A review of Pt-based electrocatalysts for oxygen reduction reaction

Abstract

Development of active and durable electrocatalyst for oxygen reduction reaction (ORR) remains one challenge for the polymer electrolyte membrane fuel cell (PEMFC) technology. Pt-based nanomaterials show the greatest promise as electrocatalyst for this reaction among all current catalytic structures. This review focuses on Pt-based ORR catalyst material development and covers the past achievements, current research status and perspectives in this research field. In particular, several important categories of Pt-based catalytic structures and the research advances are summarized. Key factors affecting the catalyst activity and durability are discussed. An outlook of future research direction of ORR catalyst research is provided.

Keywords

oxygen reduction reaction (ORR) / electrocatalysis / platinum catalyst / activity / durability

Introduction

The properties of catalyst materials are largely dependent on their crystalline and electronic structures. By well tuning the catalyst parameters such as facet, size, composition, and phase, the catalytic properties can be largely altered [1‒4]. Platinum group metals (PGM) are among the most studied catalysts for their outstanding properties in many chemical reactions, for instance oxidative amination, electrophilic cyclization, chemoselective reduction, A³ coupling, asymmetric hydrogenation, and C-H activation[5‒9]. In terms of oxygen reduction reaction (ORR) electrocatalysis, the effects of facet, structure and composition control on designed PGM nanomaterials are among the topics of intensive study [10‒14]. Since the discovery of fuel cells by German scientist Christian Friedrich Schöenbein and Welsh scientist Sir William Robert Grove, many types of fuel cells have been developed. Nowadays, the major types of fuel cells include phosphoric acid fuel cell (PAFC), polymer electrolyte membrane fuel cell (PEMFC), direct methanol fuel cell (DMFC), molten carbonate fuel cell (MCFC), and solid oxide fuel cell (SOFC) [15]. PEMFC has been recognized as one attractive solution to the growing concerns on environmental and energy-related issues such as climate change and depletion of fossil fuels [16‒20]. Indeed, the PEMFC stacks have already been applied in many transportation systems including trucks, buses, and forklifts in airports and large warehousing and logistics communities [21,22]. The essential component of PEMFCs is the embedded membrane electrode assembly (MEA) where the hydrogen oxidation reaction (HOR) occurs on the anodic side and ORR performs on the cathodic side (Fig.1) [23].

The most commonly used electrocatalyst for HOR and ORR is Platinum (Pt). Due to the sluggish reaction kinetics of ORR, a substantial amount of Pt would be required to accelerate the slow energy conversion process from oxygen to electricity and meet the specific technical requirements on both performance and durability of PEMFC. However, as one noble metal, Pt is even scarcer than gold and silver and extremely pushes up the cost of PEMFC technique [24‒27]. On the other hand, the broadly used Pt-based electrocatalysts suffer from lack of durability under the startup/shutdown conditions [23].Thus, considerable improvement in developing newly promising ORR electrocatalysts is highly desirable with respect to reduce the Pt metal usage and enhance their durability. According to the United States Department of Energy's (DOE) 2020 technological target, the total amount of PGMs at both anodic and cathodicelectrodes should be less than 0.125 mg/cm²and the ir-free mass activity should be greater than 0.44 A/mg Pt at 0.9 V verse reversible hydrogen electrode (RHE), and in terms of durability, the loss in mass activity at 0.9V should not exceed 40% [28].Great efforts have been devoted to searching new ORR electrocatalysts. Among them, PGM nanostructures with extended surface area such as de-alloyed skeletal structures, Pt-skin structures, porous metal films and thin films on high-aspect-ratio supports have been investigated. In addition, PGM alloy nanoparticles supported by carbon black, oxides, single-walled carbon nanotubes and carbon nanofibers were broadly prepared and studied. Unsupported PGM metal/alloy nanostructures such as nanowhiskers, nanodendrites, nanotube, nanowires and nanorods were also explored. To date, the shape/size-controlled Pt-alloy nanoparticles supported by carbon and thin films on nanostructured crystalline organic whisker supports (NSTF catalysts) demonstrated the highest ORR activities [ 23].The recent discovery that Pt₃Ni {111} crystal surface can exhibit an exceptionally high ORR activity of 18 mA/cm² Pt has motivated the synthesis of octahedral Pt-M (M= 3d transition metals) alloy nanoparticles, which are enclosed by the {111} planes and have a large specific active area (Fig.2) [29].

Past achievements in ORR catalyst development

Composition-controlled Pt-based ORR electrocatalysts

One long-standing challenge in the ORR electrocatalysis research is to reduce the noble metal usage by replacing Pt with other metal elements. Since the discovery that Pt₃Ni {111} crystal surface can exhibit an exceptionally high ORR activity of 18 mA/cm², great effects have been devoted to investigating the ORR properties of Pt-based alloys, PtM (M= Ni, Co, Fe, Cu, Pd, Rh, Ti, V, Cr, Mo, W and so on) [30,31]. From a theoretical point of view, alloying Pt with a carefully selected metal element would have great potential in changing the surficial electronic structure of PtM alloy and correspondingly boosting the electrocatalytic activity (Fig.3) [30].

By changing the stoichiometric Pt-Fe precursor ratio, a series of Pt-Fe alloy nanoparticles with different particle composition (Pt_0.52Fe_0.48, Pt_0.48Fe_0.52, and Pt_0.30Fe_0.70) were prepared using Pt(acac)₂ and Fe(CO)₅ as precursors by Sun and his colleagues [32]. XRD patterns show that the diffraction peaks of Pt-Fe nanoparticles position between the reference peaks of Pt and Fe, confirming the alloy formation. Also, their diffraction peaks monotonically shift to higher angles as the ratio of Fe increases, indicating the lattice constant is strongly affected by the alloy composition. Composition-controlled Pt_xNi_1-x alloy nanoparticles were also prepared using solvothermal and solid-state chemistry methods [33]. Strasser et al. synthesized octahedral Pt_1.5Ni, PtNi and PtNi_1.5 nanoparticles in dimethyfomamide (DMF) solvent by changing the ratio between Pt(acac)₂ and Ni(acac)₂ shown in Fig. 4. EELS line scan and STEM elemental mapping show the prepared octahedral Pt-Ni nanoparticles have non-uniform elemental distribution, with Pt-rich frame (corners and edges) and Ni-rich facets. After an activation process, the Ni content in these Pt-Ni nanoparticles dramatically decreases and the ORR activities follow the trend PtNi>Pt_1.5Ni>PtNi_1.5>Pt/C (The mass and specific activities of commercial Pt/C at 0.9 V_RHE  are ~0.15 A/mg Pt and ~0.23 mA/cm²_. The specific activity of polycrystalline Pt is ~1.2 mA/cm²Pt). Our group also investigated the compositional effect of Pt_xNi_1-x alloy nanoparticles prepared by a solid-state chemistry method, in which the ORR activities of Pt₄Ni, Pt₃Ni, Pt₂Ni, Pt_1.5Ni and PtNi were examined and compared with commercial Pt/C [34]. Slightly different from the results from Strasser’s group, our findings show that the Pt_1.5Ni has the highest ORR activity (Fig.5). The discrepancy on the optimal composition most likely comes from the different size and surface composition resulting from the different preparation methods, which we would elaborate in more details in later content.

Another example of compositional control of PtM alloy is Pt-Co alloy system reported by Choi et al. In that work, they synthesized Pt_xCo alloy nanoparticles with controlled composition (x = 2, 3, 5, 7, and 9) and studied the effect on the ORR activity [35]. Electrochemical measurements reveal that there is a strong correlation between ORR activity and Co composition and the Pt₃Co nanoparticles appear as the most active catalyst. Pt-Cu alloy nanoparticles with various composition (Pt₃Cu, PtCu and PtCu₃) were also synthesized and studied [36]. The results show the ORR mass activity increases as Pt₃Cu<PtCu<PtCu₃. Other studies on the compositional effect of PtM alloy on the ORR activity are reported with Pt-Cr, Pt-Mn and Pt-W bimetallic systems [37‒40]. All of them illustrate the promising capacity in replacing the noble metal Pt without sacrificing the ORR activities.

In addition to the Pt-M bimetallic systems, some ternary Pt-based alloys were also considered as ORR electrocatalyst. Among them, adding element N (N= Fe, Cu, Ni, Co) into Pt-M alloy system has been investigated aiming at further enhancement in ORR activity and durability [41]. Freshly prepared Pt-Fe/Co/Ni nanoparticles are observed with high initial ORR activity, while the dissolution of non-noble elements during the electrochemical cycling and the concurrent atomic restructuring and surficial electronic structure changes would lead to a dramatic activity loss. On the other hand, adding a third element can provide a possible synergetic effect, which might outperform their binary alloy system in terms of both activity and durability. Thus, development of ORR ternary alloy electrocatalyst such as Pt₂CuNi, Pt₃CoNi, Pt₃FeNi and Pt₃FeCo, and soon have been conducted [42,43]. For the Pt₂CuNi ternary alloy electrocatalyst, it was observed possessing both outstanding ORR activity far exceeding the DOE 2020 target and the durability comparable to that of state-of-the-art Pt/C. For the Pt₃CoNi, Pt₃FeNi and Pt₃FeCo alloy, Pt₃CoNi exhibited a higher ORR activity than Pt₃Co with an improvement factor of ~4 compared to the commercial Pt/C.

Transition metal-doped octahedral Pt₃Ni catalysts were also studied and reported with greatly improved ORR activity by Huang et al [41]. By doping 0.5%‒2% transition metal with Pt-Ni octahedral nanoparticles, an enhancement in factor of 1‒80 can be obtained regarding the area specific activity and mass activity (Fig.6). Among the sequences of metal doping using V, Cr, Mn, Fe, Co, Mo, Re, and W, Mo-doped Pt-Ni and Cr-doped Pt-Ni show the greatest activity enhancements. The Mo and Cr doping were attributed to the enhanced Pt-Metal bond strength, increased dissolution and diffusion energy barrier for Pt, and formed intact Pt-skin layer during the potential cycling.

Recently, Escudero-Escribano et al. demonstrated that the ORR activity of Pt alloy electrocatalysts could also be tuned by means of lanthanide contractions (Fig. 7) [44]. The Pt-lanthanide (lanthanum, cerium, samarium, gadolinium, terbium, dysprosium, thulium, or calcium) alloys prepared by physical vapor deposition (PVD) were observed with a shorter Pt-Pt bond length and a largely relaxed overlayer, leaving a pure Pt layer after acid leaching which boosted the electrochemical stability. It was discovered that the strain efforts can weaken the binding of H and OH species on the catalyst surface, and enthalpy contributions can help to stabilize these Pt alloys under operating conditions. A liquid half-cell using the prepared Pt_xGd alloy catalyst exhibited an outstanding activity of 3.6 A/mg Pt at 0.9 V vs. RHE, which was only suppressed by the Pt₃Ni nanoframe catalyst and Mo-doped Pt₃Ni octahedral nanoparticles. This newly found strategy also sheds light on designing the next generation of ORR electrocatalysts and suggests that ORR catalysts can be further improved and made more affordable by more appreciable destabilization of reaction intermediates.

Size-controlled Pt-based ORR electrocatalysts

Although an exact relationship between the Pt nanoparticle size and the ORR properties has not yet been established and more systematic studies need to be conducted, a tentative conclusion that the size of electrocatalyst would affect the ORR activity and durability could be drawn [45‒52]. Peles et al. measured the ORR activity of Pt nanoparticles with size range from 1 to 5 nm (Fig.8) [45]. To achieve size control of Pt nanoparticles, the Cu-UPD-Pt-replacement method was applied through a layer-by-layer growth. The results show that the mass activity of Pt nanoparticles increased by 2 times from 1.3 to 2.2 nm and then decreased with an further increase in Pt nanoparticle size. On the other hand, the area specific activity of Pt nanoparticles increased firstly rapidly by 4 times as the Pt size reached 2.2 nm and then slowly as particle size further increased. The size-dependent ORR activity of Pt nanoparticles was ascribed to the increasing ratio of {111} and {100} terrace sites and corresponding weakening averaged oxygen binding energy as the size of Pt nanoparticles increases.

Nesselberger et al. deemed that the ORR activity of Pt nanoparticles with size from 1 to 5 nm were not mainly affected by the Pt size but rather the catalyst dispersion and interparticle distance [46]. A suggesting explanation is that the oxygen binding energy on the surface of Pt particles dramatically changed when the size of Pt nanoparticles varied, especially when the size of Pt nanoparticles was below 2.3 nm, which might alter the ORR reaction pathway by changing the rate-limiting step from the first proton and electron transfer to the O-O bond breaking. Other studies implied that as the interparticle distance decreased, an overlap in the electric double layers increased, causing potential drop in the compact layer and lower adsorption energy of reaction species on Pt surfaces. A higher H₂O₂ yield has been detected and used to support their conclusion when the interparticle distance increased from well extended layer to isolated nanoparticles. Pt nanoclusters with size less than 1 nm are not commonly used in fuel cell technology and we would not discuss in details on them.

For Pt-based alloys, the size effect on the ORR properties was also studied by synthesizing the nanoparticles with size control. In our recent work, we reported strong dependence of ORR properties on the size of octahedral Pt–Ni nanoparticles, including Pt₃Ni/C and Pt_1.5Ni/C, with particle size ranging from around 4 to 8 nm synthesized using the solid-state chemistry method we innovated (Fig.9) [53]. The correlations between the Pt–Ni size and the ORR properties were investigated and attributed to alterations in the particle electronic/geometric structure and the Ni leaching behavior. The octahedral Pt₃Ni nanoparticles demonstrated a monotonous increase in the ORR activity with an increase in particle size. The area specific activity was measured as 2.47 mA/cm²  Pt for the 4.5 nm-Pt₃Ni/C and 4.06 mA/cm²  Pt for the 8.1 nm-Pt₃Ni/C at 0.9 V vs.  RHE. The significantly size-dependent ORR activity was also attributed to the varying fractions of {111} terraces on the surface of Pt-Ni nanoparticle. For octahedral Pt_1.5Ni nanoparticles, they exhibited a different size dependency of the ORR activity, with 5.8 nm-Pt_1.5Ni/C performing the best among all four catalysts and exhibiting the highest initial activity of 4.83 mA/cm²  Pt. The volcano relationship between the ORR activity and the Pt_1.5Ni size could be caused by the interplay between less stability and a higher fraction of {111} terraces of the larger octahedral Pt_1.5Ni particles.

Nanoscale morphology-controlled Pt-based ORR electrocatalysts

Since the discovery that Pt₃Ni {111} crystal surface can exhibit an exceptionally high ORR activity of 18 mA/cm²Pt, large amounts of attempts have been made to directly translate properties of single crystals to nanoparticles. While there is still huge gap between bulk crystals and nanoparticles and challenges remain in many aspects, one strategy to bridge the gap is to introduce thin film structures with controlled surface and thickness. Arenz et al. studied the ORR on Pt overlayers deposited onto Pt-Au film and explained the effects originating from ligand, strain and ensemble [54]. Using an electrochemical layer-by-layer deposition method, Pt overlayers with different thickness were deposited onto the Pt-Au substrates showing that (sub)monolayer amounts of Pt exhibit higher activity than Pt. Schmidt et al. investigated the role of strain on the ORR activity of Pt thin film catalyst with high crystalline quality fabricated by pulsed laser deposition on single-crystal substrate like {111} SrTiO₃. By tuning the interatomic distance of Pt atoms in the thin film, a decrease in the adsorption energy of oxygenated species and an improved ORR kinetics can be obtained.

To reduce the noble metal usage, preparing Pt layer structures on various substrates has also received great attention. Atomic Pt layers on transition metal nitride were prepared by a pulsed electrochemical deposition method, and the ORR measurement shows a more than 4 times increase in mass activity and 2 times increase in area specific activity compared to commercial Pt/C (Fig.10) [17]. It should be noted that the prepared TiNiN@Pt catalysts with several Pt atomic layers demonstrated a well-maintained intact core shell structures and extremely good durability with only a slight activity loss after 10000 potential cycles. Zeng et al. reported the synthesis of octahedral Pd@Pt_1.8Ni nanocrystals with an ultrathin PtNi alloy shell composed of approximately four atomic layers, which also show higher ORR activity and durability compared to commercial Pt/C [55].

Nanoscale morphology-controlled Pt-based nanostructures show a good promise in addressing the electrochemical durability issue. Under potential cycling in acidic aqueous solutions, non-noble metal component in the electrocatalyst would be oxidized and dissolved, leading to severe lattice restructuring and consequently losing their initial catalytic properties. MgO was used to prevent nanoparticles from sintering during high temperature annealing process. The ordered intermetallic Pt alloy nanostructures were found with superior long-term stability under the ORR operating conditions. The Sun group reported the preparation of ordered face-centered tetragonal (fct) PtFe nanostructure in which the iron exhibited superior antidissolution properties (Fig.11) [56]. The fct-PtFe nanoparticles demonstrated greatly enhanced ORR activity and durability compared to the disordered face-centered cubic (fcc) PtFe nanoparticles and commercial Pt/C. The fct-PtFe nanoparticles showed no obvious iron dissolution and nanoparticle degradation after even 20000 cycles between 0.6 and 1.0 V (vs.RHE) in 0.1 M HClO₄. One other example is preparation of fct-PtCo nanoparticles by Abruna’s group, with Pt-rich surface and strong Pt-Co bonding in the core being observed and accounted for the high ORR activity and long-term stability.

Another example of nanoscale morphology control on Pt-based nanostructures is the preparation of highly crystalline Pt₃Ni nanoframes with 3D elelctrocatalytic surfaces (Fig.12) [57]. Chen et al. recently reported this approach that Ni-rich PtNi₃ rhombic dodecahedron nanoparticles with size of around 20 nm were firstly made in oleylamine, followed by Ni etching and concurrent exfoliation of inner Pt onto surface of edge, forming the {111}-like Pt skin structure. The open architecture of the Pt₃Ni nanoframe allows the reactants to reach both the internal and external surfaces, giving rise to a 22 times enhancement in mass activity versus Pt/C catalyst. The mass activity was calculated as 5.7 A/mg Pt at 0.9 V, which is more than 10 times larger than the U.S. Department of Energy’s 2020 target (0.44 A/mg Pt). By applying the ionic liquid [MTBD][NTf₂] that has an O₂ solubility approximately twice that of the common HClO₄ electrolyte,  Pt₃Ni nanoframes exhibited a factor of 36 enhancement in mass activity and a factor of 22 enhancement in specific activity relative to Pt/C. The ionic liquid–encapsulated Pt₃Ni nanoframes also showed sustained superior activity upon 10000 potential cycling without noticeable decay.

One more representative work in the field of nanoscale morphology control on Pt-based nanostructures for improved ORR mass activity was reported very recently by Li et al [58]. In their work, a Pt/NiO core/shell nanowires was firstly synthesized using wet chemistry method followed by thermal annealing and then electrochemical dealloying, which generated jagged Pt nanowires with an electrochemical active surface area (ECSA) as high as 118 m²/g Pt and an area specific ORR activity of 11.5 mA/cm²and mass ORR activity of 13.6 A/mg Pt at 0.9 V shown in Fig.13. The highly stressed, under-coordinated rhombohedral-rich surface configurations of the jagged nanowires were thought to greatly contribute to the enhanced ORR activity.

Facet-controlled Pt-based ORR electrocatalysts

Since the first report that described the synthesis of cubic and tetrahedral Pt nanoparticles by El-Sayed and his colleagues, there have been a number of papers published in the field that tried to explore the physical/chemical properties of materials on nanoscale [59‒70]. An important subarea of them is the investigation on ORR activity of shape-controlled PGM metal/alloy. To mimic the Pt₃Ni{111}single crystal that shows extremely high ORR activity, octahedral, icosahedral Pt-based nanostructures enclosed by {111} facets and other high-indexed Pt nanoparticles have been prepared.

Choi et al. reported the synthesis of uniform 9 nm Pt-Ni octahedral nanoparticles using oleylamine and oleic acid as surfactants, W(CO)₆  as a source of CO for {111} facet control and benzyl ether as a solvent for reducing the surfactant coverage (Fig.14) [71]. After a further acetic acid treatment aiming at removing the surfactant, the Pt-Ni octahedral nanoparticles exhibited an ORR area specific activity as 51 times higher than that of the commercial Pt/C at 0.93 V, and mass activity of 3.3 A/mg Pt at 0.9 V. TEM, EDS, XPS, and XRD analyses demonstrated that the introduction of proper solvent could effectively eliminate the formation of Ni particles and significantly reduce the surface adsorption of surfactants leaving a clean, well-preserved {111} surface of the octahedral Pt-Ni nanoparticles, which delivered a great enhancement on ORR activity. More recently, they fabricated octahedral Pt nanocages by depositing a few atomic layers of Pt as conformal shells on palladium (Pd) nanocrystals with well-defined {111} facets followed by etching away the Pd templates, which also exhibited high ORR activity and greatly improved stability benefiting from the shape control effect.

Wu et al. reported the synthesis of Pt-M (M= Au, Ni, Pd) icosahedral nanocrystals in which CO gas and organic surface capping agents play critical roles in stabilizing the {111} surfaces enclosed icosahedral nanoparticles (Fig.15) [72]. The icosahedral Pt₃Ni had ORR area specific activity of 1.83 mA/cm² Pt and 0.62 A/mg Pt. Their results also show that the area specific activity of icosahedral Pt₃Ni catalysts was 50% higher than that of the octahedral Pt₃Ni catalysts (1.26 mA/cm² Pt). The great improvement may arise from strain-induced electronic effects explained by the density functional theory calculations and molecular dynamics simulations.

Core-shell structured Pt-based ORR electrocatalysts

Another way to improve ORR activity and stability of Pt-based catalysts is to design and prepare core-shell structures, in which the ligand effects and geometric effects are expected to be tuned separately orsimultaneously. Recently, a theoretical study comprehensively screened both the ORR activity and stability trends for materials in the following matrix in Fig. 16. Using the density functional theory calculations, 700 core-shell 2 nm transition metal nanoparticles including various Pt-based nanostructures have been identified based on the bonding energies and segregation energies for ORR [73]. Many bimetallic core-shell Pt-based catalysts revealed higher predicted ORR activities with much reduced Pt metal loadings.

In terms of synthesis Pt-based core-shell nanoparticles, a map of preparation approaches can be summarized and pictured in Fig.17 [74]. Among them, electrochemical dealloying, (electro)chemical leaching, absorbate/thermal-induced segregation, sequential deposition and galvanic displacement using Copper under potential deposition (Cu-UPD) methods have been intensively studied, with majority of them presenting different Pt mass/specific surface area based ORR activity improvement factors. Active and stable Ir@Pt core-shell catalyst developed by Jaramillo’s group has demonstrated a 2.6 times of specific and 1.8 times of mass activities compared to that of commercial Pt/C (TKK) even after 10000 stability cycles [75]. Xu and his colleagues showed that a core-shell nanostructured Au@Ni_mPt₂ catalysts can maintain excellent electrochemical durability of the Au@Ni_mPt₂ NPs even after 20000 potential cycles between 0.6 and 1.1 V (vs.RHE) in an O₂-saturated 0.1 M HClO₄. Compared with the commercial E-TEK Pt/C catalyst, the most-active Au@Ni₂Pt₂ NPs can exhibit 3–4- and 4–6-times higher Pt activity at 0.9 V before and after the 20 000 potential cycles [76].

The stability and reactivity of Pt-based core-shell nanoparticle catalysts can be further improved by careful tailoring the Pt shell thickness, core composition, and particle size [77,78]. Mechanistic insights on the degradation of core–shell nanoparticles such as Ostwald ripening, particle coalescence, shell thickness evolution, and core-shell elements redistribution are worthy to be explored in the future. The synchrotron X-ray-based spectroscopic and scattering analytics and advanced in situ (environmental) microscopic technique would be able to provide more insightful information and straightforward evidences to stress these problems.

Challenges and outlooks in ORR electrocatalyst development

Current challenge for the ORR research community is still to develop electrocatalyst with high activity, promising durability, and cost-effeteness via scalable synthesis. Many efforts have been made to address these issues in the last decades. To reduce (or even eliminate) noble metal usage and improve the electrochemical stability of a catalyst without sacrificing its electrocatalytic performance, the preparation of a highly stable single atom noble metal catalyst on electronic conductive support would be one possible direction. By further tuning the surface chemistry of elegantly selected support, a stronger metal-support interaction can be expected, and the local catalytic environment can speed up the adsorption/desorption of reaction intermediates. Design and preparation of a core-shell catalyst with a stable and intact atomic layers of noble metals and non-noble metal cores is another possible solution to the long-standing activity and durability issues, in which the stability and activity of synthesized catalysts are dominated by the mismatch and electronic interaction between top layer and core materials.

To advance the ORR catalyst development process, another common concern should also be addressed in the near future, that is how to mediate the gap in ORR performance between RDE and MEA measurements. To get a reasonably matched results, the testing procedures and local environments between them should be set as closer as possible. On the other hand, highly sophisticated modeling on the heat/mass transfer in the MEA devices and other operando/in-situ synchrotron X-ray based techniques could potentially provide foreseeable explains to those phenomena we still do not know the exact reasons yet.

As one of the final steps before implementing those techniques, mass production of active and durable catalysts, also requires much more attention. Usually, the preparation of catalysts in industry uses the impregnation method, in which metal precursors are impregnated onto supports followed by reduction in H₂at elevated temperature. Catalysts prepared by these methods exhibit problems such as a broad size distribution and poor control on crystallographic facets/composition/morphology. In the laboratory, a combination of colloidal, CVD, PVD, UPD methods are involved. For the mostly reported wet chemistry methods, there is a large amount of organic solvent and surfactant being used in the synthesis system, and labor-intensive product separation is required. Generally, the synthesis system is on the order of tens of milligrams, which is far less than the industrial requirements. Additionally, once the synthesis system is scaled up to even hundreds of milligrams, the product faces issues such as poor batch-to-batch reproducibility and lack of high quality control. For the CVD, PVD, and UPD methods, all of them are limited by the high cost and strict operation requirements which render them unsuitable for implementation at this time. For the solid-state method we developed, further strategies aiming to improve the catalysts stability such as the formation of phase ordered intermetallic alloys or partially bonded with a transition metal nitride support/framework might be suggestions.

At this moment, noble metals are still the most stable catalysts among those with considerable activities. By further tuning the geometric/composition/size factors and concomitant electronic structure and studying the degradation mechanism of current active catalysts by time-resolved in situ/operando techniques at the solid-liquid interface, next-generation ORR electrocatalyst would be developed. To further explore on ORR reaction mechanism is also worthy since it will greatly help us to obtain a better understanding of the dominating factors involved in rate-limiting steps.

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