel oxide promotes the active NiOOH species formation, resulting in remarkable electrochemical glycerol oxidation activity and selectivity for formate production. The generated NiOOH improves both the indirect GOR and the coadsorption capacity of OH* and glycerol, which is crucial to the potential-dependent GOR mechanism.

In Situ Construction of N-Doped Ni3S2@Ni(OH)2 Self-Supported Heterostructures for Highly Selective Electrooxidation of 5-Hydroxymethylfurfural to 2,5-Diformylfuran

The selective electrooxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) is promising for biomass valorization but remains challenging under alkaline conditions due to inefficient nonprecious metal catalysts. Herein, we develop a scalable N-doped Ni3S2@Ni(OH)2 heterostructure via a molten-salt-derived precursor and a one-pot hydrothermal synthesis. This catalyst achieves a low potential of 1.395 V (10 mA cm-2) and 96% DFF selectivity, with a 47.6% yield in 1.0 M K2CO3 (pH = 12). Experimental and DFT studies reveal that interfacial electron redistribution enhances HMF and hydroxyl radical adsorption, lowers the HMF-to-DFF energy barrier, and accelerates charge transfer. The hydroxyl group in HMF is more reactive than the aldehyde, boosting the DFF selectivity. The heterojunction's synergistic effect is key to achieving high-value aldehydes efficiently.

The role of iron in Ni-Fe binary catalysts for electrochemical glycerol oxidation

NiFe binary compounds have shown great potential in the electrocatalytic oxidation of biomass-derived glycerol towards valuable products. However, understanding the role of Fe on the glycerol oxidation process of Ni-based catalysts remains challenging due to the complex multi-alcohol-group oxidation. Herein, our systematic study identifies that Fe can improve both the dynamic process of Ni(OH)2 transformation into active NiOOH and the kinetic process of redox reaction between NiOOH and glycerol. Specifically, the introduction of Fe can significantly reduce the onset potential of glycerol oxidation by > 100 mV while maintaining > 95 % selectivity of HCOOH. The in situ Raman results show that the Ni2+/Ni3+ conversion potential is exactly the same as the onset potential of glycerol oxidation, indicating that NiOOH is the most important active structure, and Fe doping can optimize the dynamic process of NiOOH generation. The electrochemical dynamic analysis reveals that the oxidation of glycerol on Ni-based catalysts follows an electrochemical-chemical process, and the chemical reaction constant (k) value is greatly improved by 5 times with the presence of Fe. Density functional theory (DFT) calculation reveals that the electron transfer from Fe to Ni can help the extraction of electrons from adsorbed glycerol, thus lowering the reaction barrier and accelerating the reaction rate. Based on this, we constructed an efficient photoanode with Metal-Insulator-Semiconductor (MIS) junction structure (n-Si/SiOx/Ni3Fe1) that demonstrated the highest glycerol oxidation photocurrent and excellent selectivity for formate.

Re and Ru Co-Doped Transition Metal Alloy as a Bifunctional Catalyst for Electrooxidation of Glycerol to Formate Coupled with H2 Production

Glycerol electrooxidation (GOR), as an innovative strategy for the production of value-added chemicals, is considered a promising anodic alternative to oxygen evolution reaction in electrocatalysis. However, the high potential and the limited selectivity and faradaic efficiency impede the industrial-scale application toward GOR. Herein, we for the first time constructed rhenium and ruthenium co-doped transition metal alloy (NiCoFeRuRe) for the efficient electrooxidation of glycerol to formate. Benefiting from the rapid generation of M3+-OOH induced by Ru element and the inhibition of OER and excessive oxidation of glycerol by Re species through in situ chacterization, the optimized NiCoFeRuRe requires only 1.133 VRHE to achieve a current density of 10 mA cm-2, a faradaic efficiency of 95.6 % for formate product with a stability more than 450 h. Importantly, employing NiCoFeRuRe as a bifunctional catalyst, the cell is constructed to produce hydrogen and formate simultaneously, which is 265 mV lower than the electrolytic water splitting owning an excellent stability of 350 h. This work provides a facile strategy for rationally designing high-performance GOR catalysts for biomass upgradings.

Effect of glycerol concentration on rate and product speciation for Ni and Au-based catalysts

In this paper, we investigate the glycerol electrooxidation reaction (GEOR) on Au and Ni catalysts, specifically the effect of glycerol concentration on electrochemical activity and product speciation for GEOR in an electrochemical flow cell system. With Au foil, cyclic voltammogram behavior shifted from hysteretic to near-linear by increasing the concentration of glycerol from 0.1 M to 1 M. As a result, glycerol electrooxidation increased up to 1.4 V vs. RHE with a higher glycerol concentration. The major products were formic acid and glycolic acid, yet minor products of value-added glyceric acid, lactic acid, and dihydroxyacetone were observed at a higher glycerol concentration. Competition between glycerol and the Au surface for hydroxide inhibits the formation of poisoning Au oxide (AuOx) species and enables the formation of low degree oxidation products. With Ni foil, the GEOR peak current density in cyclic voltammetry increased with glycerol concentration, however, formation of the major product, formic acid, decreased. This study examines and utilizes differences in GEOR mechanism on Ni vs. Au catalysts to vary product speciation in flow cell systems.

Unraveling Side Reactions in Paired CO2 Electrolysis at Operando Conditions: A Case Study of Ethylene Glycol Oxidation

Replacing the oxygen evolution reaction (OER) in CO2 electrolysis with an energetically and economically favorable alternative is very promising. Yet, understanding paired organic oxidation in the environment for CO2 reduction is particularly challenging, as monitoring multiple side reactions is problematic. Herein, we examined the oxidation of ethylene glycol (EG), one of the simplest polyols, as a model reaction on a series of nickel oxyhydroxide model catalysts (β-NiMxOOH, M = Ni, Co, Fe, and Cu). Using in situ techniques, including surface-enhanced infrared absorption spectroscopy (SEIRAS) and differential electrochemical mass spectrometry (DEMS), together with various ex situ approaches, we obtained the potential-resolved and quantitative information on various side reactions comprising the OER, overoxidation to CO/CO2, catalyst dissolution, and CO2 evolution from electrolyte decarbonation. Many factors including impurity cations, pH, and potential can substantially influence the product distribution and side reactions. Such influences are nearly identical for both the electrocatalytic and chemical-electrochemical oxidation pathways. The optimized system can achieve stable and high Faradaic efficiencies of formate (∼100%), glycolaldehyde (∼86%), and glycolate (∼66%), respectively. Importantly, paired electrolysis can easily suffer from higher energy consumption than the conventional counterpart, provided side reactions are unregulated. Yet the modulated one consumed 21.1% less energy even when product separation was considered. This work reveals the unique side reactions in paired CO2 electrolysis, opening up opportunities for designing efficient systems for real-life applications.

Coupling of CuO@NiBiOx Catalyzed Glycerol Oxidation to Carbon Dioxide Reduction Reaction for Enhanced Energy Efficiency

Glycerol electrooxidation reaction (GEOR) is a promising alternative to the oxygen evolution reaction (OER) in electrolyzers, overcoming the inherent challenges of high energy demand and low-value output of water oxidation. Here, we designed a non-noble metal-based electrocatalyst (CuO@NiBiOx, CNBO) for selective and efficient GEOR. The CNBO catalyst demonstrated high selectivity and achieved nearly 100% GEOR Faradaic efficiency (FE), 80%-90% of which is conveyed into formic acid (FA). Bismuth incorporation modified the structure of the mixed oxide, increasing the surface concentration of Ni(III) species and enhancing the GEOR activity. In situ studies confirmed the formation of NiOOH, which is identified as the active site for GEOR and suggests an indirect GEOR mechanism. This study demonstrates the potential of GEOR to replace OER in Carbon dioxide reduction reaction (CO2RR) electrolyzers. Depending on the selected CO2RR catalyst (Ag or Sn), we could obtain either an easy-to-separate mixture of high-added value products (CO and FA) or a single product (FA) with FEFA > 85% at both electrodes. Moreover, we demonstrate that replacing OER with GEOR in a CO2RR-electrolyzer can save up to 25% of the electrolysis energy input, while the co-production of FA at both electrodes halves the energy per mole required for its electrosynthesis.

Highly Selective Electrooxidation of Glycerol to Tartronic Acid Over a Single-Atom Rhodium Catalyst Supported on Indium Oxide

The electrooxidation of biodiesel-derived glycerol offers an effective approach for the sustainable production of valuable C3 compounds. However, highly selective synthesis of a specific C3 compound, such as tartronic acid (TA), by glycerol electrooxidation remains a big challenge due to the competitive dehydrogenation between CαH2(OH) and CβH(OH). Herein this study reports a glycerol electrochemical oxidation reaction (GEOR) for the selective production of TA, which is catalyzed by a single-atom rhodium catalyst supported on indium oxide (Rh1-In2O3) in an alkaline medium. At a potential of 1.40 V versus reversible hydrogen electrode, the Rh1-In2O3-catalyzed GEOR achieves an optimal TA selectivity of 93.2% and a productivity of 4.6 mmol cm-2 h-1, outperforming all previously reported electrocatalytic systems for the GEOR. Experimental results, complemented by density functional theory calculation, reveal that the single-atom Rh catalyst improves glycerol oxidation by facilitating hydroxyl oxidation to active oxygen species and greatly decreasing the energy barrier for CαH2(OH) dehydrogenation in the GEOR process, thus resulting in high TA selectivity. Furthermore, an integrated electrolyzer, combining GEOR with the hydrogen evolution reaction, achieves a current density of 100 mA cm-2 at a cell voltage of 1.50 V. A techno-economic analysis demonstrates the economic feasibility of this integrated system.

Resolving Peak Overlap in HPLC Analysis of Glycerol Oxidation Products by Utilizing Various Detectors: Application to BiVO4 Photoanodes

Glycerol, often considered a waste byproduct of biodiesel production, holds the potential for conversion into chemicals of varying economic value, such as dihydroxyacetone (DHA) and formic acid (FA). Hence, accurate identification and quantification of glycerol oxidation reaction (GOR) products are crucial for glycerol valorization research and practical deployment. High-performance liquid chromatography (HPLC) is the preferred analytical method for these purposes due to its proficiency in separating and quantifying components in liquid mixtures, even in the presence of diluted solutes. On the other hand, peak overlap in chromatograms, especially among glycerol, DHA, and FA, poses a notable challenge in the analysis of GOR products. This study introduces a quantification method aimed at resolving peak overlaps in HPLC analysis of GOR products. Initially, we examine the optical properties of glycerol and GOR products to identify optimal wavelengths for spectrophotometric HPLC analysis and detection. Subsequently, we propose an algebraic approach to resolve the peak overlap of glycerol, DHA, and FA using various detectors, including the refractive index detector (RID) and the variable wavelength detector (VWD). This method is applied to analyze the GOR products of undoped, nonco-catalyzed nanoporous BiVO4 photoanodes, which have shown an intrinsic catalytic activity toward GOR products in previous studies.

Bias-Free Solar-Driven Ammonia Coupled to C3-Dihydroxyacetone Production through Photoelectrochemistry

Conversion of solar energy into value-added chemicals through photoelectrochemistry (PEC) holds great potential for advancing sustainable development but limits by high onset potential which affects energy conversion efficiencies. Herein, we utilized a CuPd cocatalyst-modified Sb2(S,Se)3 photocathode (CuPd/TSSS) to achieve an ultra-low onset potential of 0.83 VRHE for photoelectrochemical ammonia synthesis. Meanwhile, we achieved unbiased NH3 production by synthesizing major value-added C3-dihydroxyacetone (DHA) through glycerol oxidation on the BiVO4 photoanode with the loading Pd cocatalyst, instead of a traditional solar water oxidation reaction. The PEC integrated system stably produced 11.98 μmol cm-2 of NH3 and 201.9 mmol m-2 of DHA over 5 h with ~80 % faradaic efficiency without applying additional bias. In situ analysis and theoretical calculations confirmed high catalytic activity for ammonia synthesis at the CuPd/TSSS photocathode and enhanced selectivity for DHA at the Pd/BiVO4 photoanode. This design represents a breakthrough in directly utilizing solar energy, nitrate-containing wastewater, and biomass waste for ammonia and highly value-added C3 production, which addresses increasing energy demands while decreasing environmental impact.

A Transition Metal-Free Approach for the Conversion of Real-Life Cellulose-Based Biomass into Formate

Formic acid (FA) and its salt are recognized as valuable molecules for various industries such as textiles and pharmaceuticals. Currently, the global demand of FA and its salts stands at 1.137 million metric tons per year, necessitating the development of sustainable methods to meet the future demands. While numerous approaches are developed for the generation of FA but the requirement of harsh reaction conditions to achieve them is unavoidable. On the other hand, the world production of biomass is estimated at 146 billion metric tons per year and that can be considered as a prospective source of FA and their salts. Additionally, cellulose accounts for approximately 35-45% of the biomass composition. Considering this, a visible-light-mediated approach is presented to produce formate directly from biomass at room temperature as well as at atmospheric pressure. In this approach, selective generation of hydroxyl radical has been achieved which later converted sugars, cellulose, and hemicellulose into formate. Furthermore, the conversion of cellulose-rich daily-life materials such as discarded paper into the product through a series of flow experiments is demonstrated. Finally, mechanistic investigations including electron paramagnetic resonance (EPR) spectroscopy, and density functional theory (DFT) calculations are conducted to gain insights into the underlying reaction mechanism.

Efficient Alkaline-Free Electrooxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid using Electrochemically-Charged NixCo1-x(OH)2 as a Redox Mediator

Converting biomass-derived molecules like 5-hydroxymethylfurfural (HMF) into value-added products alongside hydrogen production using renewable energy offers significant opportunities for sustainable chemical and energy production. Yet, HMF electrooxidation requires strong alkaline conditions and membranes for efficient conversion. These harsh conditions destabilize HMF, leading to humin formation and reduced product purity, meanwhile membranes increase costs. Addressing these challenges, we introduce a two-step, decoupling system that operates without strong alkaline conditions and eliminates the need of membranes. In this system, nickel-cobalt hydroxides serve as effective redox mediators, driving HMF oxidation in pure water. The experimental results showed that Ni0.85Co0.15OOH effectively promotes the dehydrogenation of substrate and achieves a highly efficient oxidation of HMF in pure water, with the selectivity of product 2,5-furandicarboxylic acid (FDCA) approaching 100 %. This system has been expanded to oxidize various substrates, achieving yields exceeding 92 % for the corresponding acids of functionalized compounds such as furfuryl alcohol, furfural, benzyl alcohol, and benzaldehyde. By replacing fixed electrodes with flow electrodes, the scalability of decoupling strategy was evaluated for the harvesting of pure solid FDCA, highlighting the broad prospects in practical applications.

Understanding Biomass Valorization through Electrocatalysis: Transformation of Glycerol and Furan Derivatives

Electrochemical conversion of underutilized biomass provides a green approach for their valorization into high-value-added products, which constitutes a mild, safe, and green procedure. Moreover, the use of an electrochemical pathway in contrast to the classical routes (thermo or biochemical) offers several advantages in terms of product selectivity, safety, catalyst stability, and reusability. The highly variable number of tunable conditions in an electrochemical reaction offers a broad space for improvement until the optimum ones are achieved, including substrates, curent and voltage, electrodes, electrolytes, and cell set-up. The present Perspective aims to provide an informative overview into the biomass and waste valorization of furan derivatives and byproducts of biofuel refineries (glycerol) by reviewing the essential aspects of this field. We cover the fundamentals of electrochemical organic transformations, emphasizing the different parameters to consider during these procedures. We highlight the potential of electrochemical methods for biomass valorization and suggest new directions for more sustainable research.

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.

Mechanistic Insights into Glycerol Oxidation to High-Value Chemicals via Metal-Based Catalysts

The oxidation of glycerol offers a valuable route for producing high-value chemicals. This review provides an in-depth analysis of the current advancements and mechanistic insights into novel metal-based catalysts for glycerol oxidation. We discuss the catalytic roles of both precious metals (e.g., Pt, Pd, Au), noted for their high efficiency and selectivity, and cost-effective alternatives, such as Ni, Cu, and Fe. Bimetallic and metal oxide catalysts are highlighted, emphasizing synergistic effects that enhance catalytic performance. This review elucidates the key mechanism involving selective adsorption and oxidation, providing detailed insights from advanced spectroscopic and computational studies into the activation of glycerol and stabilization of key intermediates, including glyceraldehyde and dihydroxyacetone. Additionally, selective carbon-carbon bond cleavage to yield smaller, valuable molecules is addressed. Finally, we outline future research directions, emphasizing the development of innovative catalysts, deeper mechanistic understanding, and sustainable process scale-up, ultimately advancing efficient, selective, and environmentally friendly catalytic systems for glycerol valorization.

Delineating Catalyst Deactivation Mechanisms in Electrocatalytic Glycerol Oxidation toward Biodiesel Wastewater/CO2 Co-valorization

Biodiesel plays a key role in achieving economy-wide decarbonization but its production discharges significant amounts of CO2 and glycerol-laden wastewater. Given the increasing abundance of biodiesel wastewater and low redox potential of glycerol, coupling the glycerol oxidation reaction (GOR) with CO2 electrolysis has emerged as an attractive strategy to achieve sustainable wastewater management, CO2 utilization, and green chemical synthesis in a single unit process. Despite the need for highly stable catalysts, few studies have examined electrocatalyst deactivation in environmental waste streams. Here, we present a first-of-a-kind diagnostic study that investigates nickel (Ni) catalyst stability during the GOR in synthetic biodiesel wastewaters. A current decline of 99.7% was observed within 24 h of operation. This coincided with an 80% decrease in surface active Ni(II)/Ni(III) concentrations, 190-fold increases in interfacial impedance, and the appearance of electrode C-bonds that suggested surface coverage by GOR reactants and intermediates was likely a main contributor to loss in catalytic activity. Analyses in more complex electrolytes containing methanol and oleate suggested the emergence of distinct deactivation mechanisms through restricted NiOOH formation. Altogether, this study details several previously unreported catalyst deactivation mechanisms. These findings can ultimately help inform future catalyst design toward more practical and sustainable waste valorization.

Amorphous Ni(OH)2 Coated Cu Dendrites with Superaerophobic Interface for Bipolar Hydrogen Production Assisted with Formaldehyde Oxidation

Since formaldehyde oxidation reaction (FOR) can release H2, it is attractive to construct a bipolar hydrogen production system consisting of FOR and hydrogen evolution reaction (HER). Although copper-based catalysts have attracted much attention due to their low cost and high FOR activity, the performance enhancement mechanism lacks in-depth investigation. Here, an amorphous-crystalline catalyst of amorphous nickel hydroxide-coated copper dendrites on copper foam (Cu@Ni(OH)2/CF) is prepared. The modification of Ni(OH)2 resulted in hydrophilic and aerophobic states on the Cu@Ni(OH)2/CF surface, facilitating the transport of liquid-phase species on the electrode surface and accelerating the release of H2. The Open circuit potential (OCP) and density functional theory (DFT) calculations indicate that this core-shell structure facilitates the adsorption of HCHO and OH-. In addition, the catalytic mechanism and reaction pathway of FOR are investigated through in situ FTIR and DFT calculations, and the results showed that the modification of Ni(OH)2 lowered the energy barrier for C─H bond breaking and H─H bond formation. In the HER//FOR system, Pt/C//Cu@Ni(OH)2/CF can provide a current density of 0.5 A cm-2 at 0.36 V and achieve efficient and stable H2 production. This work offers new ideas for designing electrocatalysts for bipolar hydrogen production system assisted with formaldehyde oxidation.

Electrocatalytic Biomass Oxidation via Acid-Induced In Situ Surface Reconstruction of Multivalent State Coexistence in Metal Foams

Electrocatalytic biomass conversion offers a sustainable route for producing organic chemicals, with electrode design being critical to determining reaction rate and selectivity. Herein, a prediction-synthesis-validation approach is developed to obtain electrodes for precise biomass conversion, where the coexistence of multiple metal valence states leads to excellent electrocatalytic performance due to the activated redox cycle. This promising integrated foam electrode is developed via acid-induced surface reconstruction to in situ generate highly active metal (oxy)hydroxide or oxide (MOxHy or MOx) species on inert foam electrodes, facilitating the electrooxidation of 5-hydroxymethylfurfural (5-HMF) to 2,5-furandicarboxylic acid (FDCA). Taking nickel foam electrode as an example, the resulting NiOxHy/Ni catalyst, featuring the coexistence of multivalent states of Ni, exhibits remarkable activity and stability with a FDCA yields over 95% and a Faradaic efficiency of 99%. In situ Raman spectroscopy and theoretical analysis reveal an Ni(OH)2/NiOOH-mediated indirect pathway, with the chemical oxidation of 5-HMF as the rate-limiting step. Furthermore, this in situ surface reconstruction approach can be extended to various metal foams (Fe, Cu, FeNi, and NiMo), offering a mild, scalable, and cost-effective method for preparing potent foam catalysts. This approach promotes a circular economy by enabling more efficient biomass conversion processes, providing a versatile and impactful tool in the field of sustainable catalysis.

Tailoring the oxidation of benzyl alcohol and its derivatives with (photo)electrocatalysis

The electrochemical oxidation of alcohol molecules has gained significance as a key anode reaction, offering an alternative to the oxygen evolution reaction (OER) for hydrogen (H2) production and carbon dioxide (CO2) reduction. The (photo)electrochemical oxidation of benzyl alcohol and its derivatives serves as an important model system, not only because benzyl alcohol oxidation is a critical industrial process, but also because it offers valuable insights into electrocatalytic biomass conversion. Tailoring this reaction through electrochemical and photoelectrochemical methods using heterogeneous noble and transition metal electrocatalysts presents a green approach and the potential for uncovering new reaction mechanisms. This review article positions the electrochemical oxidation of benzyl alcohol as an alternative to the OER to produce H2, highlighting recent mechanistic studies involving noble and transition metal electrocatalysts. Furthermore, we discuss the electronic substituent effects on this reaction, which have been well-explored in organic oxidation pathways but remain underexplored in (photo)electrocatalytic contexts.

Membrane-Free Water Electrolysis for Hydrogen Generation with Low Cost

Conventional water electrolysis relies on expensive membrane-electrode assemblies and sluggish oxygen evolution reaction (OER) at the anode, which makes the cost of green hydrogen (H2) generation much higher than that of grey H2. Here, we develop an innovative and efficient membrane-free water electrolysis system to overcome these two obstacles simultaneously. This system utilizes the thermodynamically more favorable urea oxidation reaction (UOR) to generate clean N2 over a new class of Cu-based catalyst (CuXO) for replacing OER, fundamentally eliminating the explosion risk of H2 and O2 mixing while removing the need for membranes. Notably, this membrane-free electrolysis system exhibits the highest H2 Faradaic efficiency among reported membrane-free electrolysis work. In situ spectroscopic studies reveal that the new N2Hy intermediate-mediated UOR mechanism on the CuXO catalyst ensures its unique N2 selectivity and OER inertness. More importantly, an industrial-type membrane-free water electrolyser (MFE) based on this system successfully reduces electricity consumption to only 3.78 kWh Nm-3, significantly lower than the 5.17 kWh Nm-3 of commercial alkaline water electrolyzers (AWE). Comprehensive techno-economic analysis (TEA) suggests that the membrane-free design and reduced electricity input of the MFE plants reduce the green H2 production cost to US$1.81 kg-1, which is lower than those of grey H2 while meeting the technical target (US$2.00-2.50 kg-1) set by European Commission and United States Department of Energy.

Electrosynthesis of benzyl-tert-butylamine via nickel-catalyzed oxidation of benzyl alcohol

The development of sustainable synthetic methods for converting alcohols to amines is of great interest due to their widespread use in pharmaceuticals and fine chemicals. In this work, we present an electrochemical approach by using green electrons for the selective oxidation of benzyl alcohol to benzaldehyde using a NiOOH catalyst, followed by its reductive amination to form benzyl-tert-butylamine. The number of Ni monolayer equivalents on the catalyst was found to significantly influence selectivity, with 2 monolayers achieving up to 90% faradaic efficiency (FE) for benzaldehyde in NaOH, while 10 monolayers performed best in a tert-butylamine solution (pH 11), yielding 100% FE for benzaldehyde. Reductive amination of benzaldehyde was optimized on Ag and Pb electrodes, with Ag achieving 39% FE towards the amine product, though hydrogen evolution remained a competing reaction. In situ FTIR spectroscopy confirmed the formation of benzaldehyde and its corresponding imine intermediate during oxidation, while reduction spectra supported the formation of the amine product. These results demonstrate the potential of paired electrolysis for alcohol-to-amine conversion, achieving an overall 35% FE for the synthesis of benzyl-tert-butylamine. This work paves the way for more efficient and sustainable electrochemical routes to amine synthesis.

Identifying Reactive Trends in Glycerol Electro-Oxidation Using an Automated Screening Approach: 28 Ways to Electrodeposit an Au Electrocatalyst

Automated, rapid electrocatalyst discovery techniques that comprehensively address the exploration of chemical spaces, characterization of catalyst robustness, reproducibility, and translation of results to (flow) electrolysis operation are needed. Responding to the growing interest in biomass valorization, we studied the glycerol electro-oxidation reaction (GEOR) on gold in alkaline media as a model reaction to demonstrate the efficacy of such methodology introduced here. Our platform combines individually addressable electrode arrays with HardPotato, a Python application programming interface for potentiostat control, to automate electrochemical experiments and data analysis operations. We systematically investigated the effects of reduction potential (E l) and pulse width (PW) on GEOR activity during the electrodeposition (Edep) of gold, evaluating 28 different conditions in triplicate measurements with great versatility. Our findings reveal a direct correlation between E l and GEOR activity. Upon CV cycling, we recorded a 52% increase in peak current density and a -0.25 V shift in peak potential as E l varied from -0.2 to -1.4 V. We also identified an optimal PW of ∼1.0 s, yielding maximum catalytic performance. The swift analysis enabled by our methodology allowed us to correlate performance enhancements with increased electrochemical surface area and preferential deposition of Au(110) and Au(111) sites, even in disparate Edep conditions. We validate our methodology by scaling the Edep process to larger electrodes and correlating intrinsic activity with product speciation via flow electrolysis measurements. Our platform highlights opportunities in automation for electrocatalyst discovery to address pressing needs toward industrial decarbonization, such as biomass valorization.

Insights into the Electrochemical Oxidation and Reduction of Nickel Oxide Surfaces

Surface oxidation/reduction processes, driven by varying electrochemical potentials, can substantially impact catalyst effectiveness and, consequently, electrolyzer performance. This study combines theoretical and experimental approaches to explore the surface redox behavior of nickel oxides, which are cost-effective and efficient catalysts for many electrochemical reactions. Surface Pourbaix diagrams for three different phases of nickel oxides, i.e., nickel hydroxide (Ni(OH)2), nickel oxyhydroxide (NiOOH), and nickel dioxide (NiO2), were constructed using density functional theory-based simulations. Various experimental methods, including cyclic voltammetry, in situ Raman spectroscopy, and electrochemical titration, were employed to probe the surface redox processes of nickel oxide thin films. Our findings indicate that the ABAB stacking sequence of Ni(OH)2 lacks stability under oxidizing conditions to host the surface oxidation (deprotonation) events, while the AABBCC stacking sequence of NiOOH is energetically favorable due to the presence of interlayer hydrogen bonding. Rapid charge transfer facilitated by interlayer hydrogen bonding accounts for the higher reactivity of partially oxidized/reduced NiOOH (001) surfaces compared to Ni(OH)2 (001) and NiO2 (001) surfaces with the same stoichiometry, where interlayer hydrogen bonding is absent. Insights presented in this work can offer guidelines for optimizing operational conditions and tailoring the surface structures and oxidation states of nickel oxides to enhance performance in applications such as electrocatalysis and supercapacitors.

Evaluation of Active Oxygen Species Derived from Water Splitting for Electrocatalytic Organic Oxidation

Active oxygen species (OH*/O*) derived from water electrolysis are essential for the electrooxidation of organic compounds into high-value chemicals, which can determine activity and selectivity, whereas the relationship between them remains unclear. Herein, using glycerol (GLY) electrooxidation as a model reaction, we systematically investigated the relationship between GLY oxidation activity and the formation energy of OH* (ΔGOH*). We first identified that OH* on Au demonstrates the highest activity for GLY electrooxidation among various pure metals, based on experiments and density functional theory, and revealed that ΔGOH* on Au-based alloys is influenced by the metallic composition of OH* coordination sites. Moreover, we observed a linear correlation between the adsorption energy of GLY (Eads) and the d-band center of Au-based alloys. Comprehensive microkinetic analysis further reveals a volcano relationship between GLY oxidation activity, the ΔGOH* and the adsorption free energy of GLY (ΔGads). Notably, Au3Pd and Au3Ag alloys, positioned near the peak of the volcano plot, show excellent activity, attributed to their moderate ΔGOH* and ΔGads, striking a balance that is neither too high nor too low. This research provides theoretical insights into modulating active oxygen species from water electrolysis to enhance organic electrooxidation reactions.

Earth-Abundant CuWO4 as a Versatile Platform for Photoelectrochemical Valorization of Soluble Biomass Under Benign Conditions

Here, a nanosheet-structured CuWO4 (nanoCuWO4) is demonstrated as a selective and stable photoanode for biomass valorization in neutral and near-neutral solutions. nanoCuWO4 can be readily prepared by a solid phase reaction using nanosheet-structured WO3 as the template. Several substrates, including glucose, fructose and glycerol, are investigated to reveal the wide applicability of nanoCuWO4. The activity and product distribution trends in biomass valorization are investigated at different pH by taking advantage of the promising stability of nanoCuWO4 in a wide pH range. Product analyses confirm that formate production with a Faradaic efficiency (FE) of 76 ± 5% and glycolate production with an FE of 61 ± 8% can be achieved by glucose and fructose valorization at pH 10.2, respectively. On the other hand, glyceraldehyde and dihydroxyacetone (DHA) are the main products in the glycerol valorization, accounting for an FE of over 85% at pH 7. Notably, nanoCuWO4 has the highest FE of DHA in photoelectrochemical (PEC) glycerol valorization compared to WO3 and BiVO4 at neutral pH. The oxidation routes and mechanism of glycerol valorization on nanoCuWO4 are also under investigation. The co-production of H2 and value-added chemicals is finally demonstrated using a photovoltaic cell connected to a nanoCuWO4-based PEC glycerol valorization system.

Quasi-homogeneous photoelectrochemical organic transformations for tunable products and 100% conversion ratio

Photoelectrochemical (PEC) organic transformation at the anode coupled with cathodic H2 generation is a potentially rewarding strategy for efficient solar energy utilization. Nevertheless, achieving the full conversion of organic substrates with exceptional product selectivity remains a formidable hurdle in the context of heterogeneous catalysis at the solid/liquid interface. Here, we put forward a quasi-homogeneous catalysis concept by using the reactive oxygen species (ROS), such as ·OH, H2O2 and SO4•-, as a charge transfer mediator instead of direct heterogeneous catalysis at the solid/liquid interface. In the context of glycerol oxidation, all ROS exhibited a preference for first-order reaction kinetics. These ROS, however, showcased distinct oxidation mechanisms, offering a range of advantages such as ∼ 100 % conversion ratios and the flexibility to tune the resulting products. Glycerol oxidative formic acid with Faradaic efficiency (FE) of 81.2 % was realized by the H2O2 and ·OH, while SO4•- was preferably for glycerol conversion to C3 products like glyceraldehyde and dihydroxyacetone with a total FE of about 80 %. Strikingly, the oxidative coupling of methane to ethanol was successfully achieved in our quasi-homogeneous system, yielding a remarkable production rate of 12.27 μmol h-1 and an impressive selectivity of 92.7 %. This study is anticipated to pave the way for novel approaches in steering solar-driven organic conversions by manipulating ROS to attain desired products and conversion ratios.

Electrochemical and Non-Electrochemical Pathways in the Electrocatalytic Oxidation of Monosaccharides and Related Sugar Alcohols into Valuable Products

In this contribution, we review the electrochemical upgrading of saccharides (e.g., glucose) and sugar alcohols (e.g., glycerol) on metal and metal-oxide electrodes by drawing conclusions on common trends and differences between these two important classes of biobased compounds. For this purpose, we critically review the literature on the electrocatalytic oxidation of saccharides and sugar alcohols, seeking trends in the effect of reaction conditions and electrocatalyst design on the selectivity for the oxidation of specific functional groups toward value-added compounds. Importantly, we highlight and discuss the competition between electrochemical and non-electrochemical pathways. This is a crucial and yet often neglected aspect that should be taken into account and optimized for achieving the efficient electrocatalytic conversion of monosaccharides and related sugar alcohols into valuable products, which is a target of growing interest in the context of the electrification of the chemical industry combined with the utilization of renewable feedstock.

Efficient Electrocatalytic Oxidation of Glycerol to Formate Coupled with Nitrate Reduction over Cu-Doped NiCo Alloy Supported on Nickel Foam

Electrooxidation of biomass-derived glycerol which is regarded as a main byproduct of industrial biodiesel production, is an innovative strategy to produce value-added chemicals, but currently showcases slow kinetics, limited Faraday efficiency, and unclear catalytic mechanism. Herein, we report high-efficiency electrooxidation of glycerol into formate via a Cu doped NiCo alloy catalyst supported on nickel foam (Cu-NiCo/NF) in a coupled system paired with nitrate reduction. The designed Cu-NiCo/NF delivers only 1.23 V vs. RHE at 10 mA cm-2, and a record Faraday efficiency of formate of 93.8 %. The superior performance is ascribed to the rapid generation of NiIII-OOH and CoIII-OOH species and favorable coupling of surface *O with reactive intermediates. Using Cu-NiCo/NF as a bifunctional catalyst, the coupled system synchronously produces NH3 and formate, showing 290 mV lower than the coupling of hydrogen evolution reaction, together with excellent long-term stability for up to 144 h. This work lays out new guidelines and reliable strategies from catalyst design to system coupling for biomass-derived electrochemical refinery.

Universal high-efficiency electrocatalytic olefin epoxidation via a surface-confined radical promotion

Production of epoxides via selective oxidation of olefins affords a fundamental source of key intermediates for the industrial manufacture of diverse chemical stocks and materials. Current oxidation strategy generally works under harsh conditions including high temperature, high pressure, and/or request for potentially hazardous oxidants, leading to substantial challenges in sustainability and energy efficiency. To this end, direct electrocatalytic epoxidation poses as a promising solution to these issues, yet their industrial applications are limited by the low selectivity, low yield, and poor stability of the electrocatalysts. Here we report a universal electrochemical epoxidation approach via a kinetically confined surface radical pathway. High epoxidation efficiency can be achieved under mild working conditions (e.g., >99% selectivity, >80% yield and >80% Faraday efficiency for cyclohexene-to-cyclohexene oxide conversion), which can be extended to broad scope of olefin substrates. The catalytic performance originated from a surface bimolecular (L-H) reaction mechanism involving formation and surface confinement of bromine radicals due to kinetic restriction, which effectively activates inert C=C bonds while avoiding the homogenous radical side reactions. With the use of renewable energy and water as green oxygen source, successful implementation of this approach will pave the way for more sustainable chemical production and manufacturing.

Ruthenium Anchored Laser-Induced Graphene as Binder-Free and Free-Standing Electrode for Selective Electrosynthesis of Ammonia from Nitrate

Developing effective electrocatalysts for the nitrate reduction reaction (NO3RR) is a promising alternative to conventional industrial ammonia (NH3) synthesis. Herein, starting from a flexible laser-induced graphene (LIG) film with hierarchical and interconnected macroporous architecture, a binder-free and free-standing Ru-modified LIG electrode (Ru-LIG) is fabricated for electrocatalytic NO3RR via a facile electrodeposition method. The relationship between the laser-scribing parameters and the NO3RR performance of Ru-LIG electrodes is studied in-depth. At -0.59 VRHE, the Ru-LIG electrode exhibited the optimal and stable NO3RR performance (NH3 yield rate of 655.9 µg cm-2 h-1 with NH3 Faradaic efficiency of up to 93.7%) under a laser defocus setting of +2 mm and an applied laser power of 4.8 W, outperforming most of the reported NO3RR electrodes operated under similar conditions. The optimized laser-scribing parameters promoted the surface properties of LIG with increased graphitization degree and decreased charge-transfer resistance, leading to synergistically improved Ru electrodeposition with more exposed NO3RR active sites. This work not only provides a new insight to enhance the electrocatalytic NO3RR performance of LIG-based electrodes via the coordination with metal electrocatalysts as well as identification of the critical laser-scribing parameters but also will inspire the rational design of future advanced laser-induced electrocatalysts for NO3RR.

Potential cycling boosts the electrochemical conversion of polyethylene terephthalate-derived alcohol into valuable chemicals

The electrocatalytic valorization of polyethylene terephthalate-derived ethylene glycol to valuable glycolic acid offers considerable economic and environmental benefits. However, conventional methods face scalability issues due to rapid activity decay of noble metal electrocatalysts. We demonstrate that a dynamic potential cycling approach, which alternates the electrode potential between oxidizing and reducing values, significantly mitigates surface deactivation of noble metals during electrochemical oxidation of ethylene glycol. This method enhances catalyst activity by 20 times compared to a constant-potential approach, maintaining this performance for up to 60 h with minimal deactivation. In situ Raman and X-ray absorption spectroscopy show that this effectiveness results from efficient removal of surface oxide during the reaction. The strategy is applicable to polyethylene terephthalate hydrolysates and various noble metals, such as palladium, gold, and platinum, with palladium showing a high conversion rate in recent studies. Our approach offers an efficient and durable method for electrochemical upcycling of biomass-derived compounds.

Electrosynthesis of adipic acid with high faradaic efficiency within a wide potential window

Electrosynthesis of adipic acid (a precursor for nylon-66) from KA oil (a mixture of cyclohexanone and cyclohexanol) represents a sustainable strategy to replace conventional method that requires harsh conditions. However, its industrial possibility is greatly restricted by the low current density and competitive oxygen evolution reaction. Herein, we modify nickel layered double hydroxide with vanadium to promote current density and maintain high faradaic efficiency (>80%) within a wide potential window (1.5 ~ 1.9 V vs. reversible hydrogen electrode). Experimental and theoretical studies reveal two key roles of V modification, including accelerating catalyst reconstruction and strengthening cyclohexanone adsorption. As a proof-of-the-concept, we construct a membrane electrode assembly, producing adipic acid with high faradaic efficiency (82%) and productivity (1536 μmol cm-2 h-1) at industrially relevant current density (300 mA cm-2), while achieving >50 hours stability. This work demonstrates an efficient catalyst for adipic acid electrosynthesis with high productivity that shows industrial potential.

Two-Dimensional Nanorod-Shaped Co(II) Coordination Polymer on Three-Dimensional Metallic Foam: A Hybrid Platform for Electrochemical Oxidation of Glucose

This study unveils a novel electrochemical biosensor for monitoring glucose in biological fluids by employing nanorods of a cobalt-bispyridyl/dicarboxylate framework grown in a layer-by-layer manner on a highly porous nickel substrate. The hybrid microporous system has a bicatalytic effect on glucose oxidation due to the synergistic catalytic impact of the nickel and cobalt ions with varying oxidation states as electroactive sites. In addition, the controlled growth of inorganic-organic frameworks changes the mechanism of electron transfer from a diffusion-controlled process to an adsorption-controlled process, thus yielding a low onset oxidation potential (∼0.21 V/Ag-AgCl) and a high current intensity (∼1 mA) for the oxidation of glucose in alkaline media. A fast response time (∼2 s) and a reasonably high sensitivity (0.14 μA μM-1) within a broad linear range (40-360 μM) have determined the suitability and superiority of the hybrid electrode for glucose monitoring compared to many metal-organic-based biosensors. The facile fabrication process of the Co(II) coordination polymer/Ni substrate with a large surface area that benefits from the synergetic catalytic activity of nickel-cobalt hybrids may pave the way for the development of novel hybrid electrodes for biosensors and direct glucose fuel cells.

Anodic Reactions in Alkaline Hybrid Water Electrolyzers: Activity versus Selectivity

Affordable and abundant sources of green hydrogen can give a large impetus to the Energy Transition. While conventional water electrolysis has positioned itself as a prospective candidate for this purpose, it lacks cost competitiveness. Hybrid water electrolysis (HWE) has been praised for its ability to address the issues of conventional water electrolysis due to its decreased energy requirements and its ability to generate value-added products, among other advantages. In this perspective, we discuss the challenges related to the applicability of HWE, using the glycerol oxidation reaction as an example, and we identify pitfalls often found in the literature. Reported catalysts, especially those based on abundant materials, suffer from a severe selectivity-activity tradeoff, hampering their industrial applicability due to large costs associated with product separation and purification. Additionally, testing electrocatalysts under conditions that are relevant for their applications is encouraged, yet these conditions are largely unknown, as in-depth knowledge of the catalytic mechanisms is largely missing. Lastly, an opportunity to increase the amount of interdisciplinary research concerning both the engineering requirements and financial performance of HWE is discussed. Increased focus on these objectives may boost the development of HWE on an industrial scale.

Near-infrared light-driven biomass conversion

Current photocatalytic technologies mainly rely on the input of high-energy ultraviolet-visible (UV-vis) light to obtain the desired excited states with adequate energy to drive redox reactions, precluding the use of low-energy near-infrared (NIR) light that occupies ~50% of the solar spectrum. Here, we report the efficient utilization of NIR light by coupling the low-energy NIR photons with reactive biomass conversion. A unique mechanism of photothermally synergistic photocatalysis was revealed for the selective biomass conversion under NIR light. Using biomass-derived 5-hydroxymethylfurfural (HMF) conversion as a model reaction, it was found that NIR and UV-vis light featured markedly different reaction patterns. 5-Formyl-2-furancarboxylic acid (FFCA) was almost exclusively produced under NIR light, whereas UV-vis light favored the formation of 2,5-diformylfuran (DFF) as the major product. This work provides a paradigm for sustainable and selective chemical synthesis using the Earth's abundant resources, sunlight and biomass.

Selective lactic acid synthesis via ethylene glycol electrooxidation in borate buffer

Efficient and selective oxidation of ethylene glycol is challenging due to uncontrollable C-C bond cleavage. We propose an electrochemical strategy for the selective electrooxidation of ethylene glycol to sythesise lactic acid on a Ni-based electrocatalyst by controlling the pH value of the electrolyte solution.

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.

A Multienzyme Cascade Pathway Immobilized in a Hydrogen-Bonded Organic Framework for the Conversion of CO2

The reduction of carbon dioxide to valuable chemicals through enzymatic processes is regarded as a promising approach for the reduction of carbon dioxide emissions. In this study, an in vitro multi-enzyme cascade pathway is constructed for the conversion of CO2 into dihydroxyacetone (DHA). This pathway, known as FFFP, comprises formate dehydrogenase (FDH), formaldehyde dehydrogenase (FaldDH), formolase (FLS), and phosphite dehydrogenase (PTDH), with PTDH serving as the critical catalyst for regenerating the coenzyme NADH. Subsequently, the immobilization of the FFFP pathway within the hydrogen-bonded organic framework (HOF-101) is accomplished in situ. A 1.8-fold increase in DHA yield is observed in FFFP@HOF-101 compared to the free FFFP pathway. This enhancement can be explained by the fact that within FFFP@HOF-101, enzymes are positioned sufficiently close to one another, leading to the elevation of the local concentration of intermediates and an improvement in mass transfer efficiency. Moreover, FFFP@HOF-101 displays a high degree of stability. In addition to the establishment of an effective DHA production method, innovative concepts for the tailored synthesis of fine compounds from CO2 through the utilization of various multi-enzyme cascade developments are generated by this work.

Electrochemical hydrogenation and oxidation of organic species involving water

Fossil fuel-driven thermochemical hydrogenation and oxidation using high-pressure H2 and O2 are still popular but energy-intensive CO2-emitting processes. At present, developing renewable energy-powered electrochemical technologies, especially those using clean, safe and easy-to-handle reducing agents and oxidants for organic hydrogenation and oxidation reactions, is urgently needed. Water is an ideal carrier of hydrogen and oxygen. Electrochemistry provides a powerful route to drive water splitting under ambient conditions. Thus, electrochemical hydrogenation and oxidation transformations involving water as the hydrogen source and oxidant, respectively, have been developed to be mild and efficient tools to synthesize organic hydrogenated and oxidized products. In this Review, we highlight the advances in water-participating electrochemical hydrogenation and oxidation reactions of representative organic molecules. Typical electrode materials, performance metrics and key characterization techniques are firstly introduced. General electrocatalyst design principles and controlling the microenvironment for promoting hydrogenation and oxygenation reactions involving water are summarized. Furthermore, paired hydrogenation and oxidation reactions are briefly introduced before finally discussing the challenges and future opportunities of this research field.

Mind the Interface: The Role of Adsorption in Electrocatalysis

In the field of electrocatalysis, significant emphasis has been placed on developing electrode materials to enable critical energy storage reactions and sustainable chemical synthesis. However, the electrode is just one part of a complex interfacial environment that controls substrate adsorption and reactivity. In the presence of a liquid electrolyte and an electrochemical interface, adsorption processes behave substantially differently than those in the gas phase. Understanding these adsorption processes, which play an important role in all electrocatalytic reactions, is critical for the design of effective electrocatalysts. In this Perspective, we discuss the current understanding of electrochemical adsorption and its implications for catalyst design.

Electrochemical Valorization of Glycerol via Electrocatalytic Reduction into Biofuels: A Review

Electrochemical conversion of underutilized biomass-based glycerol into high-value-added products provides a green approach for biomass and waste valorization. Plus, this approach offers an alternative to biofuel manufacturing procedure, under mild operating conditions, compared to the traditional thermochemical routes. Nevertheless, glycerol has been widely valorized via electrooxidation, with lower-value products generated at the cathode, ignoring the electroreduction. Here, a review of the efficient glycerol reduction into various products via the electrocatalytic reduction (ECR) process was presented. This review has been built upon the background of glycerol underutilization and theoretical knowledge about the state-of-the-art ECR. The experimental understanding of the processing parameter influences towards electrochemical efficiency, catalytic activity, and product selectivity are comprehensively reviewed, based on the recent glycerol ECR studies. We conclude by outlining present issues and highlighting potential future research avenues for enhanced ECR application.

C-C Bond Formation Coupled with C-C Bond Cleavage during Oxidative Upgrading of Glycerol on a Nanoporous BiVO4 Photoanode

Production of biodiesel generates glycerol as a 10 wt% byproduct. Therefore, efficient and selective glycerol upgrading is critical for the sustainable production of biodiesel as well as for the production of chemicals from renewable feedstocks. In this study, the photoelectrochemical glycerol oxidation reaction (GOR) was investigated using a nanoporous BiVO4 photoanode in pH 9.3 and pH 2 buffer solutions. In both solutions, glycolaldehyde (GCAD), a C2 species, was the major product, which has never been the major product in any previous electrochemical or photoelectrochemical GOR study. To produce GCAD from the C3 species glycerol, C-C cleavage should occur to produce C2 and C1 species with a 1:1 ratio. Intriguingly, our results show that, during photoelectrochemical GOR on BiVO4, more GCAD is produced than can be explained by simple C-C cleavage, meaning that GCAD is also produced from C-C coupling of two C1 species produced from C-C cleavage. This is equivalent to converting two glycerol molecules to three GCAD molecules, which offers an extraordinary way to maximize GCAD production. To gain further insight into the nature of this unprecedented C-C coupling during GOR, photoelectrochemical oxidation of intermediate oxidation products (glyceraldehyde and 1,3-dihydroxyacetone) and glycerol-1,3-13C2 was compared to that of standard glycerol. Photoelectrochemical GOR was also compared with electrochemical GOR on BiVO4 to interrogate whether light is critical for the observed C-C coupling. Results obtained from comprehensive control experiments revealed critical information about C-C cleavage and C-C coupling during GOR on BiVO4.

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.

Halide Adsorption Enhances Electrochemical Hydrogenolysis of 5-Hydroxymethylfurfural by Suppressing Hydrogenation

Reductive upgrading of 5-hydroxymethylfurfural (HMF), a biomass-derived platform molecule, to 2,5-dimethylfuran (DMF), a biofuel with an energy density 40% greater than that of ethanol, involves hydrogenolysis of both the aldehyde (C═O) and the alcohol (C-OH) groups of HMF. It is known that when hydrogenation of the aldehyde occurs to form 2,5-bis(hydroxymethyl)furan (BHMF), BHMF cannot be further reduced to DMF. Thus, aldehyde hydrogenation must be suppressed to increase the selectivity for DMF production. Previously, it was shown that on a Cu electrode hydrogenolysis occurs mainly through proton-coupled electron transfer (PCET), where a proton from the solution and an electron from the electrode are transferred to the organic species. In contrast, hydrogenation occurs not only through PCET but also through hydrogen atom transfer (HAT), where a surface-adsorbed hydrogen atom (H*) is transferred to the organic species. This study shows that halide adsorption on Cu can effectively suppress HAT by decreasing the steady-state H* coverage on Cu during HMF reduction. As HAT enables only aldehyde hydrogenation, a striking suppression of BHMF is observed, thereby enhancing DMF production. We discuss how the identity and concentration of the halide, along with the reduction conditions (i.e., potential and pH), affect halide adsorption on Cu and identify when optimal halide coverages are achieved to maximize DMF selectivity. Our experimental results are presented alongside computational results that elucidate how halide adsorption affects the adsorption energy of hydrogen and the steady-state H* coverage on Cu, which provide an atomic-level understanding of all experimentally observed effects.

Surface Ligand Modification on Ultrathin Ni(OH)2 Nanosheets Enabling Enhanced Alkaline Ethanol Oxidation Kinetics

The ethanol oxidation reaction (EOR) is an economical pathway in many electrochemical systems for clean energy, such as ethanol fuel cells and the anodic reaction in hydrogen generation. Noble metals, such as platinum, are benchmark catalysts for EOR owing to their superb electrochemical capability. To improve sustainability and product selectivity, nickel (Ni)-based electrocatalysts are considered promising alternatives to noble-metal EOR. Although Ni-based electrocatalysts are relieved from intermediate poisoning, their performances are largely limited by their relatively high onset potential. Therefore, the EOR usually competes with the oxygen evolution reaction (OER) at working potentials, resulting in a low EOR efficiency. Here, we demonstrate a strategy to modify the surface ligands on ultrathin Ni(OH)2 nanosheets, which substantially improved their catalytic properties for the alkaline EOR. Chemisorbed octadecylamine ligands could create an alcoholophilic layer at the nanosheet surface to promote alcohol diffusion and adsorption, resulting in outstanding EOR activity and selectivity over the OER at higher potential. These non-noble-metal-based 2D electrocatalysts and surface ligand engineering showcase a promising strategy for achieving high-efficiency electrocatalysis of EOR in many practical electrochemical processes.

Challenges for Hybrid Water Electrolysis to Replace the Oxygen Evolution Reaction on an Industrial Scale

To enable a future society based on sun and wind energy, transforming electricity into chemical energy in the form of fuels is crucial. This transformation can be achieved in an electrolyzer performing water splitting, where at the anode, water is oxidized to oxygen-oxygen evolution reaction (OER)-to produce protons and electrons that can be combined at the cathode to form hydrogen-hydrogen evolution reaction (HER). While hydrogen is a desired fuel, the obtained oxygen has no economic value. A techno-economically more suitable alternative is hybrid water electrolysis, where value-added oxidation reactions of abundant organic feedstocks replace the OER. However, tremendous challenges remain for the industrial-scale application of hybrid water electrolysis. Herein, these challenges, including the higher kinetic overpotentials of organic oxidation reactions compared to the OER, the small feedstock availably and product demand of these processes compared to the HER (and carbon dioxide reduction), additional purifications costs, and electrocatalytic challenges to meet the industrially required activities, selectivities, and especially long-term stabilities are critically discussed. It is anticipated that this perspective helps the academic research community to identify industrially relevant research questions concerning hybrid water electrolysis.

Identification of Active Sites Formed on Cobalt Oxyhydroxide in Glucose Electrooxidation

Transition-metal-based oxyhydroxides are efficient catalysts in biomass electrooxidation towards fossil-fuel-free production of valuable chemicals. However, identification of active sites remains elusive. Herein, using cobalt oxyhydroxide (CoOOH) as the archetype and the electrocatalyzed glucose oxidation reaction (GOR) as the model reaction, we track dynamic transformation of the electronic and atomic structure of the catalyst using a suite of operando and ex situ techniques. We reveal that two types of reducible Co³⁺ -oxo species are afforded for the GOR, including adsorbed hydroxyl on Co³⁺ ion (μ₁ -OH-Co³⁺ ) and di-Co³⁺ -bridged lattice oxygen (μ₂ -O-Co³⁺ ). Moreover, theoretical calculations unveil that μ₁ -OH-Co³⁺ is responsible for oxygenation, while μ₂ -O-Co³⁺ mainly contributes to dehydrogenation, both as key oxidative steps in glucose-to-formate transformation. This work provides a framework for mechanistic understanding of the complex near-surface chemistry of metal oxyhydroxides in biomass electrorefining.