Oxygen Reduction Reaction Durability Research Articles

Ultradurable Pt-Based Catalysts for Oxygen Reduction Electrocatalysis

An oxygen reduction reaction (ORR) is the key half reaction of proton exchange membrane fuel cells (PEMFCs), and is highly dependent on Pt-based nanocrystals as core electrocatalysts. Despite the exceptional ORR activity from adjusting the electronic structures of surface or near-surface atoms, several serious issues, including the corrosion of carbon supports, the preferential leaching of active metal elements, the instability of surface low-coordinated atoms and the sintering/agglomeration of nanocrystals, still exist, challenging the ORR durability of developed Pt-based ORR catalysts. From the point of view of the catalyst structure design, in this review, we summarized the state-of-the-art structural regulation strategies for improving the ORR durability of Pt-based catalysts. The current limitation of Pt-based binary catalysts for ORR electrocatalysis is firstly discussed, and the detailed strategies are further classified into the optimization of supports, metal-doped alloys, core/shell structures, intermetallics and high-entropy alloys, etc. The structure–performance relationship is detailedly explained, especially emphasizing the elimination of the above restrictions. Finally, the existing challenges and future research direction are further presented, aiming at practicing the PEMFC devices of the ultradurable Pt-based catalysts.

Low‐Electronegativity Mn‐Contraction of PtMn Nanodendrites Boosts Oxygen Reduction Durability

AbstractPlatinum metal (PtM, M=Ni, Fe, Co) alloys catalysts show high oxygen reduction reaction (ORR) activity due to their well‐known strain and ligand effects. However, these PtM alloys usually suffer from a deficient ORR durability in acidic environment as the alloyed metal is prone to be dissolved due to its high electronegativity. Herein, we report a new class of PtMn alloy nanodendrite catalyst with low‐electronegativity Mn‐contraction for boosting the oxygen reduction durability of fuel cells. The moderate strain in PtMn, induced by Mn contraction, yields optimal oxygen reduction activity at 0.53 A mg−1 at 0.9 V versus reversible hydrogen electrode (RHE). Most importantly, we show that relative to well‐known high‐electronegativity Ni‐based Pt alloy counterpart, the PtMn nanodendrite catalyst experiences less transition metals’ dissolution in acidic solution and achieves an outstanding mass activity retention of 96 % after 10,000 degradation cycles. Density functional theory calculation reveals that PtMn alloys are thermodynamically more stable than PtNi alloys in terms of formation enthalpy and cohesive energy. The PtMn nanodendrite‐based membrane electrode assembly delivers an outstanding peak power density of 1.36 W cm−2 at a low Pt loading and high‐performance retention over 50 h operations at 0.6 V in H2‐O2 hydrogen fuel cells.

Low-Electronegativity Mn-Contraction of PtMn Nanodendrites Boosts Oxygen Reduction Durability.

Platinum metal (PtM, M=Ni, Fe, Co) alloys catalysts show high oxygen reduction reaction (ORR) activity due to their well-known strain and ligand effects. However, these PtM alloys usually suffer from a deficient ORR durability in acidic environment as the alloyed metal is prone to be dissolved due to its high electronegativity. Herein, we report a new class of PtMn alloy nanodendrite catalyst with low-electronegativity Mn-contraction for boosting the oxygen reduction durability of fuel cells. The moderate strain in PtMn, induced by Mn contraction, yields optimal oxygen reduction activity at 0.53 A mg-1 at 0.9 V versus reversible hydrogen electrode (RHE). Most importantly, we show that relative to well-known high-electronegativity Ni-based Pt alloy counterpart, the PtMn nanodendrite catalyst experiences less transition metals' dissolution in acidic solution and achieves an outstanding mass activity retention of 96 % after 10,000 degradation cycles. Density functional theory calculation reveals that PtMn alloys are thermodynamically more stable than PtNi alloys in terms of formation enthalpy and cohesive energy. The PtMn nanodendrite-based membrane electrode assembly delivers an outstanding peak power density of 1.36 W cm-2 at a low Pt loading and high-performance retention over 50 h operations at 0.6 V in H2 -O2 hydrogen fuel cells.

Applied Voltage‐Activated Fe─N─C Catalysts for Pem Fuel Cells

AbstractThe practical application of non‐precious Fe─N─C catalysts in proton exchange membrane fuel cells (PEMFCs) continues to remain one of the major challenges due to their relatively poor oxygen reduction reaction (ORR) performance in acid. In this work, a fast and facilely performance enhancement strategy is first proposed for various Fe─N─C catalysts by supplying different direct‐current voltages to achieve a rapid solid‐state activation at room temperature. The voltage‐activated state‐of‐the‐art Fe─N─C catalyst has demonstrated a peak power density of 1.1 W cm−2 for PEMFC and remarkably increased long‐term durability for ORR. The substantially improved performance can be attributed to the formation of highly active ketone‐decorated (edge‐hosted) FeN4 sites and substantially boosts coupled proton−electron transfer (CPET) by improving the conductivity of the as‐synthesized Fe─N─C with an interpenetrating network structure. The interconnected micro‐nodes within the interpenetrating network are the main active regions for the ORR, evidenced by an optimized structural model based on the above typical morphology. Therefore, this finding provides an innovative and facile idea for solving the activity and stability deficiency for promising Fe─N─C for commercial PEMFC.

A novel core-shell nanostructure of Ti-Au nanocrystal with PtNi alloy skin: Enhancing the durability for oxygen reduction reaction

The sluggish oxygen reduction reaction (ORR) at the cathode largely hinders the practical application of the proton-exchange membrane fuel cells (PEMFCs). Pt-based nanocrystals are the most effective electrocatalysts for the oxygen reduction reaction, however, suffer high cost, scarcity and unsatisfied durability. Herein, we report a new category of annealed multimetallic core-shell catalyst of gold core doped with titanium and platinum-nickel (PtNi) shell with Pt-rich surface as a highly active and robust electrocatalyst for ORR. The introduction of titanium can effectively stabilize the gold core, avoiding the outward migration of Au atoms and ensuring the exposure of Pt-Ni active sites. The experimental findings revealed that the annealed Ti-Au@PtNi NPs catalyst provides 19.26 and 9.84 times higher mass and specific activities than commercial Pt/C catalysts for the oxygen reduction reaction. Moreover, durability tests of annealed Ti-Au@PtNi NPs show no noticeable activity loss even after 20000 potential cycles between 0.6 and 1.0 V vs. RHE, demonstrating a promising catalyst for the oxygen reduction reaction.

In situ confined synthesis of an interlayer-riveted carbon shell encapsulated PdZnBi alloy as a highly active and durable oxygen reduction reaction catalyst

An interlayer riveted carbon shell encapsulated PdZnBi alloy is synthesized using the in situ confinement effect of small molecules. After a long-term durability test, its mass activity (MA) remained at 94% of the initial value.

Structural Effects on ORR Performances of Carbon-Free Connected Platinum–Cobalt Catalysts with Chemically Ordered Superlattice Structures

Polymer electrolyte fuel cells (PEFCs) have been considered as the promising next-generation energy-conversion technology, owing to their high energy efficiency and clean emissions. However, a sluggish oxygen reduction reaction (ORR) rate and the low durability of conventional catalysts in PEFCs limit the wider commercialization. To address these issues, our group has developed a carbon-free, connected Pt1–Fe1 catalyst with a porous hollow capsule structure. This catalyst comprises the nano-sized network formed by the connection of Pt1–Fe1 nanoparticles and exhibits an enhanced ORR specific activity (9 times higher than a commercial Pt/C) as well as excellent durability against start-stop operations due to the carbon-free structure.[1,2] However, during the load cycle operation, iron is dissolved from the catalyst, leading to the loss of ORR activity and the degradation of polymer electrolytes caused by the Fenton reaction.In this work, carbon-free and iron-free connected Ptx-Co1 catalysts with chemically ordered superlattice structures have been developed for the first time, as shown in Fig. 1(a). In addition, the metal compositions and chemically ordered (superlattice) degrees in the catalysts were controlled to investigate the structural effects on ORR activity and durability of the connected Ptx-Co1 catalysts.Herein, we employed the silica-coating method for the catalyst synthesis, which involves the combination of silica coating and high-temperature annealing. As shown in Fig. 1(b), the surface of the Ptx-Co1 nanoparticles/SiO2 sample was coated with thin silica layers to prevent the detachment of Ptx–Co1 nanoparticles from the SiO2 template as well as the large catalyst agglomeration and coalescence during the annealing process. We controlled the chemically ordered degrees of the connected Pt1–Co1 and Pt3–Co1 catalysts by using different annealing temperatures. The XRD patterns of the catalysts showed the peaks corresponding to L10 type and L12 type chemically ordered structures for the connected Pt1–Co1 and Pt3–Co1 catalysts annealed at 600°C, respectively. These two catalysts possessed ordered degrees (S) of ca. 50–60%. The TEM images of the connected Ptx–Co1 catalysts showed the formation of connected Ptx–Co1 nanonetwork and porous hollow capsule structures.The electrochemical measurements of ORR activity and durability were conducted in a 0.1 M HClO4 electrolyte solution. The connected Pt1–Co1 and Pt3–Co1 catalysts achieved 6- and 10-times higher ORR specific activities than that of Pt/C, respectively. Subsequently, the ORR activity and load cycle durability (0.6 V for 3 s Û 1.0 V for 3 s at 60 °C) of the connected Pt3–Co1 catalyst with different ordered degrees (S = 0%, 28%, and 58%) were evaluated. As the ordered degrees in the connected Pt3–Co1 catalysts were higher, the ORR specific activity was increased. Also, the retention of the specific activity of the ordered catalyst (S = 58%) was higher than that of the disordered catalyst (S = 0%) after 10,000 load cycles. The STEM-EDX line mapping results showed that 83% of the alloyed Co retained in the catalyst with a higher ordered degree even after 10000 load cycles, whereas the disordered and lower ordered catalysts retained lower Co ratios after the test, indicating the suppression effect of an ordered structure on metal dissolution. In addition, the L12 ordered Pt3–Co1 structure remained in the connected catalyst (S = 58%) after load cycle test, confirmed by the TEM diffraction analysis.In summary, we successfully developed the connected Ptx-Co1 catalysts with chemically ordered superlattice structures. In addition, we clarified the structural effects of metal composition and ordered degree in connected Ptx-Co1 catalysts on ORR activity and load cycle durability. These results provide useful guidelines for the design of advanced ORR catalysts towards high-performance PEFCs. Acknowledgement The part of this presentation is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Oxygen Reduction Reaction Properties of Dry-Process Synthesized Pt-Non PGM Transition Metal Multi-Component Alloys: Influence of Transition Metal Elements

Introduction Multi-component alloys (MCAs) have been attracted much attention in various engineering application fields. [1] By changing numbers of constituent elements (n) as well as their composition ratios (𝑥) of MCAs, the mixing-entropy (ΔSmix = −𝑅∑(𝑖=1→n) 𝑥𝑖 ln 𝑥𝑖) increases with increasing n, resulting in reduced Gibbs free-energy (ΔG = ΔH - TΔS) of the MCAs, which correlates with interesting bulk material’s properties. Furthermore, the MCAs surfaces possess specific chemical properties, for example, high thermodynamic and chemical stabilities, probably caused by the reduced ΔG, that are much different from those of binary alloys, e.g., Pt-Co or Pt-Ni. Therefore, the MCAs surfaces can be applied for novel nano-materials developments, including electrocatalytic materials. [2] Indeed, our previous study for single-crystal surfaces of the MCAs system of Pt and non-PGM Cr-Mn-Fe-Co-Ni (known as Cantor alloy [3]) demonstrates that enhanced oxygen reduction reaction (ORR) activity at a pristine state and, furthermore, electrochemical structural stability even through the potential cycle (PC)-loading. [4] Thermodynamical and chemical properties of the MCAs surfaces are, in general, a superposition of specific properties of the constituent elements and newly emerging properties that stemming from multi-component alloying. Therefore, catalytic influences of the multiple alloying-elements on ORR properties (activity and structural stability) of Pt-containing MCAs (referred to as Pt-MCAs) surfaces are much complicated, due mainly to the sluggish diffusion and segregation behaviors of the constituent elements. Thus, optimizing the constituent elements (kinds and n) and their composition ratios (𝑥) at the surface vicinity is surely essential for considering an application of the MCAs as practical ORR catalysts. In this study, we synthesized Pt-MCAs on Pt(111) substrate surface with various n and kinds of the constituent elements by controlling vacuum-deposition amounts of the elements and conducted fundamental investigations aimed for high performance cathode alloy catalyst’s developments of polymer electrolyte membrane fuel cells. Experimental In this study, we focused on equi-composition-ratio of the MCAs by picking up 2~5 elements of Cr, Mn, Fe, Co, and Ni as alloying elements of Pt. 10 monolayer (ML)-thick (1 ML = ca. 0.3 nm) MCAs layer was first deposited on the surface-cleaned Pt(111) substrate by using an arc-plasma deposition method at 300 K, and subsequently annealed in vacuum at 773 K for 30 min. Then, 4 ML-thick Pt layer was deposited on the MCAs layer at 300 K and post-annealed at 623 K in vacuum. The samples thus prepared are designated as Pt/MCAs/Pt(111). The atomic-level micro-structures and composition ratios of the surface-vicinity of resulting Pt/MCAs/Pt(111) were characterized by cross-sectional STEM-EDS and XPS. CVs and LSVs were conducted in N2-purged and O2-saturated 0.1 M HClO4 by using an RDE apparatus. ORR activity was evaluated from j k values estimated at 0.9 V vs. RHE by using Koutecky-Levich equation and structural stability (ORR durability) was discussed based upon the activity transitions during applying 5,000 potential cycles (PCs) of 0.6(3s)‐1.0(3s) V vs. RHE in O2 saturated 0.1 M HClO4 at room temperature. Results and Discussion The ORR activity trends under applying the PC loading (a), initial CVs (b), and XPS-estimated surface compositions of Pt/MCAs/Pt(111) are summarized in Fig. 1(a-c), where Cr-Mn-Fe-Co-Ni, Mn-Fe-Co-Ni, Cr-Fe-Co-Ni, and Fe-Co-Ni are adopted for the MCAs. Notably, the ORR activity of Cr containing Pt/Cr-Fe-Co-Ni/Pt(111) (green) decreased remarkably up to 2,000 PCs, compared to Cr-less Pt/Mn-Fe-Co-Ni/Pt(111) (red), Pt/Fe-Co-Ni/Pt(111) (yellow), and both Cr and Mn containing Pt/Cr-Mn-Fe-Co-Ni(Cantor alloy)/Pt(111) (blue). Furthermore, Pt/Cr-Fe-Co-Ni/Pt(111) exhibited increased electric double layer charge in the potential region of 0.4 to 0.6 V at the pristine state (b). Additionally, XPS-estimated relatively smaller and larger composition ratios of the Pt and Cr, respectively, among the tested Pt/MCAs/Pt(111) (c), suggesting that Cr of the MCAs preferentially diffuses and segregates at the surface-vicinity through the heat treatment process of the sample fabrication, which might cause severe decrease in ORR activity at the early stage of the PCs loading (up to 2,000 PCs). [5] At the meeting, correlations between the ORR properties and atomic-level microstructures of Pt/MCAs/Pt(111) surfaces will be discussed in detail. Acknowledgement This study was supported by new energy and industrial technology development organization (NEDO) of Japan, JST SPRING, Grant Number JPMJJSP2114 (Y.C.) and Grant-in-Aid for JSPS Fellows (Y.C.).

ORR Activity and Voltage-Cycling Stability of a Carbon-Supported PtxY Alloy Catalyst Evaluated in a PEM Fuel Cell

Platinum-yttrium alloys (PtxY) are suggested to have superior oxygen reduction reaction (ORR) activity and long-term stability in proton exchange membrane fuel cells (PEMFCs). However, the actual ORR activity and stability of a PtxY catalyst with a high electrochemically active surface area (ECSA) in a PEMFC remains uncertain. Here, a Ketjen black (KB) carbon supported PtxY/KB catalyst with a high ECSA (∼60 m2/g) was synthesized using a carbon nitride precursor. Based on elemental analysis, XRD, electron microscopy, and a mass-balance based model, it was shown that the acid-leached PtxY nanoparticles of the catalyst consist of a ∼0.7 nm thick Pt-shell and a Pt3Y core. Rotating disk electrode (RDE) and 5 cm2 single-cell PEMFC measurements indicated that the ORR activity of the acid-leached PtxY/KB catalyst is similar to an analogously synthesized Pt/KB reference catalyst with the same ECSA. Voltage-cycling accelerated stress tests (ASTs) between 0.6−1.0 V (in H2/N2 at 80 °C/95% RH) in 5 cm2 single-cells showed that the ORR activity and durability of the PtxY/KB catalyst is similar to that of the Pt/KB reference catalyst. Thus, the high durability of Pt-rare Earth alloys that has been claimed on the basis of RDE measurements is not observed in actual PEMFCs.

Cobalt atom-cluster interactions synergistically enhance the activity of oxygen reduction reaction in seawater

Seawater electrocatalysis is highly desired for a variety of energy storage and conversion systems, such as water splitting and metal fuel cells that directly using seawater as electrolyte. However, the adsorption of chloride ions (Cl−) on active sites of cathodes would worsen the oxygen reduction reaction (ORR) activity and stability, thus lowering the battery performance. In this work, we firstly designed an electrocatalyst model, in which the Co atomic clusters were closely surrounded by satellite Co single atoms on N-doped carbon substrates, designated as Co-ACSAs. The theoretical calculation results suggested that the Co clusters possess stronger Cl− binding energy and acted as pre-adsorption group for Cl−, thus fully exposed Co single atoms as ORR active sites with stronger O2 adsorption energy to promote the oxygen reduction reaction process. Then, we developed an ultrafast high temperature shock strategy to synthesize the Co-ACSAs by controlling the N concentration in carbon substrates. Benefiting from the moderate interacting distance between Co clusters and satellite Co single atoms in Co-ACSAs, the electronic structure of Co clusters and Co single atoms was optimized through the modulation of the interconnected hexatomic ring. As a result, Co-ACSAs exhibit superior ORR activity and durability with on-set and half-wave potentials of 0.885 V and 0.782 V, respectively, and continuously catalyzing for 485 h in seawater electrolyte. The Co-ACSAs-based seawater battery exhibits a discharge voltage of 1.41 V at 5 mA cm−2 and realized stable energy supply for more than 390 h.

Universal and Energy-Efficient Approach to Synthesize Pt-Rare Earth Metal Alloys for Proton Exchange Membrane Fuel Cell.

Traditional synthesis methods of platinum-rare earth metal (Pt-RE) alloys usually involve harsh conditions and high energy consumption because of the low standard reduction potentials and high oxophilicity of RE metals. In this work, a one-step strategy is developed by rapid Joule thermal-shock (RJTS) to synthesize Pt-RE alloys within tens of seconds. The method can not only realize the regulation of alloy size, but also a universal method for the preparation of a family of Pt-RE alloys (RE=Ce, La, Gd, Sm, Tb, Y). In addition, the energy consumption of the Pt-RE alloy preparation is only 0.052kWh, which is 2-3 orders of magnitude lower than other reported methods. This method allows individual Pt-RE alloy to be embedded in the carbon substrate, endowing the alloy catalyst excellent durability for oxygen reduction reaction (ORR). The performance of alloy catalyst shows negligible decay after 20k accelerated durability testing (ADT) cycles. This strategy offers a new route to synthesize noble/non-noble metal alloys with diversified applications besides ORR.

Tuning the surface electronic structure of noble metal aerogels to promote the electrocatalytic oxygen reduction

The sluggish kinetics of the oxygen reduction reaction (ORR) is the bottleneck for various electrochemical energy conversion devices. Regulating the electronic structure of electrocatalysts by ligands has received particular attention in deriving valid ORR electrocatalysts. Here, the surface electronic structure of Pt-based noble metal aerogels (NMAs) was modulated by various organic ligands, among which the electron-withdrawing ligand of 4-methylphenylene effectively boosted the ORR electrocatalysis. Theoretical calculations suggested the smaller energy barrier for the transformation of O* to OH* and downshift the d-band center of Pt due to the interaction between 4-methylphenylene and the surface metals, thus enhancing the ORR intrinsic activity. Both Pt3Ni and PtPd aerogels with 4-methylphenylene decoration performed significant enhancement in ORR activity and durability in different media. Remarkably, the 4-methylphenylene modified PtPd aerogel exhibited the higher half-wave potential of 0.952 V and the mass activity of 10.2 times of commercial Pt/C. This work explained the effect of electronic structure on ORR electrocatalytic properties and would promote functionalized NMAs as efficient ORR electrocatalysts.

Sulfur doped iron-nitrogen-hard carbon nanosheets as efficient and robust noble metal-free catalysts for oxygen reduction reaction in PEMFC

Transition metal-nitrogen-carbon (M-N-C) as a promising substitute for the conventional noble metal-based catalyst still suffers from low activity and durability for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). To tackle the issue, herein, a new type of sulfur-doped iron-nitrogen-hard carbon (S-Fe-N-HC) nanosheets with high activity and durability in acid media were developed by using a newly synthesized precursor of amide-based polymer with Fe ions based on copolymerizing two monomers of 2, 5-thiophene dicarboxylic acid (TDA) as S source and 1, 8-diaminonaphthalene (DAN) as N source via an amination reaction. The as-synthesized S-Fe-N-HC features highly dispersed atomic FeNx moieties embedded into rich thiophene-S doped hard carbon nanosheets filled with highly twisted graphite-like microcrystals, which is distinguished from the majority of M-N-C with soft or graphitic carbon structures. These unique characteristics endow S-Fe-N-HC with high ORR activity and outstanding durability in 0.5 M H2SO4. Its initial half-wave potential is 0.80 V and the corresponding loss is only 21 mV after 30,000 cycles. Meanwhile, its practical PEMFC performance is a maximum power output of 628.0 mW cm−2 and a slight power density loss is 83.0 mW cm−2 after 200-cycle practical operation. Additionally, theoretical calculation shows that the activity of FeNx moieties on ORR can be further enhanced by sulfur doping at meta-site near FeN4C. These results evidently demonstrate that the dual effect of hard carbon substrate and S doping derived from the precursor platform of amid-polymers can effectively enhance the activity and durability of Fe-N-C catalysts, providing a new guidance for developing advanced M-N-C catalysts for ORR.

Phase engineering of Pt-Mn intermetallics for durable oxygen reduction reaction electrocatalysis

Controlling the crystal phase of intermetallic has been demonstrated as a promising strategy for enhancing oxygen reduction reaction (ORR) performance. However, the controlled synthesis of intermetallic with specific crystal phases remains a formidable challenge. Herein, structure ordered Pt-Mn intermetallics with face-centered cubic Pt3Mn (fcc-Pt3Mn/C) and face-centered tetragonal PtMn (fct-PtMn/C) are synthesized via an impregnation-reduction method. Benefiting from the rational control of crystal phase, fct-PtMn/C with optimal surface atomic arrangement and binding environment exhibits higher activity than fcc-Pt3Mn/C and Pt/C. Moreover, fct-PtMn/C exhibits a superior mass activity retention rate of 90.3 % after 50,000 potential cycles from 0.6 to 1.0 V compared to significantly lower rate of 56.0 % and 34.8 % for fcc-Pt3Mn/C and Pt/C, respectively. Structural analysis reveals that fct-PtMn/C maintains its crystal phase, particle size, and morphology exceptionally well during the durability test, demonstrating its outstanding structural stability. Theoretical calculations indicate that phase engineering can lower the reaction energy barrier, weaken the adsorption energy of reaction intermediates on fct-PtMn/C, and thus enhance the ORR performance.

Excellent electrocatalytic performance of rice straw-derived carbon catalysts activated by zinc halides for oxygen reduction reaction

Biomass-derived carbon materials have been proved to be a prominent catalysts for oxygen reduction reaction (ORR). The chemical activator used in the preparation process of biomass-derived carbon materials has an important impact on its ORR performance. Exploration of suitable chemical activator has become extremely important. Three zinc halides (ZnF2, ZnBr2, and ZnI2) and ZnCl2 used as the activators for the preparation process of rice straw-derived carbon (SDXNC) were carefully studied for the first time. The surface area, pore volume, doping N content and surface defect of as-prepared SDXNCs were effectively improved with the pyrolysis temperature of 1100 °C, further improving the ORR activity of SDXNC samples. The four rice straw-derived carbons activated by ZnX2 presented high onset potential, half-wave potential and limited current density, which can comparable to those of Pt/C. Moreover, they also occupy the typical 4-electron reduction pathway, ultrahigh methanol tolerance, and durability for ORR. Therefore, a class of low-cost and excellent chemical activators can be used for the preparation of biomass-derived carbon materials with prominent ORR activity.

Recent Advances in Non‐Precious Metal Single‐Atom Electrocatalysts for Oxygen Reduction Reaction in Low‐Temperature Polymer‐Electrolyte Fuel Cells

AbstractFuel cells have emerged as a promising clean electrochemical energy technology with a great potential in various sectors, including transportation and power generation. However, the high cost and scarcity of the noble metals currently used to synthesise electrocatalysts for low‐temperature fuel cells has hindered their widespread commercialisation. In recent decades, the development of non‐precious metal electrocatalysts for the cathodic oxygen reduction reaction (ORR) have gained significant attention. Among those, electrocatalysts with atomically dispersed active sites, referred to as single‐atom catalysts (SACs), are gaining more interest. Nanocarbon materials containing single transition metal atoms coordinated to nitrogen atoms are active electrocatalysts for the ORR in both acidic and alkaline conditions and thus have a great promise to be utilised as non‐precious metal cathode electrocatalysts in low‐temperature fuel cells. This review article provides an overview of the recent advancements in the utilisation of transition metal‐based SACs in proton exchange membrane fuel cells (PEMFCs) and anion exchange membrane fuel cells (AEMFCs). We highlight the main strategies and synthetic approaches for tailoring the properties of SACs to enhance their ORR activity and durability. Based on the already achieved results, it is evident that SACs indeed could be suitable for the cathode of the low‐temperature fuel cells.

Recent Progress in Using Mesoporous Carbon Materials as Catalyst Support for Proton Exchange Membrane Fuel Cells.

Developing durable oxygen reduction reaction (ORR) electrocatalysts is essential to step up the large-scale applications of proton exchange membrane fuel cells (PEMFCs). Traditional ORR electrocatalysts provide satisfactory activity, yet their poor durability limits the long-term applications of PEMFCs. Porous carbon used as catalyst support in Pt/C is vulnerable to oxidation under high potential conditions, leading to Pt nanoparticle dissolution and carbon corrosion. Thus, integrating Pt nanoparticles into highly graphitic mesoporous carbons could provide long-term stability. This Perspective seeks to reframe the existing approaches to employing Pt alloys and mesoporous carbon-integrated ORR electrocatalysts to improve the activity and stability of PEMFCs. The unusual porous structure of mesoporous carbons promotes oxygen transport, and graphitization provides balanced stability. Furthermore, the synergistic effect between Pt alloys and heteroatom doping in mesoporous carbons not only provides a great anchoring surface for catalyst nanoparticles but also improves the intrinsic activity. Furthermore, the addition of Pt alloys into mesoporous carbon optimizes the available surface area and creates an effective electron transfer channel, reducing the mass transport resistance. The long-term goals for fuel-cell-powered cars, especially those designed for heavy-duty use, are well aligned with the results shown when this hybrid material is used in PEMFCs to improve performance and durability.

Carbothermal shock synthesis of CoO/N/C nanoparticles with superior durability for oxygen reduction reaction

Designing an efficient and stable non-precious metal cathode catalyst is essential to reduce fuel cell cost and promote fuel cell commercialization. Herein, ZIF-67 and carbon powder are rapidly heated using a transient Joule heat device to pyrolyze ZIF-67 to form a CoO/N/C nanocatalysts. The prepared CoO/N/C-90 is comprised of 3–4 nm CoO nanoparticles loaded on a carbon substrate. In alkaline medium the Tafel slope of CoO/N/C-90 is 50 mV dec−1, and a half-wave potential (E1/2) of 0.82 V towards oxygen reduction reaction (ORR). The E1/2 only experiences a negative shift of 4 mV after 40,000 cyclic voltammetry (CV) cycles, demonstrating excellent catalytic reaction kinetics and durability. The CoO/N/C-90 is assembled into direct borohydride fuel cells (DBFC) as a cathode, achieving a maximum power density (Pmax) of 526 mW cm−2 at 60 °C. This work indicates that the CoO/N/C-90 is an efficient and stable non-precious metal catalyst for DBFCs.

Single-Atom-Based Oxygen Reduction Reaction Catalysts for Proton Exchange Membrane Fuel Cells: Progress and Perspective.

Single-atom catalysts (SACs) are regarded as promising non-noble-metal alternatives for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells due to their high atom utilization efficiency and excellent catalytic properties. However, the insufficient long-term stability issues of SACs under the working conditions seriously hinder their practical application. In this perspective, the recent progress of SACs with optimized ORR catalytic activity is first reviewed. Then, the possible degradation mechanisms of SACs in the ORR process and effective strategies for improving their ORR durability are summarized. Finally, some challenges and opportunities are proposed to develop stable single-atom-based ORR electrocatalysts in the future.

Boosting Bifunctional Catalysis by Integrating Active Faceted Intermetallic Nanocrystals and Strained Pt-Ir Functional Shells.

PtIr-based nanostructures are fascinating materials for application in bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysis. However, the fabrication of PtIr nanocatalysts with clear geometric features and structural configurations, which are crucial for enhancing the bifunctionality, remains challenging. Herein, PtCo@PtIr nanoparticles are precisely designed and fabricated with a quasi-octahedral PtCo nanocrystal as a highly atomically ordered core and an ultrathin PtIr atomic layer as a compressively strained shell. Owing to their geometric and core-shell features, the PtCo@PtIr nanoparticles deliver approximately six and eight times higher mass and specific activities, respectively, as an ORR catalyst than a commercial Pt/C catalyst. The half-wave potential of PtCo@PtIr exhibits a negligible decrease by 9mV after 10 000 cycles, indicating extraordinary ORR durability because of the ordered arrangement of Pt and Co atoms. When evaluated using the ORR-OER dual reaction upon the introduction of Ir, PtCo@PtIr exhibits a small ORR-OER overpotential gap of 679mV, demonstrating its great potential as a bifunctional electrocatalyst for fabricating fuel cells. The findings pave the way for designing precise intermetallic core-shell nanocrystals as highly functional catalysts.