Upgrading Organic Compounds through the Coupling of Electrooxidation with Hydrogen Evolution

Abstract

The electrocatalytic splitting of water is recognized to be the most sustainable and clean technology for the production of hydrogen (H₂). Unfortunately, the efficiency is seriously restricted by the sluggish kinetics of the oxygen evolution reaction (OER) at the anode. In contrast to the OER, the electrooxidation of organic compounds (EOO) is more thermodynamically and kinetically favorable. Thus, the coupling of the EOO and hydrogen evolution reaction (HER) has emerged as an alternative route, as it can greatly improve the catalytic efficiency for the production of H₂. Simultaneously, value‐added organic compounds can be generated on the anode through electrooxidation upgrading. In this Minireview, we highlight the latest progress and milestones in coupling the EOO with the HER. Emphasis is focused on the design of the anode catalyst, understanding the reaction mechanism, and the construction of the electrolyzer. Moreover, challenges and prospects are offered relating to the future development of this emerging technology.

This Minireview summarizes the latest achievements and milestones in coupling the electrooxidation of organic compounds and the hydrogen evolution in a hybrid water electrolyzer. Emphasis is placed on exploiting the anode reaction and the design of the electrocatalyst. Prospects for the future development of this coupling technology and its potential implementation in practical organic synthesis and hydrogen production are also provided.

1. Introduction

Growing environmental pollution and the energy crisis has raised great concerns about the exploitation of sustainable energy sources as feasible alternatives to traditional fossil fuels.[ ¹ , ² ] Molecular hydrogen (H₂) has been pursued as a prospective energy carrier, thanks to its high energy density and environmentally friendly properties. ^[3] Among various H₂ production technologies, electrocatalytic water splitting (e.g. the water electrolyzer) has emerged because of its low cost and zero‐carbon emissions.[ ⁴ , ⁵ ] The overall water splitting (OWS) reaction in a water electrolyzer incorporates two half‐cell reactions: the hydrogen evolution reaction (HER) on the cathode and the oxygen evolution reaction (OER) on the anode ^[6] (Figure 1a). The thermodynamic equilibrium potential for the HER (2 H⁺/H₂) and OER (2H₂O/O₂) is 0 and 1.23 V (vs. the reversible hydrogen electrode (RHE), respectively ^[7] (Figure 1b). Thus, a theoretical minimum voltage of 1.23 V is needed to promote the OWS. However, additional energy input and highly active electrocatalysts are needed to overcome the kinetic barrier for both the HER and OER. In particular, the extremely sluggish four‐electron‐transfer OER process on the anode seriously restricts the OWS efficiency, ^[8] thereby resulting in the need for a high voltage (e.g. >1.6 V) for realizing a meaningful production of H₂ (Figure 1b).

Figure 1.

a) Schematic representation showing the water electrolyzer for the HER and OER. b) Schematic representation showing the potential ranges of the OER, EOO, and HER. c) Schematic illustration of the hybrid electrolyzer for coupling the EOO and HER.

Recently, the electrooxidation of organic compounds (EOOs) has also attracted increasing attention.[ ⁹ , ¹⁰ ] It provides a sustainable pathway that utilizes electricity from renewable energy sources for chemical upgrading, ^[11] avoiding the use of organic solvents, homogeneous catalysts, and hazardous/poisonous strong oxidants (e.g. H₂O₂, 3‐chloroperbenzoic acid, peroxy acids, oxone, or iodine) or even the elevated temperatures and pressures used in traditional organic oxidation reactions. ^[12] Importantly, the EOO is thermodynamically and kinetically favored over the OER, as it has a lower theoretical equilibrium potential (ca. 1.0 V_RHE). Thus, coupling the EOO and HER in a hybrid electrolyzer would undoubtedly lower the cell voltage for generating clean H₂ with less energy input. Importantly, value‐added compounds can be simultaneously produced on the anode (Figure 1b,c), and these can be further utilized for chemical synthesis, polymer production, and pharmaceutical manufacturing. The last decade has witnessed significant progress in the coupling of the EOO and HER. However, the catalytic activities and cell configurations are still far from sufficient for practical applications.

In this Minireview, we do not intend to be exhaustive, but rather to highlight the latest accomplishments and landmarks in the strategy of integrating the EOO with the HER to realize effective chemical upgrading and H₂ generation. We place particular interest on the EOOs on the anode side, namely, alcohols, biomass‐derived compounds (e.g. aldehydes), amines, nitro compounds, sulfides, and alkenes. In each section, key aspects of the catalysts, catalytic activity, structure–performance relationships, reaction mechanism, and electrolyzer configuration will be discussed. Additionally, challenges and perspectives for the future development of EOO‐HER systems are provided. Our goal is to provide a state‐of‐the‐art update on the development of efficient EOO‐HER systems and to point out the potential of this attractive technology for the future upgrading of organic compounds and H₂ production.

2. Electrooxidation of Organic Compounds Coupled with Hydrogen Evolution

In the past decade, significant achievements have been made in investigating the HER on the cathode of water electrolyzers. By understanding the HER mechanisms and engineering the active sites, various electrocatalysts have been developed that show excellent HER activity comparable to the benchmark Pt/C material. To date, a number of excellent reviews on the HER have already been published, examining both the design of the electrocatalysts and their potential use in practical water electrolyzers.[ ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ ] Accordingly, in this Minireview, we mainly focus on the anode electrooxidation reactions, including the reaction type, representative catalysts, catalytic performance, reaction mechanisms, and the construction of the electrolyzer.

2.1. Electrooxidation of Alcohols

Alcohol oxidation is an essential process for the synthesis of value‐added aldehydes and organic acids. Coupling the electrooxidation of alcohols and the HER in a hybrid electrolyzer is a brilliant way to upgrade alcohols and simultaneously produce H₂.[ ¹⁸ , ¹⁹ ] In the last few years, the co‐production of formate and H₂ from methanol and water was successfully realized in alkaline media. The typical electrocatalysts for the generation of formate on the anode include transition‐metal‐based materials, such as Co _x P@NiCo‐LDH, ^[20] CNFs@NiSe, ^[21] Co(OH)₂@HOS/CP, ^[22] Ni(OH)₂/NF, ^[23] and NiIr‐MOF/NF. ^[24] Similarly, when coupled with the HER, acetate ^[25] /ethyl acetate ^[26] from ethanol, acetone from 2‐propanol, ^[27] benzaldehyde ^[28] /benzoic acid[ ²⁹ , ³⁰ ] from benzyl alcohol, glycolic acid from ethylene glycol, ^[31] acrylate from 1,3‐propanediol, ^[32] glycerate/oxalate from glycerol,[ ³³ , ³⁴ ] gluconic acid/gluconolactone from glucose ^[35] and cyclohexanone from cyclohexanol ^[36] have also been achieved with good efficiencies.

2.2. Electrooxidation of Aldehydes

Aldehydes are also important precursors for the synthesis of valuable organic acids. In the past few decades, great efforts have been paid to convert biomass‐derived (e.g. cellulose, hemicellulose, and lignin ^[37] ) compounds containing −CH=O groups, namely, furfural (FUR) and 5‐hydroxymethylfurfural (HMF). The oxidation upgrading of FUR and HMF is a sustainable path for the synthesis of valuable materials, solvents, and desired chemicals. ^[38] Compared to the thermocatalytic process using oxidants (e.g. O₂, H₂O₂) and even high temperatures/pressures, electrooxidative conversion provides a clean pathway for FUR and HMF upgrading, particularly when coupled with the HER. In 2017, Sun and co‐workers realized the electrocatalytic conversion of FUR and the HER using a cost‐efficient bifunctional Ni₂P/Ni/NF catalyst. ^[39] In 1.0 M KOH, the hybrid electrolyzer achieved nearly 100 % Faraday efficiencies (FEs) and showed robust stability for the formation of 2‐furoic acid and the production of H₂. Such a device needed a cell voltage of 1.48 V to afford a current density of 10 mA cm⁻², which is about 110 mV lower than that of pure OWS (1.59 V). Furthermore, maleic acid can also be produced from FUR on a PbO₂ anode, when coupled to the HER in acid solution (pH 1.0 H₂SO₄). ^[42]

2,5‐Furandicarboxylic acid (FDCA) is one of the valuable products derived from the electrooxidation of HMF and can be used to produce important fine chemicals and polymers. ^[43] The oxidation of both the alcohol and the aldehyde group in HMF is the key to the generation of FDCA. Figure 2a displays two possible routes (route I and route II) for the formation of FDCA. ^[44] Route I begins the oxidation of the alcohol group with diformylfuran (DFF) as the first intermediate. Route II generates 5‐hydroxymethyl‐2‐furancarboxylic acid (HMFCA) as the first intermediate by the oxidation of the aldehyde group. The formed DFF and HMFCA are then converted into 5‐formyl‐2‐furancarboxylic acid (FFCA) and further to FDCA. ^[45] In 1997, Grabowski et al. reported the pioneering electrooxidation of HMF to FDCA on a nickel oxide/hydroxide anode. ^[46]

Figure 2.

a) Two possible reaction routes of HMF oxidation. SFG spectra with ssp polarizations recorded at the Ni₃N@C electrode/electrolyte interface b) after running the cell at different potentials for 90 min and c) by monitoring the reaction at different times at 1.45 V_RHE. The “ssp” indicates the polarization direction of the light source. The bands at about 1335 cm⁻¹ and 1380 cm⁻¹ indicate the formation of the HMFCA intermediate. Reproduced with permission. ^[40] Copyright 2019, Wiley‐VCH. d) Operando ATR‐FTIR spectra recorded at different applied potentials between 0.98 and 1.78 V_RHE after 20 min on a Ni _x B anode. The bands between 1335 cm⁻¹ and 1400 cm⁻¹ indicate that HMFCA is the intermediate. Reproduced with permission. ^[41] Copyright 2018, Wiley‐VCH.

Since then, various electrocatalysts have been developed, such as, PdAu alloys, ^[47] Ni/Co/Fe‐OOH, ^[48] defective CoO‐CoSe₂, ^[49] CoP, ^[50] CuNi(OH)₂, ^[51] nanocrystalline Cu, ^[52] Ni NSs, ^[53] branched Ni NPs, ^[54] Ni _x B, ^[41] Ni₂P, ^[55] Ni₃S₂, ^[56] NiCo₃O₄ NWs, ^[57] mesostructured NiO, ^[58] NiFe‐LDH, ^[59] NiCoFe‐LDH, ^[60] Ni₃N@C, ^[40] MoO₂‐FeP@C, ^[61] quinary (FeCrCoNiCu)₃O₄ NSs, ^[62] and Ir‐Co₃O₄ (LDH=layered double hydroxide, NP=nanoparticle, NS=nanosheet, NW=nanowire). ^[63] In 1.0 M KOH, these materials revealed excellent electrocatalytic activity for the upgrading of HMF with high conversion (≥98 %), high selectivity (≥99 %), and a high FE (≥95 %) toward FDCA. ^[64] The active phase was generally recognized to be the in situ formed metal hydroxide/metal oxyhydroxide species. Using in situ sum‐frequency generation (SFG) vibrational spectroscopy (Figure 2b,c), Wang and co‐workers demonstrated that route II was the most likely way for the electrooxidation of HMF in a strong alkaline medium (e.g. pH higher than 13). ^[40] The same conclusion was also reached by utilizing operando electrochemistry coupled with attenuated total reflection infrared spectroscopy (EC‐ATR‐IR; Figure 2d)[ ⁴¹ , ⁶⁵ ] and operando surface‐enhanced Raman spectroscopy (SERS)[ ⁶⁶ , ⁶⁷ , ⁶⁸ ] techniques. Furthermore, density functional theory (DFT) calculations were also used to simulate the reaction pathway.[ ⁶⁹ , ⁷⁰ ] However, the exact reaction mechanism/pathway still remains elusive, calling for more in situ/operando techniques and a detailed theoretical investigation (e.g. transition state, solvation effect). When coupled with the HER, FDCA and H₂ could be simultaneously produced on the anode and cathode, respectively. ^[71] Sun and co‐workers integrated HMF conversion and H₂ evolution in 1.0 M KOH on bifunctional electrocatalysts, such as, CoP, ^[50] Ni₂P, ^[55] and Ni₃S₂, ^[56] thereby realizing the production of FDCA and H₂ with high FEs. Other bifunctional electrocatalysts, such as, CoNW/NF, ^[72] MoO₂‐FeP@C, ^[61] NiSe@NiO _x NWs, ^[73] Cu _x S@NiCo‐LDHs, ^[74] NF@Mo‐Ni_0.85Se, ^[75] NiCo‐LDH NiCo_NSs/Cu_NWs, ^[76] and Ni₃N@C ^[40] were also reported for the hybrid electrocatalysis. On the above representative electrocatalysts, the alcohol and aldehyde conversions typically follow the inner‐sphere electrooxidation mechanism in alkaline media, where a strong interaction exists between the organic substrate and the electrode surface. The reactive oxygen species, adsorption of the substrate/intermediates, and reconstruction of the catalysts occur through the electrooxidation process. ^[77] Apart from inner‐sphere electrooxidation, outer‐sphere electrooxidation was also reported for alcohol/aldehyde conversion when using 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO), ^[43] 4‐acetamido‐TEMPO (ACT), ^[78] polyoxometalates, ^[79] and metal ions ^[80] as redox mediators.

In 2021, Wang et al. successfully realized the low‐potential electrooxidation of FUR and HMF at about 0.1 V_RHE on a Cu catalyst. ^[81] In contrast to traditional aldehyde electrooxidation, where the H atom in the −CHO group is oxidized to H₂O at high potentials (>1.0 V_RHE, 3 OH⁻+R−CHO⇌2 H₂O+R−COO⁻+2 e⁻), aldehyde oxidation at low potential combines the hydrogen atoms to generate H₂ gas (2 OH⁻+R−CHO⇌1/2 H₂+R−COO⁻+H₂O+e⁻). When coupled with the HER, the assembled electrolyzer produced H₂ on both the cathode and the anode, with an apparent FE of about 200 %. The energy input was about 0.35 kWh for 1 m³ H₂ production, much lower than the 5 kWh for H₂ generation from conventional water electrolysis. In addition, 2‐furoic acid or HMFCA was produced on the anode. This approch offers a promising avenue for hydrogen production with less electricity input. However, the catalytic performance of the low‐potential electrooxidation of FUR and HMF is still insufficient, requiring further exploration of advanced catalysts.

2.3. Electrooxidation of Amines

The electrooxidation of amines can produce valuable imines, nitriles, amides, amine oxides, and azo compounds, which are widely applied in the syntheses of pharmaceuticals and agrochemicals. ^[82] In 2018, Zhang and co‐workers coupled the electrooxidation of primary amines (−CH₂−NH₂) and the HER in a 1.0 M KOH solution (Figure 3a). ^[83] Different aromatic (3 a–3 d) and aliphatic (3 e–3 i) primary amines were electrooxidized into the corresponding nitriles (3 a′–3 i′) with excellent yields (>93 %) and selectivity (>94 %) on a NiSe nanorod array anode. Taking the conversion of benzylamine (BA, 3 a) as an example, the electrooxidation of BA to benzyl nitrile (BN, 3 a′) occurred at about 1.34 V_RHE, significantly lower than that of the OER (ca. 1.55 V_RHE). When employing NiSe as the anode for the conversion of BA and using CoP as the cathode for the HER, the as‐assembled two‐electrode electrolyzer achieved a current density of 20 mA cm⁻² at only 1.59 V, much lower than the 1.70 V for the OWS. At 1.5 V, the FE for BN and H₂ was as high as 98 % and about 100 %, respectively. Recording the Raman spectra at different potentials uncovered that the in situ generated Ni^II/Ni^III sites on the NiSe acts as the redox‐active species to facilitate the conversion of the primary amine to form nitriles. Later on, the Zhai research group reported the electrooxidation of propylamine on vacancy‐rich Ni(OH)₂ atomic layers. ^[84] In a hybrid electrolyzer, propionitrile and H₂ were produced on the anode and cathode, respectively. Similar approaches were also reported by the groups of Huang, Yan, Mondal, and Xiong for amine upgrading and H₂ generation.[ ⁸⁵ , ⁸⁶ , ⁸⁷ , ⁸⁸ ]

Figure 3.

a) Coupling the electrooxidation of primary amines on a NiSe anode with the HER. b) LSV curves in 1.0 M KOH in the presence/absence of 1 mmol BA on the NiSe anode. c) LSV curves for the conversion of BA and the HER in a NiSe//CoP hybrid electrolyzer. The inset shows a comparison of the potentials for reaching current densities of 10 and 20 mA cm⁻². Reproduced with permission. ^[83] Copyright 2018, Wiley‐VCH.

2.4. Electrooxidation of Nitroalkanes

Conjugated nitroalkenes are important substrates in organic synthesis, because of the strong electron‐withdrawing and coordinating ability of the nitro group.[ ⁸⁹ , ⁹⁰ ] In particular, conjugated nitroalkenes with an E‐configuration (4′) act as the intermediates for the synthesis of important biologically active natural products and pharmaceuticals. Up to now, the main approaches for the synthesis of 4′ are nitration of the alkene by Henry condensation using organoselenides. However, such approaches often suffer from poor selectivity for 4′ and low yields for the reactions of nitroalkanes with ketones or aldehydes, as well as the harsh conditions with expensive reagents. ^[91] Recently, Zhang and co‐workers realized the synthesis of 4′ by the electrooxidation of α‐nitrotoluene (4) on NiSe nanorod arrays in an alkaline electrolyte. ^[92] A series of nitro compounds 4 with either electron‐withdrawing or electron‐donating functional groups on the aromatic ring can be converted into the related alkenes 4′ (4 a′–4 j′; Figure 4a) with high conversions (ca. 90 %) and selectivity (ca. 90 %) . Benefiting from the good HER activity of NiSe nanorod arrays, a hybrid electrolyzer using NiSe nanorod arrays as both the anode and cathode was assembled to simultaneously produce 4′ and H₂. At 1.8 V, the FE for H₂ and the selectivity for 4 a’ was as high as 100 % and 88 %, respectively (Figure 4b). In situ and ex situ experiments uncovered that the in situ formed NiOOH was the active component. At the same time, the adsorbed SeO _x ²⁻ could promote the formation of 4′ by inhibiting the formation of the Z‐isomer and other by‐products (Figure 4c).

Figure 4.

a) Coupling the selenite‐promoted electrooxidation of α‐nitrotoluenes to E‐nitroethenes with the HER over a bifunctional NiSe nanorod arrays electrode. b) LSV curves for the conversion of α‐nitrotoluene and the HER in a NiSe//NiSe hybride electrolyzer. c) Illustration of the reaction mechanism. Reproduced with permission. ^[92] Copyright 2021, Wiley‐VCH.

2.5. Semi‐Dehydrogenation of Tetrahydroisoquinolines

Dihydroisoquinolines (DHIQs) exhibit various bioactivities towards tumor/fungal treatment, vasodilation, and nonoamine oxidase inhibition, thus suggesting potential utilization in disease treatment. ^[93] The catalytic dehydrogenation (oxidation) of 1,2,3,4‐tetrahydroisoquinolines (THIOs) is an effective approach for producing DHIQs, but suffers from complete dehydrogenation during the formation of isoquinolines (IQs). ^[94] Furthermore, such dehydrogenations typically need noble‐metal‐based catalytic materials (e.g. Pd, Ir, or Ru) and often requires hazardous and toxic strong oxidants (e.g. KMnO₄, NaIO₄, (KSO₃)₂NO, and PhSSPh), thus limiting the safe and large‐scale production of DHIQs. ^[95] In 2019, Zhang and co‐workers reported the electrocatalytic semi‐dehydrogenation (S‐DHD) of THIQs (5) to synthesize DHIQs (5′) on a Ni₂P anode in alkaline solutions (Figure 5a). ^[96] In 1.0 M KOH, the THIQ electrooxidation displayed faster kinetics than the OER. In a hybrid electrolyzer using Ni₂P as both the anode and cathode, the DHIQs and H₂ could be produced simultaneously with high activity and stability (Figure 5b,c). On the anode, the in situ formed Ni^II/Ni^III redox‐active species are essential for the S‐DHD of THIQs into DHIQs. The Ni₂P was the active component for the HER on the cathode. Similar results were also reported when using a Co₃O₄@NF anode and Pt cathode for THIQ upgrading and H₂ production. ^[97]

Figure 5.

a) Coupling the electrooxidation of THIQs and the HER. The selectivity of the electrooxidation was not given. b) LSV curves over a Ni₂P//Ni₂P electrolyzer in 1.0 M KOH without and with 0.5 mmol 5 a. c) Cyclic FEs of a Ni₂P//Ni₂P electrolyzer for the production of 5 a′ and H₂. Reproduced with permission. ^[96] Copyright 2019, Wiley‐VCH.

2.6. Electrooxidation of Sulfides

Organic sulfoxides play prominent roles in medicinal chemistry, biological processes, and material science. ^[100] To date, the oxidation of sulfides is the main pathway for the synthesis of sulfoxides, ^[101] requiring strong oxidizing agents (e.g. iodine, H₂O₂, 3‐chloroperbenzoic acid, or peroxy acids, oxone) and homogeneous catalysts (e.g. Fe‐ and Mn‐containing materials) and bringing additional issues of toxicity, operationality, and the need to avoid the use of heavy metals from the standpoint of large‐scale preparation. Recent studies demonstrated the successful electrooxidation of sulfides on Fe, ^[102] Ni^II complexes, ^[103] graphite rods, ^[104] and CoFe‐LDH ^[98] using water as the O source and MeCN as the solvent. On a CoFe‐LDH/CC anode, different kinds of aryl, heteroaryl, and alkyl sulfides (6 a–6 l, left column of Figure 6a) could be selectively electrooxidized into the relevant sulfoxides (6 a′–6 l′) with high yields (85–96 %) under ambient conditions. Importantly, such an approach was successfully expanded to the synthesis of complex pharmaceuticals with sulfoxide units, such as ricobendazole (6 m′, 78 %), omeprazole (6 n′, 70 %), sulindac (6 o′, 63 %), and the amino acid methionine (6 p′, 89 %), from the corresponding sulfides in excellent yields. Impressively, this method can be utilized in the large‐scale synthesis of 6 p′ (1.79 g) in high yield (86 %), thus highlighting its potential use in pharmaceutically relevant sulfoxides. Synchronously, the H₂ was generated on the Pt cathode. Elementary studies imply that the in situ generated amorphous CoFe‐oxyhydroxide serves as the active component for the selective electrooxidation of 6 to 6′ through a radical approach (Figure 6b). In 2021, Zhang and co‐workers utilized Ni₂P hollow nanocubes as both the anode and cathode for the electrooxidation of sulfides to sulfoxides (6 a′, 6 b′, 6 i′, and 6 r′–6 v′, right column of Figure 6a) and H₂ production in a 1.0 M KOH solution. ^[99] They also unveiled that the in situ formed Ni^II/Ni^III sites were the active species and the oxidation followed a radical‐mediated mechanism (Figure 6c).

Figure 6.

a) Coupling the electrooxidation of sulfides and the HER. b) Proposed reaction mechanism for the formation of sulfoxides and the HER in an aqueous organic electrolyte. Reproduced with permission. ^[98] Copyright 2021, Royal Society of Chemistry. c) Reaction mechanism for the selective electrooxidation of sulfides to sulfoxides and the HER on Ni₂P electrodes. Reproduced with permission. ^[99] Copyright 2021, Cell Press.

2.7. Electrooxidation of Alkenes

Alkene oxidation is an effective approach for the synthesis of vicinal diols and epoxides, which are important intermediates in the fabrication of fine chemicals, food additives, perfumes, drug intermediates, and agrochemicals. ^[105] In 2020, the Sargent group coupled the electrooxidation of ethylene with H₂ production to realize the production of ethylene glycol on a gold‐doped palladium nanostructured dendritic catalyst (PdAu‐DNT) in aqueous 0.1 M NaClO₄ media (Figure 7a). ^[106] DFT calculations and experimental results demonstrated that Au doping could effectively regulate the OH bonding energy on Pd, thus promoting the formation of ethylene glycol. As a result, the best‐performing PdAu‐DNT with an Au doping of 3.2 at. % (3.2‐PdAu‐DNT) exhibited a FE of about 80 % for the formation of ethylene glycol at a current density of about 5.7 mA cm⁻², which was much higher than that of Pd‐DNT (ca. 60 %; Figure 7b). Furthermore, the 3.2‐PdAu‐DNT revealed excellent stability for 100 hours at 1.1 V_Ag/AgCl. Simultaneously, valuable H₂ was produced on the Pt foil cathode. In 2019, Manthiram et al. demonstrated the successful integration of the electrocatalytic epoxidation of cyclooctene with the HER by using Mn₃O₄ as the anode material and Pt as the cathode material (Figure 7c). ^[107] Isotopic investigation and product analysis indicated the overall reaction consisted of the conversion of cyclooctene and water into cyclooctene oxide and hydrogen, respectively. The FE for the HER on the Pt cathode was as high as 94 %. In contrast, the FE for the epoxidation of cyclooctene on an Mn₃O₄ anode was only about 30 %, probably because of the uncontrolled over‐oxidation and OER at higher potentials. Later in 2020, the Sargent group achieved the selective partial oxidation of ethylene for the formation of ethylene oxide in a 1.0 M KCl aqueous solution. ^[108] The key idea was to utilize Cl⁻ as a redox mediator at the anode (Figure 7d). First, the Cl⁻ was oxidized to Cl₂ at the Pt anode. Second, the Cl₂ disproportionated to form HOCl and HCl. Third, the HOCl reacted with ethylene to form ethylene chlorohydrin (HOCH₂CH₂Cl). After electrolysis, the pH value of the anolyte was acidic. On the cathode, the H₂ was generated and the catholyte became alkaline. After mixing the catholyte and anolyte output streams, ethylene oxide was generated from the HOCH₂CH₂Cl and OH⁻. Furthermore, the HCl formed was also neutralized by OH⁻. At 300–800 mA cm⁻², the FEs for the production of ethylene oxide were all >70 %. This approach offers a smart way for the synthesis of epoxides by the oxidation of alkenes, while preventing overoxidation and the OER.

Figure 7.

Coupling the electrooxidation of alkenes and the HER. a) Integrating the reaction of ethylene to ethylene glycol with the HER. b) FEs for the formation of ethylene glycol with PdAu DNT and Pd DNT at different potentials. Reproduced with permission. ^[106] Copyright 2020, Nature Publishing Group. c) Pairing the epoxidation of cyclooctene and the HER. Reproduced with permission. ^[107] Copyright 2019, American Chemical Society. d) Paired system for the electrochemical reaction of ethylene to ethylene oxide and the HER. Reproduced with permission. ^[108] Copyright 2020, American Association for the Advancement of Science.

3. Conclusion and Perspectives

This Minireview summarizes the recent development of pairing the electrooxidation of organic compounds and hydrogen evolution in a hybrid water electrolyzer. The typical electrooxidation reactions, including of alcohols, aldehydes, biomass, amines, nitro compounds, sulfides, and alkenes were examined. This fascinating synthetic strategy opens a sustainable approach for upgrading organic compounds on the anode. At the same time, renewable H₂ gas is produced more effectively on the cathode. Table 1 summarizes representative examples of coupling the EOO and HER on various electrocatalysts. Clearly, this coupling process greatly lowers the cell voltage for water splitting and greatly improves the H₂ production. Despite the superior advantages of such a coupling approach and the enormous recent progress that has been made, the coupling technology is just at an early stage and the transformation efficiency is still unsatisfactory. Thus, the reasonable design of high‐performance electrodes, insightful identification of active sites, and intelligent construction of the electrolyzer is highly necessary for future practical impartments in industry. Regarding the future direction relating to the integration of the EOO and HER, we provide our perspectives as follows:

Table 1.

Electrocatalytic performance of recently reported representative EOO electrocatalysts and EOO‐HER systems.

Reaction type	Anode catalyst	Organic substrate	Concentration in electrolyte	Product	3‐electrode system		2‐electrode cell		Ref.
					E OER at j 10 [a] [VRHE]	E EOO at j 10 [VRHE]	VOER‐HER at j 10 [V]	VEOO‐HER at j 10 [V]
alcohol oxidation	Co x P@NiCo‐LDH	methanol	0.5 M in 1.0 M KOH	formate	–	1.24	ca. 1.70	1.43	[20]
	Co(OH)2@HOS	methanol	3.0 M in 1.0 M KOH	formate	1.571	1.385	1.631	1.497	[22]
	Co3O4 NSs	ethanol	1.0 M in 1.0 M KOH	ethyl acetate	1.50	1.445	–	–	[26]
	Rh nanosheets	2‐propanol	1.0 M in 1.0 M KOH	acetone	1.61	0.233	ca. 1.8	0.4	[27]
	NC@CuCo2N x	benzyl alcohol	15 mM in 1.0 M KOH	benzaldehyde	1.46	1.25	1.62	1.56	[28]
	Mo−Ni alloy NP	benzyl alcohol	10 mM in 1.0 M KOH	benzoic acid	1.49	1.345	1.49	1.38	[29]
	PdAg	ethylene glycol	1 M in 0.5 M KOH	glycolic acid	1.55	0.57		1.02 at j 20	[31]
	MoO x /Pt	glycerol	0.1 M in 1.0 M KOH	glycerate	–	–	1.6	0.7	[33]
	Co−Ni alloy	glucose	0.1 M in 1.0 M KOH	gluconic acid/gluconolactone	–	–	1.65	1.39	[35]
	Co2(OH)3Cl/FeOOH	cyclohexanol	2 mmol in 1.0 M KOH	cyclohexanone	–	–	1.56	1.46	[36]
	Ni3S2	furfuryl alcohol	10 mM in 1.0 M KOH	2‐furoic acid	ca. 1.50 (onset)	ca. 1.35 (onset)	–	–	[56]
aldehyde oxidation	Ni2P/Ni	FUR	30 mM in 1.0 M KOH	2‐furoic acid	1.55 (onset)	1.34 (onset)	1.59	1.48	[39]
	PbO2	FUR	10 mM in pH 1.0 H2SO4	maleic acid	1.85 (onset)	1.60 (onset)	–	–	[42]
	Ni2P NPs	HMF	10 mM in 1.0 M KOH	FDCA	ca. 1.50 (onset)	ca. 1.35 (onset)	1.65	1.44	[55]
	CoNW	HMF	10 mM in 1.0 M KOH	FDCA	1.563	1.311	1.866	1.504 (0.1 M HMF)	[72]
	MoO2−FeP@C	HMF	10 mM in 1.0 M KOH	FDCA	1.474	1.359	1.592	1.486	[61]
	Ni3N@C	HMF	10 mM in 1.0 M KOH	FDCA	1.60 at j 50	1.38 at j 50	1.79 at j 50	1.55 at j 50	[40]
	(FeCrCoNiCu)3O4 NSs	HMF	50 mM in 1.0 M KOH	FDCA	1.50 (onset)	1.35 (onset)	–	–	[62]
amine oxidation	NiSe	benzylamine (3 a)	1 mmol in 1.0 M KOH	benzyl nitrile (3 a′)	1.48 (onset)	1.34 (onset)	1.70 at j 20	1.49 at j 20	[83]
	Ni−Ni3N	benzylamine (3 a)	2 mmol in 1.0 M KOH	benzyl nitrile (3 a′)	1.60	1.35	ca. 1.55	1.4	[85]
	VP−Ni(OH)2 NSs	propylamine	10 mmol 1.0 M KOH	propionitrile	ca. 1.53	1.36	1.74	1.48	[84]
nitroalkane oxidation	NiSe nanorods	α‐nitrotoluene (4 a)	0.4 mmol in 1.0 M KOH	E‐nitroethene (4 a′)	–	–	1.69	1.36	[92]
S‐DHD of THIQ	Ni2P	1,2,3,4‐ tetrahydroiso‐ quinoline (5 a)	0.5 mmol in 1.0 M KOH	3,4‐dihydroiso‐ quinoline (5 a′)	ca. 1.48 (onset)	ca. 1.10 (onset)	ca. 1.62	ca. 1.35	[96]
	Co3O4 nanoribbon	5 a	20 mM in 1.0 M KOH	5 a′	ca. 1.48 (onset)	ca. 1.30 (onset)	1.620	1.446	[97]
sulfide oxidation	CoFe‐LDH	diphenyl sulfide (6 a)	0.25 M in MeCN/H2O	diphenyl sulfoxide (6 a′)	1.90 VAg/AgCl at j 5	1.39 VAg/AgCl at j 5	‐	‐	[98]

[a] j ₁₀: current density at 10 mA cm^‐2.

1. Electrode materials are always prerequisite for promoting the electrooxidative transformation of organic compounds. Exploring effective electrode materials by active‐site design and morphology engineering is essential for boosting the reaction kinetics and facilitating the reaction. Furthermore, a deep understanding of the reaction mechanism is indispensable for exploring more advanced catalysts. To achieve this goal, cutting‐edge in situ methods are required to supplement existing techniques, such as in situ X‐ray absorption fine structure (XAFS), in situ X‐ray powder diffraction (XRD), in situ Raman spectroscopy, and in situ Fourier‐transform infrared spectroscopy (FTIR) to identify the true active sites at the atomic level and to monitor the key intermediates. The in‐depth identification of the active sites and understanding of the structure–property relationships can, thus, instruct the further smart design of effective materials. Furthermore, machine learning by DFT calculations can be another powerful technique for the discovery of electrocatalysts. The new mechanistic insights gained will then serve as feedback for knowledge‐guided reactions and catalyst optimization. Moreover, the development of bifunctional materials based on earth‐abundant elements for hybrid water electrolyzers is highly desirable for cost reduction and practical applications.

2. To date, most electrooxidation reactions have worked only in alkaline media (e,g. 1.0 M KOH; Table 1). However, the HER process is kinetically more sluggish in alkaline media than in acid media. Integration of the electrooxidation reaction with the HER in an acidic electrolyzer will be more energy‐efficient. Hence, it is highly desirable to explore electrocatalysts for electrooxidation reactions with good activity, selectivity, and stability in acidic environments.

3. It is necessary to develop new electrooxidation reactions, such as, biomass upgrading, removal of industrial organic pollutants, and waste plastic reforming. In particular, the effective electrooxidation of organic pollutants offers a clean pathway for removing hazards and even for upgrading. The pairing of the pollutant electrooxidation and the HER will clearly provide a sustainable approach for the production of chemicals and the purification of water from industrial wastes.

4. Compared with the splitting of pure water, the splitting of seawater for H₂ production is more cost‐effective without desalination/purification units. ^[109] However, the splitting of seawater is seriously limited by the chloride evolution reaction (ClER) occurring at the anode, which competes with the OER and generates unfavorable chlorine and/or hypochlorite that will corrode the catalysts, membranes, and other components over time. The EOO occurs at a lower potential than the OER and ClER. ^[110] Thus, coupling the EOO with HER could enable seawater electrolysis to proceed at a high current density without the interfering ClER, thereby making it more promising for the large‐scale green production of hydrogen.

5. In terms of scale‐up, feasibility investigations, including laboratory prototypes and pilot plants for industrial applicability, are essential for commercialization. This needs close cooperation between the chemists and engineers to optimize the catalytic performance by placing a particular emphasis on the catalyst development and electrolyzer design. The flow cell could be a good option, as it can supply enough substrate to the catalyst layer and promote mass transfer. Furthermore, the long‐term operational stability of the catalyst, reaction activity and selectivity, and downstream separation, as well as purification issues need further consideration and improvement. The economics (e.g. energy cost) of any coupled process also requires careful consideration.

In summary, coupling the electrooxidation of organic compounds and hydrogen evolution can provide a sustainable and economic pathway for chemical upgrading and hydrogen production. It is expected that, through further electrode development and electrolyzer design, this appealing technology will be used in practical chemical manufacturing.

Conflict of interest

The authors declare no conflict of interest.

Biographical Information

Guangbo Chen received his PhD in chemistry from the Technische Universität Dresden (TU Dresden) in 2021 under the supervision of Prof. Xinliang Feng. Thereafter, he continued academic research as a postdoctoral fellow at TU Dresden. His research mainly focuses on the design and synthesis of carbonaceous and two‐dimensional (2D) nanomaterials, the design of electrochemical devices for energy storage and conversion, and electrochemical upgrading of organic compounds.

Biographical Information

Xinliang Feng is a full professor and the head of the Chair for Molecular Functional Materials at TU Dresden. He is also a director of the Max Planck Institute of Microstructure Physics, Germany. His scientific interests include organic synthetic methodology, organic synthesis, and supramolecular chemistry of π‐conjugated systems, bottom‐up synthesis of graphene and graphene nanoribbons, electrochemical exfoliation of 2D crystals, 2D polymers, and supramolecular polymers as well as 2D carbon‐rich conjugated polymers for optoelectronics and spintronics, energy storage and conversion, and new energy devices and technologies.

G. Chen, X. Li, X. Feng, Angew. Chem. Int. Ed. 2022, 61, e202209014; Angew. Chem. 2022, 134, e202209014.