English

• 1. Introduction

  With increasingly serious energy and environmental issues, reducing pollution at the beginning and developing sustainable chemistry has become particularly urgent. In this regard, organic electrosynthesis has been treated as a green and sustainable alternative to conventional synthetic chemistry. Notably, organic electrosynthesis uses electricity as a clean redox agent to replace the toxic and dangerous chemical redox reagents, realizing electrical-driven chemical reactions. And it is one of the research hotspots in current synthetic chemistry and possesses mild reaction conditions and high atom economy [1-10]. Moreover, chemoselectivity and reaction progress can be precisely controlled by adjusting applied potential, current, and electrode materials [2,11]. Generally speaking, the electrolysis model normally includes direct electrolysis and indirect electrolysis (Fig. 1). Direct electrolysis refers to the direct redox reaction of electroactive species on the surface of the electrode. It mainly depends on the redox potential of the substrate, and maybe suffer from the limited substrate range and difficult compatibility of sensitive groups. Indirect electrolysis provides an alternative strategy, in which an electrocatalyst was used to promote the redox reaction under a mild condition. Compared with direct electrolysis, indirect electrolysis can effectively avoid excessive oxidation/reduction of the substrate, and thus the selectivity of the product is controllable.

  Figure 1

  

  Figure 1.  Two types of electrolysis.

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  Until now, electrocatalysts that have been explored mainly include homogeneous and heterogeneous electrocatalysts [6]. As for homogeneous electrolysis, the reaction was triggered following diffusion of the electrocatalyst to an electrode surface where electron transfer occurs. Several different types of homogeneous electrocatalysts have been developed for organic electrochemical synthesis, such as transition metals-based electrocatalytic C−H functionalization [12,13], cobalt complex-based electrocatalytic organic synthesis [14], N-oxyl compounds mediated electrosynthetic reactions [15], halogen-mediated indirect electrosynthesis [16]. Heterogeneous catalysts are the ideal choice to deal with the high cost and environmental issues because of their easy recyclability. Compared with conventional inert electrodes, electrodes modifying with heterogeneous catalysts can provide greatly increased electrochemical active surface area and accelerate the electron transfer between electrode and catalysts, which in turn enhance the utilization efficiency of active sites. Hence, heterogeneous electrocatalysts have evoked an immense amount of recent interest due to their high activity, stability, and recyclability. In this review, a series of representative heterogeneous catalysts modified electrodes for organic electrosynthesis were summarized (Fig. 2), highlighting the key features and providing our perspectives on potential directions. We hope this work could promote further development for electrode materials-based versatile organic electrosynthesis.

  Figure 2

  

  Figure 2.  The representation of heterogeneous catalysts modified electrodes for organic electrosynthesis.

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  2. Anodic oxidation reaction

  Oxidative reaction is an important tool in synthetic chemistry, enabling the efficient construction of aldehydes, ketones, carboxylic acids, sulfoxides, sulfones, nitriles, etc. Traditional method commonly relys on the use of stoichiometric chemical oxidants, such as organic peroxides, hypervalent iodine, and K₂S₂O₈, which can often suffer from toxicity, poor functional group compatibility, and substantial amounts of waste. Hence, the development of environmentally benign and user-friendly approach is urgently demanded. To this regard, the adoption of heterogeneous catalysts modified anodes for oxidation reaction is a promising alternative to the traditional oxidation method. The heterogeneous catalysts can be regenerated on anodic oxidation and easily separated from reaction system and reused. It is worth noting that the main by-product is usually the clean energy molecule H₂.

  2.1 Electrocatalytic alcohol oxidation

  The selective oxidation of alcohols to aldehydes and ketones is one of the most common classes of oxidation reactions in organic chemistry. TEMPO (2, 2, 6, 6-tetramethylpiperidine N-oxyl) is a representative homogeneous catalyst for electrochemical oxidation of alcohols and it has achieved widespread application [15,17]. However, the catalytic efficiency is often limited by its slow diffusion rate to the electrode surface and dimerization inactivation. Using a hybrid strategy, several efforts have been made to immobilize the composite of TEMPO and porous materials on electrodes to improve the catalytic efficiency and facilitate recycling [18,19]. For example, Stahl and Das developed that a pyrene−TEMPO conjugate anchored on the electrode which was coated with multiwalled carbon nanotubes (MWCNTs) via noncovalent π-π stacking interactions for alcohol oxidation (Fig. 3) [18]. The resulting pyrene-TEMPO-functionalized electrode exhibited outstanding performance in electrocatalytic benzyl alcohol oxidation, far exceeding that of structurally and electronically similar homogenous 4-acetamido–TEMPO (Fig. 3B). This result demonstrated the improved rate of catalyst immobilization on the electrode, which avoided the kinetic loss associated with the transfer of catalyst to and from the electrode surface when a dissolved mediator was employed.

  Figure 3

  

  Figure 3.  Pyrene−TEMPO−functionalized electrode electrocatalytic oxidation of alcohols. Reproduced with permission [18]. Copyright 2017, Wiley Publishing Group.

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  Recently, Wang et al. designed CNT/MOL-TEMPO-CO₂^– and CNT/MOL-TEMPO-OPO₃^2–, which were prepared by TEMPO-CO₂^– and TEMPO-OPO₃^2– via ligand exchange with the remaining formates on the MOL SBUs, respectively, to promote selective electrooxidation of alcohols (Fig. 4) [19]. The modified electrodes showed efficient electrocatalytic activity for selective oxidation of primary alcohols in the presence of secondary alcohols (Fig. 4C), which may be attributed to the steric hindrance of the 4-position of TEMPO. In addition, CNT/MOL-TEMPO-OPO₃^2– could be reused six times without significant loss of activity.

  Figure 4

  

  Figure 4.  CNT/MOL-TEMPO modified electrode for electrocatalytic oxidation of alcohols. Reproduced with permission [19]. Copyright 2018, American Chemical Society.

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  In the above cases, the immobilization of molecular catalyst (TEMPO) on anodes via either noncovalent π−π stacking interactions or coordination interactions exhibit higher reaction rate and selectivity for electrocatalytic alcohol oxidation than homogenous TEMPO. These results indicate that this method could leave out the step of homogeneous catalyst diffusion to electrode surface and avoid dimerization inactivation of TEMPO. Additionally, with the steric hindrance at 4-position of TEMPO increases, the selectivity also increases. Moreover, this method facilitates the separation of catalysts from the reaction system and the modified anodes can be reused after simple solvent washing. However, long-term instability might be exist by non-covalent immobilization of molecular catalyst.

  Apart from these, transition-metal-based catalysts modified electrodes have been reported with appreciable activities for alcohol oxidation [20-22]. Zheng et al. reported hierarchical porous nitrogen-doped carbon (NC)@CuCo₂N_x in-situ grown on carbon fiber (CF) as a bifunctional electrode for both electrocatalytic oxidation of benzyl alcohol and hydrogen evolution reaction (HER) in alkaline medium [21]. The as-obtained NC@CuCo₂N_x/CF anode exhibited remarkable electrocatalytic performance for selective oxidation of benzyl alcohol to benzaldehyde with 96% yield and 95% selectivity. Furthermore, in a two-electrode electrolyzer, paired simultaneous selective electrooxidation of benzyl alcohol and HER was achieved with high conversion and selectivity by employing NC@CuCo₂N_x/CF bifunctional electrodes. This was attributed to the hierarchical architecture facilitating to expose more catalytic active sites, enhancing mass transport, and the synergistic effect between CoN and CuN modulating the adsorption energies of key species.

  Si et al. reported a PdAg bimetallic catalyst in situ grown on nickel foam (NF) electrode (PdAg/NF) to promote electro-oxidation of ethylene glycol to glycolic acid [22]. The as-prepared PdAg/NF electrocatalyst exhibited an outstanding performance for highly efficient and selective oxidation of ethylene glycol to glycolic acid with Faradaic efficiency (FE) of 92% under alkaline conditions, much superior to that of Pd/NF and Ag/NF, which could be owing to the synergic effect between Pd and Ag. The density functional theory (DFT) calculations and experimental results indicated that doping of Ag could effectively reduce the adsorption energy of the intermediate which was produced by the Pd-catalyzed dehydrogenation of ethylene glycol (Scheme 1). During the bimetallic synergistic catalysis, HOCH₂CO^* and OH^* were adsorbed on the Pd and Ag, respectively, and produced glycolic acid without breaking the C-C bond, thus significantly improving the ethylene glycol oxidation activity and glycolic acid selectivity.

  Scheme 1

  

  Scheme 1.  Schematic illustration of the proposed synergetic catalytic effect between Pd and Ag. (A, B) A probable mechanism of the synergetic catalytic effect for ethylene glycol oxidation on the PdAg/NF catalyst. (C) General pathway of ethylene glycol oxidation (the red is proposed to be the dominant pathway in this work).

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  As modifiers of anodes, inorganic heterogeneous catalysts are also proved to be the critical catalytic centers for electrocatalytic alcohol oxidation. Due to the porous structures, the reactants can be facilitated to contact with the catalytic sites as well as enhancing mass transport. By tuning the adsorption energies of metals to key intermediate species, the oxidation activity and selectivity of the reaction are regulated.

  2.2 Electrocatalytic biomass oxidation

  Catalytic synthesis of value-added chemicals from renewable biomass has received great attention in recent years. The advance of biomass oxidation has been well summarized in several previous reviews [23-26], thus this section will outlines the development of electrocatalytic biomass oxidation, aiming to draw community attention to this emerging field. As early as 1991, Grabowski et al. pioneered in this area and firstly reported Ni(OH)₂/NiOOH-covered Ni mesh as anode electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) to 2, 5-furandicarboxylic acid (FDCA) in an alkaline electrolyte (1.0 mol/L NaOH), giving 71% yield because of decomposition of HMF in strong alkaline solutions [27]. Since then, various heterogeneous catalysts modified electrodes for the electrochemical oxidation of HMF have been reported, and the oxidation pathway relates to the catalyst type and oxidation potential. For example, Chadderdon et al. reported that carbon-black-supported Au and Pd bimetallic nanoparticles (NPs) were painted onto the carbon cloth (CC) anode for electrocatalytic oxidation of HMF to FDCA (Scheme 2) [28]. The experimental results revealed that the applied electrode potential and catalyst surface composition played a pivotal role in competitive oxidation between aldehyde and alcohol groups of HMF. Au/C electrode favored the oxidation of aldehyde group to obtain 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), but required higher electrode potentials to further oxidize alcohol group to form FDCA. And Pd/C electrode followed two competitive routes to oxidize HMF to FDCA, but needed high potentials to promote this process. Interestingly, the Pd-Au bimetal modified electrode exhibited much higher catalytic performance at lower potentials than those of Au/C and Pd/C electrodes. Additionally, the bimetallic catalyst with 1:2 Pd/Au molar ratio showed the optimal electrochemical performance with 83% yield of FDCA at the potential of 0.9 V vs. RHE after 1 h.

  Scheme 2

  

  Scheme 2.  Two possible pathways of HMF oxidation to FDCA.

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  Park et al. proposed tailorable 3D hybrid multilayer electrodes for electrochemical oxidation of HMF (Fig. 5) [29]. The modified electrodes were prepared by Au, Pd, or their alloy NPs with nanosized graphene oxide (nGO) through layer-by-layer (LbL) assembly onto indium tinoxide (ITO) substrates. This approach takes advantage of precise regulation of the internal architecture of the LbL-assembled multilayer (including the thickness and position of Au and Pd NPs) to study the relationship between the nanoarchitecture of the modified electrodes and electrocatalytic activity. Interestingly, even the composition of NPs is the same, the Pd₇/Au₇ electrode with Au NPs located at the outer layer exhibited a higher yield of FDCA than that of the Au₇/Pd₇ electrode, which was attributed to the enhanced mass transfer into the inner-layer Pd NPs. In addition, the optimized LbL-assembled two-electrode system of Pd₇/Au₇ || (AuPd)₇ displayed the best full-cell electrocatalytic activity for both HMF oxidation at the anode and HER at the cathode.

  Figure 5

  

  Figure 5.  Au and Pd alloy modified electrode for electrocatalytic oxidation of HMF. Reproduced with permission [29]. Copyright 2020, American Chemical Society.

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  In a subsequent study, non-precious metal-based catalysts modified electrodes have been developed to replace noble metal-based electrodes for electrocatalytic oxidation of HMF to FDCA. For instance, transition metal phosphides [30-32], sulphides [33], borides [34-36], and nitrides [37,38] have been reported with promising electrocatalytic HMF oxidation performances and can be used several successive electrolysis cycles without obvious loss in activity. As an example, Zhang et al. developed NiB_x as a catalyst in-situ grown on NF electrode for electrochemical oxidation of HMF using H₂O as an oxygen source (Fig. 6) [36]. The as-prepared NiB_x@NF anode exhibited high conversion of HMF to FDCA and excellent FE with the value of 99.8% and 99.5%, respectively. A plausible mechanism was proposed as well. At the NiB_x@NF anode, Ni³⁺ with a newly bonded OH^– originating from H₂O was formed via the electrooxidation of Ni²⁺, and followed by reacting with HMF, DFF and/or HMFCA to obtain FDCA, accompanied by the regeneration of Ni²⁺. Additionally, in a paired electrosynthesis system, NiB_x@NF as bifunctional electrodes could simultaneously enable the oxygenation of HMF and the hydrogenation of p-nitrophenol with conversion efficiency and FE above 99%.

  Figure 6

  

  Figure 6.  NiB_x modified electrode for electrocatalytic oxidation of HMF.

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  Recently, Barwe et al. achieved a high-surface-area Ni_xB modified NF anode to electrocatalytic oxidation of HMF to FDCA in an electrochemical flow-through reactor (Fig. 7) [34], which could enable the efficient and safe scale-up of electrochemical processes for industrial applications.

  Figure 7

  

  Figure 7.  The electrochemical flow reactor for continuous HMF electrochemical oxidation. Reproduced with permission [34]. Copyright 2018, Wiley Publishing Group.

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  In recent years, metal-based oxides [39-52], hydroxides [53-56], and oxyhydroxides [57,58] exhibit fascinating electrocatalytic HMF oxidation activities. For example, Gao et al. investigated the performance of different morphologies of Ni_xCo_3-xO₄ nanostructure for electrocatalytic oxidation of HMF to FDCA [39]. Zhang et al. employed Co₉S₈–Ni₃S₂@N, S, O-tri-doped carbon heterostructures catalyst to realize efficient HMF conversion to FDCA [59]. Zhou et al. disclosed that the abundant surface defects of CoNW/NF could facilitate the electrochemical oxidation of HMF into FDCA [41]. Huang et al. achieved rich oxygen vacancies via Se doping in CoO and demonstrated outstanding electrocatalytic HMF oxidation performance due to the increased electrochemical surface area as well as reduced charge transfer resistance [43]. Liu et al. discovered bimetallic NiFe layered double hydroxide (LDH) nanosheets in-situ grown on carbon fiber paper (CFP) could promote electrocatalytic oxidation of HMF to FDCA, owing to the thin nanosheets structure feature promoting the exposure of more catalytic active sites [53].

  Mechanistically, it is widely accepted that Ni and Co species played critical roles in Ni and Co-based electrocatalysts. For the Ni-based catalysts, Choi et al. achieved NiO NPs modified carbon paper (CP) electrode for electrochemical oxidation of HMF, and demonstrated that the Ni(OH)₂ intermediate was first formed on the surface of NiO in the alkaline medium, then it was easily oxidized into Ni(III)-OOH during HMF oxidation. Adsorbed alcohol could be oxidized when Ni(III)-OOH returns to Ni(OH)₂, and the HMF oxidation occurred via the DFF pathway [44]. Zhou et al. proposed that the deposition of Pt NPs into Ni(OH)₂ modified electrode could promote electrocatalytic oxidation of HMF to FDCA [56]. The as-synthesized Pt/Ni(OH)₂ exhibited highly efficient electrocatalytic performance for HMF oxidation, 8.2 times enhancement than that of Ni(OH)₂. This study demonstrated that the doping of Pt could optimize the redox properties of Ni(OH)₂, resulting in the accelerated formation of Ni(OH)O, which was confirmed by operando methods. On the other hand, it was also proved that Pt could serve as the adsorption site of HMF, reducing the adsorption energy of HMF with optimized adsorption behavior. Recently, Zhou et al. explored the activity origin of Ni₃N via operando X-ray absorption spectroscopy (XAS), quasi in situ X-ray photoelectron spectroscopy (XPS), in situ Raman, and operando electrochemical impedance spectroscopy (EIS), and proved that Ni^2+δN(OH)_ads generated by the adsorbed hydroxyl group was the activity origin [60]. Taitt et al. systematically investigated and compared three different MOOH anodes (M = Ni, Co, Fe) for HMF electrooxidation in a 0.1 mol/L KOH solution [57]. The NiOOH anode exhibited the best catalytic performance with 96% FDCA yield at 1.47 V vs. RHE, whereas CoOOH and FeOOH anodes only achieved FDCA yield of 35% (at 1.56 V vs. RHE) and < 5% (at 1.71 V vs. RHE), respectively. Although the CoOOH anode could initiate HMF oxidation at a less positive potential than NiOOH anode benefiting from the Co(OH)₂/CoOOH couple conversion occurred at a lower redox potential, the rate of Co(OH)₂/CoOOH-mediated HMF oxidation was not fast enough to produce sufficient current density for constant potential HMF oxidation. Recently, Kornienko and Heidary studied the NiOOH catalyst using operando surface-enhanced Raman spectroscopic, demonstrating that in 10 mmol/L KOH solution, the reaction proceeded via the DFF pathway, whereas in 1 mol/L KOH solution, it followed the HMFCA pathway [58].

  For the Co-based catalysts, Lu et al. investigated Co-based spinel oxides as working electrodes for electrochemical oxidation of HMF to FDCA [61]. By the substitution of the tetrahedral site (Co_Td²⁺) and octahedral site (Co_Oh³⁺) in Co₃O₄ with Zn²⁺ and Al³⁺, respectively (Fig. 8), the authors confirmed that Co_Oh³⁺ was the optimal geometrical configuration for the oxidation of HMF and Co_Td²⁺ could provide chemical adsorption sites. Based on these, replacing Co_Td²⁺ with Cu²⁺ to enhance the exposure degree of Co_Oh³⁺ and boost acidic adsorption, the obtained CuCo₂O₄ exhibited a record performance among Co-based catalysts and four times higher than that of Co₃O₄. Similarly, based on the geometrical configuration of the metal oxides, Lu et al. prepared a 3D hierarchically nanostructured NiO-Co₃O₄ in-situ grown on the NF electrode for electrocatalytic oxidation of HMF [62]. The crystal structures of Co₃O₄ and NiO are spinel and face-center cube, respectively, thus the atom permutation cannot be one-to-one correspondence at the interface, providing abundant defects and vacancies. And these vacancies played a decisive role in electrocatalytic HMF oxidation, bringing a high yield of FDCA (98%) and FE (96%).

  Figure 8

  

  Figure 8.  Co_Oh³⁺ plays a decisive role in HMF oxidation. Copied with permission [61]. Copyright 2020, Wiley Publishing Group.

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  In addition, Deng et al. firstly reported the oxidation of HMF could be selectively regulated by changing the oxidation state of the Co-based electrocatalyst electrodeposited on CC electrode (Fig. 9) [63]. Employing CoO_xH_y as the model electrocatalyst, it was found that Co³⁺ formed at low potential acted as a sluggish oxidant for the oxidation of formyl group exclusively to carboxylate, while Co⁴⁺ generated at high potential was necessary for the initial and spontaneous oxidation of hydroxyl group in HMF with significantly faster kinetics. As a result, the oxidation products of HMFCA and FDCA could be conveniently tuned by adjusting the applied potential.

  Figure 9

  

  Figure 9.  Mechanistic illustration of potential-dependent oxidation of HMF to produce HMFCA and FDCA as mediated by electrogenerated Co³⁺ and Co⁴⁺ species

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  Porous framework materials, such as metal-organic frameworks (MOFs) [64] and covalent-organic frameworks (COFs) [65,66], always have periodic crystal structures with high surface area, porous channels, and tunable structure, making them ideal as promising candidates for catalysis [67-76]. Cai et al. firstly reported Ni-based two-dimensional (2D) MOF nanosheets grown on NF electrode for electrochemical oxidation of HMF (Fig. 10A) [77]. The as-prepared Co-doped 2D MOFs NiCoBDC/NF exhibited a superior activity in contrast with NiBDC/NF, NiFeBDC/NF, and NiMnBDC/NF, obtaining 99% yield of FDCA and 78.8% FE in a pH 13 medium at 1.55 V vs. RHE. And it remained 95% yield and 75% FE within four successive electrolysis cycles. Later, Bai et al. investigated MOF nanoarray modified NF electrode by in-situ hydrothermally deposited for electrochemical oxidation of HMF (Fig. 10B) [78]. The as-prepared ternary CoNiFe-MOFs/NF electrode exhibited highly active electrooxidation of HMF to FDCA with FE of 100% and yield of 99.76% in a 1.0 mol/L KOH solution. Furthermore, the modified electrode could be reused for 15 consecutive cycles without significant loss in activity. In 2020, Cai et al. reported Ni(II)-doped COF film immobilized on FTO electrode for electrochemical oxidation of HMF in alkaline solution (Fig. 10C) [79]. The as-prepared TpBpy-Ni@FTO gave a 96% conversion of HMF and 58% yield of FDCA.

  Figure 10

  

  Figure 10.  (A) 2D MOFs catalyst for electrocatalytic HMF oxidation. Copied with permission [77]. Copyright 2020, Royal Society of Chemistry. (B) Schematic representation for the fabrication of trimetallic MOF arrays on NF and the application of electrocatalytic HMF oxidation. Copied with permission [78]. Copyright 2021, Royal Society of Chemistry. (C) Schematic illustration of the preparation of TpBpy-Ni@FTO. Copied with permission [79]. Copyright 2020, Royal Society of Chemistry.

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  Apart from these, Wu et al. firstly described metal chalcogenides (MC) modified CP electrodes for the electrochemical oxidation of furfural to 5-hydroxy-2(5H)-furanone (HFO) using H₂O as the oxygen source (Fig. 11) [80]. As a comparison, the as-prepared CuS/CP anode exhibited the best performance with 70.2% conversion of furfural and 83.6% selectivity of HFO than the other prepared MC/CP electrodes (i.e., ZnS/CP, CdS/CP, PbS/CP, WS₂/CP and MoS₂/CP). Furthermore, CuS/CP anode provided excellent long-term stability to maintain the FE and selectivity of HFO with 77.8% and 85.3%, respectively, within a reaction time of 24 h. Mechanism studies indicated that HFO was achieved by a multi-step reaction depicted in Fig. 11B, including the C-C bond cleavage of furfural, succeeding by ring-opening and oxidation, as well as intramolecular isomerization.

  Figure 11

  

  Figure 11.  CuS modified electrode for electrocatalytic furfural oxidation.

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  Additionally, Zhang et al. explored metal phosphides (Ni₂P or Cu₃P) in situ grown on carbon fiber cloth (CFC) electrode via a facile vapor-phase hydrothermal route for electrocatalytic oxidation of furfural [81]. Compared to the Cu₃P/CFC, the as-obtained Ni₂P/CFC exhibited more superior activity for the furfural oxidation with almost 100% selectivity of furoic acid product and over 90% FE in alkaline electrolyte. These results might be mainly attributed to the formation of high valence state Ni species, such as oxides/hydroxides and oxyhydroxides.

  As for the catalysts in indirect electrolysis, transition-metal-based heterogeneous materials are proved to be critical points to improve the electrochemical performance for biomass oxidation. The material nanostructures, metal catalytic sites, surrounding chemical environment, and morphologies all benefit the development of advanced heterogeneous electrocatalysts with high performance.

  2.3 Electrocatalytic thioethers oxidation

  Sulfoxide and sulfone skeletons are invaluable structural components in pharmaceuticals and functional materials [82,83]. Traditional procedure to construct this skeleton mainly relied on selective oxidation of thioether using homogeneous catalysts with O₂ or H₂O₂ as the main oxygen sources [84,85]. Obviously, the cost, safety and product purification are the disadvantages. Recently, the electrooxidation of thioether has been developed. However, it suffers from limited noble metal electrodes, expensive electrolytes and O₂ [86,87]. To solve this issue, Han et al. investigated nickel phosphide (Ni₂P) hollow nanocubes modified NF electrode for membrane-free selective thioether electrooxidation with H₂O as the oxygen source (Fig. 12) [88]. By adjusting the potential, the controllable synthesis of sulfoxides and sulfones could be achieved with high yields and selectivity. The surface in-situ reconstruction of Ni₂P to NiOOH played a pivotal role in assisting the formation of Ni^II/Ni^III redox couple. The reaction was initiated by the oxidation of thioethers which was adsorbed on the Ni site of NiOOH, generating the radical cation. And ^•OH was formed at the same time via water electrolysis, followed by the radical cross-coupling and deprotonation to accomplish the sulfoxides. Sulfones could be obtained at a higher oxidation potential through a similar process with sulfoxides as the substrates. As a comparison, bare NF, Pt and CFP anodes were inert in the low potential system, whereas Ni₂P nanosheets and Ni₂P nanoparticles modified anodes delivered low yields of sulfoxides. These results also demonstrated that high valance Ni species were the actual active centers, and the morphologies of Ni₂P also affected catalytic performance. The excellent catalytic performance of Ni₂P hollow nanocubes might be due to its higher surface area and exposure of more catalytic active sites than Ni₂P nanosheets and Ni₂P nanoparticles. In addition, the Ni₂P hollow nanocubes modified anode could be recycled six times without a significant decrease in catalytic activity. And this method displayed broad functional group compatibility and easily synthesized deuterated sulfoxides and sulfones.

  Figure 12

  

  Figure 12.  Ni₂P hollow nanocubes modified electrode for selective thioether electrooxidation.

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  2.4 Electrocatalytic primary amines oxidation

  Nitriles are useful intermediates for the synthesis of pharmaceuticals, agrochemicals, and fine chemicals [89-91]. Selective oxidation of primary amines to nitriles could avoid the use of HCN or metal cyanides [92]. Unfortunately, most of the reported methods usually require high temperature and/or high O₂ pressure with the presence of additives for oxidation [93-95]. Hence, replacing thermocatalysis with electrocatalysis might be an effective solution. Huang et al. developed a NiSe nanorod anode prepared by direct selenization of commercially NF to promote oxidant-free electrooxidation of primary amines to the corresponding nitriles at room temperature (Fig. 13) [96]. At the NiSe nanorod anode, the surface Ni^II was first oxidized to form Ni^IIIOOH in alkaline solution, which then oxidized primary amines into nitriles, along with conversing Ni^IIIOOH into Ni^II (Fig. 13C). A wide range of structurally diverse nitriles were prepared in excellent yields. The non-water-soluble feature of nitriles could easily escape from the aqueous electrolyte/electrode interface, avoiding catalyst deactivation. Moreover, NiSe nanorod anode remained highly active within six runs and favored the continuous gram-scale nitrile production with industrial practicability. Notably, replacing sluggish oxygen evolution reaction with electrooxidation of thermodynamically more favorable primary amines in water splitting system, the cell voltage was significantly reduced relative to that of overall water splitting. Similarly, Mondal et al. declared a crystalline intermetallic nickel silicide catalyst (Ni₂Si NPs) prepared by a simple one-pot colloidal approach, aiming to produce high-value oxidation and reduction products (nitriles and H₂, respectively) [97]. The active phase Ni^IIIO_xH_y was formed by accompanying the corrosion of Si to silicate under alkaline condition. The Dissolution of Si might result in more porous Ni^IIIO_xH_y phase with a distorted layered structure and facilitate electrolyte penetration. Furthermore, the activated form of Ni₂Si exhibited highly efficient electrocatalytic oxidative dehydrogenation of primary amines with high selectivity and a wide range of substrates.

  Figure 13

  

  Figure 13.  NiSe nanorod modified electrode for primary amines electrooxidation.

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  2.5 Electrocatalytic olefins oxidation

  Epoxides as one of important class of intermediates are widely used in the manufacture of many chemical products. Typically, epoxides are synthesized via olefin epoxidation in which the use of hazardous reagents or the generation of stoichiometric side products present challenges for separation and waste streams. In this regard, electrosynthesis has provided an eco-friendly approach to achieve the selective oxidation due to the toxic reagents can be replaced with electricity.

  Previous works demonstrated that NaBr was used as a mediator for electrocatalytic epoxidation and obtained good selectivity of olefins [98-100]. However, the used Pt electrode suffers from the corrosion by Br^–, while carbon-based electrodes showed particularly low catalytic activity towards NaBr mediator. To address this issue, Zhang et al. proposed using GF-CoS₂/CoS heterostructures anchored on graphite felt (GF-CoS₂/CoS) to trigger Br^–/Br₂ redox mediator (Fig. 14) [101]. The experimental results showed GF-CoS₂/CoS anode is superior to the Pt electrode in catalyzing Br^–/Br₂ redox mediator. Under the optimized reaction conditions, the epoxidation of styrene with 97% yield was obtained using GF-CoS₂/CoS anode at 30 mA/cm². Importantly, the applied voltage on the GF-CoS₂/CoS-based electrode system was reduced to 4-5 V compared to 7.8–9.3 V on the Pt-based electrode system, saving half of the energy.

  Figure 14

  

  Figure 14.  GF-CoS₂/CoS anode for electrocatalytic epoxidation.

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  In order to facilitate the separation of catalyst from the reaction system and avoid the use of additional reagents, Jin et al. demonstrated a sustainable and green route to epoxidation reactions. They used Mn₃O₄ nanoparticles modified carbon paper as the anode and H₂O as safe oxygen atom source (Fig. 15) [102]. The experiment results indicated Mn^IV-oxo species played a pivotal role in this electrochemical oxidation and isotopic studies revealed H₂O as the sole oxygen atom source. In addition, the modified electrodes exhibited efficient catalytic epoxidation of cyclooctene with FE above 30% at the anode and coproduction of H₂ with FE above 94% at the cathode. Unfortunately, the FE and yields of electrocatalytic olefins oxidation are not high enough, and much work need to be further explored.

  Figure 15

  

  Figure 15.  Mn₃O₄ NPs modified electrode for epoxidation of olefins.

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  2.6 Electrochemical N-heterocycles dehydrogenation

  The catalytic acceptorless dehydrogenation of N-heterocycles is a dominant strategy to access quinoline and indole rings, which are frequently encountered in pharmaceuticals and bioactive molecules. Traditional dehydrogenation needs oxidants or sacrificial hydrogen acceptors, which often generate undesirable byproducts such as toxic metal salts [103,104]. At present, electrochemistry has offered an efficient way to achieve acceptorless dehydrogenation with H₂ production. In addition, replacing homogeneous catalysts with heterogeneous catalysts could avoid the tedious separation step and the contamination of the products. In 2019, Huang et al. explored the use of a Ni₂P bifunctional electrode to promote selective semi-dehydrogenation of tetrahydroisoquinolines (THIQs) (Fig. 16) [105]. Preparation of the modified electrode by immersing the nickel foam into Ni(NO₃)₂⋅6H₂O aqueous solution, followed by annealing and phosphidation. The Ni₂P nanosheet electrode exhibited high activity at controllable electrooxidation from THIQs to dihydroisoquinolines (DHIQs), as well as the production of H₂ at the same time. At the anode, Ni^III-OOH was first formed and then it oxidized 1a to 2a. A wide range of structurally diverse DHIQs were obtained in good yields and excellent selectivity. Remarkably, the Ni₂P electrode could be recycled up to 6 times without significant loss in electrocatalytic activity and maintained the nanosheet arrays morphology. Then, Li et al. prepared a Ni-Mo catalyst as both the cathode and anode to achieve hydrogenation and dehydrogenation of N-heterocycles using water as the H source [106]. Mo atom with superior hydrogen adsorption property could accelerate the generation of active H* species, promoting hydrogenation of the quinoxaline. And Ni component could be electrooxidized to NiOOH leading to the formation of Ni^II/Ni^III redox couple, which facilitated the oxidative dehydrogenation of hydrogenated quinoxaline. Subsequently, Wang et al. reported Co₃O₄@NiO bifunctional electrodes could enable electrocatalytic semi-dehydrogenation of THIQs and simultaneous electroreduction of nitrate [107]. The modified electrode was prepared by coating Co₃O₄@NiO HNTs on CP electrode, in which Co₃O₄@NiO HNTs were prepared by using adopting Co-aspartic acid nanowires (Co-Asp NWs) as precursors via the cation-exchange reaction with Ni²⁺ and followed by annealing. The supported Co₃O₄@NiO HNTs catalyst displayed excellent activity and selectivity for both anodic THIQs semi-dehydrogenation to DHIQs and cathode nitrate electroreduction to ammonia. And Ni^III-OOH and NO were the key intermediates for both reactions, respectively. In addition, Yang et al. reported the CNT/MOL-TEMPO-OPO₃^2– electrode could promote dehydrogenation of N-heterocycles (Fig. 17) [108]. A range of quinoline or indole derivatives were obtained in moderate to good yields and the hybrid catalyst could be reused three times without loss of activity.

  Figure 16

  

  Figure 16.  Ni₂P modified electrode for electrochemical semi-dehydrogenation of tetrahydroisoquinolines.

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  Figure 17

  

  Figure 17.  CNT/MOL-TEMPO-OPO₃^2– modified electrode for electrocatalytic dehydrogenation of N-heterocycles.

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  2.7 Electrocatalytic C–H functionalization

  C–H functionalization is of great interest to upgrade the primary raw materials [109,110]. However, the stubborn C–H bonds usually make their activation quite challenging, and harsh conditions such as high temperatures, toxic oxidants, and noble-metal catalysts have to be required to achieve acceptable conversions [111-113]. To address this issue, electrosynthesis instead of traditional thermocatalysis might be an effective solution. Li et al. reported W₂C/N_xC catalysts modified CC anode for alkoxylation of benzylic C–H bonds (Fig. 18) [114]. W₂C/N_3.0C electrode as the best-in-class anode exhibited excellent selectivity and good to high conversions for electrochemical alkoxylation of various aromatic C–H bonds, which was superior to the commercial electrodes (boron-doped diamond, reticulated vitreous carbon, and lead oxide). Moreover, replacing W₂C/N_3.0C catalysts with W₂C catalyst, NC sample, or a mechanical mixture of the two components obtained much lower conversions under the same conditions. This strongly implied a synergistic effect between W₂C and N_3.0C components. In addition, without an obvious decrease in FE was observed after four cycles, indicating the great electrochemical stability of W₂C/N_3.0C anode. The theoretical calculations and experimental results revealed that regulating the electron density of W₂C nanocrystals via the construction of Schottky heterojunctions with N-doped carbon could enhance the superior adsorption of benzylic C–H bonds on the W₂C surface, promoting the activation of C–H bonds, which was the rate dominating step.

  Figure 18

  

  Figure 18.  W₂C/N_3.0C modified electrode for alkoxylation of benzylic C–H bonds.

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  Recently, in another work by the Zhang's group, NiSe nanorod arrays (NAs) in-situ grown on NF electrode was employed to promote the electrooxidation of α-nitrotoluene to E-nitroethene (Fig. 19) [115]. The in-situ formed NiOOH surface layer, surface adsorbed SeO_x²⁻ through Se leaching-oxidation during electrooxidation, as well as the preferential adsorption of intermediate with two –NO₂ groups on NiOOH played a crucial role for NiSe NAs anode to highly efficient electrooxidize α-nitrotoluene to E-nitroethene with 99% selectivity, 89% FE, and the reaction rate of 0.25 mmol cm⁻² h⁻¹. A plausible mechanism was proposed as well (Fig. 19C). The α-carbon radical was generated by the oxidation of α-carbon anion at the anode, followed by self-coupling to form the intermediate with two -NO₂ groups, and then a HNO₂ molecule was eliminated to obtain the desired products. This protocol displayed broad functional group compatibility and various E-nitroalkenes were obtained with good to high selectivity and high yields. Furthermore, the reaction could also be easily achieved at the gram scale with high selectivity and conversion yield.

  Figure 19

  

  Figure 19.  NiSe NAs modified electrode for electrochemical self-coupling reaction.

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  3. Cathodic reduction reaction

  In constrast to indirect electrooxidation reations, the indirect electroreduction remains relatively underexplored. Recently, electroreduction has received increasing attention due to its tunable reducing potential and good scalability, while avoiding the use of stoichiometric reductants (reducing metals, organic compounds, etc.) in traditional reduction methods. Currently, a variety of inorganic heterogeneous catalysts (transition metal phosphides, sulfides, alloy, etc.) have been demonstrated as efficient reductants for hydrogenation reaction, deuterodehalogenation reaction and coupling reaction.

  3.1 Electrocatalytic hydrogenation of alkynes

  Selective hydrogenation of alkynes to alkenes is of great importance in chemical synthesis. Unfortunately, it usually requires harsh conditions, such as high temperature and/or high H₂ pressure. To address these issues, replacing traditional thermocatalysis with electrocatalysis and combinating with electroreducing water where hydrogen is generated in situ, might be an effective solution. Zhang et al. explored the use of a CP cathode coated with Pd-P alloy nanoparticles networks (Pd-P NNs) to achieve the selective semihydrogenation of alkynes (Fig. 20) [116]. The systems containing Pd-P NNs displayed higher activity and selectivity for electrocatalytic semihydrogenation of alkynes with H₂O to alkenes with a well-defined configuration than Pd nanoparticles. The incorporation of P could enhance the specific adsorption of alkynes as well as promote the generation of H*_ads, leading to facilitate the hydrogenation of alkynes and suppress the over-hydrogenation. Impressively, this protocol also enabled the synthesis of mono-, di-, tri-deuterated alkenes with the deuterium ratios above 99%. Furthermore, the reaction tolerated various functional groups and achieved a good yield on the gram scale. In the Pd-P||NiSe two-electrode system, 4-vinylaniline and adiponitrile were simultaneously obtained with high yields at the cathode and anode, respectively, which provided a promising prospect for industrial applications.

  Figure 20

  

  Figure 20.  Pd-P NNs modified electrode for electrocatalytic semihydrogenation of alkynes.

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  In a follow up work, they developed a new Cu-S nanowire sponges (NSs) catalyst anchored to the copper foam cathode to promote selective hydrogenation of alkynes to alkenes (Fig. 21) [117]. The experimental and theoretical results indicated that the surface-doped and -adsorbed sulfur on Cu-S NSs played a crucial role in selective alkyne semi-hydrogenation. On the one hand, the formation of sulfur anion-hydrated cation networks facilitated the production of active H* species from electrochemical H₂O splitting. On the other hand, the doping of sulfur decreased the adsorption of alkenes, avoiding over-hydrogenation. In a more recent study, Wang et al. employed a carbon-supported Cu microparticles to achieve electrocatalytic semihydrogenation of acetylene with high efficiency and selectivity [118]. By optimizing the Cu-based electrocatalyst to expose more active surfaces, it is conducive to the preferential adsorption and hydrogenation of acetylene, thereby inhibiting hydrogen adsorption and evolution. Combined with modulating the electrode potential to adjust the selectivity of the product, the excessive hydrogenation of acetylene to ethane can be completely avoided at the cathode potential higher than −0.6 V vs. RHE.

  Figure 21

  

  Figure 21.  Cu-S NSs modified electrode for electrocatalytic semihydrogenation of alkynes.

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  In addition, Ling et al. reported Ni_0.85Se nanowires with selenium vacancies (Ni_0.85Se_1-x) in-situ grown on NF electrode to promote transfer semihydrogenation of alkynes from H₂O electrolysis (Fig. 22) [119]. The Se vacancy could shift the d-band center toward the Fermi level, enhancing the surface adsorption of alkynes and H₂O as well as electron transfer. On the other hand, the Se vacancy could also decrease the activation energy barrier for H₂O dissociation which resulted in facilitating the formation of active H*. And it finally enabled the alkynes conversion up to 99% at a lower potential. Furthermore, 99% alkenes selectivity could be achieved because of the weak adsorption of alkenes as well as the thermodynamic constraints for over-hydrogenation of alkenes to alkanes. In addition, the reaction possessed excellent functional group tolerance and easily obtained deuterated alkenes with the deuterium ratios up to 99% by using D₂O as the deuterated donor.

  Figure 22

  

  Figure 22.  Ni_0.85Se_1-x modified electrode for electrocatalytic semihydrogenation of alkynes.

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  To sum up, Pd-P, Cu-S and Ni_0.85Se_1-x modified cathodes enable the efficient construction of alkenes from alkynes under mild conditions and H₂O as the H source. The doping of P, S, and Se could tune the adsorption and desorption of reactants or intermediates with the electrode surface, which will affect the thermodynamics and kinetics of subsequent reaction, and further affect the reaction selectivity. On the other hand, the incorporation of heteroatom might regular the redox capacity of reductants, which will further control the reactivity and selection. In addition, their studies demonstrated that water electrolysis is the rate-determining step, hence, developing more active heterogeneous catalysts to speed up the kinetics of water electrolysis could substantially improve the semi-hydrogenation of alkynes.

  3.2 Electrocatalytic hydrogenation of N-heterocycles

  The catalytic hydrogenation of N-heteroarenes is of great importance in chemical synthesis, material science, and hydrogen storage transfer. However, it still faces the challenges of breaking the aromaticity of substrates and catalyst poisoning. To date, a number of homogeneous systems based on metal (e.g., Ru, Ir, Pd, and Rh) complexes are active for this kind of transformation [120,121]. Nevertheless, most of them suffer from limited expensive ligands, lack of recyclability, and flammable and explosive H₂ or other expensive/toxic hydrogen sources, hindering their large-scale application. Thus, development of low-cost, safe hydrogen sources, and easily recovered and reused catalysts is urgently demanded. Recently, Li et al. exploited 3D self-supported MoNi₄ porous nanosheets on NF electrode to achieve electrocatalytic water-involving transfer hydrogenation of N-heterocycles (Fig. 23) [122]. A 99% selectivity of 1, 2, 3, 4-tetrahydroquinoxaline and 90% FE were obtained. Electron paramagnetic resonance (EPR) experiment confirmed that the in-situ generation of H* from water electrolysis was the key intermediate for quinoxaline hydrogenation via radical coupling with the quinoxaline radical anion, and the plausible mechanism was depicted in Fig. 23C. In addition, a variety of N-heterocycles hydrogenated products were achieved in moderate to good yields, and easily obtained deuterated products with the deuterium ratios up to 99% by using D₂O as the deuterated donor. Notably, MoNi₄ could be used as a bifunctional electrode for electrocatalytic dehydrogenation of N‐heterocycles, and in situ Raman tests revealed that the Ni^IIIOOH formed on the surface of MoNi₄ could promote the dehydrogenation process.

  Figure 23

  

  Figure 23.  MoNi₄ cathode for electrocatalytic hydrogenation of N-heterocycles.

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  3.3 Electrocatalytic hydrogenation of aldehydes/ketones

  The selective hydrogenation of carbonyl compounds, especially α, β-unsaturated aldehydes/ketones, is of great significance in industrial production of fine chemicals. Traditional hydrogenation focuses on H₂ as the H source and requires noble metal-based catalysts, which suffer from high cost and safety risk. Hence, it is highly desirable to develop a facile room-temperature hydrogenation method, especially with an inexpensive and safe hydrogen donor. Huang et al. employed the RuO₂−SnO₂−TiO₂/Ti as the cathode to achieve highly selective conversion of cinnamaldehyde (CAL) to cinnamyl alcohol (COL) in acidic media (Fig. 24) [123]. The as-prepared Ru_0.1Sn_0.2Ti_0.7O₂/Ti cathode exhibited the highest COL selectivity with the value of 88.86% at 58.00% conversion of CAL among the reported pure metal and other Ru_xSn_yTi_1−x−yO₂/Ti electrodes. The experimental and DFT calculation results indicated RuO₂ was the active site, which preferentially interacted with C=O of CAL and decreased the reaction barrier toward COL formation. The doping of SnO₂ could efficiently improve the FE, whereas high SnO₂ content would lead to dimers as the main product. Furthermore, low pH value and high overpotential were also facile to enhance the COL selectivity and inhibit the dimerization product.

  Figure 24

  

  Figure 24.  RuO₂−SnO₂−TiO₂ modified electrode for selective electrocatalytic hydrogenation of CAL. Reproduced with permission [123]. Copyright 2019, American Chemical Society.

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  Recently, Han et al. developed a CP cathode decorated with CoS₂ and CoS_2-x nanocapsules (NCs) for the selective hydrogenation of α, β-unsaturated aldehydes (Fig. 25) [124]. Interestingly, CoS₂ NCs and CoS_2-x NCs with rich sulfur vacancy exhibited high activity and selectivity for the hydrogenation of CAL to produce hydrocinnamaldehyde (HCAL) and hydrocinnamyl alcohol (HCOL), respectively, which was due to the specific adsorption of the styryl block on CoS₂ and C=O group on the sulfur-defective CoS_2-x and the hollow porous structures exposing abundant active sites and improving mass transfer. This work provides an efficient way for the selective hydrogenation of α, β-unsaturated aldehydes and H₂O as the low-cost hydrogen source.

  Figure 25

  

  Figure 25.  CoS₂ and CoS_2-x NCs modified electrodes for selective electrocatalytic hydrogenation of α, β-unsaturated aldehydes.

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  In addition, Chadderdon et al. proposed a Ag/C catalyst coated on CP cathode to promote the electrocatalytic hydrogenation of HMF (Fig. 26A) [125]. Though precise control of the cathode potential, the as-prepared Ag/C cathode could highly effective electrocatalytic reduction of HMF to 2, 5-bis(hydroxymethyl)furan (BHMF) under mild conditions with 96.2% FE. Unfortunately, the catalytic activity decreased slightly after the first recycle, which was probably due to Ag particle agglomeration. Li et al. reported Pd/VN hollow nanospheres coated on CF cathode for electrocatalytic hydrogenation of HMF to 2, 5-bishydroxymethyl-tetrahydrofuran (DHMTHF) (Fig. 26B) [37]. The as-obtained Pd/VN/CF cathode exhibited excellent conversion and FE of HMF into DHMTHF with the value of 92% and 91%, respectively. And the values are much higher than those obtained from CF, VN/CF, Pd/C, Pd/V₂O₅/CF and Pd/VOOH/CF electrodes. Furthermore, Pd/VN/CF cathode remained highly active within 8 successive electrolysis processes. Zhang et al. reported Cu phosphides nanosheets (Cu₃P) in situ grown on carbon fiber cloth (CFC) electrode for electrocatalytic hydrogenation of furfural to furfuryl alcohol (FAL) (Fig. 26C) [81]. The as-obtained Cu₃P/CFC demonstrated excellent catalytic performance with almost 100% selectivity of FAL and above 92% FE, much higher than that of bare CFC, Cu/CFC, Pt/CFC and Ni₂P electrodes. The DFT calculations indicated that the high concentration of H_ads was adsorbed on the surface of Cu₃P/CFC and large H₂ desorption energy, facilitating the furfural hydrogenation reaction. In addition, Yang et al. demonstrated that copper encapsulated alkaloids composite (alkaloid@Cu) cathode could achieve electrocatalytic asymmetric hydrogenation of aromatic ketones (Fig. 26D) [126]. The alkaloid@Cu directly acted as chiral inducer and could be reused for 10 cycles without obvious reduction of activity.

  Figure 26

  

  Figure 26.  (A) Ag/C and (B) Pd/VN modified electrodes for electrocatalytic hydrogenation of HMF, (C) Cu₃P/CFC electrode for electrocatalytic hydrogenation of furfural, (D) alkaloid@Cu cathode for electrocatalytic asymmetric hydrogenation of aromatic ketones.

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  In another case, Wu et al. reported PbS-based materials modified CP electrode for electrochemical reduction of levulinic acid (LA) to γ-valerolactone (GVL) (Fig. 27) [127]. Interestingly, the degree of PbS surface oxidation to PbSO₄ significantly affected the catalytic performance, which could be tuned by the calcination temperature. When the PbS was calcined at 400 ℃, the prepared PbS-400/CP electrode exhibited the best catalytic performance at a ternary electrolyte consisting of ionic liquids, H₂O and MeCN (FE of 78.6% and current density of 13.5 mA/cm²) of the electrocatalyst materials reported to date. Moreover, it also showed high selectivity with GVL being the only product. Mechanism studies indicated that LA was transformed into GVL via electrochemical hydrogenation followed by subsequent intramolecular esterification.

  Figure 27

  

  Figure 27.  PbS-400/CP electrode for electrochemical reduction of levulinic acid.

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  3.4 Electrocatalytic hydrogenation of nitriles

  NH₂ group as a valuable building block in organic synthesis is generally produced by selective hydrogenation of nitriles, which requires high hydrogen pressure and strong alkaline solutions [128]. Compared with thermal hydrogenation, electrocatalytic hydrogenation with H₂O as the proton source to selectively reduce nitriles to the corresponding amines is a promising strategy. In 2021, Zhang et al. described a Cu sheet-like nanoarrays in situ grown on Cu foil electrode (CuNAs/CuFoil) for electrochemical reduction of aliphatic nitriles to primary amines using H₂O as the H source (Fig. 28) [129]. The as-prepared CuNAs/CuFoil exhibited excellent performance for selective reduction of MeCN to EtNH₂ in a CO₂-saturated KHCO₃ aqueous solution with a selectivity of 99% and FE of 94%. The experimental and DFT calculations data demonstrated that Cu nanostructure offered preferential adsorption of the nitrile via the terminal C≡N group, promoting the hydrogenation process and at the same time suppressing the side reactions, such as HER and CO₂ reduction reaction. Furthermore, the initially formed primary amine can be protected by forming carbamic acids, thus preventing the formation of amine dimers and trimers. Additionally, the reaction tolerated various functional groups and the corresponding primary amines were obtained with exciting selectivity. Recently, Xia et al. also proposed a similar idea on MeCN electrohydrogenation in flow reactors [130]. They found that Cu NPs modified cathode exhibited the best EtNH₂ FE (~96%) at −0.29 V vs. RHE, much higher than Cu microparticles, oxide-derived Cu, and other metals catalysts, such as Ni, Pd, Pt, Sn, In and Bi. The DFT calculations indicated that these results could be attributed to the moderate binding affinity for the reaction intermediates. In addition, the flow reactor could mitigate the anticipated mass-transport limitations as well as led to significantly higher current densities (~1000 mA/cm²).

  Figure 28

  

  Figure 28.  CuNAs/CuFoil for electrochemical reduction of nitriles.

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  3.5 Electrocatalytic reduction of nitroarenes

  Selective catalytic hydrogenation of nitroarenes is of great significance for pharmaceuticals, pesticides, and dyestuff in chemical industry [131-136]. As early as 2001, Yuan et al. explored Pt/C modified cathode for electrochemical reduction of nitrobenzene [137]. As a result, 57.3% selectivity of cyclohexylanine and 28.2% selectivity of aniline with 8.2% conversion of nitrobenzene were obtained at 70 ℃ for 2 h. The limitation of the proton transportation as well as the low catalytic activity of the cathodic materials was probably responsible for the lower conversion of nitrobenzene. Hence, developing novel cathodic materials with high conductivity and electrocatalytic activity for the hydrogenation of nitroarene is necessary.

  Since then, some bimetal cathodic materials have been prepared and used to enhance the electrocatalytic activity and selectivity for electrochemical reduction of nitrobenzene to aniline [138,139]. For example, Jiang et al. reported HF-etched Cu₇₀Zr₃₀ amorphous alloy could effective electrocatalytic reduction of nitrobenzene with aniline as the dominant product, which was attributed to the formation and aggregation of Cu nanocrystals on the surface and increased electrochemically active surface area [138]. Additionally, some pure and alloyed Cu-based NPs (Cu-Cu_xO, Pt-Cu alloy and Pt NPs) supported on activated carbon modified cathodes were used for electrocatalytic reduction of nitrobenzene [140-144]. As a pertinent example, Zhang et al. investigated the ultrafine Cu_xPt_y alloying nanoparticles anchored on carbon black (donated as Cu_xPt_y/C) employed as a cathode to selectively regulate the products of electrocatalytic hydrogenation of nitrobenzene at different pH and potentials (Fig. 29). The as-obtained Cu₃Pt/C exhibited the best electrocatalytic hydrogenation activity toward nitrobenzene among the prepared Cu_xPt_y/C, owing to the adjusted electronic structure of Cu₃Pt/C could effectively facilitate the adsorption, activation and hydrogenation of nitrobenzene. As a result, the production of aminobenzene was dominant in the acidic media, irrelevant to the applied potential. However, in the alkaline media, azoxybenzene was the dominant hydrogenation product at a low reduction potential of 0.3 V (vs. RHE) with 100% conversion and 99% selectivity, while aminobenzene was absolutely dominant at a high reduction potential of −0.3 V (vs. RHE) with 100% conversion and 99% selectivity. The theoretical studies indicated that in the alkaline media, the energy barrier of the combination of Ph-NO* with Ph-N* was lower than that of Ph-NH₂*, leading to easily obtained azoxybenzene at low reduction potential and aminobenzene at high reduction potential. As the pH decreased, the free energy of the intermediate decreased, and the reaction would spontaneously obtain aminobenzene.

  Figure 29

  

  Figure 29.  (A) Schematic illustration of the synthetic process of Cu_xPt_y nanoparticles. (B) The conversion and product selectivity of Ph-NO₂ reduction over Cu₃Pt/C catalyst at different applied potentials. (C) The product selectivity of Ph-NO₂ reduction over Cu₃Pt/C catalyst at different pH solutions and different applied potential. Reproduced with permission [144]. Copyright 2021, Elsevier.

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  In recent years, several non-noble metallic oxide [145,146], boride [36], phosphide [147], and sulphides [148] have been reported as effective cathodic materials for electrocatalytic reduction of nitro-compounds. For example, Liu et al. demonstrated the defect-engineered TiO_2−x single crystals (SCs) could be used as an excellent cathodic electrocatalyst for the reduction of nitrobenzene, which was mainly attributed to the defective oxygen vacancy, high-energy [001] facet, as well as the continuous and ordered interior single crystalline structure (Fig. 30) [145].

  Figure 30

  

  Figure 30.  Defect-centered nitrobenzene reduction mechanism on the defective TiO_2−x SCs. (The mixed-valence Ti species cycling between +3 and +4 states at oxygen vacancy sites provide as an efficient electron shuttle and transport route for reductive electrons migrated from carbon cathode to TiO_2−x SCs and then to protons to generate active hydrogen (^•H/^•H⁻). Copied with permission [145]. Copyright 2016, American Chemical Society.

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  Recently, Zhang et al. reported CoP nanosheet modified NF cathode to promote the selective synthesis of azoxy-, azo- and amino-aromatics by electroreduction of nitro substrates through a potential-tuned strategy (Fig. 31) [147]. A series of desired products bearing various functional groups were obtained with excellent yields and selectivity by applying different bias input. In particular, asymmetrically substituted azoxy-aromatic compounds could also be controllably prepared with moderate to good yields. Additionally, using D₂O instead of H₂O as the sole hydrogen source provided a facile pathway to synthesize deuterated amino-aromatics with high deuterium ratios. Impressively, in a CoP||Ni₂P two-electrode configuration, azoxybenzene and octylnitrile could be simultaneously gram-scale produced with high efficiency at the cathode and anode, respectively. In a subsequent study, they achieved Co₃S₄ ultrathin nanosheets with sulfur vacancies and revealed that sulfur vacancies played a crucial role in the excellent selectivity of nitro hydrogenation, which facilitated the specific adsorption of the nitro group and the intrinsic activity of H₂O electrolysis to form active hydrogen (Fig. 32) [148]. Recently, Huang and Wang et al. studied a series of NF-supported spinel oxides MCo₂O₄ (M = Co, Cu, Mn, Fe and Zn) for electrochemical hydrogenation of nitroaromatics, and demonstrated CuCo₂O₄/NF was the most effective cathode material owing to the electrogenerated Cu species are the active intermediates [149].

  Figure 31

  

  Figure 31.  CoP modified electrode for selective electroreduction of nitro substrates.

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  Figure 32

  

  Figure 32.  Co₃S_4-x NS modified electrode for electrochemical transfer hydrogenation of nitroarenes.

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  With the development of the electrocatalytic reduction of nitroarenes, inorganic heterogeneous catalysts, as cathodes modifiers integrate the advantages of: (1) avoiding the use of strong acid and noble metal cathodes, (2) using H₂O as clean H source, (3) controlling the product by simply tuning the applied voltage. These superiorities may promote heterogeneous catalysts modified electrodes to be widely explored and applied in future.

  3.6 Electrocatalytic deuterodehalogenation of halides

  The deuterium-labeled molecules are significant for organic mechanism investigation and drug development. The C-H/C-D exchange is a straightforward deuterium incorporation strategy, which commonly required noble-metal catalysts and strong bases/acids. In contrast, the deuterodehalogenation of halides has offered an efficient way to achieve diversified deuterated molecules. However, it usually requires highly active alkyl-metal reagents, noble metal catalysts with complex ligands, and special deuterium donors. At present, electrocatalytic reduction reaction is considered to be an easy method to realize the deuterodehalogenation of halides. For example, Mitsudo et al. have achieved electrocatalytic dehalogenative deuteration of aryl halides using 9-fluorenone as an efficient mediator and expensive CD₃CN as D source in an undivided cell equipped with a Zn sacrificial anode and a Pt cathode [150]. Unfortunately, the mechanism was unclear and the yield is moderate. Hence, it is highly desirable to develop novel catalysts to realize highly efficient deuterodehalogenation of halides and elucidate the mechanism.

  Recently, Zhang et al. proposed a facile electrocatalytic deuterodehalogenation of halides using a self-supported Cu NWAs on Cu foil as the cathode which was fabricated by in-situ electrochemical reduction of CuO NWAs (Fig. 33) [151]. The reaction was initiated by the Cu NWAs cathode reduction of halides producing the carbon radical via releasing a halogen anion, and the deuterium radicals were simultaneously generated by electrocatalytic D₂O splitting. Then the desired products were formed by radicals cross-coupled reaction. This method possesses the advantages of environmental friendliness, good yields, broad substrate scopes and functional group tolerance. In addition, this electrocatalytic strategy could enable the transformation from C–H to C–D with high yields and deuterium contents via a one-pot halogenation-deuterodehalogenation process. Notably, in a Cu NWAs||Ni₂P nanosheets (NSs) two-electrode system, benzen-4-d-amine and high-value chemicals such as nitrile, aldehyde, and dihydroquinoline were successfully achieved with excellent yields at the cathode and anode, respectively.

  Figure 33

  

  Figure 33.  Cu NWAs modified electrode for radicals coupled reaction.

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  3.7 Electrocatalytic cross-coupling reactions

  Allylic alkylation is one of the most important and straightforward tools for C-C bond formation. Traditional methods usually use organometallic reagents to achieve allylic alkylation between an allylic substrate and a carbon nucleophile, which suffers from poor functional-group compatibility, and air/moisture sensitivity [152]. In terms of step economy and practicality, electrochemical allylic alkylations between allylic and alkyl halides is the most promising method. Previous reports demonstrate that electrochemical allylic alkylations need Zn anode to initiate the nucleophilic reaction as well as Pd- and Cu-salts as catalysts [152-154]. Unfortunately, the Pd- and Cu-salts will be reduced to metallic Pd and Cu under the reduction condition, leading to the actual catalytic site is unclear. To address this issue, Yin et al. explored CuPd NPs (Fig. 34), Pd NPs and Cu₃Pd NPs modified CP cathodes for the cross-coupling of alkyl halides and allylic halides without using either Zn electrode or Pd-/Cu-salts [155]. Interestingly, the Pd-catalysis was Pd/Cu composition dependent. CuPd NPs exhibited much higher catalytic activity than Pd NPs, indicating Cu played an important role in allylic alkylation. However, Cu₃Pd NPs displayed no catalytic activity for this reaction, demonstrating too much Cu would have a negative effect on Pd catalysis. These results illustrated that Pd was the main catalytic sites for C–X activation and Cu as a co-factor to promote the activation via binding to halides. And the ratio of Pd/Cu close to 1 is the most efficient catalyst. Moreover, no obvious loss of electrocatalytic activity was observed within five recycles.

  Figure 34

  

  Figure 34.  CuPd NPs modified electrode for electrochemical cross-coupling reaction.

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  4. Summary and perspectives

  Organic electrosynthesis, as a "green" technology, has developed rapidly in the past few years. As demonstrated by the above examples, employing heterogeneous catalysts modified electrodes as mediators has opened a new way for innovating organic electrosynthesis. When a suitably modified electrode is used as a heterogenous catalyst, the interaction (adsorption and desorption) of reactants or intermediates with the electrode surface can affect the thermodynamics and kinetics of electron transfer and subsequent reaction, and further affect the selectivity of the whole transformation [156]. The developments highlighted in this review exhibited that heterogeneous electrocatalysts modified electrodes could increase the selectivity and efficiency in organic electrosynthesis. To date, researchers have done lots of work on the development of heterogeneous electrocatalysts modified electrodes in synthetic methodology and designing of novel structures, however, there still present many new challenges and opportunities.

  (1) For the design of electrocatalysts: it is well known that the catalytic activity, selectivity, and stability are the main factors to evaluate the performance of electrocatalysts. Firstly, the structure such as 0D quantum dots, 1D nanowires, 2D nanosheets, and 3D framework could significantly affect catalytic activity. However, the structure–performance relationships have not been thoroughly studied. Hence, through precise structure design and fine-tuning the surrounding environment of catalytic sites to adjust the adsorption, chemical bond cleavage and formation, and desorption process is vital important for the catalytic activity and selectivity and urgently needed to explore. Furthermore, some emerging materials could be employed in the organic electrosynthesis. For example, porous framework materials (MOFs and COFs) possess high surface areas, regular arrangement of porous channels and tunable structures and inner environment of cavities, which facilitate the mass transport, electron and charge transfer process, have been applied to the electrocatalytic field [68,70,157-160]. Especially, the preparation of their nanosheets and thin films could increase the accessibility of catalytic sites, thereby improving their catalytic activity [158,161]. However, they have not received much attention in electrocatalytic organic synthesis. We anticipate a bright future for them as promising candidates for electrocatalytic organic synthesis. Secondly, the stability is also a key factor in evaluating the performance of the electrocatalysts. Partial skeleton decomposition of electrocatalysts and the electrocatalysts shedding from electrodes have been found under electrocatalytic conditions (e.g., acidic or alkaline test solution, organic solvents). Therefore, increasing their stability should be also paid close attention.

  (2) For the investigation of mechanism: mechanistic studies are crucial for every chemical reaction, and conductive to explore the factors that influence reactivity and selectivity in electrochemical reactions. Therefore, further deciphering the mechanism with the help of the construction of heterogeneous electrocatalysts with precisely controlled and well-defined structure continues to be an important subject, which would be no doubt beneficial for ab initio development of heterogeneous electrocatalysts. Recently, a review by Little et al. has focused primarily on the analytical techniques that aid in mechanism elucidation in detail, which provide in-depth analysis of ways to understand electroorganic reaction mechanisms [162]. With the rapid development of analysis tools for electrosynthesis, the reaction pathway will be accurately predicted.

  (3) For organic electrosynthesis: as mentioned above, heterogeneous catalysts modified electrodes have achieved great improvements in electrocatalytic organic synthesis in recent years. Many common classes of organic reactions are successfully accomplished by heterogeneous catalysts modified electrodes, such as oxidation reaction, reduction reaction, and cross-coupling reactions. However, some fundamental but vital organic reactions, such as the construction of complex chiral centers and the functionalization of inert C(sp³)−H bonds, are still waiting for exploration. In addition, the synergism of electrochemistry and photochemistry has received much attention, which provides a novel reaction strategy to generate super-oxidants or super-reductants to achieve the difficult molecular transformations under mild conditions [163,164]. Therefore, well-designed electro-photocatalysts to cooperate with electrocatalysis is expected to expand their application in modern organic synthesis.

  Water as the nontoxic and low-cost resource has been regarded as a potential oxygen or hydrogen source to replace the traditional oxygenating or hydrogenating agents. However, it faces a big challenge for application due to the high difficulty in activating the O−H bond of H₂O. Recently, some heterogeneous catalysts modified electrodes have been used to split water to offer reactive oxygen or hydrogen in the organic electrosynthesis [36,80,102,116,117,119,124,129,147]. For example, NiB_x and Mn₃O₄ anodes enabled the oxidation of biomass-derived platform chemicals and epoxidation of olefins using H₂O as the oxygen source, respectively [36,102]. Pd-P and CoS₂ cathodes realized the electrocatalytic semi-hydrogenation of alkynes and the selective hydrogenation of α, β-unsaturated aldehydes using H₂O as the hydrogen source, respectively [116,124]. So far, most reports have focused on the conversion efficiency and selectivity of organic reaction, whereas the activation of water, the active sites of electrocatalysts, and the oxygen/hydrogen atom transfer mechanism still remain ambiguous and need to further exploration.

  "Paired electrolysis", integrating anodic oxidation reaction with cathodic reduction reaction, enables maximum energy efficiency and the generation of value-added products. To date, the approach of paired electrolysis is mainly focus on the anodic organic oxidation reaction coupled with cathodic hydrogen evolution reaction. Recently, some new paired electrolysis reactions have been reported [115,116,147,151,165]. For example, in a Cu NWAs||Ni₂P nanosheets two-electrode system, benzen-4-d-amine and high-value chemicals such as nitrile, aldehyde, and dihydroquinoline were successfully achieved at the cathode and anode, respectively [151]. In a NiSe||NiSe two-electrode electrolyzer, α-nitrotoluenes and N-protected aminoarenes could be simultaneously synthesized with good selectivity and conversion yields [115]. In a CoP||Ni₂P two-electrode configuration, azoxybenzene and octylnitrile could be simultaneously gram-scale produced with high efficiency at the cathode and anode, respectively [147]. In the Pd-P||NiSe two-electrode system, 4-vinylaniline and adiponitrile were simultaneously obtained with high yields at the cathode and anode, respectively [116]. Despite recent advances, paired electrolysis is still very young in the organic electrosynthesis. Balancing the rates of cathodic and anodic reactions is crucial to the reaction efficiency in paired electrolysis. Hence, developing novel electrocatalysts to increase the reaction efficiency and selectivity indicate a new research trend. In addition, the relevant studies still stay in the laboratory stage, developing scalable production method for industrial settings is also urgent.

  In a word, integrating heterogeneous electrocatalysts into electrode surfaces expands the range of traditional organic electrosynthesis. Heterogeneous electrocatalysts can be specifically and rationally designed to incorporate catalytic sites and have broad application prospects. Despite there are some difficulties to overcome, heterogeneous electrocatalysts provide a promising platform for advancing electrocatalytic conversions of organic molecules to value-added products.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  We greatly appreciate the financial support from the National Natural Science Foundation of China (No. 22171154), the Youth Innovative Talents Recruitment and Cultivation Program of Shandong Higher Education, the Natural Science Foundation of Shandong Province (Nos. ZR2020QB114, ZR2020QB008 and ZR2019BB031), Jinan Science & Technology Bureau (No. 2021GXRC080). The project supported by the Foundation (No. ZZ20190312) of State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology (Shandong Academy of Sciences).

  
  
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• 

  Figure 1  Two types of electrolysis.

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  Figure 2  The representation of heterogeneous catalysts modified electrodes for organic electrosynthesis.

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  Figure 3  Pyrene−TEMPO−functionalized electrode electrocatalytic oxidation of alcohols. Reproduced with permission [18]. Copyright 2017, Wiley Publishing Group.

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  Figure 4  CNT/MOL-TEMPO modified electrode for electrocatalytic oxidation of alcohols. Reproduced with permission [19]. Copyright 2018, American Chemical Society.

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  Scheme 1  Schematic illustration of the proposed synergetic catalytic effect between Pd and Ag. (A, B) A probable mechanism of the synergetic catalytic effect for ethylene glycol oxidation on the PdAg/NF catalyst. (C) General pathway of ethylene glycol oxidation (the red is proposed to be the dominant pathway in this work).

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  Scheme 2  Two possible pathways of HMF oxidation to FDCA.

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  Figure 5  Au and Pd alloy modified electrode for electrocatalytic oxidation of HMF. Reproduced with permission [29]. Copyright 2020, American Chemical Society.

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  Figure 6  NiB_x modified electrode for electrocatalytic oxidation of HMF.

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  Figure 7  The electrochemical flow reactor for continuous HMF electrochemical oxidation. Reproduced with permission [34]. Copyright 2018, Wiley Publishing Group.

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  Figure 8  Co_Oh³⁺ plays a decisive role in HMF oxidation. Copied with permission [61]. Copyright 2020, Wiley Publishing Group.

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  Figure 9  Mechanistic illustration of potential-dependent oxidation of HMF to produce HMFCA and FDCA as mediated by electrogenerated Co³⁺ and Co⁴⁺ species

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  Figure 10  (A) 2D MOFs catalyst for electrocatalytic HMF oxidation. Copied with permission [77]. Copyright 2020, Royal Society of Chemistry. (B) Schematic representation for the fabrication of trimetallic MOF arrays on NF and the application of electrocatalytic HMF oxidation. Copied with permission [78]. Copyright 2021, Royal Society of Chemistry. (C) Schematic illustration of the preparation of TpBpy-Ni@FTO. Copied with permission [79]. Copyright 2020, Royal Society of Chemistry.

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  Figure 11  CuS modified electrode for electrocatalytic furfural oxidation.

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  Figure 12  Ni₂P hollow nanocubes modified electrode for selective thioether electrooxidation.

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  Figure 13  NiSe nanorod modified electrode for primary amines electrooxidation.

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  Figure 14  GF-CoS₂/CoS anode for electrocatalytic epoxidation.

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  Figure 15  Mn₃O₄ NPs modified electrode for epoxidation of olefins.

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  Figure 16  Ni₂P modified electrode for electrochemical semi-dehydrogenation of tetrahydroisoquinolines.

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  Figure 17  CNT/MOL-TEMPO-OPO₃^2– modified electrode for electrocatalytic dehydrogenation of N-heterocycles.

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  Figure 18  W₂C/N_3.0C modified electrode for alkoxylation of benzylic C–H bonds.

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  Figure 19  NiSe NAs modified electrode for electrochemical self-coupling reaction.

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  Figure 20  Pd-P NNs modified electrode for electrocatalytic semihydrogenation of alkynes.

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  Figure 21  Cu-S NSs modified electrode for electrocatalytic semihydrogenation of alkynes.

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  Figure 22  Ni_0.85Se_1-x modified electrode for electrocatalytic semihydrogenation of alkynes.

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  Figure 23  MoNi₄ cathode for electrocatalytic hydrogenation of N-heterocycles.

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  Figure 24  RuO₂−SnO₂−TiO₂ modified electrode for selective electrocatalytic hydrogenation of CAL. Reproduced with permission [123]. Copyright 2019, American Chemical Society.

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  Figure 25  CoS₂ and CoS_2-x NCs modified electrodes for selective electrocatalytic hydrogenation of α, β-unsaturated aldehydes.

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  Figure 26  (A) Ag/C and (B) Pd/VN modified electrodes for electrocatalytic hydrogenation of HMF, (C) Cu₃P/CFC electrode for electrocatalytic hydrogenation of furfural, (D) alkaloid@Cu cathode for electrocatalytic asymmetric hydrogenation of aromatic ketones.

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  Figure 27  PbS-400/CP electrode for electrochemical reduction of levulinic acid.

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  Figure 28  CuNAs/CuFoil for electrochemical reduction of nitriles.

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  Figure 29  (A) Schematic illustration of the synthetic process of Cu_xPt_y nanoparticles. (B) The conversion and product selectivity of Ph-NO₂ reduction over Cu₃Pt/C catalyst at different applied potentials. (C) The product selectivity of Ph-NO₂ reduction over Cu₃Pt/C catalyst at different pH solutions and different applied potential. Reproduced with permission [144]. Copyright 2021, Elsevier.

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  Figure 30  Defect-centered nitrobenzene reduction mechanism on the defective TiO_2−x SCs. (The mixed-valence Ti species cycling between +3 and +4 states at oxygen vacancy sites provide as an efficient electron shuttle and transport route for reductive electrons migrated from carbon cathode to TiO_2−x SCs and then to protons to generate active hydrogen (^•H/^•H⁻). Copied with permission [145]. Copyright 2016, American Chemical Society.

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  Figure 31  CoP modified electrode for selective electroreduction of nitro substrates.

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  Figure 32  Co₃S_4-x NS modified electrode for electrochemical transfer hydrogenation of nitroarenes.

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  Figure 33  Cu NWAs modified electrode for radicals coupled reaction.

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  Figure 34  CuPd NPs modified electrode for electrochemical cross-coupling reaction.

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