Different from the traditional high-temperature treatment of metal precursors, our synthesis of two-dimensional SC-MHEOs requires two separate steps, including the preparation of two-dimensional SC-HEBCSs and the high-temperature conversion of BCSs into oxide (BCS-oxide) in the absence of any templates (Fig. 1b). In a typical synthesis of SC-HEBCSs, metal nitrates (for example, Co2+, Ni2+, Mn2+, Cu2+, and Zn2+) were first mixed with urea in water/ethanol containing oleic acid as a stabilizer to form a homogeneous solution. After being hydrothermally treated under 160 °C, SC-HEBCS-(CoNiMnCuZn)2(OH)2CO3 nanoplates with two-dimensional morphology and high-entropy composition were prepared accordingly. Powder X-ray diffraction (XRD) pattern shows that SC-HEBCS-(CoNiMnCuZn)2(OH)2CO3 discloses a monoclinic crystalline structure with a P21/a space group (PDF 01-079-7085), which is completely same to the crystalline structure of monometallic Co2(OH)2CO3 (Supplementary Fig. S2a)59. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images further show that both SC-HEBCS-(CoNiMnCuZn)2(OH)2CO3 and Co2(OH)2CO3 are highly uniform and homogeneous with a two-dimensional plate-like nanostructure (Supplementary Fig. S2b–e). The average length, center width, and thickness are determined as 3.1 μm, 480 nm, and 90 nm, respectively (Supplementary Fig. S3). Selected area electron diffraction (SAED) pattern of a sample discloses a single set of signals, indicating the product is single-crystalline with (010) exposed facet (Supplementary Fig. S4), which is further indicated by high-resolution TEM image (Supplementary Fig. S5). High-angle annular dark-field scanning TEM (HAADF-STEM) energy dispersive spectroscopy (EDS) element mapping images further demonstrate highly distributed Co, Ni, Mn, Cu, and Zn elements within two-dimensional nanoplate (Supplementary Fig. S6). The Co/Ni/Mn/Cu/Zn atomic ratio of (CoNiMnCuZn)2(OH)2CO3 is 35.0/27.0/15.9/13.6/8.5, which is identical to the ratio obtained from inductively coupled plasma-mass spectrometry. These results confirm the successful synthesis of two-dimensional SC-HEBCSs that ensures a solid platform for the solid-phase formation of SC-MHEOs under controlled conditions.Two-dimensional SC-MHEO nanoplates were then prepared by directly treating SC-HEBCS-(CoNiMnCuZn)2(OH)2CO3 at a high temperature (300 °C) under air atmosphere in the absence of any templates. Small-angle X-ray scattering (SAXS) patterns reveal that SC-MHEO shows a characteristic peak at q value of 0.48, which is almost same to monometallic single-crystalline mesoporous Co3O4 (SC-M-Co3O4) (Supplementary Fig. S7), corresponding to an average mesoporous periodicity of 13.1 nm (Fig. 1c). In contrast, P-HEO synthesized by direct calcination of metal precursors is structurally nonporous, which has a Co/Ni/Mn/Cu/Zn atomic ratio of 33.5/26.8/16.6/15.9/7.2 (Supplementary Table S2). Meanwhile, nitrogen (N2) sorption isotherms of SC-MHEO show a pore size of 3–8 nm, further indicating mesoporous structure (Supplementary Fig. S8). The Brunauer–Emmett–Teller (BET) surface area of SC-MHEO is 82.2 m2 g−1 due to the presence of abundant mesopores, which reaches 6.8 times higher than that of nonporous P-HEO (12.1 m2 g−1) (Fig. 1d). Powder XRD patterns exhibit a single type of diffraction signals for SC-MHEO and P-HEO (Fig. 1e), which are same to the signals for SC-M-Co3O4 (Supplementary Fig. S9), confirming both of them have a single-phase spinel crystalline structure (PDF: 97-002-4210). In comparison to monometallic SC-M-Co3O4, however, all the peaks of SC-MHEO slightly shift towards the lower degrees, confirming that the Co sites in monometallic Co3O4 are randomly occupied by other four metals (Ni, Mn, Cu, and Zn) to form a HEO in the single-phase spinel structure. HEOs with homogeneous composition and Fd–3m crystalline structure are further confirmed by corresponding Raman spectra (Supplementary Fig. S10)60,61. Besides, X-ray photoelectron spectroscopy (XPS) was used to study the electronic state of anionic oxygen in HEOs. High-resolution O 1s spectra are deconvoluted into three peaks at 529.7 (O1, lattice oxygen), 530.9 (O2, defected or uncoordinated oxygen), and 531.9 eV (O3, adsorbed oxygen) (Fig. 1f)8,62. Compared with P-HEO, obviously, more O2 species is obtained for SC-MHEO, indicating rich mesoporous channels produce more defected and uncoordinated oxygen sites for electrocatalysis.The morphology and nanostructure of SC-MHEO are further characterized by various advanced electron microscopies. SEM image shows the high quality and homogeneity of SC-MHEO with a two-dimensional plate-like nanostructure (Fig. 2a), which is almost the same as its parent high-entropy (CoNiMnCuZn)2(OH)2CO3. In comparison to parent SC-HEBCS, SC-MHEO becomes slightly smaller with average length, center width, and thickness of 2.9 μm, 460 nm, and 75 nm, indicating a minor volume shrinkage during the BCS-oxide transition process (Fig. 2b and Supplementary Fig. S11). Meanwhile, the atomic force microscope (AFM) image of SC-MHEO showed a typical two-dimensional plate-like morphology with an average thickness of approximately 78 nm (Supplementary Fig. S12). HAADF-STEM image of a single SC-MHEO further shows a two-dimensional nanostructure (Fig. 2c), which is the same as monometallic SC-M-Co3O4. There are abundant and penetrated mesopores throughout the nanoplate with mesopores size of 3–10 nm, confirming they are structurally mesoporous (Fig. 2d, e). HAADF-STEM EDS mapping images clearly show uniform distributions (no element segregation) of five metal and O elements throughout the nanoplate (Fig. 2f). The Co/Ni/Mn/Cu/Zn atomic ratio is 34.8/28.1/16.0/13.2/7.9 (Supplementary Table S1), which is consistent with its parent (CoNiMnCuZn)2(OH)2CO3, indicating they are compositionally high-entropy.Fig. 2: Mesoscopic characterizations.a Low-magnification SEM image, b TEM and c HAADF-STEM images, d high-magnification TEM and e HAADF-STEM images, and f HAADF-STEM EDS mapping images of SC-MHEO.High-resolution TEM image and corresponding Fourier transform (FT) pattern show a clear and uniform interlayer spacing distance of 0.285 nm, which is slightly larger than monometallic SC-M-Co3O4, corresponding to the (220) crystalline phase (Fig. 3a–c). Moreover, the TEM image and corresponding SAED pattern observed from the top view of the nanoplate indicate a single-crystalline structure with a spinel Fd–3m space group (Fig. 3d, e and Supplementary Fig. S13). When further viewing high-resolution TEM of four positions in the sample, FT patterns exhibit completely the same signals with (002) and (220) facets (Fig. 3f), further confirming SC-MHEO is single-crystalline with (110) exposed facet. Successful synthesis of SC-MHEO is also confirmed by high-resolution XPS of metal species (Supplementary Fig. S14). Compared with SC-M-Co3O4, Co 2p XPS spectra of SC-MHEO show a negative shift of 0.40 eV, indicating that alloying other metal atoms adjusts the electronic structure of Co sites. More importantly, in the Co XPS spectra of SC-MHEO, the ratio of Co3+/Co2+ increases from 1.82 to 2.86. This shows that Mn2+, Cu2+ and Zn2+ metal ions substitute more Co2+ sites, resulting in an increase in Co3+ species. In the oxidation reaction, more adjustable Co3+/Co2+ and Ni3+/Ni2+ valence states are beneficial for electrocatalysis.Fig. 3: Atomic characterizations.a High-resolution TEM image, and b corresponding FT pattern and c structural model images of SC-MHEO. d TEM image and e corresponding SAED pattern, and f high-resolution TEM images collected from different regions in (d) and corresponding FT patterns of SC-MHEO.The above-detailed characterizations corroborate the successful synthesis of high-quality two-dimensional SC-MHEO nanoplates with uniform and homogeneous structural and crystalline features. As far as we are aware, low-dimensional and single-crystalline HEOs have never been prepared and reported in the literature. We deduce that the formation of SC-MHEO nanoplates is the result of careful control over the high-temperature treatment of two-dimensional SC-HEBCSs by a BCS-oxide transition route. There are some OH− and CO32− of BCS-(CoNiMnCuZn)2(OH)2CO3. During the high-temperature treatment, both H2O and CO2 are released accordingly, which thus self-template the formation of abundant penetrated mesopores. The nonporous structure of BCS-(CoNiMnCuZn)2(OH)2CO3 with a low BET surface area of 9.7 m2 g−1 is also confirmed by N2 sorption isotherms (Supplementary Fig. S15). A minor volume shrinkage also confirms this released process. Meanwhile, the smooth and controlled conversion of single-crystalline (CoNiMnCuZn)2(OH)2CO3 does not change the crystallinity and morphology, resulting in the in situ conversion and synthesis of two-dimensional SC-MHEO-(CoNiMnCuZn)3O4 with single-crystalline and nanoplate structure. This formation mechanism is similar to the dealloying synthesis of mesoporous metals63,64,65.Our BCS-oxide conversion route is synthetically facile and general; it can be easily applicable to the preparation of other two-dimensional SC-MHEO nanoplates with different metal compositions. First, we easily change the kinds of metal precursors and form different two-dimensional SC-HEBCSs (Supplementary Figs. S16–S19). After the high-temperature conversion under the same condition, SC-MHEO nanoplates with different compositional functions are prepared accordingly (Fig. 4a). Here, three kinds of Co3O4-like SC-MHEO nanoplates, including quinary (CoNiMnCuFe)3O4, senary (CoNiMnCuZnBi)3O4, and septenary (CoNiMnCuZnFeBi)3O4, are synthesized as the typical examples. Structural characterizations clearly reveal that all the products are morphologically two-dimensional nanoplate, structurally mesoporous, and crystallographically single-crystalline and spinel (Fig. 4b–d). Furthermore, by changing the treatment atmosphere from air to N2, the oxidation state of metal in M2(OH)2CO3 cannot be further oxidized and thus remains +2. As the calcination temperature increases, M2(OH)2CO3 structure gradually transforms into two-dimensional CoO-like SC-MHEO-(CoNiMnCuZn)O nanoplates with a Fm-3m space group (PDF#97-000-9865) (Fig. 4e, and Supplementary Figs. S19, S20). After the calculation, the configurational entropies (Sconfig) of SC-MHEO are >1.5 R, further indicating they are high-entropy materials (Supplementary Table S3). Meanwhile, both single-crystalline and mesoporous structures are maintained well, further confirming the generality of our BCS-oxide conversion route to the extended synthesis of a library of two-dimensional SC-MHEO nanoplates with rationally controlled compositional functions and phase structures (Supplementary Fig. S21).Fig. 4: Synthetic methodology.a Powder XRD patterns of SC-MEHOs with different compositions and phases. HAADF-STEM EDS mapping images, TEM images and corresponding SAED patterns, and high-magnification HAADF-STEM images of b SC-MHEO-(CoNiMnCuFe)3O4, c SC-MHEO-(CoNiMnCuZnBi)3O4, d SC-MHEO-(CoNiMnCuZnFeBi)3O4, and e SC-MHEO-(CoNiMnCuZn)O.Selective HMFOR electrocatalysis of SC-MHEO-(CoNiMnCuZn)3O4 for FDCA electrosynthesis is then performed in 1.0 M KOH containing 10 mM HMF. Meanwhile, SC-M-Co3O4 and P-HEO-(CoNiMnCuZn)3O4 are also tested as its counterpart catalysts for sharp comparisons. Compared to the linear sweep voltammetry (LSV) curve collected in the absence of HMF, all current densities electrocatalyzed by SC-MHEO are negatively shifted toward the lower potentials in the presence of HMF (Fig. 5a). Remarkably, the current density of 10 mA cm−2 in the presence of HMF is only 1.43 V, which is 0.16 V negative than that in the absence of HMF, indicating the high electrocatalytic activity of SC-MHEO for HMFOR electrocatalysis while inhibiting its competitive OER. Meanwhile, we compare the HMFOR performance of SC-MHEO, P-HEO, and SC-M-Co3O4. As shown in Fig. 5b, carbon paper (CP) as the electrocatalyst support is almost inactive for HMFOR. By contrast, both SC-MHEO and P-HEO hold a similar onset potential of 1.18 V, which is 0.17 V lower than that of SC-M-Co3O4 (1.35 V), indicating the high-entropy function for promoting HMFOR electrocatalysis. Moreover, SC-MHEO exhibits a higher current density than P-HEO at the potentials above 1.45 V, suggesting the importance of a single-crystalline and mesoporous structure that synergistically boosts HMFOR electrocatalysis. The reaction rates and kinetics are further evaluated by summarizing the Tafel slopes (Fig. 5c). Obviously, SC-MHEO discloses the lowest Tafel slope of 191.4 mV dec−1, which is lower than that of P-HEO (258.6 mV dec−1) and SC-M-Co3O4 (388.1 mV dec−1), indicating SC-MHEO with single-crystalline/mesoporous structures and high-entropy compositions accelerates the reaction kinetics and thus promotes HMFOR electrocatalysis. The accelerated kinetics of SC-MHEO is also confirmed by the smallest impedance arc diameter in electrochemical impedance spectroscopy (EIS) analysis (Supplementary Fig. S22). The double-layer capacitance (Cdl) of electrocatalysts is also tested (Supplementary Fig. S23). As summarized in Fig. 5d, SC-MHEO possesses the higher Cdl value of 26.4 mF cm−2, which is 24.9 and 1.7 folds higher than that of SC-M-CO3O4 (1.06 mF cm−2) and P-HEO (15.8 mF cm−2). The results indicate that SC-MHEO exposes more active metal sites for promoting HMFOR electrocatalysis. ECSA normalization of current densities of SC-MHEO and P-HEO is also compared in 50 mM HMF, indicating mesoporous structure exposes more undercoordinated metal and oxygen sites that further promote HMFOR electrocatalysis (Supplementary Fig. S24).Fig. 5: Electrocatalytic performance.a LSV curves of SC-MHEO collected in 1.0 M KOH with and without 10 mM HMF. b LSV curves and c summarized Tafel slopes, and d capacitive currents of SC-MHEO, SC-M-Co3O4, and P-HEO collected in 1.0 M KOH and 10 mM HMF. e Conversion of HMF and selectivity of FDCA for HMFOR electrocatalyzed by SC-MHEO (error bars are determined from five replicate trials at different potentials). f Conversion of HMF, selectivity of FDCA, and FE of FDCA electrocatalyzed by SC-MHEO for 10 consecutive cycles of HMFOR. g Comparisons of key HMFOR performance parameters for SC-MHEO, P-HEO, and SC-M-Co3O4 (A: conversion; B: FDCA yield; C: FE; D: current density at 1.43 V; E: ECSA). h Performance comparisons of SC-MHEO with the state-of-the-art electrocatalysts for FDCA electrosynthesis from HMFOR.Chronoamperometry measurements are further performed in 1.0 M KOH and 10 mM HMF at different potentials to quantitatively identify the products of selective HMFOR electrocatalysis by high-performance liquid chromatography (Supplementary Fig. S25). As summarized in Fig. 5e, SC-MHEO discloses the high HMF conversion and FDCA selectivity in the potential range of 1.385 to 1.585 V. Typically, at lower potentials (1.385 and 1.435 V), SC-MHEO achieves extremely high HMF conversion of >97% and high FDCA Faradaic efficiency (FE) of >95%. As the potential increased further (1.485, 1.535, and 1.585 V), the FE of FDCA and the conversion rate of HMF slightly decreased, mostly because of the occurrence of competitive oxygen evolution reaction (OER). Especially at a potential of 1.435 V, the HMF conversion electrocatalyzed by SC-MHEO is as high as 99.3%, with a superior FDCA FE of 97.7%. In sharp comparisons, P-HEO and SC-M-Co3O4 show a relatively slower HMF conversion rate and lower FDCA selectivity at all the potentials (Supplementary Fig. S26). For example, at 1.435 V, HMF conversion and FDCA FE are 87.2% and 84.9% for P-HEO and 78.8% and 60.5% for SC-M-Co3O4 (Supplementary Table S4). The results clearly demonstrate that SC-MHEO not only remarkably promotes HMFOR electrocatalysis but also dramatically enhances FDCA selectivity. At the same time, we have also conducted electrocatalytic HMFOR tests on other SC-MHEO nanoplates at the optimal voltage of 1.435 V (vs. RHE). Remarkably, all Co3O4-like electrocatalysts exhibited considerable conversion rates and selectivities due to the unique ‘cocktail’ effect similar to the performance on SC-MHEO-(CoNiMnCuZn)3O4 (Supplementary Fig. S27). In comparison, SC-MHEO-(CoNiMnCuZn)O disclosed the decreased activity in HMFOR electrocatalysis, which can be attributed to the absence of valence-changing metal ions in the crystal structure. Moreover, SC-MHEO shows remarkable operation stability in selective HMFOR electrocatalysis for FDCA electrosynthesis. After being tested for ten successive cycles, no significant decay is observed in both HMF conversion and FDCA selectivity (Fig. 5f). Meanwhile, SC-MHEO also maintains well in two-dimensional morphology, penetrated mesopores, and single-crystalline structure (Supplementary Fig. S28). The results clearly highlight the synergies of high-entropy effect and single-crystalline/mesoporous structures in promoting selective HMFOR electrocatalysis (Fig. 5g). Compared with the state-of-the-art electrocatalysts reported in the literature, more impressively, SC-MHEO represents one of the most active and selective HMFOR electrocatalysts for FDCA electrosynthesis (Fig. 5h and Supplementary Table S5).In general, there are two main reaction pathways in selective HMFOR electrocatalysis (Fig. 6a)9,66,67. Pathway l is the preferential electrooxidation of the aldehyde group of HMF into 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) first (2e− route). After that, HMFCA is further oxidized into formyl-2-furancarboxylic acid (FFCA) (2e− route) and finally oxidized into FDCA (2e− reaction). In Pathway ll, by contrast, HMF is first electrooxidized preferentially into 2,5-diformylfuran (DFF) by the oxidation of the hydroxyl group of HMF (2e− route). DFF is then oxidized into FFCA and finally into FDCA68. To probe the reaction pathway of our electrocatalysts, we perform potentiostatic electrocatalysis at 1.435 V and further analyze the concentrations of reactants, intermediates, and products. During HMFOR electrocatalysis, there is no DFF detected for SC-MHEO at different coulombic charges. Alternatively, trace of HMFCA is observed as the key reaction intermediate at the same time, indicating Pathway l dominates in our electrocatalyst (Fig. 6b and Supplementary Fig. S29)69,70,71. In sharp comparisons, more HMFCA amounts are detected for P-HEO and SC-M-Co3O4 in different potentials (Fig. 6c, and Supplementary Figs. S30, S31), further confirming that Pathway I dominates for HMFOR electrocatalysis. Remarkably, SC-MHEO nanoplates with high-entropy and structural advantages synergistically accelerate further HMFCA electrooxidation and thus promote HMFOR electrocatalysis via Pathway I8,72. Then, we perform HMFCA electrocatalysis to highlight the high performance of SC-MHEO in the electrooxidation of the hydroxyl group in intermediate HMFCA. As summarized in Fig. 6d, SC-MHEO completely electrooxidizes HMFCA into FDCA with a superior selectivity of >99%. However, in the same test conditions, 9.5% and 42% of HMFCA are retained when electrocatalyzed by P-HEO and SC-M-Co3O4, respectively (Supplementary Fig. S32). The results further indicate the high-entropy and structural synergies in promoting complete electrooxidation of HMF into FDCA.Fig. 6: Electrocatalytic mechanism.a Reaction pathways of HMFOR electrocatalysis to FDCA. b Product distributions at 1.435 V (vs. RHE) over SC-MHEO collected in 1.0 M KOH and 10 mM HMF. c Product distributions at different potentials over SC-MHEO, SC-M-Co3O4, and P-HEO collected in 1.0 M KOH and 10 mM HMF. d Product distributions at 1.435 V (vs. RHE) over SC-MHEO, SC-M-Co3O4, and P-HEO collected in 1.0 M KOH and 10 mM HMFCA. e Free energies of HMFOR electrocatalysis via Pathway I and Pathway II by SC-MHEO and SC-M-Co3O4.Density functional theory (DFT) calculations are also conducted to reveal the intrinsic nature of the high HMFOR performance of SC-MHEO for FDCA electrosynthesis. Here, SC-MHEO is structurally simulated by random substitutions of Ni, Mn, Cu, and Zn in Co sites of spinel Co3O4, as characterized above (Supplementary Fig. S33). The adsorption energy of the substrate molecule (HMF) was the activity descriptor of electrochemical HMFOR. We thus calculated the adsorption energies of HMF substrate molecule on different metal sites of SC-MHEO. As presented in Supplementary Fig. S34, the Co site has the strongest adsorption of HMF (−1.77 eV), which thus was considered the main active site for HMFOR electrocatalysis. In addition, we also calculated the d-band center of metal elements in SC-MHEO. The results showed that the d-band center of Co (−2.45 eV) was larger than that of other metals (Ni: 4.92 eV, Cu: 3.11 eV, Mn: 5.07 eV, Zn: 6.44 eV) closer to the Fermi level (Supplementary Fig. S35). It showed that the Co site had a strong ability to capture reaction intermediates, which further supported our conclusion. We further calculate the energy profiles of two HMFOR pathways for SC-MHEO-(CoNiMnCuZn)3O4 and SC-M-Co3O4. As summarized in Fig. 6e, both SC-MHEO and SC-M-Co3O4 show the increased energy barriers of 0.37 eV and 0.30 eV, respectively, for selective electrooxidation of the hydroxyl group of HMF* into DFF* by Pathway II. Obviously, they are thermodynamically unfavorable. In comparison, in Pathway I, energy barriers of HMF*-to-HMFCA* decrease for both SC-MHEO and SC-M-Co3O4, indicating the spontaneous process for electrooxidation of aldehyde group of HMF (Supplementary Fig. S36). Therefore, HMFOR electrocatalysis is energetically proceeded by Pathway I, as experimentally confirmed above73. Moreover, the highest energy barrier of SC-M-Co3O4 reaches 1.13 eV for the selective HMFCA*-to-FFCA* route, indicating that it is the rate-determining step of HMFOR electrocatalysis. It strongly corresponds to the high concentration of HMFCA in the reaction. By contrast, the rate-determining step of SC-MHEO is the FFCA*-to-FDCA* route, since it needs to overcome the highest energy barrier of 0.42 eV during the electrocatalysis. Remarkably, the lower energy barrier in the rate-determining step of SC-MHEO further highlights the high-entropy effect in promoting selective HMFOR electrocatalysis, and thus results in higher activity and selectivity simultaneously.In addition, the d-band center of Co in SC-MHEO is closer to the Fermi level than that of SC-M-Co3O4, indicating a strong ability to capture reaction intermediates, which is consistent with our reaction pathway diagram. Under the influence of other elements, the peak patterns of Co partial projected density of states (PDOS) tend to be more numerous and broader in the SC-MHEO structure than the SC-M-Co3O4 structure. This illustrates the impact of high-entropy systems on the central electronic structure (Supplementary Fig. S37). This illustrates the impact of high-entropy oxides on the central electronic structure. In addition, HMFOR and oxygen evolution reaction (OER) are two competing reactions due to possible water oxidation side reactions in aqueous solution. Gibbs free energies of two pathways, including the 4-electron oxygen evolution reaction and the 2-electron water oxidation reaction, are calculated accordingly (Supplementary Fig. S38). The results show that the reaction energy of the first step of both reactions (H2O → OH* + H+ +e−) reaches +1.54 eV, which is much higher than any step in the reaction pathway in HMFOR electrocatalysis. Therefore, the two side reactions do not occur preferentially.Electrocatalytic NO3−RR is then performed in 1.0 M KOH containing 0.10 M NO3− as a potential coupling cathode reaction of anode HMFOR electrocatalysis. LSV curves show that the current density of SC-MHEO electrocatalyst increases sharply in the presence of KNO3, indicating its high activity for NO3−RR electrocatalysis (Fig. 7a). Chronoamperometry measurements are further performed at different potentials to identify the products of selective NO3−RR electrocatalysis. Considering the main product of NH3, we here determine and analyze NH3 produced by a typical colorimetric method (Supplementary Fig. S39). The origin of produced NH3 was identified through the 15N isotope labeling experiments. The typical 15NH3 peak can be seen when using 15NO3− as nitrogen source, indicating that the NH3 produced comes from NO3−RR (Supplementary Fig. S40). As summarized in Fig. 7b, both the NH3 yield rate and FENH3 of SC-MHEO show the typical volcanic trends in the potential ranging from −0.30 to −0.60 V. Specifically, SC-MHEO discloses the best NO3−-to-NH3 performance at −0.40 V with the highest FENH3 of 91.5% and NH3 yield rate of 15.73 mg h−1 cm−2. In addition, SC-MHEO also shows excellent electrocatalytic NO3−-to-NH3 stability. After being performed for 12 consecutive cycles at −0.40 V, there is almost no decreasing trend of SC-MHEO in NH3 yield rates and FENH3 for NO3−-to-NH3 electrocatalysis (Fig. 7c). Physical characterizations also show that structure and crystallinity of SC-MHEO catalyst retain well (Supplementary Fig. S41). These results clearly confirm that NO3− can be efficiently electroreduced into value-added NH3 with high activity and selectivity at the cathode.Fig. 7: Two-electrode cell.a LSV curves of SC-MHEO collected in 1.0 M KOH with and without 0.10 M KNO3. b NH3 yield rates and FENH3 values of SC-MHEO collected in 1.0 M KOH and 0.10 M KNO3 (error bars are determined from five replicate trials at different potentials). c Recycling stability tests of SC-MHEO collected in 1.0 M KOH and 0.10 M KNO3. d Schematic illustration of two-electrode coupling system for anode HMFOR and cathode NO3−RR electrocatalysis. e Conversion of HMF and selectivity of FDCA for HMFOR electrocatalysis by SC-MHEO in a two-electrode coupling system. f Conversion, selectivity and FE of cathode and anode in the two-electrode cell after being tested for 10 h.Inspired by the excellent electrocatalytic performance, we finally construct a two-electrode coupling system in an H-type cell that includes selective HMF-to-FDCA electrocatalysis as the anode reaction and selective NO3−-to-NH3 electrocatalysis as the cathode reaction with SC-MHEO as a bifunctional electrocatalyst (Fig. 7d). In comparison to (+) HMFOR | | HER (−) system, (+) HMFOR | | NO3−RR (−) coupling system shows the lower onset potentials, indicating the high potential of bifunctional SC-MHEO electrocatalyst in two-electrode coupling system (Supplementary Fig. S42). Specifically, at the current density of 10 mA cm−2, the cell voltage of (+) HMFOR | | NO3−RR (−) coupling system is as low as 1.69 V. We further summarize the product selectivity and FE of two-electrode coupling system at different cell voltages. For anode HMF-to-FDCA electrocatalysis, SC-MHEO discloses superior FDCA selectivity of >95% and high FDCA FE of >90% at the lower voltages (<2.3 V) (Fig. 7e). Specifically, the best HMF-to-FDCA performance, including 96.7% of HMF conversion rate, 98.8% of FDCA selectivity, and 91.8% of FDCA FE, is achieved at the coupling voltage of 2.0 V. For cathode NO3−-to-NH3 electrocatalysis, high FENH3 of >80% is also achieved (Supplementary Fig. S43). The stability of the two-electrode coupling system is further characterized by electrolyzing a large volume of solution for a long time at a battery voltage of 2.2 V. Chronoamperometry test shows that, after being evaluated for 10 h continuous electrocatalysis, FDCA selectivity and FE at the anode still reach 98.1% and 73.7% with an HMF conversion of 98.2% (Fig. 7f). At the same time, FENH3 at the cathode is 81.3% in the same electrocatalytic condition. The results highlight that co-electrocatalysis by replacing HER with NO3−RR not only produces more value-added product (NH3) but also enhances energy efficiency in the two-electrode system, thereby demonstrating their great potential for practical application. In addition, a two-electrode coupling system continuous flow electrolyzer was used to evaluate the practicality of SC-MHEO cathode for HMFOR electrocatalysis (Supplementary Fig. S44). Impressively, the SC-MHEO electrocatalyst discloses a superior selectivity of 98.3%, a high FDCA yield of 87.5%, and a remarkable FE of 86.1% in a continuous flow electrolyzer. The result further highlights the potential application of SC-MHEO in real flow electrolyzer for producing high-value-added chemicals.
