Electrocatalytic refining of small molecules in water

Abstract

The small-molecule electrocatalytic refining technology is a sustainable and eco-friendly strategy for converting renewable feedstocks and energy into transportable fuels and high-value chemicals in water (H₂O). The key to implementing this technology hinges on designing appropriate reactions and highly efficient electrocatalysts that can selectively break and form specific chemical bonds with precision for targeted refining processes. H₂O serves as an ideal medium for clean, green, and sustainable hydrogen and oxygen sources in refining reactions. Selective hydrogenation and oxidation reactions in water provide a mild and effective approach for producing high-value chemicals through precise refining. This review introduces the fundamental theories of multiphase electrocatalysis and selective hydrogenation/oxidation reactions. We then discuss relevant reaction types, including coupling, paired, and cascade reactions. Additionally, we summarize the design principles and strategies for electrocatalysts to effectively control reaction intermediates and pathways for precise refining. Finally, we explore the challenges and future opportunities in this field.

Graphical abstract

Keywords

water / renewable energy / refining / electrosynthesis

1 Introduction

Electrocatalytic refining of small molecules in water (H₂O) has emerged as a promising strategy for transforming renewable feedstocks into high-value chemicals and clean fuels through sustainable energy integration [1-5]. This innovative approach enables precise control over reaction selectivity and pathways through bias potential modulation under mild operating conditions, offering distinct advantages over conventional thermal catalytic processes that predominantly rely on temperature and pressure adjustments [6-8]. By circumventing the dependence on fossil fuel-based chemical refineries, the electrocatalytic refining demonstrates remarkable potential for substantially reducing both greenhouse gas emissions and overall energy consumption.

Compared to non-aqueous electrochemical systems, aqueous-phase reactions exhibit unique advantages through in situ generation and utilization of reactive hydrogen/oxygen species via synergistic hydrogen/oxygen transfer mechanisms coupled with electron transfer processes, eliminating the need for external gas supplementation [9-12]. The inherent cost-effectiveness derived from simplified solvent recovery and product separation further enhances the practical feasibility of aqueous systems for small-molecule transformations. Recent advancements in fundamental electrocatalytic processes, particularly in water splitting, CO₂ reduction, and NO³⁺ reduction reaction, have demonstrated the effectiveness of active hydrogen/oxygen mediation in facilitating these conversions [13-16]. However, the extension of electrocatalytic refining strategies to synthesize complex high-value chemical architectures with enhanced functionality remains in its infancy.

Generally, high-value chemical product generation involves bond-breaking/forming and a series of multi-step electron transfer and chemical reactions at the catalyst-electrolyte interface [17-19]. These mechanisms might include adsorbed intermediates, electron transfer processes, and chemical reaction stages that can occur either concurrently or independently [20-23]. The relationship between intermediates and the catalyst surface plays a crucial role in shaping the thermodynamics, kinetics, and pathways of catalytic reactions. Understanding water’s role in reactions, analyzing possible side reactions, and identifying key intermediates and potential products are crucial for designing efficient electrocatalysts with better performance.

In this review, we focus on summarizing the fundamentals of electrocatalysis, the role of selective hydrogenation and oxidation, reaction types, catalyst design, and future perspectives (Fig.1). We first discuss the basic principles of electrocatalysis, including reaction pathways and adsorption/desorption mechanisms, as well as the mechanisms of selective hydrogenation and oxidation. We then introduce reaction types in aqueous electrocatalysis, such as coupling, paired, and cascade reactions. Next, we present design guidelines and strategies for photoelectrocatalysts used in these reactions. Finally, we summarise the current state of research in aqueous electrocatalytic refining and outline future research directions. We hope that the review will raise awareness of aqueous electrocatalytic refining and provide valuable insights into the rational design of reactions and catalysts.

2 Fundamentals of electrocatalysis

The electrocatalytic hydrogenation and oxidation of small molecules in water are closely associated with water splitting on catalysts. In these refining reactions, reactants, intermediates, and products often adsorb onto the electrode surface, and the electrode materials strongly influence the electrochemical performance [24-26]. Recently, significant progress has been made in key aspects of catalysis, such as determining the composition and adsorption configuration of critical intermediates, evaluating the adsorption strength of intermediates, regulating reaction activity and selectivity, and designing catalysts, through theoretical calculations and advanced in situ characterization techniques [27-29].

When exploring strategies for the electrocatalytic refining of small molecules in water, three major theoretical tools are primarily used for prediction: free energy diagrams, volcano plots, and linear relationships. Free energy diagrams illustrate the ease or difficulty of intermediate adsorption and desorption during the reaction process. The interaction strength between intermediates and the photocatalyst surface is determined by the adsorption free energy, which is essentially dictated by the electronic structure of the electrocatalyst and is highly sensitive to surface properties and catalyst-electrolyte interface effects. Volcano plots quantitatively reveal the relationship between catalytic activity and intermediate adsorption energy, embodying the classical Sabatier principle and offering robust guidance for catalyst design. This principle states that the optimal catalyst should neither bind reaction intermediates too weakly nor too strongly. Typically, there are strong linear correlations between the adsorption strengths of intermediates, making them difficult to decouple. Specifically, the adsorption energies of intermediates containing hydrogen exhibit a linear relationship with those of the central atom, indicating the similarity in chemical bonding. Although these linear relationships decrease the degrees of freedom in material design and enable the recognition of key intermediates as activity descriptors for high-throughput screening of various materials, they also introduce an inherent constraint on electrocatalytic activity at the peak of volcano plots. Furthermore, the thermodynamic and kinetic properties associated with the adsorption or desorption of specific key intermediates play a crucial role in determining the selectivity of various parallel pathways for particular products. Additional studies are required to elucidate the specific mechanisms through which key intermediates influence activity and selectivity. This is especially difficult in multi-electron transfer reactions involving various similar adsorbates.

To design materials with optimized intrinsic activity and selectivity for specific reactions, follow a structured two-step approach: (i) identify key descriptors using linear relationships or volcano plots; (ii) adjust material compositions or structures based on these descriptors to achieve desired properties. First, optimize the electronic structure to achieve the volcano peak, thereby adjusting the adsorption energy of critical intermediates. Second, disrupt the linear relationship through the independent modulation of adsorption energies for specific intermediates [30]. Nørskov’s team has introduced a range of material design approaches aimed at disrupting the linear relationship. These include bifunctional catalysts, the use of promoters, functional modifications, electrolyte adjustments, interfacial locations, and confinement influences. Compared to other forms of catalysis, aqueous-phase electrocatalysis is characterized by a unique electrode-electrolyte interface and an externally applied potential bias. These factors can notably affect the adsorption of intermediates and the interfacial reactions occurring at the interface.

2.1 Selective hydrogenation reaction

The selective hydrogenation reactions in water, two primary processes coexist: the hydrogen evolution reaction (HER) via water electrolysis and the electrochemical hydrogenation (ECH) reaction of small-molecule organic substrates. During HER, water molecules dissociate at the electrode surface via Volmer to form adsorbed hydrogen intermediates (H*), followed by hydrogen gas release via either the Tafel (H* recombination) or Heyrovsky (H* + H₂O + e⁻ → H₂ + OH⁻) mechanism. In contrast, organic hydrogenation proceeds through two distinct pathways: (i) direct H* addition to surface-adsorbed organic species, analogous to hydrogen atom transfer (HAT) in homogeneous catalysis, or (ii) proton-coupled electron transfer (PCET), where H⁺ and e⁻ are sequentially transferred to activate the substrate. Kinetic isotope effects (KIE) and radical quenching experiments can differentiate these mechanisms, though the actual pathway depends on electrode material, applied potential, pH, and mass transport [31, 32].

The Gibbs free energy of hydrogen adsorption (ΔG) serves as a critical descriptor for HER activity, exhibiting a volcano-shaped relationship with exchange current density. Pt-based materials, with near-zero ΔG, demonstrate optimal HER activity but suffer from excessive competitive hydrogen evolution, rendering them unsuitable for organic hydrogenation. Thermochemical studies reveal that H₂ activation into reactive hydrogen species is rate-determining. Effective catalysts must balance hydrogen dissociation/adsorption with efficient hydrogen transfer to substrates. Materials with moderate ΔG facilitate low-overpotential hydrogenation for easily reducible substrates, while those with higher ΔG preferentially activate challenging substrates at elevated potentials. This selectivity arises because higher ΔG materials suppress low-potential HER, enhance hydrogen desorption rates, and prevent catalyst surface poisoning by excessive H* coverage.

After that, the potentiostatic electrolysis is preferred for selective hydrogenation due to its precise control over reduction potentials, crucial for targeting specific functional groups with distinct redox windows. In contrast, galvanostatic operation simplifies process scaling but risks potential drift toward negative values during substrate depletion or mass transport limitations, potentially triggering side reactions. Cyclic voltammetry (CV) effectively identifies redox-active windows for organic molecules. As shown in Fig.3(a), introducing substrate R induces characteristic redox peaks, with potential a marking the onset of R hydrogenation and potential b corresponding to intrinsic HER activation. Substrate presence shifts the operational window positively, indicating preferential organic reduction over water splitting. Within the a−b window, HER competition is minimized, achieving near-theoretical Faradaic efficiency (FE) limited by H* supply at the electrode−electrolyte interface. Between b and c, enhanced HER currents reduce FE despite higher total currents, suggesting synergistic mechanisms. Below c, hydrogen bubble-induced mass transport blockage dominates, drastically lowering hydrogenation efficiency. Thus, the a−c window defines the optimal electrochemical regime for mechanistic studies and catalyst screening, providing critical insights into surface-mediated organic transformations.

2.2 Selective oxidation reaction

Selective oxidation in water involves two reactions: the oxygen evolution reaction (OER) and organic electro-oxidation reactions (EOR) involving water, which compete with each other. In OER, there are two main mechanisms. One is the adsorbed intermediate mechanism (AEM), where water or hydroxide ions (OH⁻) are electro-oxidized to form highly active oxygen species OH*, which adsorb onto catalytic sites. Subsequently, these species deprotonate to form oxygen radicals O*. Hydroxide ions (OH⁻) then attack O* nucleophilically to generate the OOH* intermediate, which after secondary deprotonation forms O₂. The other is the lattice oxygen mechanism (LOM), which involves the coupling of lattice oxygen with O* to directly form O−O bonds. For both mechanisms, the formation of O* or OOH* generally represents the rate-determining step [33]. In thermochemical oxidation reactions with O₂, the O₂ molecule adsorbs on the catalyst surface and becomes activated, forming surface-bound OOH* and O*. These species then extract hydrogen atoms from organic molecules that are adsorbed on the surface, resulting in dehydrogenated products. Alternatively, these radicals may interact with adsorbed organic compounds, leading to the formation of oxygenated derivatives.

Selective oxidation reactions in water are thus divided into two categories: (i) dehydrogenation via water on reconstructed anode surfaces; (ii) selective oxidation reactions utilizing water as an oxygen source [34-37]. Unlike cathodes, most anodes undergo in-situ reconstruction in water before OER and EOR, forming high-valent metal (hydroxide) oxides. This procedure is mainly divided into two stages. In the first stage, surface hydroxylation occurs to form metal hydroxides. In the second stage, namely oxidation-deprotonation, protons interact with hydroxyl groups, leading to the formation of water and high-valent metal (hydroxide) oxides. In the first category, high-valent metal oxides or hydroxides abstract hydrogen from reactants, generating water and forming dehydrogenated products while regenerating the electrocatalyst. In the second category, surface-adsorbed organic species react with in-situ formed oxygen species to produce oxygenated products.

Moreover, when initiating electro-oxidation reactions, it is preferable to use controlled-potential electrolysis as shown in Fig.3(b), with the potential screened based on cyclic voltammograms (typically set between potentials d and f), as previously described for EHRs (electro-hydrogenation reactions). Here, potentials d and e correspond to the oxidation reaction of R and the potential applied at the onset of OER. Increasing the anode potential beyond potential f will significantly accelerate OER, which is detrimental to organic oxidation reactions.

3 Electrocatalytic refining reactions

By utilizing the core principles of C–X bond breaking and creation, we investigate potential avenues for the next-generation photoelectrochemical processing of organic small molecules. This is achieved by integrating precursors, intermediates, products, and reaction pathways in a real-time manner [38-40]. We summarize three major classes of electrocatalytic reactions — coupling reactions, paired reactions, and cascade reactions [Fig.4(a)−(c)] — that enable efficient transformations by leveraging intermediates such as *CO, *CH_x, *NH_x, *H, *O, and *OH [41-44]. The universal strategies proposed here provide theoretical guidance for designing new reactions and open avenues for future research in related fields.

3.1 Coupling reactions

In general, the electrocatalytic conversion of small molecules typically entails a mechanism involving the breaking and creation of chemical bonds. This process occurs due to the attack of adsorbed reaction intermediates by reactive hydrogen/oxygen [45-47]. In recent years, scientists have explored the use of external reaction precursors or reactive intermediates as alternatives to reactive hydrogen or oxygen. By targeting reaction intermediates, these approaches aim to generate more complex and higher-value chemicals via coupling reactions [48-50]. Coupling reactions typically involve two main steps: the electrochemical reaction step and the chemical reaction step. Initially, raw materials are converted into reactive electrophilic or nucleophilic intermediates through an electrochemical mechanism. Subsequently, these intermediates can specifically react with either external reactive molecules or intermediates carrying an opposite charge. By taking advantage of the thermodynamic benefits of the reaction, they ultimately form the final product through a chemical reaction process in a more efficient and controlled manner. By employing the coupling strategy, the primary extension of chemical products in small-molecule transformations involves forming C−N, C−S, C−O, and C−P bonds.

Carbon dioxide (CO₂), a key greenhouse gas, has drawn significant attention as a potential carbon source for the electrochemical production of carbon-based chemical compounds. In recent years, the research findings indicate that reactive electrophiles, including *CH₂O, *CH₂CO, *CO, and *COOH, can form at the interface between the electrode and electrolyte during the electrochemical CO₂ reduction reaction (CO₂RR). In the process of CO2RR, electrophilic species react with active hydrogen species, leading to the formation of primary hydrocarbon compounds like alcohols, carboxylic acids, and carbon monoxide. However, when external nucleophilic molecules or newly formed nucleophilic intermediates are introduced into the reaction solution, the electrophilic substances tend to preferentially react with these external nucleophiles, thereby facilitating electrochemical coupling reactions.

Unlike CO₂ molecules, nitrogen can function as a renewable resource, enabling the production of nucleophilic NH₃ for coupling reactions via electrochemical reduction. Carbon and nitrogen coupling is a well-studied and important type of reaction in organic chemistry, usually forming amide products. However, the preparation of amides by electrocatalytic carbon and nitrogen coupling of inorganic small molecules is a completely new field of reaction. Wang et al. [15] successfully prepared urea by electrocatalytic coupling of nitrogen and CO₂ [Fig.5(a)]. A Faraday efficiency of 8.92% [Fig.5(b)] and a urea yield of 3.36 mmol/(g·h) were achieved at −0.4 V relative to RHE. They also investigated the reaction mechanism, finding that N₂ adsorbs side-on (*N=N*) on the PdCu alloy surface. This configuration allows the d-electrons of Pd and Cu to back-donate into the π* antibonding orbitals of N₂, thereby weakening the N≡N triple bond. DFT calculations indicate that the adjacent adsorbed *N=N* promotes CO₂ reduction to form the key intermediate *COOH, which ultimately releases CO. Simultaneously, the activated *N=N* reacts thermodynamically spontaneously (ΔG = −0.89 eV) with the as-generated CO to yield the critical urea precursor NCON [Fig.5(c)]. In addition, during the electrochemical reduction of nitrogen and nitrogen oxides, reactive nucleophiles, including *N₂, *NH₂OH, and *NH₂, are produced in situ on the electrode surface [15]. These nucleophiles are highly reactive and can be combined with electrophilic substances (e.g., CO₂RR intermediates) to realize C−N coupling reactions.

Moreover, Zhang’s team [51] developed an efficient electrochemical method to establish C−S bonds and prepared a series of C−S compounds in high yields by coupling biomass oxidation with sulfur-containing nucleophilic reagents using commercial catalysts [Fig.5(d)]. The biomass method uses methanol as a carbon source and generates reactive intermediates (e.g., *CH₂O) by anodic oxidation, which is rapidly coupled with SO₃²⁻ to avoid high energy consumption and CO₂ emissions. As shown in Fig.5(e), the system achieved a Faraday efficiency of over 95% at a low current density of 5 mA·cm⁻². Four reaction pathways with *CH₂O, *CH₃, and *HOCH₂CHO as key intermediates promote the formation of C−S bonds. Meanwhile, Stahl’s group [52] reported a case of successful C−H bond activation of arylmethyl groups with the aid of electrocatalysis, and C−O bond construction with the aid of in situ generation of benzyl radical trapping, which further enables the synthesis of benzyl alcohols and aryl aldehydes with the aid of a photocatalytic system [52]. As for C−P coupling, Suga et al. [53] utilized DABCO as a mediator to achieve C−P coupling of diarylphosphole oxides in a mixed solution of acetonitrile and water. Developing general electrocatalytic methods for various C−P bond formation in water remains challenging, often limited by functional group compatibility and organic solvents. Nonetheless, electrocatalysis shows promise for sustainable C−P bond formation, replacing chemical oxidants and reductants, offering precise redox control, and integrating with green energy.

3.2 Paired reactions

Electrochemical reactions allow the simultaneous synthesis of hydrogenation and oxidation products at the cathode and anode electrodes of the electrolyzer. Generally, paired reactions are selected based on the following criteria: (i) producing higher-value products, (ii) operating at lower cell voltages compared to the HER and OER pairs, and (iii) aligning with the reaction kinetics of either the electrochemical oxygen reduction or electrocatalytic carbon dioxide reduction [54-57]. Using these advantages, electrosynthesis enables minimal energy consumption, leading to excellent atom economy and efficient energy utilization. Several industrially established paired reactions include the chlor-alkali process, which produces chlorine and sodium hydroxide simultaneously, as well as the benzaldehyde electrochemical synthesis carried out by BASF [58].

The more developed paired reactions start with different reactants producing different products at the cathode and anode. Jin’s team [59] has designed and developed a proton-equilibrium modular electrochemical synthesis strategy based on proton-selective redox reservoirs for the sustainable production of high-value-added organic and inorganic chemicals. Nickel hexacyanoferrate (NiHCF) can be used as a redox reservoir to selectively deliver protons in suitable solutions [59]. Based on this, electrochemical synthesis with a flexible pair of organic and aqueous phases in proton equilibrium can be constructed for sustainable production of organic products such as p-tert-butyltoluene oxidation in methanol or naphthalene C−H amination in acetonitrile with H₂O₂ in water. Moreover, Li et al. [60] designed a REDOX-mediated paired system [Fig.6(a)], in which the FE of CO₂ conversion to ethylene oxide (EO) is 1.5 times higher than that of the reported benchmark electrochemical system (35%), and the operating voltage is 1.2 V lower. The IrO₂ catalyst loaded with barium oxide (BaO₂) inhibited the cracking of HOCl and improved the FE conversion of CO₂ to EO, achieving a maximum FE conversion of up to 90% of ethylene (C₂H₄) to EO. After that, they replaced the HER with the oxygen reduction reaction (ORR). At applied current densities of 100, 200, and 300 mA·cm⁻², the FE of the C₂H₄-to-EO conversion was over 80% with full-cell voltages of 2.0, 2.2, and 2.4 V, respectively [Fig.6(b)]. Cathodic ORR enhances plant-gate levelized cost (PGLC) profitability within a current density of 100−300 mA/cm², requiring only 5.3 MJ·kgEO⁻¹ of electrical energy. This marks a 3.6-fold decrease in energy intensity compared to the standard electrochemical process [Fig.6(c)]. Meanwhile, an atomically ordered intermetallic compound Pd₃Bi metallocene (i-Pd₃Biene) catalyst with strong p-d orbital hybridization has been developed for efficient electrosynthesis of caprolactam (CYCO) and electrochemical reforming of PET plastic waste. Experimental and theoretical calculations showed that the strong p-d orbital hybridization in the i-Pd₃Biene catalyst was able to optimize the adsorption equilibrium of the key intermediates, thereby enhancing the electrosynthesis efficiency of CYCO and promoting the efficient oxidation of PET-derived ethylene glycol (EG) to glycolic acid (GA). Based on the bifunctional properties of the catalyst, the constructed two-electrode asymmetric electrolyzer was able to realize the efficient electrosynthesis of CYCO and GA simultaneously at lower voltages, significantly reducing the energy consumption [61].

In addition, Wang et al. [16] provided a bipolar hydrogen production system in which hydrogen is produced simultaneously at the cathode and anode at a low voltage of approximately 0.1 V. It was achieved by coupling the low-potential anodic oxidation of biomass aldehydes with the cathode HER [Fig.6(d)]. The anodic reaction is carried out on a metal Cu catalyst, where aldehyde substances (e.g., HMF) are electrocatalyzed to the corresponding carboxylate salts and H₂ at a low initial potential of 0.05 V_RHE. Due to the low bias potential, the hydrogen atoms of the aldehyde group are released as H₂ through Tafel recombination instead of being oxidized to H₂O through the Volmer step as in traditional aldehyde electrooxidation. The initial voltage of bipolar hydrogen production is lower than 0.1 V. When the battery voltage is only 0.27 V, the current density reaches 100 mA·cm⁻² [Fig.6(e)]. Conventional water electrolysis uses over 3.5 kWh per cubic meter of H₂, increasing significantly at higher current densities. The higher hydrogen production needs increased current density, raising electricity use and lowering efficiency. To increase yield at the same current density, the bipolar system with low voltage and doubled output needs only ~0.35 kWh per m³ of H₂ at 100 mA·cm⁻² with fresh electrodes, compared to about 5 kWh for traditional electrolysers [Fig.6(f)]. This parallel paired electrosynthesis approach features the independence of reactants and products at the electrodes. It minimizes extra cell voltage requirements, as thermodynamically favorable reactions can occur at the counter electrode.

3.3 Cascade reactions

A cascade reaction is a reaction in which a product or intermediate is used to initiate a subsequent electrochemical reaction, similar to the Domino effect, allowing multiple chemical reactions to proceed in an orderly sequence [62-64]. This reaction mode not only simplifies the operation process and reduces the generation of chemical waste, but also improves the utilization of atoms and electrons, providing a safe, efficient, and green method for the photosynthesis of small organic molecules.

Our group has proposed an integrated system that combines the membrane electrode module (MEA) for reducing CO₂ to ethylene with the flow cell for oxidizing ethylene to ethylene oxide [Fig.7(a)]. This cascade reaction system can directly convert carbon dioxide into ethylene oxide, utilize renewable energy, and further reduce reliance on fossil fuels. The Faraday efficiency of ethylene in MEA and the Faraday efficiency of ethylene oxide in the flow cell under different gas flow rates are presented [Fig.7(b)]. The results show that an appropriate gas flow rate can optimize the efficiency of both, but an excessively high flow rate will reduce the concentration of ethylene, thereby affecting the generation efficiency of ethylene oxide. The cascade reaction combines the reduction of carbon dioxide and the oxidation of ethylene, achieving an efficient conversion from carbon dioxide to high-value-added chemicals and having significant potential for industrial applications. Moreover, in the electrosynthesis of organic nitrogen compounds, hydroxylamine-mediated cascade reactions are widely used. For example, the synthesis of amino acids by electrocatalytic co-reduction of nitrate and oxalic acid not only avoids the use of highly toxic cyanide, but also takes advantage of the high concentration of intermediates at the electrode interface to enhance the reaction efficiency [66]. Meanwhile, Zhang’s group [65] utilized the rich and deficient Ag obtained from the electroreduction of nano-Ag₂O as the electrocatalyst and NO, pyruvic acid and water as raw materials to achieve the efficient electrosynthesis of alanine at room temperature, and revealed the reaction pathway of *NO → *NH₂O → *OH → pyruvic acid xime (PO) → alanine. Aiming at the problem of reaction rate mismatch between the formation and reduction of pyruvate oxime, the team designed a new two-step cascade electrosynthesis system [Fig.7(c)]. Compare the five metals in the H-type electrolytic cell. The FE of OD-Ag in the direct reduction of PO was significantly higher than that of traditional Ag nanoparticles, indicating that the low-coordination Ag promoted the reduction of PO [Fig.7(d)]. Using a flow reactor with rich and absent silver as the electrocatalyst, the gram-scale preparation of high-purity alanine was achieved at an industrial current density (100 mA·cm⁻²).

In addition, Qiu’s group [67] proposed a novel liquid−liquid−solid (l−l−s) three-phase cascade system that breaks through the kinetic limitations. Taking the oxidation reaction of benzyl alcohol (BA) as a model, and taking advantage of its intermediate product, benzaldehyde (Ph-CHO), with its high solubility in C₇H₈ phase, the Ph-CHO intermediate product was separated in situ during the reaction process and extracted into the reaction inert C₇H₈ phase, effectively During the reaction, the Ph-CHO intermediate was isolated in situ and extracted into the reaction inert C₇H₈ phase, which effectively inhibited its further peroxidation reaction, thus realizing the highly selective synthesis of Ph-CHO. Under the alkaline condition of 1 mol·L⁻¹ KOH, the selectivity of Ph-CHO for benzaldehyde was as high as 80.4%@1.5 V vs. RHE, which was 200 times higher than that of the conventional two-phase system under the same condition.

4 Catalyst design

Catalysts are pivotal in modulating reaction activity and selectivity. Investigating how catalysts govern reaction outcomes and pinpointing the key factors in discovering high-potential catalysts to boost their electrochemical performance can guide efficient catalyst design.

More complex electrocatalytic reactions typically involve the simultaneous activation of multiple molecules and the mutual adsorption of multiple intermediates. For instance, the simultaneous reduction of CO₂ to *CO and NO₂/NO₃³⁺ to *NH₂, along with the formation of coupled intermediates, facilitates the electrocatalytic synthesis of urea [40, 44]. Nitrobenzene is converted to aminobenzene by efficient hydrolytic dissociation, providing *H and selective activation of the nitro group [68]. Catalysts must independently regulate intermediate adsorption in each component reaction and efficiently promote bond formation. Researchers have developed strategies like heteroatom doping [69-71], defect engineering [72], ion modulation [73-75], surface/interface engineering [45, 76], alloying [29], and ligand stabilization [77] to enhance active site activity and decouple intermediate binding energies in major reactions. Based on these advances, this review will present generic strategies for designing materials for complex electrocatalytic reactions, mainly involving [Fig.8(a)−(c)] (i) development of multi-functional sites, (ii) modulation of the microenvironment, and (iii) manipulation of intermediate adsorption and desorption by introduction of mediating elements.

4.1 Multi-site catalyst

Combining two kinds of active sites within a single system offers a straightforward and adaptable approach to separately enhance the adsorption and catalytic activity of intermediates for each component reaction. Creating well-developed volcano plots for key reactions can speed up the identification of promising active sites [78-80]. For selective hydrogenation (or oxidation), site A selectively adsorbs and activates target bonds, while site B efficiently dissociates water to supply reducing agents (like *H) or oxidizing agents (like *O or *OH). These two sites should have a good spatial arrangement to facilitate interactions between different intermediates. Materials with multiple adjacent active sites, such as metal alloys, single-atom sites in catalytically active substrates, and high-entropy nanomaterials, are very promising [29, 31, 81-83].

Hu et al. [82] introduced cerium oxide, a synergistic site capable of generating active hydrogen and assisting hydrogen transfer, into traditional copper catalysts, which was able to switch the PCET dominant mechanism in the traditional ECR process to the surface electrochemical HAT dominant mechanism [Fig.9(a)] [82]. Thus, the problem of low selectivity of ECR caused by the generation of multiple products through non-directional hydrogenation in the PCET process was overcome, and the efficient synthesis of a single product was achieved. Electrochemical tests indicated that, compared with Cu/c-CeO₂, the Cu/a-CeO_2-x catalyst significantly enhanced the ECR kinetics of furfural, achieving an efficient conversion of furfural to the single product furfuryl alcohol in a flow-type reactor, with a Faraday efficiency of (97 ± 1)% [Fig.9(b)]. In contrast, Cu/a-CeO_2-x exhibited a significantly higher j_FA of (65.7 ± 2) mA·cm⁻² and a FA yield rate of 8.9 mol·h⁻¹·m⁻² at −0.5 V_RHE, both approximately 1.9 times greater than those of Cu/c-CeO₂ [Fig.9(c)].

Recently, Wu’s team reported a new composite material for the cascade synthesis of amino alcohols. It contains two distinct monometallic sites: Fe₁ sites in nitrogen-doped carbon shells and Pd₁ sites in MOF-derived core-shell structures [85]. Constructing dual active sites in 3D space is key for multi-step chemical synthesis via tandem electro-catalytic reactions. Efficient electro-catalytic conversion of locally accumulated species and optimization of the strongly coupled metal-support interface, which can simultaneously activate multiple reactants and intermediates, are needed. The support and metal possess distinct active sites for two different reaction pathways. Their proximity facilitates the subsequent coupling of different intermediates. Strong metal-support interactions can also electronically modify the support metal, influencing adsorption behavior and ultimately affecting catalytic activity and selectivity. In C−N coupling for urea production, oxygen-vacancy-rich TiO₂ supports were found to function as parallel catalysts for nitrite reduction and enhance the catalytic activity of supported PdCu nanoparticles by optimizing their electronic structure [15]. The size and configuration of the support metal significantly impact both the metal-support interface and the catalytic performance. Supported single-cluster catalysts and fully exposed cluster catalysts, as intermediate states between single-atom catalysts and nanoparticles, are attractive candidates due to their maximized atom utilization, abundant interface sites, and diverse metal site combinations.

In addition, Lin’s team [84] proposed a high-entropy nano-surface-engineering strategy [Fig.9(d)]. They developed the PtCuCoNiMn-EC high-entropy alloy electro-catalyst, which effectively enhances glycerol oxidation to glycerate, achieving a FE of 75.2%@0.8 V vs. RHE [Fig.9(e)] [84]. In-situ spectroscopy shows that the Pt site is the main active site, where glycerol is converted into glycerate through the intermediate glyceraldehyde [Fig.9(f)]. DFT calculations show that Cu inhibits lactic acid production and enhances glycerate selectivity, while other metals adjust the electronic structure and protect the Pt site. The activity of PtCuCoNiMn-EC decreased under long-term constant potential, but the intermittent potential cycling strategy could restore its performance. In the MEA flow electrolyzer, this catalyst achieved a current density of 350 mA·cm⁻² at 1.2 V_cell, with a glyceric acid selectivity of 71.8%, and maintained stable performance for 210 hours [Fig.9(g)].

4.2 Microenvironment tuning

In catalytic refining, achieving high selectivity for specific products is crucial. Modulating the interfacial microenvironment can alter the adsorption and local concentration of products or intermediates, significantly influencing the system’s activity and selectivity [86-88]. This modulation primarily involves two approaches: one approach involves nano-confinement structural modulation of the catalyst’s geometric structure [89]. Another addition of surface promoters or blocking agents, such as ionic liquid impregnation, organic surfactant modification, and COF/MOF surface modification [90-92]. An optimized local microenvironment can regulate proton donor diffusion, enrich the local concentration of reactant molecules, and adjust the binding strength of intermediates, thereby enhancing activity and altering selectivity. Microenvironment modulation also plays a significant role in water oxidation, influencing local pH and the binding energy of oxygen species, thereby enhancing the activity and stability of photo-electrocatalysts.

Zhang’s team [76] enhanced the electric field effect through nanotips and regulated the interface microenvironment of the surfactant molecules to modify the catalyst [Fig.10(a)]. They conducted electrocatalytic deuteration of aryl acetonitrile at a current density of 100 mA·cm⁻², achieving a Faraday efficiency of 80% [Fig.10(b)]. The highest FE and yield of α,β-deuterated amine over the Cu NTs imply intrinsically high activity [Fig.10(c)]. A strong electric field generated near the tip of the needle can increase the concentration of aryl acetonitrile, reduce the activation energy of aryl acetonitrile, enhance the transfer of aryl acetonitrile, and hinder the generation of D₂ side reactions. Therefore, it breaks the inherent equilibrium problem between current density and Faraday efficiency. The in-situ ATR-FTIR spectroscopy results indicated that the addition of BTAB formed a hydrophobic interface microenvironment, reduced the amount of D₂O, and promoted the transfer of nitrile reactants. Moreover, the geometric structure of catalysts offers a new pathway for regulating multi-step electro-catalytic reactions through nano-confinement, which is as important as active sites. Nano-confinement facilitates the creation of uniform nano-active phases, modulates surface reactivity via electronic interactions, promotes reactant diffusion to active sites, stabilizes intermediate adsorption, and protects active sites.

In addition, Li’s team [93] introduced quaternary ammonium cation surfactants with varying alkyl chain lengths as electrolyte additives in the HMF electro-catalytic hydrogenation system [Fig.10(d)]. EIS and in situ ATR-SEIRAS results indicate that the surfactants assemble at the electrode-electrolyte interface, restricting water molecule migration and promoting reactant enrichment. This achieves near 100% selectivity for BHMF, enhancing its Faraday efficiency (FE) from 61% to 74% at –100 mA·cm^–2 [Fig.10(e)]. Meanwhile, Duan et al. [92] significantly enhanced the catalytic activity in the electrochemical benzaldehyde reduction coupling to construct C−C through the regulation strategy of the microenvironment at the electrode-electrolyte interface, and unexpectedly improved the stereoselectivity of racemes. The research found that CTA⁺ adsorbed on the surface of CP and exhibited a potential-dependent adsorption conformation. Specifically, when different potentials are applied, CTA⁺ undergoes a dynamic conformational transformation. The positively charged head group of CTA⁺ faces the electrode side, and the hydrophobic tail group faces the electrolyte side. After a series of electrochemical experiments for verification, it is proposed that the role of CTA⁺ ions is divided into two parts: (i) Through dipole interaction, the carbonyl radical is stabilized, enhancing the reaction rate; (ii) Through hydrophobic interactions, the interface water molecules are repelled, the hydrogen bond between the carbonyl radical and the hydroxyl group is strengthened, and the stereoselectivity of the racemic compound is enhanced.

4.3 Mediated catalysis

Mediated catalysis involves using external mediators (like catalysts or radicals) to facilitate reactions. In the selective oxidation and reduction of aqueous organic compounds, mediator catalysis has the following two main roles: (i) Reducing the reaction activation energy and improving the efficiency, the mediator can interact with the reactants and change the electronic structure and chemical bonding of the reactants, to reduce the activation energy of the reaction, thus making the reaction more straightforward to carry out [94]. (ii) Altering the reaction pathway and improving the selectivity of the reaction, the mediator offers alternative reaction pathways, improving selectivity by directing reactants more effectively toward desired products.

As shown in Fig.11(a), Sun et al. [95] developed a new method for the synthesis of nitrate by hydrogen peroxide-mediated electrochemical oxidation of nitrogen. The method has a low onset potential and achieves a Faraday efficiency of 25.6% [Fig.11(b)]. During a 190-hour steady-state current test, an electrolytic cell voltage of about 1.69 V was required to reach a current density of 15 mA·cm⁻², resulting in an estimated nitrate yield of 5.84 nmol·s⁻¹·cm⁻² [Fig.11(c)]. The authenticity of nitrogen activation was confirmed by in situ characterization techniques and revealed the key role of ·OH in the nitrogen oxidation process. In addition, the sustainability and economic analysis of the method showed its potential to replace the conventional nitrate production process.

In addition, our group [25] used chloride ions (Cl⁻) as redox mediators at the anode to promote the selective partial oxidation of ethylene to ethylene oxide. At the anode, Cl⁻ is oxidized to Cl₂, which then undergoes hydrolysis to form HCl and HClO. HClO reacts with ethylene to produce chlorohydrin, while HCl remains unconsumed, acidifying the anolyte. At the cathode, water is reduced to hydrogen gas and hydroxide ions (OH⁻), alkalinizing the catholyte. Mixing the anolyte and catholyte post-electrolysis allows OH⁻ to react with chlorohydrin to yield ethylene oxide. This offers a potential electrochemical method for producing ethylene oxide using renewable energy. Meanwhile, our group [96] has recently designed a CoO_x cluster modified IrO₂ electrocatalyst, which can electrochemically activate NO³⁻ medium into highly active NO₃ radicals. These radicals can extract hydrogen atoms from the benzyl C−H bond of toluene, thereby achieving selective oxidation of toluene to benzaldehyde [Fig.11(d)]. After the introduction of the NO³⁻ medium, the Faraday efficiency increased from 26% in direct oxidation to 86% [Fig.11(e)]. It operated stably for 100 hours at a current density of 25 mA·cm⁻² with almost no performance attenuation.

To synthesize valuable cyclic carbonates such as propylene carbonate. Mo reported a bromide-mediated membraneless electro-synthesis strategy for converting ethylene and CO₂ into vinyl carbonate [97]. By engineering the electrolyte to match the kinetics of bromide oxidation and using a chromium hydroxide membrane to protect the cathode, the system achieved a Faraday efficiency of 47%−78% for vinyl carbonate at 10−250 mA·cm⁻². Moreover, Zhang et al. [98] reported a hydroxyl radical-mediated reaction for the high-selectivity photoelectrochemical (PEC) oxidation of benzyl alcohol (BA) to benzaldehyde (BAD). They designed a BiVO₄ photoanode coated with a covalent organic framework (Ni-TpBpy) containing single Ni sites. The reaction on the Ni-TpBpy/BiVO₄ photoanode follows first-order kinetics, promoting surface-bound ·OH radical formation. This inhibits further BAD oxidation, achieving nearly 100% selectivity for BA-to-BAD conversion and a turnover rate of 80.63 μmol·h⁻¹.

5 Conclusion and perspective

This review offers a pioneering and interdisciplinary insight into the latest developments in organic molecule conversion using water as a medium. It comprehensively explores the fundamental principles, reaction types, catalyst design, and future directions of organic molecule conversion in water. As a green and sustainable method, water-based organic molecule conversion systems primarily aim for the selective transformation of organic molecules to efficiently and precisely synthesize target products. By theoretically predicting reaction types and catalysts, and regulating intermediate binding, the review demonstrates how to alter reaction pathways and obtain desired products. It covers major reaction types, including addition, coordination, and consecutive reactions. Addition reactions can form C−X bonds by introducing new elements to carbon atoms, yielding high-value products like urea. Coordination reactions involve two or more reactants interacting in a suitable electrolyte to produce complementary products, utilizing both anodic and cathodic reactions to boost efficiency. Consecutive reactions, or level-parallel reactions, enable the direct generation of valuable products without separating intermediates. The review also summarizes current catalyst developments, highlighting multi-site design, microenvironment modulation, and mediation strategies to enhance reaction efficiency and selectivity. Despite significant progress, this field is still emerging and requires further research.

5.1 Developing new small molecular reactions

In the future, there will be a need for a more extensive database of organic molecules that react with water, including more stable reagents (such as carboxylic acids, amines, and aromatic compounds) that undergo hydrogenation reactions, as well as inert alkanes that undergo oxidation reactions. Through the identification and integration of target reactions and key intermediates, it is possible to design new reactions. There are envisioned specific reactions that might be particularly promising, including cyclohexanone through C−N coupling to caprolactam, benzene to phenol, and propane to propanol. Among them, caprolactam is an essential source of industrial nylon, and the value of the substrate can be enhanced by using renewable energy. Meanwhile, phenol serves as an essential raw material for plastics, while propanol has greater economic value than propane and can be utilized as aviation fuel. This has enabled a wide range of water-based homogeneous catalysis processes, including C−N coupling, selective hydrogenation, selective oxidation, and sequential reactions. This top-down design concept has promoted the development of electrochemistry, allowing it to produce valuable goods and specialty chemicals from a diverse range of inexpensive raw materials. It has also effectively linked fundamental reactions and mechanisms with primary and complex reactions, and has provided analogous knowledge and strategies for key intermediates, including their adsorption, activity, and energy conversion efficiency.

5.2 Identifying reaction pathways

In contrast to the simplicity of the model water-based hydrolysis reaction, the electrocatalytic transformation reaction is more complex due to the presence of multiple intermediates that interact with the electron transfer mechanisms. During the electrocatalytic transformation process, numerous reaction pathways coexist. The accurate identification of these reaction pathways is crucial for the direction of the reaction process and the enhancement of the efficiency of the desired product. The latest advancements in-situ optical and microscopic techniques have provided valuable tools for the identification of authentic reaction sites and the monitoring of critical intermediates at the atomic level [99, 100]. Among them, attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) offers very high surface sensitivity in in-situ characterization and can quickly detect intermediates of small molecule conversion. However, it can experience signal saturation when detecting strongly adsorbed species. In-situ Raman spectroscopy is particularly useful for analyzing the conversion pathways of small molecules, allowing for real-time tracking of key intermediates and the development of new phases. However, it also faces certain limitations, such as sensitivity being influenced by the optical settings of the Raman instrument. In-situ X-ray absorption spectroscopy (XAS) has been essential for investigating the real-time structural changes of electrocatalysts during operation, offering important insights into their coordination environments and the oxidation states of active sites. Nevertheless, XAS faces limitations due to inter-element interference and overlapping peaks. Fourier transform infrared spectroscopy (FTIR) has high penetration, but its sensitivity is limited. To enhance the detection of conversion pathways and intermediates, it is common to combine two or three in-situ techniques to identify key intermediates. In cascade reactions, in-situ electrochemical differential mass spectrometry (DEMS) and flow electrolyzer-based mass spectrometry (FEMS) can capture gas-phase and transition-state molecules in real time, thereby precisely exploring the reaction pathways of small molecule conversion. This provides a perspective for regulating the reaction pathways of small molecules from the material design level to determine key intermediates. However, mass spectrometry poses challenges in data interpretation, and achieving a satisfactory signal-to-noise ratio can be difficult. In addition, the use of computational modeling in new contexts has facilitated a more profound understanding of the interplay between reaction and transport mechanisms in heterogeneous catalysis. This includes the potential dependence of activity, the coverage of intermediate phases, the interaction between adsorbents and other adsorbents, and the pH effect. Consequently, we advocate for a combined approach of process modeling and experimental validation. This approach will enable us to identify specific factors affecting the system under investigation. It will also allow us to refine the mechanisms of reaction. The insights gained from this approach will be used to guide the optimization of catalysts and reaction mechanisms.

5.3 Designing the industry-related catalyst and reactor

In large-scale industrial production, catalysts and reactors face multiple challenges, such as catalyst degradation and failure, secondary reactions, high electrical resistance and voltage, and low current density. Recently, intensive research has been conducted into designing catalysts with high activity, selectivity, and stability for industrial use. At the same time, reactors, like flow electrolysis cells and membrane electrode assemblies, have been optimized to adjust the catalyst’s surface environment and boost its performance [101-103]. Yet, despite these advances, catalysts, flow electrolysis cells, and membrane electrode assemblies still face unresolved challenges. One persistent issue is catalyst reduction, especially in high-temperature and high-pressure reactions. The catalyst environment can change, altering the pH value, reactant concentration, and interfacial water composition, which affects the reaction’s activity and selectivity. The current anode noble metal Ir loading in membrane electrodes is generally as high as 2−4 mg·cm⁻², accounting for over 30% of the system cost [104]. To achieve large-scale application, the Ir loading needs to be reduced to <0.5 mg·cm⁻² (DOE target), [105] which requires the development of highly dispersed catalysts or non-noble metal alternatives [106]. Currently, perfluorosulfonic acid membranes are mainly used, and the membrane material accounts for 40%−50% of the total cost of industrial electrolyzers. To reduce costs, it may be necessary to develop some new fluorine-free membranes, such as sulfonated poly(arylether ketone). Another often-overlooked factor is the impact of reactant and product penetration, which can lower product yields. Moreover, high concentrations of organic compounds can potentially destabilize electrodes and membranes. Regarding mass transfer, while higher flow rates and turbulent flows can improve mass transfer and increase current density, they can also reduce reactant conversion efficiency. Balancing mass transfer and conversion efficiency in large-scale production remains a significant challenge. Additionally, in large-scale operations, managing liquid substances can be difficult. Gas effusion reactions may hinder substrate absorption and reaction, and could compromise the stability of the catalyst and membrane.

In conclusion, overcoming these challenges will allow rational design of reactions and catalysts, and even industrial production. We hope that this review will stimulate more innovative research in small-molecule catalysis and further focus on the potential of electrocatalysis for green synthesis in water.

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