Promoting biomass electrooxidation via modulating proton and oxygen anion deintercalation in hydroxide

Electrochemical Hydrogen Production Coupling with the Upgrading of Organic and Inorganic Chemicals

Electrocatalytic water splitting powered by renewable energy is a green and sustainable method for producing high-purity H2. However, in conventional water electrolysis, the anodic oxygen evolution reaction (OER) involves a four-electron transfer process with inherently sluggish kinetics, which severely limits the overall efficiency of water splitting. Recently, replacing OER with thermodynamically favorable oxidation reactions, coupled with the hydrogen evolution reaction, has garnered significant attention and achieved remarkable progress. This strategy not only offers a promising route for energy-saving H₂ production but also enables the simultaneous synthesis of high-value-added products or the removal of pollutants at the anode. Researchers successfully demonstrate the upgrading of numerous organic and inorganic alternatives through this approach. In this review, the latest advances in the coupling of electrocatalytic H2 production and the upgrading of organic and inorganic alternative chemicals are summarized. What's more, the optimization strategy of catalysts, structure-performance relationship, and catalytic mechanism of various reactions are well discussed in each part. Finally, the current challenges and future prospects in this field are outlined, aiming to inspire further innovative breakthroughs in this exciting area of research.

High-Current-Density Glycerol Electrooxidation on S-Cu-Ni/NF Catalyst: Unveiling the Synergy between S-Promoted GOR and Cu-Inhibited OER

Electrocatalytic glycerol oxidation reaction (GOR) coupled with hydrogen evolution reaction (HER) is an ideal method to achieve efficient hydrogen production. However, realizing the stable degradation of glycerol to formate at high current densities still faces great challenges, mainly stemming from the competition between GOR and oxygen evolution reaction (OER) at high potentials. To address this issue, in this study, S-Cu-Ni/NF catalysts were successfully synthesized using Cu-Ni/NF catalysts as precursors. The introduction of Cu effectively suppressed the OER, enabling the catalysts to achieve more than 75% formate Faraday efficiency over a wide potential range from 1.3 V vs RHE to 1.6 V vs RHE. Meanwhile, S doping significantly enhanced the electron transport ability of GOR, enabling the S-Cu-Ni/NF catalyst to achieve a high current density of 100 mA cm-2 at a low potential of 1.395 V vs RHE. In addition, after 24 h of continuous operation at a high current density of 100 mA cm-2, the catalyst potential increased by only 40 mV, demonstrating excellent stability. Scanning electron microscopy with energy dispersive spectra (SEM-EDS), X-ray photoelectron spectra (XPS) and in situ Raman characterization confirmed that the precipitation of S promoted the formation of the active site NiOOH and its dynamic growth during the GOR process. The present study not only improved the selectivity of formate at high potentials by inhibiting OER but also significantly enhanced the GOR performance, demonstrated the potential of stable degradation of glycerol at high current densities, and provided a feasible strategy for the industrial application of glycerol oxidation coupled with hydrogenolysis.

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.

2D Conjugated Metal-Organic Frameworks as Electrocatalysts for Boosting Glycerol Upgrading Coupled with Hydrogen Production

The valorisation of glycerol using renewable electricity is recognised as an effective and attractive approach for producing high-value compounds from biomass byproducts. 2D conjugated metal-organic frameworks (2D c-MOFs) have emerged as promising electrocatalysts due to their tunable structures, high electronic conductivity, and efficient utilisation of well-defined active centres. In this study, we report the first systematic investigation of 2D c-MOFs containing Ni-X4 (X = O or N) moieties for the glycerol oxidation reaction (GOR), examining key factors influencing both electrocatalytic activity and selectivity. In situ 13C electrochemical nuclear magnetic resonance and Raman spectroscopies provide insights into the GOR mechanism and confirm that Ni-O4 sites are the primary active centres. Theoretical calculations further reveal that [Ni3(HHTQ)2]n (HHTQ = 2,3,7,8,12,13-hexahydroxytricycloquinazoline) exhibits superior GOR activity due to strong adsorption of reaction intermediates and weak interlayer interactions. This work highlights the potential of 2D c-MOFs as highly efficient GOR catalysts, paving the way for the rational design of advanced electrocatalysts and contributing to the development of sustainable energy conversion and storage technologies.

Facilitating OH* Generation on Atomic Pd-Cu Dual Sites towards Efficient Glycerol Electrooxidation to Formate

The electrocatalytic glycerol oxidation reaction (GOR) represents a mild and sustainable pathway to produce value-added chemicals such as formate. However, the rational design and controlled synthesis of efficient catalysts to achieve highly selective C-C bond cleavage of glycerol towards formate remains a great challenge. Herein, atomic Pd doped Cu3N hollow nanocubes with Pd-Cu dual sites were prepared via the unique Kirkendall process, exhibiting superior electrocatalytic GOR activity and selectivity towards formate with a maximum Faraday efficiency of 91.6 %. In situ spectroscopic characterization and theoretical calculations reveal that Pd atoms located in partial Cu substituent sites could facilitate the generation of active *OH and enhance the adsorption of glycerol molecule on adjacent Cu sites, while Pd atoms located in Cu3N unit center could effectively reduce the energy barrier of C-C bond cleavage for the rate determining step. Our work provides new insights into the development of well-defined dual-site catalysts to boost glycerol electrooxidation to formate.

Stable Ni(II) sites in Prussian blue analogue for selective, ampere-level ethylene glycol electrooxidation

The industrial implementation of coupled electrochemical hydrogen production systems necessitates high power density and high product selectivity for economic viability and safety. However, for organic nucleophiles (e.g., methanol, urea, and amine) electrooxidation in the anode, most catalytic materials undergo unavoidable reconstruction to generate high-valent metal sites under harsh operation conditions, resulting in competition with oxygen evolution reaction. Here, we present unique Ni(II) sites in Prussian blue analogue (NiFe-sc-PBA) that serve as stable, efficient and selective active sites for ethylene glycol (EG) electrooxidation to formic acid, particularly at ampere-level current densities. Our in situ/operando characterizations demonstrate the robustness of Ni(II) sites during EG electrooxidation. Molecular dynamics simulations further illustrate that EG molecule tends to accumulate on the NiFe-sc-PBA surface, preventing hydroxyl-induced reconstruction in alkaline solutions. The stable Ni(II) sites in NiFe-sc-PBA anodes exhibit efficient and selective EG electrooxidation performance in a coupled electrochemical hydrogen production flow cell, producing high-value formic acid compared to traditional alkaline water splitting. The coupled system can continuously operate at stepwise ampere-level current densities (switchable 1.0 or 1.5 A cm-2) for over 500 hours without performance degradation.

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.

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.

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.

Unveiling the synergistic interface effects of Ag-deposited Fe2O3/biochar catalysts to enhance wastewater degradation

Photo-Fenton reaction is a sustainable and cost-effective wastewater treatment strategy capable of removing persistent organic pollutants. We utilized biochar (referred to as C) as a key component to enhance catalytic efficiency. Using a biomimetic templating method, we synthesized Fe2O3 on the biochar and deposited Ag on its surface, resulting in the Ag@Fe2O3/C composite material (referred to as AFC). In the photo-Fenton system, the AFC material achieved a 99% degradation rate of methylene blue (MB) within 30 minutes, which is 8.5 times and 6.4 times higher than that of photocatalysis and Fenton reaction alone, respectively. Using Escherichia coli as a model, AFC achieved a 100% bactericidal rate within 6 minutes in the photo-Fenton system. This material effectively treated aquaculture wastewater, removing most impurities and colorants. By introducing Ag nanoparticles as electron transfer centers, the electron transfer efficiency was enhanced, accelerating the catalytic reaction and improving the composite's adsorption capacity for H2O2 and organic pollutants. Analysis revealed the migration pathways of photogenerated electrons and holes in AFC, providing a theoretical foundation and practical direction for the design of future photocatalytic materials and the treatment of organic pollutants.

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.

A Copper-Zinc Cyanamide Solid-Solution Catalyst with Tailored Surface Electrostatic Potentials Promotes Asymmetric N-Intermediate Adsorption in Nitrite Electroreduction

The electrocatalytic nitrite reduction (NO2RR) converts nitrogen-containing pollutants to high-value ammonia (NH3) under ambient conditions. However, its multiple intermediates and multielectron coupled proton transfer process lead to low activity and NH3 selectivity for the existing electrocatalysts. Herein, we synthesize a solid-solution copper-zinc cyanamide (Cu0.8Zn0.2NCN) with localized structure distortion and tailored surface electrostatic potential, allowing for the asymmetric binding of NO2-. It exhibits outstanding NO2RR performance with a Faradaic efficiency of ∼100% and an NH3 yield of 22 mg h-1 cm-2, among the best for such a process. Theoretical calculations and in situ spectroscopic measurements demonstrate that Cu-Zn sites coordinated with linear polarized [NCN]2- could transform symmetric [Cu-O-N-O-Cu] in CuNCN-NO2- to a [Cu-N-O-Zn] asymmetric configuration in Cu0.8Zn0.2NCN-NO2-, thus enhancing adsorption and bond cleavage. A paired electro-refinery with the Cu0.8Zn0.2NCN cathode reaches 2000 mA cm-2 at 2.36 V and remains fully operational at industrial-level 400 mA cm-2 for >140 h with a NH3 production rate of ∼30 mgNH3 h-1 cm-2. Our work opens a new avenue of tailoring surface electrostatic potentials using a solid-solution strategy for advanced electrocatalysis.

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.

Active Hydroxyl-Mediated Preferential Cleavage of Carbon-Carbon Bonds in Electrocatalytic Glycerol Oxidation

Electrocatalytic glycerol oxidation reaction (GOR) to produce high-value formic acid (FA) is hindered by high formation potential of active species and sluggish C-C bond cleavage kinetics. Herein, Ni single-atom (NiSA) and Co single-atom (CoSA) dual sites anchored on nitrogen-doped carbon nanotubes embedded with Ni0.1Co0.9 alloy (Ni0.1Co0.9@NiSACoSA-NCNTs) are constructed for electrochemical GOR. Remarkably, it can reach 10 mA cm-2 at a low potential of 1.15 V versus the reversible hydrogen electrode (vs. RHE) and realize a high formate selectivity of 93.27 % even at high glycerol conversion of 98.81 % at 1.45 V vs. RHE. The GOR mechanism and pathway are systematically elucidated via experimental analyses and theoretical calculations. It is revealed that the active hydroxyl (*OH) can be produced during the GOR. The NiSA, CoSA, and Ni0.1Co0.9 synergistically optimizes the electronic structure of CoSA active sites, reducing the energy barriers of *OH-mediated cleavage of C-C bonds and dehydrogenation of C1 intermediates. This decreases the number of reaction intermediates and reaction steps of GOR-to-FA, thus increasing the formate production efficiency. After coupling GOR with hydrogen evolution reaction in a membrane electrode assembly cell, 14.26 g of formate and 23.10 L of H2 are produced at 100 mA cm-2 for 108 h.

Enhanced Glycerol Electrooxidation Capability of NiO by Suppressing the Accumulation of Ni4+ Sites

Glycerol electrooxidation (GOR), as a typical nucleophilic biomass oxidation reaction, provides a promising anodic alternative for coupling green hydrogen generation at the cathode. However, the challenges of identifying active sites and elucidating reaction mechanisms greatly limit the design of high-performance catalysts. Herein, we use NiO and Ni/NiO as model catalysts to investigate glycerol oxidation. Electrochemical measurements and operando spectroscopic studies uncovered that Ni2+/Ni3+ species are the true active sites of NiO for GOR at lower potentials. However, the Ni2+/Ni3+ species formed on the NiO surface were easily converted to Ni4+ species (NiO2) at higher potentials, which not only contributed to the overoxidation of glycerol electrolysis products but also worked as the main active sites of the competitive oxygen evolution reaction (OER), resulting in the rapid decay of Faradic efficiencies (FEs) at high potentials. Interestingly, for Ni/NiO, only Ni3+ species were formed on the surface. Experimental and density functional theory (DFT) investigations indicated that due to the relatively lower average valence state of Ni in Ni/NiO and strong electronic interaction on the Ni/NiO interface, the surface reconstruction of Ni/NiO was effectively manipulated. Only Ni/NiO → NiOOH (Ni3+) transformation was observed, and the formation of Ni4+ species was greatly suppressed. As a result, Ni/NiO delivered superior GOR activity, and the FE did not drop apparently at high potentials. This work offers mechanistic insight into how to identify and maintain the true active sites of catalytic materials for value-added nucleophile electrooxidation reactions.

In Situ Modulation of NiFeOOH Coordination Environment for Enhanced Electrocatalytic-Conversion of Glucose and Energy-Efficient Hydrogen Production

Glucose electrocatalytic-conversion reaction (GCR) is a promising anode reaction to replace the slow oxygen evolution reaction (OER), thus promoting the development of hydrogen production by electrochemical water splitting. Herein, NiFe-based metal-organic framework (MOF) is used as a precursor to prepare W-doped nickel-iron phosphide (W-NiFeP) nanosheet arrays by ion exchange and phosphorylation, which exhibit a high electrocatalytic activity toward the hydrogen evolution reaction (HER), featuring an overpotential of only -179 mV to achieve the current density of 100 mA cm-2 in alkaline media. Notably, electrochemical activation of W-NiFeP facilitates the in situ formation of phosphate groups producing W,P-NiFeOOH, which, in conjunction with the W co-doped amorphous layers, leads to a high electrocatalytic performance toward GCR, due to enhanced proton transfer and adsorption of reaction intermediates, as confirmed in experimental and theoretical studies. Thus, the two-electrode electrolyzer of the W-NiFeP/NF||W,P-NiFeOOH/NF for HER||GCR needs only a low cell voltage of 1.56 V to deliver 100 mA cm-2 at a remarkable hydrogen production efficiency of 1.86 mmol h-1, with a high glucose conversion (98.0%) and formic acid yields (85.2%). Results from this work highlight the significance of the development of effective electrocatalysts for biomass electrocatalytic-conversion in the construction of high-efficiency electrolyzers for green hydrogen production.

Electronic Structure Engineering of RuNi Alloys Decrypts Hydrogen and Hydroxyl Active Site Separation and Enhancement for Efficient Alkaline Hydrogen Evolution

Rational design of the active sites of hydrolysis dissociation intermediates to weaken their active site competition and toxicity is a key challenge to achieve efficient and stable hydrogen evolution reaction (HER) in ruthenium-containing alloys. Density Functional Theory (DFT) simulations reveal that the transfer of the d-band electrons from Ru to Ni in RuNi alloys results in a Gibbs free energy of -0.12 eV for the Ru0.250Ni Fcc-site H*. In addition, the high spin state of the electrons outside the Ru nucleus strengthens the adsorption of OH* on the Ru─Ni bond, which weakens the active-site competition and toxicity successfully. This theoretical prediction is confirmed by electrodeposition of prepared aRuxNi, and the RuNi alloys obtained by Ru atom doping have excellent HER properties. aRu0.250Ni has overpotentials of 38 and 162.4 mV at -10 and -100 mA cm-2, respectively, and can be stably operated at -100 mA cm-2 Dual-electrode system aRu0.250Ni//bRu0Ni demonstrates an ultra-low battery voltage (1.86 V @500 mA cm-2) and excellent stability (24 h@300 mA cm-2). This holistic work resolves the mechanism of active site separation and strengthening in RuNi alloys, and provides a new design idea for the preparation of highly efficient alkaline HER electrodes.

Cation-Vacancy Engineering in Cobalt Selenide Boosts Electrocatalytic Upcycling of Polyester Thermoplastics at Industrial-Level Current Density

The past decades have witnessed the increasing accumulation of plastics, posing a daunting environmental crisis. Among various solutions, converting plastics into value-added products presents a significant endeavor. Here, an electrocatalytic upcycling route that efficiently converts waste poly(butylene terephthalate) plastics into high-value succinic acid with high Faradaic efficiency of 94.0% over cation vacancies-rich cobalt selenide catalyst is reported, showcasing unprecedented activity (1.477 V vs. RHE) to achieve an industrial-level current density of 1.5 A cm-2, and featuring a robust operating durability (≈170 h). In particular, when combining butane-1,4-diol monomer oxidation (BOR) with hydrogen evolution using the cation vacancy-engineered cobalt selenide as bifunctional catalyst, a relatively low cell voltage of 1.681 V is required to reach 400 mA cm-2, manifesting an energy-saving efficiency of ≈15% compared to pure water splitting. The mechanism and reaction pathways of BOR over the vacancies-rich catalyst are first revealed through theoretical calculations and in-situ spectroscopic investigations. The generality of this catalyst is evidenced by its powerful electrocatalytic activity to other polyester thermoplastics such as poly(butylene succinate) and poly(ethylene terephthalate). These electrocatalytic upcycling strategies can be coupled with the reduction of small molecules (e.g., H2O, CO2, and NO3 -), shedding light on energy-saving production of value-added chemicals.

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.

Time-resolved spectroscopy uncovers deprotonation-induced reconstruction in oxygen-evolution NiFe-based (oxy)hydroxides

Transition-metal layered double hydroxides are widely utilized as electrocatalysts for the oxygen evolution reaction (OER), undergoing dynamic transformation into active oxyhydroxides during electrochemical operation. Nonetheless, our understanding of the non-equilibrium structural changes that occur during this process remains limited. In this study, utilizing in situ energy-dispersive X-ray absorption spectroscopy and machine learning analysis, we reveal the occurrence of deprotonation and elucidate the role of incorporated iron in facilitating the transition from nickel-iron layered double hydroxide (NiFe LDH) into its active oxyhydroxide. Our findings demonstrate that iron substitution promotes deprotonation process within NiFe LDH, resulting in the preferential removal of protons from the specific bridged hydroxyl group (Ni2+-OH-Fe3+) linked to edge-sharing [NiO6] and [FeO6] octahedron. This deprotonation behavior drives the formation of high-valence Ni3+δ species (0 <δ < 1), which subsequently serve as the active sites, thereby ensuring efficient oxygen evolution activity. This approach offers high-resolution insights of dynamic structural evolution, overcoming the limitations of extended acquisition times and advancing our understanding of OER mechanisms.

Efficient Electrosynthesis of Valuable para-Benzoquinone from Aqueous Phenol on NiRu Hybrid Catalysts

Electrocatalytic oxidation of aqueous phenol to para-benzoquinone (p-BQ) offers a sustainable approach for both pollutant abatement and value-added chemicals production. However, achieving high phenol conversion and p-BQ yield under neutral conditions remains challenging. Herein, we report a Ni(OH)2-supported Ru nanoparticles (NiRu) hybrid electrocatalyst, which exhibits a superior phenol conversion of 96.5 % and an excellent p-BQ yield of 83.4 % at pH 7.0, significantly outperforming previously reported electrocatalysts. This exceptional performance benefits from the triple synergistic modulation of the NiRu catalyst, including enhanced phenol adsorption, increased p-BQ desorption, and suppressed oxygen evolution. By coupling a flow electrolyzer with an extraction-distillation separation unit, the simultaneous phenol removal and p-BQ recovery are realized. Additionally, the developed electrocatalytic system with the NiRu/C anode displays good stability, favorable energy consumption, and reduced greenhouse gas emissions for phenol-containing wastewater treatment, demonstrating its potential for practical applications. This work offers a promising strategy for achieving low-carbon emissions in phenol wastewater treatment.

Ozone-Assisted Cu-Based Catalysts for the Efficient Electro-Reforming Glycerol to Formic Acid

Glycerol electrooxidation reaction (GOR) to produce value-added chemicals, such as formic acid, could make more efficient use of abundant glycerol and meet future demand for formic acid as a fuel for direct or indirect formic acid fuel cells. Non-noble metal Cu-based catalysts have great potential in electro-reforming glycerol to formic acid. However, the high activity, selectivity and stability of Cu based catalysts in GOR cannot be achieved simultaneously. Here, we used ozone-assisted electrocatalyst to convert glycerol to formic acid under alkaline conditions, the onset potential was reduced by 60 mV, the Faraday efficiency (FE) reached 95 %. The catalyst has excellent stability within 300 h at the current density of 10 mA cm-2. The electron spin resonance proved that ozone produced superoxide anion during the GOR. In situ Raman spectroscopy, electrochemical studies showed that glycerol can be activated with ozone in GOR, and the C-C bond can be broken to reduce the polymerization of glycerol on the catalyst surface, so as to produce more formic acid at a lower voltage. Moreover, the removal of dissolved O3 from water can be up to 100 % after 30 minutes of GOR reaction at a solubility of 50 mg L-1 as measured by UV-VIS spectrophotometry.

Copper-Doped NiOOH for the Electrocatalytic Conversion of Glycerol to Formate via the Inhibition of the Oxygen Evolution Reaction

The combination of the electrocatalytic glycerol oxidation reaction (GOR) with the cathodic hydrogen evolution reaction serves to reduce the anodic overpotential, thereby facilitating the efficient production of hydrogen. However, the GOR is confined to a narrow potential range due to the competition of the oxygen evolution reaction (OER) at high potential. Therefore, it is necessary to develop a catalyst with a high Faraday efficiency of formate (FEFA) over a wide potential range. Herein, Cu-doped NiOOH catalysts were synthesized by electrodeposition to inhibit the competing OER during the GOR process, achieving a current density of 10 mA cm-2 at 1.278 V vs RHE, a FEFA over 70.36% within a broad potential range of 1.3 V vs RHE to 1.6 V vs RHE, and a maximum FEFA of 96.46% at 1.35 V vs RHE. In situ spectral studies and DFT calculations revealed that Cu doping slowed the *OH to *O step for the inhibition of the OER and enhanced glycerol adsorption to accelerate the GOR. A competitive reaction mechanism for boosting glycerol electro-oxidation to formate was proposed, presenting a feasible strategy for the highly selective production of electrocatalytic value-added chemicals and the sustainable production of hydrogen energy.

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.

Tackling activity-stability paradox of reconstructed NiIrOx electrocatalysts by bridged W-O moiety

One challenge remaining in the development of Ir-based electrocatalyst is the activity-stability paradox during acidic oxygen evolution reaction (OER), especially for the surface reconstructed IrOx catalyst with high efficiency. To address this, a phase selective Ir-based electrocatalyst is constructed by forming bridged W-O moiety in NiIrOx electrocatalyst. Through an electrochemical dealloying process, an nano-porous structure with surface-hydroxylated rutile NiWIrOx electrocatalyst is engineered via Ni as a sacrificial element. Despite low Ir content, NiWIrOx demonstrates a minimal overpotential of 180 mV for the OER at 10 mA·cm-2. It maintains a stable 300 mA·cm-2 current density during an approximately 300 h OER at 1.8 VRHE and shows a stability number of 3.9 × 105 noxygen · nIr-1. The resulting W - O-Ir bridging motif proves pivotal for enhancing the efficacy of OER catalysis by facilitating deprotonation of OER intermediates and promoting a thermodynamically favorable dual-site adsorbent evolution mechanism. Besides, the phase selective insertion of W-O in NiIrOx enabling charge balance through the W-O-Ir bridging motif, effectively counteracting lattice oxygen loss by regulating Ir-O co-valency.

3D Pt@ZnAl-LDH catalyst on low-grade charcoal: A novel electrochemical platform for efficient textile dye degradation and glycerol oxidation

The development of sustainable and efficient electrochemical processes is crucial for addressing global challenges related to water scarcity. In this study, we present a novel 3D core-shell electrocatalyst, Pt@ZnAl-LDH, supported on low-grade charcoal (LGC), which exhibits exceptional electrocatalytic activity for the degradation and decolorization of dye and the electrocatalytic conversion of glycerol to valuable C3 chemicals. The electrocatalytic degradation of methylene blue dye from water was investigated with a focus on the impact of temperature, pH, and dye concentration. The Pt@ZnAl-LDH/LGC anode demonstrates high selectivity for converting glucose into lactate and other C3 products, achieving an impressive 85% conversion rate at 0.5 V vs. Furthermore, the electrode achieves an exceptionally high level of selectivity for C3 products, reaching 86% at 2.2 V vs, significantly outperforming other electrodes. Theoretical calculations and electrochemical in situ techniques reveal that the incorporation of ZnAl-LDH enhances the adsorption of hydroxyl species, leading to improved glucose oxidation reaction performance. The 3D Pt@ZnAl-LDH/LGC catalyst optimizes glycerol adsorption, preventing the formation of unwanted intermediates and ensuring high activity and selectivity for C3 products. This work presents a novel electrocatalytic compound for the degradation of toxic dyes and the production of valuable C3 products using an inexpensive aqueous glucose oxidation method.

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.

Facilitating Formate Selectivity via Optimizing eg* Band Broadening in NiMn Hydroxides for Ethylene Glycol Electro-Oxidation

Ethylene glycol electro-oxidation reaction (EGOR) on nickel-based hydroxides (Ni(OH)2) represents a promising strategy for generating value-added chemicals, i.e. formate and glycolate, and coupling water-electrolytic hydrogen production. The high product selectivity was one of the most significant area of polyols electro-oxidation process. Yet, developing Ni(OH)2-based EGOR electrocatalyst with highly selective product remains a challenge due to the unclear cognition about the EGOR mechanism. Herein, Mn-doped Ni(OH)2 catalysts were utilized to investigate the EGOR mechanism. Experimental and calculation results reveal that the electronic states of eg* band play an important role in the catalytic performance and the product selectivity for EGOR. Broadening the eg* band could effectively enhance the adsorption capacity of glyoxal intermediates. On the other hand, this enhanced adsorption could lead to reduced side reactions associated with glycolate formation, simultaneously promoting the cleavage of C-C bonds. Consequently, the selectivity for formate was notably augmented by these enhancements. This work offers new insights into the regulation of catalyst electronic states for improving polyol electrocatalytic activity and product selectivity.

Wood-Structured Nanomaterials as Highly Efficient, Self-Standing Electrocatalysts for Water Splitting

Electrocatalytic water splitting (EWS) driven by renewable energy is widely considered an environmentally friendly and sustainable approach for generating hydrogen (H2), an ideal energy carrier for the future. However, the efficiency and economic viability of large-scale water electrolysis depend on electrocatalysts that can efficiently accelerate the electrochemical reactions taking place at the two electrodes. Wood-derived nanomaterials are well-suited for serving as EWS catalysts because of their hierarchically porous structure with high surface area and low tortuosity, compositional tunability, cost-effectiveness, and self-standing integral electrode configuration. Here, recent advancements in the design and synthesis of wood-structured nanomaterials serving as advanced electrocatalysts for water splitting are summarized. First, the design principles and corresponding strategies toward highly effective wood-structured electrocatalysts (WSECs) are emphasized. Then, a comprehensive overview of current findings on WSECs, encompassing diverse structural designs and functionalities such as supported-metal nanoparticles (NPs), single-atom catalysts (SACs), metal compounds, and heterostructured electrocatalysts based on engineered wood hosts are presented. Subsequently, the application of these WSECs in various aspects of water splitting, including the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), overall water splitting (OWS), and hybrid water electrolysis (HWE) are explored. Finally, the prospects, challenges, and opportunities associated with the broad application of WSECs are briefly discussed. This review aims to provide a comprehensive understanding of the ongoing developments in water-splitting catalysts, along with outlining design principles for the future development of WSECs.

A Two-Dimensional Cu-Based Nanosheet Producing Formic Acid Via Glycerol Electro-Oxidation in Alkaline Water

The sluggishness of the complementary oxygen evolution reaction (OER) is reckoned as one of the major drawbacks in developing an energy-efficient green hydrogen-producing electrolyzer. An array of organic molecule oxidation reactions, operational at a relatively low potential, have been explored as a substitute for the OER. Glycerol oxidation reaction (GOR) has emerged as a leading alternative in this context because glycerol, a waste of biodiesel manufacturing, has become ubiquitous and accessible due to the significant growth in the biodiesel sector in recent decades. Additionally, the GOR generates several value-added organic compounds following oxidation that enhance the cost viability of the overall electrolysis reaction. In this study, a low-cost, room temperature operable, and energy-efficient synthetic methodology has been developed to generate unique two-dimensional CuO nanosheets (CuO NS). This CuO NS material was embedded on a carbon paper electrode, which showcased excellent glycerol electro-oxidation performance operational at a moderately low applied potential. Formic acid is the major product of this CuO NS-driven GOR (Faradaic efficiency ~80 %), as it is formed primarily via the glyceraldehyde oxidation pathway. This CuO NS material was also active for oxidizing other abundant alcohols like ethylene glycol and diethylene glycol, albeit at a relatively poor efficiency. Therefore, this robust CuO NS material has displayed the potential to be used in large-scale electrolyzers functioning with HER/GOR reactions.

Anode-Electrolyte Interfacial Acidity Regulation Enhances Electrocatalytic Performances of Alcohol Oxidations

The local acidity at the anode surface during electrolysis is apparently stronger than that in bulk electrolyte due to the deprotonation from the reactant, which leads to the deteriorated electrocatalytic performances and product distributions. Here, an anode-electrolyte interfacial acidity regulation strategy has been proposed to inhibit local acidification at the surface of anode and enhance the electrocatalytic activity and selectivity of anodic reactions. As a proof of the concept, CeO2-x Lewis acid component has been employed as a supporter to load Au nanoparticles to accelerate the diffusion and enrichment of OH- toward the anode surface, so as to accelerate the electrocatalytic alcohol oxidation reaction. As the result, Au/CeO2-x exhibits much enhanced lactic acid selectivity of 81 % and electrochemical activity of 693 mA⋅cm-2 current density in glycerol oxidation reaction compared to pure Au. Mechanism investigation reveals that the introduced Lewis acid promotes the mass transport and concentration of OH- on the anode surface, thus promoting the generation of lactic acid through the simultaneous enhancements of Faradaic and non-Faradaic processes. Attractively, the proposed strategy can be used for the electro-oxidation performance enhancements of a variety of alcohols, which thereby provides a new perspective for efficient alcohol electro-oxidations and the corresponding electrocatalyst design.

Sulfur filling activates vacancy-induced C-C bond cleavage in polyol electrooxidation

Using the electrochemical polyol oxidation reaction (POR) to produce formic acid over nickel-based oxides/hydroxides (NiO x H y ) is an attractive strategy for the electrochemical upgrading of biomass-derived polyols. The key step in the POR, i.e. the cleavage of the C-C bond, depends on an oxygen-vacancy-induced mechanism. However, a high-energy oxygen vacancy is usually ineffective for Schottky-type oxygen-vacancy-rich β-Ni(OH)2 (VSO-β-Ni(OH)2). As a result, both β-Ni(OH)2 and VSO-β-Ni(OH)2 cannot continuously catalyze oxygen-vacancy-induced C-C bond cleavage during PORs. Here, we report a strategy of oxygen-vacancy-filling with sulfur to synthesize a β-Ni(OH)2 (S-VO-β-Ni(OH)2) catalyst, whose oxygen vacancies are protected by filling with sulfur atoms. During PORs over S-VO-β-Ni(OH)2, the pre-electrooxidation-induced loss of sulfur and structural self-reconstruction cause the in-situ generation of stable Frenkel-type oxygen vacancies for activating vacancy-induced C-C bond cleavage, thus leading to excellent POR performances. This work provides an intelligent approach for guaranteeing the sustaining action of the oxygen-vacancy-induced catalytic mechanism in electrooxidation reactions.

Surface fluorination of BiVO4 for the photoelectrochemical oxidation of glycerol to formic acid

The C-C bond cleavage of biomass-derived glycerol to generate value-added C1 products remains challenging owing to its slow kinetics. We propose a surface fluorination strategy to construct dynamic dual hydrogen bonds on a semiconducting BiVO4 photoelectrode to overcome the kinetic limit of the oxidation of glycerol to produce formic acid (FA) in acidic media. Intensive spectroscopic characterizations confirm that double hydrogen bonds are formed by the interaction of the F-Bi-F sites of modified BiVO4 with water molecules, and the unique structure promotes the generation of hydroxyl radicals under light irradiation, which accelerates the kinetics of C-C bond cleavage. Theoretical investigations and infrared adsorption spectroscopy reveal that the double hydrogen bond enhances the C=O adsorption of the key intermediate product 1,3-dihydroxyacetone on the Bi-O sites to initiate the FA pathway. We fabricated a self-powered tandem device with an FA selectivity of 79% at the anode and a solar-to-H2 conversion efficiency of 5.8% at the cathode, and these results are superior to most reported results in acidic electrolytes.

Integrated electrochemical and chemical system for ampere-level production of terephthalic acid alternatives and hydrogen

2,5-Furandicarboxylic acid (FDCA), a critical polymer platform molecule that can potentially replace terephthalic acid, coupled hydrogen coproduction holds great prospects via electrolysis. However, the electrosynthesis of FDCA faces challenges in product separation from complex electrolytes and unclear electrochemical and nonelectrochemical reactions during the 5-hydroxymethylfurfural (HMF) oxidation. Herein, an electrochemical/chemical integrated system of alkaline HMF-H2O co-electrolysis is proposed, achieving distillation-free synthesis of high-purity FDCA by acidic separation/purification and hydrogen coproduction. This system achieves ampere-level current densities of 812 and 1290 mA cm-2 at potentials of 1.50 and 1.60 V, with nearly 100% FDCA yield and HMF conversion in only 6 min at 1.50 V. The electrooxidation of HMF involves a coupling of electrochemical and nonelectrochemical reactions, wherein the aldehyde group is dehydrogenated and oxidized, followed by dehydrated and oxidized of the hydroxyl group, ultimately forming FDCA. Concurrently, nonelectrochemical reactions of intermolecular electron transfer occur in HMF and aldehyde group-containing intermediates.

Improving the Electrochemical Glycerol-to-Glycerate Conversion at Pd Sites via the Interfacial Hydroxyl Immigrated from Ni Sites

The electrochemical conversion of glycerol into high-value chemicals through the selective glycerol oxidation reaction (GOR) holds importance in utilizing the surplus platform chemical component of glycerol. Nevertheless, it is still very limited in producing three-carbon chain (C3) chemicals, especially glyceric acid/glycerate, through the direct oxidation of its primary hydroxyl group. Herein, Pd microstructure electrodeposited on the Ni foam support (Pd/NF) is designed and fabricated to achieve a highly efficient GOR, exhibiting a superior current density of ca. 120 mA cm-2 at 0.8 V vs. reversible hydrogen electrode (RHE), and high selectivity of glycerate at ca. 70%. The Faradaic efficiency of C3 chemicals from GOR can still be maintained at ca. 80% after 20 continuous electrolysis runs, and the conversion rate of glycerol can reach 95% after 10-h electrolysis. It is also clarified that the dual-component interfaces constructed by the adjacent Pd and Ni sites are responsible for this highly efficient GOR. Specifically, Ni sites can effectively strengthen the generative capacity of the active adsorbed hydroxyl (OHad) species, which can steadily immigrate to the Pd sites, so that the surface adsorbed glycerol species are quickly oxidized into C3 chemicals, rather than breaking the C-C bond of glycerol; thus, neither form the C2/C1 species. This study may yield fresh perspectives on the electrocatalytic conversion of glycerol into high-value C3 chemicals, such as glyceric acid/glycerate.

NiCo-Phosphide Bifunctional Electrocatalyst Realizes Electrolysis of Sugar Solution to Formic Acid and Hydrogen

As a promising liquid hydrogen carrier, formic acid is essential for hydrogen energy. Glucose, as the most widely distributed monosaccharide in nature, is valuable for co-electrolysis with water to produce formic acid and hydrogen, though achieving high formate yield and current density remains challenging. Herein, a nanostructured NiCoP on a 3D Ni foam catalyst enables efficient electrooxidation of glucose to formate, achieving an 85% yield and 200 mA current density at 1.47 V vs RHE. The catalyst forms a NiCoOOH/NiCoP/Ni foam sandwich structure via anodic oxidative reconstruction, with NiCoOOH as the active site and NiCoP facilitating electron conduction. Additionally, NiCoP/Ni foam serves as both an anode and cathode for the production of formate and hydrogen from wood-extracted sugar solutions. At 2.1 V, it reaches a 300 mA current density, converting mixed sugars to formate with a 74% yield and producing hydrogen at 104 mL cm2 h-1 with near 100% Faradaic efficiency.

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.

Unlocking Efficient Hydrogen Production: Nucleophilic Oxidation Reactions Coupled with Water Splitting

Electrocatalytic water splitting driven by sustainable energy is a clean and promising water-chemical fuel conversion technology for the production of high-purity green hydrogen. However, the sluggish kinetics of anodic oxygen evolution reaction (OER) pose challenges for large-scale hydrogen production, limiting its efficiency and safety. Recently, the anodic OER has been replaced by a nucleophilic oxidation reaction (NOR) with biomass as the substrate and coupled with a hydrogen evolution reaction (HER), which has attracted great interest. Anode NOR offers faster kinetics, generates high-value products, and reduces energy consumption. By coupling NOR with hydrogen evolution reaction, hydrogen production efficiency can be enhanced while yielding high-value oxidation products or degrading pollutants. Therefore, NOR-coupled HER hydrogen production is another new green electrolytic hydrogen production strategy after electrolytic water hydrogen production, which is of great significance for realizing sustainable energy development and global decarbonization. This review explores the potential of nucleophilic oxidation reactions as an alternative to OER and delves into NOR mechanisms, guiding future research in NOR-coupled hydrogen production. It assesses different NOR-coupled production methods, analyzing reaction pathways and catalyst effects. Furthermore, it evaluates the role of electrolyzers in industrialized NOR-coupled hydrogen production and discusses future prospects and challenges. This comprehensive review aims to advance efficient and economical large-scale hydrogen production.

Anchored Cobalt Nanoparticles on Layered Perovskites for Rapid Peroxymonosulfate Activation in Antibiotic Degradation

In the Fenton-like reaction, revealing the dynamic evolution of the active sites is crucial to achieve the activity improvement and stability of the catalyst. This study reports a perovskite oxide in which atomic (Co0) in situ embedded exsolution occurs during the high-temperature phase transition. This unique anchoring strategy significantly improves the Co3+/Co2+ cycling efficiency at the interface and inhibits metal leaching during peroxymonosulfate (PMS) activation. The Co@L-PBMC catalyst exhibits superior PMS activation ability and could achieve 99% degradation of tetracycline within 5 min. The combination of experimental characterization and density functional theory (DFT) calculations elucidates that the electron-deficient oxygen vacancy accepts an electron from the Co 3d-orbital, resulting in a significant electron delocalization of the Co site, thereby facilitating the adsorption of the *HSO5/*OH intermediate onto the "metal-VO bridge" structure. This work provides insights into the PMS activation mechanism at the atomic level, which will guide the rational design of next-generation catalysts for environmental remediation.

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.

Strengthening the Synergy between Oxygen Vacancies in Electrocatalysts for Efficient Glycerol Electrooxidation

Defect-engineered bimetallic oxides exhibit high potential for the electrolysis of small organic molecules. However, the ambiguity in the relationship between the defect density and electrocatalytic performance makes it challenging to control the final products of multi-step multi-electron reactions in such electrocatalytic systems. In this study, controllable kinetics reduction is used to maximize the oxygen vacancy density of a Cu─Co oxide nanosheet (CuCo2O4 NS), which is used to catalyze the glycerol electrooxidation reaction (GOR). The CuCo2O4-x NS with the highest oxygen-vacancy density (CuCo2O4-x-2) oxidizes C3 molecules to C1 molecules with selectivity of almost 100% and a Faradaic efficiency of ≈99%, showing the best oxidation performance among all the modified catalysts. Systems with multiple oxygen vacancies in close proximity to each other synergistically facilitate the cleavage of C─C bonds. Density functional theory calculations confirm the ability of closely spaced oxygen vacancies to facilitate charge transfer between the catalyst and several key glycolic-acid (GCA) intermediates of the GOR process, thereby facilitating the decomposition of C2 intermediates to C1 molecules. This study reveals qualitatively in tuning the density of oxygen vacancies for altering the reaction pathway of GOR by the synergistic effects of spatial proximity of high-density oxygen vacancies.

Redox regulation of Ni hydroxides with controllable phase composition towards biomass-derived polyol electro-refinery

Electrocatalytic refinery from biomass-derived glycerol (GLY) to formic acid (FA), one of the most promising candidates for green H2 carriers, has driven widespread attention for its sustainability. Herein, we fabricated a series of monolithic Ni hydroxide-based electrocatalysts by a facile and in situ electrochemical method through the manipulation of local pH near the electrode. The as-synthesized Ni(OH)2@NF-1.0 affords a low working potential of 1.36 VRHE to achieve 100% GLY conversion, 98.5% FA yield, 96.1% faradaic efficiency and ∼0.13 A cm-2 of current density. Its high efficiency on a wide range of polyol substrates further underscores the promise of sustainable electro-refinery. Through a combinatory analysis via H2 temperature-programmed reduction, cyclic voltammetry and in situ Raman spectroscopy, the precise regulation of synthetic potential was discovered to be highly essential to controlling the content, phase composition and redox properties of Ni hydroxides, which significantly determine the catalytic performance. Additionally, the 'adsorption-activation' mode of ortho-di-hydroxyl groups during the C-C bond cleavage of polyols was proposed based on a series of probe reactions. This work illuminates an advanced path for designing non-noble-metal-based catalysts to facilitate electrochemical biomass valorization.

Oxygen Vacancy-Laden Confinement Impact on Degradation of Metal Complexes

Oxygen vacancies (Vo) have been recognized as the superior active site for PS-mediated environmental remediation; however, the formation and activation of Vo associated with the effects of chemical and spatial environments remain ambiguous. Herein, attributing to the low defect-formation energy of Vo in the presence of sulfonate groups, an in situ nucleating Vo-laden CuO nanosheet was deliberately fabricated inside the phase of a sulfonated mesoporous polystyrene substrate (Vo-CuO@SPM). The as-prepared nanocomposite demonstrated an excellent treatment efficiency toward metal complexes [Cu-EDTA as a case] with ignorable Cu(II) leaching, and it can be repeatedly employed for 25 recycles (not limited). Mechanistically, the electron transfer and the mass transport for PDS nonradical activation were proved to be substantially enhanced by the delocalized electrons and with the assistance of the microchannel environment. This work not only establishes insight into the formation of oxygen vacancies but also reveals the PS activation mechanism in the spatially confined sites.

Electrocatalytic functional group conversion-based carbon resource upgrading

The conversions of carbon resources, such as alcohols, aldehydes/ketones, and ethers, have been being one of the hottest topics most recently for the goal of carbon neutralization. The emerging electrocatalytic upgrading has been regarded as a promising strategy aiming to convert carbon resources into value-added chemicals. Although exciting progress has been made and reviewed recently in this area by mostly focusing on the explorations of valuable anodic oxidation or cathodic reduction reactions individually, however, the reaction rules of these reactions are still missing, and how to purposely find or rationally design novel but efficient reactions in batches is still challenging. The properties and transformations of key functional groups in substrate molecules play critically important roles in carbon resources conversion reactions, which have been paid more attention to and may offer hidden keys to achieve the above goal. In this review, the properties of functional groups are addressed and discussed in detail, and the reported electrocatalytic upgrading reactions are summarized in four categories based on the types of functional groups of carbon resources. Possible reaction pathways closely related to functional groups will be summarized from the aspects of activation, cleavage and formation of chemical bonds. The current challenges and future opportunities of electrocatalytic upgrading of carbon resources are discussed at the end of this review.

Ultrathin NiV Layered Double Hydroxide for Methanol Electrooxidation: Understanding the Proton Detachment Kinetics and Methanol Dehydrogenation Oxidation

Electrochemical methanol oxidation reaction (MOR) is regarded as a promising pathway to obtain value-added chemicals and drive cathodic H2 production, while the rational design of catalyst and in-depth understanding of the structure-activity relationship remains challenging. Herein, the ultrathin NiV-LDH (u-NiV-LDH) with abundant defects is successfully synthesized, and the defect-enriched structure is finely determined by X-ray adsorption fine structure etc. When applied for MOR, the as-prepared u-NiV-LDH presents a low potential of 1.41 V versus RHE at 100 mA cm-2, which is much lower than that of bulk NiV-LDH (1.75 V vs RHE) at the same current density. The yield of H2 and formate is 98.2% and 88.1% as its initial over five cycles and the ultrathin structure of u-NiV-LDH can be well maintained. Various operando experiments and theoretical calculations prove that the few-layer stacking structure makes u-NiV-LDH free from the interlayer hydrogen diffusion process and the hydrogen can be directly detached from LDH laminate. Moreover, the abundant surface defects upshift the d-band center of u-NiV-LDH and endow a higher local methanol concentration, resulting in an accelerated dehydrogenation kinetics on u-NiV-LDH. The synergy of the proton detachment from the laminate and the methanol dehydrogenation oxidation contributes to the excellent MOR performance of u-NiV-LDH.

Unassisted Photoelectrochemical H2O2 Production with In Situ Glycerol Valorization Using α-Fe2O3

Photoelectrochemical (PEC) H2O2 production via two-electron O2 reduction is promising for H2O2 production without emitting CO2. For PEC H2O2 production, α-Fe2O3 is an ideal semiconductor owing to its earth abundance, superior stability in water, and an appropriate band gap for efficient solar light utilization. Moreover, its conduction band is suitable for O2 reduction to produce H2O2. However, a significant overpotential for water oxidation is required due to the poor surface properties of α-Fe2O3. Thus, unassisted solar H2O2 production is not yet possible. Herein, we demonstrate unassisted PEC H2O2 production using α-Fe2O3 for the first time by applying glycerol oxidation, which requires less bias compared with water oxidation. We obtain maximum Faradaic efficiencies of 96.89 ± 0.6% and 100% for glycerol oxidation and H2O2 production, respectively, with high stability for 25 h. Our results indicate that unassisted and stable PEC H2O2 production is feasible with in situ glycerol valorization using the α-Fe2O3 photoanode.

Regulation of the d-band center of metal-organic frameworks for energy-saving hydrogen generation coupled with selective glycerol oxidation

The hybrid electrolyzer coupled glycerol oxidation (GOR) with hydrogen evolution reaction (HER) is fascinating to simultaneously generate H2 and high value-added chemicals with low energy input, yet facing a challenge. Herein, Cu-based metal-organic frameworks (Cu-MOFs) are reported as model catalysts for both HER and GOR through doping of atomically dispersed precious and nonprecious metals. Remarkably, the HER activity of Ru-doped Cu-MOF outperformed a Pt/C catalyst, with its Faradaic efficiency for formate formation at 90% at a low potential of 1.40 V. Furthermore, the hybrid electrolyzer only needed 1.36 V to achieve 10 mA cm-2, 340 mV lower than that for splitting pure water. Theoretical calculations demonstrated that electronic interactions between the host and guest (doped) metals shifted downward the d-band centers (εd) of MOFs. This consequently lowered water adsorption and dissociation energy barriers and optimized hydrogen adsorption energy, leading to significantly enhanced HER activities. Meanwhile, the downshift of εd centers reduced energy barriers for rate-limiting step and the formation energy of OH*, synergistically enhancing the activity of MOFs for GOR. These findings offered an effective means for simultaneous productions of hydrogen fuel and high value-added chemicals using one hybrid electrolyzer with low energy input.

Synergistic Lewis and Brønsted Acid Sites Promote OH* Formation and Enhance Formate Selectivity: Towards High-efficiency Glycerol Valorization

As a sustainable valorization route, electrochemical glycerol oxidation reaction (GOR) involves in formation of key OH* and selective adsorption/cleavage of C-C(O) intermediates with multi-step electron transfer, thus suffering from high potential and poor formate selectivity for most non-noble-metal-based electrocatalysts. So, it remains challenging to understand the structure-property relationship as well as construct synergistic sites to realize high-activity and high-selectivity GOR. Herein, we successfully achieve dual-high performance with low potentials and superior formate selectivity for GOR by forming synergistic Lewis and Brønsted acid sites in Ni-alloyed Co-based spinel. The optimized NiCo oxide solid-acid electrocatalyst exhibits low reaction potential (1.219 V@10 mA/cm2) and high formate selectivity (94.0 %) toward GOR. In situ electrochemical impedance spectroscopy and pH-dependence measurements show that the Lewis acid centers could accelerate OH* production, while the Brønsted acid centers are proved to facilitate high-selectivity formation of formate. Theoretical calculations reveal that NiCo alloyed oxide shows appropriate d-band center, thus balancing adsorption/desorption of C-O intermediates. This study provides new insights into rationally designing solid-acid electrocatalysts for biomass electro-upcycling.