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Review

Killing Two Birds with One Stone: Upgrading Organic Compounds via Electrooxidation in Electricity-Input Mode and Electricity-Output Mode

by
Jiamin Ma
1,†,
Keyu Chen
1,†,
Jigang Wang
2,
Lin Huang
1,*,
Chenyang Dang
1,
Li Gu
3 and
Xuebo Cao
1,*
1
College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing 314001, China
2
School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo 255049, China
3
School of Materials and Textile Engineering, Jiaxing University, Jiaxing 314001, China
*
Authors to whom correspondence should be addressed.
†
These authors equally contributed to this work.
Materials 2023, 16(6), 2500; https://doi.org/10.3390/ma16062500
Submission received: 15 February 2023 / Revised: 12 March 2023 / Accepted: 13 March 2023 / Published: 21 March 2023
(This article belongs to the Special Issue Advanced Electrocatalytic Materials for Energy and Environmental Applications)
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Abstract

:
The electrochemically oxidative upgrading reaction (OUR) of organic compounds has gained enormous interest over the past few years, owing to the advantages of fast reaction kinetics, high conversion efficiency and selectivity, etc., and it exhibits great potential in becoming a key element in coupling with electricity, synthesis, energy storage and transformation. On the one hand, the kinetically more favored OUR for value-added chemical generation can potentially substitute an oxygen evolution reaction (OER) and integrate with an efficient hydrogen evolution reaction (HER) or CO2 electroreduction reaction (CO2RR) in an electricity-input mode. On the other hand, an OUR-based cell or battery (e.g., fuel cell or Zinc–air battery) enables the cogeneration of value-added chemicals and electricity in the electricity-output mode. For both situations, multiple benefits are to be obtained. Although the OUR of organic compounds is an old and rich discipline currently enjoying a revival, unfortunately, this fascinating strategy and its integration with the HER or CO2RR, and/or with electricity generation, are still in the laboratory stage. In this minireview, we summarize and highlight the latest progress and milestones of the OUR for the high-value-added chemical production and cogeneration of hydrogen, CO2 conversion in an electrolyzer and/or electricity in a primary cell. We also emphasize catalyst design, mechanism identification and system configuration. Moreover, perspectives on OUR coupling with the HER or CO2RR in an electrolyzer in the electricity-input mode, and/or the cogeneration of electricity in a primary cell in the electricity-output mode, are offered for the future development of this fascinating technology.

1. Introduction

The overuse of fossil fuels makes the economy susceptible to supply bottlenecks and price spikes and aggravates the greenhouse effect and environmental pollution worldwide. Recently, the rapid development of renewable-based electricity demonstrates great potential in mitigating the energy crisis. Unfortunately, renewable-based electricity such as wind power and solar power is only produced when the wind is blowing or the sun is shining; thus, it cannot effectively connect to the power grid. The introduction of green electricity, based on renewables, troubles direct utilization via the power grid. Therefore, it is imperative to develop clean and renewable alternatives for energy storage and chemical conversion. With the remarkable renaissance of electrochemical technology, and the constant inventions in the rapid development of renewable-based electricity technology, the technology of energy storage and conversion into chemicals is attracting great interest [1,2,3,4,5,6]. For example, hydrogen, a clean, flexible, cost-effective energy carrier for net-zero carbon (carbon-free) strategies, features a higher enthalpy of combustion (H2(g) + ½O2(g) → H2O(l) ΔH = −285.8 kJ/mol, high specific-energy-density) and enables the possibility of attaining secure and clean energy in the future. From the perspective of hydrogen production, conventional hydrogen production technology based on fossil fuels is energy-intensive and environmentally unfriendly. Meanwhile, water electrolysis for hydrogen generation has emerged and is promising because of mild reaction conditions and zero-carbon emissions (coupled with green electricity). In particular, with society progressing into renewable-based economics, water electrolysis enables the storing of electricity from intermittent and renewable wind power, solar power and nuclear power into chemicals. This wise strategy opens up great opportunities for the development of enormous energy storage capacities for excess electricity originating from renewable energy sources with the rapid increase in global energy consumption. Over the course of water electrolysis, hydrogen is generated (HER) in a cathodic chamber and oxygen is evolved (OER) in an anodic chamber. Unfortunately, the OER suffers from a relatively high theoretical oxidation potential and sluggish kinetics that severely affect the nominal efficiency of water electrolysis. Moreover, the oxygen diffusing into the cathode chamber can be reduced back to water, which reduces the overall efficiency of the process. In alkaline water electrolysis, the extensive hydrogen crossover permeation into the opposite chamber generates safety concerns (low explosive limit of 4% mol H2) [7]. In a proton exchange membrane (PEM) cell, the coexistence of the H2, O2 and Pt catalysts was observed to generate hazardous reactive oxygen species (ROS), such as hydroxyl (HO•) and hydroperoxyl radicals (HOO•), which can result in an oxidative membrane backbone degradation and side chain disintegration, thereby shortening the lifetime of PEM [8].
Recently, the kinetically favored OUR of organic compounds has been demonstrated to effectively substitute the OER to integrate with the HER in a hybrid electrolyzer (OUR||HER), which can not only generate value-added chemicals anodically for the potential manufacturing of pharmaceuticals, fine chemicals, food, pesticides, materials, etc., but also produce hydrogen cathodically [2,9,10,11,12,13,14,15,16]. In comparison to conventional water electrolysis, the OUR||HER mode requires reduced cell voltages, is more energy-saving and cost-effective and has a theoretically high utilization of electricity. Furthermore, the OUR||HER mode is more advanced than the sacrificial agent (e.g., urea [17,18], hydrazine [19,20], ammonia [21], et al.)-assisted hydrogen production that requires the constant consumption of the sacrificial agent, thereby increasing the capitalized cost, especially for large-scale applications. Notably, from the perspectives of wastes removal, there is no need to take the price of the sacrificial agent (wastes) into account over the process of environmental remediation. Instead, the additional benefit of harmful pollutants’ decomposing can be achieved as well as the hydrogen production [22].
Besides OUR||HER, OURs of organic compounds have been reported to couple with the carbon dioxide electroreduction reaction (CO2RR). The CO2RR can not only potentially curb CO2 emissions, but also generate value-added chemicals, which, however, are still limited to the extremely stable chemical bond in CO2 (C=O, 806 kJ mol−1), complicated reaction pathway, resulting poor activity and selectivity and sluggish anodic OER process. Therefore, the OUR of organic compounds is an appealing strategy to substitute the OER for effective CO2RRs (OUR||CO2RR) (Scheme 1, left). It seems that the coupling strategy of OUR||HER, OUR||CO2RR and/or other analogous pairing electrolysis is a fascinating technology that can kill two birds with one stone. Up to now, alcohols, aldehydes, amines, nitroalkanes, organic sulfides, alkenes, etc., have been reported for efficient OUR||HER and/or OUR||CO2RR [23,24,25,26,27,28,29,30,31], and could even probably couple with other important reduction reactions such as nitrogen fixation (NRR) and nitrate electro-reduction. This strategy is such an ideal electrochemical process that target products with high added value are generated at both anodes and cathodes, thus possessing great potential to enhance the utilization rate of electricity and lead to economic benefits. This kind of paired electrolysis exhibits positive aspects such as much more reduced cell voltage (reducing the energy consumption), high theoretical energy efficiency and atom economy, no production of ROS or explosive hazards, etc.
Meanwhile, the OUR can be realized via the electrooxidation reactions in a primary cell or battery from the electricity-input mode to the electricity-output, such as fuel cells and zinc–air batteries fed by biomass-derived carbonaceous fuel, as displayed in Scheme 1 (right) [11,32,33]. The oxidation reaction is also the basic unit of fuel cells that possess the merits of high efficiency, environmental friendliness and no external charging. Furthermore, traditional carbonaceous-based fuel cell technology generates low-valued products of CO2 and or CO32− that are not conducive to the realization of the goal of reducing peak emissions of carbon dioxide and carbon neutrality. In addition to alleviating the issue of energy shortages, the OUR of organic compounds in fuel cells facilitates the green and profitable cycle of waste biomass-derived resources and cuts the carbon footprint [32]. For example, glycerol manufactured from animal fats and vegetable oils can be utilized as fuel in a fuel cell with value-added product generation, which is more environmentally friendly than conventional fuel cells [32]. The OUR of organic compounds in fuel cells seems to be another fascinating strategy for electrosynthesis for both chambers (with oxygen being reduced to hydroxy peroxide) and electricity. Up to now, various alcohols (e.g., ethanol, ethylene glycol, isopropanol, glycerol and Furfural) [32,34,35,36,37] have been demonstrated to have great potential for effectively upgrading fuel cells. Besides fuel cells, upgrading organic compounds can also be realized in other devices, such as zinc–air batteries [33]. From the aforementioned description, organic compounds enabling value-added chemical generation can be realized via an electrolyzer, namely the electricity-input mode, as well as a primary cell (such as a fuel cell), namely the electricity-output mode.
This minireview intends to highlight the recent developments and landmarks of the OUR of organic compounds for value-added product generation to integrate with the HER or CO2RR in a hybrid electrolyzer and in batteries or cells for electricity generation, in terms of catalysts, performance and the structure-performance relationship. Additionally, challenges and perspectives for the future development of oxidative upgrading of organic compounds in parallel with hydrogen production or the CO2RR in a hybrid electrolyzer in electricity-input mode, and/or in a fuel cell in electricity-output mode are discussed. We hope the perspectives we have outlined in this minireview will proceed in order to develop efficient electrolyzers via the electricity-input mode and/or fuel cells (or other analogous devices) of the electricity-output mode as a cost-effective and robust solution to help in realizing the targets of effective electrosynthesis and carbon neutrality.

2. Anodic OUR of Organic Compounds in Parallel with HER (OUR||HER)

2.1. Anodic OUR of Alcohol Integrating with Cathodic HER

The OUR of methanol integrates with the HER. Compared to the traditional OER, the anodic OUR of organic molecules reduces the working voltage and possesses faster reaction kinetics [3,26]. As the simplest alcohol, methanol enables easy production via chemical or biomass industrial synthesis. Industrial methanol is inexpensive (about EUR 350 per tonne); meanwhile, methanol exhibits remarkable solubility in water. Furthermore, the methanol electrooxidation reaction (MOR) produces value-added formate salts or formic acid (near EUR 539 per tonne) and possesses fast kinetics. Thus, the OUR of methanol to replace the OER is a promising alternative for hydrogen production and chemical conversion (Table 1) [38,39,40,41,42,43]. Noble metal catalysts such as Pt and Pd have been demonstrated to be effective for the active electrooxidation of alcohols; however, these noble metal catalysts are limited by their low quantities, high price and easy CO-poisoning in alkaline medium that impedes reaction kinetics [32,44,45]. Precious metal electrocatalysts such as Ni, Co-based nanostructures have been demonstrated to be potential candidates for efficient alcohol electrocatalysts owing to their weak adsorption of CO and low price [33,43]. Liu and co-workers [43] constructed a three-dimensional Mo-doped Ni(OH)2 with increased density of active sites and quite low Ni–Ni coordination (Figure 1a), which can jointly boost the MOR activity yielding formate (Figure 1b). Meanwhile, the hybrid electrolyzer of methanol upgraded for formate generation by Mo-Ni(OH)2 anodically, and its integration with hydrogen production by Ni4Mo–MoO2 cathodically, display a relatively lower cell voltage of 1.52 V at 130 mA cm−2 and good stability (Figure 1c,d). Furthermore, an industrial level electrolysis was boosted with a decent current density of more than 500 mA cm−2 under industrial conditions (cell voltage of 2.00 V, 6 M KOH), and high selectivity of above 90%, signifying great potential for industrial application (Figure 1e–g). DFT results suggested that the excellent performance is mainly attributed to the ultralow Ni–Ni coordination effect for active Ni sites in Mo-Ni(OH)2. Notably, although methanol exhibits significant merits for upgrading with hydrogen production, designs of advanced catalysts and systems are still desirable, and methanol is toxic. Thus, attention should be paid to operation.
The OUR of ethanol integrates with the HER. Typically, aliphatic alcohol with low length of carbon chain exhibits relatively significant water solubility and relatively lower theoretical potential of oxidation, and thus shows great potential for oxidative upgrade coupling with hydrogen production. Ethanol derived from biomass fermentation has been demonstrated to be a promising alternative to renewable energy carriers and green chemicals. It serves as liquid fuel and a versatile feedstock for synthesizing fine chemicals such as acetic acid, ethyl acetate, ethylene, acetaldehyde and 1,1-diethoxyethane (DEE). For example, DEE serves as an important feedstock for the synthesis of pharmaceuticals, perfumes, polyacetal resins, etc. [27]. Unfortunately, the variety of product distribution poses great obstacles for the selective ethanol electrooxidation reaction (EOR) from the viewpoint of electrosynthesis. To achieve this goal, catalysts of fine design and synthesis, and a thorough understanding of mechanisms, are crucial. Although noble metal catalysts are limited to their high price and scarcity, they cannot be replaced for various reactions. Typically, pure Pt suffers from a weak CO anti-poisoning ability over alcohol electrooxidation. To solve this problem, an alloying strategy is often utilized to modulate the electronic structure of active sites, thus enhancing the CO anti-poisoning ability, and thereby boosting the alcohol electrooxidation activity. Alberto Rodríguez-Gómez et al. [46] developed a Pt-based bimetallic catalyst system (PtM) to boost hydrogen and value-added compound generation (acetic acid, acetaldehyde and ethyl acetate). They found that this secondary metal can influence the electrocatalytic performance as well as product distribution. For example, PtCo/C and PtNi/C possessed the highest electrochemical activity at high polarization levels. In comparison, a lower potential interval (<0.85 V) was needed for PtRu/C to promote the acetic acid production, despite sacrificing ethanol conversion. One-dimensional nanostructures such as nanowires and/or nanotubes seem to have great potential for efficient alcohol electrooxidation [32,44]. Guo and co-workers [26] reported a first example showcasing the highly efficient electro-generation of DEE in the anodic chamber (Faraday efficiency of DEE, 85%) coupling with high-purity hydrogen (highest-ever-reported Faraday efficiency of DEE, 94%) at the cathode using PtIr nanowires (denoted as PtIr NWs, diameter: about 1 nm) as the bifunctional catalysts, as shown in Figure 2a. They demonstrated that the hybrid electrolyzer of PtIr NWs||PtIr NWs achieved a lower voltage of 0.61 V at 10 mA∙cm−2 than those of the Pt NWs||Pt NWs electrolyzer (0.85 V) and the state-of-the-art commercial Pt/C||Pt/C electrolyzer (0.86 V) (Figure 2b–d). In situ infrared spectroscopy results implied that PtIr NWs is conducive to the effective activation of C−H and O−H bonds of ethanol molecules that are crucial for the formation of acetaldehyde intermediates and the final DEE (Figure 2e,f). Additionally, the hybrid electrolyzer with PtIr NWs as bifunctional catalysts delivered excellent stability without an obvious decrease of the Faraday efficiency over DEE generation. Although this kind of PtIr NWs catalyst exhibited the highest-ever-reported efficiency in pair electrolysis for ethanol electrooxidation and hydrogen production, the fact that catalysts were configurated with PGM-based catalysts cannot be neglected, which therefore presents the opportunity to develop robust and efficient noble-metal free catalysts in the future. Besides the PGM-based catalysts, earth-abundant metal catalysts have also been reported for an efficient EOR. Recently, Wang and co-workers [33] developed a heterostructured Co(OH)2@Ni(OH)2 catalyst for efficient and selective ethanol electrooxidation to acetate (1.30 V vs. RHE at 10 mA cm−2, FE: 97.9%). A relatively lower potential of the symmetric Co(OH)2@Ni(OH)2||Pt/C electrolyzer was needed (1.39 V vs. RHE at 10 mA cm−2). Although the OUR of ethanol possesses the advantages of being non-toxic, promoting flexible product distributions and showing great potential to integrate with hydrogen production, the volatility of ethanol probably hinders its practical application to some extent, and the flexible product distribution poses high requirements for catalysts and purification.
The OUR of glycerol integrates with the HER. Glycerol, a biproduct of biodiesel manufactured from vegetable oils and animal fats, has increased with an average annual growth rate of about 4.1% worldwide over the last decade [32]. Therefore, efficient utilization of glycerol is urgent and profitable. Up to now, the main strategy of utilizing glycerol has consisted of catalytic oxidation to yield value-added chemicals, such as 1,3-dihydroxyacetone, glyceraldehyde, glyceric acid and formate, which are widely applied in the pharmaceutical, food, and cosmetics industries. Glycerol electrochemical oxidation (GOR) is used in smart devices such as electrolyzers and fuel cells, which are emerging as promising platforms for the clean utilization of this alcohol [32,47,48,49,50,51]. Recently, Huang and co-workers [32] developed bifunctional PdPtAg nanowires via facile galvanic displacement reaction. The as-prepared catalysts possessed low peak-potential of the GOR oxidation at near 0.9 V vs. RHE and a relatively high formate selectivity of 81.2%. These significant merits endow the PdPtAg nanowire-based catalysts with great potential for the application of GOR-promoted hydrogen production. Recently, Chen and co-workers [50] found that GOR can be boosted via modulating proton and oxygen anion deintercalation in NiCo hydroxide, as manifested by the DFT and experimental characterizations (Figure 3a–c), which displayed a relatively low potential of 1.35 V at 100 mA cm−2, and a formate selectivity of 94.3% of GOR in a half-cell (Figure 3d). Furthermore, 1.33 and 1.58 V are required in a hybrid electrolyzer (NiCo hydroxide||NiCo hydroxide) to reach 10 and 100 mA cm−2 in GOR-promoted hydrogen production (Figure 3e,f).
Other aliphatic alcohol upgrading can also be integrated with hydrogen production in “pared electrolysis”, such as electrooxidation of ethylene glycol, isopropanol and 1,3-propandiol [52,53,54,55,56,57]. Besides the aforementioned aliphatic alcohols, some alcohols with aromatic groups and relatively good solubility of water can also be effectively coupled with the hydrogen evolution reaction, such as benzyl alcohol and its oxidized product, benzoic acid, which is a basic fine chemical utilized in synthetic fiber, resin and in the antiseptic industries [58,59,60,61]. Duan and co-workers developed a composited catalyst of Au nanoparticles supported on Co oxyhydroxide nanosheets (Au/CoOOH) for benzyl alcohol upgrade integrating with H2 production at 1.5 V vs. RHE with a current density of 540 mA cm−2. The absolute current can approach 4.8 A at 2.0 V in a hybrid flow electrolyzer. The experimental analysis combined with the theoretical calculations indicated that the benzyl alcohol can be enriched at the Au/CoOOH interface and oxidized by the electrophilic oxygen species (OH*) generated on CoOOH, resulting in higher activity than pure Au.

2.2. Anodic OUR of Aldehyde Integrating with Cathodic HER

Besides alcohol electrooxidation, electrochemically oxidative upgrading of biomass and biomass-derived platform molecules, such as aldehydes, also enables the generation of value-added chemicals and integration with the hydrogen evolution reaction in a hybrid electrolyzer. 5-hydroxymethylfurfural (HMF) is a vital building block for the pharmaceutical and other chemical industries. Its oxidized derivative of 2, 5-furandicarboxylic acid (FDCA) has been reported to be a green platform chemical to substitute terephthalic acid as a feedstock for the preparation of high-value-added polymers [62]. HMF Electrooxidation is mostly performed under alkaline conditions wherein hydroxide ions are necessities. Therefore, electrocatalytic HMF oxidation enables an integration with the HER, producing an additional product with high economic value, thus increasing the energy efficiency of hydrogen production. The pioneering work of HMF electrooxidation to FDCA was presented by Grabowski and co-workers; the yield is 71% [63]. Platinum group metal-based catalysts have also been reported with relatively high activity and selectivity to replace transition-metal-based catalysts [24,64]. Cha and Choi [64] demonstrated a FE of 100% for FDCA at 1.54 V on a 2, 2, 6, 6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated Au electrode. However, the TEMPO mediator increases the costs of downstream separation. Evidently, a high pH boosts the selective electrooxidation of HMF to FDCA [65]. At high pH values above 13, HMF is decomposed into humin-type products. Thus, to increase product yield, fine design of the catalyst system is of extreme importance, especially regarding the noble metal-free catalysts (e.g., earth-abundant metal oxides [23], nitrides [66], phosphides [67] and borides [68]). For instance, Stefan Barwe et. al. [68] developed a high-surface-area nickel boride (NixB) electrode with high FE of near 100% for the FDCA generation with a yield of 98.5%, and demonstrated HMF is preferentially oxidized via hydroxymethyl-2-furancarboxylic acid rather than 2, 5-diformylfuran through operando electrochemical–infrared spectroscopy that agrees with HPLC analysis. A relatively low potential of 1.45 V vs. RHE is achieved with a current density of 100 mA∙cm−2 over HMF electrooxidation, which is 170 mV lower than the potential necessary to evolve the OER at the same conditions. Other oxygen-contained molecules such as glucose, sodium gluconate and triclosan can also be electrooxidized to couple with the HER [69,70,71].

2.3. Anodic OUR of Carboxylates Integrating with Cathodic HER

Since the first pioneering report in the mid-nineteenth century, carboxylate electrooxidation or the Kolbe electrolysis has been extensively studied [72,73,74,75,76]. The classical manifestation of oxidative decarboxylation of an aliphatic carboxyl acid can generate a transient alkyl radical (RCO2•) which are then combined to produce a Csp3-Csp3 bond [77]. Such a heterocoupling of Csp3-Csp3 between two carboxylic acids was historically utilized as a key strategy to synthesize prostaglandin [78], jasomonic acid analogues [79], a modular route to access sugar derivatives [80] and vicinal olefin functionalizations [81], albeit to a lesser extent. Recently, Baran and co-workers developed a mildly reductive Ni-electrocatalytic strategy to integrate two different carboxylates through in situ generated redox-active esters, which was termed doubly decarboxylative cross-coupling. This operationally facile method enables heterocoupling of primary, secondary and even certain tertiary redox-active esters, thus generating a powerful new strategy for electrosynthesis. Furthermore, the reaction cannot be mimicked by applying either stoichiometric metal reductants or photochemical conditions, and it can tolerate a range of functional groups. Furthermore, this method was scalable for the synthesis of 32 compounds, which reduced overall step counts by 73%. Besides the desired Kolbe-product, a co-product of CO2 with a double-molar fraction was generated at the anode and usually stoichiometric amounts of H2 from water electrolysis at the cathode. Therefore, in an undivided electrolyzer, the respective gaseous electrolysis products of CO2 and H2 are to be obtained, and these can be directly utilized without further purification [82]. Recently, Markus Stöckl and co-workers [82] reported the first coupling of the anodic Kolbe electrolysis with a subsequent microbial conversion of the produced coproduct hydrogen at the cathode. Kolbe electrolysis of valeric acid yielded n-octane. Then, isopropanol was generated by resting Cupriavidus necator cells with gaseous products (CO2 and H2). The resting microbial cells exhibited Fes of 80% and 60% for cathodic and anodic reactions, respectively, and carbon efficiencies of up to 41%. The implementation of a hybrid electrolyzer resulted in striking process performance with overall efficiencies of up to 64.4%, as displayed in Figure 4.

2.4. Anodic OUR of Nitrogen-Contained Molecules Integrating with Cathodic HER

In addition to the aforementioned aldehydes and alcohols, amine, nitroalkane and tetrahydroisoquinoline electrooxidation are also kinetically favorable and can also generate value-added imines, amides, nitriles, azo compounds and amine oxides, which are widely utilized in the field of pharmaceuticals and agrochemicals [12,14,83,84]. In 2018, Zhang and co-workers [83] developed NiSe nanorod arrays for thermodynamically more favorable primary amine (−CH2−NH2) electrooxidation in water to replace the OER and couple the HER. The increased H2 generation can be realized at the cathode; meanwhile, the as-prepared NiSe nanorod arrays displayed significant diversity of various aromatic and aliphatic primary amines to selectively generate nitriles with decent yields (>93 %) and selectivity (>94 %) at the anode. Mechanistic investigations suggest that the reconstruction of nickel species (NiII/NiIII species) may serve as redox active sites for the transformation of primary amines. Meanwhile, the hydrophobic nitrile products enabled ready escape from the aqueous electrolyte/electrode interface, avoiding the deactivation of catalysts and thereby promoting continuous gram-scale synthesis. Later on, Zhai and co-workers [84] utilized the operando electrocatalysis variations (i.e., chalcogen leaching) to manipulate the electrocatalytic interface toward amine electrooxidation, as shown in Figure 5. Taking chalcogen-doped Ni(OH)2 as an example (Figure 5a–c), experimental results (e.g., XPS and in-situ Raman spectrum in Figure 5f,g) uncovered that chalcogens leach from the substrate and are then adsorbed on the NiOOH surface as chalcogenates over the electrooxidation process. The charged chalcogenates can induce a local electric field that drives the polar amines through the Helmholtz plane to enrich on the electrocatalytic surface. Meanwhile, the local polarization of chalcogenates and amines can boost dehydrogenated activation of amino C–N bond to nitrile C≡N bonds. Under the pushing effect of surface-adsorbed chalcogenate ions, the Ni(OH)2 displayed near-full propionitrile selectivity (99.5%) at a potential of 1.317 V vs. RHE with an ultrahigh current density, as displayed in Figure 5d,e,h.

2.5. Anodic OUR of Other Organic Molecules Integrating with Cathodic HER

Recently, two other kinds of organic molecule electrooxidation reactions have attracted chemists’ interest: electrooxidation of organic sulfides (SOR) and alkene (AEOR). (i) Regarding the electrooxidation of organic sulfides (SOR), organic sulfoxides and sulfone as the main products of sulfide oxidation play prominent roles in pharmaceuticals, biological processes and material science [85]. Unfortunately, the conventional oxidation of organic sulfides usually suffers from the required use of strong oxidizing agents (e.g., iodine, H2O2, 3-chloroperbenzoic acid, or peroxy acids and oxone), homogeneous catalysts, toxicity, etc. [3], whereas recent reports have demonstrated successful electrooxidation of sulfides using water as an oxygen source, and, e.g., MeCN, acetone and DMF as the solvent on carbon electrodes, NiII complexes, cobalt pentacyanonitrosylferrate and CoFe-LDH [86,87,88,89,90,91]. Zhang and co-workers [86] developed an electrocatalytic protocol to selectively oxidize sulfides to sulfoxides at cost-effective graphite felt electrodes. They used NaCl to serve as an electrolyte and a redox mediator to protect sensitive functional groups from oxidation. This metal-free electrocatalytic protocol is simple, green and compatible with different sensitive functional groups with acetone/water as the green solvent. The methodology could easily work on a gram or even a decagram scale, and it displays great potential regarding coupling with the hydrogen evolution reaction. (ii) Regarding the electrooxidation of alkene (AEOR), alkene oxidation is an effective strategy to synthesize vicinal diols and epoxides, which are key intermediates for the synthesis of perfumes, food additives, drug intermediates, fine chemicals and agrochemicals [92]. For example, Ethylene oxide is among the most abundantly fabricated commodity chemicals worldwide due to its wide application in the plastics industry, notably for fabricating polyesters and polyethylene terephthalates. In an early work pioneered by Meyer and co-workers in 1980 [93], cyclohexene electrooxidation was reported to produce p-benzoquinone with a Ru-based complex catalyst. Recently, Sargent group conducted a series of wonderful work on ethylene electrooxidation to produce ethylene glycol (80% selectivity) and ethylene oxide (EO), and this shows great promise to integrate with hydrogen evolution [94,95,96]. In 2020, they found that gold-doped palladium can oxidize ethylene to yield ethylene glycol with approximately 80% FE at ambient condition in aqueous media [94]. Later on in 2020 [95], they applied an extended heterogeneous:homogeneous interface (Figure 6a) and utilized chloride as a redox mediator at anode (Figure 6b) to boost the selective ethylene electrooxidation for ethylene oxide generation with a large current density of 1 A∙cm−2, high FE of ~70% and product specificities of ~97% (Figure 6c–f). In another work by the same group [96], they found that barium-oxide-loaded catalysts can obtain an ethylene-to-EO FE of 90%. This redox-mediated paired system exhibited a 1.5-fold higher CO2-to-EO FE (35%) and utilized a 1.2 V lower working voltage than the benchmark electrochemical systems.
Table
Table 1. Electrocatalytic performance of recently reported representative OUR electrocatalysts and the OUR||HER-based electricity-input system.
Table 1. Electrocatalytic performance of recently reported representative OUR electrocatalysts and the OUR||HER-based electricity-input system.
Working ModeReaction TypeAnode CatalystSubstrateElectrolyteProduct3-Electrode System2-Electrode SystemRef.
EOER at j10 (VRHE)EOUR at j10 (VRHE)VOER-HER at j10VOUR-HER at j10
OUR||HER
on electricity-input mode		Alcohol oxidationVp-Ni2P-Pt/CCmethanol2 M in 1 M KOHFormate/H20.72 at
J50		1.651 at
J50		-ca. 0.7[38]
CeO2/RuO2methanol2.5 M in 0.5 M H2SO4Formic acid/H21.4951.1951.5681.308[39]
Co(OH)2@HOSmethanol3 M in 1 M KOHformate/H21.5711.3851.6311.497[41]
PtIr NWsethanol4 M in 0.5 M HClO4DEE/H2-0.45-0.61[26]
Co(OH)2@Ni(OH)2ethanol1.0 M in 1.0 M KOHacetate/H2-1.3-1.39[33]
Goldglycerol0.1 M in 0.1 M NaOHglyceric acid/H2-1.0--[47]
NiCo hydroxideglycerol0.1 M 1 M KOHformate/H2-1.39 at J100-1.33[50]
PdAg/NFethylene glycol1 M in 0.5 M KOHglycolic acid/H21.550.57-1.02 at j20[57]
CoNCglucose0.1 M in 1.0 M KOHgluconic acid, glucaric acid/H21.7 at J1001.5 at J1001.78 V at J1000.9 V at J100[69]
Ni(OH)2benzyl alcohol benzoic acid/H2 ~1.33 at J100 [58]
aldehyde oxidationNixBHMF10 mM in 1 M KOHFDCA/H21.62 at J1001.45 at J100--[68]
carboxylate oxidationNiCl2•dme, Ligand L4carboxylic acidsNaI (0.2 M), DMFdecarboxylative products---4 mA, 4 F per mol[77]
Pt-foilvaleric acid0.5 Mn-octane----[82]
amine oxidationNiSeBenzyl-amine1 mM in 1.0 M KOHbenzyl nitrile/H21.481.341.70 at J201.49 at J20[83]
t-Ni/Co MOFBenzyl-amine0.02 M in 1.0 M KOHbenzonitrile/H2--~1.75~1.5[14]
S-Ni(OH)2Propyl-amine0.1 M in 1.0 M KOHpropionitrile/H2-1.327 at J100--[84]
sulfides oxidationCoFe-LDHsulfides0.25 M in MeCN/H2Osulfoxides/H21.90 at J51.39 at J5--[87]
Ni(ii)–bipyridinephenyl sulfideH2O (30
Equiv.), n-Bu4NBF4, MeCN		phenyl sulfoxides/H2 [89]
nitroalkanesNiSenitrotoluene0.4 mM in 1.0 M KOHE-nitroethene--1.691.36[12]
alkane oxidationPtethylene1 M KClethylene oxide/H2FE 70%product
specificities 97%		 [95]

3. Anodic OUR of Organic Compounds in Parallel with CO2RR (OUR||CO2RR)

The over-combustion of fossil fuels along with excessive anthropogenic emission of CO2 acceleratingly exacerbate global environmental and climate issues, which severely hinder the sustainable development of human society. Therefore, efficient conversion of CO2 is extremely imperative and urgent to mitigate and circumvent these challenges. With the implementation of renewable electricity, our carbon footprint could be decreased by using CO2 electroreduction (CO2RR), which can not only serve as a sustainable strategy to curb CO2 accumulation, but can also yield high-value-added fuels and chemicals. For a CO2 electrolyzer, the OER occurs in the anodic chamber, and large overpotential is required to achieve appreciable current density owing to the sluggish kinetics of the OER. Thermodynamic results suggest that an energy loss of about 90% is attributed to the OER during CO2 reduction to CO [29]. Furthermore, O2 is less valued, and the ROS caused by the OER may degrade the electrolyzer membrane and thus affect the durability of electrolyzers, as clarified above. Similar to the hybrid water electrolysis, the thermodynamically more favorable OUR integrating with the CO2RR (OUR||CO2RR) is a promising strategy to yield value-added chemicals on both anode and cathode with lower electricity input into the electrolyzer and exclude the formation of ROS, thus maximizing the utilization of electricity (Table 2) [29,30,31,97,98,99,100,101].
Up to now, the OURs of methanol [99,100], glycerol [29,31,98], 1,2-propanediol [30], hydroxymethylfurfural [98,101], glucose [29] and 2-phenoxy-1-phenylethanol [98] have been tested as anode processes to efficiently integrate with the CO2RR. In 2019, Paul J. A. Kenis and co-workers [29] found that the anodic electrooxidation of glycerol can lower the input electricity by up to 53%, which reduced the carbon footprint and operating costs, thereby opening avenues for a carbon-neutral cradle-to-gate process even when the electrolysis is driven by grid electricity (Figure 7a–c). Later in 2020, Erwin Reisner and co-workers [98] reported that glycerol can be selectively electro-oxidized to glyceraldehyde with a turnover number of near 1000 and an FE of 83 %. The cathode yielded a stoichiometric amount of syngas with a CO:H2 ratio of 1.25 ± 0.25 and an overall cobalt-based turnover number of 894 with an FE of 82 % (Figure 7d,e). This hybrid electrolyzer of OUR||CO2RR inspires the design and implementation of novel strategies for coupling the CO2RR to energy-saving, and value-added, oxidative chemistry. Shi group [99] developed a general strategy for an anodic OUR of methanol-to-formic acid integrating with a cathodic CO2RR with copper oxide nanosheets on copper foam (CuONS/CF) and mesoporous SnO2 on carbon cloth (mSnO2/CC) as cathodic and anodic catalysts, respectively. Both sheet-shaped CuONS and mesoporous SnO2 can provide significantly large electrochemical surface areas that are key for electrocatalytic reactions. CuONS/CF enabled a low potential of 1.47 V vs. RHE at current density of 100 mA cm−2 (Figure 7f), which featured a significantly boosted activity compared to the OER. The mSnO2/CC displayed a relatively high FE of 81 % at 0.7 V vs. RHE for formic-acid generation from the CO2RR (Figure 7g). A considerably low cell voltage of 0.93 V at 10 mA cm−2 was required in the hybrid CuONS/CF||mSnO2/CC electrolyzer for formic-acid production at both sides (Figure 7h). As a whole, most of the research on OUR||CO2RR were based on model studies, a few were performed in continuous-flow cell, and most of the OURs operated in strongly alkaline solutions. Therefore, there is great room to improve in terms of the electrocatalysts and the optimal operating conditions.

4. OUR-Based Fuel Cells or Other Devices

Pioneered by Sir William Robert Grove in 1839, the research and development of fuel cell technology has experienced more than 100 years of growth up to now, and great progress has been achieved. Typically, to maximize the performance of a fuel cell (e.g., output voltage, power density and efficiency), full oxidation of fuel is needed to produce and transfer more electrons. Specifically, for carbonaceous-based fuel cells with excellent electrical performance, usually CO2 and or CO32− is generated. So, does this mean that the electrical performance of a fuel cell concomitantly integrated with organic compounds upgrading will be affected? There is no doubt. In fact, the electric performance of the most advanced fuel cells fed by carbonaceous-based fuel (even those including hydrogen) fail to attain the desired end [102,103,104,105]. For example, alcohol-fed fuel cells usually output voltages ranging from 0.5 V to 0.9 V even for H2/O2 fuel cells, which are at least 0.3 V lower than the theoretical voltage. To upgrade organic compounds and generate electricity profits concomitantly, an optimum balance should and can be kept via fine designed catalysts and systems [11,32]. From the perspective of upgrading organic compounds, the intention of upgrading can be even more inclined.
Up to now, most prevailing electrodes and or photoelectrons focus on the planar structure, which suffers from limited reactants diffusion and sluggish mass transfer, thus resulting in overoxidation of valuable chemicals [106,107]. Recently, microfluidic nanostructures have been reported and fabricated to circumvent the aforementioned drawbacks due to the use of 3-D channels for precise product control and enhanced mass transfer [106,107]. Qu and co-workers [106] constructed a microfluidic photo-electrochemical architecture with 3-D microflow channels, which was fabricated with defect WO3/TiO2 heterostructures on porous carbon. The charge effectively accumulated on the nanojunction as manifested by the visualizing Kelvin probe force microscopy and photoluminescence spectroscopy (Figure 8a–f). The efficient charge separation contributed to a 3-fold enhancement of the generation of glyceraldehyde and 1, 3-dihydrocxyacetone. Meanwhile, the microfluidic platform with boosted mass transfer exhibited a reaction selectivity of 85%, which was higher than the planar protocol (Figure 8g,h). Moreover, this WO3/TiO2 heterostructured photoanode enabled effective production of high-value-added KA oil and S2O82− via photocatalytic electrooxidation of cyclohexane and HSO4− and pollutants degradation (Figure 8i,j). In a photocatalytic fuel cell with the WO3/TiO2 heterostructured photoanode, a remarkably higher open-circuit voltage of 0.9 V and a short-circuit current of 1.2 mA cm−2 were obtained (Figure 8k–o).
Cauê A. Martins and co-workers [107] developed a 3D-printed microfluidic glycerol fuel cell that generates power concomitantly to formate and glycolate. They intelligently tuned the balance between the output energy and the two carbonyl compounds generated via decorating the Pt/C/carbon paper anode in situ and before feeding reactants, or operando with Bi (while feeding reactants). For the operando method, rodlike Bi oxides dendrites were built and were inactive for the glycerol electrooxidation and covered active sites. While the in situ strategy boosted Bi decoration, which can increase the open-circuit voltage to 1.0 V, it augmented the maximum power density 6.5 times and the glycerol conversion up to 72% at ambient conditions. The authors attributed the significant performance to strong CO antipoisoning of the anode, which is conducive to a more complete reaction and harvesting more electrons at the device.
Recently, Huang and co-workers [32] developed ternary Pd–Pt–Ag nanowires for glycerol fuel cells with excellent performance. The ternary Pd–Pt–Ag nanowires were designed via the d band theory, as the-state-of-the-art Pt/C catalysts of a high d band center suffer from severe CO and CO-like intermediates poisoning. Therefore, an optimum d band center was achieved with Pt and/or Pd alloy with Ag with a low d band center. Furthermore, the Pd–Pt–Ag nanowires were also designed based on other significant merits: (i) Pt and Pd exhibit excellent intrinsic activity, (ii) Ag possesses the highest conductivity of 6.30 × 107 Sm−1 among all metals at room temperature, (iii) nanowires (NWs) possesses remarkable anisotropy that is particularly in favor of unidirectional electron transfer and anti-aggregation over catalysis compared with nanoparticles and (iv) Ag is used to break C–C bonds and form C1 products. Subsequently, Pd–Pt–Ag nanowires were generated via facile galvanic displacement on the mother Ag nanowires. The as-prepared catalysts displayed high activity and stability of glycerol electrooxidation (2.82 A mg−1, 2.03 times higher than that of the commercial benchmark Pt/C catalyst, Figure 9a) and the oxygen reduction reaction (ORR) (Figure 9b). The main product of glycerol electrooxidation on Pd0.82Pt0.56Ag nanowires was formate with a decent selectivity of 81.2% on Pd0.82Pt0.56Ag nanowires, and the proposed pathway is glycerol → glyceraldehyde → glycerate → formate (81.2%) and CO32–, as manifested by in situ Fourier transform infrared spectroscopy (FT–IR, Figure 9c) and NMR analysis. Surprisingly, a homemade glycerol fuel cell with bifunctional Pd0.82Pt0.56Ag electrodes delivered a highest-ever-recorded voltage of 1.13 V in glycerol fuel cells (Figure 9d–f), and could achieve a maximum power density of 13.7 mW cm−2 under ambient conditions that was ∼50% enhanced compared to that based on the Pt/C benchmark (Figure 9g). The excellent electrocatalytic performance and efficient glycerol upgrading implied a balance between the contradiction of high energy output and the generation of value-added products (Figure 9h,i). Therefore, the OUR of organic compounds in fuel cells provides a novel and fascinating paradigm for the electrosynthesis of high-value-added products, and helps cut our carbon footprint, as conventional carbonaceous-based fuel cells produce greenhouse gas carbon dioxide at the anodes. Notably, we found that the peak-potential of glycerol electrooxidation is near 0.9 V vs. RHE, which is far below 1.23 V, signifying the great potential of Pd0.82Pt0.56Ag electrodes for the application of glycerol upgrading assisted with hydrogen production. There is no doubt that the key to realizing the organic compounds upgrading is advanced materials, whether in a hybrid electrolyzer for cogeneration of hydrogen or in a fuel cell. Huang and co-workers concluded that decent activity and low oxidation potential of alcohol electrooxidation can be achieved via surface and interface modulation of PGM-based catalysts, such as surface electrochemically etching [44] and surface plasmon resonance [45].
In a conventional fuel cell, the hydrogen of the organic fuels is oxidized to H2O under large potential. Subsequently, value-added products and hydrogen are generated at the anode and cathode, respectively. Recently, Wang group [11] found that low-potential furfural electrooxidation drove the electrooxidation of the aldehyde group to generate value-added furoic acid with hydrogen (H) atoms of the aldehyde group to be released as gaseous hydrogen (H2) at a low potential of approximately 0 VRHE (vs. RHE). Meanwhile, the integrated electrochemical system can generate electricity of about 2 kWhm−3 of H2 yield in a cell. This interesting finding may provide a transformative strategy to convert biomass upgrading and H2 production from an electricity-input mode to an electricity-output mode.
Besides upgrading organic compounds in fuels, sporadic examples of organic compounds upgrading in other devices were reported, such as in zinc–air batteries. In a conventional zinc–air battery, the discharging process includes zinc electrooxidation on the negative electrode and ORR on the positive counterpart, and the charging process includes the corresponding reversing reactions of Zn2+ reduction and oxygen evolution (OER). Unfortunately, the sluggish kinetics of the OER tremendously increase the energy consumption over the charging process. In view of this, recently, Wang and co-workers constructed a zinc–ethanol–air battery with a heterostructured Co(OH)2@Ni(OH)2 cathode wherein the sluggish OER over the charging process was replaced by the kinetically-favored OUR of ethanol. The resulting charge voltage of the zinc–ethanol–air battery was more than 300 mV lower than the conventional zinc–air battery at the same charging conditions. Furthermore, the elimination of the oxygen bubbles from the OER also guarantees robust charging at high current density.
Table
Table 2. Electrocatalytic performance of recently reported representative OUR electrocatalysts, the OUR||CO2RR systems and OUR-based devices (fuel cell and zinc–air battery).
Table 2. Electrocatalytic performance of recently reported representative OUR electrocatalysts, the OUR||CO2RR systems and OUR-based devices (fuel cell and zinc–air battery).
Working ModeReaction TypeAnode CatalystSubstrateElectrolyteProduct3-Electrode System2-Electrode SystemRef.
EOER at j10 (VRHE)EOUR at j10 (VRHE)VOER-HER at j10VOUR-HER at j10
OUR||CO2RR
on electricity-input mode		CuONS/CFmethanol1 M KOHFormic acid-1.47 at J100-0.93[99]
CuSnmethanol1 M in 1 M KOHformate1.76 at J2001.49 at J200VOER-CO2RR: 3.84 at J100VMOR-CO2RR: 3.23 at J100[100]
PdOx/ZIF-8HMF20 mM in 0.5 M [Bmim]BF4, 1.0 M CH3CNFDCAFEco, 97%FEorganic
acid, 84.3%		--[101]
OUR-based battery on electricity output modefuel cellsPd0.82Pt0.56Agglycerol1 M in 0.5 M NaOHformate---Vocp: 1.13 V[32]
WO3/TiO2glycerol0.5 M H2SO4GLA, DHA---Vocp: 0.9 V[106]
Bi-Ptglycerol0.1 M in 0.1 M KOHglycolate and formate---Vocp: 1.0 V[107]
Zinc–air batteryCo(OH)2@Ni(OH)2ethanol1.0 Macetate----[33]

5. Conclusions and Future Perspectives

In this minireview, we summarized the recent development of the electrochemically oxidative upgrading reaction (OUR) of organic compounds to integrate with the hydrogen evolution reaction (HER) and carbon dioxide reduction reaction (CO2RR) in hybrid electrolyzers on electricity-input mode and OUR-based fuel cells and other devices under electricity-output mode to co-produce electricity.
In a hybrid electrolyzer of the HER and or CO2RR integrating with the OUR of organic compounds, such as kinetically favored electrooxidation of oxygen-contained molecules (e.g., alcohol, aldehyde, carboxylates and biomass-derived molecules), nitrogen-contained molecules (e.g., amine, nitroalkanes, and tetrahydroisoquinolines), organic sulfides and alkenes were discussed. Compared to conventional water electrolysis that suffers from inherent weaknesses including high theoretical voltages, single value-added products and low atomic efficiency, upgrading the kinetically favored organic molecules to co−generate hydrogen and value-added chemicals leads to products possessing significant theoretical energy efficiency, great atom economy, reduced cell voltage (more energy-saving), no production of ROS and explosive hazards and exogenous-oxidant-free conditions in the anodic chambers. Unlike the sacrificial agent-assisted electrolysis for the hydrogen evolution reaction and or the carbon dioxide reduction reaction, OUR||HER and/or OUR||CO2RR via hybrid electrolyzers possess the merits of high theoretical energy efficiency, great atom economy and low price, which seems to be the most fascinating synthetic strategy for hydrogen production, curbing CO2 accumulation and organic compounds upgrading, killing two birds with one stone.
In a multifunctional paradigm of batteries with the OUR, value-added chemicals are generated with the co-production of electricity, which can also kill two birds with one stone. Unfortunately, whether for OUR||HER and/or OUR||CO2RR, or the OUR of organic compounds in an electricity-output battery, these strategies based on the OUR are still at an early laboratory stage, the corresponding substrates diversities are still limited, the system design still needs to be optimized and the total efficiency is still unsatisfactory.
Therefore, cost-effective catalyst design, reaction mechanism identification, and efficient system designs of electrolyzers and/or batteries (e.g., electrolytes, diaphragms, additives, feeding systems, controlling systems, purification and further application of the products) are highly desirable for future practical developments in the industry. To efficiently upgrade the organic compounds, we provide our perspectives as follows:
  • Future work should focus on theory calculation-guided smart design and the precise synthesis of advanced catalysts. The prerequisite to realize the architecture is the advanced catalysts, especially for the anode oxidation reaction and CO2RR. The ideal catalysts must possess high intrinsic catalytic activity, large electrochemical surface areas, maximum utilization of catalytic sites, significant robustness, etc. Therefore, smart design of advanced catalysts is desirable. To screen the best catalysts, machine learning has been demonstrated to be effective. Besides the assistance of theory calculations, advanced synthesis and characterization methods are also crucial to obtain the target catalyst materials. Considering that noble metals are limited to scarce reserves in the Earth’s crust and their resulting high costs, earth-abundant metals and or carbon-based catalysts should be preferentially focused and developed;
  • Future work should work towards the optimization of reaction conditions. Besides the catalyst materials, reaction conditions (e.g., the solvent, additives and temperature) have a great influence on the thermodynamics and kinetics of substrate adsorption and conversion, the target product desorption, on processes such as mass transfer and the microenvironment, and thus on the final catalytic efficiency. Therefore, the optimization of reaction conditions is quite necessary. If conditions render them necessary, theoretical simulations and in situ and/or operando technologies can be helpful;
  • In situ and/or operando technology assisted the characterization and identification on the molecular/electronic level of the active sites, reaction pathways, important intermediates and the final structure-property relationship. In order to explore the advanced catalysts and improve the final catalytic efficiency, the study of the catalytic mechanism is indispensable. In situ and/or operando technologies (e.g., High Performance Liquid Chromatography (HPLC), Differential Electrochemical Mass Spectrometry (DEMS), Fourier-transform infrared spectroscopy (FTIR), Raman Spectra, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS)) are powerful tools able to reveal the catalytic mechanism, which conversely guides the optimization of catalysts and reaction conditions;
  • Electrolysis system optimization plays a key role in this research. The state-of-the-art systems also determine efficiency, cost, operability and the safety of these three strategies in both the laboratory and the industry. The design and optimization of systems include but are not limited to the basic cell units (e.g., electrode materials, current collector and diaphragm), feeding units, separation and purification units, controlling system, etc.
With the considerable renaissance of electrosynthesis strategies and the development of renewable-based electricity nowadays, upgrading organic compounds in a multifunctional device would be greatly boosted and more applicable in practical chemical manufacturing if the issues related to the selectivity, activity, mechanism identification, substrate universality, long-term durability of catalysts and the final electrolysis systems were solved.

Author Contributions

Conceptualization, L.H., J.M. and X.C.; investigation, J.M., K.C. C.D. and J.W.; writing—original draft preparation, L.H., J.M. and K.C.; writing—review and editing, L.G. and X.C.; visualization, C.D.; supervision, L.H. and X.C.; funding acquisition, L.H. All authors have read and agreed to the published version of the manuscript.

Funding

National Natural Science Foundation of China Fund (No. 22102063).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no competing financial interests.

Abbreviation

OURElectrochemically oxidative upgrading reaction
OERoxygen evolution reaction
HERhydrogen evolution reaction
CO2RRCO2 electroreduction reaction
PEMproton exchange membrane
ROSreactive oxygen species
MORmethanol electrooxidation reaction
EORethanol electrooxidation reaction
DEE1,1-diethoxyethane
GORGlycerol electrochemical oxidation
HMF5-hydroxymethylfurfural
FDCA2, 5-furandicarboxylic acid
SORelectrooxidation of organic sulfides
AEORelectrooxidation of alkene
LDHLayered Double Hydroxide
GLADglyceraldehyde
DHA1, 3-dihydroxyacetone
KA oilcyclohexanol and cyclohexanone
ORRoxygen reduction reaction
j1010 mA cm−2

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Materials 16 02500 sch001 550
Scheme 1. Left: polarization curves of the electrochemical oxidation upgrading reaction (OUR) via the electricity-input mode to integrate with the hydrogen evolution reaction (OUR||HER) and carbon dioxide reduction reaction (OUR||CO2RR), and the comparison with conventional electrolysis. The inset is the configuration of a simplified single hybrid electrolyzer. Right: polarization curves of the electrochemically oxidative upgrading reaction (OUR) in a smart device (e.g., fuel cell) to output the electricity. The inset is the configuration of a single fuel cell with the OUR.
Scheme 1. Left: polarization curves of the electrochemical oxidation upgrading reaction (OUR) via the electricity-input mode to integrate with the hydrogen evolution reaction (OUR||HER) and carbon dioxide reduction reaction (OUR||CO2RR), and the comparison with conventional electrolysis. The inset is the configuration of a simplified single hybrid electrolyzer. Right: polarization curves of the electrochemically oxidative upgrading reaction (OUR) in a smart device (e.g., fuel cell) to output the electricity. The inset is the configuration of a single fuel cell with the OUR.
Materials 16 02500 sch001
Materials 16 02500 g001 550
Figure 1. (a) Illustration of Ni(OH)2·xH2O nanorod array electrodes and their assembly of the hybrid electrolyzer coupling the HER and MOR. (b) The two-electrode LSV curves of Ni4Mo–MoO2 and Ni(OH)2·xH2O towards the OER and MOR, respectively. (c) The i–t curves, (d) LSV curves before and after i–t tests and (e) i–t curves in 3M methanol and 6 M KOH for the hybrid Ni4Mo–MoO2||Ni(OH)2·xH2O electrolyzer. (f) Faradaic Efficiency (FE) of the stability test of e. (g) The LSV curve of Ni4Mo–MoO2 in 1 M KOH with and without 0.5 M methanol. Reproduced with permission [43], Copyright 2022, Springer Publishing Group.
Figure 1. (a) Illustration of Ni(OH)2·xH2O nanorod array electrodes and their assembly of the hybrid electrolyzer coupling the HER and MOR. (b) The two-electrode LSV curves of Ni4Mo–MoO2 and Ni(OH)2·xH2O towards the OER and MOR, respectively. (c) The i–t curves, (d) LSV curves before and after i–t tests and (e) i–t curves in 3M methanol and 6 M KOH for the hybrid Ni4Mo–MoO2||Ni(OH)2·xH2O electrolyzer. (f) Faradaic Efficiency (FE) of the stability test of e. (g) The LSV curve of Ni4Mo–MoO2 in 1 M KOH with and without 0.5 M methanol. Reproduced with permission [43], Copyright 2022, Springer Publishing Group.
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Materials 16 02500 g002 550
Figure 2. (a) Schematic illustration of the hybrid electrolyzer with the oxidation of ethanol to DEE and hydrogen production. (b) LSV of the Pt/C||Pt/C, Pt NWs/C||Pt NWs/C and PtIr NWs/C||PtIr NWs/C cells at a scan rate of 5 mV s−1 in 0.5 M H2SO4 ethanol solution (the dotted line refers to current density of 10 mA cm−2). (c) Potential comparation (j = 10 mA cm−2), (d) the FE of DEE in various acids (V = 1.4 V). (e,f) In situ FTIR spectra characterization of the ethanol electrooxidation for DEE production over PtIr NWs/C (the different colored lines in (e,f) refer to the curves at various time in the direction of the arrow). Reproduced with permission, Copyright 2021 [26], American Chemical Society (ACS) Publishing Group.
Figure 2. (a) Schematic illustration of the hybrid electrolyzer with the oxidation of ethanol to DEE and hydrogen production. (b) LSV of the Pt/C||Pt/C, Pt NWs/C||Pt NWs/C and PtIr NWs/C||PtIr NWs/C cells at a scan rate of 5 mV s−1 in 0.5 M H2SO4 ethanol solution (the dotted line refers to current density of 10 mA cm−2). (c) Potential comparation (j = 10 mA cm−2), (d) the FE of DEE in various acids (V = 1.4 V). (e,f) In situ FTIR spectra characterization of the ethanol electrooxidation for DEE production over PtIr NWs/C (the different colored lines in (e,f) refer to the curves at various time in the direction of the arrow). Reproduced with permission, Copyright 2021 [26], American Chemical Society (ACS) Publishing Group.
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Materials 16 02500 g003 550
Figure 3. (a) The DFT−calculated Gibbs free energy profiles of glycerol oxidation reaction on Ni, NiCo and Co hydroxide. (b) In situ electrochemical–infrared spectra of NiCo hydroxide. (c) In situ electrochemical−Raman spectroscopies for NiCo hydroxide. (d) CV curves of Co hydroxide, NiCo and Ni with (solid line) or without (dash line) glycerol. (e) LSV curves for the two-electrode electrolysis coupling HER with GOR. The inset is the illustration of the electrolyzer. (f) LSV curves of NiCo hydroxide before and after a chronopotentiometry test performed at 100 mA cm−2 for 110 h. The inset is the corresponding chronopotentiometry curve of NiCo hydroxide. Reproduced with permission [50], Copyright 2022, Nature Publishing Group.
Figure 3. (a) The DFT−calculated Gibbs free energy profiles of glycerol oxidation reaction on Ni, NiCo and Co hydroxide. (b) In situ electrochemical–infrared spectra of NiCo hydroxide. (c) In situ electrochemical−Raman spectroscopies for NiCo hydroxide. (d) CV curves of Co hydroxide, NiCo and Ni with (solid line) or without (dash line) glycerol. (e) LSV curves for the two-electrode electrolysis coupling HER with GOR. The inset is the illustration of the electrolyzer. (f) LSV curves of NiCo hydroxide before and after a chronopotentiometry test performed at 100 mA cm−2 for 110 h. The inset is the corresponding chronopotentiometry curve of NiCo hydroxide. Reproduced with permission [50], Copyright 2022, Nature Publishing Group.
Materials 16 02500 g003
Materials 16 02500 g004 550
Figure 4. Schematic illustration of efficient generation of green fuels and solvents by integrating electrosynthesis and biosynthesis: carboxylic acids that originate from the mixed acid fermentation of biomass are utilized as substrate for Kolbe electrolysis process powered by renewables. Kolbe electrolysis then takes place in the hybrid electrolyzer and generates liquid products (n-octane) anodically and hydrogen cathodically. The resulted efficiencies of the process steps and the overall efficiency are shown (CE = Carbon efficiency, FE = Faradaic efficiency). Reproduced with permission [82], Copyright 2022, Wiley Publishing Group.
Figure 4. Schematic illustration of efficient generation of green fuels and solvents by integrating electrosynthesis and biosynthesis: carboxylic acids that originate from the mixed acid fermentation of biomass are utilized as substrate for Kolbe electrolysis process powered by renewables. Kolbe electrolysis then takes place in the hybrid electrolyzer and generates liquid products (n-octane) anodically and hydrogen cathodically. The resulted efficiencies of the process steps and the overall efficiency are shown (CE = Carbon efficiency, FE = Faradaic efficiency). Reproduced with permission [82], Copyright 2022, Wiley Publishing Group.
Materials 16 02500 g004
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Figure 5. (a,b) Transmission Electron Microscope (TEM) and high-resolution TEM images of the catalyst. (c) High−Angle Annular Dark Field−Scan Transmission Electron Microscope (HAAD−STEM) image and Energy Dispersive X−ray Spectroscopy (EDX) element mapping. (d) Linear sweep voltammetry curves and (e) FE and production rate under different current density of the targeted propionitrile. (f) X−ray Photoelectron Spectra (XPS) S 2p spectra of S−Ni(OH)2 and (g) in situ Raman spectra of S−Ni(OH)2. (h) Schematic illustration of manipulating the reactant interface toward efficient amine electrooxidation [84]. Copyright 2022, American Chemical Society (ACS) Publishing Group.
Figure 5. (a,b) Transmission Electron Microscope (TEM) and high-resolution TEM images of the catalyst. (c) High−Angle Annular Dark Field−Scan Transmission Electron Microscope (HAAD−STEM) image and Energy Dispersive X−ray Spectroscopy (EDX) element mapping. (d) Linear sweep voltammetry curves and (e) FE and production rate under different current density of the targeted propionitrile. (f) X−ray Photoelectron Spectra (XPS) S 2p spectra of S−Ni(OH)2 and (g) in situ Raman spectra of S−Ni(OH)2. (h) Schematic illustration of manipulating the reactant interface toward efficient amine electrooxidation [84]. Copyright 2022, American Chemical Society (ACS) Publishing Group.
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Figure 6. (a) Schematic illustration of ethylene electro−oxidation at planar interfaces. (b) Schematic illustration of the electrochemical system of ethylene−to–ethylene oxide. (c) 13C NMR spectra of ethylene oxide and ethylene chlorohydrin. (d) FE at various current densities. (e) FE of propylene chlorohydrin and propylene oxide at various current densities. (f) Half-cell potential and FE of ethylene oxide at 300 mA/cm2 (the squares refer to the FE data obtained at different time). Reproduced with permission [95], Copyright 2021, American Association for the Advancement of Science Publishing Group.
Figure 6. (a) Schematic illustration of ethylene electro−oxidation at planar interfaces. (b) Schematic illustration of the electrochemical system of ethylene−to–ethylene oxide. (c) 13C NMR spectra of ethylene oxide and ethylene chlorohydrin. (d) FE at various current densities. (e) FE of propylene chlorohydrin and propylene oxide at various current densities. (f) Half-cell potential and FE of ethylene oxide at 300 mA/cm2 (the squares refer to the FE data obtained at different time). Reproduced with permission [95], Copyright 2021, American Association for the Advancement of Science Publishing Group.
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Figure 7. Electrocatalytic performances towards the CO2RR to HCOO− on tin (a) and to C2H4 (b) and C2H5OH (c) on copper, coupled with oxygen evolution or glycerol electrooxidation at the anode, as a function of the applied cell potential for CO2RR on 1  ±  0.1 mg cm−2 tin or copper nanoparticle−coated gas diffusion layer cathodes. Reproduced with permission [29], Copyright 2019, Nature Publishing Group. (d) Scheme of hybrid electrolyzer of the OUR integrating with the CO2RR and (e) two−electrode cell energy efficiency and FE plotted as a function of the cell potential (Vcell). Reproduced with permission [98], Copyright 2020, Wiley Publishing Group. (f) LSV Polarization curves of CuONS/CF in 1.0 M KOH with and without methanol at 5 mV s−1 and (g) LSV Polarization curves of mSnO2/CC in 1.0 M KHCO3 solutions. (h) Cathodic chamber: KHCO3 solution saturated with CO2. Anodic chamber: 1.0 M KOH with and without 1.0 M methanol; reproduced with permission [99], Copyright 2020, Nature Publishing Group.
Figure 7. Electrocatalytic performances towards the CO2RR to HCOO− on tin (a) and to C2H4 (b) and C2H5OH (c) on copper, coupled with oxygen evolution or glycerol electrooxidation at the anode, as a function of the applied cell potential for CO2RR on 1  ±  0.1 mg cm−2 tin or copper nanoparticle−coated gas diffusion layer cathodes. Reproduced with permission [29], Copyright 2019, Nature Publishing Group. (d) Scheme of hybrid electrolyzer of the OUR integrating with the CO2RR and (e) two−electrode cell energy efficiency and FE plotted as a function of the cell potential (Vcell). Reproduced with permission [98], Copyright 2020, Wiley Publishing Group. (f) LSV Polarization curves of CuONS/CF in 1.0 M KOH with and without methanol at 5 mV s−1 and (g) LSV Polarization curves of mSnO2/CC in 1.0 M KHCO3 solutions. (h) Cathodic chamber: KHCO3 solution saturated with CO2. Anodic chamber: 1.0 M KOH with and without 1.0 M methanol; reproduced with permission [99], Copyright 2020, Nature Publishing Group.
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Figure 8. (a) Surface photovoltage (SPV) image of H−WO3/TiO2 and (b) the representative cross−sectional SPV profiles associated with the dashed line in panel (a). (c) The representative cross−sectional SPV profiles of a defect−free WO3/TiO2 heterostructure. Spatially resolved PL images of a defective (d) and defect-free (e) WO3/TiO2 heterostructure. (f) Schematic diagram of charge separation across the defective WO3/TiO2 interface. (g) Selectivity and (h) FE of pl−H−WO3 and m−H−WO3/TiO2 for the production of GLAD + DHA. (i) KA oil generation in the PEC oxidation of cyclohexane. (j) S2O82− generation in the PEC oxidation of HSO4−. (k) Schematic diagram of the photo−electrochemical cell with microfluidic photoanode and ORR cathode. (l) Chopped J–V curve of the system. The output power density is also displayed and calculated. (m) Chopped photocurrent−time profiles of the photo-electrochemical system at short-circuit. (n) Photographs displaying an electronic timer (left) and an electronic calculator (right) powered by two of the photo−electrochemical cells connected in series. Reproduced with permission [106], Copyright 2021, Elsevier Publishing Group.
Figure 8. (a) Surface photovoltage (SPV) image of H−WO3/TiO2 and (b) the representative cross−sectional SPV profiles associated with the dashed line in panel (a). (c) The representative cross−sectional SPV profiles of a defect−free WO3/TiO2 heterostructure. Spatially resolved PL images of a defective (d) and defect-free (e) WO3/TiO2 heterostructure. (f) Schematic diagram of charge separation across the defective WO3/TiO2 interface. (g) Selectivity and (h) FE of pl−H−WO3 and m−H−WO3/TiO2 for the production of GLAD + DHA. (i) KA oil generation in the PEC oxidation of cyclohexane. (j) S2O82− generation in the PEC oxidation of HSO4−. (k) Schematic diagram of the photo−electrochemical cell with microfluidic photoanode and ORR cathode. (l) Chopped J–V curve of the system. The output power density is also displayed and calculated. (m) Chopped photocurrent−time profiles of the photo-electrochemical system at short-circuit. (n) Photographs displaying an electronic timer (left) and an electronic calculator (right) powered by two of the photo−electrochemical cells connected in series. Reproduced with permission [106], Copyright 2021, Elsevier Publishing Group.
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Figure 9. (a) CV and (b) LSV curves to characterize glycerol electrooxidation and ORR activity, respectively. (c) In situ FT−IR spectra of GOR. (d) Photograph displaying the VOCP value measured with a multimeter. (e) VOCP values of the DGFCs evaluated with a battery testing system. (f) Discharging curves of DGFCs at a constant current of 500 μA. (g) Polarization and power density curves of the cell. (h) Photograph showing the timer powered by a single DGFC. (i) Schematic diagram of energy flow and mass transportation. Reproduced with permission [32], Copyright 2022, RSC Publishing Group.
Figure 9. (a) CV and (b) LSV curves to characterize glycerol electrooxidation and ORR activity, respectively. (c) In situ FT−IR spectra of GOR. (d) Photograph displaying the VOCP value measured with a multimeter. (e) VOCP values of the DGFCs evaluated with a battery testing system. (f) Discharging curves of DGFCs at a constant current of 500 μA. (g) Polarization and power density curves of the cell. (h) Photograph showing the timer powered by a single DGFC. (i) Schematic diagram of energy flow and mass transportation. Reproduced with permission [32], Copyright 2022, RSC Publishing Group.
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MDPI and ACS Style

Ma, J.; Chen, K.; Wang, J.; Huang, L.; Dang, C.; Gu, L.; Cao, X. Killing Two Birds with One Stone: Upgrading Organic Compounds via Electrooxidation in Electricity-Input Mode and Electricity-Output Mode. Materials 2023, 16, 2500. https://doi.org/10.3390/ma16062500

AMA Style

Ma J, Chen K, Wang J, Huang L, Dang C, Gu L, Cao X. Killing Two Birds with One Stone: Upgrading Organic Compounds via Electrooxidation in Electricity-Input Mode and Electricity-Output Mode. Materials. 2023; 16(6):2500. https://doi.org/10.3390/ma16062500

Chicago/Turabian Style

Ma, Jiamin, Keyu Chen, Jigang Wang, Lin Huang, Chenyang Dang, Li Gu, and Xuebo Cao. 2023. "Killing Two Birds with One Stone: Upgrading Organic Compounds via Electrooxidation in Electricity-Input Mode and Electricity-Output Mode" Materials 16, no. 6: 2500. https://doi.org/10.3390/ma16062500

APA Style

Ma, J., Chen, K., Wang, J., Huang, L., Dang, C., Gu, L., & Cao, X. (2023). Killing Two Birds with One Stone: Upgrading Organic Compounds via Electrooxidation in Electricity-Input Mode and Electricity-Output Mode. Materials, 16(6), 2500. https://doi.org/10.3390/ma16062500

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