Octahedral–Tetrahedral Site Synergism in Spinel Oxides Enables 1700 h‐Stable Electrosynthesis of 2,5‐Furandicarboxylic Acid in Near‐Neutral Media

ABSTRACT

The electrosynthesis of 2,5‐furandicarboxylic acid (FDCA) from 5‐hydroxymethylfurfural (HMF) in near‐neutral media offers a sustainable alternative to alkaline routes by suppressing HMF degradation and thus mitigating carbon loss. However, it remains challenging to achieve both enhanced activity and long‐term durability for the deep oxidation of HMF intermediates into FDCA in a neutral medium. Herein, we report a Ru/Ni─co‐modified spinel cobalt oxide catalyst, Ru─CoNiOx, prepared through a scalable strategy. Structural analyses reveal that atomically dispersed Ru species preferentially incorporated into distorted octahedral Co coordination environment, while Ni species are mainly stabilized at tetrahedral Co sites, forming asymmetric [Ru_oct─O₆] and [Ni_tet─O₄] coordination motifs within the Co₃O₄ lattice. This tailored coordination environment promotes surface hydroxylation and the transformation of Co(Ni)─OH_ads/Co(Ni)─OOH, while strengthening the interfacial interaction with HMF. In near‐neutral KHCO₃ electrolyte, Ru─CoNiOx achieves approximately 91.0% FDCA Faradaic efficiency and 88.2% FDCA yield at 60°C, while showing a high carbon balance of 99.4%. Furthermore, a continuously HMFOR || HER membrane electrode assembly sustains FDCA production at 200 mA cm⁻² for over 1700 h at approximately 1.9 V. This work demonstrates an effective strategy for asymmetric coordination regulation in spinel oxides and highlights the promise of near‐neutral electrocatalysis for coupled FDCA and H₂ production.

Asymmetric Ru_oct/Ni_tet coordination in spinel Co₃O₄ enables cooperative surface hydroxylation and HMF activation, delivering efficient and exceptionally durable FDCA electrosynthesis in near‐neutral media and providing a catalyst design strategy for practical biomass electrooxidation.

1. Introduction

Confronting the dual challenges of global plastic pollution and fossil resource depletion, the development of renewable and biodegradable plastic alternatives has emerged as a critical pathway toward sustainable progress. Polyethylene furanoate (PEF), a 100% renewable polyester synthesized from 2,5‐furanedicarboxylic acid (FDCA, annual potential ∼1.8 million tons) and ethylene glycol, making it a promising substitute for petroleum‐derived polyethylene terephthalate (PET, annual production >80 million tons) [1, 2, 3, 4, 5]. The widespread use of FDCA, thus holds potential to shift plastic production from a petroleum‐dependent linear model toward a sustainable paradigm [6]. Conventionally, FDCA is produced through liquid‐phase oxidation of biomass‐derived 5‐hydroxymethylfurfural (HMF) under harsh conditions (80‐150°C, 0.3–4.0 MPa) [7, 8]. In contrast, electrochemical HMF oxidation reaction (HMFOR), powered by renewable energy sources such as solar and wind, offers an efficient and sustainable route for producing FDCA [9, 10].

However, scaling up the electrochemical HMFOR process remains challenging because most highly active systems rely on strongly alkaline electrolytes, such as 1 M KOH. Although an abundance of OH⁻ in an alkaline environment can accelerate HMF oxidation and enable high FDCA selectivity, such conditions also encourage the degradation and polymerization of HMF into humins, particularly at high concentrations of HMF, leading to poor carbon balance (Figures 1a and S1a) [11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]. Conversely, near‐neutral media (e.g., 1 M KHCO₃) effectively suppress humin formation under the same conditions (Figure S1b). However, the lack of strong nucleophilic hydroxyl species in near‐neutral media slows electrooxidation compared to alkaline systems, making it hard to oxidize HMF intermediates into FDCA [22, 23, 24, 25]. So, it's vital to develop effective HMFOR catalysts that can stimulate surface hydroxylation and deep oxidation to FDCA under near‐neutral conditions.

FIGURE 1.

HMF electrooxidation in (a) strong alkaline and (b) near‐neutral media (pH ∼ 8.3) is reported in this work. (c) A comparison of the electrocatalytic HMFOR of our catalyst in this work and of previous reported catalysts in both a three‐electrode system and a flow cell system. Detailed reaction parameters are provided in Tables S7 and S8. (d) Techno‐economic analysis of the production of FDCA in strong alkaline (KOH, pH = 14) and near‐neutral (KHCO₃, pH = 8.3) electrolytes. Smooth and striped rods represent values for the production cost of FDCA in strong alkaline media and near‐neutral media, respectively. Assuming that the cell voltage remains constant at 1.90 V, the FEs of FDCA are 90% in both strong alkaline and near‐neutral media.

Spinel oxides featuring both tetrahedral (tet) sites and octahedral (oct) sites—such as cobalt oxide (Co₃O₄) with Co_tet ²⁺ and Co_oct ³⁺ sites—are promising HMFOR catalysts owing to their tunable electronic structures and abundant active sites [26, 27, 28, 29]. Substituting tetrahedral or octahedral sites with heteroatoms are an attractive design strategy for enhancing spinel oxides catalytic properties. Particularly, doping with noble metals (Ru³⁺ [30], Ir³⁺ [31], etc.) or transition metals (Cu²⁺ [32, 33], Ni²⁺ [30, 34, 35], etc.) can regulate the electronic structure, promote surface self‐reconstruction evolutions and optimize the co‐adsorption of HMF and OH⁻ on catalyst surface [23, 36], thereby accelerating the intermediate formation and promote electrooxidation of HMF. For instance, the significant enhancement of OH⁻ adsorption by single‐atom Ru at the octahedral sites of Ru─Co₃O₄ promotes the subsequent co‐adsorption of HMF and OH⁻, resulting in improved HMFOR activity [37]. Nevertheless, for near‐neutral HMFOR, the rational incorporation of different metal atoms into octahedral and tetrahedral sites, as well as the roles of these in regulating surface reconstruction, HMF adsorption and catalyst durability, remain insufficiently understood.

Herein, we propose a dual‐site regulation strategy to construct Ru─CoNiOx, in which Ru species are preferentially incorporated into octahedral Co sites, while Ni species are mainly stabilized at tetrahedral Co sites within the spinel Co₃O₄ lattice (Figures 1b and S2). The asymmetric Ru_oct/Ni_tet coordination environment synergistically promotes interfacial hydroxylation, Co(Ni)─OH_ads/Co(Ni)─OOH redox chemistry, and HMF‐related interfacial interactions, while Ni_tet incorporation helps stabilize the Co₃O₄ and atomically dispersed Ru species during HMFOR. As a result, the designed Ru─CoNiOx catalyst exhibits superior electrocatalytic performance in near‐neutral 1 M KHCO₃ electrolyte (pH 8.31), reaching a high current density of 200 mA cm⁻² at 1.64 V (versus RHE) and a Faradaic efficiency (FE) of 99.7% for 5‐formyl‐2‐furoic acid (FFCA) at 1.39 V (versus RHE) at room‐temperature. The dominant FFCA formation at room temperature indicates that FFCA‐to‐FDCA oxidation is kinetically limited in near‐neutral media. By elevating the reaction temperature to 60°C, this sluggish deep‐oxidation step is significantly accelerated, achieving an FDCA FE of ∼91.0% and an FDCA yield of ∼88.2%. The system exhibits robust stability in near‐neutral media and can operate continuously for 130 h at 100 mA cm⁻² in 0.2 M HMF, with a FE_FDCA of around 90%, ranking among the most durable and selective three‐electrode HMFOR systems reported to date (Figure 1c) [23, 24, 25, 37, 38, 39, 40]. Furthermore, a continuously membrane electrode assembly (MEA) employing Ru─CoNiOx demonstrated high FDCA space‐time yield (up to 388.8 mg cm⁻² h⁻¹ at 500 mA cm⁻²) and remarkable stability by maintaining ∼1.9 V at 200 mA cm⁻² for 1700 h in neutral electrolyte, comparing favorably with previously reported alcohol oxidation‐coupled electrolyzers, even though many of which operate in strongly alkaline media (Figure 1c) [18, 24, 41, 42, 43, 44, 45]. A preliminary techno‐economic analysis (TEA) further suggests that the KHCO₃‐based near‐neutral electrocatalytic route has a potential economic advantage for FDCA production compared with strongly alkaline processes (Figure 1d).

2. Results and Discussion

2.1. Synthesis and Structural Characterization of Ru─CoNiOx

Herein, Ru and Ni atoms were chosen as targeted dopants in Co₃O₄ to tailor the electrochemical HMFOR activity and stability. Specifically, Ru species are expected to facilitate interfacial water dissociation and surface hydroxylation under near‐neutral conditions, thereby increasing the availability of reactive hydroxylated species for HMFOR. Concurrently, the incorporation of Ni atoms could enhance the long‐term structural stability of the Co₃O₄ matrix, owing to their similar ionic radii (Ni²⁺ (0.69 Å); Co²⁺ (0.72 Å)) and the formation of stronger metal‐oxygen bonds (Ni─O bond (389 kJ/mol); Co─O bond (368 kJ/mol)) [30]. The single atom Ru─doped CoNiOx (denoted as Ru─CoNiOx), Ni doped Co₃O₄ (Ni─Co₃O₄), and Ru─doped Co₃O₄ (denoted as Ru─Co₃O₄) were synthesized via a scalable preparation method (Figure S3). Typically, the NiCo‐LDH or Co(OH)₂ nanosheets were first electrodeposited on nickel foam (NF). Next, NiCo‐LDH was then calcined in a muffle furnace to form Ni doped Co₃O₄ (Ni─Co₃O₄). Then, the final catalysts with single‐atom Ru were prepared via immersing NiCo‐LDH or Co(OH)₂ into a Ru³⁺ solution and followed by annealing treatment.

The XRD patterns of Ru─CoNiOx and Ru─Co₃O₄, as presented in Figure 2a, are in close agreement with the standard pattern of spinel Co₃O₄ (PDF#42–1467). The characteristic peak attributed to the (311) crystal plane in Ru─CoNiOx shifts slightly compared to Ru─Co₃O₄, indicating that Ni doping causes the occurrence of lattice distortion in Ru─CoNiOx [46]. The morphology of the Co₃O₄ was largely preserved upon the incorporation of Ni and Ru atoms, as evidenced by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (Figures 2b and S4, S5), which show that the nanosheets are vertically and uniformly on the NF. High‐resolution transmission electron microscopy (HR‐TEM) analysis further revealed a lattice spacing of 0.29 nm, corresponding to the (220) plane of Co₃O₄ (Figure S6). In the AC HAADF‐STEM images (Figure 2c,d), Ru species were observed to be dispersed as single atoms (highlighted by yellow circles), with these Ru single atoms exhibiting higher intensity compared to surrounding atoms (Figure 2c). In addition, a clear distinction between the atomic strengths of Ru and Co/Ni distinct atomic intensities shown in Figure 2d along the dashed rectangles, also provides strong evidence, which indicates that Ru has entered the Co₃O₄. The observation of two 0.29 nm lattice fringes intersecting at 60° in Figure 2e is characteristic of the (220) and (202) planes in Co₃O₄. The identification of these crystallographic planes is supported by the fast Fourier transform (FFT) analysis (Figure 2e inset). Energy‐dispersive spectroscopy (EDS) elemental mapping of Ru─CoNiOx further substantiated the successful incorporation of Ni and Ru into the structure (Figures 2f and S7, S8). The bulk contents of Ru, Ni, and Co in Ru─CoNiOx were determined by inductively coupled plasma optical emission spectroscopy (ICP‐OES) to be 0.19, 5.38, and 8.85 wt%, respectively (Table S1).

FIGURE 2.

(a) XRD patterns; (b) TEM image; (c, d) The spherical‐aberration correction high‐angle annular dark field‐scanning transmission electron microscopy (AC HAADF‐STEM) image of Ru─CoNiOx; the Ru single atom is highlighted by a yellow dotted circle, with the insets showing a magnified view of area 1 and the corresponding intensity profile along the green dashed rectangle. (e) Magnified image of area 2 (from d) and the selected area electron diffraction (SAED) pattern of Ru─CoNiOx. (f) Corresponding HAADF‐STEM image and elemental distribution mapping for Ru─CoNiOx; (g) Raman spectra of Ru─CoNiOx, Ru─Co₃O₄, and Co₃O₄. (h) Co 2p XPS and (i) O 1s XPS spectra of Ru─CoNiOx, Ru─Co₃O₄, Ni─Co₃O₄, and Co₃O₄.

The crystal structure of samples was investigated by Raman spectra (Figure 2g). Compared with Co₃O₄, the Raman spectrum of Ru─doped Co₃O₄ exhibits a redshift in its characteristic bands of A₁g mode, while the F₂g mode remains nearly unchanged. Since the A₁g peak position is highly sensitive to the oxygen environment in octahedral coordination [CoO₆], whereas the F₂g peak depends on tetrahedral sites [CoO₄] [47], this result suggests that Ru incorporation mainly perturbs the octahedral Co‐related coordination environment. Similarly, the A_1g peak position of Ru─CoNiOx is close to that of Ru─Co₃O₄, whereas the peak associated with tetrahedral Co²⁺ sites exhibits a slight shift after Ni incorporation. This indicates that Ni introduction primarily affects the tetrahedral Co‐related environment.

To evaluate the scalability of the proposed synthesis strategy, Ru─CoNiOx was further fabricated on a 100‐fold enlarged substrate using the optimized preparation conditions. The enlarged electrode exhibits a homogeneous appearance over the entire substrate, and XRD, SEM, and electrochemical measurements collected from five representative positions (A–E) reveal nearly identical crystal structures, nanosheet morphologies, and HMFOR activities (Figures S9–S11a), confirming excellent structural and electrochemical uniformity after scale‐up. Furthermore, enlarged electrodes prepared under different electrodeposition times (1000‐2000 s), annealing temperatures (300–400°C), and Ru immersion times (5–10 min) show only minor variations in HMFOR performance, with the potential required to reach 100 mA cm⁻² ranging from 1.49 to 1.52 V versus RHE (Figure S11b), demonstrating good process robustness and fabrication tolerance.

The surface chemical valence states for all elements were investigated by XPS. The survey spectra showed the presence of Ni, Ru, Co, and O elements (Figure S12). The high‐resolution Ni 2p XPS spectrum of the catalysts reveals the coexistence of Ni²⁺ and Ni³⁺ species, characterized by peaks at approximately 854.1 and 856.2 eV, respectively [30, 46], along with their satellite features (Figure S13). Compared to Ni─Co₃O₄, the characteristic Ni 2p peaks of Ru─CoNiOx are shifted to higher binding energies, indicating the electronic interaction between Ru and the Ni [48]. Figure S14 shows that the Ru 3p XPS of Ru─CoNiOx reveals the formation of Ru⁴⁺, Ru³⁺, and Ru satellite peaks at 461.1, 463.6, and 466.3 eV [49].

In the XPS spectrum of Co 2p (Figure 2h), the deconvoluted peaks at binding energies of approximately 779.6  and 781.6 eV are assigned to Co³⁺ and Co²⁺, respectively, and the peaks centered at 786.8 eV are attributed to the satellite peaks [47]. Notably, The Co²⁺/Co³⁺ fractions in these catalysts are as follows: Ru─CoNiOx > Ru─Co₃O₄> Co₃O₄> Ni─Co₃O₄ (Table S2). The Ni─Co₃O₄ sample displays a decreased Co²⁺/Co³⁺ ratio, indicating that Ni incorporation affects the population of tetrahedral Co²⁺ sites. Conversely, Ru─Co₃O₄ presents an elevated Co²⁺/Co³⁺ ratio, which implies that Ru incorporation influences the coordinated Co³⁺ environment. For Ru─CoNiOx, the higher Co²⁺/Co³⁺ ratio indicates a modified surface Co valence distribution, consistent with the coexistence of Ru and Ni, induced lattice distortion and charge redistribution. The O 1s spectra of samples can be deconvoluted into three sub‐bands: Lattice oxygen (M‐O), coordinative oxygen vacancy (O_v), and surface adsorbed water molecules (H₂O), which located at ≈529.5, 531.1, and 532.5 eV, respectively (Figure 2i) [50]. Ru/Ni co‐doping markedly increases the Ov‐related component, indicating the formation of abundant oxygen vacancies and a modified local coordination environment (Table S3). To distinguish the role of oxygen vacancies from the Ru/Ni dual‐site effect, oxygen‐vacancy‐enriched control samples were prepared by mild NaBH₄ treatment. Despite exhibiting comparable or even higher Ov‐related components than Ru─CoNiOx, these control samples show inferior HMFOR activities (Figure S15), demonstrating that oxygen vacancies alone are insufficient to account for the superior catalytic performance. Therefore, the enhanced activity is attributed to the synergistic coupling of oxygen vacancies with the asymmetric Ru_oct/Ni_tet coordination motifs.

2.2. Identification of Ru Octahedral Sites and Ni Tetrahedral Sites via XAS Analyses

To gain deeper insights into the local structure of spinel Co₃O₄ with Ru and Ni doping, X‐ray absorption near‐edge structure (XANES) analysis was performed. As shown in Figure 3a, in comparison to the Co foil reference (Co⁰), all four of Ru─CoNiOx, Ru─Co₃O₄, Co₃O₄, and Ni─Co₃O₄ exhibit absorption edges shifting toward higher energy levels. The inset of Figure 3a shows a slight decrease in the adsorption edge energy of Ru─CoNiOx compared to Ru─Co₃O₄, Co₃O₄, and Ni─Co₃O₄, suggesting a lower average valence state of Co in Ru─CoNiOx, which is in line with the XPS results. The Co K‐edge FT‐EXAFS spectra and the fitting results were also conducted (Figures 3b,c, S16, S17, and Table S4). Ru─Co₃O₄ exhibits a profile similar to that of Co₃O₄, featuring three characteristic peaks at 1.48, 2.45, and 3.00 Å that correspond to Co‐O, Co_oct‐Co_oct, and Co_oct/tet‐Co_tet. As shown in Figure 3b, the Co─O coordination peak at 1.48 Å in Ru─Co₃O₄ shifts negatively to 1.45 Å in Ru─CoNiOx, suggesting the presence of local compressive strain on the Co─O bonds. In addition, the structural disorder induced by Ni was revealed by the decrease in the Co_oct/tet‐Co_tet peak intensities of Ru─CoNiOx in the FT‐EXAFS curves, suggesting the successful substitution of Ni for Co at the tetrahedral sites of Co₃O₄. The wavelet transform (WT) signals in the WT EXAFS spectra are shown in Figure S18. Similarly, no obvious Co_oct/tet‐Co_tet signal is observed for Ru─CoNiOx compared to Ru─Co₃O₄, indicating weakened scattering of tetrahedrally coordinated Co and increased local disorder after Ni incorporation. These results indicate that the local coordination environment around the Co site changes due to the doping of Ni and Ru, thus prompting further investigation into their bonding configurations.

FIGURE 3.

(a) Co K‐edge XANES spectra and (b) the corresponding Fourier‐transformed (FT) EXAFS spectra of Ru─CoNiOx, with Ru─Co₃O₄, Ni─Co₃O₄, and Co foil as references. (c) FT‐EXAFS fitting curves for the Co K‐edge in R‐space. (d) Ru K‐edge XANES spectra and (e) corresponding FT‐EXAFS spectra of Ru─CoNiO_x, compared with Ru─Co₃O₄, RuO₂, and Ru foil. (f) Wavelet transform of the k ²‐weighted EXAFS signals for Ru─CoNiOx, RuO₂, and Ru foil. (g) Ni K‐edge XANES spectra of Ru─CoNiO_x (Inset: Derived average Ni oxidation state) and (h) corresponding FT‐EXAFS spectra, with Ni₂O₃, NiO, and Ni foil as references. (i) Wavelet transform of the k ³‐weighted EXAFS signals for Ru─CoNiOx, NiO, and Ni foil.

The oxidation states of Ru in Ru─CoNiOx and Ru─Co₃O₄ were further characterized using Ru K‐edge XANES (Figure 3d). The absorption edges of Ru─CoNiOx and Ru─Co₃O₄ were found to be higher than those of Ru foil, but lower than RuO₂, indicating the oxidation state of Ru atom was between 0 and +4. This finding can also be supported by XPS analyses. The Ru K‐edge FT‐EXAFS shows that the bond lengths for Ru─O bonds in Ru─CoNiOx and Ru─Co₃O₄ (2.01 Å, Figure 3e and Table S5) are slightly longer than those of Ru─O bonds in RuO₂ (1.96 Å). The variety of Ru─O bond lengths and lattice distortions are indicative of a manipulated local structure, which breaks the structural symmetry of the Co₃O₄. In addition, the lack of Ru─Ru bond at ∼2.36 Å excludes Ru nanoparticles or nanoclusters, further confirming the atomically dispersed Ru species. Additionally, the FT peaks at 2.50 and 2.98 Å in the Ru FT‐EXAFS may correspond to Ru_oct─O─Co_oct and Ru_oct─O─Ni_tet coordination, respectively. The EXAFS fitting results show that the coordination numbers of the Ru─O shell in both Ru─CoNiOx and Ru─Co₃O₄ were 5.0 ± 0.2 and 5.2 ± 0.5 (Figure S19, Table S5). These values are lower than the ideal octahedral coordination number of six, indicating that Ru is located in a distorted and undercoordinated Ru_5/6 octahedral‐like environment. Such a reduced Ru─O coordination number may originate from local oxygen vacancies, lattice distortion, and/or surface unsaturated coordination. Furthermore, it was found that the atomic distance between Ru_oct─O─Co_oct in Ru─CoNiOx (2.96 Å) is very close to that of Co_oct─Co_oct in Co₃O₄ (2.88 Å), but different from Co_tet─Co_tet (3.39 Å). Meantime, the atomic distance between Ru_oct─O─Ni_tet in Ru─CoNiOx (3.54 Å) is close to that of Co_tet─Co_tet in Co₃O₄ (3.39 Å) (Tables S4, S5). These results strongly corroborate the Raman spectroscopy findings, supporting the preferential incorporation of Ru and Ni into octahedral and tetrahedral Co‐related coordination environments, respectively. WT analysis shows that the absence of Ru─Ru scattering and the dominant Ru─O at approximately 4.0 Å⁻¹. These results exclude detectable Ru─rich nanoparticles or clusters and support the preferential incorporation of atomically dispersed Ru species into distorted octahedral Co‐related coordination environments (Figures 3f and S20).

As shown in Figure 3g, the Ni K‐edge absorption position of Ru─CoNiOx is higher than that of Ni foil, but nearly identical to NiO, the further fitting results indicate an average Ni oxidation state of +2.3 in the Ru─CoNiOx (inset in Figure 3g). Consequently, the Ni has an oxidation state close to +2, while the tetrahedral assignment of Ni is mainly based on the local coordination evidence from EXAFS. The EXAFS fitting results show that, the Ni─O coordination number in Ru─CoNiOx is approximately 4.3, which is much lower than that of commercial NiO (approximately 6.5) and is closer to the ideal tetrahedral Ni─O₄ coordination than to octahedral Ni─O₆ coordination (Figure S21 and Table S6). The slight deviation from the ideal value of four may arise from EXAFS fitting uncertainty, local lattice distortion, and minor contributions from surface hydroxylated or absorbed oxygen species. Moreover, the k ³‐weighted χ(k) oscillation in the pre‐edge region of Ru─CoNiOx differs markedly from that of NiO (Figure S22), reflecting a distinct atomic arrangement around Ni. The Ni K‐edge FT k ³‐weighted extended EXAFS spectra are shown in Figure 3h, in contrast to the Ni─Ni distance (Ni─O─Ni) of 2.57 Å in NiO, the Ru─CoNiOx present representative peaks at 1.56 Å of Ni─O bond and 2.45 Å of Ni─Co/Ru (Ni─O─Co/Ru) coordination. In addition, a weak FT peak at 3.00 Å in the Ni FT‐EXAFS may indicate Ru─O─Ni coordination, while the characteristic metallic Ni─Ni scattering peak (∼2.18 Å) is absent. In theWT EXAFS spectra (Figure 3i), Ru─CoNiOx exhibits a strong WT maximum at 4.90 Å⁻¹, which clearly differs from the signal of commercial NiO (6.86 Å⁻¹). These findings exclude a dominant NiO‐like octahedral phase and support that Ni is mainly incorporated into tetrahedral Co‐related sites as Ni─O₄ units.

2.3. Electrocatalytic Performance

Electrochemical measurements for both HMFOR and OER were conducted in a near‐neutral KHCO₃ electrolyte (pH 8.3) using a three‐electrode H‐type cell. As shown in Figures 4a and S23, Ru─CoNiOx requires a potential of 1.74 V (versus RHE) to reach 100 mA cm⁻² for OER. After adding 50 mM HMF, the potential dropped sharply by 250 mV. O₂ evolution was further quantified by gas chromatography to evaluate the HMFOR/OER competition. Compared with pure KHCO₃ electrolyte, O₂ evolution was markedly suppressed after adding HMF, while the FFCA partial current density dominated over the OER partial current density under HMFOR conditions (Figure S24). These results confirm that Ru─CoNiOx preferentially promotes HMF oxidation rather than OER in a near‐neutral electrolyte. The Ru─CoNiOx exhibited the highest activity, reaching 100 mA cm⁻² at only 1.49 V versus RHE (Figure 4b), outperforming Ni─Co₃O₄, Ru─Co₃O₄, and Co₃O₄, which required 1.59, 1.60, and 1.66 V versus RHE, respectively. These performance trends suggest that the Ru/Ni co‐modified Co₃O₄, involving distorted [Ru_oct─O₆] and [Ni_tet─O₄] coordination motifs together with oxygen vacancies, synergistically contributes to the enhanced near‐neutral HMFOR activity. The Nyquist plots in Figure S25 showed that Ru─CoNiOx had significantly lower charge transfer resistance (Rct) than that of Ni─Co₃O₄, thus validating the relatively better electronic conductivity and faster charge transfer. To examine the near‐neutral HMFOR selectivity, we then performed electrolysis at 100 mA cm⁻² in 1 M KHCO₃ with 50 mM HMF using Ru─CoNiOx catalyst (Figure S26a). After 10, 000 s of electrolysis, the product distributions of HMF detected by ¹H NMR over Ru─CoNiOx at 100 mA cm⁻² are shown in Figure S26b, the FDCA FE was only approximately 7.4%, much lower than the FFCA FE of approximately 89.0%, indicating that the further oxidation of FFCA to FDCA is kinetically sluggish under near‐neutral conditions [25, 51]. At an applied current density of 200 mA cm⁻², Ru─CoNiOx delivered a partial current density of 182 mA cm⁻² for FFCA production with a corresponding Faradaic efficiency (FE) of 91.0% (Figure 4c). Moreover, Ru─CoNiOx showed the highest FFCA FE (99.7%) than Ru─Co₃O₄ (50.0%), indicating that Ni introduction may promote the further oxidation of HMF to FFCA. This high Faradaic efficiency of FFCA could be achieved over a wide range of current densities from 50 to 200 mA cm⁻² (Figure S27).

FIGURE 4.

(a) LSV curves of Ru─CoNiOx in 1 M KHCO₃ with and without 50 mM HMF (scan rate: 5 mV s⁻¹). (b) Comparative LSV curves of various catalysts in HMF‐containing electrolyte. (c) Partial current density and FE for FFCA over Ru─CoNiOx at different applied current densities. (d) LSV curves for HMFOR at 25 and 60°C. (e) The yield changes of HMF oxidation products during electrolysis at 100 mA cm⁻² at 60°C. (f) Long‐term chronopotentiometry of various catalysts at 100 mA cm⁻² at 60°C. (g) Stability comparison of Ru─CoNiOx with recently reported catalysts in neutral/near‐neutral media.

Considering the economic value of HMF oxidation products, FDCA is regarded as the most attractive product. Direct FFCA oxidation tests show that the FFCA oxidation current density enhanced at 60°C compared with room temperature, the FDCA Faradaic efficiency increases from approximately 30.7% at 25°C to approximately 85.9% at 60°C, confirming that elevated temperature accelerates the kinetically sluggish FFCA‐to‐FDCA step in near‐neutral KHCO₃ electrolyte (Figure S28) [25]. In addition, temperature‐dependent HMFOR tests of Ru─CoNiOx show that the oxidation current density increases progressively from 25 to 60°C (Figures S29, S30). At a current density of 200 mA cm⁻², the potential required for HMF oxidation decreased from 1.64 V versus RHE at room temperature to 1.46 V versus RHE at 60°C (Figure 4d). These results suggest that elevated temperature effectively accelerates the kinetically limited FFCA‐to‐FDCA oxidation step, thereby enabling efficient FDCA production.

Accordingly, HMF electrolysis was performed at 60°C with three successive additions of 50 mM HMF at a current density of 100 mA cm⁻². Under these conditions, Ru─CoNiOx achieved an FDCA FE of approximately 91.0% and an FDCA yield of approximately 88.2%, as quantified by ¹H NMR analysis (Figures 4e, S31, S32). During electrolysis for 10 h at 60°C, FDCA gradually accumulates while 2,5‐diformylfuran (DFF), 5‐hydroxymethyl‐2‐furancarboxylic acid (HMFCA), and FFCA remain intermediates, and the carbon balance remained close to ∼99%, indicating negligible carbon loss during the reaction (Figure S33a). The more pronounced accumulation of DFF than HMFCA suggests that HMFOR over Ru─CoNiOx in KHCO₃ mainly proceeds through the DFF pathway. Meanwhile, the gradual lightening of the electrolyte color, the nearly unchanged pH, and conductivity further indicate that no obvious electrolyte degradation, severe volatilization, or HMF polymerization occurs at 60°C (Figure S33b,c). Long‐term chronopotentiometry (CP) tests were further conducted to evaluate the operational stability of Ru─CoNiOx for HMFOR at 60°C. During 90 h of electrolysis, Ru─CoNiOx retained 91.0% of its initial performance, whereas Ru─Co₃O₄ retained 85.8% under the same conditions, both of which were higher than commercial RuO₂ (Figure 4f). The superior stability of Ru─CoNiOx suggests that Ni incorporation plays a beneficial role in maintaining the structural integrity of the Ru─Co₃O₄ catalyst during HMFOR. ICP‐MS analysis further confirms the enhanced structural stability of Ru─CoNiOx during HMFOR. Compared with Ru─Co₃O₄ and RuO₂, Ru─CoNiOx exhibits significantly suppressed Ru dissolution throughout the electrolysis process. In addition, the dissolved Ni concentration gradually increases from 0.25 to 0.99 ppm after 20–90 h electrolysis (Figure S34), indicating only limited Ni dissolution during prolonged operation. The stability of Ru─CoNiOx was evaluated in 0.2 M HMF at 60°C. Ru─CoNiOx maintained stable operation at 100 mA cm⁻² for 130 h, with an average potential of approximately 1.5 V versus RHE and an FDCA FE of around 90% (Figure S35), demonstrating its robust FDCA selectivity under prolonged operation at increased HMF concentration. Considering its activity, FDCA selectivity, durability, and tolerance to relatively high HMF concentration, Ru─CoNiOx ranks among the most competitive HMFOR electrocatalysts reported under neutral conditions in a three‐electrode system (Figure 4g and Table S7) [23, 24, 25, 36, 37, 38, 45]. These results demonstrate the potential of Ru─CoNiOx for efficient and stable FDCA electrosynthesis from HMF in near‐neutral electrolytes.

The compositional and structural changes of Ru─CoNiOx following HMFOR electrolysis were also investigated. Post‐electrolysis XRD, SEM, HRTEM, XPS survey spectra, and elemental mapping results confirm that Ru─CoNiOx largely preserves its spinel phase, nanosheet morphology, and elemental distribution after long‐term HMFOR (Figures S36–S39). EXAFS also shows that Ru does not aggregate after electrolysis (Figure S40). Nevertheless, the XPS peaks of Co³⁺ and Ni³⁺ increased after long‐term electrolysis, demonstrating that the coordination environment as well as the valence of the sites change, which further confirms that abundant Co³⁺ and Ni³⁺ likely participated in the oxidative (Figure S41).

2.4. Experimental Analysis of Neutral HMFOR Mechanism

To clarify the electrocatalytic HMFOR mechanism in neutral media, electrochemical measurement technologies and in situ Raman spectroscopy were performed. As shown in the cyclic voltammetry (CV) curves of Figure 5a, the oxidation current ranges from 1.40 to 1.50 V versus RHE can be assigned to Ni²⁺/Co²⁺ oxidation to active Ni³⁺/Co³⁺ species. In addition, the Ru─CoNiOx and Ni─Co₃O₄ exhibit a Co_Td ²⁺ pre‐oxidation range of 1.25–1.40 V versus RHE, which is greater than that of Ru─Co₃O₄. This result suggests that the incorporation of Ni facilitates the early‐stage oxidation/hydroxylation of Co_Td ²⁺ sites, thereby promoting the formation of surface hydroxylated or high‐valence Co/Ni intermediates that are relevant to subsequent HMFOR catalysis [23]. After the introduction of 50 mM HMF, the anodic current densities of Ru─CoNiOx and Ru─Co₃O₄ increased rapidly, while the corresponding redox features were markedly attenuated or disappeared (Figures S42, S43). A similar but less pronounced phenomenon was observed for Ni─Co₃O₄ (Figure S44). These results indicate that HMF participates in the surface redox process and rapidly consumes the electro‐generated high‐valence Co/Ni species during HMFOR. In comparison with Ru─Co₃O₄ and Ni─Co₃O₄, Ru─CoNiOx exhibited much higher HMFOR activity, whereas the enhancement in OER activity was relatively limited (Figure S45). This contrast suggests that the superior activity of Ru─CoNiOx originates not merely from high‐valence Co/Ni species, but from a more favorable coupling between surface redox chemistry and HMF oxidation.

FIGURE 5.

(a) CV curves of Ru─CoNiOx, Ni─Co₃O₄, Ru─Co₃O₄, and RuO₂ in 1 M KHCO_3. (b) Open circuit potentials (OCPs) of Ru─CoNiOx, Ni─Co₃O₄, and Ru─Co₃O₄ in 1 M KHCO₃ before and after 0.05 M HMF was added. CV curves of Ru─CoNiOx, Ni─Co₃O₄, Ru─Co₃O₄ in (c) 1.0 M KHCO₃, (d) 1.0 M KHCO₃ + 0.05 M HMF solution. Bode plots of (e) Ni─Co₃O₄ and (f) Ru─CoNiOx in 1 M KHCO₃ with 0.05 M HMF at various potentials. In situ Raman spectra of Ru─CoNiOx at various potentials in (g) 1 M KHCO₃ and (h) 1 M KHCO₃ with 0.05 M HMF. (i) Proposed mechanism of FFCA synthesis from the electrochemical oxidation of HMF over Ru─CoNiOx.

The interfacial interaction between HMF and the catalysts was further evaluated by open‐circuit potential (OCP) measurements. As shown in Figure 5b, after adding 50 mM HMF into 1 M KHCO₃, Ru─CoNiOx displayed a larger OCP decrease than Ru─Co₃O₄ and Ni─Co₃O₄. The more pronounced OCP shift suggests stronger interfacial interaction between HMF and Ru─CoNiOx, implying that Ru/Ni co‐modification favors HMF adsorption or accumulation at the catalyst/electrolyte interface.

To further investigate the formation of surface hydroxyl species, CV measurements were performed in a 1 M KHCO₃ solution without HMF. Within the potential range of 0.8–1.05 V versus RHE, a distinct oxidation current was observed for Ru─CoNiOx, which was attributed to the formation of electrophilic adsorbed hydroxyl species, denoted as M─OH_ads or *OH [13]. This result indicates that the synergy between Ru and Ni promotes the generation of species associated with surface M─OH_ads (Figure 5c). Following the introduction of HMF, the feature was suppressed more significantly on Ru─CoNiOx than on Ru─Co₃O₄ and Ni─Co₃O₄ (Figure 5d), indicating that surface hydroxyl species rapidly participate in the oxidation reaction of HMF.

In situ electrochemical impedance spectroscopy (EIS) was then employed to investigate the interfacial kinetic behavior of the catalysts during HMFOR [40]. As shown in Figure 5e,f, the low‐frequency phase‐angle feature of Ru─CoNiOx decreased markedly under HMFOR conditions, shifting toward a higher‐frequency region. This indicates accelerated interfacial charge transfer and faster surface intermediate conversion [52, 53]. Compared with Ni─Co₃O₄, Ru─CoNiOx exhibited a lower phase angle at lower applied potentials, suggesting that Ru modification facilitates HMF oxidation kinetics. By contrast, during OER in pure KHCO₃, the low‐frequency phase‐angle response only decreased at higher potentials, which indicates sluggish OER kinetics and less efficient surface reconstruction in the absence of HMF (Figure S46) [54]. The clear distinction between HMFOR and OER further indicates that HMF plays an active role in the surface redox process, accelerating the dynamic evolution of catalytically relevant Co/Ni hydroxylated or oxyhydroxide species.

As shown in Figure 5g,h, the potential‐dependent Raman spectroscopy further elucidates the reaction behavior at the electrode/electrolyte interface during HMFOR and OER processes. Obviously, when the potential reached 1.48 V (versus RHE), two strong peaks at 476 and 556 cm⁻¹ are characteristic of high‐valence Co/Ni oxyhydroxide‐like species, such as Co(Ni)─OOH of Ru─CoNiOx catalyst [55], suggesting surface oxidative restructuring on Ru─CoNiOx under OER process (Figure 5g). In contrast, in the HMF oxidation reaction, a single Raman band near 520 cm⁻¹ emerged at a much lower potential of 1.28 V versus RHE (Figure 5h). This band is tentatively assigned to Co(Ni)─OH_ads related surface hydroxyl species or oxyhydroxide precursors [56]. The earlier appearance of this band in the presence of HMF suggests that HMF promotes the generation and turnover of surface hydroxylated intermediates. When the potential exceeded 1.58 V versus RHE, the two characteristic bands associated with high‐valence Co/Ni oxyhydroxide‐like species were observed, indicating that the generation rate of Co(Ni)–OH_ads/Co(Ni)─OOH species exceeds their consumption by HMF at higher potentials, leading to their accumulation on the catalyst surface.

Further multipotential‐step measurements were conducted to verify the involvement of high‐valence Co/Ni species in HMFOR. As illustrated in Figure S47, the catalysts were initially oxidized at 1.68 V versus RHE to increase the concentration of high‐valence Co(Ni)─OOH species and were than subjected to a low reduction potential of 0.78 V versus RHE. The Ru─CoNiOx catalyst exhibited a larger cathodic response than the Ru─Co₃O₄ and Ni─Co₃O₄ catalysts, indicating that Ru/Ni co‐modification facilitates the formation of a larger amount of reducible high‐valence Co/Ni species. The response in the presence of HMF further suggests that these electro‐generated species are involved in the chemical oxidation of HMF, which is consistent with the in situ Raman results.

Based on these results, a redox‐mediated HMFOR mechanism over Ru─CoNiOx is proposed (Figure 5i). Under anodic polarization in near‐neutral KHCO₃, Ru/Ni co‐modification promotes the early‐stage surface hydroxylation and oxidation of Co/Ni sites, generating Co(Ni)─OH_ads and subsequently Co(Ni)‐OOH high‐valence species. These electrophilic surface species act as active oxidizing intermediates for HMF conversion. Meanwhile, Ru─CoNiOx exhibits stronger interfacial interaction with HMF, increasing the local availability of HMF molecules near the active surface. The electro‐generated Co(Ni)‐OOH species then oxidize HMF and its intermediates, while being reduced back to Co(Ni)–OH_ads related species, thereby completing the catalytic redox cycle. In this process, Ru facilitates the formation of surface hydroxylated species, whereas Ni promotes the oxidative reconstruction of Co/Ni sites and strengthens the interaction with HMF‐related intermediates, accelerating HMFOR in near‐neutral media.

The selective conversion pathway of HMF on Ru─CoNiOx was investigated by monitoring the evolution of both reactants and products in a 1 M KHCO₃ electrolyte containing 50 mM HMF (Figure S48). The Raman spectra of HMF solution with the main bands assigned are presented in Figure S49. Owing to the existence of hydroxyl and aldehyde functional groups on the furan ring of HMF, there is overlap with the characteristic functional groups of electrocatalytic products, making it challenging to distinguish HMF from the products. In HMF, the characteristic peak of C═C ring appears at 1523 cm⁻¹ [39]. Upon elevating the potential to 1.48 V versus RHE, a distinctive peak corresponding to DFF and FFCA emerges at 1510 cm⁻¹ [39, 57], signifying that HMF undergoes electrocatalysis to DFF through the oxidation of a hydroxyl group in the presence of 1 M KHCO₃. With further augmentation of the potential, the aldehyde group commences oxidation to FFCA. The blue shift of alcohol C─H and aldehyde C═O groups suggests an interaction between these groups and the HMF surface or even the formation of an organic intermediate [58].

2.5. Coupled Hydrogen Production in HMFOR || HER MEA System

In light of the outstanding electrocatalytic performance of Ru─CoNiOx, which demonstrated a high FE for FDCA production at 60°C, we constructed a membrane electrode assembly (MEA) to enable the continuous synthesis of FDCA. The MEA was assembled in 1 M KHCO₃ electrolyte with an initial HMF concentration of 0.2 M to evaluate its operational stability. For this purpose, Ru─CoNiOx served as the anode, commercial Pt/C as the cathode, and an anion exchange membrane was employed to complete the assembly (active electrode area: 4 cm²; Figure S50). The experiments utilized a large‐volume (2 L) electrolyte containing a high concentration of HMF (up to 200 mM). When HMF is added to the electrolyte, the total current for the HMFOR || HER pair significantly exceeds that of the OER || HER pair at 60°C (Figure 6a). Specifically, currents of 0.5, 1.0, and 2.0 A are achieved at low cell voltages of 1.93, 2.02, and 2.19 V, respectively. In contrast, the OER || HER system requires 2.47 V to reach 0.5 A. This performance indicates that the coupled HMFOR || HER system can reduce the electricity input by approximately 1.29 kWh/m³ of H₂ produced at a current of 0.5 A (see more details in Supporting Information). As demonstrated in Figure 6b, the Ru─CoNiOx demonstrated high Faradaic efficiency (FE_FDCA > 80%) across the current densities from 100 to 500 mA cm⁻². The space‐time yield of FDCA demonstrated an increase in line with current density (reaching 388.82 mg cm⁻² h⁻¹ at 500 mA cm⁻²). Furthermore, the FE_FDCA remained above 85% as the current density of 200 mA cm⁻² (Figure 6c), with no significant decline observed as operating time extended. Remarkably, under continuous conditions of 60°C, this HMFOR || HER system demonstrated remarkable stability at low cell voltages of ∼1.9 V at the current density of 200 mA cm⁻² for a duration exceeding 1700 h (Figure 6d). During this extended operation, HMF was continuously replenished to compensate for substrate consumption, thereby enabling sustained FDCA electrosynthesis. The stable cell voltage and maintained FDCA selectivity demonstrate the robust operational durability of Ru─CoNiOx under near‐neutral MEA conditions. Compared with recently reported alcohol oxidation‐coupled hydrogen production systems, many of which were conducted in concentrated alkaline electrolytes, the present HMFOR || HER MEA exhibits competitive stability and selectivity under near‐neutral conditions (Table S8) [18, 24, 41, 42, 43, 44, 45]. The electrolyte from the HMFOR || HER system was also subjected to testing. The yellow precipitate obtained by acidifying the electrolyte was analyzed via ¹H NMR spectra, revealing only the characteristic peaks of FDCA, consistent with the standard FDCA reference (Figure S51).

FIGURE 6.

(a) LSV curves of a 4 cm⁻² electrolyzer in 1 M KHCO₃ with and without 0.2 M HMF at 60°C (inset: Schematic of the anhydrous‐mode cathode in the batch‐type AEM electrolyzer). (b, c) FDCA space‐time yield and FE of the HMFOR || HER electrolyzer (b) at different current densities and (c) over time at 200 mA cm⁻². (d) Stability test of the electrolyzer at a stepped current density of 200 mA cm⁻² (electrolyte: 1 M KHCO₃ with 0.2 M HMF, 60°C). (e) Impact of FE and current density on FDCA production cost; the estimated cost using Ru─CoNiOx at a constant cell voltage of 1.90 V is compared with the conventional benchmark of 1490$ ton⁻¹ [42].

2.6. TEA

Based on the MEA performance, a preliminary techno‐economic analysis (TEA) was conducted to evaluate the economic feasibility for electrocatalytic FDCA production in neutral media (Figure S52). Specific TEA details are provided in the Supporting Information. We proposed the complete process for the preparation of FDCA by electrooxidation of HMF in the KHCO₃ systems (Figure S53) and KOH systems (Figure S54), respectively. As discussed in the literature [59], product recovery from alkaline media necessitates acidification, which inevitably generates inorganic salt by‐products. For the KHCO₃ system, the cost structure comprises capital, operating, and material expenses. The calculation of the levelized cost for FDCA production accounts for the sales of H₂, K₂SO₄, and K₂CO₃ (Figure S55). Using excess KOH and KHCO₃ produced large amounts of K₂SO₄ (KOH systems) and K₂CO₃ (KHCO₃ systems). The TEA results show that under same operation condition (current density of 200 mA cm⁻², cell voltage of 1.9 V, FE of 91%), the levelized cost for FDCA in the KHCO₃ system (∼$1162 ton_FDCA ⁻¹) was lower than that in KOH systems (∼$1561 ton_FDCA ⁻¹) (Figure 1d, see more details in Supporting Information), indicating a higher profit potential for the KHCO₃ system. Based on the above research, A further investigation was conducted into the effects of FE and current density on the electrosynthesis levelized cost of FDCA. We found that levelized cost is highly sensitive to the FE of the FDCA (see more details in Figure S56). In addition, considering that current density and FE are two important and correlated parameters of electrocatalysis, a sensitivity analysis was performed to reveal the effects of their variations on the levelized cost for FDCA in KHCO₃ and KOH systems (Figures 6e and S57). Importantly, for the KHCO₃ system, when the current density is greater than 100 mA cm⁻² and the FE exceeds 50%, the levelized cost for FDCA is generally lower than the production price of conventional FDCA (∼$1490 ton_FDCA ⁻¹) [42], further indicating its potential economic advantage. Benefiting from the high FDCA FE of Ru─CoNiOx, the experimentally achieved operating points remain in the economically favorable region over a current‐density range of 100–500 mA cm⁻², as marked by the round dots in Figure 6e. These results indicate that improving FDCA FE and maintaining high current density are essential for reducing the cost of near‐neutral FDCA electrosynthesis.

3. Conclusion

In conclusion, we developed Ru/Ni─co‐modified spinel cobalt oxide catalyst (Ru─CoNiOx) for efficient HMFOR in near‐neutral media. Structural analyses show Ru occupies octahedral Co sites, while Ni is mainly stabilized at tetrahedral Co sites, creating asymmetric coordination motifs with abundant oxygen vacancies. This tailored local structure promotes surface hydroxylation and improves the interfacial interaction with HMF. In a three‐electrode configuration, the Ru─CoNiOx electrocatalyst exhibits a partial current density of 182 mA cm⁻² when producing FFCA, with a corresponding FE of 91.0%. The sluggish FFCA‐to‐FDCA oxidation in near‐neutral media can be effectively accelerated by elevating the temperature to 60°C. The Ru─CoNiOx maintains stable FDCA at 100 mA cm⁻² for 130 h in 0.2 M HMF, with FE of FDCA remaining around 90%. Coupled FDCA and H₂ production is achieved through continuous HMFOR || HER MEA, reaching an FDCA space‐time yield of 388.8 mg cm⁻² h⁻¹ at 500 mA cm⁻² and maintaining stability for over 1700 h at 200 mA cm⁻² with FE_FDCA > 85%. Preliminary TEA indicates the economic advantage of the KHCO₃‐based route. This study proves that asymmetric coordination of spinel oxides is effective for durable catalysts in near‐neutral FDCA electrosynthesis coupled with hydrogen production.

Author Contributions

Chuqian Xiao: writing – original draft, supervision, methodology, conceptualization. Dongqi Liu: methodology, software. Sheng Hu: methodology, software. Xiuming Bu: funding acquisition, writing – review and editing. Shaoyan Wang: software, formal analysis. Yuhang Li: writing – review and editing, resources. Xianying Wang: funding acquisition.

Conflicts of Interest

The author declares no conflicts of interest.

Supporting information

Supporting File: anie73622‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available in the Supporting Information of this article.