Hydrogen Oxidation Reaction Activity Research Articles

Improved hydrogen oxidation reaction under alkaline conditions by Au–Pt alloy nanoparticles

Abstract This work demonstrates the outstanding performance of alloyed Au1Pt1 nanoparticles on hydrogen oxidation reaction (HOR) in alkaline solution. Due to the weakened hydrogen binding energy caused by uniform incorporation of Au, the alloyed Au1Pt1/C nanoparticles exhibit superior HOR activity than commercial PtRu/C. On the contrary, the catalytic performance of the phase-segregated Au2Pt1/C and Au1Pt1/C bimetallic nanoparticles in HOR is significantly worse. Moreover, Au1Pt1/C shows a remarkable durability with activity dropping only 4% after 3000 CV cycles, while performance attenuation of commercial PtRu/C is high up to 15% under the same condition. Our results indicate that the alloyed Au1Pt1/C is a promising candidate to substitute commercial PtRu/C for hydrogen oxidation reaction in alkaline electrolyte.

Nano-sized RuO2 electrocatalyst improves the electrochemical performance for hydrogen oxidation reaction

Hydrogen oxidation reaction (HOR) can be applied to proton exchange membrane fuel cells to generate electrical energy and anode discharge. Due to its special properties, RuO2 has been applied to supercapacitors, phenolic wastewater, textile industry wastewater, and degrading organic substances. However, there is few reports on the application of the RuO2 catalyst to hydrogen oxidation reaction (HOR). In this study, we successfully obtained RuO2 NPs using a simple and eco-friendly hydrothermal method. Furthermore, the electrochemical activity of RuO2 NPs prepared at different concentration (0.15 M, 0.20 M) and different hydrothermal temperature (150 °C, 160 °C, and 170 °C) was evaluated by the hydrogen oxidation reaction. The particle size, composition, dispersion and morphology of the obtained RuO2 catalysts were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). In addition, cyclic voltammograms (CV) were utilized to investigate the electrochemical activity of the RuO2 catalysts. The results showed that the obtained catalyst at a hydrothermal temperature of 160 °C and a concentration of 0.15 M displayed a Brunauer-Emmett-Teller (BET) surface area of 26.74 m2 g−1. Meanwhile, the catalyst had a uniform distribution. The hydrogen oxidation current density of the obtained RuO2 catalysts is upto 6 mA cm−2, showing a good electrochemical activity for hydrogen oxidation reaction.

One-pot synthesis of IrNi@Ir core-shell nanoparticles as highly active hydrogen oxidation reaction electrocatalyst in alkaline electrolyte

Highly active and low cost hydrogen oxidation reaction (HOR) catalysts for anion exchange membrane fuel cells are desired, because the HOR catalytic activity under alkaline condition is much slower than that in acid. Herein, a novel and efficient IrNi@Ir core-shell HOR catalyst was synthesized by a simple one-pot approach using both iridium acetylacetonate and iridium chloride as the Ir sources, beneficial from their different reactivities. The hydrogen binding energy of the surface Ir shell is tuned by the IrNi alloy core, leading to the promoted HOR activity and stability. The mass activity of IrNi@Ir/C is 1.12 mA mgIr−1 at 0.05 V in 0.1 M KOH, which is 2.4 times that of Ir/C, and also higher than that of commercial Pt/C. It demonstrates that the fine-tuned nanostructure is propitious to the catalytic performance and has potential applications in the future clean energy device.

Palladium-ceria nanocatalyst for hydrogen oxidation in alkaline media: Optimization of the Pd–CeO2 interface

The development of Pt-free catalyst for anion exchange membrane fuel cells is limited by the sluggish hydrogen oxidation reaction (HOR) at the anode. Previously, the use of CeO2 as a catalyst promoter facilitated drastic ennoblement of Pd for the HOR kinetics in base media. However, further optimization and understanding of the Pd–CeO2 interaction, surface properties, and their influence on HOR are still needed. In this work, three types of Pd–CeO2/C catalysts are synthesized by a flame-based process, where the Pd–CeO2 interface and the HOR activity are improved as compared to catalysts prepared by wet-chemistry processes. The correlation between the Pd–CeO2 interaction and the HOR activity is established through comparisons of three types of Pd–CeO2/C synthesized catalysts using electrochemical techniques and X-ray photoelectron spectroscopy.

Enhancement of Palladium HOR Activity in Alkaline Conditions through Ceria Surface Doping

Hydrogen oxidation reaction (HOR) electrocatalysis suffers from slow kinetics at the anode of alkaline exchange membrane fuel cells (AEMFCs). A series of Pd-CeO2-x catalyst was synthesized at low temperature by the decomposition of cerium ammonium nitrate (CAN) on Pd. This simple method yields palladium with sub-stoichiometric amorphous CeO2-x islands which nucleate and grow on the surface of Pd. The vertical growth is preferred: ceria on ceria vs. ceria on palladium as evidenced by the constant values of Pd electrochemical surface areas observed for all ceria contents. The HOR activity is enhanced compared to pristine Pd. At 0.1 V vs. RHE, the specific and mass activities could be increased by a factor of 100 and 50 respectively for the highest content ceria samples. These results show that high ceria doping is requested to activate the HOR activity of palladium in alkaline medium, which can be achieved by cerium ammonium nitrate low temperature decomposition.

Impact of Strain-Induced Changes in Defect Chemistry on Catalytic Activity of Nd2NiO4+δ Electrodes.

It is well known that defect chemistry plays a vital role in determining the electronic structure, ionic conductivity, and catalytic activity of metal oxides, as demonstrated in perovskite-based oxides to achieve desired functionalities. In this work, we explored the possibility of tuning the defect chemistry and hydrogen oxidation reaction (HOR) activity of Nd2NiO4+δ model thin films by controlling the lattice strain. Highly textured Nd2NiO4+δ thin films with different strain states were prepared on (110)- and (100)-oriented single-crystal yttrium-stabilized zirconium (YSZ) substrates using pulsed laser deposition. Electrochemical impedance spectroscopy results indicated that the NNO(100) film on the YSZ(110) substrate with larger tensile strain in the a- b plane and compressive strain along the c axis exhibited higher HOR activity than the NNO(110) film on the YSZ(100) substrate at 500-600 °C. The enhancement in HOR activity is attributed to the strain-induced difference in the oxygen defect concentration, as confirmed by high-resolution X-ray diffraction analysis. We believe that the correlation among the strain state, defect chemistry, and catalytic properties is helpful for rational design of more efficient electrode materials.

Carbon Supported Pt-Co Alloy Nanoparticles As HOR and ORR Catalyst for PEM Fuel Cells

In a proton exchange membrane (PEM) fuel cell, a noteworthy part of the production cost is caused by the catalyst involving rare and costly noble metals. To meet the cost targets for the automotive PEMFC applications, the total Pt loading of state of the art membrane electrode assembly (MEA) has to be reduced from ~0.4 to 0.1 mg/cm². While on the anode side of the MEA, hydrogen oxidation reaction (HOR) is very fast in acidic media, high Pt loadings are needed to accelerate the very sluggish kinetics of the oxygen reduction reaction on the cathode side. This situation entirely turns if the environment changes from low to high pH values. For instance, in alkaline media the catalytic activity of the Pt for the HOR is insufficient and thus requires high Pt loadings, whereby the electro-activation of oxygen can occur on Pt-free catalyst materials [1]. A possible pathway to solve this issue is the implementation of bimetallic Pt-M (M = Co, Ru, Ni) alloy nanoparticles as electrocatalysts to reduce or substitute the Pt by maintaining the catalytic performance and durability [2-7]. Our approach is to tune carbon supported Pt-Co alloy nanoparticles as highly active and durable catalyst material for the HOR as well as for the ORR. We tailored the shape, size, structure, chemical composition and crystallinity of the Pt-Co particles by variation of the synthetic parameters. In particular, the structural arrangement of the Pt-Co nanoparticles was modified to form core-shell, Pt/Co-segregated or alloyed nanoparticles by using various synthetic and electrochemical conditions. The investigations of the electrocatalytic activity towards HOR and ORR on the modified Pt-Co nanoparticles were carried out by using a rotating disc electrode (RDE) setup in acidic and alkaline media. Furthermore, the structural properties and chemical composition of the Pt-Co nanoparticles were characterized by using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), CO pulse-chemisorption and X-ray diffraction (XRD). Controlling the atomic arrangement of the Pt-Co nanoparticles, superior catalytic activity for the HOR and ORR was achieved compared to commercially available Pt/C. Based on our results, we provide very promising pathways to tailor physical and electrochemical properties of the Pt-M alloy nanoparticles to boost the HOR and ORR activities for the PEM fuel cell.

Electrocatalysts for Hydrogen Oxidation Reaction in Alkaline Electrolytes

In the past 5 years, advances in anion-conductive membranes have opened the door for the development of advanced anion-exchange membrane fuel cells (AEMFCs) as the next generation of affordable fuel cells. Several recent works have shown that AEMFCs currently achieve nearly identical beginning-of-life performance as state-of-the-art proton exchange membrane fuel cells. However, until now, these high AEMFC performances have been reached with platinum-group metal (PGM)-based anode and cathode catalysts. In order to fulfill the potential of AEMFCs, such catalysts should in the near future be free of PGMs and, eventually, free of critical raw materials. Although great progress has been achieved in the development of PGM-free catalysts for the oxygen reduction reaction in basic media, significantly less attention has been paid to the catalysis of the hydrogen oxidation reaction (HOR). The much lower HOR activity of Pt in basic media compared with that in acid was itself revealed only relatively recently. While...

CO-tolerant PtRu@h-BN/C core–shell electrocatalysts for proton exchange membrane fuel cells

PtRu alloy nanoparticles (NPs) in a commercial PtRu/C catalyst were encapsulated within few-layer graphitic boron nitride (h-BN) shells to form PtRu@h-BN/C core–shell catalysts, which were used as CO-tolerant anode catalyst in proton exchange membrane fuel cells (PEMFCs). Electrochemical results reveal that the PtRu@h-BN/C catalysts show similar activity in hydrogen oxidation reaction (HOR) as the PtRu/C catalysts but present enhanced CO-tolerance in fuel cell tests. Single cell performance with the PtRu@h-BN/C catalyst has been lowered by less than 20% with 30 ppm CO/25% CO2 introduced into H2 feed gas, while the PtRu/C catalyst suffers from a decrease of 60% under the same conditions. The reaction data and characterization results suggest that h-BN ultrathin shells can achieve a good balance between durability and activity, enabling the PtRu@h-BN/C material to act as a potential anode catalyst for PEMFCs.

The Enhancement of Hydrogen Oxidation Activity and the Optimization of Alloy Composition in PdRu Nanoparticle Catalysts

Fuel cells are believed as promising power sources with many advantages including their high efficiency and environment-friendliness. Proton exchange membrane fuel cells (PEMFCs), of which usage can be expected as power sources as automobiles and so on, have yet several problems to resolve for the commercialization. Among them, the high price due to the scarcity of Pt, which is used as anode and cathode catalysts, is one of the most serious issues. Pd is a natural choice to replace Pt because the hydrogen oxidation reaction (HOR) activity of Pd is the next to that of Pt, and Pd is at least fifty times more abundant on earth than Pt. Pd is a more attractive substitute for Pt because of its higher intrinsic CO tolerance than Pt, especially as HOR catalysts for PEMFC where the tolerance of CO, a major impurity of hydrogen fuel, is crucial. Although Pd has comparable HOR activity to Pt, the exclusive use of Pd as anode catalysts of PEMFC has not been satisfactory for the replacement of Pt. We confirmed in our previous report (K. Kwon et al., Catal Today, 232 (2014) 175) that the insufficient HOR activity of Pd could be supplemented by coupling with other elements or compounds such as Ru and tungsten oxide. To investigate the effect of alloying with Ru thoroughly, the calculation of hydrogen binding energy was performed on the basis of spin polarized density functional theory. Pure Pd(001) and Ru(001), Pd monolayer on Ru, and Pd bilayer on Ru are considered as model systems in the calculation. Strain and ligand effects are only applicable to bimetallic models (Pd monolayer on Ru and Pd bilayer on Ru). The negative sign of strain and ligand effects stands for a decrease in binding energy for PdRu alloy. Meanwhile, instead of making an effort to turn the model alloy systems into nanoparticles of Pd mono/bilayer on Ru, which would not be realistic in a few nm dimension, the actual confirmation of HOR enhancement at the level of half cell and PEMFC single cell was obtained by fabricating homogeneous PdRu alloy nanoparticles on carbon support. PdRu nanoparticles of different Pd:Ru atomic ratios of 1:0, 9:1, 1:9, and 0:1 were characterized with TEM and XRD, and compared with a commercial state-of-the-art Pt-based anode catalyst (PtRu1.5/C) in electrochemical experiments. The PdRu9 catalyst shows the highest HOR activity among the PdRu samples, which is comparable to the PtRu catalyst, even though the respective Pd and Ru have lower activity than PtRu. The highest HOR activity of the Ru-richest PdRu nanoparticles could be in line with the highest HOR activity of the Ru-richest PdRu model system (Pd monolayer on Ru).

Oxygen-Tolerant Electrodes with Single-Atom Platinum Modified Covalent Triazine Frameworks for the Hydrogen Oxidation Reaction

Polymer electrolyte fuel cells (PEFC) that utilize hydrogen and oxygen as the reactants have attracted a lot of attentions as an environmentally-friendly energy conversion system, as they can be operated under relatively low temperature below 100 degree Celsius and emit only water as the reaction product. Improvement of the anode catalysts is one of the essential requirements toward the popularization of the PEFC technologies. Currently, platinum (Pt) or its alloy is used as the anode catalyst in practical PEFC. As Pt is rare and expensive, there is increasing demand for reducing the loading amount of Pt on the anode. Another problem regarding to the Pt anode catalyst is that Pt is active not only for hydrogen oxidation reaction (HOR) but for oxygen reduction reaction (ORR). During the start-up conditions of PEFC, air inevitably flows into the anode chamber and cathodic ORR proceeds on the anode Pt catalysts. Single-atom Pt catalyst is a possible candidate that solves the above two problems for the PEFC anode catalyst. Based on the single-atom nature, we can expect not only to ultimately reduce the loading amount of Pt, but also to obtain unique reaction selectivity as many reactions can proceed only on Pt ensemble sites. Recently, we demonstrated that covalent triazine frameworks (CTFs) can serve as a platform for constructing heterogeneous single-atom electrocatalysts [1-6]. Herein, we demonstrated that a Pt-modified CTF (Pt-CTF, Figure) has higher HOR and lower ORR activity compared to commercial Pt/C. The CTF was obtained by the polymerization of 2,6-dicyanopydridine on carbon nanoparticles in molten ZnCl2. Then, Pt atoms were grafted in the pore of CTFs by the impregnated with platinum chloride. The high-angle annular dark-field scanning transmission electron microscopy and extended X-ray absorption fine structure analyses revealed that Pt atoms in Pt-CTF were atomically dispersed. We evaluated the PFEC performance with 2.8 wt % Pt-CTF (Pt amount; 0.020 mg cm-2) as the anode catalyst, compared with the 20 wt % Pt/C catalyst (Pt amount; 0.10 mg cm-2). The maximum power density of PEFC with Pt-CTF was determined to be 487 mW cm-2 at 1.2 A cm-2, a value that was nearly identical to that of PEFC with Pt/C (462 mW cm-2 at 1.0 A cm-2), which contained approximately 5 times more Pt anode catalyst. These results indicated that the amount of Pt required for catalytic activity is drastically lower than that of conventional Pt/C catalysts [3]. Next, we investigated the ORR activity. The electrocatalytic ORR activity of 2.8 wt% Pt-CTF was markedly lower than that of commercial 20 wt% Pt/C, although these catalysts showed the almost similar HOR activity. The selectivity likely relied on the fact that the required number of Pt sites for HOR was significantly smaller than that for ORR. Namely, Pt-CTF selectively catalyze HOR, even in the presence of dissolved oxygen, which is critical for limiting cathode degradation during the start–stop cycles of fuel cells. [1] K. Kamiya and S. Nakanishi et al. Nature Commun. 2014, 5, 5040. [2] K. Iwase, S. Nakanishi and ± et al. Angew. Chem. Int. Ed., 2015, 54, 11068. [3] K. Kamai, K. Kamiya, K. Hashimoto and S. Nakanishi Angew. Chem. Int. Ed., 2016, 55, 13184. [4] T. Yoshioka, S. Nakanishi and K. Kamiya*g J. Phys. Chem. C,, 2016, 120, 15729. [5] R. Kamai, S. Nakanishi, K. Hashimoto*, K. Kamiya *, J. Electroanal. Chem., 2017, 800, 54 [6] S. Yamaguchi, K. Kamiya* , K. Hashimoto, S. Nakanishi* , Chem. Commun. 2017, 53, 10437 Figure 1

(Invited) Recent Progress in the Understanding of the Electrocatalysis of the CO-Tolerant Hydrogen Oxidation Reaction in Polymer Electrolyte Fuel Cells

The hydrogen anode in polymer electrolyte fuel cells (PEFCs) is vulnerable to poisoning from traces of carbon monoxide in the reformed hydrogen fuel, due to strong CO adsorption on the platinum surface. Over the past several decades, there have been efforts to develop catalysts for the hydrogen oxidation reaction (HOR) that are CO-tolerant by alloying Pt with other metals, e.g., Fe, Co, Ni and Ru. The latter is a well-known commercial HOR catalyst, but Ru can be replaced with the other, less costly metals. The CO tolerance is expected to be greater for catalysts with decreased CO adsorption strength. Recently, we have found that Pt-Fe and Pt-Co alloys with specially stabilized Pt “skins” with x atomic layers, PtxAL–PtM/C have excellent CO tolerance, HOR activity and durability.1,2 More specifically, the CO tolerance and HOR activity increase in the order Pt < Pt-Ru < Pt-Ni < Pt-Co < Pt-Fe.2 This trend is the same as that for decreasing CO adsorption strength on (110) steps and (111) terraces, as calculated with density functional theory (DFT). It is also the same trend as that for atomic H adsorption on (111) terraces, while the trend is not followed for the (110) steps: the adsorption strengths for H on (110) steps on PtxAL–PtFe(221), PtxAL–PtCo(221), and PtxAL–PtNi(221) are all very similar. Thus, we have proposed a new type of metal-metal spillover mechanism, in which H2 adsorbs dissociatively at the (110) steps and then diffuses to the (111) terraces, from which the 2Had desorb oxidatively to produce 2H+ (Fig. 1). Our understanding of these catalysts has been deepened by in situ Fourier transform infrared (FTIR) spectroscopy3 and X-ray absorption spectroscopy (XAS).4 The FTIR spectra can distinguish between step edge and terrace sites for CO adsorption and can monitor the diffusion of CO from edges to steps in real time. The XAS studies have led to a deep understanding of both the structural and electronic properties of the nanoparticulate PtxAL–PtCo/C catalyst in comparison with Pt/C. When the electrodes are held at low potentials, within the H adsorption region, the Pt particles undergo significant structural (as seen in the EXAFS) and electronic transformations (as seen in the XANES), whereas the PtxAL–PtCo particles are remarkably stable, due to the shorter Pt-Co bond lengths and rigid framework. The same is true of CO adsorption. All of these characteristics are consistent with the DFT calculations, including the weakened H and CO adsorption. We are now extending this work in order to develop even more active, CO tolerant and durable HOR catalysts. References Shi, G.; Yano, H.; Tryk, D. A.; Watanabe, M.; Iiyama, A.; Uchida, H. Nanoscale 2016, 8, 13893-13897.Shi, G.; Yano, H.; Tryk, D. A.; Iiyama, A.; Uchida, H. ACS Catal. 2017, 7, 267-274.Ogihara, Y.; Yano, H.; Matsumoto, T.; Tryk, D.; Iiyama, A.; Uchida, H. Catalysts 2017, 8 (1-13).Shi, G.; Yano, H.; Tryk, D. A.; Matsumoto, M.; Tanida, H.; Arao, M.; Imai, H.; Inukai, J.; Iiyama, A.; Uchida, H. Catal. Sci. Tech. 2017, in press. Figure 1. Atomic models of the PtxAL–PtCo(221) surface, showing (A) non-dissociative H2 adsorption, (B) spontaneously dissociated 2H at the (110) step, and (C) 2H after diffusion to the (111) terraces. Figure 1

Development of Ni-Based Bimetallic Electrocatalysts for Hydrogen Oxidation Reaction in Alkaline Fuel Cells

Anion exchange membrane fuel cells (AEMFCs) opens up the possibility of using non-platinum and/or non-platinum group metals (PGM) as electrocatalysts for both hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR). However, the slow kinetics of HOR along with the current lack of a durable and highly conducting OH- ion membrane remain major bottlenecks limiting its practical application. HOR with PGM free catalysts are still in the early stages of research unlike ORR with non-PGM catalysts1 . In order to overcome this limitation it is important to understand the mechanism of the HOR on the surface of the electro catalysts. It is considered that the two main descriptors, which enhance the electrocatalytic activity of the HOR, are the OHad or Had molecules on the surface of the catalytic materials.2,3 Ni and its alloys with transition metals would be one of the alternative choices for the HOR as Raney Ni and some of the Ni based alloys have been found showing improved HOR activity.4 Morphology, size and surface of the metallic nanoparicles have significant role on the catalytic activity in their respective applications. In this work we have synthesized monodispersed NiCo, NiCu and NiFe based bimetallic alloy nanoparticles of less than 20 nm size by high temperature solvothermal process.5 The synthesized alloy nanoparticles have been evaluated for their electrocatalytic activity and stability towards the HOR in a conventional rotating disk electrode (RDE) configuration tehnique. The nanoparticles have also been characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) to understand their physicochemical properties. The effect of surface state and alloying of the nanoparticles on the HOR kinetics is studied and analyzed in terms of mechanism and reaction steps involved. Our overall ambition in this work is to explore PGM free bi and trimetallic nanoparticles (NiM and NiMN) for the HOR and use them as anode catalysts in an operating AEM fuel cells. References Zhuang, Z. et al. Nickel supported on nitrogen-doped carbon nanotubes as hydrogen oxidation reaction catalyst in alkaline electrolyte. Nat.Commun. 7, 10141(2016).Strmcnik, D. et al. Improving the hydrogen oxidation reaction rate by promotion of hydroxyl adsorption. Nat.chem. 5,300-306 (2013).Parsons, R.The rate of electrolytic hydrogen evolution and the heat of adsorption of hydrogen.Trans. Faraday Soc. 54, 1053–1063 (1958).Kabir, S. et al. Platinum group metal-free NiMo hydrogen oxidation catalysts: high performance and durability in alkaline exchange membrane fuel cells. Jour.Mater.Chem. A, (2017).Carenco,S. et al.Controlled design of size-tunable monodisperse nickel nanoparticles.Chem.Mater.22,1340-1349(2010). Figure 1:Representative bright field scanning transmission electron microscopy (STEM) image of bimetallic NiCo nanoparticles Figure 1

Why Pt-Ru Catalyst Works Better for Alkaline Hydrogen Oxidation Reaction?

It is well-known that Pt-Ru alloy electrocatalysts perform better than Pt catalyst for the anode reaction in alkaline membrane fuel cells.1 However, it is unclear why Pt-Ru alloy catalyst performs better in alkaline membrane fuel cells. Strmcnik et al. suggested that the higher activity of Pt-Ru catalyst is due to increased oxophilicity of Pt catalyst by adding Ru component.2 Others suggested that the Ru component weakens the Pt-Had binding energy to improve hydrogen oxidation reaction.3,4 In this presentation, we demonstrate that the improved alkaline hydrogen oxidation reaction is due to the low benzene group adsorption on the catalyst surface when the Ru component is added to Pt. In order to support our hypothesis, we first compared the voltammograms of hydrogen oxidation reaction of commercial Pt and Pt-Ru alloy catalysts in 0.1 M benzyl ammonium hydroxide and tetramethyl ammonium hydroxide solutions.5 These experiments showed that the hydrogen oxidation reaction of Pt in the benzyl ammonium hydroxide solution is much slower than that of Pt in the tetramethyl ammonium hydroxide solution. More importantly, the hydrogen oxidation reaction in the benzyl ammonium hydroxide solution can be substantially increased when Pt-Ru alloy catalyst was used. Density functional theory calculations indicate that the adsorption of benzyl ammonium on the bimetallic catalyst is endergonic for all configurations of the benzyl ammonium cation, which explains the significantly better hydrogen oxidation reaction activity observed for the bimetallic catalyst. The adverse impact of benzene adsorption on electrocatalysts is further investigated in the fuel cell performance test. For this experiment, we decoupled the benzene adsorption effect from mechanistic alkaline HOR perspectives by measuring both acid and alkaline membrane fuel cell performance using polyaromatic and benzene-free perfluorinated ionomers. Both acid and alkaline fuel cells, we observed substantially better kinetic performance when Pt-Ru electrocatalyst was used with polyaromatic electrolytes. In stark contrast, Pt electrocatalyst showed better kinetic performance when benzene-free perfluorinated electrolyte was used. This indicates that the performance improvement using Pt-Ru catalyst in alkaline (and acid) conditions is not originated from the mechanistically slow hydrogen oxidation under alkaline environment but originated from the less benzene group adsorbing characteristics of Pt-Ru catalyst. Acknowledgement This work is supported by the US Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technology Office (Program Manager Dr. David Peterson).

Infiltrated Pr2NiO4 as promising bi-electrode for symmetrical solid oxide fuel cells

In-situ growth of nanoparticles on electrode surface for high temperature energy conversion devices is one of efficient ways for new electro-catalyst design. Here, perovskite-related Pr2NiO4 (PNO) has been evaluated as a novel symmetrical electrode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Nickel nanoparticles are exsolved and dispersed on the surface of PNO after exposed in H2 at 800 °C as an efficient electrocatalyst for hydrogen oxidation reaction (HOR) as anode. The electro-activity towards oxygen reduction reaction (ORR) at cathode side is further improved by infiltration process. A synergetic effect of in-situ exsolved nanoparticles as well as infiltrated ionic conducting particles have improved HOR and ORR activity at anode and cathode side, respectively. Furthermore, symmetrical SOFC (SSOFC) with infiltrated Pr2NiO4 as bi-electrode exhibits excellent short-term stability and reliable redox stability in repeated H2/air cycles.

Efficient Pt‐C MEA for PEMFC with Low Platinum Content Prepared by Magnetron Sputtering

AbstractThe presented research is focused on improvement of platinum utilization in Pt‐C composite catalysts for proton exchange membrane fuel cell (PEMFC) applications. A series of 25 nm thick nanostructured Pt‐C thin films were deposited on carbon paper gas diffusion layer substrates (GDL; Sigracet 29 BC) by a single‐step magnetron sputtering method.Total Pt loading in the thin films varied in the range 6.7–9.7 µg cm−2 as determined by X‐ray photoelectron spectroscopy (XPS). These catalysts were examined for their activity in hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR), continuously operating for periods exceeding 24 hours. The dependence of cell output power on Pt concentration and on the working pressure in the cell was examined.Membrane electrode assembly (MEA) with both magnetron sputtered electrodes and a total (anode + cathode) Pt loading 16.7 µg cm−2 was also examined during an extended 250‐hour durability test. A stable power density 0.73 W cm−2 was achieved with cell pressure of 300 kPa. Apart from reasonably small power fluctuations no long‐term efficiency loss has been detected.

Highly Selective TiN-Supported Highly Dispersed Pt Catalyst: Ultra Active toward Hydrogen Oxidation and Inactive toward Oxygen Reduction.

The severe dissolution of the cathode catalyst, caused by an undesired oxygen reduction reaction at the anode during startup and shutdown, is a fatal challenge to practical applications of polymer electrolyte membrane fuel cells. To address this important issue, according to the distinct structure-sensitivity between the σ-type bond in H2 and the π-type bond in O2, we design a HD-Pt/TiN material by highly dispersing Pt on the TiN surface to inhibit the unwanted oxygen reduction reaction. The highly dispersed Pt/TiN catalyst exhibits excellent selectivity toward hydrogen oxidation and oxygen reduction reactions. With a Pt loading of 0.88 wt %, our catalyst shows excellent hydrogen oxidation reaction activity, close to that of commercial 20 wt % Pt/C catalyst, and much lower oxygen reduction reaction activity than the commercial 20 wt % Pt/C catalyst. The lack of well-ordered Pt facets is responsible for the excellent selectivity of the HD-Pt/TiN materials toward hydrogen oxidation and oxygen reduction reactions. Our work provides a new and cost-effective solution to design selective catalysts toward hydrogen oxidation and oxygen reduction reactions, making the strategy of using oxygen-tolerant anode catalyst to improve the stability of polymer electrolyte membrane fuel cells during startup and shutdown more affordable and practical.

Determination of Hydrogen Oxidation Reaction Mechanism Based on Pt−Had Energetics in Alkaline Electrolyte

The understanding of the sluggish hydrogen oxidation reaction (HOR) kinetics in alkaline electrolyte is crucial for designing high performance electrocatalysts. Here, we report a clear and convincing result on this problem by investigating the relationship between the HOR kinetics and Pt-adsorbate energetics. Using electrochemical analysis for well-modified Pt surfaces with distinct Pt-adsorbate interactions, we establish a clear trend in activity for HOR in alkaline electrolyte, that is, the activity changes in the order Au@Pt < Pt < Pd@Pt < Ru@Pt. A decisive role of Pt−Had energetics in the HOR kinetics on Pt surfaces is determined, while no favorable effects of Pt−OHad energetics in the HOR kinetics were found. The nature of strong Pt−Had interaction for Pt/C catalyst in alkaline electrolyte is revealed by in-situ X-ray absorption near-edge structure (XANES) analysis. These insights play an important role for better understanding the mechanism and structure sensitivity for Pt catalyzed hydrogen oxidation reactions.

Benzene Adsorption: A Significant Inhibitor for the Hydrogen Oxidation Reaction in Alkaline Conditions.

Slow hydrogen oxidation reaction (HOR) kinetics on Pt under alkaline conditions is a significant technical barrier for the development of high-performance hydroxide exchange membrane fuel cells. Here we report that benzene adsorption on Pt is a major factor responsible for the sluggish HOR. Furthermore, we demonstrate that bimetallic catalysts, such as PtMo/C, PtNi/C, and PtRu/C, can reduce the adsorption of benzene and thereby improve HOR activity. In particular, the HOR voltammogram of PtRu/C in 0.1 M benzyl ammonium showed minimal benzene adsorption. Density functional theory calculations indicate that the adsorption of benzyl ammonium on the bimetallic PtRu is endergonic for all four possible orientations of the cation, which explains the significantly better HOR activity observed for the bimetallic catalysts. The new HOR inhibition mechanism described here provides insights for the design of future polymer electrolytes and electrocatalysts for better-performing polymer membrane-based fuel cells.

(Invited) Analyses of CO Tolerance at Stabilized Pt-Skin/Ptfe and PtCo Hydrogen Anode Catalysts with High Activity and Robustness for Residential PEFCs

For residential polymer electrolyte fuel cells (PEFCs) operated with H2-rich (reformate) fuel gas, the hydrogen oxidation reaction (HOR) at Pt catalysts is severely poisoned even by trace amounts of CO contained in the reformate due to a strong blocking of active sites. So far, Pt-Ru alloy catalysts have been used to mitigate such CO poisoning. However, the HOR mass activity at Pt-Ru alloys is not sufficiently high, so that a large amount of precious metals (Pt and Ru) must be used. In addition, since Ru is not stable at high potentials (≥ 0.8 V), leaching into the acidic electrolyte membrane, the anode must be protected so as not to be oxidized by air. Recently, we have developed a Ru-free hydrogen anode catalyst by forming a stabilized Pt-skin (one to two atomic layers: xAL) on PtM-alloy (M = Fe, Co, and Ni) nanoparticles supported on carbon black (PtxAL‒PtM/C).1,2 These new catalysts exhibited a high HOR activity, in both the presence and absence of CO, as well as a robustness.3,4 The apparent values of mass activity (MA app) for the HOR on PtxAL‒PtFe/C at 20 mV vs. RHE in H2-saturated 0.1 M HClO4 solution were 2‒3 times larger than those for a commercial Pt2Ru3/C (c-Pt2Ru3/C) catalyst in the absence and presence of adsorbed CO (COad) at 70 °C as shown in Fig. 1. The retention of MA app on these PtxAL-PtM/C catalysts after an accelerated durability test (ADT) simulating daily start-stop cycles repeated exposure to the reformate gas and air (2,500 potential cycles between 0.02 V and 0.95 V) in N2 purged-0.1 M HClO4 solution at 70 oC was as high as ca. 80 % as shown in Fig. 2. The apparent area-specific activity values at the PtxAL‒PtM/C remained unchanged during ADT. This suggests that the dealloying of the M component in the PtxAL‒PtM/C was suppressed by the stabilized Pt-skin layer. To clarify the mechanism of such an enhancement of HOR activity and CO-tolerance for the PtxAL‒PtM/C, we performed multilateral-analyses, by in-situ attenuated total reflection Fourier transform infrared reflection-adsorption spectroscopy (ATR-FTIRAS), density functional theory (DFT) calculations, and in-situ X-ray absorption spectroscopy (XAS) techniques. It was found that the adsorption energy of CO on terrace sites on the PtxAL skin covering the PtM alloy was weakened by the modified electronic structure. The HOR activity of PtxAL‒PtFe/C with pre-adsorbed CO was found to recover appreciably after bubbling with CO-free pure H2. Such a recovery can be ascribed to an increase in the number of active sites by a transfer of linearly adsorbed COad at terrace sites (COL, terrace) to step/edge sites (COL, step/edge) on the surface of the Pt-skin layer, without removal of COad from the surface.4,5 We will also discuss possible mechanisms for the CO-tolerant HOR. This work was supported by funds for the “Superlative, Stable, and Scalable Performance Fuel Cells (SPer-FC)” project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The synchrotron radiation experiments were performed at BL14B2 (Proposal Nos. 2015B3388 and 2016A1805) and BL16B2 (2016A5390 and 2016B5390) of SPring-8 in cooperation with the Device-Functional Analysis Department of NISSAN ARC Ltd. Figure 1