Combining anodic alcohol oxidative coupling for C–C bond formation with cathodic ammonia production

ABSTRACT Electrocatalytic oxidation of alcohols using heterogeneous catalysts is a promising aqueous, energy-efficient and environmentally friendly approach, especially for coupling different alcohols to prolong the carbon chain via co-oxidation. Precisely regulating critical steps to tailor electrode materials and electrolyte composition is key to selectively coupling alcohols for targeted synthesis. However, selectively coupling different alcohols remains challenging due to the lack of effective catalyst and electrolyte design promoting specific pathways. Herein, we demonstrate a paired electrolysis strategy for combining anodic oxidative coupling of ethanol (EtOH) and benzyl alcohol (PhCH2OH) to synthesize cinnamaldehyde (CAL) and cathodic ammonia production. The strategies involve: (i) utilizing the salt-out effect to balance selective oxidation and coupling rates; (ii) developing platinum-loaded nickel hydroxide electrocatalysts to accelerate intermediate coupling kinetics; (iii) introducing thermodynamically favorable nitrate reduction at the cathode to improve coupling selectivity by avoiding hydrogenation of products while generating valuable ammonia instead of hydrogen. We achieved 85% coupling selectivity and 278 μmol/h NH3 productive rate at 100 mA/cm2 with a low energy input (∼1.63 V). The membrane-free, low energy, scalable approach with a wide substrate scope highlights promising applications of this methodology. This work advances heterogeneous electrocatalytic synthesis through rational design principles that integrate anodic oxidative coupling with cathodic nitrate reduction reactions, having synergistic effects on efficiency and selectivity.


INTRODUCTION
The utilization of renewable energy for the synthesis of value-added chemicals wi l l be a significant part of the future development of sustainability [1 ,2 ].Electrochemical synthesis has gained momentum as a green alternative for establishing environmentally friendly and sustainable processes in recent years, since it takes advantage of clean electrons to replace dangerous and toxic redox reagents [3 -6 ].The past decade has witnessed the rapid development of numerous electrochemical reactions for synthesizing various important molecules [7 -10 ], such as a coupling reaction that can directly link two different molecules together and allow for a high atomic economy.However, most established electrochemi-cal coupling reactions use low-conductivity organic solvents, resulting in slow kinetics.Additionally, organic electrolytes lead to complex post-treatment and high separation costs [11 ,12 ].
Water is an abundant resource that can be split by renewable energy to generate active hydrogen and ox ygen species, w hich are often reactive intermediates in catalytic reactions [13 ,14 ].Utilizing these renewable H/O species to reduce or oxidize organics for value-added chemicals is a promising green route due to its low energy input, avoidance of chemical reagents and mild conditions [15 ,16 ].Electrocatalytic conversion in aqueous solutions has gained widespread interest, as it is a highly efficient route for oxidizing alcohols, aldehydes and amines into value-added products [17 -20 ].Alcohol oxidation is a primary focus of energy and chemical conversion, with potential applications in biomass utilization and fine-chemical synthesis [21 -24 ].Ni-based electrocatalysts are promising candidates, as the multivalent Ni can facilitate organic oxidation [25 -28 ].
The alcohol oxidation reaction on Ni-based electrocatalysts involves many intermediate species, such as *RCHOH, *RCHO, *RCH(OH) 2 and *RC(OH) 2 [29 ,30 ].However, a single acid product is usually the result, since intermediates with faster kinetics dominate in most reported electrocatalytic alcohol oxidation reactions.
Aldehydes are highly usef ul sy nthetic intermediates that can undergo diverse reactions to form many products [31 ,32 ].Coupling the key intermediate *RCHO from alcohol oxidation at the anode with other nucleophilic species allows the synthesis of high-value chemicals [33 ].For example, two identical or different aldehydes can produce α, β-unsaturated aldehydes by aldol condensation, which is widely used to make pharmaceutical and fine-chemical intermediates.Directly synthesizing such compounds from alcohols via co-oxidative coupling is more scientifically valuable and impactful than post-condensation approaches [34 ,35 ].However, effective methods to achieve co-oxidative C = C bond formation bet ween t wo alcohols remain elusive, limiting the utility of alcohol oxidation.In our previous work, the selective conversion of alcohol to aldehyde was achieved by reducing the alkalinity of the electrolyte and developing a salting-out strategy [36 ].While this is encouraging, matching the selective oxidation rates between two alcohols and overcoming differences in key intermediate adsorption/activation remain challenges for electrocatalytic coupling to higher-value chemicals.Our aim is to develop an electrocatalytic co-oxidative coupling strategy to directly synthesize α, β-unsaturated alde-hydes from alcohols, addressing a major challenge in synthetic chemistry.Advances in catalyst and electrolyte design are needed to realize the significant potential of co-oxidative alcohol coupling reactions.
In this work, we report a novel paired electrolysis approach that integrates the synthesis of a valueadded chemical with sustainable ammonia production (Fig. 1 ).Specifically, we develop a bifunctional electrolysis cell that couples the anodic oxidative coupling of ethanol (EtOH) and benzyl alcohol (PhCH 2 OH) to generate cinnamaldehyde (CAL) with the cathodic reduction of nitrate to ammonia.The integration of experiment, theoretical simulation and in situ electrochemical spectroscopy reveals that suppressing the over-oxidation of alcohols by the salt-out strategy and accelerating cross-coupling of key intermediates by developing efficient electrocatalysts, is vital for the coupling reaction.More importantly, the nitrate reduction reaction (NO 3 RR) was employed to construct a novel pair electrolysis system, which can not only produce more valuable NH 3 at the cathode, but also effectively improve the selectivity of the coupling reaction by inhibiting the hydrogenation and reducing energy consumption.Finally, this strategy was demonstrated to be facile for scaled-up applications.

RESULTS AND DISCUSSION
Nickel hydroxide (Ni(OH) 2 ) is a versatile, inexpensive and efficient electrocatalyst for the electrochemical oxidation of alcohols.Ni(OH)O generated by applying an external potential to the Ni(OH) 2 electrode is the active species that catalyzes the dehydrogenation of alcohols to aldehydes, ketones or acids ( Fig. S1 in Supplementary Data).For the electrochemical co-oxidation coupling reaction of alcohols, Ni(OH) 2 was chosen as the anodic electrocatalyst.It was synthesized by a hydrothermal method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), confirming its structure, morphology and composition ( Figs S2-S4).Ethanol (EtOH) and Benzyl alcohol (PhCH 2 OH) were selected as model substrates for the reaction, which are the precursors to the synthesis of CAL.The electro-oxidation of EtOH and PhCH 2 OH over the Ni(OH) 2 was carried out.Linear sweep voltammetry (LSV) was first performed to evaluate the electrocatalytic oxidation of EtOH and PhCH 2 OH over the Ni(OH) 2 electrocatalyst ( Fig. S5).The oxidation current of PhCH 2 OH was larger than that of EtOH at an identical concentration, attributed to the lower bond dissociation free energy (BDFE) of PhCH 2 OH facilitating the reaction with Ni(OH)O.In accordance w ith our prev ious work, the bulk electrolysis experiment was conducted in 1.5 M K 2 CO 3 with 100 mM PhCH 2 OH and 200 mM EtOH under 100 mA/cm 2 current density in an undivided cell.The distribution of products was analyzed by gas chromatography-mass spectrometry and 1 H-nuclear magnetic resonance ( 1 H NMR) ( Figs S6 a nd S7).However, no coupling product was observed after the reaction.Increasing EtOH concentration resulted in minor CAL, indicating selective alcohol oxidation alone does not enable cross-coupling ( Fig. S8).
The key for alcohol coupling is dehydration condensation between in-situ -generated PhCHO and CH 3 CHO intermediates.Density functional theory (DFT) calculations analyzed the thermodynamics of the non-electrochemical reaction to improve coupling efficiency.The deprotonation of CH 3 CHO was identified as the rate-determining step (RDS), with an energy barrier of 14.2 kcal/mol under alkaline conditions and 67.3 kcal/mol at neutral condi-tions.This implies that the RDS is strongly influenced by solution pH ( Fig. S9).Based on the above analysis, we investigated the effects of pH on the electrochemical co-oxidation of PhCH 2 OH and EtOH (Fig. 2 a).In the mixture of 0.1 M KOH and 0.45 M K 2 CO 3 (pH ∼13), 63% conversion but only 16% CAL selectivity was observed, along with PhCOOH generation.This is attributed to the lower aldehyde hydration barrier at high alkalinity ( Figs S10 and S11), as evidenced by NMR showing increased hydrated aldehyde content from 23% to 47% when increasing KOH from 0.01 M to 0.1 M ( Figs S12 and S13).In 1 M KOH (pH ∼14), PhCOOH became the major product.Under high alkalinity, the faster kinetics of CH 3 CHO hydration (barrier 10.5 kcal/mol) dominate over slower CH 3 CHO deprotonation (barrier 14.2 kcal/mol), resulting in oxidation to CH 3 COOH rather than coupling ( Figs S14 and S15).
Balancing alcohol over-oxidation and crosscoupling rates is key for efficient coupling reactions.Specifically, we believe that maintaining a certain alkalinity and cation concentration wi l l achieve efficient coupling reactions.The effects of varying alkalinity (0.1-0.5 M KOH) and cation concentrations (0.5-3 M K 2 CO 3 ) on the reaction were investigated (Fig. 2 b and Figs S16 and S17).In mixed KOH/K 2 CO 3 electrolytes, CAL selectivity increased with K 2 CO 3 concentration from 0.5 to 1.5 M, then decreased above 1.5 M. The optimized 0.4 M KOH and 1.5 M K 2 CO 3 electrolyte achieved 88% conversion and 55% CAL selectivity.Cations modulate hydration whi le al kalinity promotes deprotonation for cross-coupling.Evaluating other alkali metal ions showed slightly lower CAL selectivity with Na 2 CO 3 and Li 2 CO 3 versus K 2 CO 3 (Fig. 2 c).
Molecular dynamics (MD) simulations provided insights into the effect of cation concentration on reaction kinetics.With increasing K 2 CO 3 , aldehydes transitioned from disordered dispersion to aggregation ( Figs S18 and S19).Radial distribution function (RDF) and integrated RDF showed lower H 2 O density around aldehydes but higher H 2 O density around K + in concentrated electrolytes ( Figs S20-S22).The coordination number (CN) of aldehydes decreased while the CN of K + increased with higher K 2 CO 3 (Fig. 2 d).Additionally, fewer aldehyde-H 2 O hydrogen bonds (HBs) but more aldehyde-aldehyde HBs occurred at higher cation levels ( Fig. S23).This indicates that cations disrupt aldehyde-H 2 O HBs but enhance aldehyde-aldehyde interactions, causing aggregation.RDF results also showed increased CH 3 CHO density and CN around PhCHO at higher K 2 CO 3 concentrations (Fig. 2 e and f).DFT revealed larger PhCHO-CH 3 CHO binding energy with more surrounding CH 3 CHO (Fig. 2 g).In summary, cations induce aldehyde aggregation, increasing local CH 3 CHO concentration around PhCHO to kinetically favor coupling (Fig. 2 h).Precisely modulating the cation concentration is thus critical for activating intermediates for co-oxidative electrosynthesis.
Cation-induced aldehyde aggregation and increased local CH 3 CHO concentration around Ph-CHO improves coupling kinetics.Based on this, we hypothesized that a catalyst with faster alcohol oxidation could enhance PhCHO-CH 3 CHO interactions by generating more CH 3 CHO intermediates, further promoting aggregation.Thus, we focused on developing the anode catalyst.With this idea in mind, we synthesized a platinum-loaded Ni(OH) 2 catalyst (Pt-Ni(OH) 2 ) via a hydrothermal method using Ni(OH) 2 as the precursors, which is a good electrocatalyst for the alcohol oxidation reaction [37 ]. Figure 3 a shows that the diffraction peaks of Pt/Ni(OH) 2 could be indexed to cubic-phased Ni(OH) 2 ( JCPDS 14-0117) and cubic-phased Pt ( JCPDS 04-0802), respectively, indicating that Pt-Ni(OH) 2 was successf ully sy nthesized.XPS was used to probe the surface chemical composition of the materials of the prepared samples.As shown in Fig. 3 b, the Pt-Ni(OH) 2 sample exhibits prominent characteristic peaks at binding energies of 71.5 and 74.7 eV, corresponding to Pt 4 f 7/2 and 4 f 5/2 of Pt 0 , respectively.The electronic interaction between Pt and Ni(OH) 2 is proved by the positive binding energy shift of Pt 4f compared to Pt/C.It is also discerned by Ni 2p XPS spectra that there is no significant change in the chemical state of Ni (II) after Pt loading on Ni(OH) 2 surfaces (Fig. 3 b and c).The Pt content in Pt-Ni(OH) 2 is ∼11.6 wt%, which is quantitatively measured with an inductively coupled plasma mass spectrometry (ICP-MS) technique.
EtOH oxidation performance was evaluated on Pt-Ni(OH) 2 and Ni(OH) 2 .Pt-Ni(OH) 2 showed an advanced oxidation peak and larger catalytic current for EtOH oxidation versus Ni(OH) 2 , indicating faster EtOH oxidation kinetics (Fig. 3 d).The evolution of surface species on the Ni(OH) 2 electrode was carried out to analyze the active sites of the reaction.In situ Raman spectroscopy was used to monitor the bending and stretching vibrations of Ni 3 + -O at 473 and 553 cm −1 .For Ni(OH) 2 , Ni 3 + -O species are accumulated after 1.40 V (no EtOH) and 1.55 V (100 mM EtOH), respectively.For the Pt-Ni(OH) 2 , Ni 3 + -O species are accumulated after 1.35 V when no EtOH is present, meaning that Pt loading can facilitate the generation of Ni 3 + -O.Furthermore, Ni 3 + -O species cannot be accumulated within 1.55 V when 100 mM EtOH is present, indicating enhanced reaction dynamics due to the induction effects of Pt (Fig. 3 e and f).In situ electrochemical impedance spectroscopy (EIS) was further adopted to probe the interface behavior under different potentials.As shown in Fig. 3 g and Fig. S24, the presence of Pt promoted the electron transfer of Pt-Ni(OH) 2 with smaller charge transfer resistance ( R ct ) at a relatively lower voltage, verifying the positive effect of Pt loading.The phase angle value of the Pt-Ni(OH) 2 material in the low-frequency region is significantly lower than that of Ni(OH) 2 , which means that the reaction kinetics of the composite are faster (Fig. 2 h and Fig. S25).
Chronoamperometry experiments were conducted for the electrochemical co-oxidation coupling reaction, demonstrating that Pt-Ni(OH) 2 catalysts can achieve the same current density as unmodified Ni(OH) 2 with lower energy input ( Fig. S26).Using Pt-Ni(OH) 2 as the anodic catalyst resulted in increased conversion of the alcohol substrates as well as higher selectivity towards the desired product CAL (Fig. 3 i and Fig. S27).In contrast, nickel oxide (NiO) catalysts with poor alcohol   oxidation ability gave decreased yields and faradaic efficiencies.Notably, a cyclic hydrodimer (CHD) byproduct was observed during the reaction, with the CHD selectivity increasing concurrently with CAL.In particular, the Pt-Ni(OH) 2 catalyst with superior performance produced a 30% yield of CHD.Furthermore, a control experiment was conducted in the divided cell using CAL as the substrate, and the resulting gas chromatography mass spectrometry (GC-MS) spectra are presented in Fig. S28 after the reaction.The spectra reveal the presence of CHD products, indicating CHD is likely formed through reduction of CAL.
To elucidate the mechanism of CHD generation, the electrochemical process at the cathode was investigated.LSV of the hydrogen evolution reaction (HER) was performed using a platinum mesh cathode in mixed 0.4 M KOH and varying concentrations of K 2 CO 3 ( Fig. S29).Increasing the K 2 CO 3 concentration resulted in decreased HER current density.This can be attributed to the formation of hydration shells around K + ions by water molecules, reducing availability of protons required for HER.
We hypothesize that the production of CHD stems from the increased salt concentration lowering the cathode proton source, such that the hydrogen evolution current cannot match the anodic alcohol oxidation current.Under these conditions, CAL produced at the anode transfers to the cathode and undergoes reductive dimerization to generate CHD (Fig. 4 a).This provides the required cathodic current to balance the anodic current, but decreases the yield of the desired CAL product.
Inhibiting the reduction of CAL at the cathode is critical for obtaining high selectivity and yield for the electrocatalytic coupling reaction.An effective solution is to introduce a thermodynamic, more favorable alternative reaction at the cathode, which is more readily reducible than CAL and even than HER; this would avoid the reduction of CAL at the cathode.As a result, we focus our attention on replacing the HER at the cathode.The electrocatalytic nitrate reduction reaction (NO 3 − RR) has made significant progress recently, both fundamentally and for practical applications in NH 3 production.Theoretically, the potential for NO 3 − RR is + 0.69 V

Electrochemical reduction
Ra dical coup ling vs. the reversible hydrogen electrode (RHE) under alkaline conditions [38 ].This potential is much more positive than that for the HER.Given its more favorable thermodynamics, implementing NO 3 − RR at the cathode can effectively replace HER and avoid the undesired reduction of organic compounds like CAL.Compared with the twoelectron alcohol oxidation reaction, NO 3 − RR is an eight-electron process.This means the production rate of aldehydes is much faster than that of ammonia.Controlled experiments demonstrate that both PhCHO and CAL can achieve higher raw material recovery across a range of ammonia concentrations from 0 to 200 mM ( Fig. S30).Using PhCH 2 OH and NH 3 as substrates, PhCOOH was the primary product after electrolysis.Only small amounts of C −N coupled products formed even at 200 mM ammonia ( Fig. S31).The highly positive potential of NO 3 − RR render it an ideal cathodic reaction to pair with the electro-oxidation of alcohols at the anode.
Keeping the K + ion concentration at 3.4 M, the effects of nine cathode catalysts (Pt, Ni, Mo, Ag, Ti, Fe, Co, Cu and oxide-derived-Cu (OD-Cu) foil) on the reaction were evaluated in the electrolyte of 0.4 M KOH, 1.0 M K 2 CO 3 and 1.0 M KNO 3 (Fig. 4 b).Of these, eight metal foils were purchased commercially, and OD-Cu was synthesized by a wet chemical oxidation method, followed by electrochemical reduction to increase the electrochemical surface area.With nitrate addition, CAL selectivity remained largely unchanged with a Pt cathode.Ni, Mo and Ti cathodes resulted in lower CAL selectivity and faradaic efficiency.In contrast, Fe, Ag, Co and Cu increased CAL selectivity and decreased CHD.Excitingly, the OD-Cu cathode achieved 91% conversion with 85% CAL selectivity and no detected CHD.
The electrochemical activity for NO 3 − RR was investigated on different cathode electrocatalysts as shown in Fig. 4  charge transfer kinetics with OD-Cu.This agrees with and helps explain the higher NO 3 − RR activity of OD-Cu observed.As a result, the high NO 3 − RR activity and kinetics of OD-Cu enabled inhibition of CAL reduction at the cathode and improved selectivity.
The effect of nitrate concentration was evaluated (Fig. 4 e).With 0.5 M KNO 3 , 7% CHD yield was detected, decreasing CAL yield.However, CAL yield also decreased at KNO 3 concentrations above 1.0 M.This may be due to anodic oxidation of nitrogencontaining intermediates (e.g.NO 2 − , NH 2 OH and NH 3 ) formed during nitrate reduction, competing with alcohol oxidation.Using a divided cell resulted in 79% CAL yield (Fig. 4 f).Compared with a divided cell, pairing with nitrate reduction maintains almost unchanged conversion and selectivity without expensive membranes, making it a promising HER alternative in undivided electrolytic cells.Moreover, this strategy only required a cell voltage of 1.63 V, lower than Ni(OH) 2 |Pt (2.22 V), Pt-Ni(OH) 2 |Pt (2.03 V) and Pt-Ni(OH) 2 |OD-Cu (1.71 V) ( Fig. S33).The reaction temperature effect was also evaluated ( Fig. S34).Higher temperatures decreased CAL selectivity and increased phenylacetic acid, likely due to accelerated aldehyde hydration.
We investigated the yield and production rate of CAL at different current densities.As shown in Fig. 5 a, the production rate of CAL reached a maximum of 193 μmol/h at 100 mA/cm 2 .Deviating from this optimal current density decreased both the yield and production rate.Furthermore, the faradaic efficiency (FE) and production rate of NH 3 were detected by ultraviolet and visible spectrophotometry (UV-Vis) at different currents (Fig. 5 b and Fig. S36).At 100 mA/cm 2 , 60% FE of NH 3 was achieved, corresponding to 278 μmol/h production.This optimized paired electrolysis system demonstrated good stability over six c ycles, w ith no decrease in yield and FE ( Figs S37 and S38).On the other hand, after the reaction, Pt-Ni(OH) 2 maintained its original morphology, crystalline phase and surface chemical valence states, indicating decent catalyst stability ( Figs S39-S42).
The generality of this protocol was investigated using various substituted aromatic alcohols.Alcohols with electron-rich and electron-deficient substituents ( 1-10 , Fig. 5 c) successfully produced the corresponding CAL coupling products.Notably, ortho-and meta-fluorine substituents gave 2-F ( 10 , 82% yield) and 3-F ( 7 , 86% yield) CALs, respectively.Methoxy-substituent at the para-position and meta-position results in 45% yield of the 4-OMe CAL ( 2 ) and 67% yield of 4-OMe CAL ( e ), respectively.Combining experimental and DFT calculated BDFEs for the alcohols and condensation reaction barriers (BCR), alcohols with lower BDFE and especially lower BCR gave higher yields and FE (e.g. 7 and 10 ).In contrast, higher alcohol BDFE and BCR resulted in lower yields ( 4 and 6 ) ( Figs S43 and  S44).Additionally, a lower alcohol BDFE but higher aldehyde BCR also caused lower yield ( 2 ), likely due to slower condensation kinetics and buildup of unreacted aldehyde occupying active sites.Importantly, gram-scale synthesis of CAL with 67% yield was achieved under identical conditions in a 100-mL beaker, demonstrating scalability and robustness (Fig. 5 d).

CONCLUSION
In summary, efficient electrocatalytic co-oxidative coupling of ethanol and benzyl alcohol to CAL was achieved through three key strategies: (i) suppressing alcohol over-oxidation to acids using the saltout effect; (ii) accelerating cross-coupling of intermediates with a Pt-Ni(OH) 2 catalyst that has faster kinetics than Ni(OH) 2 ; (iii) replacing HER with nitrate reduction to provide more favorable thermodynamics, higher value ammonia, reduced energy use and improved selectivity by inhibiting hydrogenation.Under optimal conditions, 91% conversion and 85% selectivity to CAL were obtained along with 278 μmol/h NH 3 productive rate in a single electrolysis cell.The substrate tolerance and scaled-up experiments demonstrate practicability.This work provides a universal design principle for electrocatalytic alcohol oxidation to value-added carbon products.

Synthesis of Ni(OH) 2
Ni(NO 3 ) 2 •6H 2 O (12 mmol) was dissolved in 40 mL deionized water and stirred for 15 min.The 0.6 M NaOH aqueous solution was then added until the final pH of the mixed suspension was > 13.After stirring for 30 min, the Ni(OH) 2 suspension was transferred to a 100-mL Teflon-lined stainless-steel autoclave and maintained at 160ºC for 6 h.The autoclave was cooled down naturally to room temperature.The Ni(OH) 2 was washed three times with deionized water and anhydrous ethanol and dried at 60ºC for 10 h.Finally, the Ni(OH) 2 was obtained.

Synthesis of Ni(OH) 2 /NF
Ni(NO 3 ) 2 (10 mmol) and NH 4 NO 3 (5 mmol) were dissolved with 32 mL deionized water and stirred for 15 min.18 mL ammonia (28 wt%) was then added w ith v igorous stirring.After stirring for 15 min, the mixed solution was poured into a culture dish.The culture dish with the solution was preheated at 90°C for 2 h.Then the clean nickel foam (NF) was placed in the mixed solution and maintained at 90°C for 12 h.The Ni(OH) 2 /NF was washed three times with deionized water and anhydrous ethanol and dried at 60°C for 10 h.Finally, the Ni(OH) 2 /NF was obtained.

Synthesis of Pt-Ni(OH) 2
Ni(OH) 2 (50 mg) was dispersed in 50 mL ethylene glycol and sonicated for 30 minutes.1.66 mL H 2 PtCl 6 •6H 2 O solution (20 mg/mL) was added to achieve a 20 wt% Pt loading.The mixture was stirred for additional 30 minutes.The solution was then transferred to a 100-mL polytetrafluoroethylene (PTFE)-lined autoclave and reacted at 120°C for 4 h.After cooling to room temperature, the solid was collected by filtration and dried under vacuum overnight at 60°C.

Synthesis of oxide-derived-Cu
OD-Cu/Cu foil was prepared by a wet chemical oxidation method, followed by electrochemical reduction.Specifically, one piece of Cu foil was successively washed for 10 min in 2 M HCl solution, ethanol and water, respectively.Thereafter, the Cu foil was immersed in an aqueous solution (2.67 M NaOH and 0.13 M (NH 4 ) 2 S 2 O 8 ) for 30 min at room temperature in order to grow Cu(OH) 2 nanorods on the surface.The resulting Cu(OH) 2 /Cu foil was washed by water three times and dried at 60°C for 12 h.Before use, the Cu(OH) 2 /Cu foil was electrochemically reduced into OD-Cu/Cu foil at −3 V for 30 min in 1 M KOH solution.

Figure 1 .
Figure 1.Schematic diagram for combining anodic alcohol oxidative coupling with NH 3 production.

Figure 2 .
Figure 2. Anodic electro-oxidation coupling of EtOH and PhCH 2 OH to synthesize CAL over the Ni(OH) 2 electrode.(a) The effect of pH values on the reaction.(b) The effect of K 2 CO 3 concentration on the reaction.(c) The effect of cation types on the reaction.(d) The coordination number (CN) of O-H and K + -O as a function of cation concentration in the electrolyte with PhCHO and CH 3 CHO.(e) Radial distribution function (RDF) for the PhCHO and CH 3 CHO.(f) The coordination number curves for the PhCHO and CH 3 CHO.(g) The binding energy of PhCHO and CH 3 CHO under different molar ratios.(h) Illustration of the effect of K + ion concentration on the reaction.Other reaction conditions for (a), (b) and (c): 100 mM PhCH 2 OH, 600 mM EtOH, Ni(OH) 2 anode, Pt mesh cathode, 5 mL electrolyte, 100 mA/cm 2 , 2 h, room temperature.

Figure 3 .
Figure 3. Developing the anode electrocatalyst enhances coupling efficiency.(a) XRD of Ni(OH) 2 and Pt-Ni(OH) 2 .(b) XPS spectrum of the Pt 4f region of Pt-Ni(OH) 2 and Pt/C.(c) XPS spectrum of the Ni 2p region of Ni(OH) 2 and Pt-Ni(OH) 2 .(d) LSV (10 mV/s) of Ni(OH) 2 and Pt-Ni(OH) 2 with/without EtOH.(e) In situ Raman spectrum of Ni(OH) 2 during the OER and AOR.(f) In situ Raman spectrum of Pt-Ni(OH) 2 during the OER and AOR.(g) Nyquist plots at 1.35 V vs. RHE.(h) Bode phase plots at 1.55 V vs. RHE.(i) The product distribution of the reaction over NiO, Ni(OH) 2 and Pt-Ni(OH) 2 .

Figure 4 .
Figure 4. Cathodic replacing reaction improves coupling selectivity.(a) The mechanism and inhibition strategy of the CHD.(b) The product distributions with different cathodes.(c) LSV curves of the different cathodes with/without CAL and KNO 3 .(d) Nyquist plots at −0.2 V vs. RHE.(e) The effect of KNO 3 concentration on the reaction.(f) The yield and FE of the reaction in an undivided cell and divided cell, respectively.

Figure 5 .
Figure 5. Reaction scalability and scope of substrate.(a) The yield and production rate of CAL at different current densities.(b) The Faraday efficiency and production rate of NH 3 at different current densities.(c) Scope of the alcohol oxidation coupling reaction.(d) Scale-up experiment.Reaction conditions: substrate (100 mM aromatic alcohol; 600 mM EtOH), 5 mL 0.4 M KOH + 1.0 M K 2 CO 3 + 1.0 M KNO 3 solution, Pt-Ni(OH) 2 /NF anode, OD-Cu foil cathode, 100 mA/cm 2 , room temperature, undivided cell.