The Use of Titanium Compounds as Supports and Cocatalysts/Additives for Low-Temperature Fuel Cell Catalysts

Abstract

Among different non-carbon materials, due to their high corrosion resistance and chemical stability, titanium-based compounds, such as TiO₂, TiN, TiC and Ti₃C₂T_x, are potential supports for PEMFC catalysts. In addition to its main function as a support, due to its catalytic properties, TiO₂ is also used as co-catalyst/additive in the catalyst layer. In this work, the use of titanium compounds as catalyst supports and co-catalysts in the membrane electrode assembly of PEMFCs is overviewed and discussed.

1. Introduction

Fuel cells have attracted attention as they are eco-friendly energy generators that convert chemical energy to electrical energy electrochemically. Recent valuable reviews discuss different types of fuel cells and their history, fundamentals, and applications [1,2,3,4]. A schematic diagram of a typical polymer electrolyte fuel cell (PEMFC) and catalyst layer (CL) structural design is reported in Scheme 1.

Low-temperature PEM fuel cells (LT-FCs) operate at 60–100 °C using a solid polymer electrolyte. High-temperature fuel cells (HT-FCs), such as solid oxide fuel cells (SOFCs), operate at 600–1000 °C using a solid ceramic electrolyte. Due to the low operating temperature, the reaction kinetics in LT-FCs are slow, so platinum (Pt) and platinum group metals (PGMs) are required as the catalyst on the anode and cathode to lower the activation energy, making (LT-FCs) expensive. To reduce the cost, the PGM content has to be the low as possible, so to maximize their surface area, it is dispersed on a conductive support as carbon-based materials. Moreover, the use of PtTi alloys or TiO₂ as co-catalysts allows the reduction of the Pt content in the electrode, decreasing the cost. Conversely, as HT-FCs operate at high temperatures, due to the enhanced kinetics, they can use cheaper, non-noble catalysts, so they do not need to be dispersed on a support.

Among different metals, titanium (Ti) is the most versatile for polymer electrolyte fuel cell (PEMFC) application: indeed, bare Ti and Ti compounds can play many roles, such as bipolar plates (BPs) and BP coating, gas diffusion layer (GDL), catalyst support, co-catalyst, as well as additives in composite membranes, as shown in Scheme 2.

Among titanium compounds, titanium oxide (TiO₂), titanium nitride (TiN) and titanium carbide (TiC) are the most suitable for use in PEMFCs. TiO₂ is a cost-effective and efficient material, possessing some interesting characteristics such as low cost, high thermal stability, high mechanical–chemical stability, and high photo-activity. TiO₂ enhances the performance of the PEMFC either as a catalyst support or co-catalyst or as a membrane additive. The use of TiO₂ in composite membranes as well as a catalyst in low-temperature fuel cells was reviewed by Abdullah and Kamarudin [6]. TiN is a refractory solid exhibiting extreme hardness, thermal/electrical conductivity, and a high melting point. The use and the main properties, including electrical conductivity and corrosion resistance, of TiN as protective coatings on metal bipolar plates of PEMFCs has been discussed in recent reviews [7,8,9]. Among transition metal carbides, TiC has attracted interest because of its high stability under different harsh conditions. Furthermore, TiC possesses high corrosion resistance and high conductivity. These properties make TiC an interesting material for its use as a support in fuel cells [10]. Recently, Ti₃C₂T_x, a MXene belonging to a class of new graphene-like compounds, has attracted great interest [11]. Ti₃C₂T_x presents a large specific surface area, superior chemical and thermal stability, and excellent electrical conductivity. These characteristics make Ti₃C₂T_x a suitable catalyst support for application in PEMFC [12]. In addition, Ti can form stable PtTi alloy catalysts for oxygen reduction and low-molecular-weight alcohol oxidation. TiC, TiO₂, and TiN, as well as their doped compounds, have also been proposed as electrocatalysts for the ORR. However, TiC presents considerably lower catalytic activity than Pt [10]. On the other hand, TiO₂ is not an effective electrocatalyst because of its low conductivity and poor reactivity, due to intrinsic insulating properties [13]. Finally, TiN can facilitate the formation of a conductive path to the catalyst surface but cannot act as a catalyst itself, owing to its instability at temperature > 60 °C in acidic media [14]. Currently, these compounds are not yet capable of replacing the commonly used precious catalysts in fuel cells and therefore will not be considered in this review. In this work, the use of titanium compounds as catalyst supports and co-catalysts in the membrane electrode assembly of PEMFCs is overviewed and discussed. In the last year, the research focus shifted from fundamental feasibility to material durability, cost reduction, and system integration. Current research focuses on nanotechnology, producing cheaper catalysts (non-platinum alternatives) and non-carbon support and advanced membrane materials. The use of TiO₂ and PtTi co-catalysts decreases Pt content, reducing the cost of maintaining performance.

2. Catalyst Support

Carbon black and nanostructured carbon materials are commonly used as catalyst supports in low-temperature fuel cells, but the low stability of carbon nanoparticles under PEMFC operating conditions leads to a decrease in Pt surface area, due to both Pt particle sintering and Pt loss [15].

For these reasons, research efforts have been dedicated to non-carbon catalyst supports with high corrosion resistance in fuel cell environments [15]. Sinniah et al. [16] reported a number of works on the use of carbon-free compounds as Pt nanoparticle supports since 2015. SnO₂, TiO₂, and TiN were the most investigated compounds, followed by Mo₂C, SiO₂, and TiC. Their remarkable activity and durability were ascribed to the unique strong metal support interaction (SMSI) between the transition metals and Pt. More recently, two-dimensional MXenes also emerged as potential Pt catalyst supports. Due to their high corrosion resistance and chemical stability, titanium-based compounds, such as TiO₂, TiN, TiC, and Ti₃C₂T_x, are potential supports for PEMFC catalysts. As can be seen in Figure 1a, among them, TiO₂ was the most widely studied and used compound for a fuel cell catalyst support, followed by TiN. We have compared carbon-based and titanium-based materials in terms of cost, synthesis complexity, and scalability. Carbon-based materials are the lower-cost option for catalyst supports in industrial applications, while titanium-based supports are generally more expensive but offer superior durability and stability. Under severe operating conditions (high potential, acid environments), titanium-based supports are favored for their long-term cost-effectiveness despite higher initial material prices. A way to combine the low cost of carbon with the high durability of titanium is to use hybrid carbon-based-titanium-based supports. Carbon supports generally require less complex synthesis, although structures like CNTs require advanced methods, such as chemical vapor deposition (CVD). On the other hand, the synthesis of engineered (e.g., doped Ti-suboxides) Ti-based supports frequently requires high-temperature sintering to control crystal phases (anatase/rutile), which can lead to sintering and reduced surface area, or advanced methods like sol–gel or chemical vapor deposition to achieve specific surface properties. Carbon supports are highly scalable, with well-established production methods, whereas for Ti-based supports, the scalability is a major hurdle. While standard TiO₂ is cheap, high-surface-area engineered TiO₂ often faces challenges in industrial-scale production.

2.1. TiO₂

Among transition metal oxides, TiO₂ is the most widely used carbon-alternative fuel cell catalyst support for its low cost, high stability, low toxicity, excellent corrosion resistance, and strong metal–support interaction [15,16,17]. A drawback regarding the use of TiO₂ nanoparticles as a catalyst support, however, is mainly its poor electrical conductivity as well as its low surface area, giving rise to low Pt nanoparticle dispersion. As reported in a recent review [18], to overcome this hindrance, the use of TiO₂ mixed with or supported on carbon materials, doped TiO₂, shaped/nanostructured TiO₂, substoichiometric TiO₂, and TiO₂ mixed with inorganic oxides were proposed. As can be seen in Figure 1b, TiO₂/carbon composites and doped TiO₂ were the most investigated catalyst support. As reported by Antolini [19], the methanol oxidation under dark conditions of TiO₂-supported and conventional carbon-supported Pt catalysts were compared in different papers. For most of these works, TiO₂/carbon composites or nanostructured TiO₂ were used as catalyst supports. The specific activity (SA) of all Pt/TiO₂ catalysts was higher than that of Pt/C catalysts, due to the co-catalytic effect of TiO₂ on methanol oxidation. For most Pt/TiO₂ catalysts (76%), the mass activity (MA) was higher and the methanol oxidation onset potential was lower than those of Pt/C catalysts. In almost all of these cases, the Pt/TiO₂ catalysts with a lower MA also presented a higher onset potential, ascribed to the large Pt particle size and the low surface area of the TiO₂ particle. Moreover, the presence of TiO₂ increased the poisoning tolerance, as well as the long time durability of Pt/TiO₂ with respect to Pt/C.

2.1.1. TiO₂/Carbon Composites

There are essentially two type of ceramic–carbon composites used as a fuel cell catalyst support: (1) ceramic materials supported on a carbon substrate, where the carbon stabilizes highly dispersed ceramic particles, lead to a high specific surface area; (2) mixed ceramic–carbon materials, that is, true hybrid supports with enhanced properties, such as higher electron conductivity (by the presence of carbon) and/or higher corrosion resistance (by the presence of the ceramic oxide) than the single materials [20]. TiO₂ mixed with or supported on carbon materials, such as carbon black (CB) [21,22,23], carbon nanotubes (CNTs) [24,25,26], and graphene (G) [27,28,29], is the most common way to increase the electrical conductivity and/or the surface area of the support. The combination of conductive carbon and corrosion-resistant TiO₂ results in a suitable support for low-temperature fuel cell catalysts. In addition, as the contact between carbon and platinum in fuel cell cathodes can catalyze carbon corrosion through electrochemical oxidation, particularly at high potentials, the ceramic supported on carbon avoids direct contact between carbon and Pt, and it protects the carbon from the harsh, acidic, high-potential environment, thus reducing corrosion and increasing durability. The use of stoichiometric TiO₂ nanoparticles as part of the composite, however, not only reduces the conductivity of the support, but it also makes the formation of a strong interaction between Pt and the metal oxide difficult. Thus, in many cases, instead of stoichiometric TiO₂ nanoparticles, to further improve the electrical conductivity of the composites, either doped TiO₂, substoichiometric/defective TiO₂, or 1D nanostructures are used in composite supports. Considering that the use of TiO₂-based supported catalysts for methanol oxidation has been previously discussed [19], only the use of TiO₂/carbon composite supported catalysts for the oxygen reduction reaction (ORR) are reported in the following part of this section. Almost all these composite supported catalysts showed a higher ORR activity and a higher durability than the catalysts supported on TiO₂ and carbon alone. Li et al. [21] used a warm plasma (WP)-synthesized TiO₂ and carbon to form mixed (TiO₂)_x-C_4−x (x = 1, 2, 3) composites by a solvent evaporation method. The Pt particles were deposited on these substrates by a ethylene glycol reduction method. These composite supports gave rise to a uniform Pt dispersion and a Pt electronic reconfiguration. The Pt/TiO₂–C₃ catalyst showed an ultra-small Pt size (ca. 2.15 nm) and outperformed Pt/C catalyst, achieving 1.24 times higher electrochemically surface area (ECSA), 1.85 times higher MA, and 1.5 times higher SA than Pt/C. Moreover, the Pt/TiO_2x-C_4-x catalysts showed almost half the losses in ECSA, MA, and SA after an accelerated stress test (AST) than those of Pt/C. The outstanding ORR activity and enhanced durability were ascribed to the synergic effect of TiO₂ (strong metal support interaction (SMSI), oxygen vacancies, and corrosion resistance) and carbon (high surface area and electrical conductivity). A way to mitigate the loss in fuel cell performance, due to lower catalyst loading on the substrate, is the reduction of catalyst particle size, providing more active sites. However, the small particle size leads to particle agglomeration, giving rise to a reduction in catalyst activity. To overcome this drawback, Cao et al. [24] proposed a CNT-supported porous TiO₂ film that immobilizes PtCo nanoparticles (2 nm size) loaded on the support while protecting the CNTs inside. PtCo/TiO₂/CNT showed a MA only slightly lower than that of PtCo/CNT, but showed a MA loss of 8 and 11% following 5 k and 30 k potential cycles at 0.6–1.0 V and 1.0–1.5 V, respectively, compared to 74 and 68% MA loss for PtCo/CNT. Among various TiO₂, 1D structures have advantages such as higher aspect ratio, higher conductivity, and high dispersion on graphene than conventional TiO₂ nanoparticles [30], making them suitable for TiO₂/G composite preparation and use as a catalyst support. Baskaran et al. [27] prepared 1D TiO₂ nanowires/reduced graphene oxide (rGO) composites for their use as PEMFC catalyst supports. Pt nanoparticles were supported on TiO₂/rGO composites in the TiO₂ range of 0–75 wt% by a polyol reduction method. The optimum composition was 40 wt% TiO₂ nanowires. The improved performance was ascribed to the strong SMSI between Pt and Ti, enhancing the ORR activity. The performance of the catalyst supported on TiO₂/rGO with more than 50 wt% TiO₂ was lower than Pt/rGO, owing to the excessive agglomeration of TiO₂, resulting in a sluggish charge transport within the composite. The histogram of the ECSA, the current density (CD) at 0.6 V, and the MPD of (TiO₂)_xG_1−x-supported Pt catalysts are shown in Figure 2.

The ECSA values of the catalysts were slightly different, while CD and MPD increased with the TiO₂ content. The Pt/rGO catalyst delivered a current density of 403 mA·cm⁻² at 0.6 V and a maximum power density (MPD) of 321 mW·cm⁻², whereas the optimized (Pt/TiO₂)₄₀-rGO₆₀ catalyst delivered a performance more than twice that of Pt/rGO. However, the utilization of TiO₂/carbon-based composite materials as a catalyst support is not an optimal choice because the problems regarding the stability of the carbon during fuel cell operation remain. Moreover, it has to be remarked that most of these works have been carried out by out-of-cell tests, whose results do not always agree with those by fuel cell tests.

2.1.2. Doped TiO₂

Metal Doped Titanium Oxide (M-TiO₂, M < 0.3)

Doping with n-type dopants, such as Nb, V and Ta, is one of the best methods of improving the electrical conductivity of TiO₂. Among them, Nb is one of the most effective dopants and is used for the generation of n-type TiO₂ crystallites the most. Nb modifies the band structure of TiO₂ and generates additional carriers in its conduction band. The 4d orbitals of Nb strongly hybridize with the 3d ones of Ti and form a d-natured conduction band, originating the metallic conductivity of Nb-TiO₂ [31]. Sheppard et al. [32] concluded that the properties of Nb-TiO₂ arise from defects such as oxygen vacancies and Nb-doping in Ti sites. These defects are compensated by either electrons or Ti interstitials. However, excessive doping, above 10–20%, can lead to the segregation of Nb, the formation of second phases, or disruption of the crystal lattice, which lowers structural stability and electrical performance. The literature suggests that an Nb concentration of 5–10% is sufficient to significantly enhance conductivity without inducing severe structural damage or secondary phases. In most these works, the Nb content is 10%. Doping 10% Nb into TiO₂ would increase its conductivity ∼1600 fold [33]. Moreover, Nb doping decreases the particle size and increases surface area of TiO₂. By out-of-cell and fuel cell tests, it was observed that the catalysts supported on Nb-TiO₂ showed higher/similar catalytic activity and/or higher stability [33,34,35,36,37,38,39,40,41,42,43,44] than the same catalysts supported on carbon. The improved catalytic activity of Pt/Nb-TiO₂ was ascribed to the high dispersion of Pt nanoparticles on the Nb-TiO₂ support, and to the SMSI between Pt and Nb-TiO₂, promoting the adsorption of oxygen or fuel on the catalyst surface. The enhanced stability was ascribed to the higher corrosion resistance of Nb-TiO₂ than carbon in an acidic environment and the strong SMSI, hindering Pt sintering.

Kim et al. [45] evaluated the effect of different transition metal doping in Pt/M-TiO₂ (M = V, Cr, and Nb) on the ORR activity of Pt/Pt/M-TiO₂. Among them, the Pt/V-TiO₂ showed the best ORR activity and stability. On this basis, Barthi and Cheruvally [46] compared the performance of a V-doped TiO₂-supported Pt catalyst (Pt/V-TiO₂) as the cathode catalyst in a PEMFC to that of undoped TiO₂ supported Pt, prepared in the same way.

The single PEMFCs with Pt/V-TiO₂ and Pt/TiO₂ delivered a cell voltage at 50 mA·cm⁻² of 0.817 and 0.732 V, respectively. The histogram of the A_Pt/V-TiO2-to-A_Pt/TiO2 ratio (A: ECSA, charge transfer resistance for cathode (Rc), current density at 0.85 V (j_0.85) from chronoamperometry (CA), and MPD) is reported in Figure 3. As can be seen in Figure 3, the ECSA was not affected by the presence of V. Instead, the presence of V remarkably increased the conductivity, thus decreasing the Rc and considerably increased the j_0.85. An increase in the MPD can also be observed. The better performance of Pt/V-TiO₂ than Pt/TiO₂ was ascribed to the higher electrical conductivity and stronger SMSI between Pt and V-TiO₂ support.

Non-Metal Doped Titanium Oxide

Nitrogen doping generates defects and modifies the electronic band structure of TiO₂, improving the interaction with catalysts like Pt. Carbon doping can further enhance the electronic conductivity of the TiO₂ support. C- and N-doped TiO₂ strongly enhances the stability and activity of the catalyst. Doping hinders catalyst particle agglomeration, resulting in high fuel cell durability, especially under harsh operating conditions. Two papers were addressed to C and N co-doped TiO₂ as Pt supports in PEMFCs. Dhanasekaran et al. [47] evaluated the performance as cathode catalysts of Pt supported on N- and C-doped TiO₂. Pt supported on optimum levels of N- and C-doped TiO₂ in the optimum composition showed improved cell performance compared to Pt supported on undoped TiO₂. Lee et al. [48] prepared N,C co-doped mixed-phase TiO₂ nanoparticles with a decreased (compared to that of pristine TiO₂) band gap and containing Ti³⁺ ions, oxygen vacancies, and Ti–X bonds (X = O, OH, N, C). The doping level was controlled by dopant (urea) loading, while the abundance of defect sites resulted in an enhanced metal–support interaction for Pt/N,C co-doped TiO₂. This catalyst showed high ORR activity and durability. The MPD decreased by only 4% against 52% for Pt/C under single PEMFC operation conditions.

2.1.3. Ti_nO_2n−1 and TiO_2−x

Sub-Stoichiometric Ti_nO_2n−1 (4 ≤ n ≤ 10)

The Ti–O system presents a sub-stoichiometric composition of the general formula Ti_nO_2n−1 (Magneli phases), with n between 4 and 10 [49]. Among this series of distinct oxides, Ti₄O₇ presents the highest electrical conductivity (>10³ S·cm⁻¹ at room temperature) [50]. In addition, these oxides show high oxidation resistance in an acid environment. On the basis of these conductive and oxidation-resistant properties, Ti_nO_2n−1 phases, particularly Ti₄O₇, have attracted attention as a possible support for fuel cell catalysts [51,52,53,54,55,56,57,58]. First, Ioroi et al. [51,52] evaluated the performance of Ti₄O₇-supported Pt as an anode catalyst in a single PEMFC and compared it to that of Pt/C. The performance of the single PEMFC with 5 wt% Pt/Ti₄O₇ as the anode catalyst was similar to that of the cell with 20 wt% Pt/C, while the performance as the cathode catalyst was lower than that of Pt/C, due to the larger Pt particle size on Ti₄O₇ than that on carbon, resulting in a lower ECSA. A high-potential holding test showed that the Pt/Ti₄O₇ catalyst is stable up to 1.5 V. A single PEMFC with a Pt/Ti₄O₇ cathode operating at 80 °C for 350 h showed remarkable voltage stability. Chisaka et al. [53] prepared Ti₄O₇ particles from titanium oxysulfate (TiOSO₄) and polyethylene glycol by a carbothermal reduction reaction, obtaining fine Ti₄O₇ particles, forming aggregates with a broad pore size distribution. When commercial TiO₂ powders were used as precursors, much larger Ti₄O₇ particles (Ti₄O₇-L) were obtained. Tests in PEMFCs indicated that the optimum 20 wt% Pt/Ti₄O₇ is effective for both the anode hydrogen oxidation (HOR) and cathode ORR. ADTs under PEMFC conditions were performed on the Ti_nO_2n−1 and Pt/C catalysts [54,55,56,57,58]. The Pt/Ti_nO_2n−1 catalysts showed outstanding electrochemical stability. The Pt particles supported on the carbon aggregated, while the morphology of the Pt supported on Ti_nO_2n−1 remained almost unchanged, resulting in a higher ECSA loss of Pt/C with respect to that of Pt/Ti_nO_2n−1. The superior durability of Pt/Ti₄O₇ was ascribed to the high stability of Ti₄O₇ and the SMSI between Pt and Ti₄O₇.

TiO₂ with Oxygen Vacancies (TiO_2−x)

Oxygen vacancies (O_V) are a kind of crystal defect, which can essentially change the crystal structure of transition metal oxides and then modify their intrinsic properties [59]. Defective TiO₂ with oxygen vacancies recently gained intensive attention due to its abundant electron-rich sites, enhancing electronic interaction between supports and metals [60]. Pt 4f core-level spectra of Pt-TiO_2−x shifted to lower binding energy, which indicates a d-band center shift relative to Fermi level, reducing the adsorption of the intermediates on the catalyst surface and improving ORR activity [61]. In addition, oxygen vacancies on TiO₂ can be used as anchor sites to adsorb metal ions and immobilize the metal nanoparticles. Different studies reported the use of bare TiO_2−x [62,63,64,65,66,67] and TiO_2−x–carbon composites [23,25,68] as fuel cell catalyst supports, mainly for the ORR but also for the MOR. As reported in these works, the catalysts supported on TiO_2−x and TiO_2−x–carbon composites showed a higher catalytic activity and durability than the same catalysts supported on stoichiometric TiO₂ and conventional carbon. Two times higher activity and three times higher stability in methanol oxidation reaction, a 0.12 V negative shift of the CO oxidation peak potential and a 0.07 V positive shift of the oxygen reaction potential were obtained for Pt supported on TiO_2−x support compared to Pt supported on pristine TiO₂ support [62]. Naik et al. [63] evaluated two-dimensional TiO_2−x nanosheet (NS)-supported Pt nanoparticles as ORR catalysts in PEMFCs. TiO_2−x NSs showed enhanced electronic conduction compared with TiO₂ NSs. Pt/TiO_2−x NSs showed outstanding ORR activity, due to a strong SMSI between the Pt and the TiO_2−x NSs. Both MA and SA of the Pt/TiO₂ NSs and Pt/TiO_2−x NSs catalysts were higher than that of Pt/C, due to the SMSI between TiO₂ and Pt, particularly in the presence of oxygen vacancies.

The CO adsorption on Pt/TiO₂ NSs and Pt/TiO_2−x was weakened by electron transfer to Pt from TiO₂ NSs support, due to the negative shift in Pt 4f peaks. The performance of cathode catalysts in PEMFC, as well as the stability of the Pt/TiO_2−x NSs and Pt/TiO₂ NS catalysts were evaluated. The histograms of the MPD and the ECSA loss are reported in Figure 4. The fuel cell using the Pt–TiO_2−x NSs catalyst delivered a higher power density than the cell with Pt/TiO₂. The ECSA decrease of the Pt–TiO_2−x NSs was smaller than that of the Pt–TiO₂ NSs, showing outstanding stability during fuel cell operation. Yan et al. [25] evaluated a TiO_2−x/CNT composite material as a support for Pt nanoparticles. The oxygen vacancy facilitated interactions between TiO_2−x and Pt, enhancing the anchoring of Pt and avoiding particle growth. The catalytic activity and the durability of the Pt/TiO_2−x/CNT catalyst were higher than those of Pt/C. Si et al. [68] prepared N-doped carbon materials containing TiO₂ via pyrolysis of MIL-125(Ti)–NH₂ in either N₂/(TiO₂-NC)_N or H₂/Ar gas (TiO_2−x-NC)_HA. A high content of oxygen vacancies were obtained on TiO₂ surface in H₂/Ar gas, enhancing the interaction between TiO₂ and Pt. The Pt/(TiO_2−x-NC)_HA catalysts showed higher activity and stability for methanol oxidation than Pt/(TiO₂-NC)_N and Pt/C catalysts. However, the stability of these vacancies is a major concern, as they tend to vanish when exposed to oxidative environments, such as the cathode of a PEMFC, where the vacancies can be re-oxidized. Several strategies can be used to stabilize them: (i) Introducing acceptor-type dopants, such as Fe³⁺, Mo⁶⁺ and Nb⁵⁺, into the lattice, making oxygen vacancies less prone to filling in oxidative environments; (ii) doping with non-metals like N or F, lowering the formation energy of vacancies; (iii) using conductive carbon materials to form composite supports, helping in anchoring the metal particles more robustly and improving the overall conductivity, which can help maintain the defective structure of TiO₂. Moreover, the vacancies can be partially regenerated through potential cycling techniques in a hydrogen-containing atmosphere, which can reduce the surface and re-introduce oxygen vacancies.

The similarities and differences in the promoting effect of TiO_2−x support and sub-stoichiometric Ti_nO_2n−1 were compared [18]. The similarities include improved electronic conductivity and SMSI, resulting in enhanced electrocatalytic performance. The differences are mainly due to the crystal phase and morphologies. The sub-stoichiometric Ti_nO_2n−1 is composed by the Magnéli phase with a spherical shape. Conversely, TiO_2−x supports with oxygen vacancies are mainly composed of the rutile and anatase phases with tunable morphologies like rod, sheets, and porosity. TiO_2−x has a higher proportion of oxygen vacancies, leading to their superior electronic conductivity. The oxygen vacancies induce SMSI and then promote electrocatalytic performance. However, the formation of Magnéli phase at higher temperatures is energy-consuming. Moreover, TiO_2−x tends to be oxidized into TiO₂ during long-term fuel cell operation.

2.1.4. One-Dimensional (1D) TiO₂

Nanostructured materials, with a typical dimension of less than 100 nm, have attracted attention for their use as a catalyst support. Such materials include spheroidal nanoparticles, nanosheets, and nanofibers together with 1D nanotubes, nanowires, and nanorods. In view of their high stability, high surface area, and moderate electrical conductivity, 1D TiO₂ nanotubes (TONTs) were tested as fuel cell catalyst supports [69,70,71,72,73,74,75,76,77,78,79]. The nanotube structure presents many advantages such as support of electrocatalysts associated with specific geometry. The geometry of the arrays enhances the loading and dispersion of catalysts, and the highly porous structure facilitates the permeation of reactant species, resulting in an enhancement of catalytic activity. Pt, Pd, PtRu, PtNi, PtCo, and PtAg supported on TONTs were investigated for MOR and ORR by both out-of-cell tests and as an anode or cathode catalyst in low-temperature fuel cells, showing higher catalytic activity and durability than the same catalysts supported on TiO₂ nanoparticles as well as on carbon-supported catalysts. For example, Manidakam et al. [69] deposited Pt nanoparticles on TiO₂ nanotubes (Pt/TONT) by a chemical impregnation method. They compared the electrochemical performance of Pt/TONT for both the ORR and the MOR with that of TiO₂ nanoparticle-supported Pt (Pt/TiO₂-NP) and commercial Pt/C. The ORR onset potential showed a shift of 50 mV towards higher values and a 12% increase in SA with respect to Pt/C. Pt/TONT also showed a MA 17-fold higher than that of Pt/C and 3.4-fold higher than that of Pt/TiO₂-NP. The ADT test showed a 12% ECSA loss for Pt/TONT, 40% for Pt/TiO₂-NP, and 76% for Pt/C following 10,000 cycles. The Pt/TONT also showed enhanced MOR activity compared to both Pt/TiO₂-NP and Pt/C. The enhanced performance of Pt/TONT was ascribed to an increase in oxygen defects on its surface, improving the charge transfer between Pt and TONT.

Pisarek et al. [70] used Pd/TONT as an anode catalyst for a direct formic acid fuel cell (DFAFC). Polarization and power density curves of DFAFCs with Pd/TONT and Pd/C anode catalysts are shown in Figure 5. The MPD of the cell with Pd/TONT was 70% higher than that of the cell with Pd/C. This is surprising considering that the Pd/C catalyst has much smaller and well-dispersed Pd nanoparticles (~3 nm) than the Pd/TONT (~10 nm) and, as a consequence, a remarkably higher ECSA (~60 m²·g⁻¹ vs. ~10 m²·g⁻¹). Many factors could explain this result such as the SIMS, the hydrophilicity of TiO₂, and the thin layer of Pd nanoparticles on top of TONT, which allows a better diffusion of formic acid to the catalyst and removal of CO₂. Moreover, small carbon-supported crystallites, having higher contribution of edges, are less effective for formic acid electrooxidation than the larger TONT-supported Pd nanoparticles, which have a higher amount of crystal planes. As previously reported, to increase the conductivity, TONT/carbon-based composites, such as TONT/C [80,81,82] and TONT/graphene [27], were investigated as catalyst supports, and the results were very promising, showing enhanced activity and durability of the catalyst for the MOR [80,81,82] and the ORR [27].

2.1.5. TiO₂-MO_x Mixed Oxide Supports

TiO₂-MO_x (M: transition metal) binary oxides used as fuel cell catalyst supports can be in the form of either solid solution [83,84,85] or separate oxides [86,87,88]. In the case of Ti_1−xM_xO₂ solid solutions, what distinguishes M-doped TiO₂ from TiO₂-MO_x mixed oxide is the amount of M. The transition threshold between M-doped TiO₂ and TiO₂-MO_x mixed oxide can be set at a value of M = 0.3 mol. The most used mixed oxides as catalyst supports for fuel cell applications were TiO₂–RuO₂ [83,86,87,88,89,90,91]. In most of these works, the Ru content was 30%. These materials showed high electrical conductivity and high chemical stability in an acidic and oxidative environment.

Ti_0.7Ru_0.3O₂, as a catalyst support, provides various advantages in terms of its high surface area, as well as the SIMS between Pt and the co-catalytic metals in the support, leading to highly dispersed and well-anchored Pt catalyst particles. Ti_0.7Ru_0.3O₂ acts as a co-catalyst, improving Pt activity by the ‘bifunctional mechanism’, related to the high proton conductivity of hydrated Ti_0.7Ru_0.3O₂, and the electronic effect between Pt and Ti_0.7Ru_0.3O₂, decreasing the interaction between Pt and the intermediate species. Ho et al. [83] evaluated the performance of DMFCs and PEMFCs with Pt/Ti_0.7Ru_0.3O₂, Pt/C, and PtRu/C as anode catalysts. The histograms of the MPD of these fuel cells operating at 60 °C are shown in Figure 6. As can be seen in Figure 6, both DMFC and PEMFC with Pt/Ti_0.7Ru_0.3O₂ as the anode catalyst delivered the highest MPD. Lo et al. [86] evaluated the stability of hydrous and anhydrous TiO₂–RuO₂ (TRO) powders. Through an ADT test, it was observed that the anhydrous TRO-a support has a 10-fold higher electrochemical stability than carbon. Unlike with carbon, Pt supported on TRO does not accelerate the corrosion rate of the support. The Pt particle size on the surface of the TRO support was in the 4–8 nm range. The performance of the PEMFC with Pt/TRO-a (40 wt% Pt) as anode and cathode catalysts was only slightly lower than that with Pt/C. The lower performance was attributed both to the lower MA of the Pt/TRO-a catalyst (larger Pt particle size) as well as enhanced ohmic and transport losses. The PEMFC with Pt/TRO-h instead delivered a poor performance due to flooding. The performance at 100% RH of PEMFCs with Pt/TRO-a and Pt supported on high-surface-area carbon (Pt/HSAC) was evaluated. Despite the lower ECSA, the initial performance of the PEMFC with Pt/TRO-a was only slightly lower (especially at lower current densities) compared to that with Pt/HSAC. Whereas the cell with Pt/HSAC showed a very remarkable loss in performance, the PEMFC with Pt/TRO showed minimal loss in performance upon exposure to 1000 start–stop cycles.

2.2. TiN

2.2.1. Pure and Doped TiN Nanoparticles

Transition metal nitrides (TMNs) are considered as promising supports for low-temperature fuel cell catalysts [4]. Among TMNs, the high electrical conductivity and corrosion resistance make TiN the most investigated [16,92,93]. Unlike carbon, electron transfer from TiN to supported Pt enhances Pt electronic density [94,95], resulting in a more weak adsorption of surface-blocking oxygen species, as well as higher catalytic activity and durability than Pt/C.

TiN nanoparticles were tested as fuel cell catalyst support by out-of-cell tests [94,96,97,98,99,100,101,102]. A higher ORR and MOR activity, as well as a higher stability of TiN-supported catalysts than carbon-supported catalysts, was observed. Yue et al. [101,102] compared the ORR activity of Pt/TiN, Pt/TiC, and Pt/C. The histograms of the half-wave potential (E_1/2) of Pt/TiN, Pt/TiC, and Pt/C before and after ADT are shown in Figure 7. The E_1/2 of Pt/TiN was higher than that of Pt/C and Pt/TiC, particularly after ADT, indicating a higher stability of Pt/TiN compared to Pt/TiC and Pt/C. By TiN doping with 5–10 at% M (M = Nb, Mo, Co, Ni, V), the catalytic activity and durability of TiMN-supported catalysts was higher than that of TiN-supported catalysts, ascribed to the modification of the electronic structure of the catalyst in the presence of a second metal [103,104,105,106,107]. However, the poor charge transfer and/or mass transfer of TiN nanoparticles is a limitation for the catalytic activity and durability. Moreover, the low surface area of TiN nanoparticles generally leads to a low dispersion of the catalyst and, in turn, to poor catalytic activity. To address these issues, the use of either pure and doped 1D TiN or pure and doped TiN–carbon composites was proposed.

2.2.2. Pure and Doped 1D TiN

The high surface area and excellent conductivity of 1D nanostructures make 1D TiN materials, such as TiN nanotubes (NTs) [108,109,110,111,112], TiN nanofibers [113], and TiN nanorods [114], suitable for their use as supports for fuel cell catalysts. The high catalytic activity and the good CO tolerance of 1D TiN-supported catalysts were due to the co-catalytic effect of TiN, the strong TiN-Pt interaction, and the unique structure of 1D TiN [109]. Further enhancement of the activity and stability was obtained by doping 1D TiN with transition metals (TMs) in the atomic fraction 0.05–0.1 [115,116,117,118,119]. Generally, pure and doped 1D TiN and carbon-supported catalysts have similar particle size and ECSA. However, following aging tests, the catalysts supported on doped 1D TiN showed a lower ECSA loss than that of the same catalysts supported on undoped 1D TiN and on carbon. Chen et al. [115] evaluated the MOR activity and durability of Pt supported on TiN NTs, TiCoN NTs, and carbon black.

The Pt/Ti_0.95Co_0.05N NTs catalyst delivered a noticeable increase in MOR activity and durability compared to Pt/TiN NTs and Pt/C. An enhancement of both the maximum current density (MCD) and ECSA retained in the order Pt/C < Pt/TIN NTs < Pt/TiCoN NTs can be observed in the histogram in Figure 8. The outstanding stability of Pt/TiCoN NTs was ascribed to the strong electronic coupling between Pt and Ti_0.95Co_0.05N NTs, due to Co doping and the NT supports. Pt/TiCoN NTs showed the lowest onset potential for CO oxidation, followed by Pt/TiN NTs and Pt/C. Co donated electrons to Pt, leading to a decrease in Pt d-band vacancy, weakening reaction intermediate bonding [116] and facilitating CO removal with respect to TiN support.

2.2.3. Pure and Doped TiN-Carbon Material Composites

To increase the specific surface area of pure and doped TiN nanoparticles, they were supported on high-surface-area carbon materials, such as carbon black [120,121,122], carbon nanotubes (CNTs) [123,124,125,126,127,128], graphene (G) [129,130], and mixed carbon nanotube-reduced graphene oxide (CNT-rGO) [131]. These composites presented a higher catalytic activity and durability of the supported catalysts than those supported on single nitride and carbon-based supports. For example, Zhang et al. [120] combined the outstanding corrosion resistance and SMSI of TiN with the high conductivity of carbon, forming a TiN@C composite with a TiN core and a porous carbon shell. The composite TiN@C support showed a higher corrosion resistance than bare carbon. Liu et al. [131] synthesized a TiNiN-decorated three-dimensional (3D) CNT-rGO support, resulting in the formation of large accessible pores, facilitating the contact of catalyst and electrolyte. The surface area of TiNiN/CNT-rGO was remarkably higher than that of TiNiN.

The MCD increased in the order Pt/CNT-rGO < Pt/TiNiN < Pt/TiNiN/CNT-rGO (Figure 9). The Pt/TiNiN and Pt/TiNiN/CNT-rGO catalysts presented a higher MCD than Pt/CNT-rGO, related to the co-catalytic effect of TiNiN. The higher activity of Pt/TiNiN/CNT-rGO than that of Pt/TiNiN was due to improved electrical conductivity. These catalysts have the same Pt particle size and Pt loading [126,131], but given the surface area of TiNiN/CNT-rGO is higher than that of TiNiN, the Pt interparticle distance (x_i) on the TiNiN/CNT-rGO support is higher than that on the TiNiN one. As catalytic activity can decrease with decreasing x_i [132], a higher x_i can result in higher MOR activity. On the other hand, the high ECSA-retaining properties of both Pt/TiNiN and Pt/TiNiN/CNT-rGO catalysts (Figure 9) was ascribed to strong Pt-TiNiN interactions.

2.3. TiC

2.3.1. TiC Nanoparticles

Among TM carbides, TiC attracted growing interest because of its outstanding stability under a variety of harsh conditions [10] and its high conductivity (300–800 S·m⁻¹), making it a promising candidate as a fuel cell catalyst support. Due to a strong overlap between metal d-bands and C-2p orbitals, noble metal (NM) atoms preferentially adsorb at C sites while favoring the electron transfer from C to NM. The strong interaction between NM atoms and TiC surface allows us to retain the configuration as well as avoid clustering, thus enhancing the stability of electrodes while retaining its catalytic activity. Despite these positive features for the use as catalyst/catalyst support, a facile synthesis process for porous TiC with high surface area is still a challenge. Mirshekari and Shirvanian [133] carried out a comparative study on the ORR activity and stability of TiO₂, TiN, and TiC-supported Pt catalysts in PEMFC environment. The ECSA values for the Pt/TiO2, Pt/TiN, and Pt/TiC catalysts were 7.15, 3.35, and 26.41 m²_gPt, respectively. The ORR current for the Pt/TiO₂ and Pt/TiC catalysts was nearly the same within the activation region (0.8–0.95 V), while Pt/TiC showed a higher ORR current at a lower overpotential. The E_1/2 value for the Pt/TiC catalyst was 0.81 V, whereas that for Pt/TiO₂ and Pt/TiN was 0.78 V and 0.62 V, respectively, indicating a higher ORR activity for Pt/TiC than Pt/TiO₂ and Pt/TiN. The number of electrons transferred (n) for the Pt/TiO₂, Pt/TiN, and Pt/TiC catalysts were 3.0, 3.9, and 4.0, respectively, indicating that Pt/TiN and Pt/TiC approach the n values related to the four-electron transfer pathway, while the Pt/TiO₂ catalyst has the highest fraction of the two-electron transfer pathway. The highest ORR activity of the Pt/TiC catalyst can be explained by the higher ECSA and/or the strong electronic interaction between Pt and TiC support. It should also be remarked that TiC can be oxidized at high potentials (above 0.95 V vs. RHE), forming a TiO_x layer on the surface. Thus, it is possible that the ORR performance of the Pt/TiC catalyst was affected by the presence of the TiO_x layer. The Pt/TiC catalyst also showed higher stability in PEMFC acidic media than Pt/TiO₂ and Pt/TiN, with a slight increase in ECSA and no reduction in ORR current after ADT. Chamgordani et al. [134] synthesized Pt/TiO₂, Pt/TiO_2−x, and Pt/TiC catalysts, and their ORR activity and stability were compared to those of Pt/C. Among the synthesized catalysts, Pt/TiC shows superior ORR activity in acidic media with a higher E_1/2 and a lower Tafel slope. Both Pt/TiC and Pt/TiO_2−x catalysts showed higher stability compared to Pt/C. Through fuel cell tests, the Pt/TiC catalyst delivered good performance with a MPD of 573 mW·cm⁻² and a current density of 694 mA·cm⁻² at 0.6 V, despite a low cathode loading of 0.1 mg_Pt·cm⁻². Roca-Ayas et al. [135] observed an improved catalytic activity towards the CO and methanol electrooxidation on TiC, ascribed to the presence of surface oxides at the Pt/support interface. In particular, the current density obtained for Pt/TiC, activated up to 1.0 V, was 2-fold higher than that achieved with the commercial PtRu/C catalyst, the benchmark catalyst for methanol oxidation. Ou et al. [136] used TiC as Pt support for methanol electrooxidation. The CO tolerance of Pt/TiC was improved compared to that of Pt/C. The forward anodic peak current (I_f) to the reverse anodic peak current (I_b) ratio of Pt/TiC was compared with that of Pt/C. The I_f/I_b ratio was used to evaluate the catalyst tolerance to methanol oxidation intermediate species. A higher I_f/I_b value implies a higher amount of complete oxidation of methanol to carbon dioxide. A higher I_f/I_b value was observed for Pt/TiC, indicating that most of the methanol oxidation intermediate species were oxidized to CO₂ in the forward scan, ascribed to a co-catalytic effect of TiC. A lower peak current of Pt/TiC electrode than Pt/C may depend on the lower ECSA of TiC-supported Pt. Chiwata et al. [137] prepared TiC-supported Pt nanoparticles with and without heat treatment at 600 °C in 1% H₂/N₂. Hemispherical Pt nanocrystals were found to be dispersed uniformly on the TiC support after heat treatment. They found that the ORR mass activity of Pt/TiC at 0.85 V was comparable to that of Pt/C, but the durability of Pt/TiC was much higher than that of Pt/C. It was reported that, if clustered and segregated noble metals electrocatalysts are loaded on TiC support, there is a fair possibility of formation of surface oxycarbide or oxide layers over TiC, which are formed by oxidative electrochemical conditions at >0.92 V vs. RHE as pointed out earlier [10,138]. Unfortunately, these layers are insulating as opposed to highly conducting TiC, which may lower the specific electrochemical activity of the electrocatalyst. To increase the surface area, 1D TiC and TiC/carbon composites were proposed as fuel cell catalyst supports.

2.3.2. One-Dimensional TiC

Qiu et al. [139] proposed bark-structured TiC nanowires (NWs) as a Pt support. The Pt nanoparticles deposited onto the TiC NWs showed higher ECSA, higher MOR activity, and long-term durability than Pt/C. The unique 1D nanostructure of the TiC NWs provides fast transport and a short diffusion path for electroactive species, as well as a high utilization of catalysts. The enhanced electrocatalytic properties were also due to the high electrical conductivity and excellent chemical/electrochemical stability of TiC. Liu et al. [140] utilized mechanically resilient mats consisting of overlaid electrospun nanofibers with self-generated TiC crystallites embedded in a carbon matrix as a novel support for Pt nanoparticles. The ORR activity of Pt was remarkably improved, ascribed to the high specific surface area of the support and the synergetic effect of TiC and Pt.

2.3.3. TiC/Carbon Composites

Zhao [141] used an epitaxial TiC/nanodiamond (ND) composite as a support for Pt. The Pt/TiC/ND catalyst showed much higher activity and stability for the MOR and the ORR than the Pt/ND catalyst. The high durability of Pt/TiC/ND was ascribed to the chemical stability of the ND core and the anchoring effect of the TiC layer to Pt. Zhao et al. [142] prepared an ordered mesoporous carbon/titanium carbide composite (OMC/TiC) and utilized it as a Pt support. The Pt/OMC/TiC catalyst showed a higher activity for the MOR and the ORR than that of Pt/OMC and Pt/C catalysts. Zheng et al. [143] grew TiC on the surface of carbon black by the sol–gel method coupled with high-temperature treatment (TiC-C). Based on the SMSI between TiC and Pt, the prepared Pt/TiC-C catalyst showed twice the mass ORR activity than that of Pt/C. Moreover, Pt/TiC-C showed higher durability than Pt/C, preventing the aggregation and dissolution of Pt nanoparticles.

2.4. Ti MXenes

2.4.1. Pure Ti MXenes

M_n+1X_nT_n, (MXenes) are layered metal carbide, nitride, and carbonitride compounds, where M is a transition metal, X is C and/or N, T is a surface termination group (–OH, –O and –F), and n can be associated with three different integers 1,2,3 [144]. They are formed by a unique sandwich structure, where X is the central layer and M is the outer layer, surrounded by the terminal groups). The electronic properties of MXenes mainly depend on the outer metal layer and surface functionalized substituents. MXenes have higher electrical conductivity compared to other 2D materials, where Ti-based MXenes have conductivities of up to 2.4·10³ S·cm⁻¹ [145]. MXenes possess excellent stability and hydrophilicity due to terminal groups. These characteristics make them appropriate support materials. In Ti-based layered MXenes, Ti layers make the catalyst resistant to corrosion, and C layers give electronic conductivity for charge transfer. In addition, the layered Ti MXenes provides a large surface area for metal nanoparticle deposition, and –F termination reduces carbon corrosion in fuel cell environment and prevents Pt dissolution and Ostwald ripening [144]. Two-dimensional Ti₃C₂T_x can modify the electronic structure of supported metal nanoparticles, improving catalytic activity and CO poisoning tolerance. Furthermore, a SMSI between the 2D MXene and supported nanoparticles was pointed out both by theoretical simulations and experimental results, attesting that MXene can be a suitable fuel cell catalyst support material. The use of MXenes as supports for oxygen reduction and low-molecular-weight alcohol oxidation fuel cell catalysts has been reported in recent valuable reviews [10,12,144,146]. In the first works, Xie et al. supported Pt nanoparticles on Ti₃C₂X₂ (X = OH, F) nanosheets [147] and on-surface Al leached Ti₃AlC₂ [148] as a carbon-alternative supports for their use in a fuel cell corrosive environment.

These Ti MXene-supported Pt catalysts showed much enhanced activity and durability for the ORR than Pt/C. Zhang et al. [149] investigated the ORR activity of Pt nanoparticles supported on accordion-like Ti₃C₂T_x nanosheets in both alkaline and acidic solutions. The Pt/Ti₃C₂T_x catalysts displayed enhanced ORR activity and stability compared to Pt/C, especially under alkaline conditions. As can be seen in Figure 10, before cycling, E_1/2 was 0.853 V for Pt/Ti₃C₂T_x, 21 mV more positive than 0.832 V of Pt/C. After ADT, the E_1/2 of Pt/Ti₃C₂T_x negatively moves to 0.847 V, 30 mV higher than Pt/C (0.817 V), indicating the outstanding ORR activity and durability of the Pt/Ti₃C₂T_x catalyst. Wang et al. [150] prepared a Ti₃C₂ MXene-supported Pt catalyst and used it for MOR for the first time. Pt/Ti₃C₂Tx showed outstanding MOR activity, ca. three times higher than that of Pt/C.

2.4.2. Ti₃C₂T_x/CNT(rGO) Composites

A drawback of the use of Ti₃C₂T_x nanosheets as a catalyst support is related to its aggregation or restacking, which decreases the number of the active sites and limits electron transport, reducing the catalytic performance. An effective strategy to prevent restacking of MXene nanosheets and enhance the conductivity is the addition of CNT to MXenes [151,152]. The presence of CNT, acting as spacer between the Ti₃C₂T_x nanosheets, can effectively prevent Ti₃C₂T_x restacking. Another drawback is the structure distortion of the Pt cluster induced by MXene, which is disadvantageous to oxygen desorption. This problem can also be overcome by the addition of CNT to Ti₃C₂T_x, stabilizing the Pt cluster [151]. Xu et al. [151] utilized a Ti₃C₂T_x-CNT (1:1) composite as a Pt support. The Pt/Ti₃C₂T_x-CNT catalyst showed higher ORR activity and durability than the conventional Pt/C catalyst. The mass activity of Pt/Ti₃C₂T_x-CNT was 3.4-fold greater than that of Pt/C, and the ECSA of the Pt/Ti₃C₂T_x-CNT catalysts showed only a 6% drop compared to Pt/C (27%) after 2000 potential cycling. Zhang et al. [152] prepared a (Ti₃C₂T_x)_y-(MWCNTs)_1−y (0 ≤ y ≤ 1) composite by a solvothermal method, where 2D Ti₃C₂T_x nanosheets were intercalated and connected by conductive 1D MWCNTs, avoiding Ti₃C₂T_x restacking. Then, Pt nanoparticles were deposited on the composite support. The Pt/(Ti₃C₂T_x)_0.5-(MWCNTs)_0.5 catalyst showed the maximum ECSA value, ascribed to the higher dispersion of Pt nanoparticles, and outstanding MOR activity and CO tolerance, higher than that of Pt/Ti₃C₂T_x and Pt/MWCNTs. Yang et al. [153,154] fabricated 3D hybrid architectures formed by RGO and Ti₃C₂T_x nanosheets as cobuilding blocks. This structure presented highly interconnected porous carbon networks, large specific surface areas, and good electron conductivity. Then, Pt and Pd were deposited on this composite for methanol [153] and formic acid [154] oxidation, respectively. The resulting 3D Pt(Pd)/RGO–Ti₃C₂T_x showed higher catalytic activity, CO tolerance, and long-term stability than the same catalysts supported on carbon black, CNT, RGO, and Ti₃C₂T_x.

3. Co-Catalyst/Additive

First, it has to be remarked that, in the literature, the difference between catalyst-metal oxide dispersion and metal oxide supported catalyst is often insufficiently underlined. In the former case, the metal oxide only acts as an additive/co-catalyst and not as a support, and the size of metal catalyst and metal oxide particles are not correlated. Whereas in the latter case, the particle size of the metal catalyst depends on, and is considerably lower than, the particle size of the metal oxide, and, in addition to its main function as a support, as previously reported, it can also have catalytic properties [15]. Generally, pure and doped TiN and TiC are not used as additives/co-catalysts in polymer electrolyte fuel cells. Thus, in the following sections, only the use of TiO₂ as co-catalyst/additive in the catalyst layer will be reported and discussed.

3.1. TiO₂ Co-Catalyst/Additive

When TiO₂ acts as a co-catalyst, it facilitates the typical fuel cell reactions of oxygen reduction and hydrogen and low-molecular-weight alcohol oxidation, and enhances the catalytic activity of the catalyst, ascribed to the hypo-d-electron character of TiO₂, generating strong interactions with hyper-d-electron character metals, such as platinum [155]. These interactions give rise to a Pt–Pt bond decrease, the inhibition of hydroxyl chemisorption, and the shift in the formation of Pt-OH to higher potentials, thus facilitating the ORR [155]. Moreover, synergistic effects between TiO₂ and Pt also promote the oxidation of small organic molecules, according to the bifunctional mechanism. The bifunctional mechanism can be ascribed to the co-catalytic role of TiO₂: the intermediate alcohol oxidation products, such as CO coming from the incomplete electrooxidation of methanol, adsorbed on Pt react with the OH^– ions adsorbed on TiO₂, forming non-poisonous CO₂ and refreshing the Pt active site. In the bifunctional mechanism, the following reaction takes place:

CO_ads + OH_ads → CO₂ + H⁺ + e⁻

(1)

TiO₂ can also act as an additive: in this case, it does not participate in the catalytic process, but can improve the catalytic activity in different ways, such as increasing Pt dispersion during synthesis, protecting Pt from agglomeration, enhancing the hydrophilic properties of the catalyst layer, and enhancing the proton transport.

3.1.1. TiO₂ as a Cathode Co-Catalyst/Additive for the ORR

First, Shim et al. [156] evaluated the catalytic activity of carbon-supported Pt–TiO₂ (TiO₂ 5–20%) catalysts by cyclic voltammetry and PEMFC tests. Compared to Pt/C, all the Pt-TiO₂/C catalysts showed higher ECSA, MA and PEMFC performance, with a maximum performance for 15% TiO₂. The adsorption strength of oxygen on a Pt surface was weakened by TiO₂ presence. The enhancement of ECSA and PEMFC performance were ascribed to synergic effects, based on the adlineation model, with the formation of an interface between Pt and TiO₂, and by spillover, due to surface diffusion of intermediates. Selavarani et al. [157] investigated the performance of a PEMFC with a carbon-supported Pt-TiO₂ as the cathode catalyst. The PEMFC with Pt-TiO₂/C in the Pt:TiO₂ atomic ratio 2:1 annealed at 750 °C as the cathode catalyst delivered a power density at 0.6 V of 750 mW·cm⁻², higher than that of the cell with Pt/C as the cathode catalyst (620 mW·cm⁻²), ascribed to the better dispersion of Pt in the Pt-TiO₂ catalyst. Moreover, Pt-TiO₂/C showed only 6% loss in the ECSA after 5000 potential cycles compared to 25% for Pt/C. To enhance the durability of Pt/C as cathode catalyst for PEMFCs, Chung et al. [158] prepared a TiO₂-coated Pt/C (TiO₂/Pt/C) by atomic layer deposition (ALD). The TiO₂/Pt/C catalyst with 2 nm TiO₂-protective layer showed similar ORR activity with Pt/C, but with higher durability. After ADT, the reduction of ECSA value for the Pt/C catalysts was ca. 90%, while a lower value of ECSA reduction (60%) for the TiO₂/Pt/C catalysts was observed. The decrease in the PEMFC performance of the PEMFC with the Pt/C catalysts was larger than that of the cell with the TiO₂/Pt/C catalysts. The TiO₂ layer was effective in preventing carbon corrosion and enhancing the interaction between Pt and carbon support, avoiding Pt agglomeration, but was not effective for suppressing Pt dissolution. Chaisubanan et al. [159] incorporated three types of commercial metal oxides (CeO₂, MoO₂, and TiO₂) on the PtCo/C catalyst layer. Among them, the catalyst with TiO₂ showed the highest ORR activity and stability. The incorporation of TiO₂ at low loading (0.03–0.06 mg·cm⁻²) increased the power density at 0.6 V of the PEMFC, with TiO₂-PtCo/C as the cathode catalyst, than that of the cell with PtCo/C, due to higher ECSA, lower internal contact resistance, and better hydrophilic properties [160]. Moreover, TiO₂ remarkably improved the stability in acid solution and during PEMFC operation. Pt low-coordination facets (100), (110), and (111) are the main Pt crystal planes, with (111) being the most stable due to its higher coordination number. The lower coordination sites of Pt, (100) and (110), are more soluble under harsh PEMFC operating conditions than Pt (111) [161]. The (100) and (110) sites will prompt Pt surface atoms to peel off by electrochemical oxidation and dissolution, giving rise to Pt electrochemical corrosion. A way to overcome this drawback is the use of a protective layer that selectively protects low-coordinated Pt sites [161]. On this basis, Liu et al. [162] proposed a nitrogen-doped titania (N-TiO₂)-stabilized Pt/C catalyst, achieved by selective ALD of TiO₂ followed by nitrogen doping. N-TiO₂ selectively deposits on the low-coordination sites of Pt and keeps the exposure of Pt (111) sites, without changing their size and distribution. The selective decoration of N-TiO₂ shields the low coordination sites of Pt, hindering the degradation of low-coordinated sites and preventing the electrochemical degradation and aggregation of Pt. The N-TiO₂ shield enhances the ORR activity and durability of Pt/C. Pt/C@N-TiO₂ was tested as the cathode catalyst. In a PEMFC, in a wide potential range, the current density of Pt/C@N-TiO₂ was higher than that of Pt/C. The initial MPD of the cell with Pt/C@N-TiO₂ (1300 mW·cm⁻²) was slight higher than that of the cell with Pt/C (1210 mW·cm⁻²). After 30,000 potential cycles, the MPD of the PEMFC with Pt/C showed a decrease of 19.8%, remarkably higher than the decrease of MPD of the cell with Pt/C@N-TiO₂ (3.1%). The MA loss of the cell with Pt/C@N-TiO₂ was only 8.3%, far less than that of the cell with Pt/C (37.9%).

A drawback of direct alcohol fuel cells is the crossover of the fuel from the anode to cathode across the proton exchange membrane. The alcohol crossover gives rise to parasitic alcohol oxidation on the cathode, leading to a mixed potential that not only lowers fuel utilization efficiency but also adversely affects the cathode performance and consequently the overall fuel cell efficiency [163]. Methanol/ethanol crossover in a DEFC can be mitigated either by using an alcohol impermeable membrane or by employing an alcohol-tolerant ORR catalyst. [163,164]. There are two types of alcohol-tolerant catalysts, that is, catalysts with lower MOR activity than Pt, reducing methanol adsorption and catalysts with higher MOR activity to decrease CO poisoning [163]. It was observed that the addition of TiO₂ enhances the alcohol tolerance of Pt/C [165,166,167]. First, an improved methanol tolerance of Pt-TiO₂/C (Pt:Ti = 1:1) than Pt/C was observed by Xiong and Manthiram [165], but the reason for the higher methanol tolerance was not well clarified. Then, Selvarani et al. [166] investigated the methanol tolerance of Pt-TiO₂/C with different atomic ratios, prepared by the sol–gel method, followed by heat treatment at 750 °C. Pt-TiO₂/C with the Pt:Ti atomic ratio of 2:1 showed enhanced methanol tolerance than Pt/C. The DMFC with Pt-TiO₂/C (2:1) as the cathode catalyst delivered a higher MPD (180 mW·cm⁻²) than the cell with P/C as the cathode catalyst (80 mW·cm⁻²). The methanol adsorption–dehydrogenation process requires at least three neighboring Pt atoms (ensemble effect). As the probability of finding three neighboring Pt atoms on the surface of Pt-TiO₂/C (2:1) and Pt-TiO₂/C (1:1) is lower than that on Pt/C and Pt-TiO₂/C (3:1) surfaces, the methanol oxidation current is lower for Pt-TiO₂/C (2:1) and Pt-TiO₂/C (1:1). Meenakshi et al. [167] prepared Pt-TiO₂/C catalysts in different Pt:Ti atomic ratios, namely 1:1, 2:1, and 3:1, followed by heat treatment at 750 °C. Among them, the Pt-TiO₂/C (2:1) catalyst showed the highest ORR activity both in the presence and the absence of ethanol. The direct ethanol fuel cell (DEFC) with Pt-TiO₂/C (2:1) as the cathode catalyst delivered a MPD of 41 mW·cm⁻², higher than that of the cell with Pt/C (21 mW·cm⁻²). The higher ethanol tolerance of the Pt-TiO₂/C catalyst than that of Pt/C was ascribed to two effects. The spillover effect, where reactive species (like OH radicals) generated at the TiO₂ site migrate (spillover) to the Pt site, reducing the Pt–CO bond strength and removing adsorbed CO from alcohol oxidation, and the ensemble effect, due to the presence of TiO₂ around Pt active sites, blocking ethanol adsorption on Pt sites.

To summarize, the presence of TiO₂ gives rise to two counteracting effects, both increasing the alcohol tolerance: a decrease in MOR/EOR activity by the ensemble effect, decreasing alcohol adsorption, and an increase in MOR/EOR activity due to TiO₂ presence, enhancing the oxidation of CO. The former effect prevails at low TiO₂ contents, while the latter is predominant for high TiO₂ contents.

SO₂, an air contaminant, may be fed with air at the PEMFC cathode, leading to Pt site deactivation. Thus, the improvement of the SO₂ tolerance of cathode catalysts is a necessity to avoid this drawback. Liu et al. [168] tested Pt-TiO₂/C as an ORR SO₂-tolerant catalyst. Among different TiO₂ contents, the Pt-TiO₂/C catalyst with 20 wt% TiO₂ showed the highest ORR activity and SO₂ tolerance: the ORR kinetic current density was 20.5% higher and the degradation rate in SO₂ presence was 50% lower than those of Pt/C. The fast removal of poisonous SO₂ from the Pt surface was ascribed to the interaction between Pt and TiO₂ as well as the high amount of hydroxyl groups on the surface of TiO₂.

3.1.2. TiO₂ as Anode Co-Catalyst/Additive for HOR, MOR and EOR

The use of Pt-TiO₂/C and Pt-WO₃-TiO₂/C catalysts in the PEMFC anode electrode was reported by Shim et al. [156] and Nagarajan et al. [169,170], respectively. In all these works, the higher hydrogen oxidation (HOR) activity of Pt-TiO₂ than Pt was not due to a co-catalytic effect, but to a larger ECSA. Carbon-supported Pt-WO₃-TiO₂ catalysts in various compositions (Pt-W-Ti 0:5:5, 2:4:4, 4:3:3, 6:2:2, 8:1:1, and 10:0:0 wt%) were investigated as anode materials [169]. Among them, the Pt-W-Ti 4:3:3 composition presented the highest HOR activity. The amount of Pt was reduced from 1.76 to 0.704 mg·cm⁻². The presence of WO₃-TiO₂ additives improves the proton transport within the Pt surface through the inclusion of WO₃-TiO₂ molecules between Pt particles. In the presence of WO₃-TiO₂, the effective utilization of platinum was much higher than 10 wt% Pt/C. This catalyst delivered a higher performance in single PEMFC than bare Pt/C.

Many works addressed MOR [171,172,173,174,175,176,177] and EOR [175,178,179,180] on carbon-supported Pt-TiO₂ catalysts, using both conventional spherical/cubo-octahedral TiO₂ nanoparticle and TiO₂ nanotubes (TONTs), for their use as anode catalysts in DMFCs and DEFCs, respectively. Wang et al. [171] as well as Yu and Xi [178] evaluated the effect of the amount of TiO₂ nanoparticles in the Pt-TiO₂ and PtRu-TiO₂ catalysts on MOR and EOR activity, respectively. The dependence of the maximum current density in the presence of TiO₂ (MCD_{Pt(Ru)-TiO2/C}) to the MCD of bare Pt(Ru)/C (MCD_Pt(Ru)/C) ratio on TiO₂ content is shown in Figure 11. An optimal value of TiO₂ content, that is, 10 wt% for the MOR and 20 wt% for the EOR, can be observed in Figure 11. The difference in the optimal TiO₂ content could depend on the different sizes of the TiO₂ nanoparticles. The improvement in MOR activity by TiO₂ addition to PtRu/C was low because the MOR activity of PtRu/C is considerably higher than that of Pt/C, reducing the effect of TiO₂ presence. The enhancement of MOR/EOR activity for low TiO₂ content was ascribed to a synergistic effect between Pt(Ru) and TiO₂ nanoparticles, facilitating the removal of CO-like intermediates from the surface of the catalyst: OH species can easily form on the surface of TiO₂ during the oxidation process, facilitating the conversion of poisonous CO into CO₂, according to the bifunctional mechanism. For TiO₂ content above the optimal value, a percolation effect takes place: the formation of TiO₂ clusters in the presence of too much TiO₂ can block the path of electron transportation to the catalyst. Moreover, the alcohol adsorption–dehydrogenation process, requiring at least three neighboring Pt atoms, is reduced by the ensemble effect. In the case of PtRu/C catalysts, Ru can dissolve or be dealloyed from PtRu alloy particles during fuel cell operation [181], resulting in activity loss. In the presence of TiO₂, the dissolved Ru species during fuel cell operation may be trapped on the surface of TiO₂ nanoparticles, preventing dissolved Ru from diffusing into the electrolyte, improving the stability of DMFC performance [171]. This positive effect of TiO₂ was also reported by Saida et al. [172]. Following an ADT by potential cycling, almost all of MOR activity of PtRu/C was lost. Conversely, in the case of PtRu- TiO₂/C, methanol oxidation was still observed after ADT, with the best catalyst showing ca. 10-fold higher MOR activity than PtRu/C. The enhanced durability of the catalyst was ascribed to TiO₂ suppressing the Ru loss from the catalyst.

The research group of the Shanghai Jiao Tong University investigated the effect of the addition of TONTs to low-Pt content (5%) carbon-supported PtNi on both MOR [173] and EOR [179]. The MOR activity of carbon-supported Pt(5%)Ni(10%)-TONT (10%) was higher than that of 20% Pt/C. The ECSA of PtNi-TONT/C was larger than that of the commercial PtRu/C catalyst. A lower onset potential for the EOR was observed for the PtNi-TONT/C catalyst compared to PtRu/C. The EOR current density delivered by the PtNi-TONT/C catalyst was roughly 75% of that by the Pt-Ru/C catalyst. However, the Pt content of the former was only 25% of the latter. In conclusion, the presence of TONTs modifies the electronic structure of Pt, generating more metallic sites. More hydroxide groups are also generated, facilitating the oxidative removal of the reaction intermediates.

Song et al. [180] compared the effect of TiO₂ nanoparticle and TONT addition to Pt/C on the EOR activity. The ECSA for Pt-TONT/C and Pt-TiO₂/C catalysts was almost the same, being 1.39 times larger than Pt/C catalyst, showing that both TONT and TiO₂ can increase the ECSA of Pt. The EOR activity of Pt-TONT/C, however, was much higher than that of Pt-TiO₂/C, indicating that, with respect to TiO₂, the addition of TONT has other advantages besides preventing catalyst from aggregating. A reason for the higher EOR activity of Pt-TONT/C is that TONTs contain much more structural water in itself than TiO₂. The TONT–OH bonds on Pt-TONT/C can directly donate hydroxide species to Pt sites to oxidize CO, leading to outstanding EOR activity. Conversely, TiO₂ must activate the H₂O in the electrolyte at a certain potential to form hydroxide species to oxidize CO. Another reason was the nature of hydrophilic tube-structure of TONT, which favors faster mass diffusion in the catalytic layer during ethanol oxidation. Xiao et al. [174] added titanate compounds such as nanotubes, nanobelts and nanorods to Pt/C. Following addition of these nanomaterials, the MOR activity of Pt/C remarkably increased. The 1D co-catalysts with suitable porous channels and higher proton conductivity could accelerate the methanol oxidation. Moreover, the co-catalysts contain more adsorbed and structural water could increase the oxidation of intermediates. In other works, the higher MOR/EOR activity of Pt-TiO₂/C than that of Pt/C was ascribed to a uniform and enhanced Pt dispersion along with narrow size distribution with a smaller particle size [175,176], as well as decreased charge transfer resistance [177]. Finally, Zhang et al. [182] observed that the addition of TiO₂ to rGO-supported Pd improved the MOR. The OH^– ions adsorbed on TiO₂ nanoparticles contributed to converting poisonous CO to non-poisonous CO₂ to refresh the active surface of Pd.

3.2. Ti as Co-Catalyst in PtTi Alloys

3.2.1. PtTi Alloys as Cathode Catalysts for the ORR

Combinatorial screenings of PtM binary and PtTiM ternary alloys for the ORR were carried out by He et al. [183,184]. Among different alloys, PtCo, PtNi, PtZn, and PtCu showed the highest ORR activity, but presented poor chemical stability in acid media. On the other hand, PtW, PtTi, and PtSe showed a low ORR activity improvement than Pt, but good chemical stability [183]. To improve the ORR activity of PtTi while maintaining high stability, PtTiM (M = Co, Cr, Cu, Fe, Mn, Mo, Ni, Pd, Ta, V, W and Zr) ternary alloys were evaluated by a thin film-based combinatorial high throughput screening. Among these PtTiM alloys, PtTiNi, PtTiCu, and PtTiV, showed the highest ORR activity, with a tenfold, an eightfold, and a sixfold enhancement compared to that of bare Pt, while maintaining a high chemical stability [184]. Different theoretical studies of PtTi nanoparticles for potential use as PEMFC catalysts were carried out by using density functional theory (DFT [185,186,187,188]. Jennings et al. [185,186] investigated the influence of PtTi alloy composition on structural properties and on the ORR kinetics and poisoning effects. Changes in d-band properties, related to changes in PtTi composition, were correlated with differences in OH- and CO-binding energies. They found that a Pt-rich PtTi alloy gives rise to a downshift in d-center, compared to bare Pt, which correlates with a weakening of the OH and CO adsorption energies. Duan et al. [187] investigated the Pt segregation on the (111) surface of an ordered Pt₃Ti crystal using DFT calculations and a Monte Carlo (MC) simulation method. Through MC simulation, when Pt concentration is above 75 % (off-stoichiometry), Pt atoms tend to segregate to the surface, forming a Pt outermost layer (Pt-skin), while the second and lower layers maintain the ordered Pt₃Ti structure. In addition, DFT calculations indicated that the d-band center of the Pt-segregated Pt₃Ti (111) surface would downshift by 0.21 eV with respect to that of a pure Pt (111) surface. Accordingly, O adsorption energy on the Pt-segregated Pt₃Ti (111) surface was weaker than that on the pure Pt (111) surface, resulting in an enhanced ORR activity on Pt₃Ti alloy. The results of a DFT study on the ORR pathway on a Pt-segregated Pt₃Ti (111) surface indicated that the ORR proceeds via a H₂O₂ dissociation mechanism, whose activation energy for the rate-determining step was remarkably lower than on the pure Pt (111) surface, explaining the experimental higher ORR activity of Pt₃Ti than Pt [188]. Moreover, the DFT calculations predicted an enhanced stability of the Pt₃Ti (111) surface against Pt dissolution and oxidation compared to the Pt (111) surface.

On this basis, experimental works were addressed to the evaluation of disordered and ordered PtTi alloys as ORR catalysts for their use as cathode materials in fuel cells. In all cases, the ORR activity and stability of PtTi catalysts were higher than those of bare Pt.

Disordered PtTi Alloys

The ORR activity of disordered PtTi alloys was evaluated in some works [189,190,191,192]. First, Stamenkovic et al. [189] reported a relationship in electrocatalytic trends on Pt₃M (M = Ni, Co, Fe, Ti, V) surfaces between the experimentally determined surface electronic structure (the d-band center) and ORR activity. This relationship exhibits ‘volcano-type’ behavior, and among these Pt₃M catalysts, the ORR activity of Pt₃Ti was the lowest, only slightly higher than that of bare Pt, confirming the results of the combinatorial screening of PtM alloys [183]. Ding et al. [190] synthesized carbon-supported disordered PtTi alloy nanoparticles with different Pt:Ti ratios by heat treatment at temperatures from 500 °C to 950 °C, a H₂/N₂ atmosphere using Pt₂(dba)₃ (dba = bis-dibenzylidene acetone), and TiCl₄(THF)₂ as precursors. Among the various compositions, Pt₇₅Ti₂₅ showed the highest ORR activity, with a twofold improvement compared to Pt/C. Duan et al. [191] fabricated hierarchical nanoporous (HNP) PtTi alloy with bimodal pore/ligament size distributions and controllable bimetallic ratio from a Pt₁₀Ti₁₀Al₈₀ alloy by a two-step dealloying process. The HNP Pt₃Ti alloy showed higher specific activity than Pt/C and nanoporous Pt. Moreover, the mass activity of HNP Pt₃Ti was nearly 3.0 times of that of Pt/C and 2.5 times of that of nanoporous Pt. This result was ascribed to the hierarchical ligament/pore structure and the alloying effect of Ti in the Pt–skin surface alloy. The stability of HNP PtTi, nanoporous Pt and Pt/C was also evaluated by a potentiostatic method. At the beginning the potentiostatic currents decreased rapidly for all catalysts due to the formation of double layer capacitance. Then, the current decreased with time due to the loss of surface active sites caused by the adsorption of intermediate species on the catalyst surface. The current decay on HNP Pt₇₅Ti₂₅ was much slower than on nanoporous Pt and Pt/C catalysts. After 3600 s, the currents of the HNP Pt₃Ti alloy showed a value of 24% of the initial current, while that of nanoporous Pt and Pt/C decreased to 16% and 7%. Kim et al. [192] fabricated glass carbon (GC)-supported Pt and Pt/TiO₂ catalysts by plasma-enhanced atomic layer deposition (PEALD) at a substrate temperature of 250 °C. Trimethyl(methyl-cyclopentadienyl) platinum (IV) and tetrakis(dimethylamido) titanium were used as the Pt and Ti precursors. Then, the Pt deposited on TiO₂ layer was submitted to thermal annealing in H₂ at temperatures between 500 and 900 °C, leading to the formation of a Pt₃Ti alloy. The catalyst annealed at 800 °C showed a MA increase of more than 2 times compared to pure Pt nanoparticles prepared by ALD and commercial Pt/C. A 3-fold improvement of the specific activity for the ALD Pt₃Ti catalyst compared to a polycrystalline Pt electrode was observed. The improvement of the ORR activity of the Pt₃Ti alloy was ascribed to a combination of strain and ligand effects. Moreover, the ORR activity of Pt₃Ti was retained following 10,000 ADT cycles, due to the intrinsic stability of the bulk Pt₃Ti, as supported by a near-negligible amount of Ti dissolution.

Ordered PtTi Alloys

As reported in a recent review [193], the ORR-specific activity and the stability of ordered A₃M, AM₃, and AM (A = Pt, Pd; M = first row transition metal) catalysts were higher than those of the disordered ones. However, the synthesis of ordered platinum titanium nanoparticles with small particle size is a challenge, as they have to be synthesized at high temperatures (≥700 °C) under H₂ atmosphere, due to the high oxophilicity of Ti and the significantly different reversible potentials for the reduction of Pt vs. Ti ions [192]. Nanoparticle agglomeration occurs during the thermal treatment, limiting the use of traditional synthesis techniques. To address the aggregation of nanoparticles, Pt₃Ti alloy nanoparticles were synthesized at 700 °C in a surfactant-free-KCl matrix [194], or sulfur anchoring was used to limit particle aggregation [195]. Herzog et al. [196] varied wet-chemical synthesis parameters such as reductive annealing temperatures and precursor ratios and studied their effect on the phase structure, particle size, alloying degree, and ordering degree. They observed that, by adjusting the initial Ti amount at a reduction temperature of 700 °C, it is possible to prepare a PtTi/C catalyst, with a maximum amount of Ti being alloyed in the disordered PtTi and ordered Pt₃Ti phases without the major formation of TiO₂ crystallites and excessive particle growth. Jeon and McGinn [197] prepared Pt_100−xTi_x/C (x = 0, 25, 50, and 75) catalysts by sequential impregnation of Pt and Ti on the carbon support followed by annealing at 900 °C under H₂/Ar. The sizes of the Pt-Ti crystallites were 2.4, 4.2, 5.1, 5.7, and 6.7 nm for the Pt/C, Pt/C-900, and annealed Pt₇₅Ti₂₅/C-900, Pt₅₀Ti₅₀/C-900, and Pt₂₅Ti₇₅/C-900 catalysts, respectively. The annealed Pt-Ti catalysts were mixtures of Pt, Pt₃Ti, Ti₃O₅, and a small amount of TiO₂. The presence of the ordered Pt₃Ti phase was observed in both the Pt₅₀Ti₅₀/C-900 and Pt₂₅Ti₇₅/C-900 catalysts, whereas the Pt₃Ti phase was not observed in the Pt₇₅Ti₂₅/C-900 catalyst, although the composition of this catalyst is the same than the Pt₃Ti phase. The ORR-specific activity of the Pt₇₅Ti₂₅/C catalyst only slightly increased to 15 μA·cm⁻² compared with 11.9 and 13.8 μA·cm⁻² values of the Pt/C and Pt/C-900 catalysts, respectively. Conversely, the ORR-specific activity of the Pt₅₀Ti₅₀/C-900 and Pt₂₅Ti₇₅/C-900 catalysts remarkably increased to 32.3 and 30.2 μA·cm⁻², respectively. The high ORR activity of these catalysts was ascribed both to TiO₂ and Pt₃Ti presence. The methanol tolerance of these catalysts was also investigated: as expected, due to the higher MOR activity, in the presence of methanol, both the Pt₅₀Ti₅₀/C-900 and Pt₂₅Ti₇₅/C-900 catalysts showed poor ORR activity. Park et al. [198] synthesized carbon-supported ordered Pt-Ti alloy nanoparticles by wet chemical reduction of Pt and Ti precursors, followed by heat treatment at 800 °C. The catalysts with Ti precursor molar compositions of 40% and 25% formed ordered Pt₃Ti and Pt₈Ti phases, respectively. The ORR polarization curves of the catalysts before and after 1500 electrochemical cycles between 0.6 and 1.1 V showed little change, indicating the high stability of the ordered Pt₃Ti and Pt₈Ti alloys, while Pt/C showed a low stability. Farid et al. [199] prepared PtTiC catalysts by plasma magnetron co-sputtering of Pt and Ti at high Ar pressure. The formation of the intermetallic Pt₃Ti phase was observed. Deposition at 0.15 mbar Ar pressure results in the formation of conical nanopillars like structures with a high ECSA, playing an essential role in the electrochemical activity, durability, and cell performance of the catalyst. The best assembled MEA showed a MPD of 604 mW·cm⁻², close to that of the MEA with Pt/C (618 W·cm⁻²). Moreover, for the PtTi catalyst, nine-times higher MA power density and five-times higher SA power density were obtained compared to Pt/C. Kim et al. [200] prepared carbon-supported ordered Pt₃Ti nanoparticles using a commercial Pt/C catalyst as the starting material. A TiO₂ overlayer was formed on the Pt/C by hydrolysis of titanium (IV) butoxide in the presence of benzyl alcohol, followed by annealing at 700 °C in flowing H₂. The Pt₃Ti nanoparticles were well distributed on the carbon surface with an average size of 4.2 nm. The presence of TiO₂ layers avoided nanoparticle aggregation during the thermal annealing. Conversely, when the Pt/C was annealed in the absence of TiO₂ layers, the particle size increased from 3.2 to 6.2 nm.

As can be seen in the histograms of Figure 12, among all catalysts, the Pt-Ti/C prepared with 10 vol% H₂ flow had the highest E_1/2. This catalyst showed a lower change in E_1/2 than Pt/C after ADT, indicating a higher stability of Pt-Ti/C 10% H₂ than Pt/C. The intermetallic Pt–Ti/C showed enhanced mass activity and durability for the ORR, and, following an ADT, unlike Pt/C, the ECSA showed little change. Gunji et al. [201] prepared carbon-supported Pt-Ti alloy nanoparticles. The as-prepared spherical Pt-Ti NPs were disordered and surrounded by high-index facets. After vacuum-annealing, ordered Pt_0.75Ti_0.25 alloy nanoparticles, surrounded by the low-index (111) facets and with low surface area, were obtained. Notwithstanding the ORR activity of the ordered structure needing to be higher than that of the disordered one, the spherical Pt-Ti alloy nanoparticles showed higher ORR activity than pure Pt or even faceted Pt_0.75Ti_0.25 alloy nanoparticles.

Compared to binary alloys, ternary alloys present a higher flexibility in tuning the Pt electronic structure for catalyst optimization [202]. Zhao et al. [203] synthesized ordered Pt₃Co_0.6Ti_0.4 nanoparticles (∼3 nm) supported on ZIF-8-derived mesoporous carbon (Pt₃Co_0.6Ti_0.4/DMC). The Pt₃Co_0.6Ti_0.4/DMC catalyst showed faster ORR kinetics than Pt₃Co/DMC and Pt/C with minimal activity loss (20.1%) and only 5 mV decay in half-wave potential after 20,000 potential cycles. More importantly, its improved performances was confirmed by a H₂/air PEMFC test. Theoretical calculations indicated that the substitution of Ti for Co gives rise to a strengthened ligand effect and optimizes the surface electronic structure of Pt₃Co_0.6Ti_0.4, leading to a remarkably enhanced ORR activity.

3.2.2. PtTi Alloys as Anode Catalysts for the MOR

Both atomically disordered and ordered Pt-Ti alloy nanoparticles show a much lower affinity for CO adsorption than either pure Pt or PtRu nanoparticles: The interaction with Ti modifies the electronic structure of Pt, altering the d-band center, which weakens CO binding and promotes faster reaction kinetics. As reported in the previous section, Jeon and McGinn [197] prepared Pt_100−xTi_x/C (x = 0, 25, 50, and 75) catalysts by sequential impregnation of Pt and Ti on the carbon support followed by annealing at 900 °C under H₂/Ar. The presence of the ordered Pt₃Ti phase was observed in both the Pt₅₀Ti₅₀/C-900 and Pt₂₅Ti₇₅/C-900 catalysts. The Pt₅₀Ti₅₀/C-900 and Pt₂₅Ti₇₅/C-900 catalysts showed 103% and 198% higher MOR activity, respectively, than Pt/C-900 catalyst at 0.7 V vs. RHE. Abe et al. [204] synthesized Pt₃Ti alloy nanoparticles by reducing Pt(1,5-cyclooctadiene)Cl₂ and Ti(tetrahydrofuran)₂Cl₄ with sodium naphthalide in tetrahydrofuran, forming atomically disordered Pt₃Ti nanoparticles (particle size 3 nm). These atomically disordered Pt₃Ti nanoparticles were transformed to larger atomically ordered Pt₃Ti nanoparticles (particle size = 37 nm) by annealing at temperatures > 400 °C. Atomically ordered Pt₃Ti nanoparticles showed higher methanol oxidation current densities than pure Pt, PtRu, or atomically disordered Pt₃Ti nanoparticles. Cui et al. [189] prepared ordered Pt₃Ti nanoparticle catalysts with ultra-small particle sizes by a KCl-nanoparticle method. This catalyst showed enhanced catalytic activity and stability for methanol oxidation compared to pure Pt.

Two papers reported an effective strategy of coupling Pt-Ti intermetallics with TiO_x to develop highly active MOR catalysts [200,201]. Sanetuntikul et al. [205] prepared Pt-titanium intermetallic nanoparticles on a unique hierarchical carbon/TiO₂ structure, consisting of Pt₈Ti and TiO₂ nanoparticles dispersed in N-doped carbon. The Pt₈Ti-TiO₂/C catalyst showed a 50 mV positive onset potential and 10 times higher SA for the MOR than Pt/C. A DMFC with Pt₈Ti-TiO₂/C as the anode catalyst showed 4.6 times higher power density than that with Pt/C as the anode catalyst at 0.35 V and 60 °C. Moreover, Pt₈Ti-TiO₂/C showed an ECSA decay of 23% after 3000 CV cycles, whereas Pt/C catalyst showed a decay of 90%. The outstanding stability of Pt₈Ti-TiO₂/C was ascribed to TiO₂, which is chemically resistant in the electrolyte medium. Similarly, Zhang et al. [206] prepared a carbon-supported intermetallic Pt₃Ti nanocatalyst coupled with amorphous TiO_x (Pt₃Ti-TiO_x/C). The TiO_x not only confines Pt₃Ti nanoparticles during the synthesis and electrocatalytic process by a strong metal–oxide interaction but also promotes H₂O dissociation to form more OH species, supporting the conversion of adsorbed CO. The Pt₃Ti-TiO_x/C catalyst showed a remarkable enhancement of MA (2.15 A·mg_Pt^–1) for methanol oxidation compared with Pt₃Ti/C and Pt/C, as well as a high MA retention (~71%) after the durability test.

4. Conclusions and Perspectives

Due to their high corrosion resistance and chemical stability, titanium-based compounds, such as TiO₂, TiN, TiC, and Ti₃C₂T_x, are potential supports for PEMFC catalysts. Among them, TiO₂ was the most widely investigated fuel cell catalyst support for its co-catalytic effect, not present in the other Ti-based compounds. Among transition metal oxides, TiO₂ is the most widely used carbon-alternative fuel cell catalyst support for its low cost, high stability, low toxicity, excellent corrosion resistance, and strong metal–support interaction. A drawback regarding the use of TiO₂ nanoparticles as catalyst supports, however, is mainly their poor electrical conductivity as well as their low surface area, giving rise to low Pt nanoparticle dispersion. To overcome this hindrance, the use of TiO₂ mixed with or supported on carbon materials, doped-TiO₂, shaped/nanostructured TiO₂, substoichiometric TiO₂, and TiO₂ mixed with inorganic oxides were proposed.

TiN and TiC were used in the form of pure nanoparticles and, for TiO₂, as pure and doped carbon composites. More recently, two-dimensional MXenes also emerged as potential Pt catalyst supports. However, few tests were carried out in fuel cells for TiN-, TiC-, and Ti Xenes-supported catalysts.

The use of Ti-based compound/carbon-based composites is useful in improving the catalytic activity of Ti-base compounds, but they present the same problems of stability during fuel cell operation observed for pure carbon materials. The limited use of TiC as a PEMFC catalyst support in PEMFC can be related to its instability at temperatures > 60 °C and/or acidic condition with the formation of a non-conductive TiO₂ layer. To summarize, all Ti-based supported catalysts showed considerably higher stability than the same catalysts supported on carbon. However, they also present, albeit to a lesser degree, instability under acidic or high-temperature/high potential conditions. TiN behaves passively in sulfuric acid media at temperatures higher than 60 °C, due to the formation of –OH groups on its surface, which reduces its electrical conductivity, thereby inhibiting its electron transportation properties. After a solubility test at 95 °C, the TiC phase remained unchanged. However at 200 °C, some TiO₂, which is insoluble in acidic media, was observed, indicating that TiC is not stable at 200 °C in 1.0 M H₂SO₄. TiC is electrochemically unstable and undergoes a surface oxidation at around 0.95 V. The formation of insulating titanium oxycarbide or oxide can be catalyzed by the presence of Pt, modifying the metal–support interface. Therefore, the electron-blocking oxycarbide or oxide layer lowers the overall activity of the catalyst. In addition, the gaseous product of TiC electrooxidation cannot cause the detachment of Pt from the support surface, seriously decreasing the catalyst performance. TiO₂ is the most stable: it is insoluble in dilute alkali and dilute acid, but soluble in hot concentrated sulfuric acid.

Some types of Ti-based compounds and the catalytic activity of supported catalysts are reported in

Table 1. Types of Ti-based compounds and catalytic activity of supported catalysts. NM: noble metal.

Compound	Type	Reaction	Activity	Ref.
TiO2	TiO2/carbon composites, 1D TiO2	MOR	NM/TiO2 > NM/C	[19]
	TiO2/carbon composites	ORR	NM/TiO2-C > NM/C, NM/TiO2	[21,22,23,24,25,26,27,28,29]
	Doped-TiO2	ORR (10) MOR, EOR	NM/Doped-TiO2 ≥ NM/C > NM/TiO2	[33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48]
	TinO2n−1 TiO2−x	HOR ORR ORR, MOR	NM/TinO2n−1 = NM/C NM/TinO2n−1 = NM/C NM/TiO2−x > NM/TiO2, NM/C	[51,52,53] [23,25,62,63,64,65,66,67,68]
	1D TiO2	ORR, MOR	NM/TONT > NM/TiO2, NM/C	[69,70,71,72,73,74,75,76,77,78,79,80,81,82]
	TiO2-MOx mixed oxide	MOR,	NM/TRO > NM/C, NM/PtRu	[83,84,85,86,87,88,89,90,91]
TiN	Bare TiN	ORR, MOR	Pt/TN > Pt/C	[94,96,97,98,99,100,101,102]
	Doped-TiN	ORR	NM/Doped-TiN > NM/TiN	[103,104,105,106,107]
	1D pure/doped TiN	MOR	NM/doped TiNNT > NM/TiNNT > NM/C	[108,109,110,111,112,113,114,115,116,117,118,119]
	Pure/doped TiN/carbon composites	MOR, ORR	NM/TiN-C > NM/C, NM/TiN	[120,121,122,123,124,125,126,127,128,129,130,131]
TiC	TiC nanoparticles	ORR, MOR	NM/TiC > NM/C	[133,134,135,136,137]
	1D TiC	MOR, ORR	NM/TiC > NM/C	[139,140]
	TiC/carbon composites	ORR, MOR	NM/TiC > NM/C	[141,142]
Ti MXenes	Bare Ti3C2X2	ORR, MOR	NM/Ti3C2X2 > NM/C	[147,148,149,150]
	Ti3C2X2/carbon composites	ORR, MOR	NM/Ti3C2X2 carbon > NM/C	[151,152,153,154]

. A concise comparison of Ti-based supports, conventional carbon supports, and Ti–carbon hybrid systems in terms of key properties such as electrical conductivity, durability, and catalytic performance is reported in

Table 2. Concise comparison of some properties of C-based (C), Ti-based (T), and hybrid C-based–Ti-based (C-T) supports.

Property	Order
Conductibility	C > C-Ti > Ti
ECSA	Pt/C > Pt/C-Ti > Pt/T
Catalytic activity	Pt/C-T > Pt/C ≥ Pt/T
Durability	Pt/T > Pt/C-T > Pt/C

.

Future research directions should address hybrid support systems that combine titanium-based materials with highly conductive nanostructures, such as MXenes, graphene, or carbon nanotubes. These hybrid systems may simultaneously improve electrical conductivity and corrosion resistance while maintaining strong metal–support interactions.

TiO₂ was also used as a co-catalyst/additive in carbon-supported catalysts for the ORR and MOR with promising results. Due to their high stability, PtTi alloys were investigated as ORR catalysts, but their activity for oxygen reduction was lower than that most PtM alloys (M = transition metal). The ORR and MOR activities of ordered PtTi alloys were higher than those of disordered ones, but ordered alloys have to be prepared at high temperatures, leading to a decrease in their surface area. The catalytic activity of carbon-supported catalyst with TiO₂ and PiTi co-catalyst/additives is reported in

Table 3. Catalytic activity of carbon-supported catalyst with TiO₂ and PiTi co-catalyst/additives.

Compound	Type	Reaction	Activity	Ref.
TiO2		ORR	Pt-TiO2/C ≥ Pt/C Pt-TiO2/C > Pt-MoO2/C, Pt-CeO2/C PtCo-TiO2 > PtCo/C Pt-TiO2/C ≥ Pt/C (high alcohol tolerance)	[156,157,158] [159] [160] [165,166,167]
		HOR	Pt-TiO2/C ≥ Pt/C Pt-WO3-TiO2/C	[156] [169,170]
		MOR	Pt-TiO2/C ≥ Pt/C PtRu-TiO2/C ≥ PtRC PtNi-TONT/C > PtRu/C Pt-TONT/C > Pt-TiO2/C ≥ Pt/C	[171,172,173,174,175,176,177] [178] [173]
		EOR	Pt-TiO2/C ≥ Pt/C PtNi-TONT/C < PtRu/C	[175,178,179,180] [179]
PtTi alloys	Disordered alloy	ORR	Pt3Ti/C > Pt/C	[189,190,191,192]
	Ordered alloy	ORR	Pt3Ti/C > Pt/C	[197,199,200]
	Ordered alloy	MOR	Pt3Ti/C > PtRu/C, Pt/C, disordered Pt3Ti/C	[194,197,204]

.

Some challenges, such metal leaching and scalability, are related to the use of non-carbon support for fuel cell catalysts. Up till now, no complete non-carbon supports that do not leach in the catalyst layer have been reported. Metals used in non-carbon supports, such as TiO₂ and Ti₄O₇, leach as cations to some extent. The leached cations exchange H⁺ in the sulfate groups in catalyst layers and membranes, decreasing proton conductivity. It was reported that Ta-doped TiO_x, possessing radical scavenging properties, can suppress leaching [207]. On this basis, future works should deal on doped Ti-based supports to reduce the amount of Ti leaching. To suppress titanium ion dissolution in acidic PEMFC environments and improve the durability of titanium-based supports, several surface protection technologies can be applied beyond titanium doping. These technologies focus on creating dense, conductive, and corrosion-resistant protective layers or on modifying the surface chemistry to enhance SMSI.

The synthesis of supported catalysts can be difficult to scale up without affecting their characteristics. Few studies have addressed large-scale production of carbon- and non-carbon-supported catalysts. Many catalysts reported in the literature use samples no larger than a few dozen milligrams. Few works on the scalability of Ti-base supports have been reported [208,209,210]. Ti₄O₇ is a promising carbon-free conductive material due to its high electrical conductivity and electrochemical stability, but this compound was not widely used, due to the difficulty of its mass production. To overcome this drawback, Chisaka et al. [208] developed a novel and inexpensive way to increase the production batch size, that is, the mass of carbon-free Ti₄O₇ synthesized in a single reaction. By doping Ti₄O₇ with vanadium by using a conventional high-temperature synthesis route, the production batch size was successfully increased. V cations were found to not exist in the V⁵⁺ form, instead being reduced to V⁴⁺/V³⁺ in the Ti₄O₇ lattice without segregating to form other phases, and the production batch size increased with increasing V/Ti atomic ratio up to 0.10, resulting in the synthesis of 2 g of (Ti_0.91V_0.09)₄O₇. By increasing the V/Ti ratio to 0.20, the vanadium species segregated to form a mixture of Ti₄O₇ and (Ti_0.5V_0.5)₂O₃ or V₂O₃ phases. Scaling the production of 2D materials is a challenge due to synthesis bottlenecks. This challenge typically comes from bottom-up synthesis, limiting the production to the substrate size or precursor availability for chemical synthesis and/or exfoliation. Conversely, MXenes, a large class of 2D TMN and/or TMC, are obtained by a top-down synthesis. The selective wet etching process does not have similar synthesis constraints as with some other 2D materials. The reaction occurs in the whole volume; the process can be readily scaled with reactor volume. On this basis, Schuck et al. [209] prepared 2D Ti₃C₂T_x using two batch sizes, 1 and 50 g, to be evaluated if large-volume synthesis influences the structure or composition of MXene flakes. They observed that the materials obtained in both batch sizes are essentially identical, indicating that Ti MXenes structure or properties do not change with scaling synthesis. Liu et al. [210] used a molten-salt flux method to synthesize large quantities of single-crystalline TiO₂ nanowires with controllable dimensions. In addition, in situ dopant incorporation of different transition metals allows us to modulate the catalytic properties. Future works should focus the identification of new methods, such as metal doping, to increase batch size of Ti-based supports, maintaining their properties.