Review—Electrocatalytic Oxidation of Alcohols Using Chemically Modified Electrodes: A Review

Chemically modified electrodes such as conducting polymer-based, noble metal nanoparticles based, metal oxides or metal oxides as a nanoparticles-based, and reduced graphene oxide-based modification are found to be offering high selectivity, enhanced sensitivity and above all superior stability.⁵⁶ But the utilization of modified electrodes ensured high economical value along with environmental friendliness. Similarly, substituting traditional methods for the synthesis of heterocyclic aldehydes from the oxidation of primary alcohols utilizing KMnO₄, K₂Cr₂O₇, pyridinium chlorochromate (PCC), and pyridinium dichromate (PDC) have been done. These reagents with modified electrodes significantly solved the issues of high toxicity, carcinogenicity, and exothermic properties and made the reactions more practical, feasible and efficient.^57–61 Also, the use of halogenated organic solvents, for such oxidation processes makes those methods environmentally unfriendly and raises issues regarding the safe disposal of waste products.⁶² Currently, the dispersion of Pt nanoparticles onto the carbon-based electrodes has been gaining great attention among researchers as compared to the use of two platinum sheets as electrodes in a biphasic medium as the former being inexpensive.⁶³

The illustration shown in (Fig. 1) represents the developments in the electrocatalytic oxidation of alcohols in different phases. The initial phase involved the usage of simple electrolysis method. In the second phase, Biphasic electrolysis was utilised which involved both In-direct electrolysis (using metal oxides) and non-metal mediators. The examples of metal oxides and non-metal mediators used were MnO₂, TiO₂, RuO₂ and NO₃⁻, NO₂⁻ respectively. The third phase and fourth phase involved usage of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) as a mediator and air oxidation method respectively for electrocatalytic oxidation of alcohols. Further, in the fifth phase researchers are now making use of different types of modified electrodes for achieving the electrocatalytic oxidation of alcohols.

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The organocatalytic reagent, TEMPO is a highly active and stable compound which is extensively used in the selective oxidation of primary alcohols to yield aldehydes and also holds the potential to be used as mediators in several oxidation reactions.^64,65 TEMPO is stable in both aqueous and non-aqueous media and this stability is linked to its steric factors and makes the reagent environmentally benign.The radical scavenging properties of TEMPO makes it a versatile catalyst in biochemical applications.⁶⁶

TEMPO has been used as a sustainable organic catalyst or mediator in various oxidation procedures which obey the Anelli–Montanari reaction which requires aqueous NaOCl or bleach as a co-oxidant in a bi-phasic medium.⁶⁷ With advancements in this field, a practical Cu^I/TEMPO-based catalyst was designed by Stahl for the selective oxidation of primary alcohols to aldehydes under normal aerobic conditions.⁶⁸ This gives phenomenal results concerning both chemoselectivity and reaction yield with a relatively simple procedure to follow. This methodology was further improved by Gao by replacing the bpy/Cu^I/NMI catalyst system with Fe (NO₃)₃·9H₂O, which is much cheaper, ligand-free co-oxidant, thereby making the oxidative process more appealing for pharmaceutical applications, preparation of fragrances and food additives.⁶⁹

The low-cost aminoxyl, 4-acetamidoTEMPO (ACT) is revealed as a highly effective mediator for electrocatalytic oxidation of various simple alcohols.⁷⁰ Electrocatalytic alcohol oxidation with TEMPO and bicyclic nitroxyl derivatives was found to be highly selective due to the steric effects developed in the formation of an alkoxide adduct with the oxoammonium species during the reaction under basic conditions.⁷¹ However, under acidic conditions, the selectivity changes, wherein the mechanism is proposed to involve bimolecular hydride transfer, which favors more electron-rich substrates.⁷² Nitroxyl-catalyzed alcohol oxidation has recently gained significant interest and advances via the use of bicyclic nitroxyls, such as 2-azaadamantane N-oxyl (AZADO) and 9-azabicyclo [3.3.1] nonane N-oxyl (ABNO).⁷³ These nitroxyls exhibit excellent activity for the oxidation of both 1° and 2° alcohols. However, due to the high cost and multi-step synthesis of these reagents, they are underutilized and less appreciated in large scale applications. The cyclic voltammograms revealed that ACT exhibited more than 2-fold higher catalytic current than the other nitroxyls whereas the AZADO, ABNO, and TEMPO had performed similarly. The applicability of ACT was further established by Rafiee et al. in developing a constructive and scalable route to generate carboxylic acids from alcohols and aldehydes using electrochemical oxidation. The environmentally benign nature of the electrosynthesis makes this reaction a favourable one for the synthesis of aldehydes and carboxylic acids in the chemical and pharmaceutical industry.⁷⁰

The drawbacks of using organic solvents in oxidation of benzyl alcohol were addressed by the use of anionic micellar system in aqueous acidic medium as green and facile method. Sodium-dodecyl sulphate has been used to solubilise the reagent in the aqueous acidic medium. Moreover, β-cyclodextrin-polypyrrole-modified carbon fibre paper (β-CD-PPy/CFP) electrode was employed as electrode (Fig. 2). The characterization of the modified electrode was done utilizing various electro and physio-chemical techniques like cyclic voltammetry (CV), EIS, SEM, FTIR and Raman spectroscopy. Cyclic voltammetric studies confirmed that the β-CD-PPy modified CFP electrode exhibited a robust electrocatalytic activity towards the TEMPO mediated benzyl alcohol oxidation. Furthermore, TEMPO mediated oxidation of benzyl alcohol to benzaldehyde was established as an efficient and high yielding method.⁷⁴ TEMPO mediated electrocatalytic oxidation of pyridyl carbinol using carbon nanospheres (CNSs) derived from Areca nut decorated with Pd nanoparticles revealed that the mesoporous nano carbonaceous structures provide enhanced surface sites. These high energy surface sites promoted electrodeposition of Pd nanoparticles and thereby stimulating the accessibility of analytes to the entire pore surface. The modified electrode has been successfully utilised for the electrooxidation of pyridyl carbinol to pyridyl carbinal and the product formation was confirmed using GC-MS, ¹H NMR, and IR. The studies conducted reflects that the developed method was single step, facile, economic, and selective towards electrochemical oxidation of pyridyl carbinol mediated by TEMPO.⁷⁵

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Polythiophene is widely regarded as a versatile conducting polymer in electrocatalytic applications due to its characteristic features like high stability, conductivity, and processability at ambient conditions.^76,77 Pt nanoparticles dispersed on polythiophene obtained in the form of nanoclusters was successful in attracting benzyl alcohol onto the electrode surface (Fig. 3). From the cyclic voltammetric studies, it was observed that Pt–PTh/stainless steel (SS) electrode was more efficient than the bare SS electrode towards benzyl alcohol. Also, single-step direct oxidation of benzyl alcohol to benzaldehyde could be achieved at Pt–PTh/SS electrode using TEMPO as a mediator in the method.⁷⁸

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Usage of modified electrode along with aqueous micellar medium and TEMPO as organic mediator showed to have a futuristic approach which gains advantages over indirect biphasic method involving toxic metals as mediators. Even though the methods with modified electrodes are highly selective, affordable, environmentally benign, their high electrocatalytic performances are yet to be explored in the field of oxidation of alcohols.

The electrooxidation of alcohols employing TEMPO modified polyaniline electrode prepared by the electrochemical polymerization was found to be highly efficient, environmentally benign, and also allowed the selective oxidation of alcohols. High selectivity and rate conversion achieved utilizing alkaline media allowed the conversion of alcohols to their specific aldehydes.⁷⁹ One of the major advantages of this study was that water as a solvent during metal-free reaction conditions will be of great benefit to product purification. This method thus gave rise to the possibilities of exploring TEMPO-modified electrodes in an aqueous system, retaining high electrocatalytic activity for alcohol oxidation in water.

The mechanism of TEMPO-mediated oxidation of benzyl alcohol was found to be having the following steps (Fig. 4). TEMPO undergoes oxidation forming TEMPO⁺ cation, which acts as an initiator for the oxidation of benzyl alcohol. Furthermore, TEMPO selectively acts on benzyl alcohol converting to benzaldehyde and prevents over oxidation to benzoic acid. TEMPO⁺ is then reduced to TEMPOH, a hydroxyl amine obtained by 2e⁻ transfer at the electrode surface which gets converted to TEMPO.^74,78

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Immobilized quinone-hydroquinone redox mediator on poly-pyrrole modified electrode showed rough surface which enhances the electrocatalytic activity. Moreover, cyclic voltammogram capacitive currents and solution-surface interface were found higher for a modified electrode as compared to the bare Pt, which reflected upon the significance of poly-pyrrole. Benzaldehyde thus obtained at the end using a modified electrode with fixed redox mediators had satisfactory yield and could also be reused without separation.⁸⁰

Yi and co-workers were able to synthesize the derivative monomer of pyrrole having the side chain nitroxyl radical, 4-(3-(pyrrol-1-yl) propionamido)-2,2,6,6-tetramethylpiperidin-1-yloxy (PyATEMPO).⁸¹ Also, its polymer PPyATEMPO was efficaciously developed by cyclic voltammetry on a Pt electrode in NaClO₄/CH₃ solution and was later characterized utilizing SEM. Cyclic voltammograms and in situ FTIR spectroscopic techniques were able to predict the electrocatalytic activity of PPyATEMPO and the underlying mechanism in the electrochemical oxidation of benzyl alcohol on PPyATEMPO electrode respectively. Based on the results, PPyATEMPO was found to be having significantly high electrocatalytic activity in the presence of the Lewis base 2,6-lutidine. Also, the possibility of ameliorating the benzyl alcohol to benzaldehyde oxidation may be due to the proton transfer from benzyl alcohol to 2,6-lutidine forming 2,6-lutidine cation. FTIR results revealed that PPyATEMPO⁺ (oxoammonium ion) responsible for selective oxidization of benzyl alcohol to benzaldehyde was formed by oxidation of PPyATEMPO. Also, the synthesis of benzaldehyde via benzyl alcohol oxidation was performed on the PPyATEMPO film in place of the Pt substrate electrode, as the latter was producing benzoic acid as the major product.

The patterns of CVs of conducting polymer and TEMPO mediated modified electrodes are shown in (Fig. 5). From this figure, six points can be summarized^78,82—(1) The oxidation current density increases with increasing alcohol concentration and levels off at certain higher concentrations of alcohol, (2) The anodic peak current is proportional to the concentration of alcohol and cause linear enhancement in current density, (3) The cathodic peak current is less in magnitude compared to anodic peak current, (4) The presence of high-energy active catalytic sites such as metal, metal nanoparticles, conducting polymers attracts the alcohol readily towards the electrode surface, (5) Efficient catalyst such as TEMPO for alcohol oxidation is highly desirable due to its versatility, high selectivity to aldehydes or ketone, enhancement in current density, (6) Increase in the scan rate induces an increase in the electrocatalytic peak current for the catalytic oxidation of alcohol.

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Conducting polymers are found to offer a potential increase in operational efficiency, increase the electrical conductivity, and is equally involved in providing an adequate amount of electron transfer kinetics for electrocatalytic reactions.⁸⁴ Few of the conducting polymers used are polypyrrole (PPY),⁸⁵ polyaniline (PANI),⁸⁶ poly(3-methylethiophene) (PMT),⁷⁷ poly(3,4-ethylenedioxythiophene) (PEDOT),⁸⁷ poly(2-Methoxyaniline)-sodium dodecyl sulfatecomposite,⁸⁸ poly(2-aminodiphenylamine),⁸⁹ poly(vinylferrocene),⁹⁰ poly(1,5-diaminonaphthalene,⁹¹ polyindoles⁹² and poly(o-aminophenol).⁹³ Due to their superior electrical, optical and mechanical properties, they are found to have applications in the field of electronics and biosensors. Few of these conducting polymers have been reported to have enhanced performance in DMFCs and are found to be acting as catalysts in methanol oxidation.^94–98 Advantages offered by conducting polymers such as high dispersibility, improved conductivity, mechanical strength, and adhesive property are yet to be explored in the field of electroorganic conversions of alcohols to aldehydes. The electrode-electrolyte interfacial properties increase upon the electrodeposition of conducting polymers on a substrate electrode. This enhancement is attributed to the induction of porosity and roughness by the conducting polymer due to their three-dimensional growth on the substrate, which enhances the surface area of CP substrate.

Among the conducting polymers, PANI is found to be an interesting candidate due to various applications such as high stability, conductivity, ease in preparation and good environmental stability. Few of its exceptional properties like acting as a protective matrix, high surface area, and superior stability in acid media has allowed it to gain attention from academic as well as technological fields.^99–106 Moreover, many studies have been conducted on the electrochemical oxidation of methanol employing PANI modified electrodes and their modification effect.^107–112 One of the most prevalent types of composites consists of materials containing PANI and multi-walled carbon nanotubes (MWCNTs). PANI has profound applications in electronics, supercapacitors, batteries, and sensors.

Multiwalled carbon nanotube (MWCNT)–PANI composite films via in situ electrochemical polymerizations were used as substrates for electrodeposition of Pt-NPs for finding out its electrocatalytic activity towards glycerol oxidation. Effective parameters involved in the electrocatalytic oxidation of standard alcohols such as the effect of scan rate, switching potential on electro-oxidation of glycerol, glycerol concentration, and Pt mass were investigated to optimize the experimental conditions necessary for glycerol oxidation. Also, electrochemical evaluations involving cyclic voltammetric studies and potential scan rates revealed high electrocatalytic activity, improved reaction kinetics, enhanced stability, and storage properties. These results also gave an insight into the mechanism involved, which was found to be purely based on mass transfer.⁸³ Carbon electrodes modified by PPy acts as conductive bridge by providing pathway for continuous electron transfer between base electrode and Pt or RuO nanoparticles. CPs also enhances the binding stability of metal nanoparticles during electrocatalytic process.^113,114

Dispersion of metal nanoparticles improves the interfacial properties between electrodes and electrolyte. Metal nanoparticles have excellent catalytic properties as the chemical, structural and electronic properties of nanoparticles are different from those of the bulk materials. Also, they are chemically more active due to their high surface energy. Nanoparticles are great initiators of electron transfer reactions.

The combination of Pd-PANI modified electrodes paved the way for a new research in the field of electroxidation heterocyclic alcohols. The porous morphology of PANI improves the surface area for Pd deposition on PANI/CFP electrode. TEMPO-mediated aqueous phase electrooxidation of pyridyl methanol at Pd-decorated PANI on carbon fiber paper electrode and bare CFP electrode revealed that the presence of high energy surface sites offered by agglomerated Pd nanonetworks assisted the molecules of pyridyl methanol to undergo oxidation mediated by TEMPO, along the polymer matrix (Fig. 6). Also, the proposed method could achieve single step direct oxidation of pyridyl methanol to pyridyl carboxaldehyde which has significant applications in synthesis of drugs, dyes, sanitizers, and in agrochemical industry.¹¹⁵

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Recent evidences show that Pd is widely being used due to its higher availability, tolerance to surface poisoning, catalytic property, hydrogen storage capacity and sensors. It has also been observed that the Pd has superior catalytic activity than Pt for electro-oxidation of alcohols in the alkaline media. It also facilitates an even dispersion of electrochemically deposited metal nanoparticles which helps in improving the catalytic activity of metal particles.¹¹⁶ Pd has the essential prospective as an efficient substitute for Pt especially in the development of direct formic acid fuel cell (DAFC). It is therefore required to synthesize porous Pd nanostructures containing nanosized particles on suitable substrates to utilize the high surface area to volume ratio and improved catalytic activity.¹¹⁷ To improve the activity of pure Pd catalyst, Pd alloy catalysts such as PdSn, PdRu, and PdNi have been used due to their effect on the oxidation of adsorbed CO intermediates to CO₂ during methanol oxidation at lower potentials. Out of the above-mentioned catalysts, Pd/TiO₂ nanocomposites have shown high electrochemical activity and exceptional stability in methanol oxidation.¹¹⁸ One complication in the case of metal nanoparticles such as Pd nanoparticles itself is that due to their high curvature, they can't be grown on one-dimensional (1D) carbon spheres and 1D carbon nanotubes. Therefore, the usage of layered structures as folded polymers for anchoring these metal nanoparticles is gaining attention due to their planar structures, good conductivity, and high porosity. The hybrid Pd@PDCX catalysts synthesis involved the preparation of hyper-crosslinked polydichloroxylene solid nanoflakes that were hybridized with PdCl₂ for electrochemical oxidation of alcohols such as methanol and ethanol. For the fabrication of Pd nanoparticle-decorated hyper crosslinked polydichloroxylene, the hybrid was reduced using aqueous NaBH₄. The final step involved the thermal pyrolysis of polymer-supported palladium nanoparticles to form Pd-decorated carbon nanoflakes. The fact that the Pd nanoparticles could be well deposited within the crevices of HDCX nanoflakes by hyper-crosslinking-mediated self-condensation polymerization method gave rise to such exclusive constructions. The result of such convoluted and extensive constructions led to improved surface area and stability due to reduced resistance between the nanoflakes (Pd@PDCX). The major reason for such an improved surface area is attributed to the dual benefits offered by mesoporous frameworks as ion-buffering reservoirs also providing cracks to anchor the Pd nanoparticles. These frameworks also ensured structural stability by relaxing the volume expansion developing during the prolonged methanol oxidation process. The advancement in the electrocatalytic activity of the hybrid catalyst in the oxidation of alcohol at higher concentrations ensured unprecedented catalytic performance and stability which in turn increases its application as sensors and catalysts and in DAFCs.¹¹⁶

Soleimani-Lashkenari and co-workers were able to develop a novel method for the electrodeposition of Pd nanoparticles on the PANI/TiO₂ nanocomposites modified glassy carbon electrode to form the Pd/PANI/TiO₂/GC working electrode for methanol oxidation (Fig. 7). The electrochemical activity of the fabricated palladium/Polyaniline/titania (Pd/PANI/TiO₂) electrocatalyst for the methanol electrooxidation was determined by cyclic voltammetry and chronoamperometry in an alkaline media consisting of potassium hydroxide (KOH). From the experimental results, Pd/PANI/TiO₂ was found to be promoting the electrochemical activity of methanol oxidation in terms of current density and onset potential, increased surface area, improved methanol oxidation kinetics, ensued higher stability as compared to the Pd/GC electrode. Therefore, due to the exceptional characteristics, this nanocomposite along with Pd catalyst can be recommended for further research and also as an application for DMFCs.¹¹⁸

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The catalytic activity and stability of nano-dendritic Pd on PEDOT has been demonstrated for electrooxidation of 1,2- propanediol (1,2-PD) in an alkaline medium.¹¹⁹ Presence of PEDOT was found to be the major factor towards high catalytic efficiency. The success of this research later led to a continuation of the electrocatalytic oxidation of C₃-aliphatic alcohols such as propanol (PA), (1,2-PD), 1,3-propanediol (1,3-PD), and glycerol (GL). The Pd-PEDOT/C electrode was found to be having an enhanced catalytic activity as compared to the Pd-PEDOT/C which is attributed to the presence of a thin film layer of PEDOT. The electrodeposited thin film of the PEDOT layer possesses a large number of surface defects, which are uniformly distributed on the carbon paper electrode. Additionally, each defect acts as a nucleation center for the formation of metal atoms which later grows into nanosized dendrites. The presence of short branches, kinks, steps, crevices in the Pd layers ameliorates the surface area of the catalyst. On the contrary, Pd was non-dendritic in the absence of PEDOT. The electrochemical active surface area (EASA) of the Pd-PEDOT/C and Pd/C electrodes was estimated by CO adsorption followed by the oxidative stripping method.¹¹⁹ Consequently, Pd-PEDOT/C electrode demonstrated greater electrochemical activity than Pd/C electrode. Moreover, the oxidation kinetics rate of these alcohols was seen to be increasing in the following order: PA < 1, 2-PD < 1, 3-PD < GL. The formation of adsorbed species (OH and alcohols) was considered as the rate-determining step in cyclic voltammetric oxidations of PA, 1,2-PD, 1,3-PD, and GL. The presence of enhanced electrocatalytic activity in the case of poly-alcohol was credited to the electronic and steric factors, adsorption effects, and formation of different reaction intermediates, products. Glycerol leading the electrooxidation kinetics comparison order reflects on the possibility of its usage in the DAFCs as a promising fuel.

MnO₂ nanowire arrayed electrode was chosen as a transition metal due to its low cost and environmental friendliness. Utilizing electrodeposition technique, Pt nanoparticles composited MnO₂ nanowire arrayed electrodes (PME) having "brush shape" 3D architecture on Si substrate was prepared. For comparison of electrochemical performance, Pt nanowire arrayed electrodes which also had the same amount of Pt nanoparticles were prepared and compared with PME in terms of electrochemical activity for methanol oxidation employing CV and chronoamperometry in sulfuric acid solution. Cyclic voltammetric studies demonstrated that PME had a higher peak current density and lower overpotential than Pt nanowire arrayed electrode when they have the same quality catalysts. Besides, PME was also able to achieve higher resistance and displayed better tolerance towards catalytic poisoning and bought down the electrode cost. Therefore, PME was able to solve the cost of the metal employed and catalytic poisoning from adsorbed CO and CHO species.¹²⁰

A simple sol-gel technique was employed to synthesize the carbon ceramic electrode (CCE), which was later modified electrochemically with the addition of Pt particles for oxidation of alcohols. Effective parameters involved in the electrocatalytic oxidation of alcohols such as the effect of Pt loading, methanol, and ethanol concentration, the effect of H₂SO₄ medium temperature, and forward potential scan rate were investigated and analyzed. Moreover, due to the large surface area and increased resistance towards CO poisoning, the catalytic currents were found to be larger than those observed at the same electrocatalyst anchored on other substrates. Additional properties such as sustained stability and high reproducibility make it an ideal candidate as an anode in DAFCs and similar applications.¹²¹

Electrooxidation of ethanol at platinum electrodes has been extensively studied for understanding the mechanism. The mechanism is supported by spectroscopic and electrochemical studies. One such generally accepted mechanism involves the following steps-

$$\begin{eqnarray}&&{\rm{Pt}}+{{\rm{CH}}}_{3}-{{\rm{CH}}}_{2}{\rm{OH}}\,\leftrightarrow \,{\rm{Pt}}-{\rm{CHOH}}-{{\rm{CH}}}_{3}+{{\rm{H}}}^{+}+{\rm{e}}\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{\rm{Pt}}-{\rm{CHOH}}-{{\rm{CH}}}_{3}\,\leftrightarrow \,{\rm{Pt}}-{\rm{CHO}}-{{\rm{CH}}}_{3}+{{\rm{H}}}^{+}+{\rm{e}}\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{\rm{Pt}}-{\rm{CHO}}-{{\rm{CH}}}_{3}\,\leftrightarrow \,{\rm{Pt}}-{\rm{CO}}-{{\rm{CH}}}_{3}+{{\rm{H}}}^{+}+{\rm{e}}\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&{\rm{Pt}}-{\rm{CO}}-{{\rm{CH}}}_{3}+{\rm{Pt}}\,\leftrightarrow \,{\rm{Pt}}-{\rm{CO}}+{\rm{Pt}}-{{\rm{CH}}}_{3}\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&{\rm{Pt}}+{{\rm{H}}}_{2}{\rm{O}}\,\leftrightarrow \,{\rm{Pt}}-{\rm{OH}}+{{\rm{H}}}^{+}+{\rm{e}}\end{eqnarray} \tag{ 5 }$$

There are certain limitations to this mechanism due to disagreements pointed out by various researchers. One such case is that there is no agreement regarding the nature of the adsorbed species. Another is the presence of one or two carbon atoms in the intermediates formed.^122–125 One major issue related to the ethanol oxidation is the breaking of the C–C bond for total oxidation to CO₂ which leads to the increased production of partial oxidation products like CH₃CHO and CH₃COOH. But the production of these partial oxidation products is seen to be reducing the electrical cell efficiency.¹²¹

Likewise, the most commonly accepted mechanism for methanol oxidation at platinum electrodes that result in the formation of carboxyl intermediates and strongly adsorbed CO species is-

$$\begin{eqnarray*}&&\left({{\rm{CH}}}_{3}{\rm{OH}}\right){\rm{solution}}\,\leftrightarrow \,{\rm{Pt}}-\left({\rm{CO}}\right){\rm{ads}}+4{{\rm{H}}}^{+}+4{\rm{e}}\end{eqnarray*}$$

Bimetallic Pt-Ru nanoclusters on PEDOT was electrochemically co-deposited on carbon paper substrate in an acidic electrolyte consisting of chloroplatinic acid and ruthenium chloride. The presence of thin layer PEDOT on a carbon paper substrate was detected to be implicated in the synthesis of homogenous, distinct, and dispersed nanoclusters of Pt-Ru having nanoparticle spectrum. Whereas in the absence of PEDOT, the nanoclusters assembled were seen to be less homogenous, agglomerated, and complex. Cyclic voltammetric study revealed that peak currents of methanol oxidation were several times greater for PtRu–PEDOT electrode than Pt–Ru electrode. This work reveals the significance of the presence of PEDOT for uniform distribution of catalysts and for conducting further research for more feasible applications within sensors, catalysts, and in DAFCs.¹²⁶ Also, electropolymerization of EDOT to PEDOT can be carried out in a non-aqueous medium because of the high solubility of EDOT monomer in non-aqueous media than in aqueous media. The presence of SDS (anionic surfactant) has shown to be improving the solubility of EDOT monomer in an acidic aqueous solution. SDS also has an application in improving the solubility of pyrrole in an aqueous media and also enhancing the rate of its electropolymerization.¹¹⁹

The zirconium oxide in ZrO₂-ERGO composite produces bridging molecules that allow easy anchoring of Pt particles to form a functional ERGO multilayer film produced through a co-electrochemical deposition procedure (Fig. 8). Both ERGO and ZrO₂ enhance the electro-catalytic performance by increasing surface area and improving the number of Pt active sites respectively. Cyclic voltammetric and amperometric studies reflected on the high electro-catalytic performance in methanol oxidation. The Pt/ZrO₂-ERGO modified film was found to be exhibiting better catalytic activity and stability for methanol oxidation as compared to the Pt/ERGO modified film or commercially available Pt/C electrocatalysts. Cost-effectiveness, high efficiency in methanol oxidation, and better resistance towards adsorbed CO species make it suitable for application in highly stable methanol fuel cells. The study also revealed the significance of ERGO as a promising and effective electrocatalyst in electrochemical sensors and catalysis.¹²⁷

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Different kinds of carbon materials, such as mesoporous carbons, hollow graphitic nanoparticles, carbon nanotubes, carbon nanocoils, and graphitic carbon nanofibers were also used as efficient catalysts. The carbon spheres are used as support for Pt particles for ethanol electrooxidation in acid media. To increase the surface area and make complete use of it, the catalyst (Pt) is dispersed on a conductive support like carbon. The porous nature helps to increase the effective surface area of deposited platinum while its low cost brings down the cost of fuel cell anodes. The use of carbon-ceramic as supporting material is effective to reduce the use of expensive platinum and to attain ideal catalytic performance.¹²¹

Wang and co-workers came up with a unique and novel approach called composite-molten salt (CMS) for the preparation of carbon nanospheres. Advantages like simple, economic, environmentally benign, and operation under normal atmospheric pressure and low temperature gave these carbon nanospheres an edge over those which were prepared by already known hydrothermal synthesis or commercially available ones. Carbon nanospheres coated with Pt nanoparticles were employed for electro-oxidation of methanol and ethanol in alkaline media. Electrocatalytic performance of Pt catalyst supported on the CMS carbon nanospheres in terms of methanol and ethanol electro-oxidation was significantly higher as compared to its counterparts. This enhancement was majorly ascribed to the high carbonization, conductivity, and development of porous structure by nanospheres due to their CMS mode of synthesis. This greatly diminished the liquid sealing effect allowing efficient gas diffusion.¹²⁸ This work also showed that the carbon nanospheres were more efficient than the amorphous powder– carbon black, having a low degree of graphitization, thereby making it prone to electrochemical corrosion.

Nickel cobaltite nanoparticles (NiCoO₄ NPs) synthesized via a facile hydrothermal method in presence of urea and NiCo₂O₄ nanoparticles were incorporated into the zeolite-4A. Calcination of the physical combination of NiCo₂O₄ and zeolite-4A gave rise to the NiCo₂O₄@zeolite-4A. Cyclic voltammetric studies and chronoamperometric studies conducted in a conventional three-electrode system gave us an insight into the significantly high electro-catalytic activity and rate kinetics respectively. The incorporation of NiCo₂O₄ nanoparticles into zeolite-4A in the modified electrode was able to enhance the electro-catalytic oxidation of methanol due to its rich binary electro-active sites of Ni and Co species, high intrinsic electron conductivity, long term stability, and superior surface structures.¹²⁹

The electrocatalytic activity of the modified nickel hydroxy/glassy carbon electrode (MNGC) towards methanol oxidation was determined by employing cyclic voltammetry and chronoamperometry in an alkaline media. The modified electrode was found to be catalyzing the methanol oxidation occurring at the MNGC electrode via nickel oxyhydroxide or simple Ni³⁺ species. Moreover, the methanol oxidation mechanism was found to be changing from diffusion control at lower methanol concentration to rate controlled at higher methanol concentration. The reaction kinetics between methanol and NiOOH was found to be slow due to a decrease in the scan rate with increase in the (IPaI/IPcII) ratio. Besides, an increase in current was observed to be occurring parallelly to that of methanol concentration which proves the catalytic role performed by NiOOH during methanol oxidation. The exceptional stability showcased by the modified electrodes can have direct and wide range applications in the hydrogen storage materials and DAFCs.¹³⁰

Experimental studies have reported that hydrogen has a profound effect on the electro-catalytic oxidation of CO adsorbed on Pd. Also, Pd₄Pt catalyst was found to have a high tolerance to CO. Moreover, exceptional catalytic behavior was observed for the electrochemical oxidation of formic acid on Pt/Pd electrocatalysts, where formic acid oxidation occurs at lower potentials than at Pt or Pt/Ru. Both the CO tolerance and activity for formic acid oxidation of these Pd containing catalysts may be due to the presence of occluded hydrogen. The influence of hydrogen occluded into a Pd electrode coated with a PdPtRu catalyst has been demonstrated on the electrochemical oxidation of methanol in an acidic medium. It exhibited robust electro-catalytic activity even at low potentials. The low consumption of hydrogen during the electrochemical oxidation of methanol was attributed to the catalytic effect of the occluded hydrogen. Moreover, better resistance and decreased sensitivity towards the adsorbed CO species have been exhibited by the modified electrodes. The enhancement in the electrochemical oxidation of methanol was attributed to the occluded hydrogen. The occluded hydrogen converts adsorbed CO to CH₂O in a chemical reaction, thereby producing reaction sites for methanol oxidation. Later, the excess CH₂O produced is oxidized to CO₂ before it can escape from the porous PdPtRu catalyst employed. The study conducted reflects upon the usage of the hydrogen occluded modified electrodes in the application of DMFCs.¹³¹

Ni and Pt have been used as important electrocatalysts in the direct oxidation of small organic molecules. A lot of interest has recently been shown regarding the choice of materials cheaper than platinum. A few studies have been reported on the application of less expensive materials such as Co, Pd, and Fe.^132–134 The electrochemical oxidation of methanol on a Ni-Co alloy modified glassy-carbon electrode in an alkaline medium was cost-effective and efficient modifier. Cyclic voltammetric and double-step chronoamperometric studies revealed high electro-catalytic activity and irreversible nature of this reaction. The Ni-Co alloy modified electrode exhibited a higher activity towards methanol oxidation in the Ni (III) and Co (IV) oxidation states and showcased first-order kinetics in terms of methanol oxidation. Effective parameters involved in the electrocatalytic oxidation of alcohols such as mole ratio of Ni-Co in the alloy in modification step, potential scan rate, methanol concentration and solution temperature on the electro-oxidation of methanol were investigated and analyzed. The study also revealed the catalytic role exhibited by NiOOH in methanol oxidation and that methanol oxidation starts once the formation of the first amount of NiOOH on the electrode surface takes place.¹³⁵

The pattern of SEM and TEM images obtained from various electrocatalytic oxidation studies have been shown in the (Fig. 9). From the figure it can be summarised that the coating of conducting polymers on the electrode surface acts as nucleation centres for the growth of metal nanoclusters, nanoflakes, and nanostructures. The nanostructures thus formed; act as high-energy surface sites in attracting the molecules of alcohol toward the surface of the electrode. These metal nanoclusters also provide channels for alcohol molecules to diffuse into the polymer matrix, therefore leading to an effective electrocatalytic oxidation process.^74,78,115

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Metallic nanoparticles have very high catalytic activity and selectivity which make them suitable for catalyzing reactions such as electrochemical, oxidation, reduction and coupling reactions. Metal oxide nanoparticles are widely used in industries as an active component, promoter, or support. The high density of defect sites in the nanometer range is due to its unique physical and chemical properties. Both metallic and nanoparticles are universally being used as catalysts in fuel cell and other electrochemical applications. The past few decades have witnessed an ameliorated growth in the development of metals and their oxides as nanoparticles.¹³⁶ Another efficient method for enhancing the electrocatalytic activity in methanol oxidation is the use of support materials which include carbon-based materials like carbon nanotubes (CNT), carbon nano-fibers (CNF), graphene, and metal oxides like TiO₂, SnO₂, RuO₂, MnO₂ and CeO₂.¹¹⁸

Zirconium oxide (ZrO₂) nanomaterials like in Pt/ZrO₂-electrochemically reduced graphene oxide (ERGO) modified electrode¹²⁷ are industrially very significant because of their distinctive blend of properties like thermal stability, chemical inertness, and lack of toxicity. They are used in varied applications like fuel cells, optical coatings, flat panel displays with low-energy excitation sources, solar energy converters, optical amplifiers, and photonic devices. Nickel cobaltite nanoparticles (NiCo₂O₄ NPs) are an important class of nanoparticles being used as they have inherent redox and tuneable chemical, morphological, and textural properties, which can be used to effectively bind on the substrate materials. NiCo₂O₄ acquires an inverse spinel structure in which the nickel ions fill the octahedral sites whereas cobalt ions are scattered across the tetrahedral and octahedral sites. NiCo₂O₄ exhibits higher electronic conductivity and electrochemical activity than those of nickel and cobalt oxides and as a result higher redox property than every single component. This makes it a notable material to be used in various fields like magnetic materials, electro-catalysts, optical limiters and switches, chemical sensors, and lithium-ion batteries.¹²⁹

The high activity of Mn (IV) oxides is due to the presence of the Mn⁴⁺/Mn³⁺redox couple and the role of oxygen in methanol oxidation. Also, MnO₂ is more efficient as it is cost-effective and more sustainable in nature than other transition metal oxide systems. It has been noted that compounds such as RuO₂, SnO₂, WO₃, and CeO₂ assist the oxidation of CO or CHO species by adsorption of oxygen-containing species close to the poisoned Pt sites, thereby operating in a "bifunctional approach" and inhibiting catalysts poisoning by CO and CHO species. Experimental studies have reported an improvement of the catalytic activity of Pt-Ru electrodes towards methanol electrooxidation due to the introduction of transition metal oxides as co-catalysts.^120,137 Characterization by XRD and X-ray photoelectron spectroscopy (XPS) of Pt-RuO₂/Ti electrodes revealed that the films only consisted of Pt nanoparticles dissipated in RuO₂ and that neither metallic Ru nor Pt-Ru alloy was present on the surface. Chronoamperometric and potential sweeps in cyclic voltammetric studies confirmed the enhancement in the electrocatalytic oxidation of methanol which was majorly governed by the presence of hydrous ruthenium oxide (RuO_2-δ(OH)_δ) playing the role of oxygen species donor thereby promoting CO to CO₂ oxidation reaction.¹³⁷ RuO₂ coated titanium electrodes, commonly known as dimensionally stable anodes (DSAs), are the most extensively used anodes in the chloroalkali industry due to their excellent stability, corrosion resistance, and very high electrocatalytic activity. The RuO₂-coated electrodes have charge storage and transport capacities in both the inner and outer surface-active sites, which facilitates electrochemical reactions to occur at different positions inside the pores. These electrodes are also widely used as anodes of the electrochemical capacitor due to large capacitive currents coupled with faradaic currents. These have been used as electrodes for the generation of both hydrogen and oxygen as well as for oxygen reduction.¹³⁸ Experimental evidence also suggests the electrochemical properties of thermally prepared oxide film electrodes can be different depending on their support materials. Therefore, it was proposed that the so-called significant species may be formed on the electrode surface. Numerous interactions seem to be operating between support materials and film components, leading to the formation of various species at the electrode surface. Hence, the RuO₂ system on Ni support is expected to increase the electrocatalytic activity of the RuO₂-Ni electrode during the oxidation of organic compounds.¹³⁸ When thermally prepared RuO₂-Ni electrode in alkaline media¹³⁸ was examined using SEM before and after oxidation and it was found that the grains became coarser after the lengthy oxidation. The agglomeration of small particles must have taken place during the cell operation. This might have occurred because of the dissolution of RuO₂ upon oxidation of the electrode, followed by the recrystallization of RuO₂, which is produced during the mediated oxidation of ethanol. This electron shuttling process enables the stabilization of the electrode as RuO₂ produced during the oxidation is redeposited in the form of RuO₂ after the mediated oxidation of ethanol. Thus, the RuO₂-Ni electrode is stable initially but found to change its morphology overtime. From the primary cyclic voltammetric experiments conducted on bare Ni, RuO₂-modified Ti, and RuO₂-modified Ni electrodes in alkaline media, RuO₂-modified Ni electrode was found to be exhibiting the highest electro-catalytic properties for ethanol oxidation. Advantages like moderately high stability and effective electron transfer mediator reflects on the possibility of its potential usage in the DAFCs.

Over the years, the carbon-based supports have gained huge attention due to the electrocatalytic activity, dispersion, stability, mass transfer kinetics at the electrode surface, and electric conductivity. Among the commonly used carbon-based materials such as activated carbon and graphite, reduced graphene oxide has certainly been in more demand due to interesting characteristics such as high surface area and presence of a special electronic structure.¹³⁹ Various studies involving rGO and graphene supported metal catalysts in alcohol oxidation have also been conducted and analyzed.^140,141 The properties of the graphene—a 2D carbon candidate ranges from possessing high surface area, superior chemical, thermal properties and mechanical stability to flexibility which can be achieved by different reduction methods involving the removal of oxygen functional groups.^142,143 Consequently, these reductions lead to the formation of reduced graphene oxide. The rGO possessing relatively high electro-conductivity solves the limitation of graphene. The homogenous distribution of certain nanoparticles on rGO sheets have given rise to the experimentation of various combinations of nanostructures supported on rGO such as Pt-ZrO₂-ERGO, Pd nanodendrites-rGO for studying and understanding their combined electro-catalytic activity and efficiency for oxidation of ethylene glycol and glycerol.¹²⁷

A lot of experimental studies were done on understanding the kinetics and mechanism of methanol oxidation under diverse conditions and on different electrodes such as Pt, binary and ternary alloys, modified electrodes, nanofibers, and nickel.^{130,144–147} One of the widely used methods for minimizing the overvoltage effects of methanol oxidation is the use of chemically modified electrodes (CMEs). Different metals and their complexes have been used as modifiers for preparing modified electrodes. It is observed that Ni-based electrodes have more advantages as compared to other metal based electrodes. The cyclic voltammetric studies using carbon paste electrode modified with Ni (II)-BS complex as a modifier and rGO nano sheets (Fig. 10) demonstrated high electrocatalytic activity and efficiency towards methanol oxidation. NiOOH groups generated by further electrochemical oxidation of nickel (II) hydroxide on the surface of the modified electrode were found responsible for methanol oxidation. The usage of rGO as nanosheets and Ni (II)-BS complex as a modifier ensured heightened sensitivity and surface area for methanol oxidation at the modified electrode surface. Analysis of effect of pH and methanol concentration, low detection rate and a wider range limit showcased by the modified electrode in terms of methanol oxidation in presence of ethanol ensured its high selectivity and potential applicability in the field of sensors.¹⁴⁸

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Different modified electrodes employed for the electrocatalytic oxidation of various alcohols shown in Table I clearly highlight that TEMPO supported catalyst system has been able to showcase ameliorated efficiency.^74,78,79,115 However, TEMPO as a homogenous system has been underutilized due to its high cost. The incorporation of various types of conducting polymers such as PPY,^74,81 PANI,^{82,97,115,116,118} and PEDOT^119,126 along with nanoparticles for electrodeposition in most of the works shows their utmost importance as a type of modification which has also proved to be highly economic. Few researchers also had tried to highlight the usage of water as a solvent which was found to be more efficient, produced fewer toxic by-products, and was in-expensive.⁷⁹ The modified electrodes involved in oxidising the various types of alcohols to aldehydes with their potential scope of application is shown in Fig. 11.

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Last three decades witnessed a tremendous growth in research focusing on the electrocatalytic oxidation of alcohols using various electrodes. Later, improvisations employing modified electrodes proved to be more efficient and benign as compared to the conventional electrodes. Therefore, the production of carbonyl compounds by electro-oxidation of alcohol under eco-friendly and mild conditions is gaining huge attention to tackle present-day scenarios like energy crises and environmental pollution. The advancements in this field can be attributed to a plethora of applications offered by both alcohols and aldehydes in fuel cells, food, chemical, and cosmetic industries. The different electrode modifications involve the usage of conducting polymers, nanoparticles, metal oxides, and reduced graphene oxide revealed to be more robust and gave better results. This review article reveals that only limited number of methods are available for electro-oxidation of heterocyclic alcohols utilizing modified electrodes. Moreover, due to ameliorated efficiency and a wide range of applications, modified electrodes can also be employed for electrosynthesis, detection, and degradation of various organic compounds including olefins, toluene, ethylene, and phenol for various applications. Applying better developmental techniques, gaining further insights into the working of modified electrodes, and making them more economic and reproducible is highly desirable.

Utilizing electricity as a source of energy instead of oxidative reagents is a prominent green advantage in electrosynthesis method. The use of chemically modified electrodes has a lot to offer to a significantly distinct and tenable approach to synthesis of various organic compounds such as benzoxazoles and nitriles from aldehydes. Similarly, electrochemical synthesis of triazolopyridines and related triazole-fused poly (hetero) aromatic structures which are extensively used in pharmaceuticals can be made using modified electrodes. This eliminates the certain failures such as harsh reaction conditions, use of expensive catalysts, limited substrate scopes and low chemo-selectivity which are part of the conventional methods. Electrochemical synthesis of nitrogen heterocycles and their derivatives if achieved by employing modified electrodes can be extremely important as they hold the structural units of many medicinally important compounds found in the biological system.