Electro-Oxidation of Glycerol to High-Value-Added C1-C3 Products by Iron-Substituted Spinel Zinc Cobalt Oxides

NH4Cl-Assisted Electrosynthesis of P-Doped Co(OH)2 Nanosheet on Cu2S Hollow Nanotube Arrays for Glycerol Electrooxidation

The glycerol oxidation reaction (GOR) for producing high-value-added organic compounds is of great research interest due to its potential in alleviating the energy crisis. Herein, a facile NH4Cl-assisted electrodeposition strategy is reported to fabricate 3D nano-forest array-like hollow nanostructures. The hierarchical heterojunction by combining phosphorus doping Co(OH)2 nanosheets with Cu2S nanotube arrays (P-Co(OH)2@Cu2S NTs/CF) is developed to realize the optimization on GOR. The optimized P-Co(OH)2@Cu2S NTs/CF catalyst exhibits an exceptional activity with a formate Faradaic efficiency (FE) of 97.40% at a potential of 1.30 V (vs RHE). The experimental results indicate that this unique hollow nano-forest structure, grown on a conductive support, can expose more active sites and facilitate electron transfer, thereby demonstrating excellent GOR performance. This work provides new opportunities for the design of electrocatalysts of high-activity and low-cost hollow heterostructure electrocatalysts for glycerol electrooxidation.

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.

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.

Electrocatalytic Glycerol Upgrading into Glyceric Acid on Ni3Sn Intermetallic Compound

Electrochemical glycerol oxidation features an attractive approach of converting bulk chemicals into high-value products such as glyceric acid. Nonetheless, to date, the major product selectivity has mostly been limited as low-value C1 products such as formate, CO, and CO2, due to the fast cleavage of carbon-carbon (C-C) bonds during electro-oxidation. Herein, the study develops an atomically ordered Ni3Sn intermetallic compound catalyst, in which Sn atoms with low carbon-binding and high oxygen-binding capability allow to tune the adsorption of glycerol oxidation intermediates from multi-valent carbon binding to mono-valent carbon binding, as well as enhance *OH binding and subsequent nucleophilic attack. The Ni3Sn electrocatalyst exhibits one of the highest glycerol-to-glyceric acid performances, including a high glycerol conversion rate (1199 µmol h-1) and glyceric acid selectivity (62 ± 3%), a long electrochemical stability of > 150 h, and the capability of direct conversion of crude glycerol (85% purity) into glyceric acid. The work features the rational design of highly ordered catalytic sites for tailoring intermediate binding and reaction pathways, thereby facilitating the efficient production of high-value chemical products.

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.

Electrocatalytic Mechanism of Defect in Spinels for Water and Organics Oxidation

Spinels display promising electrocatalytic ability for oxygen evolution reaction (OER) and organics oxidation reaction because of flexible structure, tunable component, and multifold valence. Unfortunately, limited exposure of active sites, poor electronic conductivity, and low intrinsic ability make the electrocatalytic performance of spinels unsatisfactory. Defect engineering is an effective method to enhance the intrinsic ability of electrocatalysts. Herein, the recent advances in defect spinels for OER and organics electrooxidation are reviewed. The defect types that exist in spinels are first introduced. Then the catalytic mechanism and dynamic evolution of defect spinels during the electrochemical process are summarized in detail. Finally, the challenges of defect spinel electrocatalysts are brought up. This review aims to deepen the understanding about the role and evolution of defects in spinel for electrochemical water/organics oxidation and provide a significant reference for the design of efficient defect spinel electrocatalysts.

Photoassisted Strategy to Promote Glycerol Electrooxidation to Lactic Acid Coupled with Hydrogen Production

Electrocatalytic oxidation of glycerol (GLY; from a biodiesel byproduct) to lactic acid (LA; the key monomers for polylactic acid; PLA) is considered a sustainable approach for biomass waste upcycling and is coupled with cathodic hydrogen (H₂) production. However, current research still suffer from issues of low current density and low LA selectivity. Herein, we reported a photoassisted electrocatalytic strategy to achieve the selective oxidation of GLY to LA over a gold nanowire (Au NW) catalyst, attaining a high current density of 387 mA cm^-2 at 0.95 V vs RHE, together with a high LA selectivity of 80%, outperforming most of the reported works in the literature. We reveal that the light-assistance strategy plays a dual role, which can both accelerate the reaction rate through the photothermal effect and also promote the adsorption of the middle hydroxyl of GLY over Au NWs to realize the selective oxidation of GLY to LA. As a proof-of-concept, we realized the direct conversion of crude GLY that was extracted from cooking oil to attain LA and coupled it with H₂ production using the developed photoassisted electrooxidation process, revealing the potential of this strategy in practical applications.

Glycerol Electro-Oxidation in Alkaline Medium with Pt-Fe/C Electrocatalysts Synthesized by the Polyol Method: Increased Selectivity and Activity Provided by Less Expensive Catalysts

We have investigated platinum catalysts containing iron as a modifier to obtain catalysts with superior electrocatalytic activity toward glycerol electro-oxidation in an alkaline medium. The electrocatalysts, supported on carbon Vulcan, were synthesized by the polyol method. The physicochemical characterization data showed that the metals were well distributed on the carbon support and had small particle size (2 nm). The Pt:Fe metal ratio differed from the nominal composition, indicating that reducing iron with platinum was difficult, even though some parameters of the synthesis process were changed. Electrochemical analyses revealed that PtFe/C was more active and stable than commercial Pt/C was, and analysis of the electrolysis by-products showed that iron addition to Pt/C boosted the glycerol conversion and selectivity for glyceric acid formation.

Electrocatalytic Upcycling of Biomass and Plastic Wastes to Biodegradable Polymer Monomers and Hydrogen Fuel at High Current Densities

Transformation of biomass and plastic wastes to value-added chemicals and fuels is considered an upcycling process that is beneficial to resource utilization. Electrocatalysis offers a sustainable approach; however, it remains a huge challenge to increase the current density and deliver market-demanded chemicals with high selectivity. Herein, we demonstrate an electrocatalytic strategy for upcycling glycerol (from biodiesel byproduct) to lactic acid and ethylene glycol (from polyethylene terephthalate waste) to glycolic acid, with both products being as valuable monomers for biodegradable polymer production. By using a nickel hydroxide-supported gold electrocatalyst (Au/Ni(OH)₂), we achieve high selectivities of lactic acid and glycolic acid (77 and 91%, respectively) with high current densities at moderate potentials (317.7 mA/cm² at 0.95 V vs RHE and 326.2 mA/cm² at 1.15 V vs RHE, respectively). We reveal that glycerol and ethylene glycol can be enriched at the Au/Ni(OH)₂ interface through their adjacent hydroxyl groups, substantially increasing local concentrations and thus high current densities. As a proof of concept, we employed a membrane-free flow electrolyzer for upcycling triglyceride and PET bottles, attaining 11.2 g of lactic acid coupled with 9.3 L of H₂ and 13.7 g of glycolic acid coupled with 9.4 L of H₂, respectively, revealing the potential of coproduction of valuable chemicals and H₂ fuel from wastes in a sustainable fashion.