Pt-based Cathode Research Articles

Optimized mass transfer in a Pt-based cathode catalyst layer for PEM fuel cells

Different strategies of mass transfer enhancement in a Pt-based cathode catalyst layer for PEM fuel cells.

Investigating Oxygen Reduction Performance of Fe-N-C Catalysts for Proton Exchange Membrane Fuel Cells

Non-precious Fe-N-C catalysts have reached high activity levels for acidic oxygen reduction reaction (ORR) in rotating ring disk electrode (RRDE) setup approaching to that of the platinum group metals (PGM) when compared at high catalyst loadings of 0.6-1.0 mgFe-N-C cm-2. The improvements in the ORR activity of Fe-N-Cs can be attributed to new synthetic routes developed in recent years that have led to increased Fe-N4 active site density and utilization and/or better intrinsic activity/turnover frequency (TOF) of those sites.1-3 Despite impressive activities in half-cells, Fe-N-Cs still require quite high loadings in the range of 4.0-5.0 mg cm-2 in proton exchange membrane fuel cells (PEMFCs), which makes the Fe-N-C cathodes 4-5x thicker than Pt-based cathodes (~100 µm for Fe-N-C layer vs ~10-20 µm for Pt layer). Thick Fe-N-C cathodes suffer from quick performance drop in medium to high current density region due to mass-transport related losses. Reducing the Fe-N-C loading levels in RRDE and in PEMFC without comprising the ORR activity would need further improvements in active site density, site utilization and TOF. In this talk, I will present our work on how morphological tailoring of carbon framework in Fe-N-Cs affects the utilization of active sites and water adsorption-desorption behaviour with the aim of improving the ORR performance and mass-transport properties. Active-site imprinted carbon frameworks are used to prepare Fe-N-Cs that consist of purely Fe-N4 sites without any other iron species.1,4,5 Besides activity, performance stability against extensive voltage cycling in RRDE and long-term durability in PEMFC single-cells is another important criterion that Fe-N-Cs need to fulfil for becoming a viable substitute to PGM catalysts. Very recently, encouraging results on the improvement of Fe-N-C durability in PEMFC tests have been reported using radical scavengers and by coating a thin carbon layer on the surface of Fe-N-C.6,7 However, significantly more efforts are required in that direction for reaching the durability targets of thousands of hours. Second part of the talk will discuss the performance stability of Fe-N-Cs.

Single Cell Performance and Durability of Pt and Pt-Based Catalysts Supported on Mesoporous Carbon

Introduction In Japan, an extremely high cell performance, i.e., 0.84 V at 0.2 A/cm2 and 0.70 V at 3.0 A/cm2, is required for polymer electrolyte fuel cells (PEFCs) used in fuel cell electric vehicles after 2030 [1]. In addition to high oxygen reduction reaction (ORR) activity of the Pt and the Pt-based cathode catalysts for increasing cell voltage in the low current density (LCD) region, high oxygen diffusivity is crucial for achieving the cell voltage in the high current density (HCD) region [2]. Therefore, it is considered that carbon supports for the catalyst NPs would play an important role for realizing the high cell performance. In this study, a mesoporous carbon (MPC) was selected as the support for Pt and Pd@Pt core-shell structured catalyst NPs and cell performance of the catalysts were compared with that of the catalyst supported on a conventional Ketjen Black (KB). Furthermore, durability of the MPC support against a high potential range was investigated. Experimental KB-600JD (SBET: 1,335 m2/g, LION) and MPC (CNovel MH-18, SBET 1,334 m2/g, TOYO TANSO [3]) were used as carbon supports. The Pt/MPC catalyst, Pt diameter of 2.5 nm and metal loading of 50 wt.%, was synthesized through impregnation of Pt(NO2)2(NH3)2 followed by thermal reduction in N2 at 400oC. Pt/Pd/KB and Pt/Pd/MPC core-shell catalysts were synthesized by a direct displacement reaction (DDR) method [4]. Diameter of Pd@Pt NPs was 5.8 nm and Pt and Pd metal loading were 20 wt.% and 30 wt.%, respectively. Cyclic voltammogram was recorded in Ar-saturated 0.1 M HClO4 at 25oC with a scan rate of 50 mV/s and ORR activity was evaluated in O2-saturated 0.1 M HClO4 at 25oC with a positive potential scan rate of 10 mV/s at 1,600 rpm. Fuel cell performance was evaluated by using a single cell with an active area of 1 cm2. Nafion DE2020 was used as an ionomer for the catalyst ink preparation (I/C ratio was set to 0.83-1.2) and Nafion NRE 211 with 25 µm in thickness was used as a membrane. Pt loading in the cathode catalyst layer was set to 0.1 mg/cm2. H2 and air were supplied to the anode and the cathode by 418 NmL/min. and 988 NmL/min., respectively, and pressurized to 150 kPa at gas outlets. The cell temperature was set to 80oC and humidified from 35% to 95% relative humidity (RH). H2 utilization was 5% at a current density of 3.0 A/cm2. Durability of the carbon supports was evaluated by using the single cell. H2 and N2 were fed to anode and cathode, respectively, and cathode potential was swept from 1.0 to 1.5 V vs. RHE with a scan rate of 500 mV/s for 1,000 cycles. Results and Discussion Pore size distribution of the carbon supports is shown in Figure 1. Both KB-600JD and MPC (CNovel MH-18) carbon supports have mesopores, and the MPC exhibits a narrower pore size distribution centered at 4 nm. Figure 2 demonstrates I-V performance of Pd@Pt core-shell catalysts, indicating that Pt/Pd/MPC catalyst shows a higher cell voltage in HCD region (> 2 A/cm2) than that of Pt/Pd/KB-600JD catalyst. Interconnectivity of the mesopores analyzed by 3D-TEM observation of the carbon supports is depicted in Figure 3. It is clear that density of interconnected mesopores is much higher in the MPC support, which is considered to contribute to the higher cell voltage in HCD region of the Pt/Pd/MPC catalyst. Figure 4 displays cross-sectional SEM image of the Pt/Pd/MPC catalyst. In addition to the well-interconnected mesopores (Figure 3), macropores coexist in the MPC support, which is also considered to contribute to the higher cell voltage in HCD region.A high potential durability test was performed by using potential cycling of 1.0-1.5 V vs. RHE for 1,000 cycles at 80oC. Thickness of Pt/MPC cathode catalyst layer was decreased by 36% after the durability test, while the decrease was mitigated to 14% with a heat treatment of the MPC support at 2,200oC in Ar. (Figures 5). As is well known, it was demonstrated that the high temperature heat treatment has a positive effect for durability improvement of the carbon support against the high potential range.This study was partly supported by NEDO, Japan.

(Poster Award - 3rd Place) Synthesis of Spherical Mesoporous Carbon (SMPC) Support and Electrochemical Properties of Pt/SMPC Catalyst

Introduction In Japan, extremely high cell performance is required for polymer electrolyte fuel cells (PEFCs) used in fuel cell electric vehicles after 2030 [1]. Therefore, it is of great importance to improve not only ORR activity, but also oxygen diffusivity of Pt and Pt-based cathode catalysts in PEFCs. In this study, a spherical mesoporous carbon (SMPC) with central mesopore’s diameter of 7 nm was synthesized and used for support of the Pt NP catalyst. Electrochemical properties of the Pt/SMPC catalyst were investigated. Experimental The SMPC support was synthesized by silica template method in which SiO2 precursor was in-situ formed [2]. 7 mL of tetrapropyl orthosilicate was added in a mixed solution (6 mL conc. NH3, 150 mL EtOH and 10 mL H2O) and stirred for 15 min. Then, 0.8 g resorcinol and 1.12 mL formaldehyde (37 wt.% aq.) were added and stirred at room temperature for 24 h. The precipitate was filtered and dried at 60°C overnight in air. The powder was heat-treated in a tube furnace at 850°C for 3 h in Ar atmosphere to obtain SiO2/SMPC. The SiO2/SMPC was dispersed in 10% HF aq. and stirred at room temperature for 2 h to remove silica template. The Pt/SMPC catalyst was synthesized by alcohol reduction method [3]. 300 mg SMPC was dispersed in a EtOH/H2O mixed solution (37.5 mL/250 mL), and Pt(NO2)2(NH3)2 (corresponding to 300 mg metallic Pt) dissolved in aqueous HNO3 was added, followed by refluxing at 90°C for 4 h in N2 atmosphere. The catalyst was filtered, washed and dried at 60°C overnight in air. Characterization of SMPC and Pt/SMPC was conducted by N2 gas adsorption, TG-DTA, XRD, TEM and TEM-EDX. The catalyst was loaded on GC-RDE. CV was recorded in Ar-saturated 0.1 M HClO4 at 25oC and at 50 mV/s. The ORR activity was evaluated by LSV performed in O2-saturated 0.1 M HClO4 at 25oC at 10 mV/s in the positive direction while rotating the GC-RDE at 1,600 rpm. Results and Discussion The weight percentage of silica template was 70% in SiO2/SMPC. Figure 1 shows SEM and TEM images of SiO2/SMPC before and after HF treatment. The SEM images clearly show that SiO2/SMPC and SMPC have spherical shape with mean diameter of 200 nm. In TEM images of SiO2/SMPC, center of the sphere is darker than edge and the edge became darker than the center after HF treatment, suggesting that the silica template NPs resided at the center of the sphere. TEM-EDX composition analysis of SiO2/SMPC is demonstrated in Fig. 2. Before HF treatment, Si was detected at the center of the sphere. After HF treatment, only C was detected with high intensities at the edge of the sphere. These results suggest that the SiO2/SMPC has a silica rich core-carbon rich shell structure. The thickness of the carbon rich shell was ca. 60 nm. The surface area of SiO2/SMPC was 510 m2/g and the area increased to 1,590 m2/g after HF treatment. Figure 3 shows pore size distribution of SiO2/SMPC. After HF treatment, SPMC support exhibited clear existence of mesopores with a central diameter of 7 nm.A cross-sectional TEM image of Pt/SMPC catalyst is depicted in Fig. 4. Pt NPs with Scherrer’s diameter of 2.3 nm were well dispersed over the carbon-rich shell, which indicates that mesopores in the carbon shell were well interconnected. The ECSA and the ORR activity of the Pt/SMPC catalyst were 92 m2/g and 450 A/g@0.9 V, respectively, the latter of which was similar to that of a commercially available Pt/C (TKK, TEC10E50E). Single cell tests using Pt/SMPC is now underway, and the results will be presented at the meeting.This study was partly supported by NEDO, Japan. Reference [1] M. Suzuki, https://www.nedo.go.jp/content/100888556.pdf (in Japanese).[2] Z. Deng et al., Nano Lett., 22, 9551 (2022).[3] M. M. Rahman et al., RSC Adv., 11, 20601 (2021). Figure 1

Degradation of Pt-Based Cathode Catalysts Upon Voltage Cycling in Single-Cell PEM Fuel Cells Under Air or N2 at Different Relative Humidities

A major degradation mechanism of polymer electrolyte membrane fuel cells (PEMFCs) in transportation applications is the loss of the electrochemically active surface area (ECSA) of platinum cathode catalysts upon dynamic load cycling (resulting in cathode potential cycles). This is commonly investigated by accelerated stress tests (ASTs), cycling the cell voltage under H2/N2 (anode/cathode). Here we examine the degradation of membrane electrode assemblies with Vulcan carbon supported Pt catalysts over extended square-wave voltage cycles between 0.6-1.0 VRHE at 80 °C and 30%-100% RH under either H2/N2 or H2/Air; for the latter case, differential reactant flows were used, and the lower potential limit is controlled to correspond to the high-frequency resistance corrected cell voltage, assuring comparable aging conditions. Over the course of the ASTs, changes of the ECSA, the hydrogen crossover current, the proton conduction resistance and the oxygen transport resistance of the cathode electrode, as well as the differential-flow H2/O2 and H2/Air performance at 80 °C/100% RH were monitored. While the ECSA loss decreases with decreasing RH, it is independent of the gas feeds. Furthermore, the H2/Air performance loss only depends on the ECSA loss. ASTs under H2/N2 versus H2/Air only differ with regards to the chemical/mechanical degradation of the membrane.

Design of Noble-Metal-Free Membrane Electrode Assemblies Based on Metal Chalcogenides for Electrochemical Hydrogen Production Via Alkaline Seawater Electrolysis

The production of large amounts of green hydrogen (H2) by electrochemical water splitting represents one of the key pillars to reach net-zero by 2050. As one of the major drawbacks, both low-temperature proton-exchange membrane (PEM) as well as anion-exchange membrane (AEM) electrolysis still rely on the utilization of noble metals in the catalyst layers driving the evolution of H2 and oxygen (O2) on the cathode (HER) and on the anode (OER), respectively. Beyond that, the requirement for highly pure water feeds to prevent degradation of the cell performance over time was previously addressed as one of the main concerns. In particular, this holds true for arid regions located close to the ocean. Advantageously, these coastal arid zones provide essentially unlimited access to seawater, coupled with ample solar irradiation and wind throughout the entire year.[1]One of the major challenges for direct seawater electrolysis lies in the development of selective catalysts, especially for the anode due to the inherently faster reaction kinetics of the competing two-electron chlorine evolution reaction (ClER). Nickel-iron layered double hydroxides (NiFe-LDH) were identified as active and selective OER electrocatalysts in a previous study[2] by our group. Dionigi et al.[3] recently reported a general design principle for selective seawater electrolysis, in which alkaline pH values > 7.5 are claimed to favor OER over ClER for overpotentials < 480 mV.At the cathode, state-of-the-art electrolyzers utilize Pt-based catalysts for an efficient HER.[2] For direct seawater electrolysis, however, not only the high cost but also the weak stability due to chloride-induced corrosion restrict the applicability of Pt-based electrodes. In our contribution, we thus present noble-metal free catalyst materials based on metal chalcogenides with high HER activity and improved stability in alkaline seawater electrolyte in a single-cell electrolyzer setup. Membrane-electrode assemblies (MEAs) with superior corrosion-resistance by a modification of the porous transport layers (PTLs) for both cathode and anode will be presented, which outperform reference MEAs containing Pt-based cathodes in alkaline seawater electrolysis. The optimized noble-metal-free electrode design (fig. 1) combined with well-controlled electrolyte feeding enables alkaline seawater electrolysis operating at industrially relevant current densities.

Anion Exchange Membrane Electrolyser Performance with Ni Ferrite Anodes Calcined at Different Temperatures

Hydrogen has a critical role in enabling European countries to achieve net-zero carbon targets. Hydrogen production via water splitting, using an electrolyser, is considered the "greenest" way because it does not produce any direct carbon emissions when powered by renewable sources. Among the different technologies of electrolysers (liquid alkaline, proton exchange membrane, etc.), there has been a recent surge in interest in that one based on anion-exchange membranes (AEMs). With respect to the state-of-the-art electrolysers that employ conventional acid polymer electrolyte separators (e.g., perfluorinated systems such as Nafion®), electrocatalysis with AEMs is much more promising. Accordingly, inexpensive catalysts can be used.In this work, NiFe2O4 catalysts are prepared by an "oxalate route". Nickel and iron nitrates in suitable stoichiometric ratios are added in successive steps to the oxalic acid solution. An oxidizing solution of hydrogen peroxide is then added. The obtained precipitate is first filtered and then dried. Different calcination temperatures (350, 450 and 550°C) are applied to obtain the Ni Ferrite with different surface and bulk characteristics. The catalysts are then deposited by a spray technique onto a commercial FAA3-50 (from FuMa-Tech) anion-exchange membrane, combined with a Pt-based cathode electrode, and investigated for the water electrolysis process in a single cell of 5 cm2 geometrical area. A comparison between the performance recorded with the different anode catalysts is reported and discussed. Acknowledgments The authors thank the Italian Ministry for University and Research (MUR) for funding through the FISR 2019 project AMPERE (FISR2019_01294).

Lattice-strained Pt nanoparticles anchored on petroleum vacuum residue derived N-doped porous carbon as highly active and durable cathode catalysts for PEMFCs

High cost and poor durability of Pt-based cathode catalysts for oxygen reduction reaction (ORR) severely hamper the popularization of proton exchange membrane fuel cells (PEMFCs). Tailoring carbon support is one of effective strategies for improving the performance of Pt-based catalysts. Herein, petroleum vacuum residue was used as carbon source, and nitrogen-doped porous carbon (N-PPC) was synthesized using a simple template-assisted and secondary calcination method. Small Pt nanoparticles (Pt NPs) with an average particles size of 1.8 nm were in-situ prepared and spread evenly on the N-PPC. Interestingly, the lattice compression (1.08%) of Pt NPs on the N-PPC (Pt/N-PPC) was clearly observed by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), which was also verified by the shift of (111) crystal plane of Pt on N-PPC to higher angles. The X-ray photoelectron spectroscopy (XPS) results suggest that the N-PPC support had a strong effect on anchoring Pt NPs and endowing surface Pt NPs with lowered d band center. Thus, the Pt/N-PPC as a catalyst simultaneously boosted the ORR activity and durability. The specific activity (SA) and mass activity (MA) of the Pt/N-PPC at 0.9 V reached 0.83 mA cm−2 and 0.37 A mgPt−1, respectively, much higher than those of the commercial Pt/C (0.21 mA cm−2 and 0.11 A mgPt−1) in 0.1 M HClO4. The half-wave potential (E1/2) of Pt/N-PPC exhibited only a minimal negative shift of 7 mV after 30,000 accelerated durability tests (ADT) cycles. More importantly, an H2–O2 fuel cell with a Pt/N-PPC cathode achieved a power density of 866 mW cm−2, demonstrating that the prepared catalyst has a promising application potential in working environment of PEMFCs.

Kirkendall effect-driven formation of hollow PtNi alloy nanostructures with enhanced oxygen reduction reaction performance

The design of Pt-based cathode catalyst for the oxygen reduction reaction (ORR) is critical for fabricating high-performance polymer electrolyte membrane fuel cells. In this study, hollow PtNi alloy nanostructures (h-PtNi/C-X) deposited on a carbon black support are prepared using a facile synthesis method at various pH value (X). The degree of alloying of the hollow nanostructures is modulated with increasing pH. Among these h-PtNi/C catalysts, h-PtNi/C-11 exhibits superior ORR activity (five-fold mass activity and higher power density) and improved durability both in half and unit cells, compared to a commercial Pt/C catalyst.

(PEFC&E 2022 Student Poster Award Winner - 1st Place) MEA Performance of Pd@Pt Core-Shell Catalysts Supported on Different Particle Sizes of Mesoporous Carbon

1. Introduction Very high performance, especially on output power density (6 kW/L), for PEFCs in FCVs is targeted for 2030 in Japan [1]. Therefore, we need to improve not only ORR activity, but also oxygen diffusivity of Pt-based cathode catalysts for PEFCs. In this study, highly interconnected mesoporous carbon (MPC) [2] was selected as a support of Pd@Pt core-shell catalyst. The Pd@Pt core-shell catalyst NPs were supported on different particle size of the MPC and cell performance was investigated. 2. Experimental We selected CNovel®MH-18 (central mesopore diameter: 4 nm, SBET: 1,334 m2/g, TOYO TANSO) as a MPC support. As-received MPC with an average size of ca. 2 μm was pulverized to ca. 800, 450, and 250 nm by a wet bead-milling process. The Pd core NPs were loaded on MPCs by an impregnation-thermal reduction method and the Pt shell was formed on the Pd core NPs by a direct displacement reaction method [3]. Characterizations of the MPC, the Pd/MPC cores and the Pt/Pd/MPC catalysts were conducted by N2 gas adsorption, TG-DTA, XRD, XRF, TEM, SEM, CV and the ORR activity of the catalysts was evaluated by the RDE method. MEAs with 1×1 cm2 were fabricated by a decal method using a reinforced membrane (12 µm in thickness, Gore) and a GDL (TORAY). The Pt metal loading at the cathode was 0.1 mg-Pt/cm2. Nafion® DE2020 was used as an ionomer for the catalyst ink preparation (I/C was set to 0.83). I-V performance was evaluated at 80oC, 35-95% RH. H2 gas (418 mL/min.) and air (998 mL/min.) were supplied to the anode and the cathode, respectively, and H2 gas was pressurized to 150 kPa (ambient + 50 kPa) at gas-outlet. 3. Results and Discussion N2 gas adsorption data were analyzed by the t-plot method and the changes in the total, external and internal surface area of the MPCs pulverized for different times are shown in Figure 1. It was shown that the total and the external surface area increased, while the internal surface area decreased with pulverization time. These results suggest that fresh surface was newly formed by the pulverization, and the fraction of the internal surface was decreased. Figure 2 shows cross-sectional TEM images of the Pt/Pd/MPC catalysts using different sizes of MPC supports (ca. 2,000, 800, 450 and 250 nm), which indicates that the catalyst NPs were uniformly supported throughout the interior of MPC regardless of the particle size of MPC. The ECSA of the Pt/Pd/MPC catalysts changed little with the particle size of MPC (ca. 120 m2/g-Pt), while the ORR mass activity at 0.9 V was the highest (1,533 A/g-Pt) when the catalyst NPs were supported on relatively large MPC (800 nm).I-V curves at 80oC, 35% RH of the MEAs using the Pt/Pd/MPC cathode catalysts using different sizes of MPC supports are demonstrated in Figure 3. Interestingly, the cell voltage was higher over the entire current density range for Pd@Pt catalysts using MPCs with larger particle sizes (2000 and 800 nm). This tendency was also observed at a high relative humidity of 95% RH. These results suggest that the Pd@Pt catalyst NPs supported on larger MPC (2,000 and 800 nm) contributed to ORR in the high current density range even when the relative humidity was low. Figure 4 compares the connectivity of mesopores in Ketjen Black EC-600JD (KB-600JD, LION) and CNovel® MH-18 visualized from 3D-TEM observation results. It is clear that the MPC has a higher mesopore’s connectivity than KB-600JD. Figure 5 shows cross-sectional SEM images of the Pt/Pd/MPC. Larger pores were observed inside of the MPC support. Therefore, it is considered that the high connectivity of the mesopores and the presence of larger pores inside of the MPC support contributed to the superior I-V performance observed in MEAs using the catalysts supported on MPCs with particle size of ca. 2,000 and 800 nm. At present, it is not clear why the catalysts supported on smaller MPCs (450 and 250 nm) exhibited inferior I-V performance, though the oxygen diffusion inside MPCs should be easier than that for larger ones.This study was partly supported by NEDO, Japan. The CNovel®MH-18 supports with different size were supplied by Dr. Shodai of TOYO TANSO.

Performance of a Pd-Zn Cathode Electrode in a H2 Fueled Single PEM Fuel Cell

A 21.7 wt.% Pd—7.3 wt.% Zn/C electrocatalyst prepared via the wet impregnation (w.i.) method was deposited onto commercial carbon cloth (E-TEK) and tested towards its electrocatalytic performance as a cathode electrode material for oxygen reduction reaction (ORR) in a H2 fueled single proton-exchange membrane fuel cell (PEMFC). A commercial PtRu electrode (E-TEK) was used as PEM anode for hydrogen oxidation reaction (HOR). The performance of the aforementioned PEMFC was compared with that of the same PEMFC with two different Pt-based cathodes, which were prepared by deposition onto commercial carbon cloth (E-TEK) of 29 wt.% Pt/C synthesized via w.i. and of commercial 29 wt.% Pt/C (TKK). The metal loading of the tested cathode electrodes was 0.5 mgmet cm−2. Comparison was based both on polarization curves and on electrochemical impedance spectroscopy (EIS) measurements at varying cell potential. In terms of power density, the lowest and highest performance was exhibited by the PEMFC with the 21.7 wt.% Pd—7.3 wt.% Zn/C cathode and the PEMFC with the commercial 29 wt.% Pt/C (TKK) cathode electrode, respectively. This behavior was in accordance with the results of EIS measurements, which showed that the PEMFC with the 21.7 wt.% Pd—7.3 wt.% Zn/C cathode exhibited the highest polarization resistance.

Lewis Acid Site Assisted Bifunctional Activity of Tin Doped Gallium Oxide and Its Application in Rechargeable Zn-Air Batteries.

The enhanced safety, superior energy, and power density of rechargeable metal-air batteries make them ideal energy storage systems for application in energy grids and electric vehicles. However, the absence of a cost-effective and stable bifunctional catalyst that can replace expensive platinum (Pt)-based catalyst to promote oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the air cathode hinders their broader adaptation. Here, it is demonstrated that Tin (Sn) doped β-gallium oxide (β-Ga2 O3 ) in the bulk form can efficiently catalyze ORR and OER and, hence, be applied as the cathode in Zn-air batteries. The Sn-doped β-Ga2 O3 sample with 15% Sn (Snx =0.15 -Ga2 O3 ) displayed exceptional catalytic activity for a bulk, non-noble metal-based catalyst. When used as a cathode, the excellent electrocatalytic bifunctional activity of Snx =0.15 -Ga2 O3 leads to a prototype Zn-air battery with a high-power density of 138 mW cm-2 and improved cycling stability compared to devices with benchmark Pt-based cathode. The combined experimental and theoretical exploration revealed that the Lewis acid sites in β-Ga2 O3 aid in regulating the electron density distribution on the Sn-doped sites, optimize the adsorption energies of reaction intermediates, and facilitate the formation of critical reaction intermediate (O*), leading to enhanced electrocatalytic activity.

Fabrication and Performance Evaluation of Tubular Catalyst Layer for Proton Exchange Membrane Fuel Cell

Over the past several decades, planar proton exchange membrane fuel cells (PEMFCs) received considerable attention due to their simple structure, manufacturing, and easy integration. However, current PEMFC electrode designs are disadvantaged by high cost, insufficient mass transport, nonuniform reactant, current and temperature distribution, and limited water removal. The development of a novel tubular fuel cell design could address these challenges. Tubular-shaped PEMFC offers several advantages over planar one, including lower pressure drop, efficient water removal, reduced mass transport losses, and cost reduction owing to removing one of gas diffusion layer/bipolar plate (GDL/BPP) side.This work developed a carbon nanofiber (CNF)/Pt-based cathode for a tubular fuel cell. The tubular CNF support for Pt catalyst was fabricated employing the electrospinning method. Polyacrylonitrile (PAN) was used as the precursor. The electroless deposition of Pt using the chloroplatinic acid solutions was applied to produce well-dispersed low Pt loading nanowires. The tubular CNF/Pt electrodes were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), ex-situ cyclic voltammetry to determine the CF and Pt morphology and feasibility of the prepared layer in PEMFC application. SEM images indicated that increasing the Pt precursor concentration in the plating bath leads to higher Pt loading and larger Pt clusters on the CNF surface. The calculated electrochemical active surface area (ECSA) trend imparts that high Pt loading is less favorable as thick Pt layers lead to low Pt utilization and increased material costs. ECSA results agreed with SEM images, suggesting that the medium Pt concentration (1.5 g L-1 Pt precursor) developed a homogenous Pt distribution and comparable Pt surface area of 24 m2 gPt -1 for the Pt loading of 0.046 mg cm-2, proving the feasibility of the proposed process.

Recent progress in carbon nanotubes support materials for Pt-based cathode catalysts in PEM fuel cells

Support materials have a significant impact on catalytic activity, stability, and performance of catalysts toward the oxygen reduction reaction (ORR). The properties of carbon-based materials have made them an excellent alternative for use as support for nanosized catalysts. Recently, carbon nanotubes (CNTs) have been explored as catalyst support materials, and their properties make them a promissory alternative. Furthermore, catalysts supported on CNTs exhibit higher resistance to electrochemical oxidation, better catalytic performance, and higher durability than catalysts supported on carbon black. In recent years, CNTs have acquired great relevance as catalysts support materials for ORR in acid media. This review addresses the most relevant studies on CNTs modification using methods such as functionalization, doping, and hybrid supports (CNTs-metal oxide) used as supports for Pt-based cathode catalysts in proton exchange membrane fuel cells.

PtCo Nanoalloy Co-Doped Atomically Dispersed Co-N4 Catalyst for Oxygen Reduction in Fuel Cell

Fuel cells are the trending renewable energy source for sustainable future; however, the insufficient performance of Pt-based cathode catalysts particularly, the limited durability hampers their broader applications. Recently, Pt nanoalloys and transition metal-based single-atom catalysts have been emerging catalysts for ORR in fuel cells. However, the high cost of the Pt group metals (PGM) and the limited performance of non-PGM catalysts are huge challenges to meet the desired performance of the fuel cells. Here we report a PtCo nanoalloy catalyst encased by the atomically dispersed Co-N4 doped graphitic carbon nanosheets for the oxygen reduction reaction. The catalyst is prepared by a facile molten salt synthesis method to confine PtCo nanoalloy into atomically dispersed Co and nitrogen co-doped graphitic carbon nanosheets through high-temperature annealing. The catalyst is a unique combination of PtCo nanoparticles and atomically dispersed Co-N4 in a modified carbon nanosheets matrix which endows a synergistic response leading to a high oxygen reduction activity and stability. The as-prepared PtCo@gCNS nanoparticles along with atomically dispersed Co-N4 active sites possess remarkable oxygen reduction performance demonstrating a mass activity of 1.268 AmgPt -1 and superior durability with a negligible decay after 30000 potential cycles. Density functional theory calculations reveal the richly accessible PtCo nanoalloy and atomically dispersed Co-N/C in the ultra-thin graphitic carbon nanosheets as dual active sites responsible for the improved ORR performance. Moreover, ultra-thin graphitic carbon nanosheets provide enhanced electronic conductivity and mass transport, while carbon-coated architecture provides improved durability against catalyst nanoparticles dissolution. Such a unique and multi-components catalyst system with PtCo nanoalloy, Co-N4 single atomic sites and upgraded carbon support altogether provide an effective ORR catalyst. The synergistic catalysis resulting from the atomically dispersed Co-N4 single atomic sites and PtCo nanoparticles engrossed in graphitic carbon nanosheets through a facile and scalable synthesis approach could step up the realistic applications of oxygen electrocatalysts in practical fuel cells. Figure 1

Pd@Pt Core-Shell Structured Catalyst Supported on Mesoporous Carbon for Achieving a High Cell Performance

1. Introduction An extremely high performance is required for polymer electrolyte fuel cells (PEFCs) installed in fuel cell electric vehicles after 2030 as shown in Figure 1 [1] and a high oxygen accessibility [2] in addition to a high ORR activity and a durability is crucial for Pt-based cathode catalysts in PEFCs. In order to achieve the cell performance, it is of great importance to suppress cell voltage drop especially in a high current density (HCD) region and we consider that carbon support for the Pt-based catalysts would play an important role. In this study, mesoporous carbon (MPC) with a large specific surface area was selected for the support of Pd@Pt core-shell structured catalyst and cell performance of the catalyst supported on the MPC was compared with the catalyst supported on a conventional carbon support of Ketjen Black (KB). 2. Experimental KB-600JD (SBET: 1,335 m2/g) and MPC (CNovel MH-18, SBET 1,278 m2/g, TOYO TANSO [3]) were used as carbon supports for synthesis of Pd@Pt core-shell structured catalyst. The Pd@Pt core-shell structured catalyst was synthesized by a direct displacement reaction method [4]. The metal loadings of the catalyst were 20 wt.% for Pt and 30 wt.% for Pd. CV was recorded at 25oC in Ar-saturated 0.1 M HClO4 and ORR activity was evaluated by RDE method performed in O2-saturated 0.1 M HClO4 at 25oC with a rotation speed of 1,600 rpm and a positive potential scan rate of 10 mV/s. Fuel cell performance was evaluated by using a single cell with an active area of 1 cm2 (5 serpentine flow paths). Nafion® DE2020 was used as an ionomer for the catalyst ink preparation (I/C was set to 0.83) and Nafion® NRE 211 (25 µm in thickness) was used as a membrane. The Pt loading in cathode catalyst layer was set to 0.1 mg/cm2, and H2 and air were supplied to anode and cathode by 418 NmL/min. and 988 NmL/min., respectively (utilization was 5% at current density of 3.0 A/cm2). The cell temperature was set to 80oC and humidified by 95% RH. 3. Results and Discussion Figure 1 shows cell performances of a reference Pt catalyst supported on KB-300J (TEC10E50E, mean diameter: 2.8 nm, metal loading: 48 wt.%, TKK) and Pd@Pt core-shell structured catalyst supported on KB-600JD evaluated with 1.0 atm at gas outlet. Although the cell performance of the Pt/Pd/KB-600JD catalyst overcame the performance of the reference Pt/KB-300J catalyst, cell voltage significantly dropped in HCD region (> 2.0 A/cm2) compared to cell voltage required in 2030. In order to suppress the voltage drop, gas diffusion layer (GDL) was altered to a high current use (22BB, SGL Carbon Inc.) from a stational use (28BC, SGL Carbon Inc.) and the cell performances are overwritten on Figure 1. It is clear that the voltage drop in HCD region was well suppressed by using the GDL 22BB, which arose from a three-fold higher gas permeability of the GDL 22BB compared to the GDL BC28.Next, CNovel MH-18 was used as a carbon support and pore size distribution is demonstrated in Figure 2. The KB-600JD possesses mesopores broadly ranging from 2 to 20 nm in diameter, while the CNovel MH-18 has mesopores with a narrower size distribution centered at 4 nm in diameter. Figures 3 and 4 summarize cell performances using the Pt/KB-300J, the Pt/Pd/KB-600JD and the Pt/Pd/CNovel MH-18 cathode catalysts. When the cell performance was evaluated with 1.0 atm at gas outlet, the Pt/Pd/KB-600JD catalyst exhibited a superior cell performance to that of the Pt/Pd/CNovel MH-18 catalyst. On the contrary, when the cell performance was evaluated with 1.5 atm at gas outlet, the performance using the Pt/Pd/CNovel MH-18 catalyst was much improved and exhibited a higher cell voltage in a low current density region (0.01-1.0 A/cm2) as shown in Figure 5. Since more than 60% of the Pd@Pd NPs were deposited at interior part of the CNovel MH-18 MPC support, it is considered that the cell performance using the Pt/Pd/CNovel MH-18 catalyst exhibited the significant improvement under the pressurized condition at 1.5 atm and the higher cell voltage in the low current density region due to suppression of a direct ionomer adsorption onto the catalyst surface.This study was partly supported by NEDO, Japan.

Stabilization of Pt on Carbon Supports Doped with Atomically-Dispersed Metal/Metal-Oxide Additives

The shift in focus of Proton Exchange Membrane Fuel Cell (PEMFC) development for use in heavy-duty truck applications has accelerated the need for highly durable and efficient Membrane Electrode Assemblies (MEA) in order to meet the long lifetime requirements (~30,000 hours of runtime or 1 million mile driving distance) and high efficiency to minimize the total cost of ownership dominated by the H2 fuel cost.1 These requirements for heavy-duty truck are far stringent compared to the light-duty vehicle applications (~8,000 hours of run time). This brings into focus the need to develop highly-durable catalyst and membrane materials to satisfy the longer lifetime demanded by heavy-duty trucks.The primary cause of cell voltage loss in an operating fuel cell largely arises from the electrochemical surface area (ECSA) loss of the Pt-based cathode catalyst layer. Two mechanisms, namely Pt migration/coalescence and dissolution/redeposition are responsible for the coarsening of Pt nanoparticles leading to surface area loss.2,3 In this presentation, we will explore the concept of using metal/metal-oxide (M/MOx) based dopants in the carbon support as additives to mitigate the Pt coarsening behavior. Specifically, we evaluate dopants that are in the form of M/MOx nanoclusters and/or atomically-dispersed M in the graphitic carbon lattice (M-Cy). We will discuss fuel cell MEA results by investigating the stability when subjected to accelerated voltage cycling tests along with a suite of ex situ characterizations to understand the chemical nature of the catalyst materials with dopants. Acknowledgements: This work is partially supported by U.S Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0008821. References Marcinkoski, J. et al. Hydrogen Class 8 Long Haul Truck Targets. Subsection of the Electrified Powertrain Roadmap. Technical Targets for Hydrogen-Fueled Long-Haul Tractor-Trailer Trucks. DOE Advanced Truck Technologies 1–31 (2019). Available at: https://www.hydrogen.energy.gov/pdfs/19006_hydrogen_class8_long_haul_truck_targets.pdf. Cherevko, S., Kulyk, N. & Mayrhofer, K. J. J. Durability of platinum-based fuel cell electrocatalysts: Dissolution of bulk and nanoscale platinum. Nano Energy 29, 275–298 (2016). Yu, K. et al. Degradation mechanisms of platinum nanoparticle catalysts in proton exchange membrane fuel cells: The role of particle size. Chem. Mater. 26, 5540–5548 (2014). Figure 1

Bifunctional Oxygen Electrocatalysis on Mixed Metal Phthalocyanine-Modified Carbon Nanotubes Prepared via Pyrolysis.

Non-precious-metal catalysts are promising alternatives for Pt-based cathode materials in low-temperature fuel cells, which is of great environmental importance. Here, we have investigated the bifunctional electrocatalytic activity toward the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) of mixed metal (FeNi; FeMn; FeCo) phthalocyanine-modified multiwalled carbon nanotubes (MWCNTs) prepared by a simple pyrolysis method. Among the bimetallic catalysts containing nitrogen derived from corresponding metal phthalocyanines, we report the excellent ORR activity of FeCoN-MWCNT and FeMnN-MWCNT catalysts with the ORR onset potential of 0.93 V and FeNiN-MWCNT catalyst for the OER having EOER = 1.58 V at 10 mA cm–2. The surface morphology, structure, and elemental composition of the prepared catalysts were examined with scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The FeCoN-MWCNT and FeMnN-MWCNT catalysts were prepared as cathodes and tested in anion-exchange membrane fuel cells (AEMFCs). Both catalysts displayed remarkable AEMFC performance with a peak power density as high as 692 mW cm–2 for FeCoN-MWCNT.

Iron and cobalt containing electrospun carbon nanofibre-based cathode catalysts for anion exchange membrane fuel cell

The use of Pt-based cathode catalyst materials hinders the widespread application of anion exchange membrane fuel cells (AEMFCs). Herein, we present a non-precious metal catalyst (NPMC) material based on pyrolysed Fe and Co co-doped electrospun carbon nanofibres (CNFs). The prepared materials are studied as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts in alkaline and acidic environments. High activity towards the ORR in alkaline solution indicated the suitability of the prepared NPMCs for the application at the AEMFC cathode. In the AEMFC test, the membrane-electrode assembly bearing a cathode with the nanofibre-based catalyst prepared with the ionic liquid (IL) (Fe/Co/IL–CNF–800b) showed the maximum power density (Pmax) of 195 mW cm−2, which is 78% of the Pmax obtained with a commercial Pt/C cathode catalyst. Such high ORR electrocatalytic activity was attributed to the unique CNF structure, high micro-mesoporosity, different nature of nitrogen species and metal-Nx active centres.

Transition Metal-Containing Nitrogen-Doped Nanocarbons Derived from 5-Methylresorcinol for Anion Exchange Membrane Fuel Cell Application

Development of novel energy conversion and storage technologies is highly relevant for the future renewable energy-based economy. Low-temperature polymer electrolyte fuel cells are clean and efficient devices, but their commercialization is hindered by high cost and scarcity of Pt-based cathode catalysts for the electrochemical oxygen reduction reaction (ORR). Among the various non-precious metal catalysts that have been intensively explored through the last decades, transition metal-containing nitrogen-doped carbon nanomaterials have shown the most promising results. One of the most common and simple strategies to prepare these catalysts is high-temperature pyrolysis of organic carbon precursors in the presence of nitrogen and metal sources, where carbonization and N-doping occur simultaneously.1 This work describes preparation of highly active ORR electrocatalysts from 5-methylresorcinol, Co and/or Fe salts and dicyandiamide (DCDA) via a simple one-step pyrolysis procedure at 800 °C. It has been proposed that DCDA forms graphitic carbon nitride below 600 °C, which acts as a reactive template during carbonization of organic precursors, decomposing at above 750 °C and yielding N-doped graphene-like carbon structures.2 The SEM and TEM analysis showed that the materials consist of wrinkled carbon structures doped with nitrogen and metals, and also contain metal nanoparticles encapsulated in N-doped carbon layers. According to XRD analysis, these nanoparticles are mainly composed of Co, Fe/Fe3O4 and FeCo alloy for the CoNC, FeNC and FeCoNC catalysts, respectively. The specific surface area of the materials was between 290 and 410 m2 g−1. The XPS analysis revealed that the pyridinic- N is the most abundant N species in all materials and the catalysts also contain metal-coordinated N centers.The electrocatalytic activity of the catalysts for ORR was evaluated in alkaline solution using the rotating disc electrode (RDE) method and the optimal ratio of the precursors was determined. The bimetallic catalysts were more active toward ORR than their monometallic counterparts. Treatment of the materials in acid solutions followed by a second pyrolysis slightly increased their ORR activity. The acid-treated bimetallic catalyst (FeCoNC-at) showed a remarkable ORR performance, comparable to that of commercial Pt/C (20 wt%), which can be attributed to the high surface metal and nitrogen contents (Figure 1a). This material also demonstrated a rather high stability in short-time tests (15000 potential cycles) and good tolerance to methanol. The FeCoNC-at catalyst was further tested in anion exchange membrane fuel cell (AEMFC) with hexamethyl-p-terphenyl poly(benzimidazolium) (HMP-PMBI)3 membrane, where a high value of peak power density (P max= 415 mW cm–2) was achieved (Figure 1b). References A. Sarapuu, E. Kibena-Põldsepp, M. Borghei, and K. Tammeveski, J. Mater. Chem. A, 6, 776-804 (2018).X.H. Li, S. Kurasch, U. Kaiser, and M. Antonietti, Angew. Chem. Int. Ed., 51, 9689-9692 (2012).A.G. Wright, J. T. Fan, B. Britton, T. Weissbach, H. F. Lee, E. A. Kitching, T. J. Peckham, and S. Holdcroft, Energy Environ. Sci., 9, 2130-2142 (2016). Figure 1