Ni‐Induced Low‐Valence Tungsten Oxide for Highly Selective and Stable Glycerol Electro‐Oxidation to Formic Acid

ABSTRACT

Electrocatalytic upgrading of renewable biomass‐derived glycerol represents a sustainable method to produce value‐added chemicals, but the complex reaction network of key intermediates during the glycerol oxidation poses challenges to product selectivity. Herein, we report a nickel‐doped tungsten oxide catalyst grown on nickel foam (Ni‐WOx/NF) for the highly selective electrooxidation of glycerol to formic acid (FA). This optimized catalyst achieves an FA Faraday efficiency (FE) of 95.1% and operates stably for over 18 h, outperforming conventional non‐precious metal catalysts. Mechanistic understanding revealed that nickel doping effectively modulates the valence states of tungsten, increasing the proportion of low‐valent W⁴⁺ species. This promotes the desorption of FA from the catalytic interface, thereby inhibiting its over‐oxidation and enhancing selectivity. Taking a step forward, an integrated electrocatalytic system coupling the anodic glycerol oxidation reaction (GOR) with the cathodic nitrate reduction reaction (NO₃−RR) is constructed. This system enables the simultaneous coproduction of FA and ammonia (NH₃), achieving a high FE for FA of 96.9% along with sustained stability over 12 h, demonstrating significant potential for practical applications. This work demonstrates a sustainable catalytic system for selective glycerol oxidation by precisely regulating the reaction pathways of key intermediates.

This study demonstrates a nickel‐induced low‐valence tungsten oxide for selective electrocatalytic glycerol oxidation with a formate Faraday efficiency of 96.9% in a flow cell. Mechanistic investigations reveal that low‐valence tungsten promotes key intermediate desorption, thereby suppressing over‐oxidation.

1. Introduction

The depletion of traditional fossil fuels and severe environmental issues have accelerated the exploration of alternative clean and renewable energy [1]. Biomass upgrading, which involves transforming sustainable chemical resources into high‐value products and fuels, has garnered widespread attention [2]. Among various biomass derivatives, glycerol (GLY), the primary byproduct of the biodiesel industry, has experienced a global surplus, leading to its diminished market value [3]. The electrochemical oxidation of glycerol provides an attractive pathway for its valorization under mild conditions, holding promise for converting this abundant raw material into valuable chemicals [4]. Previous studies have demonstrated that the glycerol oxidation reaction (GOR) can yield a spectrum of products, including glyceraldehyde [5, 6], dihydroxyacetone [7, 8], glyceric acid [9, 10], lactic acid [11, 12], glycolic acid [13, 14], and formic acid [15, 16]. However, the complex reaction network of key intermediates during the glycerol oxidation poses challenges to the selectivity of target products [17]. Therefore, it is crucial to develop an efficient electrocatalyst that exhibits selective adsorption and activation towards reaction substrates or intermediate species.

Tungsten oxides have garnered considerable attention in various catalytic applications, including water splitting [18], ammonia synthesis [19], carbon dioxide reduction [20], and alcohol oxidation [21], owing to their tunable electronic structures. This tunability stems from their nonstoichiometric nature, where adjustable W valence states and abundant oxygen vacancies confer promising catalytic potential for reactions such as glycerol oxidation [22]. However, in alkaline media, which is a common environment for glycerol electrooxidation, tungsten oxides often suffer from dissolution [23], leading to poor durability and thus limiting their practical application. Doping tungsten oxides with transition metals has proven to be an effective strategy to enhance their stability and optimize their catalytic activity [24]. Nickel‐based materials are particularly attractive owing to their low cost, high stability, and notable catalytic activity. Incorporating nickel into tungsten oxides can simultaneously improve the structural stability in alkaline solutions and modulate the catalytic properties. For instance, Zhang et al. [25] prepared a bifunctional Ni/WO₃ catalyst for urea electrolysis and hydrogen evolution in an alkaline electrolyte, which demonstrated remarkable durability. Despite this, there is still a gap in the literature regarding the use of tungsten oxide‐based catalysts for the electrocatalytic upgrading of biomass‐derived glycerol.

In this work, we synthesize a nickel‐doped tungsten oxide electrocatalyst supported on nickel foam (Ni‐WO_x/NF) via a two‐step hydrothermal method for the efficient electrooxidation of glycerol. Comprehensive characterization confirms that nickel doping modulates the electronic structure of tungsten, increasing the proportion of low‐valence W⁴⁺ species. Electrochemical evaluation reveals that the Ni‐WO_x/NF electrode achieves a FE of 95.1% for formic acid (FA) at 1.4 V versus RHE, along with an operational stability of over 18 h, significantly outperforming the undoped counterpart. In situ Raman and infrared spectroscopic analyses elucidate the reaction pathway and demonstrate that the introduced W⁴⁺ sites facilitate FA desorption from the catalytic interface, thereby suppressing its over‐oxidation and enhancing selectivity (Figure 1). Taking a step forward, an integrated electrocatalytic system coupling the anodic GOR with the cathodic nitrate reduction reaction (NO₃ ⁻RR) is constructed. This system enables the simultaneous coproduction of FA and ammonia (NH₃), achieving a high FE for FA of 96.9% along with sustained stability over 12 h, demonstrating significant potential for practical applications. By developing this Ni‐doped catalyst, we establish a valence‐state modulation strategy that can guide the design of efficient electrocatalysts for converting biomass into value‐added chemicals.

FIGURE 1.

Schematic illustration of the decisive role of nickel in modulating the valence state of tungsten oxide for selective glycerol electrooxidation.

2. Results and Discussion

2.1. Synthesis and Characterization of Electrocatalysts

The Ni‐WO_x/NF catalyst was synthesized through a two‐step hydrothermal method (Figure 2a). Initially, a WO_x precursor was directly grown on nickel foam (NF) via hydrothermal treatment at 180 °C. Subsequently, Ni²⁺ ions were introduced into the precursor through a second hydrothermal step at 120 °C, yielding the final Ni‐WO_x/NF catalyst. For comparison, control samples were prepared in parallel: Ni/NF was obtained by subjecting bare NF to the second hydrothermal step only, while WO_x/NF was prepared solely via the first step. The surface morphology and microstructure of the catalysts were first examined by scanning electron microscopy (SEM). As shown in Figure 2b, Ni‐WO_x/NF presents densely packed, vertically aligned nanosheet arrays that are firmly anchored on the NF substrate. This well‐defined architecture creates abundant inter‐sheet channels, which are conducive to electrolyte diffusion and mass transport, thereby potentially exposing more active sites. In contrast, the Ni/NF sample exhibits a typical but sparser nanosheet morphology (Figure S1a), and WO_x/NF shows a dense yet irregularly oriented sheet‐like structure (Figure S1b). These comparisons indicate that the introduction of Ni during the second hydrothermal step not only preserves the high‐surface‐area framework but also promotes a more ordered vertical growth of the nanosheets. To probe the detailed interfacial structure, transmission electron microscopy (TEM) was employed. The high‐resolution TEM (HRTEM) image in Figure 2c reveals distinct lattice fringes with spacings of 0.243 and 0.151 nm, which correspond to the (002) plane of WO₂ and the (113) plane of Ni(OH)₂, respectively. This provides clear evidence for the coexistence and intimate contact of WO_x and Ni(OH)₂ phases, suggesting the formation of a heterointerface. Consistent with this, energy‐dispersive x‐ray spectroscopy (EDX) elemental mapping (Figure 2d) demonstrates a homogeneous distribution of Ni, W, and O elements across the selected area, confirming the successful and uniform incorporation of Ni into the WO_x matrix.

FIGURE 2.

(a) Schematic illustration of the synthesis process. (b) SEM images, (c) high‐resolution TEM (HRTEM) images, (d) EDS elemental mapping of Ni‐WO_x/NF. (e) XRD patterns, and (f) Raman spectra of Ni‐WO_x/NF, WO_x/NF, and Ni/NF. (g) XPS survey spectrum of Ni‐WO_x/NF. High‐resolution XPS spectrum of (h) O 1s, (i) W 4f for Ni‐WO_x/NF and WO_x/NF. High‐resolution XPS spectrum of (j) Ni 2p for Ni‐WO_x/NF and Ni/NF.

The crystalline phases of the samples were identified by x‐ray diffraction (XRD). As depicted in Figure 2e, the diffraction peaks at 12.19°, 24.61°, 59.98°, and 61.25° can be indexed to Ni(OH)₂ (PDF#38‐0715) [26], while the peak at 23.17° is characteristic of W₁₈O₄₉ (PDF#36‐0101) [27]. Notably, in the pattern of Ni‐WO_x/NF, the distinct diffraction peaks attributable to crystalline WO_x phases are significantly attenuated or absent. This observation is likely due to the surface coverage and interfacial interaction with the newly formed Ni(OH)₂, which aligns with the HRTEM results indicating a heterojunction structure. Further insights into the local chemical environment and bonding were gained through Raman spectroscopy (Figure 2f). The spectrum of WO_x/NF shows characteristic peaks at 773 and 814 cm⁻¹ (assigned to W–O stretching vibrations) and at 958 cm⁻¹ (terminal W = O stretch), consistent with the signature of W₁₈O₄₉ [28]. The spectrum of Ni/NF displays prominent peaks at 459 and 490 cm⁻¹, typical of Ni(OH)₂ [29], which remain clearly visible in the Ni‐WO_x/NF spectrum. Importantly, significant changes are observed in the tungsten‐oxide‐related regions upon Ni doping. In Ni‐WO_x/NF, the W = O stretching vibration shifts to 933 cm⁻¹, and a new peak emerges at 913 cm⁻¹, which is attributable to the [WO₄] tetrahedral unit in NiWO₄ [30]. The characteristic peak at 995 cm⁻¹ corresponds to the vibrational mode typical of tungstate‐like species [31]. Moreover, the appearance of a distinct band at 883 cm⁻¹, plausibly associated with W–O–Ni stretching vibrations or lattice distortions induced by Ni incorporation, serves as direct spectroscopic evidence for a strong chemical interaction between the Ni species and the tungsten oxide host structure [32].

To elucidate the surface chemical states and interfacial electronic interaction induced by Ni doping, x‐ray photoelectron spectroscopy (XPS) analysis was conducted. The survey spectrum of Ni‐WO_x/NF (Figure 2g) confirms the co‐existence of Ni, W, and O elements, consistent with the successful synthesis of the composite. The high‐resolution O 1s spectrum (Figure 2h) was deconvoluted into three components corresponding to lattice oxygen (O_L, 530.6 eV), surface chemisorbed oxygen (O_H, 531.4 eV), and oxygen in adsorbed water (O_W, 532.7 eV) [33]. Compared with WO_x/NF, the relative area of the O_H component increases significantly from 8.14% to 15.66% in Ni‐WO_x/NF, indicating that Ni doping promotes the formation of chemisorbed oxygen on the catalyst surface. The W 4f spectra provide direct evidence for the alteration of tungsten valence states (Figure 2i). The spectra were fitted with three pairs of spin‐orbit doublets, assigned to W⁶⁺ (35.72/37.88 eV), W^δ+ (4 < δ < 6) (35.18/37.14 eV), and W⁴⁺ (33.71/36.26 eV) [34, 35]. Notably, the proportion of low‐valence W⁴⁺ increases from 6.39% in WO_x/NF to 13.38% in Ni‐WO_x/NF, demonstrating that Ni incorporation facilitates the reduction of W species, thereby enriching the catalyst surface with more W⁴⁺ sites. The electronic interaction between Ni and W was further probed by the Ni 2p spectrum (Figure 2j). The peaks at 856.39 eV (Ni 2p_3/2) and 873.99 eV (Ni 2p_1/2), along with their satellites, are characteristic of Ni²⁺ in Ni(OH)₂ [36]. Compared with the Ni/NF reference, these binding energies shift positively by about +0.86 eV in Ni‐WO_x/NF, indicating a decreased electron density on Ni. This shift suggests electron transfer from Ni to adjacent W atoms, which aligns with the observed increase in W⁴⁺ content and further confirms the strong electronic coupling at the Ni(OH)₂/WO_x interface [37]. In summary, the XPS results collectively reveal that Ni doping effectively induces the valence state distribution of tungsten, leading to a pronounced increase in surface W⁴⁺ species. These electronic structure modifications, consistent with the structural insights from TEM and Raman, are anticipated to critically influence the catalytic behavior of Ni‐WO_x/NF in the subsequent electrochemical glycerol oxidation.

2.2. Electrocatalytic Glycerol Oxidation Performance

The electrocatalytic performance of Ni‐WO_x/NF for glycerol oxidation was evaluated in a standard H‐type cell using linear sweep voltammetry (LSV). Figure 3a compares the LSV curves of Ni‐WO_x/NF in 1 M KOH with and without 0.1 M glycerol. In the absence of glycerol, the anodic current at potentials below 1.60 V versus RHE remains low, with a noticeable peak around 1.43 V versus RHE corresponding to the oxidation of surface Ni²⁺–Ni³⁺ [38]. Upon introducing glycerol, the onset potential shifts negatively to 1.28 V versus RHE, and the current density increases substantially. At a current density of 50 mA cm⁻², the required potential is reduced by 330 mV (from 1.72 to 1.39 V vs. RHE), indicating that glycerol oxidation proceeds more readily than the oxygen evolution reaction (OER) on Ni‐WO_x/NF and confirming its high activity toward GOR.

FIGURE 3.

(a) LSV curves of the Ni‐WO_x/NF in 1 M KOH with and without 0.1 M glycerol addition at the scan rate of 5 mV s⁻¹. (b) LSV curves of various catalysts in 1 M KOH containing 0.1 M glycerol at the scan rate of 5 mV s⁻¹. (c) Comparison of anodic potentials at varied current densities for various catalysts. (d) Tafel plots, (e) double‐layer capacitance (Cdl), and (f) Nyquist plots of various catalysts. (g) Comparison of Faradaic efficiency for glycerol oxidation products among different catalysts at 1.4 V versus RHE. (h) Faradaic efficiency for FA on Ni‐WO_x/NF and WO_x/NF at different applied potentials. (i) Comparison of the catalytic performance of different catalysts. (j) Cycling stability test at 1.5 V versus RHE. (k) Comparison of the Raman spectra for the pristine and post‐GOR Ni‐WO_x/NF and WO_x/NF.

To benchmark its performance, the GOR activities of WO_x/NF, Ni/NF, and bare NF were also measured under identical conditions (Figure 3b). Ni‐WO_x/NF exhibits the highest current density across the applied potential range. The potentials required to reach current densities of 10, 50, 100, and 200 mA cm⁻² are summarized in Figure 3c. Notably, Ni‐WO_x/NF achieves these current densities at significantly lower overpotentials than the other catalysts, demonstrating that the synergy between Ni and WO_x effectively enhances GOR activity. Kinetic insights were obtained from Tafel analysis (Figure 3d). Ni‐WO_x/NF shows a Tafel slope of 101.90 mV dec⁻¹, which is markedly lower than those of WO_x/NF (128.64 mV dec⁻¹), Ni/NF (137.58 mV dec⁻¹), and NF (153.02 mV dec⁻¹). This smaller Tafel slope suggests more favorable reaction kinetics for glycerol oxidation on Ni‐WO_x/NF, consistent with its superior activity observed in the LSV measurements.

The electrochemical surface area (ECSA), estimated from double‐layer capacitance (Cdl) measurements in a non‐Faradaic potential window, provides further understanding of the activity origin. As shown in Figure 3e and Figure S2, Ni‐WO_x/NF exhibits a Cdl value of 11.89 mF cm⁻², which is comparable to that of WO_x/NF (11.09 mF cm⁻²) and much larger than that of Ni/NF (0.99 mF cm⁻²). This indicates that the dense nanosheet architecture of Ni‐WO_x/NF offers a substantially larger number of accessible active sites, contributing to its high current density. Electrochemical impedance spectroscopy (EIS) was employed to probe the charge‐transfer characteristics at the electrode‐electrolyte interface. The Nyquist plot in Figure 3f reveals that Ni‐WO_x/NF possesses the smallest semicircle diameter among all samples, corresponding to the lowest charge‐transfer resistance (R_ct) [39]. This facilitates faster interfacial electron transfer during GOR, further corroborating its enhanced kinetic performance. Collectively, these electrochemical analyses demonstrate that Ni‐WO_x/NF is a highly active GOR catalyst, benefiting from its large electrochemical surface area, favorable reaction kinetics, and efficient charge transfer.

The product distribution and selectivity of the GOR on Ni‐WO_x/NF were quantified by ¹H NMR spectroscopy after chronoamperometric tests. As summarized in Figure 3g, at an applied potential of 1.4 V versus RHE, formic acid (FA) is identified as the major oxidation product. Ni‐WO_x/NF achieves a remarkable Faradaic efficiency (FE) of 95.1% for FA and a total FE of 97.5%, substantially outperforming the control catalysts. In particular, the Ni/NF electrode shows a much lower FA FE of 65.3% and a considerable accumulation of glycolic acid (GA), an intermediate in the oxidation cascade, indicating its inferior capability in further oxidizing GA to FA. This pronounced contrast underscores the essential role of WO_x species in facilitating the complete oxidation pathway toward FA. The potential‐dependent FE of FA was further investigated for Ni‐WO_x/NF and WO_x/NF (Figure 3h). Both catalysts experience a gradual decline in FE with increasing potential, primarily due to the growing competition from the OER. Nevertheless, Ni‐WO_x/NF maintains a high FA FE of 86.8% even at 1.7 V versus RHE. In contrast, WO_x/NF exhibits a lower overall FE at lower potentials, which is attributed not to OER but to the likely over‐oxidation of FA to CO₂. This observation suggests that Ni doping effectively mitigates undesired over‐oxidation, thereby enhancing the operational potential window for selective FA production. Furthermore, Ni‐WO_x/NF exhibits a larger partial current density for FA than WO_x/NF across different applied potentials (Figure S3), indicating its higher catalytic activity toward FA production. The performance of Ni‐WO_x/NF compares favorably with previously reported non‐noble‐metal‐based GOR catalysts in terms of both activity and selectivity (Figure 3i and Table S1).

To probe the interplay between reactant adsorption and reaction efficacy, the effects of glycerol and KOH concentrations were systematically studied [40]. As shown in Figures S4a and S4b, both current density and FE of FA increase with glycerol concentration up to 0.1 M, beyond which they decline, likely due to increased solution resistance and competitive adsorption that hinder the necessary adsorption of OH⁻ species. Similarly, elevating KOH concentration enhances the current density (Figure S4c) by promoting the availability of OH⁻, while the FA FE reaches a plateau under optimized conditions (Figure S4d). These results highlight that fine‐tuning the reaction microenvironment is crucial for achieving high reaction rates and selectivity.

Long‐term stability is a critical metric for practical application. As shown in Figure 3j, the catalyst maintains stable current density and FA FE (88%–91%) over nine consecutive electrolysis cycles at 1.5 V versus RHE, demonstrating excellent operational stability. SEM images (Figure S5) reveal that the vertically aligned nanosheet architecture of Ni‐WO_x/NF remains intact after the reaction, whereas the structure of WO_x/NF collapses completely. Correspondingly, Raman spectra (Figure 3k) confirm that the surface phases of Ni‐WO_x/NF are well preserved, while all characteristic signals of WO_x/NF disappear after electrolysis, indicating severe dissolution in the absence of Ni stabilization. The W content of the catalysts before and after the reaction was further determined by inductively coupled plasma optical emission spectroscopy (ICP‐OES). The results confirmed that the W content on the surface of Ni‐WO_x/NF exhibited no significant change after the reaction, whereas almost all the W on the surface of WO_x/NF was lost (Table S2). This enhanced durability is attributed to the strong interfacial interaction between Ni(OH)₂ and WO_x, as well as to the increased proportion of stable W⁴⁺ species induced by Ni doping [41].

2.3. Mechanism Investigation

Based on product analysis and literature reports, a plausible reaction pathway for the electrooxidation of glycerol to FA is proposed [42], as illustrated in Figure 4a. The process is believed to initiate with the selective oxidation of a primary hydroxyl group to form glyceraldehyde, followed by further oxidation to glyceric acid. Subsequent oxidative cleavage of the C–C bond in glyceric acid yields GA and FA. GA, as a key intermediate, can be further oxidized to produce additional FA. It is noteworthy that at sufficiently high applied potentials, FA itself may undergo over‐oxidation to CO₂, which dissolves and forms carbonate species in the alkaline electrolyte.

FIGURE 4.

(a) Pathway of electrocatalytic glycerol oxidation to the FA with Ni‐WO_x/NF in a basic medium. (b) Yield of FA and conversion of GA from the oxidation of GA. (c) Carbon balance of Ni‐WO_x/NF and WO_x/NF as a function of potential. (d) Open circuit potentials (OCPs) of Ni‐WO_x/NF and WO_x/NF in 1 M KOH solution before and after FA addition. In‐situ Raman spectroscopy of Ni‐WO_x/NF during (e) GOR and (f) OER. (g) In situ infrared spectra of Ni‐WO_x/NF during the electrocatalytic GOR.

To gain deeper mechanistic insight, GA was employed as a substrate to compare the oxidation performance of different catalysts at a fixed potential. As shown in Figures S6 and S7, the oxidation of GA on both Ni‐WO_x/NF and WO_x/NF proceeds with the production of FA and carbonate species derived from the over‐oxidation of FA to CO₂ dissolved in the solution. However, Ni‐WO_x/NF demonstrates higher GA conversion and FA yield than WO_x/NF, and both outperform Ni/NF significantly (Figure 4b). These results clearly indicate that the WO_x species, rather than the Ni(OH)₂ component, serves as the primary active site responsible for the efficient conversion of GA to FA. The superior performance of Ni‐WO_x/NF can be attributed to its enhanced intrinsic activity and, importantly, its ability to suppress the over‐oxidation of FA—a limitation observed with the WO_x/NF catalyst.

The carbon balance during GOR was further analyzed to quantify carbon loss, particularly from FA over‐oxidation [40]. As depicted in Figure 4c, the carbon balance for Ni‐WO_x/NF remains around 95% across a wide potential range, approaching the theoretical maximum. Only at a very high potential of 1.8 V versus RHE does it drop below 90%, indicating noticeable carbon loss due to FA over‐oxidation. In stark contrast, the carbon balance for WO_x/NF remains consistently low (80%–85%) at all tested potentials, suggesting that a significant portion of FA is continuously over‐oxidized to CO₂ independent of the applied potential. This persistent carbon loss implies that FA over‐oxidation on WO_x/NF is governed primarily by the intrinsic properties of the catalyst rather than by external potential. The open‐circuit potential (OCP) was utilized to preliminarily investigate the adsorption behavior of FA at the catalyst interface, revealing the influence of organic adsorbates on the inner Helmholtz layer. Upon the addition of 1 M FA, the OCP change for WO_x/NF (Δ = 0.164 V) was more significantly negative than that for Ni‐WO_x/NF (Δ = 0.117 V), indicating a stronger affinity of FA for WO_x/NF (Figure 4d). This also demonstrates that after nickel doping, the adsorption of FA within the inner Helmholtz layer is weaker, thereby effectively preventing its overoxidation. To further verify this, the adsorption energies (E _ads) of FA on Ni‐WO_x/NF and WO_x/NF were subsequently calculated, as shown in Figure S8. The adsorption energy of *FA on WO_x/NF (−1.603 eV) is significantly stronger than that on Ni‐WO_x/NF (−1.195 eV), which is primarily attributed to the shift of W species toward lower valence states on the catalyst surface after Ni doping; the enrichment of W⁴⁺ weakens the adsorption of FA on the catalyst. This is consistent with the above OCP results. Collectively, these findings elucidate that the high‐valence W species in undoped WO_x possess strong electron‐withdrawing character, which excessively stabilizes the adsorbed FA intermediate and impedes its desorption, thereby promoting its further oxidation to CO₂. The introduction of Ni into the WO_x matrix modulates the electronic structure of tungsten, significantly increasing the proportion of low‐valence W⁴⁺ species. The enhanced surface concentration of W⁴⁺ weakens the interaction with FA, facilitating its timely desorption and effectively suppressing over‐oxidation. Therefore, the strategy of Ni doping, by inducing a controlled reduction of tungsten valence states, steers the reaction pathway toward selective FA formation and presents an effective electronic‐structure‐based approach for achieving high activity and selectivity in glycerol electrooxidation.

To dynamically monitor the surface evolution and identify key intermediates during the GOR, in situ Raman spectroscopic investigations were carried out on the Ni‐WO_x/NF electrode. As shown in Figure 4e, when the potential is increased to 1.5 V versus RHE in a glycerol‐containing electrolyte, characteristic peaks emerge at 474 and 558 cm⁻¹, corresponding to the formation of NiOOH [43]. Concurrently, a signal associated with FA appears at 1349 cm⁻¹ [44]. Importantly, a broad feature attributed to WO_x species remains observable within the 840–950 cm⁻¹ region throughout the potential window, indicating that the WO_x phase remains stable under operating conditions after Ni doping. Furthermore, upon glycerol addition, the anodic current is primarily directed toward glycerol oxidation rather than water oxidation, which effectively suppresses the over‐oxidative environment that would otherwise cause WO_x dissolution, thereby further protecting the structural stability of WO_x. In contrast, under OER conditions without glycerol (Figure 4f), the WO_x‐related bands disappear as the potential rises, and no FA‐related signal is detected, confirming that the presence of glycerol not only protects the catalyst surface but also validates FA as a specific product of GOR.

Complementary insights were obtained from in situ attenuated total reflection Fourier‐transform infrared (ATR‐FTIR) spectroscopy (Figure 4g). Characteristic vibrational bands associated with key species were identified: 1110 cm⁻¹ (C–O stretch of glycerol) [45], 1224 cm⁻¹ (CH₂ bending of glycerol) [46], 1383 and 1354 cm⁻¹ (C–H in‐plane bending and O–C–O vibration of FA) [38], 1580 cm⁻¹ (glyceric acid) [33], 1470 cm⁻¹ (glyceraldehyde) [47], and 1083 cm⁻¹ (glycolic acid) [48]. The sequential appearance and evolution of these signals—from glycerol and its partially oxidized intermediates (glyceraldehyde, glyceric acid, and glycolic acid) to the final product FA—provide direct spectroscopic evidence supporting the proposed stepwise reaction pathway illustrated in Figure 4a. These in situ spectroscopic results collectively demonstrate that WO_x species act as the primary active sites for glycerol oxidation and that inducing the valence state of tungsten through Ni doping effectively controls the reaction pathway and product selectivity. The preserved WO_x phase under reaction conditions, coupled with the observed intermediate sequence, corroborates the earlier hypothesis that increasing the proportion of W⁴⁺ facilitates FA desorption and inhibits its over‐oxidation, thereby enabling highly selective FA production.

2.4. Paired‐Electrolysis NO₃ ⁻RR//GOR System

Motivated by the excellent GOR performance of Ni‐WO_x/NF, we further constructed a paired electrolysis system by integrating the anodic GOR with the cathodic nitrate reduction reaction (NO₃ ⁻RR), aiming to achieve simultaneous production of two value‐added chemicals [49]. Considering the high activity and selectivity of copper‐based materials for NO₃ ⁻RR, Cu(OH)₂ grown on copper foam was selected as the cathode catalyst [50, 51]. As shown in Figure S9, Cu(OH)₂/CF displays a nanorod morphology and high nitrate reduction activity, as confirmed by LSV curves (Figure S10). The resulting NO₃ ⁻RR//GOR system, schematically depicted in Figure 5a and Figure S11, employs Ni‐WO_x/NF as the anode and Cu(OH)₂/CF as the cathode, with the two compartments separated by an anion exchange membrane.

FIGURE 5.

(a) Scheme of NO₃ ⁻RR//GOR paired‐electrolysis system. (b) The LSV curves of overall water‐ splitting systems and the NO₃ ⁻RR//GOR paired‐electrolysis system. Potential‐dependent Faradaic efficiency and yield rate of FA (c) and NH₃ (d) in AEM electrolyzer. (e) Stability measurement of NO₃ ⁻RR//GOR paired‐electrolysis system at a cell voltage of 1.8 V.

The feasibility of this coupled strategy was first examined in an AEM electrolyzer. As shown in Figure 5b, the LSV curve of the paired system shows a significantly lower cell voltage compared to that of conventional water splitting across all current densities. For instance, at 10 mA cm⁻², the required voltage is reduced by 896 mV, highlighting the markedly enhanced energy efficiency enabled by replacing the anodic OER with GOR and the cathodic HER with NO₃ ⁻RR. Product quantification under varying applied cell voltages further confirms the system's efficacy. As presented in Figure 5c, the Faradaic efficiency for FA reaches a maximum of 96.9% at 1.5 V, and remains high (92.4%) even at 2.1 V, corresponding to an FA yield rate of 583 µmol h⁻¹ cm⁻². Concurrently, the FE for NH₃ stays stable within the range of 50%–60% over a wide voltage window (Figure 5d), achieving a maximum NH₃ yield rate of 6.3 µmol h⁻¹ cm⁻² at 2.7 V. Moreover, the system demonstrates promising operational stability, maintaining consistent co‐production of both FA and NH₃ during an extended electrolysis test at 1.8 V for 12 h (Figure 5e). Collectively, the increased content of low‐valent tungsten species, induced by nickel doping, plays a key role in steering the selective glycerol oxidation pathway on Ni‐WO_x/NF. Building on this fundamental understanding, the constructed NO₃ ⁻RR//GOR paired electrolysis system successfully demonstrates the feasibility of simultaneously generating valuable FA and NH₃ with high efficiency and stability, showcasing a practical and energy‐advantageous route for coupled biomass upgrading and nitrogen resource utilization.

3. Conclusion

In summary, we have successfully synthesized a nickel‐doped tungsten oxide electrocatalyst (Ni‐WO_x/NF) via a two‐step hydrothermal method for the efficient electrooxidation of glycerol. Comprehensive characterizations confirm that nickel doping effectively modulates the electronic structure of tungsten, leading to an increased proportion of low‐valence W⁴⁺ species. The Ni‐WO_x/NF electrode demonstrates exceptional electrocatalytic performance, achieving a high Faradaic efficiency of 95.1% for formic acid at 1.4 V versus RHE with excellent operational stability over 18 h. In situ spectroscopic studies reveal that the enhanced selectivity originates from the facilitated desorption of formic acid at the W⁴⁺‐enriched interface, which effectively suppresses its over‐oxidation. Taking a step forward, an integrated electrocatalytic system coupling the anodic GOR with the cathodic nitrate reduction reaction (NO₃ ⁻RR) is constructed. This system enables the simultaneous coproduction of FA and ammonia (NH₃), achieving a high FE for FA of 96.9% along with sustained stability over 12 h, demonstrating significant potential for practical applications. This work demonstrates a sustainable catalytic system for selective glycerol oxidation by precisely regulating the reaction pathways of key intermediates.

Author Contributions

Hua Wang and Shengbo Zhang: conceptualized and guided this work. Hua Wang, Shengbo Zhang, and Mei Li: designed the experiments. Lang Chen and Shen Yan: performed the experiments. Hua Wang, Shengbo Zhang, Mei Li, Lang Chen and Shen Yan: wrote the paper. All the authors participated in the data analysis and commented on the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Supporting information

Supporting File: smll74650‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.