Biaxially strained PtPb/Pt core/shell nanoplate boosts oxygen reduction catalysis.

Electrocatalysis Platinum is an excellent but expensive catalyst for the oxygen reduction reaction (ORR), which is critical for fuel cells. Alloying platinum with other metals can create shells of platinum on cores of less expensive metals, which increases its surface exposure, and compressive strain in the layer can also boost its activity (see the Perspective by Stephens et al. ). Bu et al. produced nanoplates—platinum-lead cores covered with platinum shells—that were in tensile strain. These nanoplates had high and stable ORR activity, which theory suggests arises from the strain optimizing the platinum-oxygen bond strength. Li et al. optimized both the amount of surface-exposed platinum and the specific activity. They made nanowires with a nickel oxide core and a platinum shell, annealed them to the metal alloy, and then leached out the nickel to form a rough surface. The mass activity was about double the best reported values from previous studies. Science , this issue p. [1410][1], p. [1414][2]; see also p. [1378][3] [1]: /lookup/doi/10.1126/science.aah6133 [2]: /lookup/doi/10.1126/science.aaf9050 [3]: /lookup/doi/10.1126/science.aal3303

Trimetallic Pt–Pd–Ni octahedral nanocages with subnanometer thick-wall towards high oxygen reduction reaction

Spectroscopic insights into the nature of active sites in iron–nitrogen–carbon electrocatalysts for oxygen reduction in acid

Development of cathode catalysts with high oxygen reduction reaction (ORR) activity and high durability is essential for the large-scale commercialization of polymer electrolyte fuel cells (PEFCs), which are applied to fuel cell vehicles (FCVs) and residential co-generation systems. At the present stage, costly Pt or its alloys have been employed as the cathode catalyst, having appreciable ORR activity and durability in strong acidic electrolyte at low operating temperatures <100 °C. The mass activity MA of Pt-based catalysts is defined as MA (A gPt −1) = j S (A m−2) × ECA (m2 gPt −1). To increase the area-specific activity j S (current density per active surface area), Pt-M alloys (Pt-Fe, Pt-Co, Pt-Ni, Pt-Cr, etc.) have been examined. Assuming spherical catalyst particles, the electrochemically active surface area ECA is inversely proportional to the particle diameter. To increase the ECA, it is effective to disperse Pt or Pt-alloy nanoparticles on high-surface-area carbon supports (Pt/C or Pt-M/C). However, for developing such catalysts having both high ORR activity and high durability, there has been long-standing controversy surrounding important issues such as the crystal structures (ordered and disordered) and the chemical composition of Pt-M alloys, as well as their optimum particle size. Furthermore, because the mechanism of enhancement of the ORR activities at Pt-M alloys is still unclear, the strategy for designing new potential catalysts has not yet been established. This presentation focuses on the following topics for the cathode catalysts from bulk single crystals to practical nanoparticle catalysts. 1) Pt particle-size effect on the durability for an accelerated test, which simulates load-cycles for FCVsSo far, much research has suffered from trade-offs in determining the optimal particle-size of the catalysts, due to the different trends of Pt particle-size effects on the MA and durability. However, by the use of our n-Pt/C catalysts with a very narrow size distribution (σd ≤ 10%), the most durable catalyst with the highest MA over the whole test period (65°C, 0.6 ↔ 1.0 V, up to 30,000 cycles) was found to be n-Pt2 nm/C.1 2) Enhanced ORR activities at Pt-M alloys2, 3 2-1) Enormously enhanced ORR activity at Pt-skin layer formed on Pt3Co(111) single crystal electrode Pt-skin/Pt–Co(111) single crystals exhibited extremely high ORR activity, j S. The j S value at 0.9 V reached a maximum at Pt73Co27(111), the value of which is ca. 27 times higher than that on a pure Pt(111) electrode.4 By the use of in situ STM, in situ SXS, ex situ XPS, and DFT calculation, we have succeeded in correlating such a high j S with the specific surface structure. 2-2) ORR activity and durability of ordered- and disordered-Pt3Co/C5 We have recently clarified the effect of the crystal structure of Pt3Co alloy nanoparticles on the ORR activity, H2O2 yield, and durability, for the first time, by the use of the ordered- and disordered-Pt3Co/C catalysts with the nearly identical average particle size, size distribution, and composition.5 3) Enhancement in the ORR activity and durability at stabilized Pt-skin–PtCo alloy catalysts6, 7 We have successfully prepared PtCo alloy nanoparticles, having a stabilized Pt skin (one to two atomic layers: xAL), supported on carbon black or graphitized carbon black (PtxAL–PtCo/C or PtxAL–PtCo/GCB). These new catalysts exhibited high MA for the ORR, together with superlative durability. This work was supported by funds for the ‘‘Superlative, Stable, and Scalable Performance Fuel Cell (SPer-FC)’’ project and “High Performance Fuel Cell (HiPer-FC)” project from the NEDO of Japan. References H. Yano, M. Watanabe, A. Iiyama, and H. Uchida, Nano Energy, 8, 13893 (2016).H. Uchida, H. Yano, M. Wakisaka, and M. Watanabe, Electrochemistry, 79, 303 (2011).M. Watanabe, D. A. Tryk, M. Wakisaka, H. Yano, and H. Uchida, Electrochim. Acta, 84, 187 (2012).S. Kobayashi, M. Wakisaka, D. A. Tryk, A. Iiyama, and H. Uchida, J. Phys. Chem. C. 121, 11234 (2017).H. Yano, I. Arima, M. Watanabe, A. Iiyama, and H. Uchida, J. Electrochem. Soc., 164, F966 (2017).M. Watanabe, H. Yano, D. A. Tryk, and H. Uchida, J. Electrochem. Soc., 163, F455 (2016).M. Chiwata, H. Yano, S. Ogawa, M. Watanabe, A. Iiyama, H. Uchida, Electrochemistry, 84, 133 (2016).

Polymer electrolyte membrane fuel cells (PEMFCs) are energy conversion devices that change chemical energy into electric energy. PEMFCs have high power density and efficiency wiht no pollution due to hydrogen as a fuel [1]. However, their high cost and low stability hindered PEMFCs from full-commercialization. This issue is originated from large amount of noble platinum catalyst in the cathode electrode to overcome the sluggish oxygen reduction reaction (ORR) [2]. To increase ORR activity with small amount of Pt, Pt-M alloy (M: Fe, Ni, Co, etc) catalysts have been studied over the past years. Particularly, Pt-M intermetallic catalysts showed significant enhancement of ORR activity and stability because of their abundant active sites and strong interactions between Pt and metal [3]. In this report, Pt3Mn intermetallic nanoparticles on carbon support (Pt3Mn/C) for ORR in PEMFCs was produced by using a facile synthesis. By using commercial Pt/C and Mn precursor, Pt3Mn/C catalyst could be easily synthesized. Total synthesis can be separated into 2 steps; first, Pt/C was sonicated in hexane, and Mn(acac)2, HDD, diphenyl ether was mixed and heated for 30 min. at 250 oC. Secondly, As-synthesized Pt3Mn/C was heat-treated under 700 oC for 4 hours in H2/Ar atmosphere to form Pt3Mn intermetallic nanoparticles. Pt3Mn intermetallic catalyst demonstrated a high mass activity compared to commercial Pt/C. While initial mass activity of Pt/C was 0.125 A/mgPt, Pt3Mn/C showed 0.386 A/mgPt, which was more than 3 times higher activity. Furthermore, Pt3Mn/C not only exhibited a high initial ORR activity, but also showed a great stability in a voltage-cycling test. After accelerated degradation test for 10k cycles, mass activity of Pt/C was reduced into 0.028 A/mgPt (77.6 % decrease), when Pt3Mn/C showed 0.276 A/mgPt (28.5% decrease). Additionally, Pt3Mn intermetallic catalyst showed a fast kinetic in alcohol oxidation reactions compared to commercial Pt/C. In methanol oxidation reaction, mass activity of Pt/C was about 0.10 A/mgPt, when Pt­3Mn/C showed 0.15 A/mgPt, which was 1.5 times higher. In ethanol oxidation reaction, Pt/C showed 0.035A/mgPt, when Pt3Mn/C showed 0.070 A/mgPt, which was 2 times higher than Pt/C. Further details of Pt3Mn intermetallic catalysts was characterized by adopting XAS analysis and DFT calculations to verify their high catalytic activity and stability. Through XAFS data, heat-treated Pt3Mn intermetallic nanoparticle showed higher coordination number, which implies its higher ordering than as-synthesized Pt3Mn. From XANES analysis, stability of catalysts can be compared. DFT calculation was also performed to compare ORR dissociative mechanism between Pt3Mn(111) and Pt(111). While ORR spontaneously occurs Pt3Mn(111) under potential of 0.87 V, Pt(111) needs potential of 0.53 V, which indicates high ORR activity of Pt3Mn. Finally, we demonstrated opportunity of utilizing in actual applications by the single-cell test. These results verified that intermetallic Pt3Mn/C catalyst can be used as an active and stable ORR catalyst for PEMFCs. Catalyst loading was 0.15 mgPt/cm2 for both anode and cathode. Initial performance of MEA with Pt3Mn/C cathode was 551 mA/cm2, when Pt/C showed 420 mA/cm2 @ 0.7 V. After long term durability test of each MEA, single cell performance of Pt3Mn catalyst decreased into 486 mA/cm2, which was 88% of initial performance. However, Pt/C catalyst decreased into 308 mA/cm2, which was 73% of initial performance, larger degradation than Pt3Mn/C.

Aiming at developing a highly active electrocatalyst with high platinum utilization efficiency, we report a facile synthesis of carbon supported Pd3Au@Pt electrocatalyst by chemical reduction of K2PtCl4, K2PdCl4, and aq NaAuCl4 with ascorbic acid (AA) under ambient conditions in the absence of surfactants. The resultant Pd3Au@Pt/C electrocatalyst comprises of a thin platinum layer less than 1 nm in thickness deposited on the outer surface of Pd3Au alloy core with an average diameter of 3.4 nm. Remarkably, Pd3Au@Pt/C exhibited a high mass activity (MA, 939 mAmg(Pt)(-1)) toward oxygen reduction reaction (ORR), which is 4.6 times that of commercial Pt/C (203 mAmg(Pt)(-1)). The durability of Pd3Au@Pt/C is close to that of commercial Pt/C. According to X-ray diffraction (XRD) patterns, the lattice constant of the Pd3Au alloy supported on carbon is determined to be 3.950 angstrom close to yet slightly larger than that of Pt/C (3.920 angstrom), inducing a lateral tensile strain of the platinum shell. Meanwhile, electrons from the Pd3Au core appear transferred to the platinum shell as evidenced by X-ray photoelectron spectroscopy (XPS). We propose that the lateral tensile strain (geometric effect) and the electron transfer (electronic effect) as well as the high platinum utilization efficiency have contributed to the significantly improved electrocatalytic activity of Pd3Au@Pt/C. The coexistence of the lateral tensile strain and the electron transfer in the electrocatalyst with a high ORR activity has not been reported prior to this study.

Bifunctional oxygen electrocatalysts that facilitate the acidic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the same electrode are critical components of unitized regenerative fuel cells (URFCs) and metal-air batteries. Obtaining bifunctional oxygen catalysts that simultaneously provide high ORR and OER activity, high stability, and lower precious metal content remains a significant challenge. Our recent work has explored bimetallic two-dimensional (2D) nanoframes as an approach to obtain carbon-free, unsupported nanostructures and demonstrated nickel-platinum and nickel-iridium 2D nanoframes function as high activity, separate ORR and OER catalysts respectively.1,2 To obtain high activity bifunctional oxygen catalysts, we investigated the combination of NiPt and CoIr 2D nanoframes. Bimetallic 2D nanoframes were prepared by microwave-assisted synthesis of noble metal-decorated transition metal hydroxide nanosheets followed by thermal reduction and chemical leaching steps. The 2D nanoframes utilize noble metal (Pt,Ir)-transition metal (Ni, Co) interactions to alter surface electronic structure, and the unsupported nanostructure provides a carbon-free matrix with three-dimensional accessibility to the catalytically active sites. Electrochemical testing using a rotating disk electrode configuration showed the two-component 2D nanoframe bifunctional oxygen electrocatalysts have higher ORR mass activity, OER mass activity, and round-trip efficiency compared with baseline Pt-IrO2 catalysts. Accelerated durability testing consisting of repeated voltage cycles over ORR/OER potential ranges showed that the 2D nanoframe catalysts exhibit similar ORR stability and improved OER stability compared with Pt-IrO2. The ability to combine highly catalytically active surfaces within a carbon-free 3D nanoarchitecture provides the opportunity to design bifunctional catalysts with improved activity and stability. 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

Bimetallic alloys based on Pt and Y are potential cathode catalysts for proton exchange membrane fuel cells (PEMFCs) due to their high oxygen reduction reaction (ORR) activity. Nevertheless, the synthesis of carbon supported PtxY catalysts is challenging due to the low standard reduction potential of yttrium compared to platinum. Hence, extended electrochemical testing in actual PEMFCs remains elusive, especially with respect to catalyst degradation upon voltage-cycling. In this publication, we present the synthesis of a bimetallic PtxY/C catalyst via impregnation of commercial Pt/C with an yttrium halide precursor and subsequent heat-treatment in H2 at 1200°C. This catalyst showed a high specific ORR activity, at a mass activity similar to Pt/C due to its comparably low electrochemical surface area (ECSA). On the other hand, the large particle size of the here synthesized PtxY/C catalyst (≈10 nm) resulted in a significantly enhanced stability versus degradation in an accelerated stress test (AST) based on voltage-cycling between 0.6 and 1.0 VRHE at 50 mV s−1, showing a superior ECSA, ORR activity and H2/air performance after 30000 cycles compared to a standard Pt/C catalyst.

Preparation of porous PtAuCu@Pt core-shell catalyst for application to oxygen reduction

High activity and durability of carbon-supported core-shell PtPx@Pt/C catalyst for oxygen reduction reaction

Nanosize platinum clusters with small diameters of 2–4 nm are known to be excellent catalysts for the oxygen reduction reaction. The inherent catalytic activity of smaller platinum clusters has not yet been reported due to a lack of preparation methods to control their size (<2 nm). Here we report the synthesis of platinum clusters (diameter ≤1.4 nm) deposited on genomic double-stranded DNA–graphene oxide composites, and their high-performance electrocatalysis of the oxygen reduction reaction. The electrochemical behaviour, characterized by oxygen reduction reaction onset potential, half-wave potential, specific activity, mass activity, accelerated durability test (10,000 cycles) and cyclic voltammetry stability (10,000 cycles) is attributed to the strong interaction between the nanosize platinum clusters and the DNA–graphene oxide composite, which induces modulation in the electronic structure of the platinum clusters. Furthermore, we show that the platinum cluster/DNA–graphene oxide composite possesses notable environmental durability and stability, vital for high-performance fuel cells and batteries.

PEMFCs have long been considered to be among the most promising next-generation energy conversion systems for portable devices and transportation vehicles due to their zero or low pollution, high power density and low temperature operation1,2. However, the widespread commercialization of PEMFCs remains challenged by the high cost of cell components and limited cell durability. Therefore, there is a significant need to develop new catalyst materials with high intrinsic oxygen reduction reaction (ORR) activity and stability, as well as a fabrication process that leads to high cell performance and durability with low catalyst loadings. In this work, we investigate the activity and stability of Pt-Ni nanocage oxygen reduction electrocatalysts (Figure 1a) both ex-situ and in-situ in an operating PEMFC. Ex-situ, the catalysts were both physically and electrochemically characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and both cyclic and linear sweep voltammetry using a rotating disk electrode (RDE). The nanocages showed 2 and 4 times greater specific and mass activity than Pt/C, respectively, for the ORR with high durability. The catalysts were transitioned from the RDE platform to operating PEMFCs by application onto a Nafion® 212 membrane to form a catalyst-coated membrane (CCM) with a low cost, custom-built air-assisted cylindrical liquid jet spraying system (ACLJS). Several ACLJS parameters were investigated and the optimized parameters were used to produce CCMs that were tested by scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), electrochemical impedance spectroscopy (EIS), PEMFC polarization and cyclic voltammetry. The CCM prepared with the Pt-Ni nanocage catalyst showed no obvious Pt and Ni dissolution and redeposition in the membrane even after 30K cycles. The performance and mass activity retention of the Pt-Ni nanocage catalyst was far superior to commercial Pt/C and just short of the US Department of Energy (DOE) 2020 targets3 (Figure 1 b,c), suggesting that Pt-alloy nanocages are very promising candidates for high-performing commercial PEMFCs.

Two-dimensional iron-porphyrin sheet as a promising catalyst for oxygen reduction reaction: a computational study

Thermal reduced graphene oxide enhanced in-situ H2O2 generation and electrochemical advanced oxidation performance of air-breathing cathode

Introduction Development of non-noble metal cathodes as alternative materials of platinum is required for practical application of PEFCs. We focused on the oxide-based compounds of group 4 and 5 metals because of their high chemical stability. We reported that carbonitrides of tantalum, niobium, zirconium and titanium oxidized under low oxygen partial pressure had high catalytic activity for oxygen reduction reaction (ORR) in acidic solution [1-3].Recently, we found that titanium oxide-based electrocatalysts (Ti-CNOs) made from organic titanium complexes with heat treatment under low oxygen partial pressure showed high oxygen reduction reaction (ORR) activity [4].In this study, we prepared two kinds of Ti-CNOs, that is, Ti-CNO(TPPz_NMP) prepared by the impregnation method using N-methylpyrrolidone (NMP) as solvent and Ti-CNO(TPPz_dry) prepared by ball-milling in dry process, and compared them to obtain principle of catalyst design. Experimental Oxy-titanium tetrapyrazino porphyrazine (TiOTPPz) was used as a starting material.TiOTPPz was supported on multi-walled carbon nanotube (MWCNT) as a conductive support by impregnation method using NMP as solvent. The powder was heat-treated at 900oC under N2 containing 2% H2 + 0.05% O2 to prepare oxide-based catalyst powder (Ti-CNO(TPPz_NMP)). TiOTPPz was mixed with MWCNT and the mixture was conducted ball-mill in dry process. The powder with dry ball-mills was also heat-treated under the same condition to obtain Ti-CNO(TPPz_dry).All electrochemical measurements were performed in 0.1 mol dm-3 H2SO4 at 30oC with a 3-electrode cell. A reversible hydrogen electrode (RHE) and a glassy carbon plate were used as a reference and a counter electrode, respectively. As pre-treatment, the potential was scanned 200 cycles from 0.05 to 1.2 V with a scan rate of 150 mV s-1 in O2 atmosphere. Then, the rest potential was measured in O2 atmosphere. Next, the potential was scanned 3 cycles from 0.2 to 1.2 V at a scan rate of 5 mV s-1 in both O2 and N2 atmosphere, and the cathodic current density of 3rd cycle was used for the evaluation of the ORR activity. Results and discussion Figure 1 shows the potential – ORR current density curves and the rest potential of the Ti-CNO(TPPz_dry) and Ti-CNO(TPPz_NMP) which were oxidized 10 h. The ORR current density, i ORR, of the Ti-CNO(TPPz_dry) was larger than that of the Ti-CNO(TPPz_NMP). The rest potential of the Ti-CNO(TPPz_dry) was also higher than that of Ti-CNO(TPPz_NMP).Figure 2 shows the relationship between the rest potential and the |i ORR@0.85 V| of the Ti-CNOs(TPPz_NMP) and the Ti-CNOs(TPPz_dry). Positive correlation was observed. The Ti-CNOs (TPPz_dry) had higher rest potentials and larger ORR current than the Ti-CNOs(TPPz_NMP). The secondary particle size of the Ti-CNOs(TPPz_dry) was smaller than that of the Ti-CNO(TPPz_NMP). Therefore, the precursor prepared by ball-milling oxidized more uniformly due to small secondary particles, resulting that the Ti-CNOs(TPPz_dry) had high ORR activity. The highest rest potential was over 1.0 V vs. RHE. Acknowledgements The authors wish to thank ORIENT CHEMICAL INDUSTRIES CO., LTD for supply of oxy-titanium tetrapyrazino porphyrazine (TiOTPPz), and the New Energy and Industrial Technology Development Organization (NEDO) for financial support. Reference [1] A. Ishihara, et al., J. Phys. Chem. C, 117, 18837 (2013).[2] Y. Ohgi, et al., J. Electrochem. Soc., 160, F162 (2013).[3] K.-D. Nam, et al.,Electrochim. Acta, 55, 7290 (2010).[4] K. Suito, et al., J. Fuel Cell Technol., 12, 130 (2013). [in Japanese]