Progress in Cathode Materials for Methanol Fuel Cells

Abstract

Methanol fuel cells are the most viable alternative to lithium-ion batteries for portable and other applications. The performance of methanol fuel cell depends in part on the microstructure, contact at the electrode-electrolyte interface, and oxygen reduction reactions (ORR) taking place at the cathode, which requires highly efficient cathode materials. The cathode materials have a significant impact on the performance of methanol fuel cells, making their selection and development an important field of research. This review paper provides a comprehensive overview of the progress made in cathode material selection for methanol fuel cells over the past decade. The development of different classes of cathode materials and cathode support is extensively discussed with particular emphasis on structure and electrochemical properties and performance. Also presented are research challenges and opportunities in developing new cathode materials and future trends. Finally, this review paper provides valuable insights into advancements in cathode material selection for methanol fuel cells, sheds light on hybrid composites support materials, and paves the way for further innovation in the pursuit of efficient and commercially viable methanol fuel cell technologies.

1. Introduction

Methanol fuel cells (MFCs) are gaining popularity as an appealing alternative to traditional sources of energy due to their high energy density, low emissions, and ease of methanol management [1]. The cathode is critical in MFCs since it is where the oxygen reduction (ORR) procedure happens. To ensure effective ORR kinetics and overall cell performance, proper cathode material selection plays an essential role [2]. The efficiency of the ORR also relies on efficient catalysts at the cathode, which facilitate the process. The performance of the cathode is depended on the microstructure, high electronic conductivity, and the contact at the electrode-electrolyte interface in addition to being expected to be compatible with the cathode support [3, 4, 5, 6, 7]. The electrochemical oxygen reduction reaction occurring at the cathode is as follows:

$$\frac{1}{2}{\mathrm{O}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}={\mathrm{H}}_2\mathrm{O}$$E1

High catalytic activities at the cathode-electrolyte interface are desired for optimum performance of the MFCs. In addition, the cathode catalyst arrangement is expected to be stable, have high selectivity, and also durable under operating conditions for a long time.

Cathode fabrication process and technique are important for microstructure optimization and improvement in the triple phase boundary (TPB), where the methanol, air, and the electrolyte react to produce the needed power for the intended application. A comprehensive review on methods of catalyst deposition of different support systems are presented by Meille [8], Di Noto et al. [7], and Singh et al. [4]. The overall performance of the cathode is mainly hindered by methanol cross-over from the anode through the membrane [9, 10, 11, 12, 13]. The methanol cross-over leads to the poisoning of the electrode surface and hence affect the ORR kinetics needed for power production. Further, ORR and methanol oxidation reaction (MOR) occur simultaneously at the cathode producing mixed potentials, which reduces the cell voltage and, generate additional water. Also, high cost and low durability associated with cathodes continue to be the two most important challenges restricting the commercialization of MFCs [4, 10, 14, 15].

2. MFC cathode catalyst materials

MFC cathode catalyst are mostly based on noble metals such as platinum (Pt), palladium (Pd), and ruthenium (Ru), which are expensive and hence requires efficient usage resulting in the provision of support in order to reduce cost [6, 14, 15, 16, 17]. Furthermore, the slow reaction kinetics and surface poisoning makes it necessary to provide cathode support with improved surface area and porosity to enhance the electrochemical processes in the MFCs. Notwithstanding the setbacks, Pt-based catalysts have widely been used for ORR in both directly and supported carbon electrodes in acidic and alkaline media [11, 18, 19]. However, Pt has the highest ORR catalytic activity but develops high overpotential, which reduces the performances considerably. Also, the Pd and Ru have found wide applications in both alkaline and acidic media due to their effectiveness [6, 20].

The development of alternate catalysts as replacements for the precious metals-based is on the increase. These precious metal-based composites and non-precious metal-based cathodes are methanol and carbon monoxide (CO) tolerant with a high catalytic activity that is close to that of Pt and other precious-metal-based catalysts [5, 6, 21]. Most of these non-precious metal-based ORR catalysts are metal oxides and transition metal nitrides and sulfides and noble metals. Recently, perovskite structured metal oxides catalysts, ABO₃, which are chemically and structurally versatile, have also been proposed [13, 22]. They are reported to be ORR efficient and methanol tolerant.

3. Precious metal-based cathode materials

Precious metal-based MFC electrodes, comprising platinum (Pt), palladium (Pd), and ruthenium (Ru), meet all the basic requirements as cathodes for MFCs. The advances and significant researches into the MFCs have made precious metal-based cathode materials the most significant electrode catalyst due to their high catalytic activity, stability, and selectivity [4, 14, 20]. However, there are limitations such as methanol crossover, CO poisoning and low durability is hindering their applications. The catalytic activity of the precious metal-based catalysts is influenced by factors such as crystallographic orientation, particle size, surface morphology, alloying effects, and catalyst-support interaction [23, 24, 25]. There have alloying of precious metals with each other to take advantages of the strengths while overcoming their weakness to enhance their activity the cathode [15, 26, 27]. For instance, Pt-Ru [26] and Pt-Pd [27] nanocomposite catalyst have been proven to improve electrocatalytic activity and long-term stability for ORR when they were used as cathodes for polymer.

3.1 Platinum-based cathode materials

Platinum (Pt) is widely regarded as the most efficient catalyst for the ORR in methanol fuel cells in MFCs. Pt-based cathodes’ excellent electrochemical properties and stability make them a preferred choice for commercial applications [4, 21, 28]. Pt-based catalysts have been supported on multi-walled carbon nanotubes, carbon black, mesoporous carbon spheres, and graphene systems [4, 17, 29, 30]. Despite the importance of Pt-based cathode for MFCs and the strides made, there remain drawbacks to its utilization. The high cost and limited availability of Pt pose significant challenges for widespread application. Improvement in Pt utilization and reduction in Pt loading have been suggested to overcome these limitations.

Alloying Pt with transition metals like cobalt, nickel, or iron, synthesizing Pt-based nanocomposites, and developing advanced nanostructured catalysts have been used in addressing these concerns [11, 14, 31]. These approaches aim to enhance the catalytic activity while reducing the amount of Pt required, making the catalysts more cost-effective. Glüsen et al. [14] using these approaches reported high electrochemical performance and long-term stability for the resulting catalyst after 3000 h of operation compared to commercial Pt catalyst under the same operating conditions. Mazzapioda et al. [13] synthesized CaTiO_3-δ and used it as a co-catalyst with Pt/C for the cathode of MFC in order to improve the ORR for better overall performance. The presence of the CaTiO_3-δ introduces oxygen vacancies which serve as active sites for oxygen adsorption, and improves the ORR resulting in an increase of 40% in the maximum power density at 90°C compared to Pt/C under the same operating conditions (Figure 1).

Figure 1.

Comparison of DMFC polarization and power density curves for the MEAs equipped with the bare Pt/C (black) and the composite Pt/C:CTO (red) cathode catalysts at different temperatures (30, 60 and 90°C) [13].

Also, the electrochemical performance of the Pt-based cathode is meaningfully affected by methanol concentration (methanol crossover) and CO poisoning [4, 13, 20, 32]. Methanol crossover occurs when methanol molecules diffuse across the polymer electrolyte membrane, leading to decreased fuel efficiency and catalyst degradation. CO poisoning occurs when CO molecules adsorb onto the Pt catalyst surface, inhibiting the ORR. Development of improved catalyst formulations and exploring catalyst support materials with enhanced methanol tolerance and CO tolerance have been suggested [13, 22, 32, 33].

Further, the long-term stability and durability of Pt-based cathode materials are critical for their practical implementation in methanol fuel cells. Harsh operating conditions, such as high potentials, fuel impurities, and temperature fluctuations, can lead to catalyst degradation and performance decay [23, 25, 34, 35]. Catalyst degradation mechanisms include particle agglomeration, dissolution, and surface reconstruction. Researchers are actively working on improving the stability and durability of Pt catalysts by developing novel synthesis methods, surface modifications, and support materials to enhance catalyst resistance to degradation [35, 36].

3.2 Palladium-based cathode materials

Palladium (Pd), a precious metal alternative to Pt-based catalyst, exhibits excellent catalytic activity for the ORR vital in methanol fuel cells [20]. Pd facilitates the electrochemical reduction of oxygen, and promotes the conversion of oxygen molecules to water. The cost-effectiveness and comparable catalytic activity to platinum make them attractive candidates for commercialization [20]. Pd-based catalysts can be utilized in direct methanol fuel cells (DMFCs) for portable electronics, transportation systems, and stationary power generation. The development of Pd-based cathode materials, along with advancements in system design and integration, can contribute to the widespread adoption of methanol fuel cells as a clean and efficient energy solution.

Although Pd is more abundant than Pt, its stability and performance can be compromised by methanol crossover and carbon monoxide poisoning. Pd has been alloyed with other metals, developing Pd-based nanomaterials with enhanced stability, and exploring novel surface modifications to improve the selectivity and durability of Pd catalysts [37]. Transition metals such as gold (Au), silver (Ag), and copper (Cu) has been alloyed with Pd and have led to enhanced catalytic activity and stability of the composite catalyst while increasing the cathode resistance to methanol crossover and carbon monoxide poisoning [38, 39]. Li et al. [40] synthesized PdCu nanowires with improved methanol tolerance and enhanced ORR performance. Also, PdCu alloy catalysts have shown superior activity and selectivity for the ORR in the presence of methanol compared to pure Pd catalysts [41]. Such advancements demonstrate the potential of Pd-based cathode materials for methanol fuel cells.

Methanol crossover, CO poisoning, and long-term stability remain significant concerns with Pd-based electrodes, which need to be addressed. Methanol crossover can reduce fuel efficiency and degrade catalyst performance, while CO poisoning can inhibit the ORR. The stability and durability of Pd-based catalysts under harsh operating conditions require further investigation. One method suggested to overcome these drawbacks to improve Pd-based cathode performance and durability is to structure them in core-shell bifunctional catalysts [42, 43].

To improve the performance and overcome the challenges associated with Pd-based cathodes, research focused on optimizing catalyst composition, alloying strategies, and catalyst-support interactions to enhance performance should be vigorously pursued. Also, the development of novel synthesis methods, exploration of advanced characterization techniques, and investigation of new support materials will also contribute to the progress of Pd-based cathode materials [44].

3.3 Ruthenium-based cathode materials

Ruthenium (Ru) is a less commonly explored precious metal for methanol fuel cell cathodes. However, recent studies have shown that Ru-based catalysts possess notable ORR activity and exhibit superior resistance to methanol crossover and carbon monoxide poisoning [6]. The unique properties of Ru, such as its high affinity for oxygen and low reactivity towards methanol, make it an attractive alternative to Pt and Pd catalysts. For instance, Wang et al. [45] developed a Ru-based catalyst supported on a carbon nanotube matrix, which exhibited excellent catalytic performance and stability in methanol fuel cells [45]. Structural modification such as crystal phase control, nanoscale size effects, and defect engineering have played a crucial role in enhancing the ORR catalytic activity of Ru-based cathode materials [46]. These modifications optimize the active sites, surface area, and electronic properties of ruthenium catalysts, leading to improved ORR activity and stability.

Besides, Ru alloying with other transition metals has emerged as a strategy to enhance the catalytic activity and stability of Ru-based cathode materials. The alloying Ru with other transition metals enhanced the electrochemical performance since improved resistance to methanol crossover and CO poisoning are discussed, highlighting the potential of these alloy catalysts in methanol fuel cells [6]. RuSe supported on CNT cathode assembly used as cathode of DMFC for instance exhibited better electrochemical performance due to enhanced ORR resulting from the presence of Se, which improved the oxygen reduction of the RuSe/CNT compared with Pt/C [6].

To make Ru and Ru-composite-based cathode material commercially viable with improved performance and durability, further research is needed to optimize Ru-based catalysts and explore their potential in practical applications. This can be attained by exploring novel alloy compositions and understanding the underlying mechanisms to address these challenges and advance the field of ruthenium-based cathode materials for methanol fuel cells.

4. Bifunctional catalyst

To improve the selectivity and catalytic activity of MFCs, bifunctional catalysts have been investigated as potential alternatives to conventional catalysts. Bifunctional catalysts have the ability to simultaneously facilitate the ORR and MOR [19, 21]. They typically consist of a combination of metals, metal alloys, or metal oxides core and catalytically active shell, which exhibit desirable properties such as high electrochemical surface area, good electrical conductivity, and excellent stability [47]. The core material, often composed of platinum (Pt) or palladium (Pd), serves as a platform for anchoring the active sites and providing stability, while the shell material enhances the electrocatalytic activity [4, 14, 42, 43]. As cathodes for MFCs, the performance of bifunctional catalysts relies on the optimization of active sites for both the ORR as well as the promotion of intermediate species adsorption and conversion [4].

The cost associated with the use of expensive and limited supply of precious metals also makes the adoption of MFCs prohibitive. To overcome this, MFC electrodes based on bifunctional catalysts, have been designed with the core-shell consisting of a mixture of Pt or Pd with less expensive metals such as cobalt (Co), nickel (Ni) and other nanoparticles [13, 14, 30]. The resulting electrodes showed enhanced catalytic activity and stability compared to pure Pt catalyst. Zhu et al. [48], for instance, synthesized a PtCu/CeO₂ core-shell catalyst that exhibited excellent bifunctional catalytic activity for MOR and ORR. Also, Chen et al. [49] synthesized a PdCu/C catalyst that exhibited excellent bifunctional catalytic activity at a significantly lower cost compared to Pt-based catalysts. Various synthesis methods have been employed to fabricate efficient bifunctional catalysts for methanol fuel cell cathodes. These methods include wet chemical synthesis, solvothermal/hydrothermal methods, electrodeposition, and physical deposition techniques [13, 50]. The choice of synthesis method depends on factors such as catalyst composition, morphology, and desired catalytic properties [51]. Controlling the catalyst structure at the nanoscale level is crucial for achieving enhanced catalytic activity [52].

Bifunctional catalysts have demonstrated significant improvements in the performance of methanol fuel cell cathodes. The integration of both the ORR and MOR functionalities in a single catalyst enhances the overall cell efficiency, reduces the reliance on separate catalysts for each reaction, and mitigates issues related to catalyst poisoning [47]. These have been attained through the use of multi-element catalysts. For instance, Pt, the most commonly used ORR catalyst, has been alloyed with transition metals such as gold (Au), ruthenium (Ru), iridium (Ir), and others to form a bifunctional catalyst in order to improve the catalytic activities and MOR tolerance [5, 19]. Zhang et al. (2021) synthesized a PtAuCu/C catalyst with significantly enhanced MOR and ORR activities compared to pure Pt catalysts. The alloy catalyst showed improved tolerance to CO poisoning, making it a promising candidate for methanol fuel cells. Pérez et al. [19] synthesized Pt/Cr/Ru on carbon and studied their MOR and ORR. The Pt/Cr/Ru substrate prepared by simultaneously depositing them on the carbon electrode showsthe best activity for ORR and is less affected by methanol. Baglio et al. [53] fabricated a bifunctional Pt-Fe/C cathode and reported enhanced catalytic activity, methanol tolerance, and enhanced ORR. Also gaining prominence for ORR is a Pt-free cobalt spinel oxides M_XCo_3−xO₄ where M is transitional metal made up of Fe, Co, Ni, and Mn [5]. Usually, carbon, graphene, and other conducting surfaces are attached to or used as support, thus forming the shell for these spinel oxides [54, 55]. These compounds have comparative catalytic activity in addition to their low cost due to their abundance and environmental friendliness. Bian et al. [5] reported a highly porous nanohybrid cobalt spinel oxide CoFe₂O₄ on graphene electrode structure with large surface area, good adhesion, excellent pore distribution provided a large TPB for ORR, and stability compared with Pt/C operating under the same conditions (Figure 2).

Figure 2.

Current-time (i-t) chronoamperometric responses for the ORR on CoFe₂O₄ (mixed with AB), rGO, CoFe₂O₄/rGO nanohybrid and commercial Pt/C in O₂-saturated 0.1 M KOH at 0.4 V (vs. Ag/AgCl) [5].

Bifunctional catalysts have shown great potential for enhancing the performance and stability of methanol fuel cell cathodes. Their ability to simultaneously catalyze the ORR and the MOR offer advantages such as improved cell efficiency, reduced catalyst reliance, and enhanced tolerance to methanol crossover. Despite the progress made in bifunctional catalyst research, several challenges remain. The optimization of catalyst composition, design, structure, and synthesis techniques to achieve synergistic effects between the ORR and the MOR is essential for performance optimization. Also, enhancing the stability of bifunctional catalysts under the harsh operating conditions of MFCs is also a critical area that needs much attention and focus. Moreover, the development of cost-effective and scalable synthesis methods for large-scale production of bifunctional catalysts is necessary for their commercial viability.

5. Transition metal nitrides and sulfides (TMNS)

Material properties and electrode characteristics are important in the performance of MFCs. To improve performance, Transition metal nitrides and sulfides (TMNS) have attracted significant attention and are proposed as potential catalyst materials for methanol fuel cell cathodes. TMNS are usually Pt-free electrocatalysts consisting of transition metals such as Fe, Mn, Co, etc., and non-metal groups, mainly N and C [5, 7, 50] . TMNS and its electrodes have desirable properties such as high conductivity, abundant active sites, and good electrocatalytic activity required for effective ORR at the cathode [13, 56, 57]. In addition, TMNS electrodes are durable and methanol tolerant [57]. TMNS cathodes have been developed for ORR commercially and reported to be inactive to MOR, and have desirable properties such as high conductivity, abundant sites for catalytic activity, and good electrocatalytic activity required for ORR effective ORR at the cathode [13, 56, 57]. Vecchio et al. [58] studied the intrinsic activity of commercial Fe-N-C electrocatalysts towards ORR and MOR inactivity by rotating disk electrode electrochemical characterization technique. They reported MOR tolerance and high activity for ORR. Fajardo et al. [50] also synthesized and studied the ORR activity of nitrogen and sulfur-doped Co₃O₄ nanoparticles on graphene for MFC. The resulting electrode had lower area-specific resistance and, higher power and current densities than commercial Pt-based catalysts. The electrode assembly exhibited good stability and durability after a long run.

WN, MoN, CoS, and MoS₂, TMNS used as MFC cathode, have also been studied and reported to efficiently enhance ORR kinetics resulting in improved MFC performance due to their material characteristics and microstructure of the electrode [32, 57]. Tungsten nitride (WN) and molybdenum nitride (MoN), for instance, have been reported to exhibit excellent electrical conductivity and high chemical stability [59]. Besides, they excellently possess catalytic activity towards ORR due to the presence of nitrogen vacancies that provide active sites for the oxygen reduction process. Further, cobalt sulfide (CoS) and molybdenum sulfide (MoS₂), possess a layered structure with abundant edge sites that promote catalytic activity [60]. They also exhibit good stability and high electrical conductivity, making them attractive candidates for methanol fuel cell cathodes. The specific material properties desired for enhanced catalytic performance inform the choice of method for TMNS MFC cathode fabrication [60]. Several methods such as chemical vapor deposition, solvothermal synthesis, hydrothermal synthesis, and electrodeposition have been adopted in order to control the morphology, composition, and particle size of the TMNS [61].

Generally, the performance of the Pt-metal-free cathodes is greatly influenced by methanol concentration, catalyst loading, and operation temperature [43]. High methanol concentrations cause high rates of methanol crossover, hence reduced performance [57]. However, for Pt-metal-free cathodes, the effect of high methanol crossover due to higher methanol concentration is countered by increasing the catalyst loading and temperature, where the performance improves remarkably since the ORR kinetics and MOR tolerances rise. The power density of Fe-C-N catalyzed cathode in MFC increased from about 33 to 45 mW cm⁻² when the catalyst loading was 2–6 mg/cm² [58], and also the power density increased with temperature (60–90°C), catalyst loading (2–6 mg/cm²) and methanol concentration (1–10 M) with the open circuit potential fairly constant throughout.

Although the use of TMNS for MFC cathodes have seen progressive development, there remain several challenges. Optimized catalyst composition, morphology, and surface structure are vital to further enhance the catalytic activity and stability of TMNS. Additionally, understanding the reaction mechanisms and the role of specific active sites needs to be thoroughly explored. Also, the integration of these catalysts into practical fuel cell systems is needed and requires addressing issues such as scalability, cost-effectiveness, and long-term stability. To address these, further optimization and tailoring of the TMNS cathode microstructure through the exploration of electrode fabrication mechanisms is recommended to further improve the overall cell performance. Also, further exploration of synthesis methods, catalyst design, and mechanistic studies will enable the realization of efficient and cost-effective methanol fuel cells.

6. Support materials for MFC cathode catalyst materials

The cathode support material plays a crucial role in enhancing the catalytic activity, stability, and durability of catalysts for ORR in methanol fuel cells [62]. Carbon-based materials, metal oxides, and hybrid composites have demonstrated promising results in enhancing catalytic activity and durability of cathode catalysts. The physical properties of the catalyst support, such as porosity, pore size distribution, adhesion of catalyst, and others, affect the performance of the cell [22, 63, 64] . Nanostructured cathode support assembly improves the porosity and provides more active sites for oxygen species adsorption, dissociation, partial reduction, and the combination of oxygen species with oxygen vacancy on the electrode surface [65, 66]. Carbon-based catalyst support systems are widely used. CNFs and CNTs, carbon-based nanostructured cathode support for MFCs cathode, are reported to improve ORR, methanol tolerance, and stability of MFC cathodes [3, 29, 67]. Also, metal oxides [22, 64, 68] and hybrid composites [69, 70, 71] have also been used and reported to improve ORR and long-term stability of the cathode support systems.

6.1 Carbon-based support materials

Carbon-based materials have been extensively investigated as cathode support materials for methanol fuel cells due to their excellent electrical conductivity, chemical stability, and high surface area. Additionally, the high corrosion resistance, uniform particle size distribution, strong adhesion of catalyst particles, and uniform dispersion of catalyst particles on support make carbon-based materials more desirable [5, 29, 67, 72]. They support and stabilize the catalyst nanoparticles, facilitate ORR, and improve the overall performance of MFCs. Carbon black, Carbon nanotubes (CNTs), Graphene, Mesoporous carbon, and Carbon nanofibers (CNFs) are the main carbon-based materials for MFC cathode support [17, 29, 62, 73]. Carbon-supported cathodes become unstable leading to loss of catalyst activity during long operation. The loss of activity and hence reduced ORR is due to oxidation of carbon, which splits the catalyst particles leading to reduced cell performance [67]. Also, the dissolution, sintering, and agglomeration of Pt leads to the corrosion of carbon support materials leading to the low durability of the cell [15].

Carbon black is a commonly used carbon-based support material due to its high electrical conductivity, large surface area, and well-established commercial availability [17, 72]. It provides a three-dimensional network structure that enhances the dispersion and stability of catalyst nanoparticles. Surface modifications, such as acid treatment or nitrogen doping, have been employed to modify the surface properties of carbon black, leading to improved catalytic activity and methanol tolerance [74, 75]. For instance, nitrogen doping introduces active sites and modifies the electronic structure of carbon, enhancing the catalytic activity and stability of the catalyst [7, 76]. The carbon/nitrogen hybrid cathode support composites can be synthesized through various methods such as pyrolysis of carbon precursors in the presence of nitrogen-containing compounds. Carbon/nitrogen-doped carbon composites exhibit improved ORR activity, enhanced methanol tolerance, and increased durability [77]. Zhang et al. [78] reported that nitrogen-doped carbon nanofiber composites exhibited superior catalytic performance and stability in methanol fuel cells.

To improve methanol tolerance and cathode stability during the operation of MFCs, carbon nanotubes (CNT) with highly conductive pathways for electron transport and effectively promoting the ORR kinetics have been fabricated and used as cathode support [79]. The unique structure of CNTs also allows for strong interaction with catalyst nanoparticles, enhancing catalyst dispersion and stability. Surface functionalization of CNTs with functional groups, such as carboxyl or hydroxyl, further improves their compatibility with catalyst nanoparticles and enhances catalytic activity [80, 81]. Luo et al. reported that functionalized CNTs-supported Pt catalysts exhibited superior methanol tolerance and stability compared to traditional carbon black supports. Also, nitrogen-doped CNT supported Pt-Ru [43].

Carbon nanofibers (CNF) are three-dimensional porous structures synthesized through catalytic growth or electrospinning techniques [3, 27]. CNF offers unique physical properties such as porous structure for enhanced mass transport and electrochemical performance, and large surface area for easy accessibility of reactants to catalysts. CNFs, just like any carbon-based support material, have high electrical conductivity and have been proven to enhance methanol tolerance, and excellent durability [82]. For example, Zhang et al. reported that Pt-loaded CNFs exhibited higher catalytic activity and better stability in methanol fuel cells compared to carbon black supports [83]. Similar reported enhanced catalytic activity and long-term stability have been reported by several authors [27].

Graphene is a two-dimensional carbon material with high electrical conductivity, large surface area, high porosity, and excellent mechanical strength [5, 84]. It has emerged as a promising cathode support material for methanol fuel cells due to its physicochemical properties. Graphene-based support materials can be synthesized through chemical vapor deposition and exfoliation techniques [16, 67, 85, 86]. Graphene supports exhibit improved catalytic activity and stability due to enhanced charge transfer and efficient dispersion of catalyst nanoparticles. Wang et al. [45] demonstrated that graphene-supported Pt catalysts exhibited higher ORR activity and improved methanol tolerance compared to traditional carbon supports [45]. Also, Brian et al. [5] reported enhanced ORR catalytic activity and stability when a bimetal CoFe₂O₄ was supported on graphene. Moreover, a survey on the performance of polymer electrolyte fuel cells (PEMFCs) showed that Pt and Pt-alloyed catalysts and non-precious metal catalysts on graphene and graphene-doped cathode supports have superior catalytic activity and stability for ORR resulting in improved cell performance [26, 67].

Mesoporous carbon is porous structured carbon fabricated in such a manner in order to optimize the morphology, porosity, and intimate adhesion of the catalyst to the surface for enhanced ORR activity [29, 62]. Mesoporous carbon support has been reported to have increased the power density by about 30% compared to normal carbon structured support due to the high specific surface area, porosity, and adhesion leading to extended TPB and enhanced ORR [29].

6.2 Metal oxide support materials

Metal oxides such as titanium dioxide (TiO₂), tin dioxide (SnO₂), and cerium oxide (CeO₂) have been investigated and used as cathode catalyst support materials in MFCs [64, 73, 87, 88]. They offer unique properties, such as high thermal stability, corrosion resistance, and strong interaction with catalyst nanoparticles. Metal oxide support systems provide stable platforms for catalyst anchoring, improving the catalytic activity, methanol tolerance, and durability of the catalyst [23, 64, 88]. However, the ability to control the metal oxide morphology, optimize the metal-support interaction, and mitigate of catalyst poisoning and methanol crossover effects, coupled with the stability and long-term durability of metal oxide supports under harsh operating conditions, limits its practical implementation. Several methods, such as surface modifications and other engineering techniques, have been proposed to enhance their electrocatalytic properties and subsequent performance when used as cathode supports [23].

Titanium dioxide (TiO₂) is a widely explored metal oxide support material in MFCs due to its exceptional chemical stability, high surface area, and low cost [64, 89, 90, 91]. TiO₂ provides a stable and conductive platform for catalyst immobilization, allowing for efficient charge transfer and improved catalytic activity [68, 88, 91]. They also show long-term durability, electrochemical oxidation, and corrosion resistance [23, 88, 91]. Ioroi et al. [91] used Ti₄O₇ as support for Pt (Pt/Ti₄O₇) and reported good catalytic activity for ORR and has the potential to be used as corrosion resistant cathode. Huang et al. [88] also reported the exhibition of good electrochemical performance under full cell operation and corrosion resistance and enhanced stability of the cathode system due to the presence of TiO₂ support. Surface modifications, such as doping with metals or nitrogen, have been employed to enhance the electrocatalytic properties of TiO₂ supports [89]. For instance, Hassen et al. [92] reported that nitrogen-doped carbon-supported TiO₂ nanofiber catalysts exhibited enhanced ORR activity and methanol tolerance. A mixed metal oxide of tantalum (Ta) modified TiO₂ prepared by the sulfite complex route was used as a support for Pd catalyst and evaluated for stability and durability as an ORR catalyst [23]. The presence of the Ta modified the structural and surface properties leading to an increase the oxygen deficiency and improved morphology, which led to high ORR activity, stability, and durability compared with other catalysts.

Tin dioxide (SnO₂) has emerged as a promising cathode support material due to its high electrical conductivity, chemical stability, and strong interaction with catalyst nanoparticles [64, 87]. SnO₂ supports promote better catalyst dispersion and stability, enhancing the catalytic performance of methanol oxidation. Zhang et al. [87] synthesized mesoporous SnO₂ as cathode support for Pt to investigate the electrochemical performance, stability, and durability in PEMFCs. The Pt/SnO₂ cathode assembly presented a good surface area, good electrochemical activity for ORR, and long-term durability and stability compared to Pt/C. The electrochemical surface area loss was only 50% compared with 90% for the Pt/C. Surface modifications, such as alloying with other metals or metal oxides, have been employed to improve the catalytic activity and methanol tolerance of SnO₂-based supports [78]. For example, Wang et al. [93] demonstrated that Pt-SnO₂ nanocomposites exhibited enhanced catalytic performance and durability in methanol fuel cells [93].

Cerium oxide (CeO₂), also known as ceria, with excellent oxygen storage capacity, redox properties, and high surface area, has attracted attention as a cathode support material for MFCs. CeO₂ supports facilitate the oxygen supply and storage during the ORR, improving the catalytic activity and methanol tolerance of the catalyst. To improve the catalytic properties of ceria support, it has either been doped with metals or defects introduced into its structure in order to modify the catalyst surface to tailor it for ORR [94]. Liu et al. [95] reported that metal-doped CeO₂-supported Pt catalysts exhibited enhanced catalytic activity and stability in methanol fuel cells.

6.3 Hybrid composite support materials

Hybrid composite materials, formed by combining carbon-based materials with metal oxides or other nanomaterials, have gained attention as promising cathode support materials. Carbon/metal oxide composites, carbon/polymer composites, and carbon/nitrogen-doped carbon composites have been synthesized and used as cathode support to enhance catalytic, improve stability, and synergistic effects [64, 96]. They have enhanced electrical conductivity, improved catalyst dispersion, and increased catalytic activity.

Carbon/metal oxide composites cathode support takes advantage of the physiochemical properties of carbon and metal oxide to form a superior catalyst support to over the individual limitations associated with these individual materials. These composites can be synthesized using various methods such as sol-gel, chemical vapor deposition, or hydrothermal approaches [96]. Carbon/metal oxide composites exhibit improved catalyst dispersion, enhanced catalytic activity, and stability compared to individual components [93]. For example, Zhu et al. [97] reported that carbon/manganese oxide composites showed enhanced catalytic activity and durability for the oxygen reduction reaction in methanol fuel cells. Nanostructured ZrO₂/nitrogen-doped graphene nanosheets were synthesized and used as support for Pt exhibited higher electrochemical surface area, durability, and better ORR attributable to the unique structure and chemical interactions between the support and the catalyst [96].

Also, polymers such as polyvinyl alcohol (PVA), polyacrylonitrile (PAN), or polypyrrole (PPY) have been used in combination with carbon-based materials to form composite supports [98]. The integration of polymer into carbon-based materials improved the mechanical strength, stability, and facile processability, while carbon impacts electrical conductivity and a large surface area for improved ORR. Carbon/polymer composites exhibit enhanced catalyst dispersion, improved methanol tolerance, and excellent durability [98]. For instance, PAN-based carbon composites are reported to exhibit superior performance and stability in methanol fuel cells [99, 100]. Also, polymers have solely been used as support materials for Pt, Ru, and Pd catalysts for ORR in polymer electrolyte fuel cells. For instance, polypyrrole, due to its electrical conductivity and environmental stability among others, have been used as support for Pt and Pd with corresponding improvement in ORR activities [98, 101]. The methods of the PPY-catalyst preparations greatly influence the ORR activity.

Like all other cathode support materials, the hybrid composites face challenges such as long-term stability, methanol crossover, and catalyst poisoning effects although they have unique properties that enhance the catalytic activity for MFCs. Careful understanding of the fundamental principles governing the catalytic performance of hybrid composite cathode support materials, and further optimization of the microstructure of these hybrid composite supports through continued research to pave the way for efficient and durable methanol fuel cells.

7. Summary and future outlook

Notwithstanding the significant advances in materials selection for MFC cathodes, the use of Pt and Pt/composite electrodes still dominates as the material of choice. Sluggish oxygen reduction reaction, due to methanol crossover from the anode and inherently slow reaction kinetics, has been identified as the main problem associated with the cathode of MFCs coupled with the associated catalyst cost. Efforts should be geared towards the development of new catalysts aimed at improved reaction kinetics through the use of new cathode and cathode-support materials. Deliberate catalyst design, synthesis techniques optimization, and mechanistic understanding of the catalytic activity are crucial for performance optimization and commercialization of MFCs.

Furthermore, the development of low-cost but effective ORR catalysts, with high methanol tolerance and CO tolerance or high selectivity for ORR is highly desirable. The development of low-cost catalyst alternatives with comparable performance is crucial for commercial viability. However, challenges such as stability and durability must be addressed to ensure their widespread adoption.

In conclusion, creating novel cathode materials is crucial to tackling the world’s energy issues. Despite the inherent difficulties and limitations, the discipline is set to make tremendous progress. To realize the full potential of cathode materials, researchers are increasingly relying on multidisciplinary approaches, better characterization techniques, and novel synthesis methods. Future advances in cathode material research promises to transform the landscape of energy technology, allowing a cleaner and more sustainable energy future as demand for efficient and sustainable energy storage and conversion solutions continues to climb.