Metal oxide-based materials as an emerging platform for fuel cell system: A review

Co-precipitation method

The unique and simplest methodology in fabrication is for the preparation of MO nanoparticles. A solvent is used for oxo-hydroxide precipitation while chloride, nitrates, etc is used as an intermediate to salt. Thus, it is preferable for using surfactant or high or high-gravity sensitive precipitation to improve the morphology since regulation of size and chemicals during the synthesis of mixed metal oxides homogeneity gets questioned [43,44].

The co-precipitation method is generally used for the synthesis of nanoparticles in which several parameters such as reaction time, pH, the temperature of the reaction, concentration of the initial solution, and material play a crucial role in obtaining ceramic powders for the required shape and size. For example, nano aggregates of cobalt oxide (Co₃O₄) was successfully synthesised via co-precipitation method. The as-synthesised product is evaluated for its catalytic characteristics in the thermal breakdown of ammonium perchlorate and is demonstrated to be an effective catalyst in fuel application [45].

Sol–gel technique

This technique is a wet-chemical procedure in which the sol (or solution) progressively advances in this process to form a gel-like network including both solid and liquid phases. The approach of sol–gel technique enables the design of oxo-hydroxide through precursor hydrolysis which usually contains alcohol [46]. Following the molecules’ condensation, metal hydroxide is formed, and subsequently hydroxyl species are condensed by polymerisation to form a thick porous gel. Adequate drying and calcination process results in oxide that is ultrafine, porous, and pure. [47]. Figure 3(b) displayed the sol–gel process schematics model. Sol is generated either via polymerisation reactions or hydrolysis processes by allocating enough reagents to the precursor solution. The sol–gel process could be used to acquire the necessary material types, shapes, regulated phases, and also the size of the materials extracted [48].

The variables which could be regulated by the sol–gel technique involve (1) the nature of the solvent used (2) the saturation of the precursor used, (3) the solution pH, (4) the form and concentration of additives used, (5) the pre and post-heat treatment of the products, (6) the aging of the solution, and (7) the composition of the polymer applied for oxidation [50]. The magnetic, electrical, optical, and other inherent properties of the materials are reinforced by particles formed in the gel matrix, which have a uniform general shape. The sol–gel method has currently been used to fabricate GO-based nanocomposite materials, particularly throughout the manufacture of self-cleaning film and glass coating. Since sol–gel generated inorganic composites provide help to evaluate temperature, it is possible to maintain the physical characteristics of the doping portion by regulating the blending between the guest molecule and the host formulae. The sol–gel approach is a simple and quick procedure that is more flexible and practicable. The fabrication via the sol–gel method could be accomplished at low temperatures and is successful in achieving varied MO morphology, based on the initial precursor situation [51]. Santiago et al. investigated the performance of Nafion-TiO2 hybrids prepared using sol–gel process for high temperature PEMFC. Using a low-temperature sol–gel method, hydrophilic anatase nanoparticles were successfully incorporated into the Nafion matrix. Fuel cell tests at temperatures up to 130°C indicated that hybrid membranes display a greater ohmic drop than Nafion membranes [52].

Hydrothermal method

Hydrothermal development is a specific antidote process for setting up nanoarrays of MO on complex substrates for use [53,54]. Karl Emil von Schafhäutl, a German geologist who created minuscule quartz crystals in a pressure cooker, published the first case of hydrothermal crystal development in 1845 [55]. The cycle helps to show the soluble metal salts in an organic or aqueous solution or a combination of organic and water solutions at elevated temperatures and high circumstances in an autoclave. Hydrothermal chemical methods are known as easy and efficient routes on a water network and are becoming more common today in the manufacture of arranged nanoarray structures. Its approach is based on chemical reactions and changes in the solubility of materials in a sealed heated aqueous solution beyond ambient temperature and nanocrystal growth pressure. According to BET results from previous literature, which show a high specific surface area of 543 m²/g and pore volume of 0.5 cm³/g, the hydrothermal approach was discovered to be the best way for synthesising a high surface area ZrO₂. At a scan rate of 100 mV s⁻¹, the ZrO₂ nanoparticles maintain their rectangular form with good cycling and reversibility stability. With an enhanced redox peak current caused by the transfer of electrons, the CV's rectangular form exhibits the optimal capacitive behaviour. Therefore, ZrO₂ nanoparticles embedded in Nafion membrane are a potential electrolyte for fuel cell applications [56].

Deposition method

Deposition method is also one of the simple methods to produce metal oxide. The method to prepare nanoparticles is quite similar to the Au/TiO₂ catalyst preparation. One benefit associated with this method is its ability to generate tiny nanoparticles of gold that are less than 10 nm in size [57]. In addition to ensuring convenience in the management of synthesis, electrochemical deposition is introduced which contributes to the best results in terms of synthesis and has many advantages in the development of metal oxide structure. This is demonstrated by the fact that ZnO can be successfully developed using appropriate electrolytes from the preparation of ZnO [58]. Rajalaksimi et al. reported that the durability of catalyst is one of the major issues in PEMFC application. Therefore, they prepared nano TiO₂ as a candidate for catalyst support for Pt electrocatalyst. The deposition of TiO₂ in the Pt matrix is anticipated to prevent the aggregation of Pt particles, efficiently scatter the Pt atoms in the clusters, manage the nanostructure of the catalyst, and offer corrosion-resistant heat and oxidative stability. Compared to typical Pt supported on carbon catalysts, these catalysts’ electrochemical activity and thermal stability were shown to be superior [59].

Soak-deoxidise-air oxidation

This approach can be used to process compounds such as ZnO/Cu₂O especially use in photocatalysis activity. Here some methods of the experiment will be discussed where ZnO has diluted in 50 ml CuSO₄·5H₂O solution containing Cu²⁺ at a concentration of x mol/L and sonicated to produce a homogenous suspension. After that, the solution is then vigorously agitated for 30 min at room temperature to ensure that the Cu²⁺ ions are deposited on the ZnO nanoparticles’ surface. The solvent is solubilised and treated respectively three times using alcohol solution and deionised (DI) water to extract free Cu²⁺. 30 mL of DI water is slowly poured for suspension while being gradually heated for 5–10 min in a water bath with constant stirring. Slowly apply a solution of N₂H₄·H₂O (1 M) to the suspension until a red-brown precipitate has developed. Once the temperature reaches a stable equilibrium, the solution was filtered in the water bath for 30 min while stirring, products are obtained by filtration, cleaned multiple times with alcohol and DI water, and gradually dried in air for 3 h at 60°C [60]. Regarding this technique, it is a big opportunity to be used in the fuel cell application which may bring high performance in the achievement.

Impregnation and reduction

The method is straightforward economical and friendly handling. In this method, low loading of precious metal is attained by adsorption of the precursor or through the exchange of ions. According to that, if higher loadings are demanded, the washing step is skipped and the support is dried directly, ensuring that all precursor remains on the support (impregnation and drying). Generally, a metal salt solution is impregnated in a mesoporous substratum and the metal complexes are reduced to metallic nanoparticles. Several nonporous metal-incorporated products were developed by the impregnation and reduction process. The metal salt solution adsorbs directly onto mesoporous substratum or associates with it chemically. As demonstrated by Wen et al. [61] fabricated metal nanoparticles were embodied via the impregnation route in the ceria microchannels. Additional research Lysine-assisted modulated mesoporous alumina strongly supports Au nanoparticles about 2 nm in size [62]. Campelo et al. [63] documented a simple and environmentally friendly approach to prepare highly active and dispersed silica-supported Au and Pd nanoparticles using a microwave process without using any reduction agent which also provides some benefits over the traditional reduction in the concentration of hydrogen at high temperatures [64]. Rather than use a metal solution to wet the mesoporous substratum, Gao and Ying first fabricated SBA-15 with a dendrimer [65]. The technique of impregnation and reduction may be used to load multiple metals on the same mesoporous support. Futamura and his team [66] proved that methods impregnated with noble metal give up to 95% utilising performance beneficial to the fuel cell application. In conclusion, lead by wetting later the precursor solution in this method will only fill the support pores to avoid deposition on the catalyst external and beneficial in slight waste (Table 2). The variety of such methods of synthesis can be advantageous to attaining high performance in fuel cells (Figure 4). OPEN IN VIEWER

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Table 2

The production of various MO materials and the advantages and disadvantages of each method are described.

Type of MO material	Preparation methods	Advantageous	Disadvantageous
TiO2	Co-precipitation [67]	Simple process
		Easy to perform in the laboratory
		Has a significant surface area per unit mass and has the potential to improve physical, chemical, and electrical characteristics when compared to the corresponding qualities displayed in the material's bulk form.
	Hydrothermal [68]	It can generate massive, high-quality crystals while keeping tight control over their chemical structure.	The requirement for costly autoclaves
		Enhance photocatalytic activity as the proportion of photocatalytically active anatase phase in derived TiO2 increases.
	Chemical vapour deposition [69]	At low or high rates, produce consistent, pure, repeatable films.	High price for substances with enough purity
		Good compatibility with adhesion	Low deposition rates
SiO2	Sol gel [70]	It may sinter at low temperatures ranging from 200 to 600° C.	Fine pores
		Straightforward process, cost-effective, and quick technique of producing high-quality coverage	Long processing time
	Chemical precipitation [71]	Employs less expensive materials	Sustaining consistent product quality is challenging
		Potentially industrialisable
ZnO	Chemical Process [72]	Low manufacturing costs, tiny particle sizes, low tendency for particles to aggregate, and great uniformity of crystalline structure and morphology.	Constant powder grinding and grain size reduction, which decreases with increasing milling duration and energy
	Hydrothermal [73] [74]	Simple equipment, mild preparation conditions and minimal cost.	The reaction takes a long time
		May transform the microstructure of ZnO powders from short prisms to flower-like formations.
		The size and form of nanomaterials can be varied by hydrothermal processing.

Titanium dioxide (TiO₂)

Since its utilisation by Fujishima and Honda for electrochemical photolysis of water [75], TiO₂ has been commonly used in electrical and photoelectric fields. TiO₂ has been explored as promising support for fuel cells, owing to its stability in the operation condition, minimal production cost, commercial availability, non-toxicity, and simplicity in regulating their size and structure. The existence of strong connections between TiO₂ and metal nanoparticles is a great advantage [76], as it can prevent the metal particle from aggregating together, disperse metal atoms in clusters, and affect the electrical properties of metal nanocatalysts. TiO₂ was used for oxygen reduction in acidic and alkaline mediums [77], early in the 1980s and 1990s.

Huang et al. [78] prepared Pt-Ir/TiO₂ electrocatalysts and their physical incorporation was evaluated as a bifunctional oxygen electrode (BOE) catalyst for unitised regenerative fuel cells (URFCs). The Ir-TiN performance was tested and the findings indicated that it is suitable to be used as a corrosion-resistant microporous layer. The URFC performance also showed a significant enhancement in round-trip efficiency of supported Pt-Ir/TiO₂ (42.2%) compared to unsupported Pt-Ir black (29.8%). Moreover, the Pt-Ir/TiO₂ also demonstrated higher UFRC performance than the Pt-Ir black catalyst with a metal loading of 1.0 mg cm⁻² and 2.0 mg cm⁻², respectively on the oxygen electrode. The improvement of URFC performance was ascribed to the support effect of offering a high specific surface area. Even though TiO₂ as a catalyst or support has a certain catalytic feature, its poor electrical conductivity and surface area limit its usage in PEFCs. These issues have been alleviated by the change in morphology and composition of TiO₂ material are discussed in detail below.

First, the composition of TiO₂ can be modified into sub-stoichiometric Ti_n O_2n-1. The electronic conductivity of TiO₂ must be increased for PEFC electrochemical application. One technique that was discovered to increase the electrical conductivity of TiO₂ is by decreasing pure TiO₂ and forming oxygen vacancies to obtain sub-stoichiometric TiO₂. The existence of titanium interstitials and oxygen vacancies helps to reduce the band gap of TiO₂, hence increasing their electrical conductivity. This sub-stoichiometric TiO₂ also known as the Magnéli phase possesses a higher conductivity than graphite and has a general formula of Ti_nO_2n-1 (4 < n < 10) [79]. The conductivity of Ti₄O₇ rises as high as ca. 10³ S cm⁻¹ at room temperature. Furthermore, there were no oxidation peaks obtained between 0 and 2.0 V vs. NHE in 1 M H₂SO₄ solution for Magnéli phase TiO₂ [30]. Magnéli phase TiO₂ can be applied as PEFC catalyst support materials because of their high conductivity and oxidation resistance. Since Chen et al. revealed Pt–Ru–Ir catalysts deposited on Ebonex (several sub-oxides of TiO₂, primarily Ti₄O₇ and Ti₅O₉) and Ti₄O₇ as bifunctional oxygen reduction/water oxidation catalysts [80] the group of Ioroi has conducted some studies on Ti₄O₇ as catalyst support for fuel cell applications. They synthesised Ti₄O₇ by decreasing TiO₂ powder at high temperatures in an H₂ condition and discovered that it has a substantially higher onset oxidation potential (ca. 1.8 V) than Vulcan XC-72 (ca. 0.9 V) and is extremely stable under the PEFC working system [81]. The HOR and ORR-specific activity of Pt nanocatalysts supported on Ti₄O₇ are comparable to those of the Pt/C catalyst. They also examined the impact of high-potential holding and cycling on the Pt/Ti₄O₇ catalyst's stability and electrochemical activity [82]. The Pt/Ti₄O₇ catalyst is highly stable with high potentials of up to 1.5 V. Even after 350 h of operation at 300 mA cm⁻², there is no voltage deterioration when Pt/Ti₄O₇ was used as the cathode in pure H₂/O₂ fuel cells. Ioroi et al. [83,84] invented a novel pulsed UV laser irradiation technique to create nanometric Magnéli phase TiOx with a specific surface area larger than 20 m² g⁻¹ to enhance the surface area of the TiO₂ support. After cyclic tests sweeping from 0.05 to 1.0 V at a scan rate of 10 mV s⁻¹ in 0.1 M perchloric acid (HClO₄) solution using the rotating disk electrode method at room temperature, the resulting Pt–TiOx and Pt–Ti/TiOx catalysts had 2-fold higher specific activity for the ORR than Pt/Vulcan XC72 catalyst. Figure 5(a) revealed that they still maintain their initial electrochemically active area even after 10,000 potential sweep cycles between 1.0 and 1.5 V. Zhang et al. [85] used a pyrolysis process followed by microwave radiation to develop Ti₄O₇ supported Pt@Ru core–shell. The purpose of this study is to increase the durability of PtRu by including a core–shell structure and a strong bonding with Ti₄O₇ particles. The first stage in this approach is to co-reduce a combination of TiO₂ and ruthenium precursor under heat treatment in H₂ reducing environment to produce a Ru core on Ti₄O₇ support, and the second stage is to build a platinum shell using microwave irradiation. The Ru@Pt/Ti₄O₇ catalyst displays a higher carbon monoxide (CO) tolerance capability towards H₂ oxidation than the PtRu/C alloy catalyst. In PEFCs, sub-stoichiometric TiO₂ exhibit significant co-catalytic activity with Pt. It must be noted that they tend to oxidise to stoichiometric TiO₂ over long-term cell operation, which is resulting in a decrease in electrical conductivity and also electrocatalytic performance. As a consequence, improving the long-term durability of sub-stoichiometric TiO₂ is still a concern among researchers. OPEN IN VIEWER

Doping TiO₂ with other metals is another method to enhance the electrical conductivity properties of TiO₂. The material that has received the greatest attention is niobium (Nb) doped with TiO₂, which achieves electrical conductivity around 0.2–1.5 S cm⁻¹ [80]. Despite that, the charge compensation for Nb⁵⁺ replaces Ti⁴⁺ is accomplished either by stoichiometric reduction of Ti⁴⁺ to Ti^3+. or by creating one Ti cation vacancy for every four Nb atoms when TiO₂ is doped with Nb. Nb_0.1Ti_0.9O₂ was discovered by Chen et al. [80] as catalyst support for Pt-Ru-Ir, and they observed that it has a more extensive range of −0.4 to 2 V vs. NHE and it is more stable than Ebonex and Ti₄O_7. Garcia et al. [86] doped Nb into anatase TiO₂ support by using the sol–gel technique. Then, the PtRu/Nb_0.1Ti_0.9O₂ catalyst was prepared using lithium borohydride (LiBH₄) as a reducing agent in tetrahydrofuran solvent. In comparison to the PtRu/C catalyst, the PtRu/ Nb_0.1Ti_0.9O₂ catalyst demonstrated a 6% increase in mass activity and a 100% increase in the specific activity. The mass activity of the PtRu/Nb_0.1Ti_0.9O₂ membrane electrode assembly (MEA) is 46% higher than the E-TEK MEA.

The use of Nb-doped with TiO₂ was investigated by Park et al. [87] with a 10 nm particle size. These catalysts were synthesised via a hydrothermal approach followed by a borohydride reduction procedure to support Pt nanoparticles on the substrate. As seen in Figure 5(b), the ORR of the Pt/Nb-TiO₂ catalyst has a higher mass activity with a value of 235.3 mA mg⁻¹ and onset potential (1.0 V) than Pt/Vulcan XC-72 with a value of 58.8 mA mg⁻¹ and 0.9 V onset potential. The improvement in ORR performance is owing to the excellent Pt dispersion on Nb-TiO₂ catalyst support and great interaction between metal catalyst and oxide support. Other than Nb, molybdenum doping also can improve the electrical conductivity of TiO_2. This process may increase the TiO₂’s electrical conductivity by producing more vacancies for Ti, whereas it can transport more electrons to Pt from the TiO₂ support via a strong metal catalyst-oxide support interaction. These findings suggest that if doping TiO₂ with other metal ions may contribute more electrons to Pt or produce more Ti vacancies, it may be able to dope TiO₂ to produce highly co-catalytic TiO₂ [4].

Owing to their high efficiency and environmental friendliness, PEMFCs have sparked a lot of interest in recent years. Rajalakshmi et al. [59] investigated TiO₂ as alternative catalyst support for electrocatalyst Pt and can withstand the corrosive environment of PEMFC application. The incorporation of TiO₂ support in the Pt matrix was designed to prevent Pt particle aggregation, efficiently disperse the Pt atoms in clusters, regulate the catalyst's nanostructure, and enhance oxidative and thermal stability against corrosion. The Pt/TiO₂ catalyst was reported to achieve better thermal stability compared to the conventional Pt/C catalyst. This catalyst demonstrated high electrochemical activity, durability, and fuel cell performance. Most of the ORR on TiO₂ occurs through a 4-electron route. Kim et al. [88] synthesised TiO₂ catalyst as the non-Pt catalyst via heat treatment of titanium sheets at a temperature ranging from 600°C to 1000°C. In 0.1 mol dm⁻³ H₂SO₄ solution, the prepared catalysts were chemically and electrochemically stable. For the ORR, the TiO₂ catalysts have varied catalytic activity and the ORR of TiO₂ catalyst heat-treated at 900 °C happened at a potential of 0.65 V vs RHE. The difference in ORR catalytic activity of heat-treated TiO₂ catalysts is attributed to a change in the material property of the catalyst surface caused by the heat-treatment condition. It was discovered that the ORR catalytic activity of TiO₂ improved as the specific crystalline structure, such as TiO₂ (rutile phase) (110) plane and the work function also increased. A change in surface state such as crystalline structure and work function is expected to impact the ORR catalytic activity.

The other metal that is currently studied to dope with TiO₂ particles is the bismuth (Bi) element. Bhowmick et al. [89] impregnated Bi element on pure TiO₂ (Bi-TiO₂) and evaluated it as photocathode catalyst in microbial fuel cell (MFC). Based on electrochemical impedance spectroscopy (EIS), Bi-TiO₂ achieved twofold greater exchange current density and a lower charge transfer resistance than pure TiO₂, indicating that it is a superior ORR catalyst. MFC performance was conducted by using Bi-TiO₂ and could produce a maximum power density of 224 mW m⁻², which was higher than MFC that operated with Pt as cathode catalyst (194 mW m⁻²). However, both of these performances are much higher than MFC that operated with TiO₂ catalyzed cathode and without any cathode catalyst which is 68 and 60 mW m⁻², respectively. These findings are proposed Bi-doped TiO₂ catalyst as a superior low-cost alternative to the highly expensive Pt.

Tungsten oxides (WO_x)

Tungsten oxides (WO_x) gave much attention and have been produced using several approaches and employed in many fuel cell systems. Lalande et al. [90] found that Pt nanocatalysts on Pt/WOx exhibit CO tolerance comparable to commercial PtRu black catalyst in fuel cell tests with H₂ and 100 ppm at the anode. Shafia Hoor et al. [91] and Lee et al. [92] used an electrodeposition approach to make Pt/WO₃ and PtRu/WO₃ nanocatalysts. In contrast, Park et al. used a multi-sputtering deposition process to reach Pt/WO₃ and PtRu/WO₃ nanocatalysts [93,94]. They discovered that Pt/WO₃ and PtRu/WO₃ have more incredible catalytic activity than Pt and PtRu for the MOR. The existence of alloy nanophases and WO_x spillover impacts are intimately related to the improved catalytic activity of the MOR. The role of WO_x as a co-catalytic was suggested by Hobbs and Tseung [95] in the anodic hydrogen oxidation reaction on Pt/WO₃ electrodes by forming H_xWO_3. Many research on WO_x in fuel cells reported since the 1960s. After that, the Tseung group conducted many experiments using PtRu and Pt nanocatalysts modified with WO₃ in the electrooxidation process of formic acid [96], methanol [97], CO and H₂ [98]. They discovered that Pt and PtRu as anode and cathode electrodes modified with WO₃ are more efficient and poison resistant than Pt and PtRu alloy catalysts. Several factors have affected the promotion of WO_3, such as (1) noble metal nanoparticles dispersed uniformly on WO₃, (2) the effect of ‘hydrogen spillover’, and (3) a high level of CO tolerance towards poisoning. The dispersion of nanoparticles depends on the various production processes. The term ‘hydrogen spillover’ refers to the process of producing bronze hydrogen tungsten and then oxidising hydrogen on tungsten-hydrogen bronze. This ‘hydrogen spillover’ ensures that Pt-active sites may operate more effectively as dehydrogenation catalysts. The existence of WO₃ may promote the adsorbed water (OH_ad) development, which would oxidise intermediate impurities efficiently.

Tin oxides (SnO)

MO may be suitable as support materials for future PEFC cathodes in their most significant oxidation state. SnO and TiO₂ are two promising catalyst support materials with good potential stability that might be used to make iridium-based nano-size electrocatalysts similar to PEFC. According to Yasutake and his team [99] indium-decorated SnO₂ supported on Vapor-Grown Cabon Fiber (IrO₂/Sn(Nb)O₂/VGCF) and carbon-free Ir/TiOx/Ti sheet, where Ti sheet with naturally occurring oxide surface is employed as the catalyst support and gas diffusion layer (GDL) [100]. The Pt nanoparticles’ kinetic activity supported by SnO₂-based MOs was investigated using a multilayer method in this study. Model electrodes consisting of SnO₂ thin films supporting Pt nanoparticles, as well as porous catalytic systems comprised of Pt nanoparticles supported by Sb-doped SnO₂ high surface area powders, were studied in particular. Takabatake et al. [101] study the MO supports for PEFC electrocatalysts have been shown to have high cycle durability. In terms of stability, electrochemical activity, and resistance to dissolution, Pt/SnO₂ performs well. Pt dissolution rates in Pt/SnO₂ electrocatalysts are equivalent to those in Pt/C electrocatalysts. These findings show that SnO₂ could be a credible replacement electrocatalyst support. Pt-SnO₂ nanocatalysts were introduced by Matsui et al. [102] and Waki et al. [103], and they are shown to have a lesser onset potential for CO oxidation compared to Pt/C. Founded on features such as outstanding chemical stability, economical, electro-catalytic activity, wide band gap (3.5 eV), and ease of production, SnO₂, a well-known n-type MO semiconductor, has been extensively used in DMFCs technologies. Fan and co-workers [104] came up with a hierarchical framework decorated Pt nanoparticles on SnO₂ support. Zhang et al. [105] demonstrated a Pt-SnO2 flower for methanol oxidation. The synthesis process of Pt-SnO₂/C via a microwave-aided polyol was discovered by Sandoval-Gonzalez et al. [106]. The results revealed that the designed catalyst achieved superior stability and catalytic activity for MOR performance compared to the PtRu/C catalyst owing to the obvious bifunctional characteristics of Pt and SnO₂.

Molybdenum oxides (MoOx)

There are five Magnéli phases in molybdenum oxides, with compositions ranging from MoO₂ to MoO₃. The structure of MoO₂ is rutile, with MO₆ octahedra sharing cores and edges. Yan et al. [107] placed Pt nanoparticles on the composite support after synthesising MoO₂ on carbon. The obtained Pt/C-MoO₂ displayed good stability and higher mass activity (mA mgPt⁻¹) at 0.9 V vs. RHE for the ORR performance compared to conventional Pt/C catalyst owing to the MoO₂ promoting impact and the strong connection between Pt and MoO₂ (98.4 mA mgPt⁻¹). According to Wang and his colleagues [108], an electrochemical co-deposition process can discover Mo⁴⁺, Mo⁵⁺ and Mo⁶⁺ valence states in Pt/MoOx catalysts. The MoOx for MOR promotion mechanism was postulated to give OH species with decreased capacity for the elimination of adsorbed CO poisons. As high impurities tolerant electrocatalysts for PEFC systems, Liu et al. [109] explored into MoOx@Pt core–shell and PtMo particles. The MoOx@Pt core–shell nanocatalysts are displayed in Figure 6(a) discrete underpotential hydrogen adsorption/desorption peaks, whereas the Pt_0.8Mo_0.2 alloy catalyst does not. As shown in Figure 6(b), the MoOx core on the Pt shell nanocatalyst had a substantially decreased onset H₂ oxidation potential (0.14 V vs. SCE) compared to conventional Pt and PtRu electrocatalysts and Pt_0.8Mo_0.2 alloy. Furthermore, it achieves the diffusion-limited current at 0.1 V vs. SCE, which is significantly lower than commercial electrocatalyst. More critically, the electrical action of MoOx@Pt core–shell was thought to diminish the oxidation overpotential and degrade the connection of Pt–CO. This is owing to the MoOx cores’ inadequacy to supply OH species to the Pt shells, which are required for the bifunctional process to regenerate the Pt surface and remove CO. OPEN IN VIEWER

Carbon black

Fuel cells have a lot of potential for constructing a future electrical power generating system that is both efficient and clean. Related to its small size and low operating temperature, the PEMFC is the most viable contender for replacing combustion engines in cars and stationary applications. Munakata and his team [112] discovered the enhancements of nanoparticle composite performance for Pt-loading PEMFCs. There have been a variety of carbon-supported platinum catalysts (Pt/Cs) commercially available and employed. However, because carbon supports erode over time during PEMFC operation, metal compounds such as oxides have been investigated as catalyst supports to increase PEMFC durability. Zhang et al. [113] reported the development of a high-performance electrocatalyst for methanol oxidation employing Pt nanoparticles supported on nitrogen-doped carbon-TiO₂ composite (Pt/TiO₂/NDC) using a microwave-assisted polyol process. The synergy impact of their distinct form and composition, as well as the electrical interaction between the TiO₂/NDC, likely led to the development of Pt/TiO₂/NDC catalysts. Further exploring study regarding metal oxide with carbon material by Pongpichayakul and his colleague [114] study the CeO₂ was decorated onto a variety of carbons, involving carbon black (CB), graphene oxide (GO), reduced graphene oxide (rGO), carbon nanotubes (CNT) and mixed carbons, and then Pt was electrochemically deposited. A face-centred cubic structure was employed to study the dispersion of Pt and CeO₂ nanoparticles onto carbon surfaces. Essentially, the fabrication of MO with carbon materials as a potential for new material is studied by the right synthesis in fabrication has a high tendency in reaction performance and be used as a problem solving by the new formulation applied.

Graphene

Graphene and graphene-based materials have unique qualities such as a large surface area, great durability, thermal stability, strong conductivity, and high mechanical stability, making them ideal for use as catalyst support in acidic DMFCs. Graphene has a larger surface area (2630 m² g⁻¹) than carbon nanotubes (CNTs) and graphite, making it appropriate for energy applications such as DMFCs [115]. Pt supported on holey-reduced graphene oxide frameworks [116], PtCu-rGO [117], graphene oxide-PVP [118], graphene-fullerene [119], and graphene-nanofibre [120] also demonstrate great efficiency as an electrocatalyst for methanol oxidation. Pt with nanoflowers structure evenly distributed over rGO-SnO₂ is studied by Sha and his colleagues [121] for optimal utilisation of Pt nanostructures and better catalytic activity. In order to achieve Pt nanoflowers with uniform dispersion and high density properties, Pt nanostructures are synthesised using a new, ecologically hydrothermal procedure using lemon extract followed by Pt electro-deposition process. Yu et al. [122] published a paper describing the synthesis of a porous molybdenum dioxide (MoO₂)/graphene oxide (POMOFs/GO) composite from a polyoxometalate-based MOFs/graphene oxide (POMOFs/GO) precursor. Owing to the presence of phosphorus (P) in the POMOFs precursor, the final composite product was made up of MoO₂, phosphorus-doped nanocarbon (PC), and RGO support (referred to as MoO₂@PC-RGO). Since of their distinctive 2D nanostructure, outstanding electrical conductivity, and high electrochemical stability, graphene sheets are a viable component to act as a support material for electrocatalysts in this POMOFs/GO-assisted method.

Carbon nanotube

Carbon nanotubes (CNTs) are tube-shaped carbon structures that come in a variety of lengths, thicknesses, and layer counts. One of the most important strategies for producing composite support is to use carbon or CNT to improve the MO's properties and surface area. For example, a hybrid of titanium cobalt nitride (TiCoN) and CNT has recently been employed as a supported catalyst to improve methanol oxidation reaction corrosion resistance [123]. After 5000 s of operation, the Pt catalyst supported on TiCoN-CNT showed increased activity and corrosion resistance [124]. The current density per mass of Pt was as high as 1400 mA mg⁻¹ in a study utilising a hybrid catalyst of Pt and nickel phosphide (Ni₂P) support on CNT [125]. TiN-CNT [126], a unique network connecting Pt and MWCNT [127], and dihydroxypolybenzimidazole (2OH-PBI)-MWCNT [128] as support for the methanol oxidation reaction in DMFC are two more studies that changed or added other materials to CNT. The incorporation of TiO₂ into Pt-Ru matrix was investigated by Chen et al. [129]. When compared to PtRu/C catalysts, this new Pt-Ru-Ti/C catalyst had a greater catalytic performance for the MOR. The researchers discovered that the TiO₂ particles added to the catalyst reduce the grain size to roughly 12 nm and improve intra-particle dispersion significantly. X-ray absorption fine spectra (EXAFS) analysis was used to create a Pt-Ru-Ti/C catalyst with a multi-scale design.

PtRu/C catalysts from Johnson-Matthey (Pennsylvania, USA), E-TEK (New Jersey, USA), and in-house preparations contain a Pt-rich encircled by a Ru-rich shell, as shown in Figure 7. This research showed that there is no discernible Pt and Ru segregation in the Pt–Ru–Ti/C catalyst, showing that TiO₂ plays a role in boosting Pt and Ru atom dispersion in the catalytic clusters and encouraging intra-particle dispersion [4]. In addition, because of their high conductivity, remarkable stability, and huge specific area, CNTs have been combined with TiO₂ nanoparticles as composite catalyst support for PEFC systems. According to Muhamad et al., Pd/C catalysts modified with TiO₂ have greater catalytic activity for CO/H₂ performance, than commercial Pt/C catalysts [130]. It has been shown that the addition of TiO₂ nanoparticles enhances both metal particle dispersion and surface area, and it also improves CO tolerance. In a study conducted by Selvarani et al. [131], they investigated the heat treatment of Pt-TiO₂/C catalysts at 750°C with 2:1 atomic ratio of Pt:Ti exhibits improved tolerance towards methanol while sustaining excellent catalytic activity. The peak power density of the DMFC with a Pt– TiO₂/C cathode was 180 mW cm⁻², which was greater than the peak power density of the DMFC with Pt/C, which was 80 mW cm⁻². Song et al. used a sol–gel approach to cover a thin layer of amorphous TiO₂ on CNTs and an ethylene glycol reduction process to create Pt composite nanoparticles [132]. Comparing the Pt–TiO₂/CNTs catalyst to conventional Pt/CNTs and Pt/C electrocatalyst, the Pt–TiO₂/CNTs catalyst displayed excellent CO tolerance and higher electrocatalytic activity for EOR performance. Table 3 shows the electrodes-based MO that gives good potential as used in fuel cell applications where the performance more stable rather than commercial catalyst. OPEN IN VIEWER

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Table 3

Electrode metal-based in fuel cell application.

Electrodes	Method	Application	Power Density (mW cm−2)	Ref
Graphitised nano fibre -PtRu	Polyol	Anode in MFC	19.2	[133]
NiO–CuO/Gt	Co-precipitation method	ORR in MFCs	21.25	[134]
CNTs/CNFs	Electrospinning	Anode in MFC	362 ± 20	[135]
Ni2Co-rGO	Physically mixed	Anode in DGAFC	40.44	[136]
TiO2-C	Supramolecular self-assembly with in situ crystallisation process	Anode in DMFC	91	[137]
(Pd1Sn3)10(CeO2 NR)20(Vn)70	Chemical reduction method	Anodes in ADEFC	30	[138]
u-SnO2/Co3O4	Facile method	Anode in DBFC	52.5	[139]
Pt/TNTS-Mo	Polyol	ORR in PEMFC	0.52	[140]
Cu and Mo-doped Ca3Co4O9−δ	Liquid displacement	Cathode in SOFC	0.367	[141]
MoO2-based	Electrostatic spray deposition (ESD)	Anode in SOFC	3000	[142]

TiO₂-based composite membranes

Owing to its outstanding stability, inexpensive, excellent photocatalytic activity, and non-toxicity, TiO₂ nanoparticles have been the subject of various research. Polyvinylidene fluoride (PVDF) is a prominent polymer for fabricating membranes but this polymer is easily contaminated by proteins and other contaminants, resulting in a significant decline in membrane fluxes [160]. Various synthesis methods can be used to enhance hydrophilicity properties, however, this will damage the PVDF molecule's main chain and reduce the PVDF membrane's benefits. The incorporation of TiO₂ material in the PVDF membrane not only enhances the hydrophilicity properties but also inhibits biofouling in membrane bioreactor systems and PVDF membranes [161]. Aside from that, PVDF/TiO₂ has the potential to considerably enhance the rate of decomposition of isoproturon, a phenyl urea herbicide [162].

In the other study, PVDF/TiO₂ ultrafiltration membranes were fabricated via the technique of phase inversion and evaluated by blood serum albumin (BSA) retention and pure water flux effectiveness [163]. The major goal of this research was to examine and compare the influence of TiO₂ diameters on the PVDF structure and performance using a variety of methods. Smaller-sized TiO₂ nanoparticles provide improved antifouling properties in the PVDF composite membrane. Besides, the PVDF/TiO₂ membrane with a smaller nanoparticle diameter has a smaller mean pore size, decreased surface roughness, and more apertures inside the membrane, according to SEM and AFM measurements. The PVDF/TiO₂ membrane with smaller nanoparticles exhibited lower mean pore sizes, reduced surface roughness, and more pores inside the membrane, as revealed by AFM studies (Figure 9(a)). Figure 9(b) displayed the XRD graphs of PVDF/TiO₂ membrane, PVDF membrane, and TiO₂ nanoparticles with different granular diameters. Based on XRD characterisation, smaller TiO₂ nanoparticles demonstrated a greater influence on the crystallisation of PVDF. OPEN IN VIEWER

In the other study, Hasegawa et al. [164] investigated the dispersion of TiO₂ into Nafion polymer membrane through liquid phase deposition (LPD) and this method had been carried out through titanium-fluoro complex reaction in an aqueous solution. The findings revealed that the dispersion of TiO₂ increases the hydrophilicity and hygroscopic properties of PEM in PEFC applications operated at high temperatures. The TiO₂ nanoparticles deposited on the Nafion membrane ranged from 0.07 to 0.18 wt-% for LPD reaction times varying from 3 to 9 h. Figure 10(a) shows the impedance plots for Nafion and Nafion/TiO₂ membranes. It can be seen from the figure that Nafion/TiO₂ displayed better conductivity than Nafion 117. This enhancement in conductivity is caused by nanoparticles that are constrained in a cluster of Nafion at the surface. These nanoparticles also participate in water retention since the majority of TiO₂ is present as nanoparticles. The incorporation of TiO₂ has no effect on the Nafion's ion-exchange capacity but improved water retention capacity of this composite membrane from 63% to 83%. The results proved that the incorporation of a small amount of TiO₂ enhanced water uptake and also increase the membrane performance. OPEN IN VIEWER

The fabrication of Nafion membrane with TiO₂ as inorganic filler had been studied by Mousavi et al. [165]. A simple solution casting method was used to fabricate Nafion/TiO₂ composite membrane. It was made with different solvents such as dimethylformamide (DMF), dimethylacetamide (DMAc), and N-methyl-2-pyrrolidone (NMP) for microbial fuel cell. The morphologies of these Nafion/TiO₂ composite membranes were characterised using SEM microscopy. SEM images of Nafion composite membranes in Figure 10(b) displayed homogeneous morphology with 1 wt-% TiO₂ particles. The lower gap between the solubility characteristics of the polymer material and solvent, suggests a greater interaction between the solvent used and the polymer. The interaction between Nafion polymer and DMF solvent can give an impactful role in micro phase-separated structures, owing to the existence of strong hydrogen bonding. This makes the polymer-solvent interaction stronger compared to the interaction between Nafion polymer and NMP solvent. When DMF solvent is used to make a Nafion/TiO₂ composite membrane, it has the highest porosity, the best morphology, and the highest membrane conductivity. After testing the membranes in an MFC system, it was determined that Nafion/TiO₂ produced in DMF solvent offered the maximum power density. This can be concluded that filler like TiO₂ can influence membrane activity either by physical or chemical interaction.

Jang et al. [166] prepared Nafion/TiO₂ composite membrane with prism-patterned structure for the PEMFC application at elevated temperature and low relative humidity conditions. This composite membrane was created by dispersing evenly formed TiO₂ layers over a Nafion membrane and then imprinting it using a micro-sized prism mould. The use of a spin-coating method to produce the TiO₂ layers resulted in well-distributed TiO₂ layers with no aggregation issues. Other than that, more proton routes are also enabled by the extra prism patterning process. This process is done by deeply embedding the pre-inserted TiO₂ particles in the Nafion polymer matrix and this technique could improve the interfacial surface area between the composite membrane and catalyst layer, and also increase the distance between TiO₂ nanoparticles. This prism-patterned Nafion/TiO₂ outperformed Nafion 211 polymer membrane under low humidity and high temperature circumstances owing to the hygroscopic properties of TiO₂ and also the prism patterning effect.

Other than Nafion, polymer membranes such as polybenzimidazole (PBI), polyvinyl alcohol (PVA), and polyaniline (PANI) also used to be incorporated with TiO₂ particles to increase PEM performance. Aparico et al. [167] developed a solution-casting approach to fabricate PVA/TiO₂ composite membranes with glutaraldehyde solution (GA) as a linking agent to improve the physical, thermal, and chemical properties of the membranes. The degree of cross-linking in this study was varied by changing the reaction time. The results showed that by increasing cross-linking reaction time (h), the value of proton conductivity increases by several magnitude orders, achieving a maximum of 0.016 S cm⁻¹ at 130 °C for a PVA/TiO₂ composition of 1:12% that was cross-linked with GA for 42 h and subsequently immersed in a 32 wt-% KOH solution for another 24 h. Nano TiO₂ doped with PVDF and phosphonated PVA (PPVA) was studied by Yagizatli et al. [168] to enhance fuel cell properties. The TiO₂ was incorporated at varying wt-% (2, 5, 8, 10, and 15%) to increase the performance of membrane and the result demonstrated that the PPVA/PVDF membrane with 8 wt-% nano-TiO₂ had an outstanding outcome, with a current density of 382.7 mA cm⁻² and a power density of 225.4 mW cm⁻² at 0.6 V cell potential. The performance of the synthesised membranes that are similar to Nafion 117 shows that these composite membranes are potential for PEMFC. The presence of TiO₂ material enhanced the hydrophilicity and mechanical stability of the composite membrane, therefore researchers concluded that TiO₂ inorganic material is a promising material to enhance fuel cell properties.

SiO₂-based composite membranes

Other than TiO₂ nanoparticles, silicon dioxide (SiO₂) has been extensively studied and found to be appropriate owing to its properties such as being thermally and chemically stable, large surface area, being relatively environmentally inert, and being highly suspendable in aqueous solution [169]. SiO₂ is a group IV MO and has excellent electrical insulation, abrasion resistance and good thermal stability. The SiO₂ surface enhances the dispersion of nanoparticles in aqueous solutions, making it extremely miscible because of its electrostatic stability. Polysulfone (PSF) membranes are now widely used in a variety of fields because of their resistance to chlorine and oxidation and superior physicochemical stability. Despite that, they have a weaker ability of anti-fouling because of high hydrophobicity properties and low surface energy. One of the most frequent approaches for improving their hydrophilic properties is to dope the inorganic oxide into the polymer to generate organic–inorganic composite membranes, which is a combination of organic and inorganic components. This technique has been extensively explored because of its simple process technology and working process.

Some studies have focused on non-stoichiometric nano-SiO₂ doped with Ce that was synthesised first and subsequently mixed with PSF to produce new and unique composite membranes [170]. Nevertheless, it is worth noting that the SiO₂ composition in the PSF membrane does not vary for various compositions in this experiment. The SiO₂ composition was purposely kept constant at 10 wt-% to discriminate between none addition of SiO₂ nanoparticles on PSF membranes, the impacts of modified SiO₂ nanoparticles, and unmodified SiO₂ nanoparticles. The presence of nanoparticles can improve the permeability, mechanical stability, and other properties of the membrane. Unfortunately, a higher nanoparticles weight percentage in membranes may degrade membrane performance and lead to losses. Tensile strength is increased to some extent when inorganic oxide nanoparticles are added. The rationale given was that the intermolecular forces between inorganic oxide nanoparticles and polymeric chains dispersed evenly in PSF limit the free motion of polymeric chains, and increase the membrane's tensile strength consecutively. The tensile strength of the membrane is increased as a result of the inorganic oxide nanoparticles being placed in an interlocking polymer chain [171]. Furthermore, the incorporation of inorganic oxide nanoparticles can improve the hydrophilic property of the membrane. This is because inorganic oxide nanoparticles have hydrophilic hydroxide radicals on their surface. Based on porosity analysis, there is improvement in porosity by incorporating inorganic oxide nanoparticles. The addition of inorganic oxide nanoparticles decreases the PSF crystallinity and raises the amorphous portion, resulting an increment in membrane porosity. Finally, it can be stated that the PSF membrane with modified nano-SiO₂ is not only tensile force tolerant but also fouling resistant and practical as well as desirable in treating wastewater containing oil.

Apart from the addition of SiO₂ nanoparticles to PSF membranes, various polymeric membranes have been studied, and some of them have shown an improvement with the addition of SiO₂ nanoparticles. Poly(vinyl alcohol) (PVA) is another common polymeric membrane that has been modified by SiO₂ nanoparticles. As widely known, PVA is a semi-crystalline glassy polymer that is employed in technological and scientific applications. Unfortunately, the PVA membrane also has disadvantages such as its hydrophilic nature in water systems that cause the mechanical and chemical features to become unstable, restricting its application in aqueous conditions [172]. Numerous studies have investigated the implementation process of blending, grafting, or cross-linking into PVA membranes to improve mechanical properties and membrane stability. The introduction of inorganic filler materials into polymer membranes is another way to enhance mentioned properties. Inorganic fillers in PVA composites have been found to sustain higher water concentrations, improve membrane thermal stability, and enhance pervaporation selectivity. Some research showed that polymeric inorganic filler composites in gas separation applications demonstrated high permeability and enhanced selectivity.

The methanol crossover problem is one of the most challenging concerns for DMFC. Nafion/Palladium-silica nanofibre (N/Pd-SiO₂) composite membranes with varied fibre loadings were fabricated using a solution casting process to solve this problem without affecting membrane conductivity [173]. The palladium nanofibres with SiO₂ support were made using a simple electrospinning approach and have diameters between 100 and 200 nm. All composite membranes outperformed Nafion 117 in terms of water absorption and ion exchange and were shown to be thermally stable for use as PEM. The composite membranes with 3 wt-% of fibre content had optimum proton conductivity of 0.1292 S cm⁻¹ and decreased methanol permeability of 8.36 × 10⁷ cm² s⁻¹. The maximum power density of the composite membrane was found to be higher than that of commercial Nafion 117 in single-cell tests.

A novel bioinspired Nafion (Bio-Nafion) membrane composed of Nafion matrix and biofunctional SiO₂ (Bio-SiO₂) nanofibres was developed by Wang et al. [174]. Electrospinning process of silica sol produced from tetraethyl orthosilicate resulted in the formation of SiO₂ nanofibres. Meanwhile, bio-SiO₂ nanofibres were created by immobilising amino acids (lysine, serine, glycine and cysteine) on SiO₂ nanofibres, which functioned as excellent proton conductors with many protons (H⁺) transport sites. The composite membranes were designated as Nafion-Lys, Nafion-Ser, Nafion-Gly, and Nafion-Cys, and the distinct amino acids polar groups (-NH_2, -SO₃H, and -OH,) that contributed to membrane characteristics were examined in detail. Owing to glycine having the lowest number of hydrophilic groups among the amino acid, it showed the lowest value of proton conductivity and water uptake while Nafion-Cys achieved the highest value of proton conductivity at 80°C. In general, the addition of Bio-SiO₂ nanofibres to composite membranes increased methanol permeability, dimensional stability, and proton conductivity considerably.

Amjadi et al. [175] also studied the incorporation of SiO₂ nanoparticles in Nafion 117 membrane via sol–gel reaction. The main goal of this study is to decrease the hydrogen permeation rate across the composite membrane in PEMFC application. The findings stated that Nafion/SiO₂ displayed low hydrogen permeability (Figure 11(a)). Lower gas permeabilities are caused by doped particles that have been higher loaded. The decrease of void spaces in the membrane could be attributed to the lowering of H₂ permeability by the addition of SiO₂ particles to the Nafion membrane. In reality, mineral particles embedded in the Nafion matrix network inhibit hydrogen from penetrating by making the diffusion channel more torturous. As seen from Figure 11(b), water absorption rises as SiO₂ doping level increases up to 7 wt-%. The Nafion with 7% SiO₂ absorbs 36% more water compared to the pure Nafion membrane. This is owing to the presence of hydrophilic inorganic particles embedded in a Nafion matrix. Owing to the masking of hydrophilic HSO3 groups in the Nafion clusters, an increase in doping quantity reduces water absorption. Tg value in DSC measurements for composite membrane also shows an increment compared to unmodified Nafion membrane. In general, the proton conductivities of composite membranes are lower compared to unmodified Nafion membranes at room temperature. However, the modified membranes outperform the pure Nafion membrane at 110°C and low humidity. The PEMFC performance of Nafion/SiO₂ at various temperatures and relative humidity levels is shown in Figure 11(c). The Nafion membrane with a SiO₂ content of 5–7% absorbed more water and performed better in fuel cell application. OPEN IN VIEWER

Owing to the prevalence of significant levels of CO in the reformats, strong CO tolerance or resistance is necessary for practical PEMFCs paired with onboard reformers for automotive industries. Increasing the working temperature of PEMFCs is the most effective method of improving their CO tolerance, and as a result, it is of significant technological relevance. Cheng et al. [176] investigated high-temperature polymer membrane, polybenzimidazole (PBI) doped with phosphoric acid and incorporated with SiO₂ nanoparticles. These composite membranes are capable to operate at temperatures more than 200°C. The phosphoric acid in the polymer matrix is held in place by nanoclusters of PA and phosphosilicate that were formed by polarising the membrane cells at 250°C for 24 h at 0.6 V cell voltage. This allows the membrane to be more stable at higher temperatures than traditional PA/PBI membranes. The PBI/SiO₂ composite membrane doped with PA has a high CO tolerance, with no performance degradation at CO concentrations as high as 11.7% at 240°C. With anhydrous circumstances, the single cell has a peak power density of 283 mW cm⁻² and is stable at 240°C for 100 h under a cell voltage of 0.6 V in 6.3% CO-contained H₂ fuel. Based on the results obtained by many researchers, the presence of SiO₂ nanoparticles enhanced fuel cell performance, proton conductivities, and water uptake compared to unmodified polymer membranes.

ZrO₂-based composite membranes

Zirconium dioxide (ZrO₂), similarly recognised as zirconium oxide or zirconia, is a crystalline MO that is widely used in ceramics. Its achieved mechanical resistance, heat resistivity, and abrasive abilities. In membrane application, ZrO₂ is widely used in methanol permeability in sulphonated polyetherketone to reduce water. The ZrO₂ and polyetherketone combination is a promising contender for use in fuel cells with this property. The fuel cell was created as a potential energy conversion solution, and PEFC is especially appealing for mobile applications. It is a significant research field for all of the main automobile firms. Despite that, storage and delivery remain to be a challenge. This challenge can be solved via the usage of converters to create hydrogen from liquid fuels like gasoline or methanol. Assuming methanol might be fed directly to the fuel cell, which would be ideal, the reformer might be omitted, resulting in greater technical simplicity and space savings. Membranes having good low permeability properties and proton conductivity to water and methanol remain desired for the breakthrough of DMFC development. Nafion membranes have been widely utilised in fuel cells because of their excellent proton conductivity and chemical resistance, however, their methanol permeability and production costs are still too high. Various non-fluorinated membranes for fuel cells have been studied, with the potential for lower costs. Some reports describe sulphonated polyether ether ketone, sulphonated polysulfone, sulphonated polyamides and sulphonated polyphosphazane demonstrating hydrogen fuel cells have a good performance [177,178]. Nonetheless, methanol permeability is still rather high in many circumstances, and it has been shown that altering TiO₂ and ZrO₂ leads to a decrease in methanol and water permeability.

In another study [179], hydrolysis and tetrabutylzirconate (TBZ) condensation precursor was used to create ZrO₂ nanoparticles with 6.3 ± 0.5 nm diameter via in situ method in Nafion. In this research, Nafion resin was dissolved in NMP solvent to produce Nafion solution. Then, the obtained solution undergoes solvent evaporation technique at 60°C beneath vacuum. In an inert nitrogen atmosphere at 80°C, the appropriate amount of TBZ solution in n-butanol was progressively poured into the Nafion solution under continuous stirring. After adding the required quantity of HCl solution (2 M), the mixture was continuously stirred for 1 h at 80°C and then allowed to cool to room temperature. The mixture was constantly agitated for another 8 h after adding the necessary amount of deionised water, to allow the TBZ to completely condense and yield a transparent sol containing Nafion–ZrO₂ nanoparticles. The ultimate concentration of Nafion was reported to be around 2 wt-%, and the amount of ZrO₂ was reported to be around 5 wt-%.

Figure 12(a) displayed a TEM image for Nafion-ZrO₂ nanocomposite dispersion with 2 wt-% of Nafion content in NMP solution. A drop of the solution was directly placed on a thin carbon sheet supported by a copper grid to prepare samples for TEM measurement. The in situ produced ZrO₂ nanoparticles were uniformly dispersed in Nafion with 6.3 ± 0.5 nm diameter [179]. The FTIR spectra of a Nafion–ZrO₂ nanocomposite membrane are shown in Figure 12(b), which was obtained from a previous report [179]. The FTIR spectrum of the recast Nafion membrane was also plotted in the graph for comparison. The Zr-O vibrational mode has a notable peak at 1016 cm⁻¹ in the FTIR spectrum of the Nafion-ZrO₂ nanocomposite membrane, indicating the existence of ZrO₂ nanoparticles. Some typical absorption peaks for ZrO₂ in the FTIR spectrum are difficult to discern attributed to overlap with Nafion absorption peaks. Since all of the typical absorption peaks for Nafion were observed in the composite membrane without a shift in wavenumbers, the introduction of ZrO₂ nanoparticles did not influence the structure and crystallinity of the Nafion membrane. Owing to electrostatic interactions, established Nafion molecules can self-assemble onto ZrO₂ particles, preventing the initial ZrO₂ nanoparticles from becoming any larger. At increased temperatures and relative humidity, the Nafion-ZrO₂ nanocomposite membrane outperforms the recast Nafion in terms of water retention characterisation. OPEN IN VIEWER

Sacca et al. [180] reported that recast the Doctor-Blade casting method was used to create Nafion with three varying percentages (5, 10, and 20 wt-%) of commercial ZrO₂ as an inorganic oxide filler is needed to conduct in PEFC at temperatures about 120–130°C. Nanocomposite membranes were made by dispersing inorganic oxide components in 10 wt-% Nafion. The prepared membranes were investigated in terms of water uptake capacity, ion exchange capacity, X-ray diffraction, and thermos-gravimetric analyses. The addition of the inorganic oxide component increased the mechanical properties of the developed composite membranes. Aside from that, the membranes were tested in a 5 cm² commercial single cell at temperatures ranging from 80°C to 130°C in humidified H₂/air at 3 bar pressures, and the findings were contrasted to a recast Nafion membrane using the same process. At 130°C (85% relative humidity) and 110°C, Nafion-ZrO₂ with a 10% concentration attained power densities of 387 and 604 mW cm⁻² (100% relative humidity), respectively.

Mohammadi et al. [181] recast Nafion composite membrane containing ZrO₂ and TiO₂ nanoparticles with a 75 nm mean size diameter for PEMFC application was explored. The in situ sol–gel method was used to make Nafion/TiO₂ composite membranes while tiny ZrO₂ nanoparticles were manufactured. A Nafion/ZrO₂ composite membrane was made by mixing the inorganic component with a 5% (w/w) Nafion-water dispersion. All nanocomposite membranes retained more water than non-modified membranes were compared to non-modified membranes. ZrO₂ increased proton conductivity, but TiO₂ (with a mean size of 25 nm) improved water retention. The physical structures of the membranes were then investigated using XRD SEM and AFM. The proton conductivity and water uptake for these modified nanocomposite membranes were also determined. The single-cell test in terms of I-V polarisation beneath 0.6 V and 110°C, Nafion/TiO₂-ZrO₂ demonstrated excellent performance, and 30% relative humidity and 1 atm. This study shows that the use of nanoparticles in the manufacture of MEA has a significant impact on the performance of fuel cells and that a more careful selection of the nanoparticle and its manufacturing process merits future investigation.

Other than Nafion, the addition of ZrO₂ into other polymer membranes was also investigated. Rambabu et al. [182] show the inclusion of ZrO₂ into a PSF anion exchange membrane (AEM) in a fuel cell application. In this study, innovative imidazolium functionalised polysulfone (ImPSF) membranes with ZrO₂ nanoparticles were fabricated using a solution casting technique, and the adhesion and property enhancement by the introduction of ZrO₂ were determined using structural, morphological, mechanical, and thermal analysis. ImPSF revealed better water absorption with intact morphology. These modified membranes also achieved 47% and 21% higher values of ionic conductivity and ion exchange capacity, respectively as compared to pure ImPSF membrane. With increasing contents of ZrO₂ in the nanocomposite membrane composition, the performance of fuel cells using Pt/C catalysts improved in terms of OCP and power density. ImPSF nanocomposite membrane containing 10% ZrO₂ displayed the highest OCP and power density of 1.04 V and 270 mW cm⁻², respectively, which was about 35% and 39% greater than pure ImPSF membrane. Vinodh et al. [183] prepared a novel quaternised polysulfone incorporated with ZrO₂ nanoparticles (QPSU/ZrO₂) via a solution casting procedure. Then, the characteristic properties of QPSU/ZrO₂ were studied by thermal-gravimetric analysis, electrochemical impedance spectroscopy (EIS), and XRD while the morphological properties of this membrane were observed via SEM and TEM images. Fuel cell performance was evaluated using membrane electrode assemblies (MEA) that are made up of carbon-supported platinum as electrocatalyst at the anode while platinum as cathode catalyst and hot pressed with QPSU/ZrO₂ composite membrane. Based on the obtained findings, a cheap QPSU/ZrO₂ nanocomposite membrane demonstrated better electrochemical performance in AMFC application. At 60 °C, the QPSU containing 10% ZrO₂ achieved maximum power density of 250 mW cm⁻². This finding demonstrated that QPSU/ZrO₂ is a viable choice for AMFC applications.

Membrane water uptake

In membrane performance, water is a critical component because it serves as a proton conductor. However, exceeding the appropriate water content harms the membrane and its function [196,197]. Water uptake performance may be determined by examining the change in the membrane's weight (thickness) between wet and dry circumstances. The polymer membrane is immersed in deionised water for 24 h or two days at room temperature and the weight after immersing is calculated. Then, the wet membrane is dried in a vacuum oven at a certain time and temperature. The water uptake is calculated using the below equation:

WU = mas s wet − mas s dry mas s dry × 100 %

(4)

Muliawati et al. [201] also stated that an effective membrane typically achieved high proton conductivity with high water uptake. This is because water intake significantly contributes to the establishment of hydrophilic characteristics in membranes that facilitate proton transport even at low humidity levels. The rate of water absorption in the polymer matrix may be improved by increasing the ionic group content(-OH and -SO₃H) in the polymer chain, however excessive water absorption causes the membrane to inflate, resulting in a higher methanol permeability and loss of mechanical stability. After all, the water uptake also depends on the types of MO used as inorganic filler in the polymer chain and their properties in order to enhance fuel cell performance.

Membrane proton conductivity

Proton conductivity is the most critical performance criterion in fuel cell membrane applications, with a required value of >0.01 S cm⁻¹ [202]. Four electrodes or two electrodes are employed to obtain proton conductivity values that connected potentiostat with a frequency set from 1 MHz to 0.1 Hz. The membrane was prepared rectangular with a dimension of 1 cm . 4 cm or in a circle shape with a diameter of 2 cm depending on the type of electrode used. After that, the membrane was soaked in deionised water overnight to keep the membrane in a hydrated state. The equation of proton conductivity for four electrodes was described by Shaari et al. [196], whereas the proton conductivity by using two electrodes was explored by Yusoff et al. [147]. Protons’ poor conductivity poses a significant obstacle to today's fuel cell membranes, except for the commercial Nafion membrane. One option to boost proton conductivity is to increase the ion exchange capacity by synthetic methods; nevertheless, this strategy reduces the membrane's mechanical stability. The addition of MO materials as inorganic filler is expected to significantly increase the conductivity of protons.

Zeng et al. [203] found that the addition of SiO₂ with different content displayed better performance compared with pure Nafion. They studied the proton conductivity value on pure and annealed Nafion with different content of SiO₂ (3, 5, 10 and 15 wt-%) at room temperature and anneal completed over a temperature range of 240–300°C. The results indicate that lower SiO₂ materials (<5 wt-%) slightly affected the membrane performance compared to pristine Nafion. Unfortunately, excessive SiO₂ nanoparticle addition (>10 wt-%) drastically lowered the proton conductivity values. Therefore, they only focused on Nafion with 5 wt-% SiO₂ nanoparticles. The temperature has a considerable effect on proton conductivity in composite Nafion/SiO₂ membranes (as seen in Figure 13(a)). The proton conductivity evaluated for pure Nafion, Nafion-5 wt-% SiO₂@240°C, Nafion-5 wt-% SiO₂@270°C and Nafion-5 wt-% SiO₂@300°C were 0.12, 0.11, 0.13 and 0.12 S cm⁻¹, respectively. The values indicated that high-temperature annealing had a significant effect on the proton conductivity of the composite membranes, particularly at 270°C. OPEN IN VIEWER

The other study was conducted by Wang et al. [174] for the modification of Nafion with biofunctional SiO₂ materials. Bio-SiO₂ nanofibres were created by immobilising amino acids on SiO₂ nanofibres, which functioned as excellent proton conductors with multiple H⁺ transport sites. Figure 13(b) demonstrated the proton conductivity for Nafion and bio-SiO₂ at the temperature range from 20 to 80°C. Notably, the pristine commercial Nafion membrane has proton conductivities of 0.075, 0.087, 0.106, and 0.132 S cm⁻¹ at temperatures of 20, 40, 60, and 80°C, respectively. All Nafion with bio-SiO₂ achieved higher proton conductivity values than pristine Nafion membrane. Bio-SiO₂ addition may provide acid–base pairs into PEMs as proton donors and acceptors, resulting in the creation of nanofibre proton channels with regularly spaced amino acids. Thus, this design facilitates proton hopping and increases proton conductivity. Proton conductivity values for all composite membranes are as follows: Nafion-Cysteine > Nafion-Serine > Nafion-Lysine > Nafion-Glycine, with Nafion-Cysyeine having the greatest value of 0.2424 S cm⁻¹ at 80°C. Thus, conductivity is largely reliant on the number of proton acceptors and donors in amino acids, as well as the ability of these amino acids to bind to and dissociate from H ⁺.