Oxygen Reduction Reaction Mass Activity Research Articles

Pt1.4Ni(100) Tetrapods with Enhanced Oxygen Reduction Reaction Activity

Precisely tuning structure and selectively exposing active surfaces of Pt-based alloys are critical to improving the utilization of precious Pt and promoting electrocatalytic performance. It is worth noting that (100) facets of Pt alloys are universally recognized as less active towards the oxygen reduction reaction (ORR). Herein, a simple and mild approach is proposed to prepare Pt1.4Ni tetrapods (PtNi-TPs) whose surface is predominantly covered by (100) facets via simultaneously reducing the Ni and Pt precursors. In spite of the large proportion of exposed (100) facets, the Pt1.4Ni tetrapods show superior electrocatalytic activity, demonstrating an enhanced ORR mass activity of 1.23 A mg−1Pt and specific activity of 2.01 mA cm−2, which are 12.6- and 17.2-fold higher than those of commercial Pt/C catalysts, respectively. Density function theory calculations suggest that the incorporation of Ni can weaken Pt-OH overbinding effect on the (100) facets, promoting hydrogenation of *OH. The free energy changes in the potential-limiting step demonstrate that the (100) facets are more active. This work is expected to provide new view on the rational designing of nanocatalysts facets. Pt1.4Ni tetrapods enclosed by (100) facets as efficient oxygen reduction reaction catalysts.

Solvothermal Doping of Lanthanum on Nanoscale Platinum Surfaces to Improve Oxygen Electroreduction Performance

AbstractPlatinum (Pt) nanocatalysts, doped or alloyed with lanthanum (La), are promising candidates to promote the sluggish oxygen reduction reaction (ORR) kinetics. However, owing to the very low standard reduction potential of La3+ (−2.38 V), most synthetic routes to reduce La are costly and complex. Therefore, the preparation of highly active bimetallic PtLa nanocatalysts by simple and low‐cost synthesis methods is a challenging task. This work offers a feasible synthesis procedure for doping Pt nanocrystals with La in a simple solvothermal method. Elemental mapping and X‐ray photoelectron spectroscopy measurements indicate that metallic La is doped on the Pt catalyst surfaces. The La‐doped Pt nanocatalysts exhibit ORR mass activity of 0.49 A mgPt−1 at 0.9 V, which is 2.3 and 2.7 times compared to Pt nanocatalysts without La dopant and commercial Pt/C, respectively.

Nitrogen-doped Pt3Co intermetallic compound nanoparticles: A durable oxygen reduction electrocatalyst

Pt-based ordered alloy catalysts have been extensively investigated to improve the catalytic performance for proton exchange membrane fuel cells. Herein, we report N-doped ordered Pt3Co nanoparticles supported on XC-72R carbon (O-Pt3CoN/C) for oxygen reduction reaction (ORR). Both the X-ray photoelectron spectroscopy and the energy-dispersive X-ray spectroscopy elemental mapping analyses confirm the successful N-doping within the Pt3Co intermetallic compound. Consequently, the N-doped catalyst exhibits a significantly higher ORR performance in comparison with Pt3Co and commercial Pt/C catalysts. Importantly, the ORR mass activity of N-doped Pt3Co intermetallic compound is almost 2.4 times of that obtained on commercial Pt/C. Moreover, after accelerated durability test over the potential range of 0.6 to 1.1 V (vs. RHE), ORR activity decay of N-doped Pt3Co/C is only 21.5%, lower than those of ordered Pt3Co and commercial Pt/C catalysts. Thus, N-doping could be utilized as an effective strategy for further improving the ORR performance of Pt-M/C catalysts.

Boosting Oxygen and Peroxide Reduction Reactions on PdCu Intermetallic Cubes

AbstractPalladium‐based nanocatalysts have the potential to replace platinum‐based catalysts for fuel‐cell reactions in alkaline electrolytes, especially PdCu intermetallic nanoparticles with high electrochemical activity and stability. However, unlike the synthetic methods for obtaining the nanoparticles, the effect of PdCu shape on the performance is relatively less well studied. Here, we demonstrate the facet dependence of PdCu intermetallics on the oxygen reduction reaction (ORR) and peroxide reduction, and reveal that the {100} dominant PdCu cubes have a much higher ORR mass activity and specific activity than spheres at 0.9 V vs. RHE, which is four and five times that of commercial Pd/C and Pt/C catalysts, respectively, and show only a 31.7 % decay after 30 000 cycles in the stability test. Moreover, cubic PdCu nanoparticles show higher peroxide electroreduction activity than Pd cubes and PdCu spheres. Density functional theory (DFT) calculation reveals that the huge difference originates from the reduction in oxygen adsorption energy and energy barrier of peroxide decomposition on the ordered {100} PdCu surface. Given the relationship between the shape and electrochemical performance, this study will contribute to further research on electrocatalytic improvements of catalysts in alkaline environments.

Oxygen Reduction Reaction on Metal and Nitrogen–Doped Carbon Electrocatalysts in the Presence of Sodium Borohydride

Metal and nitrogen–doped carbon electrocatalysts (M–N–C, where M is Fe or Co) were investigated for the oxygen reduction reaction (ORR) in alkaline conditions in the presence of borohydride ions (BH4−). The electrochemical properties of the Fe–N–C and Co–N–C catalysts were investigated by cyclic voltammetry and rotating disk electrode techniques: their ORR electrocatalytic activity was bridged to their physicochemical properties, mainly type of metallic center (Fe vs. Co), structure (atomic dispersion vs. nanoparticles), and BET surface area. It is found that Fe–N–C catalysts have the best performances for the ORR electrocatalysis, even in the presence of BH4− anions. The atomically dispersed Fe- and Co-containing electrocatalysts reach better BH4−-tolerance than their counterparts baring nanoparticles. For the atomically dispersed Fe–N–C electrocatalysts, the lowest BET surface area material generates a slight advantage of ORR mass activity and a poor (and desired) activity for borohydride oxidation reaction (BOR).

Improving the High-Current-Density Performance of PEMFC through Much Enhanced Utilization of Platinum Electrocatalysts on Carbon

We report an effective approach to the synthesis of high-content and high-dispersion Pt nanoparticles (NPs) on XC-72 carbon black as cathode electrocatalyst with improved high-current-density performance in proton exchange membrane fuel cells (PEMFCs). While exceptionally high catalytic activity for oxygen reduction reaction (ORR) was reported based on rotating disk electrode (RDE) technique, such catalysts do not deliver at nearly the same level performance in PEMFC due to the lack of optimized design of catalyst structures on carbon support. We recently developed a synergistic synthesis method to make ex-ceptionally high-content and finely dispersed Pt catalysts, which showed the highest Pt-electroactive surface area and the high-est Pt mass activity for ORR among the electrocatalysts tested. More importantly, the membrane electrode assembly (MEA) made with this catalyst had a superb performance at the current density higher than 1200 mA cm-2 in a hydrogen-air PEMFC measurement. 195Pt NMR was used to analyze the molecular structures of the metal precursors and to understand the mecha-nisms for the formation of Pt catalysts at high dispersity and uniformity.

High Durability of Pt3Sn/Graphene Electrocatalysts toward the Oxygen Reduction Reaction Studied with In Situ QEXAFS.

To prevent the corrosion of carbon and to enhance corrosion resistance, charge transfer, and mass transfer, graphene, which exhibits a high surface area and good conductivity, was used as an electrocatalyst support for a fuel cell. Pt3Sn/G electrocatalysts for the oxygen reduction reaction (ORR) were prepared with alcohol reduction. The characterization of synthesized catalysts was analyzed according to the energy-dispersive spectrometer (EDS), X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), and extended X-ray absorption fine structure (EXAFS). The electrochemical performance was analyzed with cyclic-voltammetry (CV), linear sweep voltammetry (LSV), and accelerated degradation test (ADT) measurements. The Pt3Sn/G electrocatalysts showed more positive onset potential and larger ORR mass activity than commercial Pt/C catalysts after 5000 cycles of ADT, indicating that in an acidic environment, Pt3Sn/G is more chemically stable than Pt/C. Graphene has effective acid tolerance, is more stable against corrosion, and shows increased stability through preventing PtSn nanoparticles from detaching from the surface. According to the in situ quick EXAFS (QEXAFS) under a CV test to clarify the potential-dependent state of the Pt3Sn/G electrocatalyst, the results show that the electrode surface is reproducible; there is no perceptible change in the oxidation state of the Pt3Sn/G electrocatalyst. The radial distribution function (RDF) of the EXAFS spectra shows that the adsorption and desorption of H+ and OH- cause no structural change in the Pt3Sn crystallites. This work provides insight into the reaction mechanism of proton electroreduction and hydrogen adsorption on a Pt3Sn/G electrocatalyst surface.

Pre-deposited Co nanofilms promoting high alloying degree of PdxAu nanoparticles as electrocatalysts in alkaline media

High alloyed and well dispersed PdxAu nanoparticles are deposited on pre-deposited Co nanofilm substrates (PdxAu/Co-nanofilms/C) by using an immiscible ionic liquid (IL)/water interface. However, low alloyed PdxAu/C catalysts are formed without pre-deposited Co nanofilms through the same synthesis method. The high-alloyed PdxAu/Co-nanofilms/C catalysts demonstrate significantly increased activity for ethanol oxidation reaction (EOR) than low-alloyed PdxAu/C of the same Pd/Au composition, with the catalysts of low Pd/Au atom ratio (Pd:Au = 1:1) demonstrating the optimal activity. Notably, high alloyed degree of the PdxAu nanoparticles in the PdxAu/Co-nanofilms/C catalysts brings the lattice expansion of Pd, causing an up-shift of the d-band center, which results in the enhancement of OH− adsorption and correspondingly promotes electro-oxidation of ethanol excluding the effect of Co nanofilm substrates. Differently, the enhancement activities of the Pd-Au bimetallic system for oxygen reduction reaction (ORR) are almost not affected by the alloying degree and only dependent on Pd/Au atom ratios, with the catalysts of low Pd/Au atom ratio (Pd:Au = 1:1) displaying the highest ORR mass activity, respectively. These results exhibit the specific dependence between the different electrochemical process and the physical parameters for the Pd-Au bimetallic system.

A general strategy for bimetallic Pt-based nano-branched structures as highly active and stable oxygen reduction and methanol oxidation bifunctional catalysts

The morphology and size of Pt-based bimetallic alloys are known to determine their electrocatalytic performance in reactions relevant to fuel cells. Here, we report a general approach for preparing Pt-M (M = Fe, Co and Ni) bimetallic nano-branched structure (NBs) by a simple high temperature solution-phase synthesis. As-prepared Pt-M NBs show a polycrystalline structure and are rich in steps and kinks on the surface, which promote them favorable bifunctional catalytic properties in acidic electrolytes, specifically in terms of the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR). Specially, Pt-Co NBs/C catalyst shows 6.1 and 5.3 times higher in specific activity (SA) and mass activity (MA) for ORR than state-of-the-art commercial Pt/C catalysts, respectively. Moreover, it exhibits a loss of 4.0% in SA and 14.4% in MA after 10,000 cycles of accelerated durability tests (ADTs) compared with the initial activities. In addition, we also confirmed the superior MOR activity of Pt-Co NBs/C catalyst in acidic electrolytes. For Pt-M NBs with other alloying metals, the ORR and MOR activities are both higher than commercial catalysts and are in the sequence of Pt-Co/C > Pt-Fe/C > Pt-Ni/C > commercial Pt/C (or PtRu/C). The improved activities and durability can benefit from the morphological and compositional effects. This synthesis approach may be applied to develop bifunctional catalysts with enhanced ORR and MOR properties for future fuel cells designs.

High-Performance Pt-Co Nanoframes for Fuel-Cell Electrocatalysis.

Pt-based alloy catalysts are promising candidates for fuel-cell applications, especially for cathodic oxygen reduction reaction (ORR) and anodic methanol oxidation reaction (MOR). The rational design of composition and morphology is crucial to promoting catalytic performances. Here, we report the synthesis of Pt-Co nanoframes via chemical etching of Co from solid rhombic dodecahedra. The obtained Pt-Co nanoframes exhibit excellent ORR mass activity in acidic electrolyte, which is as high as 0.40 A mgPt-1 initially and 0.34 A mgPt-1 after 10 000 potential cycles at 0.95 VRHE. Furthermore, their MOR mass activity in alkaline media is up to 4.28 A mgPt-1 and is 4-fold higher than that of commercial Pt/C catalyst. Experimental studies indicate that the weakened binding of intermediate carbonaceous poison contributes to the enhanced MOR behavior. More impressively, the Pt-Co nanoframes also demonstrate remarkable stability under long-term testing, which could be attributed to the negligible electrochemical Co dissolution.

Hollow PtFe Alloy Nanoparticles Derived from Pt-Fe3 O4 Dimers through a Silica-Protection Reduction Strategy as Efficient Oxygen Reduction Electrocatalysts.

The development of efficient and stable electrocatalysts for the oxygen reduction reaction (ORR) is critical for the large-scale production of fuel cells. Platinum (Pt) nanoparticle catalysts show excellent performance for ORR, though the high cost of Pt is a limiting factor that directly impacts fuel cell production costs. Alloying Pt with other transition metals is an effective strategy to reduce Pt utilization whilst maintaining good ORR performance. In this work, novel hollow PtFe alloy catalysts were successfully synthesized by high-temperature pyrolysis of SiO2 -coated Pt-Fe3 O4 nanoparticle dimers supported on carbon at 900 °C, followed by SiO2 shell removal and partial dealloying of the PtFe nanoparticles formed using HF. The obtained hollow PtFe nanoparticle catalysts (denoted herein as PtFe-900) showed a 2.3-fold enhancement in ORR mass activity compared to PtFe nanoparticles synthesized without SiO2 protection, and a remarkable 7.8-fold enhancement relative to a commercial Pt/C catalyst. Further, after 10 000 potential cycles, the ORR mass activity of PtFe-900 remained very high (90.9 % of the initial mass activity). The outstanding ORR performance of PtFe-900 can be attributed to the modification of Pt lattice and electronic structure by alloying with Fe at high temperature under the protection of the SiO2 coating. This work guides the development of improved, highly dispersed Pt-based alloy nanoparticle catalysts for ORR and fuel cell applications.

Electrochemical Properties and Single Cell Performance of Pd Core-Pt Shell Structured Catalyst Synthesized by a Simple Direct Displacement Reaction

A carbon supported Pd core-Pt shell structured catalyst (Pt/Pd/C) was synthesized by a very simple direct displacement reaction (DDR), in which Pd core nanoparticles (NPs) were directly displaced with [PtCl4]2− on stirring in N2 saturated H2SO4 aqueous solution at 70 °C instead of a modified Cu under potential deposition (Cu-UPD)/Pt displacement method. In DDR, potential difference between Pd core and [PtCl4]2− was decreased compared with the difference between Cu shell and [PtCl4]2− in Cu-UPD/Pt displacement method, which suppressed non-uniform Pt shell formation and increased Pt shell coverage, enhancing oxygen reduction reaction (ORR) activity. Furthermore, fine Pd core NPs were preferentially dissolved out by Cl− anions released from [PtCl4]2− during DDR performed in acidic H2SO4 solution at 70 °C and mean diameter of Pd@Pt core–shell NPs increased, improving durability of Pt/Pd/C catalyst by the size effect. Large Pd particles, however, were formed in Pt/Pd/C catalyst through a disproportionation reaction of Pd2+ cations generated in DDR, which was suppressed by addition of Br− anions as complexing agent. A single cell using Pt/Pd/C cathode catalyst synthesized by DDR with Br− anions and activated by H2–O2 chemical pretreatment showed 2.4-fold ORR mass activity of a commercially available Pt/C catalyst at a current density of 1.0 A cm−2. Thus, DDR was considered to be a suitable synthetic method for a mass production of highly active and durable Pt/Pd/C catalyst for ORR.

Oxygen reduction reaction mechanism and kinetics on M-NxCy and M@N-C active sites present in model M-N-C catalysts under alkaline and acidic conditions

M-N-C electrocatalysts (where M is Fe or Co) have been investigated for mitigating the dependence on noble metals when catalyzing the oxygen reduction reaction (ORR) for fuel cell technologies in acidic or alkaline conditions. Rotating disk and rotating ring-disk electrode measurements for Fe-N-C and Co-N-C catalysts demonstrate promising performances and stability for the ORR, while the activity of main suspected active sites (M-NxCy and M@N-C) has been discussed on the basis of the known physical-chemical properties of the catalysts in acid and alkaline media. Thereupon, it is observed that atomically dispersed Fe-NxCy sites reach the highest ORR activity in acid media when amplified by an adequate energy binding between the metallic center and the oxygenated reaction intermediates. In contrast, Fe@N-C core-shell sites reach a maximum ORR mass activity in alkaline media through a synergistic effect involving catalyst particles with metallic iron in the core and nitrogen-doped carbon in the shell.

Elucidating the Dynamic Nature of Fuel Cell Electrodes as a Function of Conditioning: An ex Situ Material Characterization and in Situ Electrochemical Diagnostic Study.

To increase the commercialization of fuel cell electric vehicles, it is imperative to improve the activity and performance of electrocatalysts through combined efforts focused on material characterization and device-level integration. Obtaining fundamental insights into the ongoing structural and compositional changes of electrocatalysts is crucial for not only transitioning an electrode from its as-prepared to functional state, also known as "conditioning", but also for establishing intrinsic electrochemical performances. Here, we investigated several oxygen reduction reaction (ORR) electrocatalysts via in situ and ex situ characterization techniques to provide fundamental insights into the interfacial phenomena that enable peak ORR mass activity and high current density performance. A mechanistic understanding of a fuel cell conditioning procedure is described, which encompasses voltage cycling and subsequent voltage recovery (VR) steps at low potential. In particular, ex situ membrane electrode assembly characterization using transmission electron microscopy and ultra-small angle X-ray scattering were performed to determine changes in carbon and Pt particle size and morphology, while in situ electrochemical diagnostics were performed either during or after specific points in the testing protocol to determine the electrochemical and interfacial changes occurring on the catalyst surface responsible for oxygen transport resistances through ionomer films. The results demonstrate that the voltage cycling (break-in) step aids in the removal of sulfate and fluoride and concomitantly reduces non-Fickian oxygen transport resistances, especially for catalysts where Pt is located within the pores of the carbon support. Subsequent low voltage holds at low temperature and oversaturated conditions, i.e., VR cycles, serve to improve mass activities by a factor of two to three, through a combined removal of contaminants, surface-blocking species (e.g., oxides), and rearrangement of the catalyst atomic structure (e.g., Pt-Pt and Pt-Co coordination).

Unconventional p-d Hybridization Interaction in PtGa Ultrathin Nanowires Boosts Oxygen Reduction Electrocatalysis.

Alloying 3d transition metals with Pt has been discovered as an effective strategy to boost the catalytic activity in oxygen reduction reaction (ORR), which, however, often raises the insufficient catalyst durability issue due to rapid leaching of the 3d metal elements. To overcome this issue and realize enhancements in both the activity and the durability properties, here we report a new catalytic structure based on PtGa ultrathin alloy nanowires (NWs), which feature an unconventional strong p-d hybridization interaction. Relative to commercial Pt catalyst, the optimum Pt4.31Ga NWs catalyst exhibited 10.5- and 12.1-fold enhancement in the ORR mass activity and specific activity, respectively. Particularly, the Pt4.31Ga NWs catalyst showed only 15.8% loss in the mass activity after 30 000 cycles of durability test, as compared to a big decrease of 79.6% for the commercial Pt catalyst. Our mechanistic studies find a strong p-d hybridization interaction between Ga and Pt that accounts for the improved ORR performance via synergistically optimizing the surface electronic structure, enhancing the oxidation resistance of Pt, and suppressing the leaching of lattice Ga. We believe this work provides new perspectives to design active and durable electrocatalysts toward ORR.

Oxygen reduction reaction on thin-film Ag electrodes in alkaline solution

Thin Ag films with nominal thicknesses from 0.5 to 10 nm deposited on a glassy carbon (GC) substrate were prepared by electron beam evaporation and the oxygen reduction reaction (ORR) on the Ag/GC electrodes in alkaline solution was studied employing the rotating disk electrode (RDE) technique. Transmission electron microscopy (TEM) was used to examine the surface morphology of Ag films and it was found that the average particle size for the thinnest film was about 5.5 nm. The Ag particle size increased with increasing the film thickness and for 5 nm film the silver particles merged to a uniform coating. Electrochemical testing showed that the thin-film Ag electrodes are efficient catalyst for oxygen reduction and peroxide reduction reactions. The electron transfer number was calculated to be slightly lower than four for thinnest Ag film and it was 4 for all the other films. Tafel analysis showed similar slope value for all thin-film Ag electrodes revealing that the rate-determining step does not change with the Ag film thickness. The electro-oxidation of Ag films and Pb underpotential deposition experiments were also conducted. Finally, Ag coatings were electrochemically stripped from the GC electrode to determine the mass of Ag on the substrate surface, which was in good agreement with the calculated value. The 1 nm Ag film showed the best utilisation of silver, as mass and specific activities for ORR were the highest for that coating. Nanostructured Ag electrodes might be promising catalysts for alkaline membrane fuel cells.

Oxygen Reduction Reaction Activity of Cobalt Platinum Bronze Nano-Particles Prepared By a Silica Template Method

Platinum metal-based materials have been mostly used as the catalysts for polymer electrolyte fuel cells and extensively studied to improve their performance by alloying and shape-controlling. Although these advanced technologies are pursued mainly for enhancing the catalytic activities and showed significant improvement in deed, their durabilities under realistic conditions do not often accompany. For wider distribution of fuel cell vehicles, however, both high activity and high durability should be compatibly achieved. Recently we have focused on metal-platinum oxide bronzes as a durable cathode catalyst and reported a study on their fundamental properties [1,2], in which a platinum bronze containing cobalt (Co-Pt bronze) was synthesized by solid state reaction using platinum dioxide and cobalt nitrate hexahydrates as the raw materials. Its oxygen reduction reaction (ORR) was enhanced after potential cycles down to the hydrogen region, which formed a thin Pt metallic layer over the oxide. While its specific activity was as high as that of pure platinum nanoparticles, its durability against the potential cycles was much higher. However, its mass-specific electrochemical surface area (ECSA) and mass activity were much lower than those of pure platinum nanoparticles because of its large particle size (20-50 nm). In the present study, a novel preparation process was applied to obtain smaller Co-Pt bronze nanoparticles and their characteristics and electrochemical performances for PEFC application were examined. Co-Pt bronze nano powder was prepared by a template method using a mesoporous silica, TMPS-4R (pore diameter: 3.8 nm, Taiyo Kagaku Co., Ltd.). Hexachloroplatinate hexahydrates and cobalt nitrate hexahydrates were dissolved in ultrapure water and then mixed with the silica template powder. The mixed powder was heat-treated under O2 flow (200 mL min-1) at 650 ºC for 1 hour. The obtained powder was dispersed in NaOH to dissolve the silica template and subsequently washed with ultrapure water. Co-Pt bronze nano power was collected from the supernatant of the washed suspension after centrifuged at 5,000 g (large Co-Pt bronze and metallic platinum particles were precipitated.). The obtained powder was characterized by XRD, SEM, TEM and the nitrogen sorption method. The electrochemical behaviors including ORR activity of the obtained Co-Pt bronze nano particles were evaluated by the thin-film RDE method in 0.1 M HClO4 at 30 ºC. XRD measurement confirmed that the heat-treated powder is comprised of platinum bronze and platinum metal phases and that the latter was removed by the centrifuge process. The nanoparticles collected from the supernatant was comprised of the platinum bronze only and showed a much larger N2-sorption surface area than the precipitate (58 and 7.3 m2 g-1 for the supernatant and precipitation, respectively). The SEM observation revealed that the platinum bronze particles collected from supernatant are cylindrically shaped with ca. 4 nm in diameter (Fig. 1A). This shape reflects the mesopores of the templated silica. Electrochemical measurement showed that the surface of the prepared nanoparticles was gradually metallized over potential cycling. The ECSA and ORR mass activity after potential cycling were respectively larger than those of large particles made by the solid state reaction. This work was financially supported by NEDO, Japan.

Synthesis of Pd Core-Pt Shell Structured Catalyst By a Very Simple Direct Displacement Reaction

Introduction Carbon supported Pd core-Pt shell structured catalyst (Pt/Pd/C) is a promising candidate for decrease of Pt usage in PEFCs owing to its high Pt utilization and oxygen reduction reaction (ORR) activity [1, 2]. Previously, we synthesized Pt/Pd/C catalyst by a modified Cu-UPD/Pt displacement method [3], and also developed an electrochemical high activation protocol (HAP) and a H2-O2 chemical treatment to enhance ORR activity of the catalyst [4]. Besides ORR activity enhancement, synthetic method suitable for a mass production of the catalyst is highly demanded. In this study, we synthesized the Pt/Pd/C catalyst by a very simple direct displacement reaction (DDR) in which a carbon supported Pd core (Pd/C) was directly displaced with a Pt precursor. We believe that DDR is a much simpler synthetic method compared with the modified Cu-UPD/Pt displacement one and suitable for mass production of the catalyst. Features and electrochemical properties of Pt/Pd/C catalyst synthesized by DDR were investigated. Experimental Pt/Pd/C catalyst was synthesized by DDR. 300 mg of Pd/C core (mean size: 4.4 nm, metal loading: 32 wt.%, ISHIFUKU Metal Industry) was dispersed in 200 mL of water and pH was adjusted to 1 by 2 M H2SO4 at 5°C under N2 gas atmosphere. K2PtCl4 corresponding to Pt monolayer shell was added and stirred at 5°C for 0.5 h, followed by stirring at 70°C for 3 h, by which Pt/Pd/C catalyst was synthesized. Pt/Pd/C catalyst was activated by HAP (rectangular potential cycling of 0.05 V (300 s)-1.0 V (300 s) vs. RHE performed in Ar saturated 0.1 M HClO4 at 80°C for 50 cycles [4]) and by H2-O2 chemical treatment (H2 and O2 gasses were alternately introduced in 2 M H2SO4 containing Pt/Pd/C catalyst at 80°C, in which equilibrium potentials of H2 (ca. 0.00 V) and O2 (ca. 0.98 V) were alternately applied to the catalyst [4]). Durability of Pt/Pd/C catalyst was evaluated by an accelerated durability test ADT (rectangular potential cycling of 0.6 V (3 s)-1.0 V (3 s) vs. RHE performed in Ar saturated 0.1 M HClO4 at 80°C for 10,000 cycles). Pt/Pd/C catalyst was characterized by TG, XRD, XRF, TEM, TEM-EDX and CV measurements. ORR activity of Pt/Pd/C catalyst was evaluated by RDE technique performed in O2 saturated 0.1 M HClO4 at 25°C with a positive scan rate of 10 mV/s at 1,600 rpm. Results and Discussion Irregular Pt shell formation on Pd core observed in Pt/Pd/C catalyst synthesized by a modified Cu-UPD/Pt displacement method was suppressed in Pt/Pd/C catalyst synthesized by DDR as demonstrated in TEM images in Fig. 1. CVs of Pd/C core and Pt/Pd/C catalysts are summarized in Fig. 2 with focusing on anodic scan in a potential range of 0.05-0.5 V. Large hydrogen desorption wave observed in Pd/C core was much reduced in Pt/Pd/C catalyst synthesized by DDR compared with catalyst synthesized by Cu-UPD method, indicating that Pt shell coverage was increased by DDR. It is considered that potential difference of Pd core and [PtCl4]2- in DDR was decreased compared with that of Cu shell and [PtCl4]2- in Cu-UPD method, which slowed down speed of displacement reaction and suppressed irregular Pt shell formation, leading to increase of Pt shell coverage. Furthermore, fine catalyst particles disappeared and particle density decreased, which improved durability of the Pt/Pd/C catalyst. ORR mass activity of Pt/Pd/C catalysts is summarized in Fig. 3. Initial ORR mass activity of Pt/Pd/C catalyst synthesized by DDR exhibited 1,028 A/g-Pt and the activity was enhanced to 1,685 A/g-Pt by HAP, which was 5.3-fold of a reference Pt/C catalyst (320 A/g-Pt, TEC10E50E, TKK). The observed high ORR mass activity is considered to arise from the high Pt shell coverage of Pt/Pd/C catalyst synthesized by DDR. Activation of Pt/Pd/C catalyst by H2-O2 chemical treatment and a single cell performance will be presented in the meeting. Acknowledgement This study was supported by NEDO, Japan.

Effect of Catalyst Loading on the Degradation of d-PtCo/C Cathode Catalyst

The Consortium for Fuel Cell Performance and Durability (FC-PAD) was formed by the U.S. Department of Energy Fuel Cell Technologies office (FCTO) in 2015 to improve the understanding of performance and durability issues in polymer electrolyte membrane fuel cells (PEMFCs) and to help meet the DOE 2020 targets. This talk will summarize some of the durability work performed within the consortium on commercial dealloyed-PtCo/C catalyst-based membrane electrode assemblies (MEAs). Commercial MEAs using Nafion®HP membranes, ElystPt300670 (30wt.%PtCo/high surface area C) cathode catalyst, and ElystPt200380 (20wt.%Pt/Graphitized carbon) anode catalyst were obtained from Umicore. The MEAs were assembled in a 5cm2differential cell reported by D. R. Baker et al.1and conditioned extensively before performing beginning of life (BOL) characterization. The conditioning protocol included a recovery step where the cell was operated at high RH and low temperatures and voltages to flush out impurities and maximize the performance.2,3The samples were then subjected to the DOE-recommended square wave catalyst accelerated stress test (AST) which is a 3 sec hold at 0.6V and 3 sec hold at 0.95V at 80oC and 100%RH.4Characterizations were performed after 15,000 and 30,000 cycles of the AST with the recovery protocol applied to the MEA before characterization. Electrochemical characterization included mass activity, electrochemical surface area, polarization curves, Electrochemical Impedance Spectroscopy (EIS), and oxygen transport measurements. The BOL and end-of-test catalysts were also characterized by electron microscopy, energy dispersive X-ray spectroscopy, X-ray absorption spectroscopy, and small-angle X-ray scattering to determine particle size and Pt to Co content. The oxygen reduction reaction mass activity, electrochemical surface area, and performance in the kinetic region of the catalysts degrade with AST cycling. This loss can be attributed to both Co leaching from the catalyst and catalyst particle size increase. The effect of this degradation was most pronounced in the high current region for the MEA with low Pt loading. For example, in Figure 1, the voltage loss at 1A/cm2after 30,000 AST cycles is ~50mV for the 0.15mgPt/cm2MEA and ~200mV for the 0.05mgPt/cm2MEA. This is primarily due to increases in the pressure-independent oxygen transport resistance. The non-Fickian oxygen transport resistance of low-loaded (0.05 mgPt/cm2) MEAs is significantly larger than those of the high-loaded MEAs even at BOL and worsens significantly with ageing of the catalyst. EIS experiments in HelOx (21% O2, balance He) also revealed that there was an increase in the pressure-dependent gas transport during the AST cycling. However, little or no change was observed in the ionomer sheet resistance over the course of the durability testing, independent of the catalyst loading. These results will be compared with recent results reported for pure Pt catalyst-based MEAs subjected to similar tests.5In this talk, the analysis of the cathode catalyst composition and morphology before and after the testing will be presented in detail and correlated to the observed performance losses. Acknowledgements This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Fuel Cells Program Manager: Dimitrios Papageorgopoulos and Technical Development Manager: Greg Kleen). References D. R. Baker, D. A. Caulk, K. C. Neyerlin, and M. W. Murphy, J. Electrochem. Soc.,V156(9), B991-B1003 (2009). https://doi.org/10.1149/1.3152226.Zhang, B. A. Litteer, F. D. Coms, and R. Makharia,. J. Electrochem. Soc., V159(7),F287-F293 (2012). https://doi.org/10.1149/2.063207jes.Zhang et al, U.S Patent Application Publication US 2011/0195324 A1, August 11 (2001).United States Department of Energy Fuel Cells Technologies Office Multi-Year Research, Development, and Demonstration Plan. https://www.energy.gov/sites/prod/files/2017/05/f34/fcto_myrdd_fuel_cells.pdf.G. S. Harzer, J. N. Schwammiein, A. M. Damjanovic, S. Ghosh, and H. A. Gasteiger, J. Electrochem. Soc.,V165(6), F3118-F3131 (2018). https://doi.org/10.1149/2.0161806jes Figure 1

Effects of Catalyst and Electrode Composition on Performance of Unitized Regenerative Fuel Cells

A unitized regenerative fuel cell (URFC) that uses a single cell that functions in electrolysis or fuel cell mode provides potential advantages including lower cost, mass, and volume compared with discrete electrolyzer and fuel cell systems. However, significant challenges for URFCs include lower performance, lower round-trip efficiency, and higher stability/degradation issues compared with separate systems. The lower performance and enhanced degradation of URFCs results from a combination of factors including catalyst degradation and water management issues that result from switching between electrolyzer and fuel cell modes. To address these challenges and improve performance of proton-exchange membrane (PEM) URFCs in both fuel cell and electrolyzer modes, we evaluated the effects of catalyst and electrode composition of the bifunctional oxygen electrode. To improve catalyst activity and efficiency of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), we explored using bifunctional oxygen catalysts composed of bimetallic nanoframes derived from our prior studies.1,2 Electrochemical testing using a rotating disk electrode configuration showed that the nanoframe bifunctional oxygen electrocatalysts have higher ORR mass activity, OER mass activity, and round-trip efficiency and reasonable stability compared with baseline Pt-IrO2 catalysts. URFC membrane electrode assemblies (MEAs) were fabricated and tested to determine the effect of the composition of the oxygen electrode on fuel cell and electrolyzer performance and evaluate the effect of switching between operating in fuel cell and electrolyzer modes. Performance comparisons were made with MEAs composed of baseline commercial Pt black-IrO2 catalysts. References Godinez-Salomon, F.; Mendoza-Cruz, R.; Arellano-Jimenez, M. J.; Jose-Yacaman, M.; Rhodes, C. P., Metallic Two-Dimensional Nanoframes: Unsupported Hierarchical Nickel-Platinum Alloy Nanoarchitectures with Enhanced Electrochemical Oxygen Reduction Activity and Stability. ACS Appl. Mater. Interfaces 2017, 9, 18660-18674. DOI: 10.1021/acsami.7b00043Godínez-Salomón, F.; Albiter, L.; Alia, S. M.; Pivovar, B. S.; Camacho-Forero, L. E.; Balbuena, P. B.; Mendoza-Cruz, R.; Arellano-Jimenez, M. J.; Rhodes, C. P., Self-Supported Hydrous Iridium–Nickel Oxide Two-Dimensional Nanoframes for High Activity Oxygen Evolution Electrocatalysts. ACS Catal. 2018, 8, 10498-10520. DOI: 10.1021/acscatal.8b02171