Synergistic Enhancement of Electrochemical Alcohol Oxidation by Combining NiV-Layerd Double Hydroxide with an Aminoxyl Radical

doi:10.21203/rs.3.rs-4722149/v1

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Synergistic Enhancement of Electrochemical Alcohol Oxidation by Combining NiV-Layerd Double Hydroxide with an Aminoxyl Radical

https://doi.org/10.21203/rs.3.rs-4722149/v1

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Electrochemical alcohol oxidation (EAO) is a compelling method for producing high-value carbonyl products. However, its industrial viability is hindered by suboptimal efficiency stemming from low reaction rates. Herein, we present a novel synergistic electrocatalysis approach that integrates an active electrode and aminoxyl radical to enhance the performance of EAO. The optimal aminoxyl radical (4-acetamido-TEMPO, ACT) and Ni_0.67V_0.33-layered double hydroxide (LDH) were screened as cooperative electrocatalysts by integrating theoretical predictions and experiments. The Ni_0.67V_0.33-LDH facilitates ACTH adsorption and activation via ketonic oxygen interactions, thereby enhancing selectivity and yield at high current densities. The electrolysis was scaled to demonstrate a 200 g synthesis of the steroid carbonyl product 8b resulted in a 91% yield and a productivity of 243 g/h. These results represent a promising method for accelerating electron transfer to enhance alcohol oxidation, highlighting its potential for practical electrosynthesis applications.

Physical sciences/Chemistry/Electrochemistry/Electrocatalysis

Physical sciences/Chemistry/Chemical engineering

Physical sciences/Chemistry/Green chemistry

electrochemical alcohol oxidation

synergistic effect

layer double hydroxides

aminoxyl radical

electrocatalysis

Electrochemical alcohol oxidation (EAO) has gained increasing attention in recent years as it enables efficient alcohol oxidation for pharmaceutical synthesis and biomass conversion^1–4. In general, EAO can be performed directly at the electrode surface or indirectly in solution using a mediator^5,6. In the direct EAO process, the reacting molecules are oxidized directly at the electrode surface, and metal oxides that serve as heterogeneous electrocatalysts have been extensively studied (Fig. 1a)^7–12. Previous studies have systematically investigated the electrocatalytic mechanism of nickel-based oxides/(oxy)hydroxides for EAO and have demonstrated that the Ni²⁺/Ni³⁺ redox couple can promote the EAO process^13–16. Particularly, the incorporation of different metallic elements with various valence states into Ni-based hydroxides can form flexible redox-centers toward boosting the activity of electrocatalysts^17–20. However, this approach is susceptible to unforeseen side reactions or over-oxidation problems at high current densities, resulting in lower selectivity or yield of the target product. In the indirect (mediated) EAO process, aminoxyl radicals have been studied in detail as homogeneous electrocatalysts (Fig. 1b), such as TEMPO (2,2,6,6-tetramethyl-1-piperidine-N-oxyl) and ACT (4-acetamido-TEMPO)^21–23. The mechanism for electrochemical aminoxyl radical-mediated alcohol oxidation under basic conditions has also been investigated^2,6,24,25. The oxoammonium cation forms through the electrochemical oxidation of the aminoxyl radical at the electrode surface, which then reacts with alcohol to produce the corresponding hydroxylamine and carbonyl product²⁵. The indirect EAO process operates at moderate potentials and can tolerate diverse functional groups, but the low current density of the oxidation reaction is due to the slow electron transfer rate of the reaction intermediates. Therefore, it is imperative to enhance the optimization of the reaction kinetics in the EAO process to meet the prerequisites for industrial application.

To achieve fast electrochemical kinetics in the EAO process, it is essential to facilitate electron transfer between the electrocatalysts and the organic intermediates. The regeneration of the oxoammonium cation from the hydroxylamine in the indirect EAO process can proceed via through two different pathways: comproportionation of an oxoammonium and hydroxylamine or direct 2e⁻/1H⁺ proton-coupled electron transfer (PCET) oxidation of the hydroxylamine^24,26. Efficient PCET oxidation of the hydroxylamine could provide a means to enable alcohol oxidation with reduced overpotential and potentially could lead to increased EAO reaction rates^27,28. This concept has been demonstrated through an “integrated cooperativity” mechanism using a Cu/aminoxyl catalyst, in which Cu^II and the aminoxyl participate together to mediate alcohol oxidation²⁸. The reaction rate and turnover frequency of the (bpy)Cu/TEMPO catalytic system was significantly higher than those of TEMPO alone. Hydroxylamine oxidation also may be achieved by a “serial cooperativity” mechanism, identified with an iron(III) nitrate/aminoxyl co-catalyst system. This pathway features a redox cascade wherein the alcohol is oxidized by an in situ-generated oxoammonium species²⁹. Recently, several studies have been reported that show synergistic effects between metal oxides or other heterogeneous electrocatalyst and aminoxyl radicals for alcohol oxidation^30–33. The complexity of the reaction has, however, resulted in significant challenges in the development process, largely due to the intricate nature of the cooperative catalyst-mediated EAO, which involves heterogeneous electrocatalyst and aminoxyl radicals. Despite the considerable efforts made, the mechanism of EAO remains elusive, making it particularly difficult to develop novel EAO systems based on this mechanism. Consequently, the search for a synergistic electrocatalyst-mediated EAO reaction mechanism is of paramount importance for the resolution of the aforementioned development dilemma.

Herein, we introduce an alternative strategy to enhance EAO through a synergistic electrocatalytic system using Ni-based layered-double-hydroxide (LDH) electrocatalysts paired with aminoxyl radicals (Fig. 1c). A series of different heterogeneous Ni-based LDH electrocatalysts, including Ni alone, NiV, NiMn, NiFe, NiCo, NiCu materials, are evaluated with different aminoxyls, including TEMPO, 4-HO-TEMPO, 4-MeO-TEMPO, ACT, 4-oxo-TEMPO. The results indicate, which are also analyzed using density-functional theory (DFT) calculations, reveal that the NiV electrocatalyst in combination with ACT shows excellent performance. The results suggest that ACTH adsorbs onto the NiV surface. A Ni_0.67V_0.33-LDH/ACT electrochemical co-catalyst system demonstrates excellent EAO performance at high current densities in a continuous flow electrolyzer, with benzaldehyde selectivity and yield reaching up to 99%. This cooperative electrocatalyst system demonstrates broad substrate scope and functional-group tolerance. The electrolyzer was scaled up to enable synthesis of a high-value steroid carbonyl product with a high productivity of 243 g/h at large current electrolysis of 60 A. This work highlights the utility of synergistic heterogeneous and homogeneous electrocatalysts and provides mechanistic insights into the basis for this behavior.

Preliminary reactivity studies. TEMPO and related aminoxyl radicals have been widely used for electrochemical alcohol oxidation (EAO)^{21–23,25,26,34,35}. In this study, we selected five aminoxyl radicals as model electrocatalysts (Fig. 2a) due to their varying oxoammonium/aminoxyl redox potentials resulting from differences in their R-functional groups. Nickel oxyhydroxide materials (NiOOH) are also promising heterogeneous electrocatalysts for EAO^{8,9,13,30,31,36–38}. The resting state of NiOOH is Ni(OH)₂ during the open-circuit conditions. We wondered whether the two electrocatalysts could be combined to achieve synergistic benefits for EAO. Benzyl alcohol (BA) was chosen as a model substrate for EAO, and the BA oxidation reactions were conducted under constant-potential conditions (1.45 V vs. RHE) in a continuous-flow parallel-plate electrolyzer with electrode area of 10 cm², in which Ni-based materials as the anode and nickel foam as the cathode (see section Methods for details). Gas chromatography (GC) was utilized to monitor and quantify the concentration changes of the organic compounds. The synthesis and characterization of the NiM-LDH materials are elaborated below and in the Supporting Information (see section Methods for details).

Attempts to perform direct oxidation of BA at 1.45 V vs. RHE with a Ni(OH)₂-impregnated graphite-felt electrode resulted in only low yield of the target product benzaldehyde (PhCHO) (5%, Table 1, entry 1). Better results were obtained with 5 mol% AcNH-TEMPO (ACT) rather than with a heterogeneous electrocatalyst, with the yield of PhCHO increasing to 38% (entry 2). The yield increased further when both Ni(OH)₂ and ACT were used together (52%, entry 3), raising the prospect of a synergistic effect that exceeds the sum of the two individual catalysts. Analysis of heterogeneous Ni(OH)₂-based materials that feature other metals within the lattice were only marginally better as heterogeneous electrocatalysts (e.g., entry 4); however, a NiV-LDH material with a Ni_0.67V_0.33 stoichiometry (described further below) paired with ACT led to nearly quantitative yield of PhCHO (entry 5). Other NiM-LDH materials with ACT were not as effective as the NiV-LDH/ACT catalyst system (M = Mn, Fe, Co, Cu; entries 6–9). In addition, ACT is superior to TEMPO (entry 10), 4-HO-TEMPO (entry 11), 4-MeO-TEMPO (entry 12) and 4-oxo-TEMPO (entry 13).

Computational analysis of aminoxyl interactions with NiM-LDH materials. Density functional theory (DFT) calculations were performed to investigate the electrocatalytic reactivity. Figure 2a displays the reactivity of various aminoxyl radicals on a clean NiOOH electrode surface. Adsorption energies for H-TEMPOH, 4-HO-TEMPOH, 4-MeO-TEMPOH, AcNH-TEMPOH (ACTH), and 4-oxo-TEMPOH were calculated to be -0.44 eV, -0.96 eV, -0.15 eV, -0.73 eV, and − 0.68 eV, respectively (Supplementary Fig. 2). The adsorption energy for the above components follows the order of 4-HO-TEMPOH > AcNH-TEMPOH > 4-oxo-TEMPOH > H-TEMPOH > 4-MeO-TEMPOH. Despite the strongest interaction between 4-HO-TEMPOH and NiOOH surface, the oxidative activity of 4-HO-TEMPO is attenuated in alkaline environments, attributed to deprotonation of the HO- group, reducing in the electron density of the aminoxyl radical and impairing its oxidative capability. Therefore, the ACT shows exhibits good adsorption and superior electrocatalytic activity. Further calculations were performed to assess ACTH adsorption to various NiM-LDH materials, with M = V, Mn, Fe, Co and Cu. The V-doped NiOOH material exhibited the strongest adsorption of ACTH (Fig. 2b), with an adsorption energy calculated to be -2.04 eV, and the NiV-LDH material binds ACT most strongly among the different aminoxyls (Fig. 2c). The amide carbonyl group of ACT binds to an exposed Ni ion on the NiV-LDH surface, with a calculated Ni-O bond length of 1.98 Å.

Synthesis and characterization of NiM-LDH materials. The heterogeneous NiM-LDH electrocatalysts (M = V, Mn, Fe, Co, Cu) were distributed uniformly on graphite felt (GF) via a hydrothermal method³⁹, as schematically illustrated in Fig. 3a. NiV-LDH/GF electrocatalysts with different Ni/V molar ratios were obtained by controlling the dosage of nickel nitrate and vanadium (III) chloride during the hydrothermal synthesis process. Ni(OH)₂/GF was also synthesized by a similar procedure but without V dopants. The scanning electron microscopy (SEM) displayed that the Ni_0.67V_0.33-LDH has a three-dimensional (3D) nanosphere morphology composed of ultrathin double hydroxide nanosheets with a diameter of approximately 600 nm (Fig. 3b, c), sharply contrasting the smooth surface of the pristine GF (Supplementary Fig. 3). The Ni(OH)₂ also shows that the entire surface of the GF is uniformly coated with the dense nanosheets (Supplementary Fig. 4). Such porous structure exposes large number of active sites, facilitating penetration and diffusion of electrolytes and increasing the effective contact between the electrode and electrolytes during the EAO process^40,41. Similarly, other Ni-based LDHs were evenly distributed on the surface of the GF (Supplementary Fig. 5). Transmission electron microscopy (TEM) images illustrate that the Ni_0.67V_0.33-LDH has a nanosphere structure consisting of thin interconnected nanosheets (Fig. 3d, e) that are also reflected in the SEM results. The lattice fringe spacing of the Ni_0.67V_0.33-LDH is approximately 0.205 nm^42–44, as revealed by the high-resolution TEM (HRTEM) in Fig. 3f. The corresponding selected area electron diffraction (SAED) image in the inset of Fig. 3f shows that the Ni_0.67V_0.33-LDH contains multiple crystal planes in the form of circles of bright dots^43,45. The energy dispersive X-ray spectrum (EDX) analysis reveals that the Ni_0.67V_0.33-LDH contains Ni, V, C and O as the main elemental components, with a V:Ni atomic ratio of approximately 1:2.38 (Supplementary Fig. 6). The elemental analysis in Fig. 3g also confirms that the elements Ni, V, and O are evenly distributed in the Ni_0.67V_0.33-LDH. The mass content of Ni and V was determined using inductively coupled plasma spectroscopy (ICP) to be 28.44% and 13.21%, respectively.

The powder X-ray diffraction (XRD) patterns of Ni_0.67V_0.33-LDH are shown in Fig. 3h. To avoid interference from the strong signal of the GF, powders synthesized by the same method were characterized. The diffraction peaks of Ni(OH)₂ can be assigned to the standard brucite (space group: P3m1) crystal phase of β-Ni(OH)₂ (JCPDS No. 73-1520)^46,47, without any other peaks. After incorporating V into the structure of Ni(OH)₂, the diffraction peaks of Ni_0.67V_0.33-LDH centered at 11.46°, 23.26°, 33.46°, 34.41°, 38.76° and 59.80°, corresponding to the (003), (006), (101), (012), (015) and (110) facets of hexagonal α-Ni(OH)₂ (JCPDS No. 38–0715) with larger interlayer spacing^17,48,49, respectively. This result indicates that the incorporation of V transforms Ni(OH)₂ from β phase to α phase⁴⁴. In addition, Brunauer-Emmett-Teller (BET) analysis probed the specific surface area of Ni_0.67V_0.33-LDH (Supplementary Fig. 7). The nitrogen adsorption-desorption curves of Ni_0.67V_0.33-LDH display a type IV isotherm with a hysteresis loop in the range of relative pressure (P/P₀ = 0.45-1.0) and pore size in the range of 2 to 8 nm, indicating that Ni_0.67V_0.33-LDH is a mesoporous material. Moreover, the Ni_0.67V_0.33-LDH has a larger BET surface area (9.66 m² g^− 1) compared to β-Ni(OH)₂ (2.81 m² g^− 1), demonstrating that V-doping alters the morphology of the nanospheres and is expected to improve the EAO efficiency by facilitating the mass transfer of the reactants and electrolytes. The surface chemical composition and electronic state of the elements in the Ni_0.67V_0.33-LDH and β-Ni(OH)₂ powders were investigated by using X-ray photoelectron spectroscopy (XPS) (Supplementary Fig. 8a). The Ni 2p XPS spectrum of Ni_0.67V_0.33-LDH and β-Ni(OH)₂ showed two Ni 2p_3/2 peaks (Fig. 3i), including the dominant Ni²⁺ species and a Ni³⁺ species with a central binding energy of 855.9 and 858.1 eV, respectively^48,50. Moreover, the V 2p_3/2 signal exhibits two peaks at 516.6 and 517.5 eV (Fig. 3j), indicating the existence of a large number of V⁴⁺ species and a small number of V⁵⁺ species, respectively¹⁷. The latter data confirm that V is doped into the Ni_0.67V_0.33-LDH. The O 1s peaks observed for Ni_0.67V_0.33-LDH at 531.1 and 532.5 eV are attributed to metal-oxygen (Ni-O-V) bonds and adsorbed H₂O, respectively (Supplementary Fig. 8b).

Analysis of synergistic NiM-LDH/ACT electrocatalytic alcohol oxidation. To evaluate the electrochemical performance of NiM-LDH/ACT for EAO, linear sweep voltammetry (LSV) analysis was carried out in a batch reactor using a typical three-electrode system (0.5 M Na₂CO₃ aqueous electrolyte with pH 11.5). As displayed in Fig. 4a, the LSV of Ni_0.67V_0.33-LDH electrocatalyst exhibits significantly higher electrocatalytic activity compared to other NiM-LDHs (M = Mn, Fe, Co, Cu). The oxidation peak at 1.53 V vs. RHE was identified as the Ni²⁺/Ni³⁺ oxidation peak related to the oxidation of Ni(OH)₂ to NiOOH^51,52, indicates that the incorporation of V into Ni-based LDH can effectively promote the oxidation of Ni²⁺ sites. Figure 4b demonstrates that the V content in the NiV-LDHs electrocatalysts has a volcano-like relationship with the oxidation current density, with a Ni/V atomic ratio of ~ 2:1 (Ni_0.67V_0.33-LDH) exhibiting the highest current density (Supplementary Fig. 9).

Use of 5 mol% ACT reveals an oxidation peak at 1.62 V vs. RHE (Fig. 4c), corresponding to oxidation of ACT to the oxoammonium species, ACT⁺. The oxidation peak intensity is significantly enhanced when both Ni_0.67V_0.33-LDH and ACT are present, reaching up to 130 mA/cm² at 1.58 V vs. RHE. Flow oxidation studies conducted with benzyl alcohol (BA) reveal complementary results. Use of 100 mM BA reveals a much higher current density (336 mA cm^− 2) for the cooperative Ni_0.67V_0.33-LDH/ACT electrolysis system relative to ACT alone (153 mA cm^− 2) or Ni_0.67V_0.33-LDH alone (3 mA cm^− 2) at 1.45 V vs. RHE in the flow electrolyzer (Fig. 4d, e). The corresponding Tafel slope of Ni_0.67V_0.33-LDH/ACT is 47 mV dec^–1 (Fig. 4f), which is significantly lower than that of the ACT (64 mV dec^− 1) or Ni_0.67V_0.33-LDH (88 mV dec^–1), reflecting more favorable reaction kinetics for BA oxidation, relative to the two individual catalysts, Ni_0.67V_0.33-LDH and ACT. Electrochemically active surface area (ECSA) measurements were performed by systematically measuring CVs within the non-faradaic potential range to quantify the double-layer capacitance (C_dl, Supplementary Fig. 10). The NiV-LDH shows an increase in bilayer capacitance relative to Ni(OH)₂, 90.8 mF cm^− 2 vs. 31.2 mF cm^− 2 (Supplementary Fig. 11), suggesting that increase access to surface sites in the Ni_0.67V_0.33-LDH material at least partially contributes to its improved electrocatalytic performance.

A series of chronoamperometry experiments were conducted at different potentials (1.30–1.50 V vs. RHE in a 0.5 M Na₂CO₃ electrolyte) in the continuous-flow electrolyzer to investigate further the origin of Ni_0.67V_0.33-LDH/ACT cooperativity. Figure 4g shows that the target product, benzaldehyde (PhCHO), was produced with a selectivity of 99% at a voltage of 1.45 V vs. RHE using the cooperative Ni_0.67V_0.33-LDH/ACT electrode. Nearly 1930 C (2 F/mol) of charge is required to completely convert BA into PhCHO (Supplementary Fig. 12). Benzoic acid (PhCOOH) formation is negligible during the process (Supplementary Fig. 13). Independent testing of Ni_0.67V_0.33-LDH and ACT for EAO performance at 1.45 V vs. RHE leads to only 9% and 38% yield of PhCHO, respectively (Fig. 4h), confirming the synergistic benefits of Ni_0.67V_0.33-LDH and ACT for alcohol oxidation. The selectivity and yield of PhCHO remains consistently above 95%, even after 10 consecutive electrochemical oxidation cycles (Fig. 4i). The morphology and structure of Ni_0.67V_0.33-LDH were not significantly altered after electrochemical oxidation experiments, as shown by SEM, TEM, and elemental mapping (Supplementary Figs. 14 and 15). EDX analysis after the reaction revealed a slight loss of Ni and V and an increase in O content compared to the pre-reaction Ni_0.67V_0.33-LDH electrocatalyst (Supplementary Fig. 16); however, high-resolution XPS spectra of Ni 2p and V 2p exhibted a slight increase in Ni³⁺ and V⁵⁺ after EAO compared to the as-prepared Ni_0.67V_0.33-LDH material (Supplementary Fig. 17). These observations are rationalized by partial reconstruction of the LDH material during EAO.

Mechanistic analysis of NiV-LDH/ACT co-catalytic alcohol oxidation. A series of experiments and computations were performed to probe mechanistic features of the EAO reaction. In situ electrochemical impedance spectroscopy (EIS) was conducted at various potentials to investigate the interfacial behavior of Ni_0.67V_0.33-LDH/ACT^8,13,36. For Ni_0.67V_0.33-LDH in 0.5 M Na₂CO₃, the results reveal a shift to lower phase and higher frequencies with increasing applied potentials, reflecting an increase in reaction rates (Supplementary Fig. 18). As shown in Fig. 5a, the phase angle decreases significantly and frequency shifts higher after the addition of ACT and BA (blue line), indicating a faster oxidation rate and faster kinetics of EAO of BA. Meanwhile, the corresponding Nyquist plots displayed approximately vertical lines in 0.5 M Na₂CO₃ (Supplementary Fig. 19), suggesting a high charge transfer resistance for the Ni_0.67V_0.33-LDH electrode at 1.37 V vs. RHE (Fig. 5b). The Nyquist plots show a smaller semicircle when both ACT and BA are present in the electrolytes (blue line), reflecting minor charge transfer resistance that is consistent with efficient Ni_0.67V_0.33-LDH/ACT-mediated EAO⁵³. In addition, the Bode plot and Nyquist plot were also analyzed for the β-Ni(OH)₂/ACT in the batch reactor (Supplementary Figs. 20 and 21), the results revealed that the larger semicircle of the Nyquist plot compared with Ni_0.67V_0.33-LDH/ACT, implied that the transfer resistance can be reduced after the introduction of V. In situ Raman experiments were performed at various potentials to monitor the changes in the surface structure of Ni_0.67V_0.33-LDH and β-Ni(OH)₂ electrodes (Fig. 5c-f and Supplementary Fig. 22). With β-Ni(OH)₂, no discernible change is observed upon increasing the potential up to 1.37 V vs. RHE (Fig. 5c), while two new peaks become evident at 473 and 557 cm^− 1 upon increasing the potential to 1.47 V vs. RHE^8,54. These peaks are assigned to surface Ni³⁺-O vibrations upon oxidation of Ni(OH)₂ to NiOOH. Ni_0.67V_0.33-LDH exhibits a Raman signal at approximately 767 cm^− 1 that is absent in the β-Ni(OH)₂ catalyst and is attributed to a V-O stretch (Fig. 5d). Increasing the applied potential to 1.37 V vs. RHE leads to gradual enhancement of the 767 cm^− 1 peak, in addition to emergence of the NiOOH peaks at 473 and 557 cm^− 1, indicating that NiOOH forms at a lower potential in to form in Ni_0.67V_0.33-LDH catalysts at a lower potential than in β-Ni(OH)₂, thus the introduction of V facilitates the formation of NiOOH, consistent with the LSV and EIS. The peaks of NiOOH were attenuated at 1.37 V vs. RHE after the addition of ACT compared to without ACT in situ Raman spectroscopy (Fig. 5e), indicating that Ni³⁺ was initially formed on the surface of Ni_0.67V_0.33-LDH, then quickly captured the electrons, and partially transformed back into Ni²⁺ again⁵⁵. Because the reduction of the Ni³⁺ by ACT forms ACT⁺, which can oxidize BA to PhCHO, these results align with the synergistic benefits of NiV-LDH and ACT for EAO. DFT calculations illuminate the increase in electron density upon electron transfer from an adsorbed ACT to a Ni³⁺ site (Fig. 5g, h). Taken together, these results raise the possibility that enhanced formation of the more oxidizing and Lewis acidic Ni³⁺ by the V ions in the NiV-LDH promotes ACT or ACTH adsorption and oxidation to ACT⁺ species, and the adsorbed ACT⁺ species could then promote rapid alcohol oxidation (Fig. 5i).

Electrochemical oxidation of other alcohols with the synergistic NiV-LDH/ACT co-catalyst system. EAO performance was then evaluated with Ni_0.67V_0.33-LDH, ACT, and Ni_0.67V_0.33-LDH/ACT with a series of different alcohol substrates using a recirculating-flow parallel-plate electrolyzer (Fig. 6). Due to the different solubilities of the substrates, we chose different ratios of aqueous sodium carbonate and acetonitrile to ensure that the substrates completely dissolved and the electrolyte retained good conductivity. The cooperative Ni_0.67V_0.33-LDH/ACT electrocatalyst shows superior performance relative to Ni_0.67V_0.33-LDH and ACT used independently in the oxidation of primary alcohols (1a-7a) to the corresponding aldehydes, affording yields of ≥ 87% for 1b-7b in ≤ 46 min. Oxidation of the pharmaceutically relevant steroid 19-hydroxyandrost-4-ene-3,17-dione, 8a, with the Ni_0.67V_0.33-LDH/ACT co-catalyst generated the aldehyde 8b in 96% yield, which is significantly better than Ni_0.67V_0.33-LDH (2%) and ACT alone (30%). Efforts then focused on synthesis of a precursor of tibolone [(7α,17α)-17-hydroxy-7-methyl-19-norpregn-5(10)-en-20-yn-3-one]. The substrate 9a (7α-methyl-19-aldehyde-4-androstene-3,17-dione) was evaluated on 50 mmol scale. Despite the large molecular weight and poor solubility of the substrate, a current density of 150 mA/cm² (based on the electrode two-dimensional surface area) was accessible and showed stable anode potential of 1.4–1.6 V vs. RHE during 15 A constant current electrolysis with a flow rate of 2000 mL/min (Fig. 6B-a). Time-course analysis by HPLC showed that the concentration of 9a decreases steadily with parallel formation and conversion of intermediates 9a-1 and 9a-2, ultimately converging on the oxidation product 9b (Fig. 6B-b). High selectivity and yield of 9b (95% and 94%, respectively) were achieve within 25 min.

We then focused in increate the reaction productivity by increasing the reactor volume and establishing a large-scale electrochemical synthesis system for oxidation of sterol 8a to 8b with H₂ production at the counter electrode. An anode consisting of Ni_0.67V_0.33-LDH/GF and a Ni foam cathode were integrated within three electrolyzers, with two-dimensional electrode surface areas of 10, 100, and 400 cm² (Fig. 7a, b). Computational fluid dynamics (CFD) simulation revealed that enlarging the electrolyzer caused non-uniform flow rates within the reactor, particularly at the corners of the flow channels where the fluid flow is essentially absent (Fig. 7c). Introducing a cavity structure in the filled electrodes enables significantly improved fluid flow, resulting in a more uniform flow rate distribution (Fig. 7d). The LSV curve in Fig. 7e shows that the onset potential for the oxidation of sterol 8a (200 g) is 1.2 V vs. RHE, significantly lower than that of OER (Fig. 7f). Optimization of the electrolysis method, including the reaction current, electrolyte flow rate and electrolyte temperature, led to high productivity in formation of the steroid carbonyl product 8b from 8a (Fig. 7g). The resulting product exhibited a selectivity of 94% during a constant current electrolysis of 60 A in the 400 cm² electrolysis cell (Supplementary Fig. 23). The productivity associated with this reactor is 243 g/h with an energy consumption of approximately 0.71 kWh/kg. This reaction was repeated on hectogram-scale (182.7 g), affording 8b in 91% isolated yield on the basis of HPLC and ¹H NMR analysis (Supplementary Figs. 24–27). Collectively, these results demonstrated the utility of this cooperative Ni_0.67V_0.33-LDH/ACT-mediated for larger scale EAO applications.

In summary, we have demonstrated that two previously known electrocatalyst compositions, heterogeneous Ni-based LDH materials and molecular aminoxyls, show improved performance when used together for electrochemical alcohol oxidation. Experimental and computational analysis suggests that synergistic benefits arise from multiple factors. Vanadium ions in the optimal NiV-LDH material facilitates oxidation of Ni²⁺ to Ni³⁺ and adsorption of the ACT aminoxyl species to the NiV-LDH surface promotes electron transfer from the aminoxyl to Ni³⁺. The adsorbed oxoammonium, formed upon reduction of Ni³⁺ sites can promote alcohol oxidation. This cooperativity enables the Ni_0.67V_0.33-LDH/ACT electrocatalyst to support high yields and selectivities in the oxidation of primary alcohols to aldehydes (up to 99%, in both cases). The Ni_0.67V_0.33-LDH/ACT electrocatalyst shows good scope and functional group tolerance, and its utility is showcased in the oxidation of complex steroid pharmaceutical intermediates. The latter application has been demonstrating in a recirculating parallel-plate flow reactor, exhibiting a productivity of 243 g/h and a current density of 150 mA/cm². This demonstration of synergistic benefits between heterogeneous and homogeneous electrocatalysts provides a foundation for many future studies targeting practical applications of organic electrosynthesis.

Pre-treatment of GF

The Graphite Felt (GF 065) was sliced into pieces measuring approximately 3 cm × 3.5 cm × 0.6 cm. It was then subjected to successive ultrasonic treatments in 40 wt% HNO₃, acetone, ethanol, and deionized water, each for 30 min. Subsequently, the GF was dried under vacuum at 60°C overnight.

Preparation of NiM-LDHs (M = V, Mn, Fe, Co, Cu)

As an example, NiV-LDH/GF was first prepared, which was synthesized using a one-step hydrothermal method. For the preparation of NiV-LDH, a solution with different molar ratios of Ni/V (1:1, 2:1, 3:1, 4:1, 5:1 and 1:0 for the synthesis of Ni_0.50V_0.50-LDH, Ni_0.67V_0.33-LDH, Ni_0.75V_0.25-LDH, Ni_0.80V_0.20-LDH, Ni_0.83V_0.17-LDH and pure Ni(OH)₂, respectively) was synthesized by combining Ni(NO₃)₂·6 H₂O and VCl₃ in 80 ml H₂O, keeping the total amount of metal ions (Ni + V) was kept at 2 mmol. After adding 0.36 g urea (6 mmol), 0.148 g NH₄F (4 mmol), and the GF, which was uniformly dispersed by ultrasound, the mixture was transferred to a 100 mL stainless steel autoclave. The autoclave was securely sealed and the mixed solution was maintained at 120°C for 12 h. Afterwards, the autoclave was cooled to room temperature, and the NiV-LDH/GF was washed three times in succession with deionized water and ethanol three times in sequence and subjected to vacuum drying at 60°C overnight.

Moreover, the preparation of NiMn-LDH, NiFe-LDH, NiCo-LDH and NiCu-LDH was the same except that 50% Mn(NO₃)₂, Fe(NO₃)₃·9 H₂O, Co(NO₃)₂·6 H₂O and Cu(NO₃)₂·3 H₂O were used instead of VCl₃. The preparation of Ni(OH)₂ is identical to that of NiV-LDH, with the exception of the omission of the V source.

Materials Characterization

Scanning electron microscopy (SEM, Hitachi FE–SEM S–4700) and scanning transmission electron microscopy (STEM, JEM-ARM300F) were conducted to investigate the morphology and microstructure of the electrocatalyst. X-ray diffraction (XRD) was performed using an X-ray diffractometer with graphite monochromatized Cu Kα radiation (λ = 1.54 Å). The corresponding cell parameters of each sample were then calculated and analyzed using Jade software. X-ray photoelectron spectroscopy (XPS) data were collected using a Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectrometer. Brunauer–Emmett–Teller (BET) specific surface area and pore distribution analysis were conducted using an ASAP2460 analyzer. Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP–AES) analysis was conducted to accurately measure the weight loading of nickel (Ni) and vanadium (V). In situ Raman spectra were characterized using a confocal Raman imaging microscope (Renishaw InVia, 532 nm laser) under different potentials (1.27 to 1.77 V vs. RHE) using an Ivium-n-Stat electrochemical workstation in a three-electrode system.

Electrochemical measurements

Cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronopotentiometry (CP), chronoamperometry (CA), and electrochemical aminoxyl-mediated oxidation were conducted using an Ivium-n-Stat electrochemical workstation in a three-electrode system, with an Ag/AgCl electrode employed as the reference electrode. The electrolysis experiments were investigated using a continuous-flow parallel-plate electrolyzer with electrode area of 10 cm², in which Ni-based materials as the anode and nickel foam as the cathode. The anolyte comprised of 10 mmol benzyl alcohol (BA, 1.08 g) and 5 mol% ACT in 100 mL 0.5 M Na₂CO₃ solution, with the 0.5 M Na₂CO₃ serving as the catholyte. The mixture was stirred to ensure uniformity, and a peristaltic pump was used to transport the electrolyte into the reaction systems. The electrolysis reaction was conducted at room temperature using a constant potential of 1.45 V vs. RHE. In addition, the small-scale up and large-scale up continuous flow experiments were carried out at a constant current, with electrode areas of 100 and 400 cm², respectively. In the enlarged electrolyzer, the electrodes are not fully distributed across the entire PTFE chamber (Fig. 5a). Instead, its upper and lower extremities form a cavity structure, which is favorable to the distribution and flow of electrolyte.

The CV, LSV, CP and electrochemical aminoxyl-mediated oxidation were carried out in a batch reactor using a three-electrode system consisting of Ni_0.67V_0.33-LDH/GF as the working electrode, a Pt wire as the counter electrode, and Ag/AgCl as the reference electrode. The anolyte consisted of 10 mmol BA (1.08 g) and 5 mol% ACT in 100 mL 0.5 M Na₂CO₃ solution.

In situ electrochemical impedance spectroscopy (EIS)

In situ EIS measurements were performed using an Ivium-n-Stat electrochemical workstation with a three-electrode system. The frequency range was 0.1 Hz to 100,000 Hz with an amplitude of 5 mV, and the applied potential was varied from 1.27 V to 1.67 V vs. RHE with a 0.05 V interval¹³.

Calculation of Conversion, Selectivity, Faradaic Efficiency

The Conversion (%), the Selectivity (%) and Faradaic efficiency (FE) were calculated as shown in (1), (2) and (3):

$$\:\text{C}\text{o}\text{n}\text{v}\text{e}\text{r}\text{s}\text{i}\text{o}\text{n}\:\left(\text{%}\right)=\:\frac{mol\:of\:substrate\:consumed}{mol\:of\:initial\:substrate}\:\times\:100\text{%}\:,\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(1\right)$$

$$\:\text{S}\text{e}\text{l}\text{e}\text{c}\text{t}\text{i}\text{v}\text{i}\text{t}\text{y}\:\left(\text{%}\right)=\:\frac{mol\:of\:\text{m}\text{a}\text{i}\text{n}\:\text{p}\text{r}\text{o}\text{d}\text{u}\text{c}\text{t}\:formed}{mol\:of\:substrate\:consumed}\:\times\:100\text{%}\:,\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(2\right)$$

$$\:\text{F}\text{E}\:\left(\text{%}\right)=\:\frac{mol\:of\:\text{m}\text{a}\text{i}\text{n}\:\text{p}\text{r}\text{o}\text{d}\text{u}\text{c}\text{t}\:\:formed}{total\:charge\:passed\:/\:(n\:\times\:F)}\:\times\:100\text{%}\:,\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(3\right)$$

Where n and F are the number of electron transfers and the Faraday constant (96485 C mol^− 1), respectively.

Theoretical calculations

Periodic DFT calculations were performed using the Vienna ab initio simulation package (VASP)⁵⁶. The exchange correlation function of Perdew-Burke-Ernzerhof (PBE) and the projected augmented wave (PAW) method⁵⁷ were used to describe the ion-electron interaction. The cut-off energy for the plane wave basis sets was 400 eV. The O 2s2p and V 3p4s3d electrons are treated as valence states. The convergence criteria of geometry and energy were set to 0.02 eV/Å and 10^− 6 eV, respectively. In addition, strong correlation effects due to charge localization are modelled by adding a Hubbard U-like term⁵⁸ to the PBE function. And the effect U value of 5.50 eV for the Ni 3d states was applied Spin polarization was considered for all calculations in this work. For the NiOOH (001) surface, the p-(5×5) supercell with a thickness of one layer was constructed based on the bulk phase (space group: P3m1), the Gamma (1×1×1) k-point mesh was used to reduce the computational cost due to the large cell. The vacuum space of 15 Å was added to the z-direction of the slab modules.

Data availability

All data that support the findings of this study are present in the paper and the Supplementary Information. Further information can be acquired from the corresponding authors.

Acknowledgements

Suiqin Li, Shibin Wang and Yuhang Wang contributed equally to this work. The authors acknowledge the financial support from the National Key R & D Program of China (2022YFA1504200), Zhejiang Provincial Natural Science Foundation of China (No.LR22B060003), National Natural Science Foundation of China (22322810, 22078293, 22141001, and 22008211), the Fundamental Research Funds for the Provincial Universities of Zhejiang (RF-C2023004), China Postdoctoral Science Foundation (2024M752866), Postdoctoral Fellowship Program of CPSF (GZB20230662), and Zhejiang Provincial Postdoctoral Research Project Merit-based Funding (ZJ2023016).

Author contributions

S.Q.L. synthesized the samples and performed the characterizations. S.B.W. contributed to theoretical calculation. Y.H.W. performed the partial experiments. J.H.H. and K.L. contributed to the discussion of the results. J.B.Gerken and S.S.Stahl. contributed to the discussion of the mechanism. X.Z. and J.G.W. designed the research, and wrote the paper. All the authors commented on and revised the manuscript.

Competing interests

The authors declare no competing interests.

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Mo Y et al (2020) Microfluidic electrochemistry for single-electron transfer redox-neutral reactions. Science 368:1352–1357
Nutting JE, Rafiee M, Stahl SS (2018) Tetramethylpiperidine N-Oxyl (TEMPO), Phthalimide N-Oxyl (PINO), and Related N-Oxyl Species: Electrochemical Properties and Their Use in Electrocatalytic Reactions. Chem Rev 118:4834–4885
Ma C et al (2021) Recent advances in organic electrosynthesis employing transition metal complexes as electrocatalysts. Sci Bull 66:2412–2429
Liu C, Chen F, Zhao BH, Wu Y, Zhang B (2024) Electrochemical hydrogenation and oxidation of organic species involving water. Nat Rev Chem 8:277–293
Rafiee M, Mayer MN, Punchihewa BT, Mumau MR (2021) Constant Potential and Constant Current Electrolysis: An Introduction and Comparison of Different Techniques for Organic Electrosynthesis. J Org Chem 86:15866–15874
Wang F, Stahl SS (2020) Electrochemical Oxidation of Organic Molecules at Lower Overpotential: Accessing Broader Functional Group Compatibility with Electron – Proton Transfer Mediators. Acc Chem Res 53:561–574
Ge R et al (2022) Selective Electrooxidation of Biomass-Derived Alcohols to Aldehydes in a Neutral Medium: Promoted Water Dissociation over a Nickel-Oxide-Supported Ruthenium Single-Atom Catalyst. Angew Chem Int Ed 61:e202200211
Chen W et al (2020) Activity Origins and Design Principles of Nickel-Based Catalysts for Nucleophile Electrooxidation. Chem 6:2974–2993
Chen X et al (2019) Defect engineering of nickel hydroxide nanosheets by Ostwald ripening for enhanced selective electrocatalytic alcohol oxidation. Green Chem 21:578–588
Li S et al (2019) Biomass Valorization via Paired Electrosynthesis Over Vanadium Nitride-Based Electrocatalysts. Adv Funct Mater 29:1904780
Zheng J et al (2017) Hierarchical Porous NC@CuCo Nitride Nanosheet Networks: Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting and Selective Electrooxidation of Benzyl Alcohol. Adv Funct Mater 27:1704169
Yan Y et al (2023) Electrocatalytic Upcycling of Biomass and Plastic Wastes to Biodegradable Polymer Monomers and Hydrogen Fuel at High Current Densities. J Am Chem Soc 145:6144–6155
Zhou B et al (2021) Platinum Modulates Redox Properties and 5-Hydroxymethylfurfural Adsorption Kinetics of Ni(OH)₂ for Biomass Upgrading. Angew Chem Int Ed 60:22908–22914
Bender MT, Lam YC, Hammes-Schiffer S, Choi K-S (2020) Unraveling Two Pathways for Electrochemical Alcohol and Aldehyde Oxidation on NiOOH. J Am Chem Soc 142:21538–21547
Zhang H et al (2024) On the Activity and Selectivity of 5-Hydroxymethylfurfural Electrocatalytic Oxidation over Cation-Defective Nickel Hydroxides. ACS Catal 14:9565–9574
Zeng L et al (2023) Cooperative Rh-O₅/Ni(Fe) Site for Efficient Biomass Upgrading Coupled with H₂ Production. J Am Chem Soc 145:17577–17587
Jiang J et al (2018) Atomic-level insight into super-efficient electrocatalytic oxygen evolution on iron and vanadium co-doped nickel (oxy)hydroxide. Nat Commun 9:2885
Huang H et al (2020) Co hydroxide triggers electrocatalytic production of high-purity benzoic acid over 400 mA cm^– 2. Energy Environ Sci 13:4990–4999Ni
Yang G et al (2022) Unraveling the mechanism for paired electrocatalysis of organics with water as a feedstock. Nat Commun 13:3125
Wang C, Wu Y, Bodach A, Krebs ML, Schuhmann W, Schüth F (2023) A Novel Electrode for Value-Generating Anode Reactions in Water Electrolyzers at Industrial Current Densities. Angew Chem Int Ed 62:e202215804
Rafiee M, Konz ZM, Graaf MD, Koolman HF, Stahl SS (2018) Electrochemical Oxidation of Alcohols and Aldehydes to Carboxylic Acids Catalyzed by 4-Acetamido-TEMPO: An Alternative to Anelli and Pinnick Oxidations. ACS Catal 8:6738–6744
Rafiee M, Alherech M, Karlen SD, Stahl SS (2019) Electrochemical Aminoxyl-Mediated Oxidation of Primary Alcohols in Lignin to Carboxylic Acids: Polymer Modification and Depolymerization. J Am Chem Soc 141:15266–15276
Rafiee M, Miles KC, Stahl SS (2015) Electrocatalytic Alcohol Oxidation with TEMPO and Bicyclic Nitroxyl Derivatives: Driving Force Trumps Steric Effects. J Am Chem Soc 137:14751–14757
Taitt BJ, Bender MT, Choi K-S (2020) Impacts of the Regeneration Pathways of the Oxoammonium Cation on Electrochemical Nitroxyl Radical-Mediated Alcohol Oxidation. ACS Catal 10:265–275
Goes SL, Mayer MN, Nutting JE, Hoober-Burkhardt LE, Stahl SS, Rafiee M (2021) Deriving the Turnover Frequency of Aminoxyl-Catalyzed Alcohol Oxidation by Chronoamperometry: An Introduction to Organic Electrocatalysis. J Chem Educ 98:600–606
Cardiel AC, Taitt BJ, Choi K-S, Stabilities (2019) Regeneration Pathways, and Electrocatalytic Properties of Nitroxyl Radicals for the Electrochemical Oxidation of 5-Hydroxymethylfurfural. ACS Sustain Chem Eng 7:11138–11149
Ryan MC, Whitmire LD, Mccann SD, Stahl SS (2019) Copper/TEMPO Redox Redux: Analysis of PCET Oxidation of TEMPOH by Copper(II) and the Reaction of TEMPO with Copper(I). Inorg Chem 58:10194–10200
Badalyan A, Stahl SS (2016) Cooperative electrocatalytic alcohol oxidation with electron-proton-transfer mediators. Nature 535:406–410
Nutting JE, Mao K, Stahl SS (2021) Iron(III) Nitrate/TEMPO-Catalyzed Aerobic Alcohol Oxidation: Distinguishing between Serial versus Integrated Redox Cooperativity. J Am Chem Soc 143:10565–10570
Li S et al (2023) Chromium-Doped Nickel Oxide and Nickel Nitride Mediate Selective Electrocatalytic Oxidation of Sterol Intermediates Coupled with H₂ Evolution. Angew Chem Int Ed 62:e202306553
He J et al (2023) Highly efficient electrocatalytic oxidation of sterol by synergistic effect of aminoxyl radicals and Se–Ni₅P₄. AIChE J 69:e18153
Wang M et al (2024) Process intensification enhanced continuous flow for the effective coupling of sterol electrooxidation with H₂ evolution using NiMo-based electrocatalysts. Chem Eng Sci 285:119589
Wang H et al (2023) Boosting 5-hydroxymethylfurfural electrooxidation in neutral electrolytes via TEMPO-enhanced dehydrogenation and OH adsorption. Chin J Catal 46:148–156
Chadderdon XH, Chadderdon DJ, Pfennig T, Shanks BH, Li W (2019) Paired electrocatalytic hydrogenation and oxidation of 5-(hydroxymethyl)furfural for efficient production of biomass-derived monomers. Green Chem 21:6210–6219
Li S et al (2022) Reaction and transport co-intensification enhanced continuous flow electrocatalytic aminoxyl-mediated oxidation of sterol intermediates by 3D porous framework electrode. Chem Eng J 446:136659
Chen W et al (2022) Activated Ni–OH Bonds in a Catalyst Facilitates the Nucleophile Oxidation Reaction. Adv Mater 34:2105320
Li S et al (2023) Doped Mn Enhanced NiS Electrooxidation Performance of HMF into FDCA at Industrial-Level Current Density. Adv Funct Mater 33:2214488
Yang Z, Zhang B, Yan C, Xue Z, Mu T (2023) The pivot to achieve high current density for biomass electrooxidation: Accelerating the reduction of Ni³⁺ to Ni²⁺. Appl Catal B-Environ 330:122590
Zhao T et al (2021) Situ Reconstruction of V-Doped Ni₂P Pre-Catalysts with Tunable Electronic Structures for Water Oxidation. Adv Funct Mater 31:2100614
Ma L et al (2016) Self-assembled ultrathin NiCo₂S₄ nanoflakes grown on Ni foam as high-performance flexible electrodes for hydrogen evolution reaction in alkaline solution. Nano Energy 24:139–147
Paul R et al (2019) 3D Heteroatom-Doped Carbon Nanomaterials as Multifunctional Metal-Free Catalysts for Integrated Energy Devices. Adv Mater 31:1805598
Wang D, Li Q, Han C, Lu Q, Xing Z, Yang X (2019) Atomic and electronic modulation of self-supported nickel-vanadium layered double hydroxide to accelerate water splitting kinetics. Nat Commun 10:3899
He D et al (2020) In-situ optimizing the valence configuration of vanadium sites in NiV-LDH nanosheet arrays for enhanced hydrogen evolution reaction. J Energy Chem 47:263–271
Tyagi A, Joshi MC, Shah A, Thakur VK, Gupta RK (2019) Hydrothermally Tailored Three-Dimensional Ni–V Layered Double Hydroxide Nanosheets as High-Performance Hybrid Supercapacitor Applications. ACS Omega 4:3257–3267
Liu Q et al (2019) Tuning the coupling interface of ultrathin Ni₃S₂@NiV-LDH heterogeneous nanosheet electrocatalysts for improved overall water splitting. Nanoscale 11:8855–8863
Gong M et al (2013) An Advanced Ni–Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation. J Am Chem Soc 135:8452–8455
Yang W et al (2019) Rapid room-temperature fabrication of ultrathin Ni(OH)₂ nanoflakes with abundant edge sites for efficient urea oxidation. Appl Catal B-Environ 259:118020
Fan K et al (2016) Nickel–vanadium monolayer double hydroxide for efficient electrochemical water oxidation. Nat Commun 7:11981
Dinh KN et al (2018) Ultrathin Porous NiFeV Ternary Layer Hydroxide Nanosheets as a Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting. Small 14:1703257
Sun X et al (2021) In Situ Construction of Flexible V–Ni Redox Centers over Ni-Based MOF Nanosheet Arrays for Electrochemical Water Oxidation. Small Methods 5:2100573
Luo R et al (2022) A dynamic Ni(OH)₂-NiOOH/NiFeP heterojunction enabling high-performance E-upgrading of hydroxymethylfurfural. Appl Catal B-Environ 311:121357
Sun H et al (2017) Porous Multishelled Ni₂P Hollow Microspheres as an Active Electrocatalyst for Hydrogen and Oxygen Evolution. Chem Mater 29:8539–8547
Gu K et al (2021) Defect-Rich High‐Entropy Oxide Nanosheets for Efficient 5‐Hydroxymethylfurfural Electrooxidation. Angew Chem Int Ed 133:20415–20420
Gao L et al (2021), Operando unraveling photothermal-promoted dynamic active-sites generation in NiFe₂O₄ for markedly enhanced oxygen evolution. Proc. Nat. Acad. Sci. 118, e2023421118
Huang C, Huang Y, Liu C, Yu Y, Zhang B (2019) Integrating Hydrogen Production with Aqueous Selective Semi-Dehydrogenation of Tetrahydroisoquinolines over a Ni₂P Bifunctional Electrode. Angew Chem Int Ed 58:12014–12017
Kresse G, Furthmüller J (1996) Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comp Mater Sci 6:15–50
Kresse G, Joubert D (1999) From ultrasoft pseudopotentials to the projector augmented-wave method. Phys Rev B 59:1758–1775
Dudarev SL, Botton GA, Savrasov SY, Humphreys CJ, Sutton AP (1998) Electron-energy-loss spectra and the structural stability of nickel oxide: An LSDA + U study. Phys Rev B 57:1505–1509

Table 1 is available in the Supplementary Files section.

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Synergistic Enhancement of Electrochemical Alcohol Oxidation by Combining NiV-Layerd Double Hydroxide with an Aminoxyl Radical

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Abstract

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Introduction

Results

Discussion

Methods

Pre-treatment of GF

Preparation of NiM-LDHs (M = V, Mn, Fe, Co, Cu)

Materials Characterization

Electrochemical measurements

In situ electrochemical impedance spectroscopy (EIS)

Calculation of Conversion, Selectivity, Faradaic Efficiency

Theoretical calculations

Declarations

References

Tables

Additional Declarations

Supplementary Files

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