Cellulose Valorization via Electrochemical Oxidation: Efficient Formate Generation for Green Energy Storage

Abstract

Achieving efficient electrocatalytic oxidation of cellulose-derived biomass is a pivotal strategy for advancing bioenergy utilization and achieving carbon neutrality. This study addresses the challenges of low conversion efficiency caused by cellulose’s high crystallinity and excessive energy consumption in conventional processes by proposing a novel integrated system combining solid heteropoly acid catalytic pretreatment and electrocatalytic oxidation. By preparing the (C₁₆TA)H₂PW solid acid catalyst, we successfully achieved hydrolysis of microcrystalline cellulose under 180 °C for 60 min, attaining a glucose yield of 40.1%. Furthermore, a non-noble metal electrocatalyst system based on foam copper (CuF) was developed, with the Co₃O₄/CuF electrode material demonstrating a Faradaic efficiency of 85.3% for formate production at 1.66 V (vs. RHE) in 1 mol L⁻¹ KOH electrolyte containing the pretreated cellulose mixture, accompanied by a partial current density of 153.2 mA cm⁻². The mechanism study indicates that hydroxyl radical-mediated C-C bond selective cleavage dominates the formate generation. This integrated system overcomes the limitations of poor catalyst stability and low product selectivity in biomass conversion, offering a sustainable strategy for green manufacturing of high-value chemicals from cellulose.

1. Introduction

The global surge in energy demand and intensifying climate crisis underscores the unsustainable nature of traditional fossil energy systems. According to the International Energy Agency (IEA), fossil fuels accounted for 79.6% of global primary energy consumption in 2022, directly contributing to 36.8 Gt of CO₂ emissions and driving atmospheric CO₂ concentrations to 50% above pre-industrial levels [1]. Concurrently, fossil energy reserves face depletion risks, with current technologies projecting only 54 years of sustainable supply for oil and natural gas [2]. Against this backdrop, the Paris Agreement’s carbon neutrality goals necessitate accelerated transitions toward low-carbon energy systems. Biomass energy, characterized by renewability and carbon cycling, has emerged as a pivotal alternative to fossil resources for achieving emission reductions [3].

Cellulose, the most abundant polysaccharide in nature with an annual production exceeding 1.3 × 10¹² tons, consists of glucose units linked by β-1,4-glycosidic bonds. Its high carbon content and renewable nature position it as an ideal feedstock for synthesizing high-value chemicals [4]. Formic acid (HCOOH), a critical platform chemical for hydrogen storage, pharmaceutical synthesis, and clean fuels, currently relies on fossil-derived methanol carbonylation processes that suffer from high energy consumption (reaction temperature > 160 °C) and significant carbon emissions (2.8 tons CO₂ per ton formic acid) [5]. Thus, developing green conversion technologies for cellulose-derived formic acid synthesis is critical for advancing energy transition and achieving carbon neutrality.

The research on the conversion of sugar-based lignocellulosic waste into formic acid has also been systematically studied, primarily focusing on biomass conversion under oxidative conditions using oxygen or hydrogen peroxide as oxidants. Tatsumi Ishihara’s research group at Kyushu University [6] employed calcium oxide as a solid catalyst in a 1.4% hydrogen peroxide aqueous solution at 70 °C, achieving formic acid yields of 50% and 66% within 30 min using glucose and xylose as substrates, respectively. Additionally, the addition of acids enables the integration of cellulose hydrolysis and glucose oxidation into a single reaction system, thereby simplifying process complexity. Peter Wasserscheid’s team at Friedrich-Alexander University Erlangen-Nuremberg developed a heteropoly acid oxidation system using Keggin-type H₅PV₂Mo₁₀O₄₀ heteropoly acid, converting lignocellulose into formic acid under 90 °C and 30-bar oxygen atmosphere. Furthermore, their group developed a method employing long-chain primary alcohols as solvents to extract formic acid from aqueous phases, shifting reaction equilibrium. Using homogeneous H₈PV₅Mo₇O₄₀ as the catalyst under 90 °C and 20-bar oxygen, they achieved an ~85% formic acid yield [7,8,9]. Jakob Albert’s research group at Friedrich-Alexander University [10] further advanced previous studies by developing a continuous production method for formic acid from biomass feedstock, representing a significant step toward preliminary industrialization.

In China, research on the conversion of sugar-based lignocellulosic waste has also been highly active. Li Yang’s research group at Xi’an Jiaotong University [11] abandoned oxygen and hydrogen peroxide, instead directly using dimethyl sulfoxide (DMSO) (1 v%) as an oxidant to promote selective hydrolysis and oxidation of lignocellulosic biomass into formic acid. Deng Xisheng’s research group at National Cheng Kung University (Taiwan) [12] utilized functionalized graphene quantum dots and platinum co-catalysts to achieve simultaneous hydrogen and formic acid production via catalytic reforming of cellulose under alkaline conditions with 1-sun light irradiation. Shen Feng’s research group at the Ministry of Agriculture and Rural Affairs’ Environmental Protection Scientific Monitoring Institute [13] aimed to develop vanadium-free reaction systems, synthesizing MnO_x catalysts under hydrothermal conditions. Using this catalyst at 160 °C with glucose as a substrate, they achieved an 81.1% formic acid yield, successfully replacing vanadium-based catalysts. Our research group has been actively engaged in hydrothermal glucose oxidation studies. Systematic investigations into hydrothermal formic acid production from glucose revealed that alkali addition serves dual roles: promoting active oxidant generation and suppressing organic acid decomposition (e.g., formic acid to CO₂). Under 2.5 M NaOH conditions at 150 °C for 20 min, we achieved an 85% formic acid yield [14]. To optimize reaction conditions and reduce temperature, we developed a high-efficiency glucose-to-formic acid protocol: homogeneous alkalis (LiOH, NaOH, or KOH) effectively activate glucose, while hydrogen peroxide facilitates oxidation. This approach achieved near-stoichiometric glucose conversion with 91.3% formic acid yield within 8 h under ambient temperature [15]. Although conventional thermochemical methods demonstrate considerable efficiency in converting carbohydrates to valuable chemicals, they require large amounts of expensive oxidants (H₂O₂ or compressed O₂) and/or high temperatures, imposing significant technical and cost constraints that limit practical applications. This highlights the urgent need for greener, more sustainable pathways and technological strategies.

Electrocatalytic oxidation of biomass feedstocks offers the advantage of co-producing high-value chemicals and hydrogen with low energy consumption and reduced carbon dioxide emissions. Compared to traditional thermochemical methods, this process eliminates the need for high temperatures and additional oxidants/reductants while generating fewer byproducts and pollutants, thereby making it a more sustainable and eco-friendly alternative. According to literature reports, researchers have explored various biomass oxidation reactions to replace the oxygen evolution reaction (OER) for improved energy efficiency and economic viability, such as methanol oxidation [16], ethanol oxidation [17], and glycerol oxidation. Currently, significant attention has been directed toward electrocatalytic resource conversion of sugar-based lignocellulosic waste. Cellulose, as one of the most abundant biomass resources in nature, along with its bio-derived compounds, has been extensively investigated for electrocatalytic valorization. This approach not only aligns with circular economy principles but also provides a promising pathway for sustainable chemical production.

Traditional cellulose conversion technologies primarily rely on high-temperature and high-pressure thermochemical catalysis, requiring strong acid/base media or precious metal catalysts, which suffer from low atomic utilization efficiency, poor reaction selectivity, and severe equipment corrosion. In recent years, electrocatalytic oxidation technology has gained significant attention for its ability to precisely regulate reaction pathways under mild conditions. By generating reactive oxygen species (ROS) to selectively cleave C-C bonds, this approach substantially enhances reaction efficiency and product selectivity. Current research focuses on cellulose pretreatment and catalyst design. Solid heteropoly acids, combining Brønsted acid sites and redox activity, enable efficient depolymerization of cellulose into soluble monosaccharides. In the meantime, transition metal oxides, leveraging multivalent states and high electrochemical activity, exhibit excellent performance in electrocatalytic oxidation [18]. However, the synergistic optimization mechanisms between cellulose pretreatment and electrocatalytic oxidation remain unclear, and the structure–performance relationships of catalyst active sites and reaction pathways require further elucidation.

To achieve high-value conversion of cellulose, pretreatment technologies must first disrupt its inherent crystalline structure and hydrogen-bonding network. This process increases the accessibility of cellulose in subsequent reactions by exposing more reactive sites, thereby effectively enhancing conversion efficiency. Such structural modification is critical for overcoming the natural recalcitrance of cellulose and enabling efficient biochemical or thermochemical valorization pathways. While conventional hydrolysis methods have achieved industrial applications, their inherent limitations continue to hinder technological advancement: acid hydrolysis faces challenges in catalyst recovery and byproduct inhibition; enzymatic hydrolysis, though environmentally friendly, suffers from high costs of cellulase enzymes and limited substrate adaptability; supercritical hydrolysis requires extreme conditions, leading to high energy consumption. Against this backdrop, heteropoly acid (HPA) systems—which combine the catalytic efficiency of homogeneous systems with the easy separation of heterogeneous catalysts—have emerged as a research focus due to their unique Brønsted/Lewis dual acidity, redox synergy, and thermal reversibility, offering a promising solution to overcome existing technical bottlenecks.

Extensive literature has reported cellulose solid-acid catalytic hydrolysis methods for monosaccharide production. For instance, the Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, innovatively developed one-dimensional nano-layered solid acids, achieving 85% glucose selectivity in cellulose hydrolysis at 150 °C with minimal activity loss after four reuse cycles [19]. Academician Chen Kefu’s team at South China University of Technology developed a lignin-based solid acid (LC-SO₃H), derived from corn stover enzyme residue and functionalized with -SO₃H and -COOH groups via sulfonation. This catalyst enabled mild-condition cellulose hydrolysis to produce glucose and 5-hydroxymethylfurfural (HMF), with the catalyst demonstrating five reuse cycles and 30% higher lignin utilization efficiency [20]. Xuejun Pan’s group at the University of Wisconsin–Madison constructed a sulfonated carbon-based solid acid (KOH-activated activated carbon) with a surface sulfonic acid group density of 2.1 mmol/g, achieving 78% glucose yield in cellulose hydrolysis at 180 °C and retaining 92% activity after five cycles [21]. Prof. Fu’s group at the Shanghai Institute of Ceramics, Chinese Academy of Sciences, made breakthroughs in biomass catalysis by integrating magnetic Fe₂O₃ nanoparticles with sulfonated mesoporous silica SBA-15, creating a magnetic solid acid catalyst (Fe₂O₃-SBA-SO₃H) with ordered mesoporous structures (pore size ~2–3 nm). This design synergizes Fe₂O₃’s inherent magnetism for easy recovery, SBA-15’s high specific surface area (>800 m²/g), and -SO₃H groups providing strong acidic sites (Hammett acidity H₀ ≈ −12.5). The multi-component synergy overcomes the limitations of single-function catalysts, offering hierarchical support for cellulose hydrolysis [22].

Our study proposes a “solid heteropoly acid pretreatment–electrocatalytic oxidation” coupled process to achieve efficient and selective conversion of cellulose into formic acid. We first develop a phosphotungstic acid/surfactant composite solid acid catalyst to disrupt cellulose crystallinity and enable mild preparation of electrolyzable monosaccharides by overcoming the limitations of cellulose’s recalcitrant structure. By designing non-precious metal oxide electrocatalysts (e.g., Co₃O₄/CuF), a Faradaic efficiency of 85.3% for formate production at 1.66 V (vs. RHE) in 1 mol L⁻¹ KOH electrolyte was achieved. Temporal-resolved experiments and spectroscopic characterizations unravel the active site-dependent selectivity mechanisms for formic acid generation during electrocatalytic oxidation, including the C-C bond cleavage kinetics and formic acid production pathways in cellulose electrooxidation. This integrated approach not only provides a novel technological solution for high-value utilization of biomass resources but also establishes a theoretical framework for constructing carbon-neutral systems driven by electrocatalysis, thereby advancing global efforts toward carbon neutrality.

2. Experimental

2.1. Analytical Methods

Liquid samples were analyzed using an Agilent 1260 HPLC system (Agilent Technologies, Santa Clara, CA, USA), a Bruker Avance III 400 MHz 1H NMR spectrometer (Bruker Biospin, Billerica, MA, USA), and an in situ FTIR setup (Bruker Tensor 27, Bruker Optics, Ettlingen, Germany) to monitor dynamic reaction processes. Solid samples were characterized via JEOL JSM-7600F SEM (JEOL Ltd., Tokyo, Japan), Bruker D8 Advance XRD (Bruker AXS, Karlsruhe, Germany), FEI Tecnai G2 F20 TEM (FEI Company, Hillsboro, OR, USA), and Thermo Fisher Scientific Nicolet iS50 FTIR (Thermo Fisher Scientific, Wilmington, DE, USA). Detailed analytical protocols, including instrument calibration and validation procedures, are provided in the Supplementary Information (SI).

2.2. Synthesis of the Polyoxometalate (C₁₆TA)H₂PW and Cellulose Degradation Experiments

At room temperature, two solutions are prepared: 10 mL of 40 mM H₃PW₁₂O₄₀ aqueous solution and 10 mL of 40 mM cetyltrimethylammonium bromide (CTAB) aqueous solution. The two solutions are rapidly mixed to form a white precipitate. The mixture is centrifuged to collect the precipitate, followed by freeze-drying to obtain a white solid powder.

0.05 g of pretreated microcrystalline cellulose and 0.1 g catalyst are accurately weighed and transferred into a reaction vessel, followed by addition of 5 mL deionized water. The reaction is initiated under magnetic stirring, and the system is heated to the target temperature. Timing is started upon reaching the set temperature. After a predetermined reaction period, the reaction is terminated, and the mixture is promptly transferred to centrifuge tubes. Centrifugation separates the mixture into an upper layer (containing glucose) and a lower layer (unreacted cellulose and catalyst). Due to variations in catalyst activity and experimental conditions, other byproducts may be present in the reaction solution in varying proportions.

2.3. Synthesis of Catalytic Electrode Sheets

Prior to electrode preparation, foam copper (CuF, pore size range: 0.2–0.6 mm, mass density: 280–420 g/m², thickness: 1.5 mm) was cut into 2 × 2 cm square sheets. The sheets were sequentially ultrasonically cleaned with 3 M HCl solution, anhydrous ethanol, and ultrapure water for 10 min each to remove surface impurities and oxides. The cleaned foam copper sheets were then dried in a vacuum oven at 60 °C for 6 h prior to further use.

0.4 mmol of a metal nitrate salt (e.g., Co(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, or Ni(NO₃)₂·6H₂O) and 300 mg urea were dissolved in 14 mL of ultrapure water. The solution was stirred and ultrasonically treated at room temperature to form a homogeneous mixture. This solution, along with a pretreated CuF electrode sheet, was transferred into a 25 mL PTFE-lined stainless-steel autoclave (SANYO Electric Co., Ltd., Osaka, Japan), ensuring the CuF was vertically positioned inside. The autoclave was sealed and heated at 120 °C for 16 h in an oven. After cooling naturally to room temperature, the electrode sheet was removed. It was repeatedly ultrasonically rinsed with anhydrous ethanol and ultrapure water until no powder residue remained on the surface. Finally, the electrode was dried in a vacuum oven at 60 °C for 6 h to obtain the metal hydroxide electrocatalyst loaded on CuF.

The prepared electrode catalytic material was placed in a crucible and transferred into a muffle furnace under an air atmosphere. The sample was heated at a rate of 2 °C/min to 300 °C and maintained at this temperature for 2 h to obtain the metal oxide electrocatalyst supported on foam copper.

2.4. Electrolysis Experiment

In this study, a two-compartment H-type electrolytic cell was employed for electrochemical testing under ambient pressure. The cell comprises electrodes, a proton exchange membrane (Nafion 117 (DuPont China Co., Ltd., Shanghai, China)), and a cell body. Electrochemical analyses were conducted using a CHI660E electrochemical workstation (Shanghai Chenhua Co., Ltd., Shanghai, China). A three-electrode system—comprising a working electrode, reference electrode, and counter electrode—was utilized. Voltage measurements revealed the potential difference between the working and reference electrodes, while the counter electrode balanced charge distribution within the system. The working and reference electrodes were positioned on one side of the membrane, with the counter electrode on the opposite side, separated by the proton exchange membrane.

Prior to the experiments, the Nafion proton exchange membrane underwent pretreatment of sequential soaking at 80 °C for 1 h each in 5 wt% H₂O₂ solution, ultrapure water, and 5 wt% H₂SO₄ solution, followed by storage in ultrapure water to ensure readiness for use.

3. Results and Discussion

3.1. Characteristics of Solid Heteropoly Acids

The as-prepared Keggin-type polyoxometalates (C₁₆TA)H₂PW were applied in infrared spectroscopic analysis (Figure S1). The stretching vibration peaks of P-Oa, W-Od, W-Ob, and W-Oc in 12-tungstophosphoric acid were identified at 1080, 980, 890–850, and 800–760 cm⁻¹, respectively. The infrared spectrum of the synthesized catalyst exhibited distinct peaks at 1095, 975, 880, and 750 cm⁻¹, which align closely with reference data and previously reported values [23], confirming the preservation of the Keggin-type polyoxometalate anionic framework in the catalyst. Furthermore, the absorption bands at 2900 and 2800 cm⁻¹ in the infrared spectra can be attributed to C-H stretching vibrations from the surfactant CTAB.

X-ray diffraction (XRD) analysis of the synthesized solid heteropoly acid catalyst revealed characteristic peaks at 2θ = 14.6°, 20.6°, 21.9°, 23.1°, 36.1°, and 39.9°, corresponding to the (200), (220), (221), (310), (422), and (432) crystal planes of H₃PW₁₂O₄₀ (standard reference card PDF#01-075-2125). As shown in Figure 1, the XRD pattern of the catalyst after synthesis exhibited close agreement with the diffraction planes of H₃PW₁₂O₄₀, confirming the retention of the original heteropoly acid structure.

The solid heteropolyacid catalyst was further analyzed using SEM, with the corresponding microstructural morphology illustrated in Figure 2. The catalyst exhibits a porous, sponge-like three-dimensional cluster structure, which arises from the surfactant templating method (modified with [C₁₆H₃₃(CH₃)₃N]⁺ cations) during sol–gel or hydrothermal synthesis. This unique morphology significantly increases the catalyst’s specific surface area, enhances cellulose adsorption capacity, and facilitates substrate diffusion during reaction processes.

As shown in Figure 3, HRTEM analysis of the catalyst synthesized using phosphotungstic acid (H₃PW₁₂O₄₀) as a support revealed lattice fringes with a spacing of 0.134 nm, corresponding to the (0 1 0) crystallographic plane. This observation aligns with the characteristic diffraction peak at 6.74° in the XRD pattern of phosphotungstic acid, providing further evidence that the acid retains its fundamental structure during the catalyst synthesis process.

The pretreatment experiment used prepared (C₁₆TA)H₂PW catalyst to process microcrystalline cellulose under optimized conditions (180 °C for 1 h). The HPLC chromatogram (Figure S2) revealed a glucose conversion rate of 40.1%. Notably, the catalyst exhibited exceptional stability and reusability, as evidenced by its efficient separation from the reaction system and retention of nearly full catalytic activity over repeated cycles. To quantitatively evaluate the catalytic stability, a standardized procedure was established using 0.1 g cellulose mixed with 0.05 g solid heteropoly acid catalyst in 5 mL aqueous medium under controlled conditions (180 °C, 1 h). After each reaction cycle, the catalyst was systematically recovered via centrifugation filtration and directly reused for subsequent reactions. As demonstrated in Figure S2, the glucose yield remained consistently stable at 40% throughout three consecutive reaction cycles, confirming negligible performance degradation. This systematic evaluation confirms that the solid heteropoly acid catalyst possesses both high catalytic efficiency and operational durability under the specified reaction conditions.

3.2. Characterizations of Different Electrodes

In the electrolysis experiment, six catalytic electrode samples were synthesized, all supported on pretreated foam copper (CuF). The precursors included cobalt nitrate hexahydrate [Co(NO₃)₂·6H₂O], iron nitrate nonahydrate [Fe(NO₃)₃·9H₂O], and nickel nitrate hexahydrate [Ni(NO₃)₂·6H₂O]. Through sequential hydrothermal reaction and muffle furnace calcination, the following catalysts were obtained: Co₂(CO₃)(OH)₂/CuF, Co₃O₄/CuF, Fe₆(CO₃)(OH)₁₂/CuF, Fe₃O₄/CuF, Ni(OH)₂/CuF, and NiO/CuF. X-ray diffraction (XRD) analysis was first conducted on all six catalysts to investigate their crystalline structures. For the synthesized cobalt-based electrocatalyst, as shown in Figure 4, characteristic peaks at 2θ = 43.3°, 50.4°, and 74.1° correspond to the (111), (200), and (220) diffraction planes of metallic copper (Cu), respectively (standard reference card PDF#00-004-0836), confirming the preservation of the foam copper substrate’s crystalline structure during both hydrothermal synthesis and calcination. Although the XRD spectra exhibited limited resolution, the pattern of the sample on the foam copper substrate after hydrothermal synthesis but prior to calcination aligns with the diffraction planes of Co₂(CO₃)(OH)₂ (standard reference card PDF#48-0083). Following calcination, distinct peaks at 31.3°, 36.9°, 38.5°, 44.8°, and 65.2° in the XRD spectrum correspond to the (220), (311), (222), (400), and (440) planes of Co₃O₄ (standard reference card PDF#01-071-4921), demonstrating the successful conversion of Co₂(CO₃)(OH)₂ to Co₃O₄ during the calcination process.

The XRD patterns of both iron-based and nickel-based electrocatalysts were also analyzed to elucidate their structural evolution during synthesis (Figures S3 and S4). For the iron-based catalyst, characteristic peaks at 2θ = 43.3°, 50.4°, and 74.1° (PDF#00-004-0836) correspond to the metallic copper (Cu) substrate in both the hydrothermally synthesized precursor and the final calcined product. However, prior to calcination, additional diffraction signals at 17.8°, 33.2°, and 54.6° (PDF#00-046-0098) confirm the presence of Fe₆(CO₃)(OH)₁₂. After calcination, these peaks diminish, while new peaks emerge at 31.2°, 36.8°, 38.5°, 44.7°, and 59.3°, indexing to the (220), (311), (222), (400), and (511) planes of Fe₃O₄ (PDF#00-026-1136), confirming complete phase conversion. Similarly, the nickel-based catalyst exhibits identical Cu substrate peaks (PDF#00-004-0836) but distinct hydrothermal-phase signatures: broad diffraction bands at 12.8°, 23.5°, and 34.1° (PDF#00-046-0098) attributed to Ni(OH)₂. Calcination induces a structural shift, with sharp peaks at 37.2°, 43.2°, 62.9°, and 75.4° (PDF#00-004-0835) corresponding to the (111), (200), (220), and (311) planes of NiO, demonstrating full conversion from hydroxide to oxide. This comparative analysis highlights the critical role of calcination in stabilizing metal oxides while preserving the underlying Cu substrate, a pivotal strategy for optimizing electrocatalytic performance.

In addition to the XRD characterization, both the unmodified foam copper electrode and the Co₃O₄/CuF-loaded electrode were analyzed using SEM and TEM. Representative microstructural images are shown in the following figures.

As shown in Figure 5, the pristine foam copper exhibited a three-dimensionally interconnected porous structure, with a relatively smooth surface at higher magnification. This morphology likely facilitates catalyst adhesion and growth. In contrast, after hydrothermal treatment, the electrode surface was densely covered with nanosheet clusters directly grown on the smooth foam copper framework. This hierarchical architecture provides abundant active sites while reducing interfacial contact resistance for efficient charge/mass transfer across the electrode–electrolyte interface.

The focused physical characterization of Co₃O₄/CuF was prioritized because subsequent electrochemical experiments confirmed its superiority as the optimal catalytic electrode.

As shown in Figure 6, TEM analysis revealed that the Co₃O₄/CuF electrocatalyst displayed a nanorod-like structure. Such morphological features enhance the accessible surface area and strengthen interfacial interactions between the electrolyte and electrode. High-resolution TEM (HRTEM) images further demonstrated lattice fringes with a spacing of 0.251 nm, corresponding to the (311) crystal plane of Co₃O₄. This observation is consistent with the characteristic diffraction peak at 36.9° in the XRD pattern of Co₃O₄, providing additional evidence for the successful synthesis of Co₃O₄ on foam copper.

3.3. Catalytic Performance of Different Electrodes

To evaluate the catalytic activity of the electrode materials, we conducted electrochemical performance tests on CuF, Co₂(CO₃)(OH)₂/CuF, Co₃O₄/CuF, Fe₆(CO₃)(OH)₁₂/CuF, Fe₃O₄/CuF, Ni(OH)₂/CuF, and NiO/CuF electrodes, followed by comparative analysis based on the results. A three-electrode system was employed, with the working electrode as the target anode material, a platinum foil counter electrode, and an Hg/HgO reference electrode (1 M KOH). The anolyte consisted of a mixture of the pretreated cellulose solution and 1 M KOH aqueous solution (pH 14.0), while the catholyte was 1 M KOH. The cathodic and anodic compartments were separated by a Nafion proton exchange membrane.

Building upon our research group’s prior studies on electrocatalytic bond cleavage and oxidation of biomass such as methanol and glycerol [24], we first compared and screened the electrochemical performance of Co₂(CO₃)(OH)₂/CuF, Fe₆(CO₃)(OH)₁₂/CuF, and Ni(OH)₂/CuF metal hydroxide electrocatalysts in glucose oxidation reactions. Current density comparisons under varying applied potentials directly reflect catalytic activity. As shown in Figure 7a, linear sweep voltammetry (LSV) tests were conducted on four electrodes: Co₂(CO₃)(OH)₂/CuF, Fe₆(CO₃)(OH)₁₂/CuF, Ni(OH)₂/CuF, and unloaded CuF (scanning rate: 5 mV s⁻¹). The data reveal that all three metal hydroxide catalysts exhibit significantly higher catalytic activity than unloaded CuF during cellulose solution oxidation. Notably, all three catalysts achieved a current density of 10 mA cm⁻² at 1.37 V vs. RHE, whereas unloaded CuF reached the same current density at 1.4 V vs. RHE. This indicates that hydroxide loading does not substantially enhance oxidation performance compared to bare CuF. However, Fe₆(CO₃)(OH)₁₂/CuF and Ni(OH)₂/CuF demonstrated superior anodic polarization current densities at elevated potentials, suggesting relatively higher catalytic efficacy for cellulose solution oxidation.

The three compared hydroxide-based electrocatalysts did not demonstrate significant improvements in enhancing the oxidation of the reaction solution; furthermore, the applied potentials required to achieve a current density of 10 mA cm⁻² showed no substantial performance enhancement compared to the unloaded CuF catalyst [25]. Therefore, preliminary electrochemical evaluations suggest limited catalytic activity improvement for cellulose solution oxidation using different metal hydroxide electrocatalysts [26]. The prior literature indicates that metal oxide catalysts exhibit superior performance in biomass oxidation applications [27]. Consequently, we will prioritize evaluating the electrocatalytic performance of metal oxide-loaded electrodes [28].

To compare and screen the electrochemical performance of different metal oxide electrocatalysts in cellulose solution electrolysis, linear sweep voltammetry (LSV) tests were conducted on the Co₃O₄/CuF, Fe₃O₄/CuF, and NiO/CuF electrodes at a scanning rate of 5 mV s⁻¹. The results were benchmarked against the unloaded CuF electrode. As shown in Figure 8a, all three metal oxide electrocatalysts exhibited superior catalytic activity compared to the bare CuF electrode, achieving significantly higher current densities at the same applied potentials. Among these, Co₃O₄/CuF demonstrated exceptional catalytic performance, reaching a current density of 10 mA cm⁻² at an applied potential of 1.16 V vs. RHE. In contrast, Fe₃O₄/CuF and NiO/CuF required higher potentials of 1.24 V and 1.31 V vs. RHE, respectively, to achieve the same current density.

Furthermore, as shown in Figure 8b, the Tafel slope analysis based on polarization curves revealed that Co₃O₄/CuF, Fe₃O₄/CuF, and NiO/CuF exhibited Tafel slopes of 171, 239, and 230 mV dec⁻¹, respectively, during the electrolysis of cellulose solutions. This result confirms that Co₃O₄/CuF demonstrates superior catalytic activity among the three metal oxide electrodes. Compared to the unloaded CuF electrode, Co₃O₄/CuF achieves a 10 mA cm⁻² current density at a significantly reduced applied potential, indicating enhanced electrolysis efficiency.

In addition to linear sweep voltammetry (LSV) tests, we conducted comprehensive electrode reaction kinetics analysis on the oxide-loaded electrodes. As shown in Figure 8c, the electrochemical impedance spectroscopy (EIS) spectra of these electrodes exhibited typical semicircular patterns. Among them, the Co₃O₄/CuF electrode displayed the smallest charge transfer resistance (8.25 Ω), whereas the NiO/CuF, Fe₃O₄/CuF, and unloaded CuF electrodes exhibited resistances of 12.2 Ω, 23.6 Ω, and 186.9 Ω, respectively. These results confirm the superior catalytic reaction kinetics of Co₃O₄/CuF in the tested system. The CV curves of all the electrodes at varying scan rates were also analyzed to evaluate electrochemical active surface area (ECSA) via double-layer capacitance (C_dl) fitting. As shown in Figure 8d and Figure 9, and Table S1, Co₃O₄/CuF demonstrated the highest C_dl value and ECSA among all the electrodes. The CV curves of Co₃O₄/CuF exhibited distinct oxidation/reduction peaks with increasing peak currents at higher scan rates, indicating excellent electrochemical reversibility and catalytic activity. In contrast, Fe₃O₄/CuF and NiO/CuF showed smaller peak currents and broader redox peaks, suggesting slower reaction kinetics and lower catalytic activity.

Based on the comprehensive experimental results, the Co₃O₄/CuF electrocatalyst demonstrates exceptional catalytic activity and superior electrochemical performance among the metal oxide electrocatalysts, highlighting its significant application potential. The catalyst exhibits a low Tafel slope and minimal charge transfer resistance (8.25 Ω) in glucose oxidation reactions, indicating excellent reaction kinetics. Furthermore, Co₃O₄/CuF maintains stable electrochemical reversibility and catalytic activity across varying scan rates, further validating its advantages as a robust electrocatalyst for glucose oxidation.

Following the initial screening and comparison of the electrochemical performance of the prepared metal electrocatalysts, further analysis was conducted to evaluate their catalytic activity and product formation during the electrocatalytic oxidation of cellulose into formate. To compare the electrocatalytic performance of the different metal oxide catalysts and quantitatively/qualitatively analyze reaction products, constant-potential electrolysis experiments (1.66 V vs. RHE) were performed over 60 min using Co₃O₄/CuF, Fe₃O₄/CuF, and NiO/CuF as anodes. NMR analysis of the liquid-phase products collected post-reaction confirmed the presence of formate (Figure S5), demonstrating the successful establishment of a cellulose-to-formate electrocatalytic oxidation pathway in the developed system.

Quantitative analysis of formate produced during the electrocatalytic reactions of different metal oxide catalysts was performed using HPLC. Combined with electrochemical workstation data, partial current densities and Faradaic efficiencies for formate generation were calculated for each catalyst. As shown in Figure S6, the calcined Co₃O₄/CuF catalyst exhibited exceptional catalytic activity, achieving a partial current density of 153.2 mA cm⁻² and a formate Faradaic efficiency of 85.3%. In contrast, the pre-calcined Co₂(CO₃)(OH)₂/CuF precursor displayed significantly lower performance, with a partial current density of 56.3 mA cm⁻² and a Faradaic efficiency of 51.3% under identical conditions. Comparative analysis further revealed that Fe₃O₄/CuF and NiO/CuF delivered partial current densities of 143.8 mA cm⁻² (FE = 80.1%) and 114.3 mA cm⁻² (FE = 77.5%), respectively, underscoring the critical role of calcination in optimizing both activity and selectivity for glucose oxidation to formate. In contrast, under identical experimental conditions, electrolysis of cellulose without solid heteropoly acid pretreatment yielded a formate concentration of only 6 mmol L⁻¹, with a Faradaic efficiency of 17.9% and a production yield of merely 1%. These results unequivocally demonstrate the superiority of the experimental design proposed in this work.

3.4. Oxidation Mechanism of Pretreated Cellulose

To investigate the electrochemical oxidation mechanism of pretreated cellulose on the Co₃O₄/CuF electrode, we employed HPLC to track and analyze liquid-phase products generated during prolonged constant-potential electrolysis experiments. The applied potential was maintained at 1.66 V vs. RHE to precisely control the reaction rate and extent.

The time-resolved liquid-phase product distribution and concentration changes during constant-potential electrolysis are illustrated in Figures S7 and S8. Figure S8 displays the temporal concentration profiles of formate, arabinose, and erythrose during the electrocatalytic glucose oxidation process. The reaction time spanned 0–40 min, with the concentration range set to 0–30 mmol L⁻¹. The black curve represents formate, whose concentration surged rapidly from near 0 mmol L⁻¹ at the initial stage (0 min) to ~28.5 mmol L⁻¹ at 40 min, exhibiting a pronounced exponential growth trend. In contrast, the red curve (arabinose) and blue curve (erythrose) exhibited more gradual changes: the arabinose concentration reached a maximum of 5.2 mmol L⁻¹ at 30 min, followed by a deceleration in accumulation rate, while the erythrose concentration remained below 2 mmol L⁻¹ throughout the reaction. This observation implies that formate is directly produced via C1-C2 bond cleavage in the electrocatalytic glucose oxidation pathway.

Furthermore, given arabinose’s structural similarity to glucose (both are aldoses with five and six carbon atoms, respectively), it is hypothesized that arabinose may also undergo C1-C2 bond cleavage to yield formate and erythrose (a four-carbon aldose). Subsequently, erythrose could further decompose into formate and glyceraldehyde (a three-carbon aldose), continuing this cascade until the carbon chain is reduced to one carbon unit. Theoretically, this pathway allows one molecule of glucose (C₆H₁₂O₆) to be fully converted into six molecules of formate (C₆H₁₂O₆ → 6 HCOO⁻).

Based on these results, this study proposes a plausible reaction mechanism for the electrocatalytic oxidation of pretreated cellulose to formate over the Co₃O₄/CuF electrode, as illustrated in Figure 10. Here, glucose was used as the representative of cellulose to elucidate the reaction mechanism. Initially, under applied potential, the Co₃O₄ catalyst loaded on CuF interacts with hydroxyl groups in the solution, leading to electron loss and the formation of a hydroxyl-coordinated cobalt active species (Co²⁺δO(OH)_ₐd) [29]. Simultaneously, the aldehyde group at the α-carbon of glucose, the hydrolysis product of cellulose, undergoes a reversible hydration reaction, generating a hydrated glucose intermediate (I) [30]. Subsequently, the hydrated glucose is oxidized by Co²⁺δO(OH)_ₐd, facilitating C1-C2 bond cleavage to yield one molecule of formic acid and a hydrated arabinose intermediate [31]. The formic acid is further dehydrated to formate, which is stabilized under alkaline conditions. Meanwhile, the hydrated arabinose continues to be oxidized by Co²⁺δO(OH)_ₐd, producing another molecule of formate and a hydrated erythrose intermediate. This iterative process repeats until all carbon atoms are converted into formate. Consequently, within the constructed reaction system, one molecule of glucose is fully electrocatalytically oxidized to yield six molecules of formate.

4. Conclusions

This study successfully synthesized a solid heteropoly acid catalyst using 12-tungstophosphoric acid and cetyltrimethylammonium bromide as precursors, demonstrating exceptional catalytic activity in cellulose hydrolysis. The catalyst exhibited high selectivity, reusability, and environmental compatibility, with minimal activity loss after repeated cycles. To evaluate the feasibility of anodic formate production within this electrochemical system, various cobalt-, iron-, and nickel-based hydroxide/oxide electrocatalysts were synthesized via hydrothermal or combined hydrothermal–calcination methods. Among these, the cobalt-based oxide catalyst (Co₃O₄/CuF) displayed superior glucose oxidation activity. Constant-potential electrolysis tests at 1.66 V vs. RHE revealed Co₃O₄/CuF’s outstanding performance, achieving a partial current density of 153.2 mA cm⁻² and a formate Faradaic efficiency of 85.3%.

To elucidate the reaction mechanism, HPLC was employed to track liquid-phase products during prolonged electrolysis. The results confirmed that arabinose, alongside formate, increased with applied potential and charge transfer, identifying arabinose as a critical intermediate in the glucose-to-formate pathway. Based on these findings, a stepwise C1-C2 bond cleavage mechanism was proposed, wherein glucose undergoes sequential oxidative cleavage via Co₃O₄/CuF to ultimately yield six molecules of formate per glucose unit. This work establishes a sustainable and efficient strategy for biomass valorization through electrocatalytic oxidation.