Open AccessRenewablesREVIEWS1 May 2024

Glycerol Electrooxidation to Value-Added C₁–C₃ Chemicals: Mechanism Analyses, Influencing Factors, Catalytic Regulation, and Paired Valorization

Chaogang Zhou

College of Metallurgy and Energy, North China University of Science and Technology, Tangshan 063210

Hebei Iron and Steel Laboratory, Tangshan 063210

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Jiajun Chen

College of Metallurgy and Energy, North China University of Science and Technology, Tangshan 063210

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Jingjing Zhao

School of Pharmacy, North China University of Science and Technology, Tangshan 063210

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Yuxuan Meng

Hebei Provincial Laboratory of Inorganic Nonmetallic Materials, College of Materials Science and Engineering, North China University of Science and Technology, Tangshan 063210

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Zhenhua Li

State Key Laboratory of Chemical Resource Engineering, College of Chemistry, Beijing University of Chemical Technology, Beijing 100029

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Xianguang Meng

*Corresponding authors:

E-mail Address: [email protected]

Hebei Iron and Steel Laboratory, Tangshan 063210

Hebei Provincial Laboratory of Inorganic Nonmetallic Materials, College of Materials Science and Engineering, North China University of Science and Technology, Tangshan 063210

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Jiacheng Wang

*Corresponding authors:

E-mail Address: [email protected]

Institute of Electrochemistry, School of Materials Science and Engineering, Taizhou University, Taizhou 318000

State Key Laboratory of High-Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899

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https://doi.org/10.31635/renewables.024.202400052

Glycerol, as a byproduct of biodiesel production, can be used to produce a variety of high-value C₁, C₂, and C₃ chemicals by electrocatalytic glycerol oxidation reaction (EGOR). Further coupling EGOR with CO₂ reduction reaction (CO₂RR) or hydrogen evolution reaction (HER) in paired electrolyzers is increasingly attractive due to the reduced input energy for the simultaneous formation of value-added products on both sides of the cell. This review article introduces the main reaction path of EGOR and the influencing factors of the reaction conditions of EGOR. The catalysts for the highly selective formation of glyceric acid, lactic acid, tartaric acid (TA), or formic acid (FA) from EGOR are highlighted. The latest research progress on design strategies of catalysts required for these reactions was reviewed. Subsequently, the paired electrolyzers coupling EGOR with HER or electrocatalytic CO₂RR were evaluated. Finally, the challenges and prospects in the field of EGOR are pointed out to move forward with the future development of glycerol electrocatalysis.

Introduction

Promoting the development of renewable energy has become the current global consensus.¹ As a clean energy substitute for fossil fuels, biodiesel has the characteristics of renewable and biodegradable, and its output has increased year by year.¹^–³ Biodiesel is a mixture of monoalkyl esters extracted from natural resources and is most commonly produced by the base-catalyzed or acid-catalyzed transesterification reaction of vegetable oils and fats (such as triglycerides) with alcohols (such as methanol) to produce fatty acid alkyl esters (biodiesel) and glycerol, as shown in Figure 1. Glycerol is the main by-product; usually, 110 kg is produced for every ton of biodiesel produced.¹^,⁴^,⁵ Since the purity of the crude glycerol is only 40–88%, it must be purified and optimized before it can be used. However, the cost of purifying glycerol is too high, and many biodiesel plants even burn glycerol directly as waste.¹^,⁶^,⁷ Therefore, it is vitally important to explore the new process of glycerol utilization and to convert glycerol into high-value-added chemical products.

Figure 1 | Catalytic transesterification reaction for the conversion of triacylglycerol and methanol to glycerol and biodiesel.

Glycerol is a highly functional molecule, which has three hydroxyl functional groups (–OH) and relatively strong reducibility. As shown in Figure 2, under the action of a catalyst, its symmetrical primary and secondary hydroxyl groups are prone to undergo various reactions, including catalysis, hydrogenolysis, hydrogenation, dehydration, esterification, etherification, polymerization, carbonylation, and so on.²^,⁶^–¹⁰ Among these glycerol conversion pathways, glycerol catalytic oxidation is one of the current promising pathways that can efficiently produce high-value-added products. Glycerol catalytic oxidation includes electrocatalysis, thermal catalysis, and photocatalysis. Oxidation experiments on Pt/C catalysts have shown that the yield of glyceraldehyde (GLYD), glyceric acid (GLAC) and hydroxypyruvate in electrocatalytic glycerol oxidation reaction (EGOR) is 1.7 times higher than that of thermal catalytic glycerol oxidation.¹¹ Moreover, the electrocatalytic process is relatively gentle with less energy consumption.¹² Also, it does not need the addition of chemical oxidants, thereby reducing environmental pollution.¹³ EGOR can obtain various C2 and C3 high-value chemicals such as tartaric acid (TA), dihydroxyacetone (DHA), GLAC, GLYD, lactic acid (LA), glycolic acid (GLYCA), oxalic acid (OA), and so on.¹⁴^–¹⁶ In addition, the C–C bond can be cleaved twice to generate C₁ products. These products are usually used in industrial fields such as sewage treatment, food additives, detergents, medicine, and cosmetics.² Their economic value is much higher than that of glycerol. Therefore, EGOR plays a very significant role in the development of the industrial chain of glycerol.

Figure 2 | All value-added products, formed by the valorization of glycerol *via* different reaction pathways. It is to be noted that glycerol-derived products show great potential in different fields.

Herein, the authors analyzed the influence of external conditions such as glycerol concentration, temperature, pH value, and applied voltage on EGOR, and reviewed the research progress of highly active transition metal-based and precious metal-based catalysts used for EGOR in recent years. Then the adsorption process and interaction relationship between the functional groups of the reactants and intermediate products and the catalyst were analyzed, all of which provided a reference for the subsequent efficient preparation of the catalyst. In addition, the influence of the structure, composition, as well as the morphology of the electrodes and catalysts of the electrolysis reactor on the catalytic activity, stability, and consumption of the reaction energy was introduced. These factors provided a reference for the subsequent efficient production of EGOR combined with hydrogen evolution reaction (HER) or electrochemical CO₂ reduction (ECR). Finally, the challenges faced by EGOR technology and the difficulties encountered in the industrial application were analyzed, and the catalytic design strategy and development of the glycerol oxidation industrial chain prospected.

Parameters of EGOR

Overpotential (η), conversion ratio (α), charge transfer resistance (R_ct), Tafel slope, electrochemical active surface area (ECSA), selectivity, Faradaic efficiency (FE), and stability are important parameters to study and evaluate the performance of electrocatalysts.¹⁷^–²³ η refers to the difference between the equilibrium potential and the actual potential when the current density reaches a specified level, which directly reflects the activity of the electrocatalytic or photo electrocatalytic reaction. The closer it is to 0 V, the better the performance of the catalyst, and the lower the required external voltage to achieve the same current density, meaning lower consumption of energy. The Tafel diagram shows the relationship between η and current density i. The smaller the Tafel slope, the faster the current density increases, indicating that the catalyst has better catalytic performance, faster kinetics, and lower energy consumption. Charge transfer resistance (R_ct) is an important parameter when performing electrochemical impedance spectroscopy characterization, which reflects the resistance of the charge transfer between reactants. The magnitude of R_ct depends on the density of active sites of the catalyst surface, the diffusion efficiency of reactants, the external environment, and other factors. ECSA is the effective region involved in the electrochemical reaction, which is a significant indicator for judging the changing characteristics of the electrode reaction.

η = E_{i} - E_{t}

(1)

η = a + b \cdot \log (i)

(2)

ECSA = C_{dl} / C_{s}

(3)

In the above formula, E_i is the actual potential and E_t is the theoretical potential; a and b are constants, where b is called the Tafel slope; C_dl is the double layer capacitance, which can be obtained by electrochemical impedance spectroscopy (EIS); C_s represents the specific capacitance of the smooth surface sample under the same conditions, which can be obtained by reviewing the literature. The conversion (%), selectivity (%), and FE (%) of EGOR can be determined by the following formula, respectively:

α (%) = \frac{N_{0, glycerol} - N_{glycerol}}{N_{0, glycerol}} \times 100 %

(4)

Selectivity (%) = \frac{N_{product}}{n \times (N_{0, glycerol} - N_{glycerol})} \times 100 %

(5)

FE (%) = \frac{N_{product} \times z \times F}{Q_{total}} \times 100 %

(6)

where the value of n is determined by the product type, and for C₃, C₂, and C₁ compounds, n is 1, 3/2, and 3, respectively; Q_total is the total amount of charge that passes through the electrode, F is the Faradaic constant, and z is the number of electrons transferred.

Overview of EGOR Path

EGOR is a multielectron transfer process to form a variety of high-value products. EGOR proceeds in five steps: (1) diffusion of glycerol from the liquid phase to the catalyst surface; (2) adsorption of glycerol on the catalyst surface; (3) catalytic oxidation of glycerol by active oxygen; (4) product detachment from the catalyst surface; (5) product diffusion to the liquid phase. Various intermediate products and reaction pathways during the oxidation of glycerol are shown in Figure 3. The catalyst could directly affect the pathway and selectivity of the EGOR product. All the design and control strategies for catalysts mainly aimed to reduce cost, improve the inherent activity of each active center, and increase the number of catalyst active centers.²⁴^,²⁵ In addition, during the oxidation of glycerol, some products or intermediates (such as CO) could be strongly adsorbed on the surface of the catalyst, hindering the adsorption and deprotonation process of glycerol molecules on the catalyst surface. As a result, the catalytic activity is significantly reduced.²⁶^,²⁷

Figure 3 | Overview of main pathways leading to the formation of various products in EGOR.

The oxidation of glycerol to DHA is completed by oxidizing the secondary hydroxyl group, which requires the transfer of 2e⁻. DHA exists as a dimer structure, which is safe and biodegradable.²⁸ The primary hydroxyl group of glycerol could generate GLYD by oxidizing 2e⁻. During this process, introducing oxygen and adding an appropriate amount of alkali can reduce the breakage of C–C bonds and increase the reaction rate.²⁵^,²⁹ GLYD and DHA can spontaneously isomerize with each other and further oxidize to form GLAC. Then the hydroxyl group at the third carbon atom of GLAC can be oxidized to produce TA. TA can effectively inhibit the conversion of carbohydrates into fat in the human body. Also, it could be used to lose weight and prevent coronary heart disease.³⁰^–³² GLYD or DHA could be converted to pyruvaldehyde by dehydration rearrangement and can be further converted to lactate (LA) through hydration reaction and disproportionation reaction.³³^–³⁵ LA is another high-value organic molecule, which is usually used in pharmaceuticals and food, and it is gaining attention as a raw for the biodegradable plastic polylactic acid (PLA).³⁶ Since LA tends to form under high-temperature conditions, we need to focus on developing highly active catalysts for converting glycerol into LA under a milder environment in the future.³⁷^,³⁸

The formation of C₂ products in EGOR undergoes a C–C bond fracture process. For example, a C–C bond fracture in GLAC could lead to producing GLYCA, glyoxylic acid, acetic acid, hydroxypyruvate, and other compounds.³⁹^,⁴⁰ C₃ compounds could break two C–C bonds, and C₂ compounds could break one C–C bond to form C₁ compounds such as FA, carbon monoxide (CO), and so on. FA is a raw material usually used in chemical industry, textile, and pharmaceuticals. Especially, FA has a high hydrogen storage capacity (53 g_H2L⁻¹) and can be decomposed under a mild environment condition to produce H₂. Thus, it is usually used as a fuel for direct FA fuel cells.⁴¹^–⁴⁴ CO₂ is the C₁ product of the complete oxidation of glycerol, but the reaction has a high energy barrier and slow kinetic process.²⁶^,⁴⁵

Influence of Reaction Conditions on EGOR

Temperature, pH value of electrolyte, concentration of glycerol electrolyte, and applied voltage are important variables that affect the choice of EGOR paths and product types.⁴⁶^,⁴⁷ Within a certain range, the activity of EGOR increases with increasing glycerol concentration. Once the concentration exceeds this range, the electrolyte viscosity increases accordingly, hindering the effective movement of ions onto the surface of the catalyst. Specifically, the current density begins to decrease after reaching saturation. Related studies have shown that the spatial effect has a significant impact on the relative adsorption affinity of secondary and primary hydroxyl groups.¹⁷ When the glycerol concentration is high, the steric hindrance of primary hydroxyl groups is usually less than that of secondary hydroxyl groups because of the limited number of accessible adsorption sites. Thus, it has a greater chance of being adsorbed on the electrocatalyst surface and oxidized to generate aldehyde groups.⁴⁸

Generally speaking, the conductivity of the solution increases with the increase in temperature, and thus, the overall resistance of the system could be reduced.⁴⁹ With the increase in temperature, the activation energy required for EGOR could be decreased, thus obtaining higher reaction activity and reducing the energy barrier. When the temperature is too high, the conversion mechanism and product distribution of EGOR could be changed, showing no longer activity increase or even decrease.¹⁷^,¹⁸ For example, in the range of 20–40 °C, the activity of the Cu₂O catalyst could increase with increasing temperature, yielding mainly DHA, GLYD, and GLAC as products. When the temperature reaches or exceeds 50 °C, GLAC is the main product.¹⁷ Although within a certain temperature range, the reaction rate could be accelerated with increased temperature, applied potentials also affect the reaction kinetics significantly. When Cu₂O catalyst was used in EGOR, the transformation processes from GLYD to GLAC and from GLAC to TA were significantly accelerated when the potential was increased from 1.8 V versus reversible hydrogen electrode (vs RHE) to 2.0 V (vs RHE). TA with two adjacent carboxyl groups was relatively stable and was no longer oxidized. But when the voltage increase was continued to 2.2 V (vs RHE), the energy was enough to overcome the reaction barrier, which caused TA to further break the C–C bond and generate a new product, GLYCA.¹⁷ Therefore, it was concluded that the changes in potential could affect the glycerol oxidation pathway.

EGOR is hypersensitive to the pH value. Regarding common noble metal-based catalysts, often higher current densities are observed under alkaline conditions while keeping the glycerol concentration and voltage unchanged. As the pH value increases, the catalytic activity is also enhanced. Lowering the pH value could significantly inhibit the glycerol oxidation process on Pt and Au catalysts.⁴⁶ In addition, the selectivity of the catalyst and the reaction path also fluctuate with changes in pH value.¹⁹^,²² When studying the impact mechanism of Bi atom doping on Pt catalysts, it was found that glycerol has different reaction pathways in acidic and alkaline environments. In an alkaline environment, the adsorption of Bi atoms can inhibit the C–C bond cleavage of glycerol on the surface of the Pt catalyst, hinder the generation of CO, increase the yield of the main product GLAC, and enhance the activity by about 5 times. However, the addition of Bi in acidic media could hinder the oxidation pathway of primary carbons of glycerol, thereby inhibiting the oxidation process.¹⁹ Since acidic conditions are more common in these fields, including industrial electrocatalytic hydrogen production, the development of acid-resistant and highly active catalysts is of great significance.⁵⁰^,⁵¹ MnO₂ has been reported to be a highly stable EGOR catalyst, which has high activity in acidic media, but it is a metallic oxide that is easily soluble in acidic media.⁵² Density functional theory (DFT) calculations and in situ Raman spectroscopy analysis indicated that glycerol suppressed the lattice oxygen mechanism, which could damage the catalyst structure. Also, the formation of MnO₄⁻ was suppressed, which avoided the deactivation of the MnO₂/CP catalyst in an acidic environment. This work provided a reference for the subsequent design of acid-resistant EGOR catalysts with high selectivity and high activity.

Selectivity of EGOR

The selective oxidation of primary and secondary hydroxyl groups and the control of the oxidation depth of oxygen-containing functional groups play an important role in the path selection of the highly selective oxidation of glycerol.⁵³^,⁵⁴ Pt-based, Au-based, and Pd-based precious metal catalysts are considered to be highly active due to their lower d-band center (−2.25 eV) and are conducive to deprotonation. Different catalysts provide different active sites for glycerol or intermediates in the reaction process, and the interaction is also very different, so the electrocatalysis process will be accompanied by the formation of a variety of by-products, which brings problems to detection, separation, and purification.²⁴^,⁵⁵ Designing the reasonable composition and structure of the catalyst and analyzing its catalytic reaction mechanism and conversion path have important significance to achieving efficient utilization of glycerol. There are two main methods for adjusting catalysts: (1) Increasing the number of active sites per unit area and enhancing intrinsic activity; specifically, active sites can be increased by adjusting the morphology of the catalyst. (2) The Fermi level can be changed by doping atomic or other means and the adsorption process can be further adjusted, thereby increasing its intrinsic activity.

Production of DHA from EGOR

DHA is highly functionalized and active which has one carbonyl group and two hydroxyl groups. It can participate in various chemical reactions such as the Mannich and Maillard reactions. Also, it could serve as a raw material for the synthesis of macromolecular compounds, usually used in cosmetics, medicine, health products, and other fields.⁵⁶ The glycerol catalytic oxidation method is mainly used in the current industry to produce DHA. A large number of reports indicate that Pt-based catalysts are more likely to preferentially oxidize the primary hydroxyl groups of glycerol to produce GLYD, which is further oxidized to obtain GLAC and C₁ and C₂ products.⁵⁷^,⁵⁸ It is not easy to achieve precise oxidation of glycerol secondary hydroxyl groups by catalysts and generate DHA. Moreover, precious metal-based catalysts usually have better catalytic activity under alkaline media, but DHA is not stable under alkaline conditions, making it difficult to have high catalytic activity and high DHA selectivity at the same time.⁵⁹^,⁶⁰ Using noble metal-free cobalt oxide (CoO_x) catalyst for EGOR, at a current density greater than 3 mA cm⁻², the yield rate of DHA could reach 9.6 μmol h⁻¹ cm⁻² with a selectivity of 45%, and no oxygen evolution reaction (OER) occurred.²⁸ Furthermore, Operando Raman spectroscopy was used to characterize the potential-induced structural shifts between oxyhydroxides and cobalt oxides (CoO_x; Figure 4a–d), so that the relationship among surface structure of electrocatalyst, applied potential, and product distribution can be established. The preliminary mechanism for the formation of C₃ products by EGOR in mild alkaline media is shown in Figure 4e. CoO_x is the first electrochemically oxidized to amorphous (hydroxyl) hydroxide, which acts as an active species for the oxidation of glycerol or its intermediates. Glycerol is oxidized by breaking C–H bonds to form DHA and GLYD, part of which can be further oxidized to form GLAC, and finally oxidized to form FA. After statistical research, DHA and GLYD account for the largest proportions of the product. This work explores the structural evolution and the origin of the activity of non-noble metal catalysts and has a certain reference for subsequent research.

Figure 4 | (a) Operando Raman measurement setup; (b–d) on the left is the time-varying operando Raman spectra; on the right is Lorentz fitting of the Raman spectra in fixed regions; (e) EGOR pathway of CoO_x electrocatalyst in mild alkaline medium. Reproduced with permission from ref 28. Copyright © 2021 Elsevier B.V.

Production of GLAC from EGOR

The GLAC molecule contains two hydroxyl groups and a carboxyl functional group, which can further undergo a series of reactions such as esterification, acetal, and dehydration. It has extremely broad application prospects in the chemical industry. Moreover, it is an important raw material and intermediate in polymer polymerization and condensation reactions. Through polymerization, oxidation, and amination, a battery of high-value products such as malonate, pyruvic acid, and amino acids can be further obtained. The study on the adsorption behavior and oxidation mechanism of glycerol on single crystal platinum catalyst in an alkaline medium confirmed the sensitivity of EGOR to the atomic morphology of Pt. Compared with Pt(110) and Pt(111), Pt(100) has good catalytic activity, which was not easily poisoned after multiple cycle scans (Figure 5a–c). Fourier transform infrared (FTIR) shows that there are two broad bands centered on 1400 and 1600 cm⁻¹ on the surface of three crystals (Figure 5d–f), which indicates that the mechanism of EGOR is to generate alkoxides through glycerol dehydrogenation and then form aldehyde intermediates. In the oxidation process, the arrangement of surface atoms directly leads to different selectivity, and the Pt(100) surface is more inclined to selectively produce GLAC.⁶¹^–⁶³

Figure 5 | (a) Volt-ampere curve at Pt(111). Reproduced with permission from ref 61. Copyright © 2017 Elsevier B.V. Voltammetric profiles of EGOR on (b) Pt(110) and (c) Pt(100). Reproduced with permission from ref 62. Copyright © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (d) In situ FTIR spectra at Pt(111). Reproduced with permission from ref 61. Copyright © 2017 Elsevier B.V. In situ FTIR spectra at (e) Pt(110) and (f) Pt(100). Reproduced with permission from ref 62. Copyright © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. FTIR, Fourier-transform infrared spectroscopy.

In addition to the adjustment from the metal aspect, the appearance of the catalyst carrier also makes a great difference in the catalytic activity.⁵³^,⁶⁴^,⁶⁵ For example, as shown in Figure 6b,c, there are structural differences between (101) and (001) faces of TiO₂ and there FTIR spectra is shown in Figure 6a. By regulating the ratio of (001) crystal face and (101) crystal face of the TiO₂ support in the bimetallic Au₁Pt₃ catalyst, the O_2c-Ti_5c-O_2c coordination unsaturated dominant active site matching the reaction molecular orbital was selectively exposed, and the adsorption mode of intermediate products was changed, so that the C=O bond in GLYD was adsorbed on the carrier surface in the form of a bidentate species (Figure 6d,e). It promoted the deep oxidation of the aldehyde group to carboxylic acid, significantly improving the selectivity of GLAC.⁵³ The current catalysts used for GLAC production through EGOR are still mainly precious metals, but recent research by Choi and coworkers⁶⁶ has proven that Mn²⁺ in MnSb₂O₆ can significantly inhibit C–C cracking. The relative selectivity of GLAC reaches an unprecedented 82%.⁶⁶ The mechanism can be further studied in the future.

Figure 6 | (a) In situ CO₂ FTIR spectra of TiO₂–101, TiO₂–101–001, TiO₂–001; (b) Structure models (101) plane of anatase TiO₂ and illustration of monodentate carbonate on (101) plane; (c) Structure models of (001) plane of anatase TiO₂ and illustration of bidentate carbonate on (001) plane; Illustration of GLYD isomerization and oxidation and corresponding catalysts terminated with (101) plane (d) and (001) plane (e). Reproduced with permission from ref 53. Copyright © 2022 American Chemical Society. FTIR, Fourier-transform infrared spectroscopy; GLYD, glyceraldehyde.

Production of TA from EGOR

TA has extremely high economic value and is an important pharmaceutical raw material and important food additive raw material. It can make food sour, and its sourness is better than malic acid, LA, and so on. It is also an important pharmaceutical raw material.⁶^,³⁰ It was found that the electron interaction between the transition metal-nitrogen-carbon (M-N/C) carrier and Pt is helpful in enhancing the catalytic activity of Pt by enhancing the electron transfer kinetics.⁶⁷ Chu and coworkers¹⁶ prepared Pt nanoparticles decorated, Fe- and N co-doped carbon nanotube (Pt/FeNC) catalyst for the highly selective EGOR to TA (Figure 7a,b). Valence band analysis and kinetic experiments indicate that the electronic structure of Pt could be adjusted by the FeNC carrier. The d-band center moves upward, and the adsorption of glycerol and GLAC on Pt is observably enhanced. This enhances the electrocatalytic activity of Pt for EGOR and accelerates the conversion of glycerol to GLAC and from GLAC to TA. In addition, the plentiful Fe-N_x species in the FeNC carrier can be used as anchoring sites of Pt particles, which can increase the number of active sites and availably avoid the excessive aggregation of Pt particles at a certain point to ensure the catalyst maintains high stability. In a 5.0 mol/L high-concentration glycerol solution, the Pt/FeNC catalyst achieved 94% conversion and over 47% tartrate yield under 0.9 V versus RHE conditions, which amounted to a TA yield of 1182 mmol/h/g Pt.¹⁶

Figure 7 | Reaction mechanism of TA formation from glycerate over Pt/CNT (a) and Pt/FeNC (b). Reproduced with permission from ref 16. Copyright © 2022 Elsevier B.V. TA, tartaric acid; CNT, carbon nanotube.

In addition to the method of increasing the active site of the catalyst, the addition of other heteroatoms to the Pt catalyst can activate the low coordination Pt atoms to enhance the activity of the catalyst. At the same time, it can also achieve the purpose of adjusting the glycerol oxidation pathway and improving the selectivity of the product. For example, in an alkaline environment, adding Bi and Pb to a polycrystalline platinum (Pt_p) catalyst can increase the catalytic activity of polycrystalline platinum (Pt_p) by ∼5 times.⁶⁸ The adatoms can inhibit the pathway of complete C–C bond fracture (producing carbonate) (Figure 8), and increase the yield of GLAC, TA, and FA. This is due to the fact that the catalyst surface modification reduces the serviceable area for the formation of multi-bond intermediates, changes the electron configuration of the surface of the catalyst, and produces positively charged substances that can act as stable reaction intermediates. Wang and coworkers⁶⁹ loaded Ag onto a porous Au catalyst and found that while maintaining high electrocatalytic activity, it could also adjust the reaction pathway and improve the selectivity of substances such as formate and glycolate. The mechanism of forming GLAC, LA, and TA in EGOR still needs further clarification. The reaction mechanism can be further explored through in situ characterization combined with theoretical calculations.

Figure 8 | Reaction pathways of the bare P_tp electrode, and after modification by Bi and Pb. (Black arrows indicate the pathway for the clean P_tp, and the green and gray arrows represent the pathways for the modified electrode.) Reproduced with permission from ref 68. Copyright © 2020 American Chemical Society.

Production of FA from EGOR

Among various products generated in EGOR, almost all intermediates could be eventually oxidized to FA.²⁷^,⁴³^,⁴⁴^,⁷⁰ According to relevant data, FA and formate ions (HCOO⁻) are very easily degraded by microorganisms, causing minimal pollution to the environment.⁴⁵^,⁷¹^–⁷³ With the development of the FA fuel cell industry, the global demand for FA could increase dramatically. The traditional industrial production of FA or formate is mainly through the multistep reaction process of methanol and CO, which needs a good deal of energy consumption and costs.⁴¹^,⁷⁴^–⁷⁶ Compared with this method, the production of 3 mol FA from 1 mol glycerol by EGOR is milder and environmentally friendly, which is in line with the research direction of green chemistry.

Various oxides or hydroxides of transition metals (Cu, Co, Fe, Ni, etc.) can promote the breakage of C–C bonds on the catalyst surface, thereby improving formate productivity.⁵³^,⁷⁷ Jin and coworkers⁷⁸ also confirmed this conclusion by testing batteries of nanostructured cobalt-based spinel oxide (MCo₂O₄, M=Mn, Fe, Co, Ni, Cu, Zn) catalysts. The catalytic activity of CuCo₂O₄ is the best in 0.1 mol/L KOH solution, then NiCo₂O₄, CoCo₂O₄, FeCo₂O₄, ZnCo₂O₄, MnCo₂O₄, respectively. The FA selectivity of up to 80.6% was achieved by EGOR with the CuCo₂O₄ catalyst directly integrated into the carbon fiber paper electrode. The total functional electrical stimulation (FES) of all value-added products was 89.1%.⁷⁸ Chen and coworkers⁷⁹ found that doping Co in Ni hydroxide could promote the removal of protons and oxygen anions from the catalyst surface by combining spectral analysis with DFT calculation. The formation of oxygen vacancies in NiCoOH during EGOR increased the d-band filling at the Co site and promoted the charge transfer from the surface of the catalyst to the lysed molecule during the second C–C bond cleavage. Figure 9 shows that NiCo hydroxide has enhanced the activity of EGOR. With the current density of 100 mA/cm² and applied voltage of 1.35 V, the FA selectivity could reach 94.3% (Figure 9c).⁷⁹

Figure 9 | (a) Effect of glycerol on cyclic voltammetry curves; (b) Tafel curves; (c) Selectivity of different products; (d) In situ infrared spectra; (e) The Gibbs free energy profiles of EGOR. Reproduced with permission from ref 79. Copyright © 2022 Nature Communications. EGOR, glycerol oxidation reaction.

The detailed reaction pathway was determined by DFT calculation. Figure 10a shows that the oxygen anion and proton (de)intercalation in hydroxides actively participated in the basic reaction steps. These include electrochemically driven proton delamination from the electrocatalyst lattice (IS→0) and oxygen-anion delamination during reaction product desorption. The desorption of the product is accompanied by the desorption of oxygen anions in the hydroxide lattice, that is, the formation of oxygen vacancies. Taking the desorption of FA after the first C–C cleavage (7A→7B) as an example, they found a strong correlation between the energy barrier of the desorption step and the E_{f_vac} value (Figure 10c). Co hydroxide has the lowest oxygen anion removal energy, and its products have the lowest potential barrier for desorption from the surface of the catalyst. Also, the products easily leave the surface rather than continue to oxidize, resulting in poor selectivity in the reaction of Co hydroxide to FA (Figure 10c). Ni and NiCo hydroxide have higher oxygen anion deintercalation energy, which is beneficial for further oxidation of glycerol to FA. Figure 10b shows the free energy barrier for the second C–C bond breakage and metal site d-band filling after oxygen anion deintercalation. The results showed that as more electrons were collected on the metal sites, the second C–C bond breakage was mightily promoted, and the reaction barrier was lowered. It was found that NiCo hydroxide has the highest d-band filling at the Ni and Co sites, corresponding to its lowest second C–C bond-breaking barrier. The energy barrier of NiCo hydroxide was the lowest during the second C–C bond breakage process and it was the facilitation step for FA formation.

Figure 10 | (a) The pathway and theory of EGOR; (b) The free energy barrier (ΔG) of the second C–C bond breakage step and the d-band filling; (c) The ΔG of the desorption step and the energy at which an oxygen anion is removed from the hydroxide lattice. Reproduced with permission from ref 79. Copyright © 2022 Nature Communications. EGOR, glycerol oxidation reaction.

Coupling EGOR with Typical Cathodic Reactions

The ECR and electrocatalytic HER use electrons produced by electrolytic water as a direct reducing agent. Their anodic reaction is usually OER, which is a relatively complex 4e⁻ transfer process with slow kinetics and needs a very large overpotential to drive the electrolysis, leading to high energy consumption.⁸⁰^,⁸¹ Although the theoretical minimum voltage for electrolyzing water is 1.23 V (vs RHE), in actual applications, electrolyzers usually require a voltage higher than 1.80 V vs RHE to work effectively. Through suitable catalytic design and electrolysis device design, using EGOR instead of OER as an anode reaction can produce high value-added products on both sides of the electrolyzer with extremely low energy consumption.

Coupling EGOR with HER

Hydrogen is a nonpolluting and high-energy-density sustainable energy carrier and is regarded as the most promising alternative to fossil fuels.⁸²^–⁸⁴ At present, the most widely used hydrogen production processes are fossil fuel reforming and electrolytic water splitting.⁸⁵^–⁸⁷ The former could consume large amounts of fossil fuels and emits a lot of greenhouse gases.⁸⁸^,⁸⁹ Figure 11 summarizes the production route of EGOR-assisted water splitting for hydrogen production, which can realize the coproduction of FA and hydrogen. The production process could greatly reduce energy consumption and economic costs, which meet the requirements of green and sustainable developments.

Figure 11 | Comprehensive demonstration of glycerol oxidation-assisted electrolytic hydrogen production in a renewable electricity-powered electrolytic cell to obtain high-valued hydrogen, formic acid, and other products.

The energy consumption of H₂ production from the electrolysis of water is mainly concentrated in the anode OER, accounting for approximately 80% of the total energy consumption.⁹⁰ In this process, no matter what kind of exchange membrane is used to separate the two-compartment reactor, the low-value O₂ product generated by the anode could be inevitably mixed with hydrogen produced by the cathode. Thus, there is a risk of explosion.⁹¹^–⁹³ Moreover, H₂ and O₂ can easily form reactive oxygen species (ROS) such as O₂H and OH radicals under the combined action of catalysts, which can easily cause the membrane to be degraded, thereby reducing the life of the electrolyzer.⁸²^,⁹⁴ From the thermodynamic view, the potential required for the electrooxidation reaction of small organic molecules is often lower than OER, as shown in Figure 12b. Figure 12c outlines the advantages of glycerol oxidation in place of OER. Thus, using small molecules such as glycerol, urea, and ammonia as reducing agents to assist H₂ production is a valid strategy to solve the problems of energy consumption and stability.⁹⁶^–¹⁰⁰ Considering the high cost of reducing agents, it is more common to use EGOR as an anode semicell reaction by which the energy cost is reduced by ∼20% compared with single electrolyzed water.¹⁰¹^,¹⁰² As shown in Figure 12a, the introduction of small organic molecules such as glycerol is not only suitable for hydrogen production from electrolytic water but also EGOR replaces OER with slow kinetics in the process of nitrate reduction. This reduces energy consumption by ∼20% and simultaneously produces high-value-added ammonium and FA.⁹⁵

Figure 12 | (a) The electrochemical NO₃RR, coupled with the anodic OER or EGOR. Reproduced with permission from ref 95. Copyright © 2022 Royal Society of Chemistry. (b) LSV curves of HER || EGOR with 0.1 M glycerol and HER || OER without 0.1 mol/L glycerol. Reproduced with permission from ref 96. Copyright © 2022 Wiley-VCH GmbH. (c) The advantages of replacing OER by glycerol oxidation reaction. NO₃RR, nitrate reduction reaction; OER, oxygen evolution reaction; EGOR, glycerol oxidation reaction; HER, hydrogen evolution reaction; LSV, linear sweep voltammetry.

The cathodic HER reaction is a process involving double electron transfer with a catalytic intermediate H^* in a wide pH range. HER involves three electrochemical processes: first, proton (H^*) is adsorbed on the catalyst surface (Volmer step), and then the adsorbed hydrogen (H^*) is formed by hydrogen desorption process (Heyrovsky step) or chemical combination of two H^* (Tafel step) to form H₂.⁸¹^,¹⁰³^–¹⁰⁵ The rate of the entire reaction is largely determined by ΔG_H, which is the free energy of the catalyst for hydrogen adsorption. When the binding of hydrogen to the surface is too weak, the Volmer step is the limiting link; when the binding is too strong, the desorption step (Heyrovsky/Tafel) will be the limiting link. The ΔG_H of highly active HER catalysts towards reaction intermediates is close to 0.¹⁰⁶^,¹⁰⁷ Reasonable control of the binding energy of catalyst surface reaction intermediates is the key to the design of materials to improve performance. In combination with DFT calculation, previous studies have presented graphs of the relationship between ΔG_H and the exchange current density (i₀) of a large number of catalyst materials.¹⁰⁴^,¹⁰⁸^,¹⁰⁹ As can be seen from these volcanic maps, precious metals are mostly located near the top and have efficient electrocatalytic activity such as Pt and Rh, but their high cost hinders their commercial application as precious metal catalysts.¹⁰⁴^,¹¹⁰ Previous reports on Pt single atoms, atomic clusters, and nanoparticles (NP) in HER usually found ways to anchor noble metals on carriers to enhance electrochemical activity.¹¹¹^,¹¹² Intuitively, by reducing the particle size of Pt, the utilization efficiency of active sites can be improved and the load of Pt can be reduced. However, the structural characterization and electrochemical test results of amorphous and crystalline NiFe layered double hydroxides (LDH) catalyst showed that the crystallinity of the substrate significantly affected the size of anchored platinum NPs and the intrinsic activity of the catalyst (Figure 13).¹¹³ In the future design process of precious metal-based catalysts, not only the precious metal itself must be optimized but also the substrate must be reasonably adjusted.

Figure 13 | Study on the effect of crystallinity of substrate. Reproduced with permission from ref 113. Copyright © 2021 American Chemical Society.

The activity and selectivity of non-noble metal catalysts mainly depend on their composition, morphology, and electronic structure.⁶⁵^,¹¹⁴ Various transition metal nitrides, sulfides, phosphides, selenides, and their alloy exhibit high catalytic activity due to their inherent activity, tunable electronic structure, and high stability.¹¹⁵^–¹¹⁸ However, its electrical conductivity is poor, and it is usually compounded with a conductive substrate to reduce the impedance of the electron transmission path. This type of catalyst is also widely used in OER, HER, and oxygen reduction reactions. For example, in an alkaline medium, Ni-Mo-N nanoplates on carbon fiber cloth can be used in both EGOR and HER to produce high-purity H₂ and FA with Faradaic efficiencies of 99.7% and 95.0%, respectively. Compared with pure water electrocatalysis, the energy consumption of the reaction is reduced by >16%.¹¹⁹ Yu and coworkers¹⁰² synthesized the NiV LDH catalyst by hydrothermal method and further prepared the E-NiV LDH by electrooxidation process. Also, P-NiV LDH was obtained by using low-temperature N₂/H₂ radio frequency plasma to regulate the activation properties of the Ni site of the catalyst. The preparation method is shown in Figure 14a.¹⁰² The electrolytic cell assembled with E-NiV LDH and P-NiV LDH as the anode and cathode catalysts (Figure 14b), which simultaneously produced formic acid and hydrogen. The performance test results are shown in Figure 14c–f. It only requires 1.25 V to reach 10 mA·cm⁻² current density and is 320 mV lower than the overpotential of electrolytic water, and the energy consumption can be reduced by ∼30%.

Figure 14 | (a) A scheme to obtain H₂ and formate simultaneously; (b) Schematic diagram of P-NiV LDH and E-NiV LDH dual-electrode system; (c) Contrast of catalytic activity; (d) In situ FTIR spectra; (e) comparisons of Tafel slopes; (f) Changes in the content of glycerol and its products with time. Reproduced with permission from ref 102. Copyright © 2023 Springer Nature. P-NiV LDH/E-NiV LDH, photo/electrocatalytic nickel-vanadium layered double hydroxide; FTIR, Fourier-transform infrared spectroscopy.

Transition metal elements have abundant valence electrons and adjustable electronic structures, so they are widely used as doping elements in various electrocatalysts. The crystal structure, electronic structure, and the Fermi level of the host metal can be adjusted by doping the transition metals, so as to effectively optimize the physical, chemical, and electrical properties of the catalyst to improve the activity of the catalyst.¹²⁰^–¹²² For example, V doping has a regulating effect on the interface structure of the Pt/NiFeV catalyst, which can enable self-reduction of Pt⁴ and optimize the electronic interaction between Pt clusters and NiFeV LDH, accelerating the kinetics of HER and OER.¹²³ Zhang and coworkers¹⁰¹ prepared Mox-Ni₃Se₂/NF and NiSe-Ni₃Se₂/NF catalysts by doping Mo atoms in Ni₃Se₂ nanoparticle and synthesizing heterojunction, respectively (Figure 15a–d). This electrolyzer built with these catalysts only requires a very low voltage of 1.40 V to achieve the current density of 40 mA cm⁻², and the FES for FA production reached 97%.¹⁰¹ The DFT theoretical calculation indicated that due to the introduction of Mo (Figure 15e,f), the d-band center of Ni₃Se₂ deviated from the Fermi level, which weakened the adsorption capacity of hydrogen on its surface. Meanwhile, as shown in Figure 15g, the construction of heterogeneous interfaces resulted in the deformation of the surface structure of NiSe, thus exposing more highly active Ni sites for glycerol adsorption and obtaining outstanding point catalytic performance.

Figure 15 | (a) Synthesis of Mo_0.2-Ni₃Se₂/NF and NiSe-Ni₃Se₂/NF catalysts; (b) electrolytic cells producing formic acid and hydrogen; (c) LSV curves of Mo_0.2-Ni₃Se₂/NF || NiSe-Ni₃Se₂/NF; (d) the free energy of HER for Mo-Ni₃Se₂ and Ni₃Se₂; Top and side view of atomic structure models for (e) Ni₃Se₂ and (f) Mo_0.2-Ni₃Se₂; (g) the theoretical structure models of glycerol adsorption on NiSe-Ni₃Se₂ heterojunction. Reproduced with permission from ref 101. Copyright © 2023 Wiley-VCH GmbH. LSV, linear sweep voltammetry; HER, hydrogen evolution reaction.

Coupling EGOR with ECR

As a rich, cheap, and clean C₁ resource, CO₂ has the advantages of being nontoxic, stable, and renewable.⁹⁸^,¹²⁴^–¹²⁶ The main step of ECR is to use an external voltage to overcome the high redox potential of CO₂ to CO₂^*− (CO₂+ e⁻ = CO₂^*−).¹²⁷^–¹³¹ This process uses water molecules as electron donors. The electrons transfer to CO₂ molecules or solvated ions of CO₂ on the surface of the cathode through an external circuit, thus reducing CO₂ to high-value-added products without adding a reducing agent.¹³²^,¹³³ As the highest valence compound of C, CO₂ reduction could produce various products through 2e⁻, 4e⁻, 6e⁻ and 8e⁻ transfer pathways such as CO, HCOOH, HCOO⁻, CH₂O, CH₄, CH₃OH, and other C₁ products, as well as OA, acetic acid, ethanol, ethylene, and other C₂ products (Table 1).¹²⁷^–¹³³

**Table 1 | The Standard Electrode Potential of CO₂ Reduction to Different Products Under the Conditions of 25 °C and 1 atm**
CO₂ Reduction Reactions	Electron Transfer Quantity	Standard Electrode Potential (V vs RHE)
${CO}_{2} {+ e}^{-} \to {CO}_{2}^{*_{-}}$	1	−1.900
$2 {CO}_{2} + 2 e^{-} = C_{2} {O_{4}}^{2 -}$	2	−0.590
${CO}_{2} + 2 H^{+} + 2 e^{-} \to CO + {HO}_{2}$	2	−0.106
${CO}_{2} + 2 H_{2} O + 2 e^{-} \to {HCOO}^{-} + {OH}^{-}$	2	−1.078
${CO}_{2} + 2 H^{+} + 2 e^{-} \to HCOOH$	2	−0.250
${CO}_{2} + 4 H^{+} + 4 e^{-} \to HCHO + {HO}_{2}$	4	−0.070
$2 {CO}_{2} + 8 H^{+} + 8 e^{-} \to {CH}_{3} COOH + 2 H_{2} O$	4	0.119
${CO}_{2} + 6 H^{+} + 6 e^{-} \to {CH}_{3} OH + {HO}_{2}$	6	0.016
$2 {CO}_{2} + 12 H^{+} + 12 e^{-} \to C_{2} H_{4} + 3 H_{2} O$	6	0.064
${CO}_{2} + 8 H^{+} + 8 e^{-} \to {CH}_{4} + 2 {HO}_{2}$	8	0.169

The differences in the catalyst and potential may affect the path of ECR, resulting in different products.¹³⁰^,¹³¹^,¹³⁴ CO₂ first produces CO₂^*− intermediate through single-electron reduction. CO₂^*− intermediate further forms ·OCHO intermediate through the proton transfer process, then HCOOH is obtained by proton transfer. On the other hand, CO₂^*− can also undergo proton transfer with water molecules to form ·COOH intermediates, which are further reduced to ·CO, and finally desorbed to CO. The selectivity of the products of ECR hinges on the intensity of the interaction between the catalyst and the intermediates of ·CO, ·COOH, ·OCHO, and so on.¹³¹ When the binding strength between ·CO and catalyst is moderate, ·CO could be further reduced to yield products with more than two electrons transferred, and it is even possible to form C₂ and above products by C–C bond coupling.¹³⁴ The carbon–carbon coupling mechanism is more complex, and factors such as the initial activation of CO₂, catalyst nanostructure, and mass transfer conditions need to be considered. The introduction of machine learning predictions based on in situ characterization and theoretical calculation analysis can efficiently calculate key data on the electronic structure of catalytic materials, which has broad prospects in catalyst design.¹³⁵ The catalyst poisoning phenomenon mentioned earlier is due to the strong interaction between ·CO and catalyst so it is difficult for CO to desorb.²⁶^,²⁷

In a typical ECR, anode OER accounts for 94.5% of the total energy consumption, and no value-added chemicals are produced (formula 7–9).¹³¹^,¹³⁶^–¹³⁸ Combining ECR with other reactions, which is more favorable for anodization in thermodynamics can significantly reduce the reaction activation energy and simultaneously produce other value-added products besides oxygen on both sides of the cell. By introducing reducing molecules such as dextrose,¹³⁹ methyl alcohol,¹⁴⁰^,¹⁴¹ ethanol,¹²⁴ octylamine,¹⁴² 5-hydroxymethylfurfural (HMF),¹⁴³^,¹⁴⁴ glycerol, and so on into the electrolyte, energy consumption can be reduced by about 20–70%, and FA can be produced simultaneously on both sides of the cell (Table 2). Among them, glycerol is used as an anodic reactant, and the reaction formula is shown in Figure 16e.

Cathode : {CO}_{2} + 2 [H] \to HCOOH Δ G_{1}^{0} = 13.7 {kJ mol}^{- 1} (5.5 %)

(7)

Anode : H_{2} O (l) \to 2 [H] + \frac{1}{2} O_{2} Δ G_{2}^{0} = 237.1 {kJ mol}^{- 1} (94.5 %)

(8)

Overall : {CO}_{2} + H_{2} O (1) \to HCOOH + \frac{1}{2} O_{2} Δ G^{0} = 250.8 {kJ mol}^{- 1} (100 %)

(9)

**Table 2 | The Overview of Energy-Saving Efficiency for Various Small-Molecules Oxidation Reactions**
Reducing Molecule	Cathode \|\| Anode	Cathode Product	Anode Product	Energy Saving Efficiency	Ref.
Ethanol	CuPc/GDE \|\| Pd Tiweb	HCOO⁻ CH₄, etc.	CH₄, HCOO⁻ etc.	40.8%	124
Methanol	mSnO₂/CC \|\| CuONS/CF	FA	FA	35%	141
HMF	Cu1Bi \|\| NiCo LDH	FA	2,5-furandicarboxylic acid	22.9%	144
Glycerol	Sn/C GDE \|\| Pt/C	FA, LA	FA, LA	75%	142
Glycerol	BiOBr/GDE \|\| NixB	FA	FA	41%	143
Glycerol	Ni_0.33Co_0.67(OH)₂@HOS/NF \|\| BiOI/CP	FA	FA	11.8%	145

GDE, gas diffusion electrode.

The doping element V, which is widely used in HER and OER catalysts, can also be used in ECR catalysts. V doping into the Bi₂O₃ catalyst can cause partial amorphization of Bi₂O₃ and reduce the electron density near the active center of Bi₂O₃. The former provides more active sites, while the latter enhances the adsorption of CO₂ (Figure 16a). At a voltage of −1.1 V, the selectivity of FA is as high as 94.2%.¹⁴⁶ Jin and coworkers¹⁴⁵ paired Ni_0.33Co_0.67(OH)₂@HOS/NF with BiOI/CP catalyst to establish an electrolyzer, as shown in Figure 16c. The introduction of Co can reduce the potential of oxidation of Ni(OH)₂ to active NiOOH, thereby reducing the overpotential of EGOR. Moreover, Co doping can adjust the electronic form of Ni, thus increasing active sites to speed up the adsorption process of oxygen-containing intermediates and improving EGOR kinetics. The performance test results are shown in Figure 17a–i, when the voltage is 1.9 V, the current density can be reached at 22.4 mA cm⁻², and 110% of the total electric energy to FA energy conversion efficiency is obtained.¹⁴⁵ Among Ni-based materials, Nickel boron compound (NixB) catalyst is proved to be a highly active catalyst for EGOR.¹⁴⁷^,¹⁴⁸ Combining NixB and BiOBr catalysts to build a paired reactor, which reaches an FES of 141% and a current density of 200 mA cm⁻², as shown in Figure 16b,d.¹⁴³ However, these results are realized in simple batch reactors with low electrolyte volume and small electrodes and are non-applicable to industrial production. The electrochemical reaction process is affected not only by the catalyst but also by many factors such as the three-phase interface and the mass transfer channels for electrons, protons, gas, and water. However, there are relatively few studies on flow cell reactor or membrane electrode assembly systems for EGOR-coupled cathodic reduction, so it is necessary to further optimize the structural parameters of the catalytic layer, proton exchange membrane, and gas diffusion layer.

Figure 16 | (a) Catalytic mechanism of Bi₂O₃-V for CO₂ reduction. Reproduced with permission from ref 146. Copyright © 2022 American Chemical Society. The schematic representation of (c) an EGOR || ECR paired electrolysis reactor. Reproduced with permission from ref 145. Copyright© 2022 Royal Society of Chemistry. The paired electrolysis electrolyzer (b, d). Reproduced with permission from ref 143. Copyright © 2023 The Authors. ChemSusChem published by Wiley-VCH GmbH. (e) The related chemical reaction equations. EGOR, electrocatalytic glycerol oxidation reaction; ECR, electrocatalytic CO₂ reduction.

Conclusions and Outlook

The preparation methods and catalytic mechanisms of highly selective electrocatalysts used to prepare DHA, GLAC, TA, FA, and other products were reviewed, with emphasis on the adjustment methods of Pt-based catalysts. In addition to noble metals, transition metal sulfide, selenide, nitride, and carbide catalysts show relatively good catalytic activity. Through doping and other means, the original d-band structure and the electronic structure can be changed, and the catalyst stability and activity further improved. Further, the electrolytic cell design, comprehensive energy consumption, electrode products, and conversion efficiency of EGOR-coupled cathodic reduction reaction were introduced and summarized. Finally, the development of a crude glycerol reuse industrial chain is prospected reasonably.

Figure 17 | Average potential of BiOBr (a) and NixB (b) recorded during electrolysis; (c) Average formate FES from paired electrolysis; (d) Formate production rate of the paired electrolyzer. Reproduced with permission from ref 143. Copyright © 2023 The Authors. ChemSusChem published by Wiley-VCH GmbH. (e) LSV polarization curves of Ni_0.33Co_0.67(OH)₂@HOS || BiOI/CP for the ECR; (f) Chronoamperometric curve of electrolyzer powered; (g) FES of FA production at different potentials on Ni_0.33Co_0.67(OH)₂@HOS/NF; (h) FES and partial current density of HCOO⁻ from ECR; (i) Catalyst stability. Reproduced with permission from ref 145. Copyright © 2022 Royal Society of Chemistry. FES, functional electrical stimulation; LSV, linear sweep voltammetry; ECR, electrochemical CO₂ reduction.

The intermediate products of EGOR and the interaction between their functional groups and catalysts still need to be further determined, so we still need to carry out new experiments, using advanced in situ characterization techniques and DFT calculations to study. Pt-based catalysts are the most widely used noble metal catalysts and show good EGOR catalytic activity and selectivity in both alkaline and acidic media, but their cost is too high. By adjusting the surface morphology of the catalyst, introducing defects, doping with other elements, controlling the formation of heterojunction structures, amorphization or alloying with metals such as Bi, Sb, Sn, the catalytic performance and the selectivity of products (GLAY, GLAC, and TA) can be further improved. This can reduce precious metal usage. For example, because of the different arrangement of atoms on the surface, the Pt(100) crystal face shows better stability and GLAC selectivity during the oxidation process. The doping of Bi atoms can change the electron configuration on the surface of the catalyst and greatly improve the catalytic activity of polycrystalline platinum. In addition, Pd, Au, and Ag also show the ability to generate various products such as hydroxypyruvic acid, glycolate, and formate. In the future, a more detailed exploration of their catalytic mechanisms and pathways will be needed. Transition metal catalysts (Cu-based, Ni-based, and Co-based catalysts) can promote the breakage of C–C bonds, thereby improving the productivity of C₁ products, but they often suffer from poor stability and selectivity. Non-noble metal catalysts are still in the early stages of research, and their catalytic activity lags far behind that of precious metal catalysts. However, through reasonable adjustments such as doping Co atoms in Ni-based catalysts or CuCo alloying or NiCo alloying, the catalyst can show better catalytic performance for a single specific product. In the future, it is necessary to further determine the interaction mechanism between transition metals to promote the industrial application of non-noble metal catalysts.

The coupling of EGOR with cathode reactions can reduce the anode reaction potential and produce high-value formate and H2. Coupling EGOR with HER can also prevent the generation of explosive H₂/O₂ mixtures and ROS, and mitigate anode wastewater pollution. Although great progress and breakthroughs have been made in identifying reaction intermediates and products, detailed pathways, especially the transient control of energy distribution are needed for further understanding. In the future, many details of the electrode-electrolyte interface need to be further elucidated at the molecular level. The adjustment of the reaction device mainly starts from the perspective of the membrane electrode and catalytic layer structure. By exploring new membrane electrode preparation methods and processes, we can coordinate the reaction process as a whole and improve fuel cell performance.

Actual crude glycerol contains impurities such as methanol, NaOH, heterogeneous or homogeneous catalyst residues, and other salts, which could cause damage to the EGOR catalyst surface, thus affecting the selectivity and stability of the catalyst. There are only a few studies on the design of impurity-resistant catalysts or electrodes. Finding efficient catalysts that can be used to catalyze the oxidation of crude glycerol is very significant to the development and application of such electrocatalytic systems.

Conflict of Interest

All authors contributed to this article and have consented to its publication. This article has no conflict of interest with any institution or individual.

Funding Information

This paper was made possible as a result of a generous grant from the National Natural Science Foundation of China (NSFC; grant no. 52074128), Basic Scientific Research Business Expenses of Colleges and Universities in Hebei Province, China (grant no. JYG2022001), and Hebei Provincial Natural Science Foundation of China (grant no. H2022209089).

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Volume 2

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Page: 89-110

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Glycerol Electrooxidation to Value-Added C₁–C₃ Chemicals: Mechanism Analyses, Influencing Factors, Catalytic Regulation, and Paired Valorization

Introduction

Parameters of EGOR

Overview of EGOR Path

Influence of Reaction Conditions on EGOR