Fuel Cell Conditions Research Articles

Analysis of alkaline exchange membrane fuel cells performance at different operating conditions using DC and AC methods

Membrane electrode assemblies (MEAs) for anion exchange membrane fuel cells (AEMFCs) were manufactured from commercial materials: Pt/C catalyst, A201 AEM and AS4 ionomer by using an industrial mass-production digital printing method. The MEA designs selected are close to those recommended by US Department of Energy, including low loading of platinum on the cathode side (0.2 mg cm−2). Polarization curves and electrochemical impedance spectroscopy (EIS) were applied for MEA evaluation in fuel cell conditions with variation of gas humidification and oxygen partial pressure (air vs oxygen). The typical impedance curves recorded at H2/O2 gas configuration consist of high- and medium-frequency arcs responsible for hydrogen oxidation and oxygen reduction, respectively. Operation with air as a cathode feed gas resulted in a decrease in AEMFC performance due to possible CO2 poisoning and mass transfer losses. At the same time, EIS demonstrated formation of a low frequency loop due to diffusion limitations. Despite the low loading of platinum on the cathode (0.2 mg cm−2), a peak power density of ∼330 mW cm−2 was achieved (at 50/50% of RH on anode and cathode), which is substantially higher performance than for AEMFC MEAs tested at similar conditions.

Cobalt Platinum Bronze As a Catalyst for Polymer Electrolyte Fuel Cells

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 started fundamental studies on mixed oxides of platinum (so called "platinum bronze") consisting of platinum, oxygen and other metal ions (such as Li, Na, Mg, Ca, Zn, Cd, Co, Ni [1]), which were reported by Shannon et al. as an ORR catalyst for fuel cells. Although their activities were not extremely high, they were reported not to be dissolved with hot aqua regia. This remarkable property made us expect that these materials can exhibit excellent durability under fuel cell conditions (high temperature, high potential and also chloride contaminant). Here, we present the preparation, characterization and electrochemical behavior of Co-Pt bronze for PEFC application. Co-Pt bronze was prepared according to the literature [1] with some modifications. Platinum oxide and Co(NO3)2·6H2O powders were mixed and heat-treated under air conditions at 650 ºC for 5 hours. The obtained powder was washed with heated (80 °C) aqua regia for 3 times and the remaining powder was then thoroughly rinsed with pure water. The electrochemical behaviors including ORR activity of the obtained Co-Pt bronze were evaluated by the thin-film RDE method in 0.1 M HClO4 at 30 ºC. Then, the electrode was subjected with potential cycling (0.05-1.2 V, 500 mV s-1, 100 cycles) before another electrochemical behavior measurement. The Co-Pt bronze powders were characterized as prepared and before and after electrochemical measurement by nitrogen sorption method, XRD, XPS, SEM etc. XRD measurement confirmed that the as-prepared powder after the heat treatment contained a bronze phase and a platinum metal phase and the latter was completely removed by the heated aqua regia treatment. The particle size of the obtained Co-Pt bronze powder was 20-80 nm from SEM observation and the specific surface area evaluated by BET analysis was 18.8 m2 g-1. The elemental composition was Co0.5Pt3O4 from EDX analysis. The ORR current for the Co-Pt bronze was low in the 0.6-1.0 V potential sweep (Figure 1a, before potential cycling). Cyclic voltammogram under anaerobic conditions showed no hydrogen sorption and platinum redox currents before the potential cycling but these currents gradually increased (Figure 1b) over the potential cycling. The ORR current of the Co-Pt bronze after potential cycling was comparable to that of Pt/Vulcan (Figure 1a, after potential cycling). While XRD measurement did not exhibit any change before and after the potential cycling, XPS spectrum indicated presence of metallic platinum after the potential cycling. These results clearly indicated that a thin metallic platinum phase was formed on the surface of the Co-Pt bronze by potential cycling and the metallic Pt was the main active site for ORR. Moreover, the Co-Pt bronze also exhibited an HOR activity due to the metallic site. Therefore, Co-Pt bronze can be used not only for the cathode but also for the anode. The durability of the Co-Pt bronze will be also discussed at the meeting. This work supported by NEDO, Japan.

Quantification of Active Sites at Spinel Type Metal Oxide Catalysts by Surface Interrogation Scanning Electrochemical Microscopy

Nanostructured spinel type metal oxides are promising catalysts for the oxygen reduction reaction (ORR) in fuel cells as well as for the oxygen evolution reaction (OER) in electrolysers under alkaline conditions [1]. For a more comprehensive understanding of the oxygen electrochemistry the enlightenment of the interaction between catalyst structure and occurring surface reaction will be a major step [2]. Therefore, new experimental approaches are needed to identify material patterns that determine the electrocatalytic activity. Especially, following the oxidation states of metal atoms as function of potential and time will provide new insights into the ORR and OER mechanisms. A versatile and powerful method in the assessment of electrocatalysts is scanning electrochemical microscopy (SECM). The spatial activity and selectivity of a catalytic material is immediately visualized by analyzing the current response of a movable probe electrode [3]. Recently, Rodríguez-López et al. [4] described a surface interrogation mode of SECM that allows the detection of adsorbates depending on time and substrate potential. This method is based on a transient feedback of a redox mediator with which a coulometric titration is performed. By integrating the current response at the tip electrode the quantification of active catalytic sites is accessible. In this study, we apply the surface interrogation mode to detect the amount of active sites at spinel type metal oxide catalyst under alkaline fuel cell conditions. Therefore, the catalyst powder is supplied in cavity-microelectrodes which were prepared by electrochemical etching of a metal wire [5]. Through the quantification of active sites and the examination of their reactivity new insights to the ORR of alkaline fuel cells are achieved. The results will be compared to a commercial carbon-supported platinum fuel cell catalyst. 1. X. Deng and H. Tüysüz, ACS Catal., 4, 3701 (2014). 2. I. Katsounaros, S. Cherevko, A. R. Zeradjanin and K. J. J. Mayrhofer, Angew. Chem. Int. Ed., 53, 102 (2014). 3. G. Wittstock, M. Burchardt, S. E. Pust, Y. Shen and C. Zhao, Angew. Chem. Int. Ed., 46, 1584 (2007). 4. J. Rodríguez-López, M. A. Alpuche-Avilés and A. J. Bard, J. Am. Chem. Soc., 130, 16985 (2008). 5. A. Minguzzi, C. Locatelli, O. Lugaresi, A. Vertova and S. Rondinini, Electrochim. Acta, 114, 637 (2013).

Durabilty of Pt-Based Alloy Nanoparticles Supported on Functionalized Carbon Materials for the ORR – Tuning the Interaction between Particles and Support Material

In the last decades, proton exchange membrane fuel cells (PEMFC) have attracted large attention as promising renewable and clean technology for energy conversion. Designing nanomaterials with the aim to improve their electrocatalytic performance and long-term durability is still a critical challenge. Today´s PEMFC research has focused on Pt-based alloy nanoparticles combined with high surface area carbon materials due to its improved activity and stability for the ORR compared to pure Pt/C.[1,2,3] Although many studies have focused on the relations between the structural and chemical properties of the catalytically active metal nanoparticles and their catalytic ORR performance, the interactions of these nanoparticles with the carbon substrate have been rarely investigated up to date. In particular, the immobilization and stabilization of the nanoparticles on the support material have a strong influence on the long-term durability of the ORR catalysts. Therefore, one crucial point is the functionalization of the carbon support with hydroxide and carboxylate groups which seems to improve nucleation and dispersion of the Pt alloy nanoparticles; on the other hand, these oxygen-containing surface groups can accelerate the carbon corrosion associated with particle detachment during the potential cycling. Therefore, another strategy is the introduction of hetero-atoms as N-C-, C-B, C-F and C-S-bonds into the carbon framework improving their electrochemical oxidation resistance.[4,5,6,7]One target of this work is the stabilization of the carbon support material by functionalization to prevent carbon corrosion under the operating fuel cell conditions. We modified the morphology, structure and chemical properties of the porous carbon materials by using different methods such as acid treatment or thermal treatment in a reactive gas atmosphere. On the other hand, we optimized the design of Pt-based alloy (Pt-Ni, Pt-Co) nanoparticles supported on these functionalized porous carbon support materials. The properties and behavior of these materials are monitored by using high resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX) and X-ray absorption spectroscopy (XAS). The electrochemical stability and durability of the catalysts were also investigated by applying various accelerated test protocols such as potential cycling between 0.5 - 1.0 (10.000 cycles), 0.5 - 1.5 (2000 cycles) and 1.0 - 1.5 V vs. RHE (2000 cycles). We monitored the electrochemically active surface area (ECSA) of the Pt and Pt alloy nanoparticles supported on these functionalized carbon materials during the runs to correlate its loss to the number of cycles, scan rate and potential range. Based on these results we evaluate the relationship between the carbon modification, size and chemical composition of the bimetallic nanoparticles, electrocatalytic properties and durability.Based on this study, we developed a model, describing the critical parameters to improve the interaction between the particles and the support material. This approach serves to enhance the catalytic properties and long-term durability of the ORR electrocatalysts for PEMFC applications.

Rhodium–Tin Binary Nanoparticle—A Strategy to Develop an Alternative Electrocatalyst for Oxygen Reduction

A Rh–Sn nanoparticle is achieved by combinatorial approaches for application as an active and stable electrocatalyst in the oxygen reduction reaction. Both metallic Rh and metallic Sn exhibit activities too low to be utilized for electrocatalytic reduction of oxygen. However, a clean and active Rh surface can be activated by incorporation of Sn into a Rh nanoparticle through the combined effects of lateral repulsion, bifunctional mechanism, and electronic modification. The corrosion-resistant property of Rh contributes to the construction of a stable catalyst that can be used under harsh fuel cell conditions. Based on both theoretical and experimental research, Rh–Sn nanoparticle designs with inexpensive materials can be a potential alternative catalyst in terms of the economic feasibility of commercialization and its facile and simple surfactant-free microwave-assisted synthesis.

V-doped TiO2 supported Pt as a promising oxygen reduction reaction catalyst: Synthesis, characterization and in-situ evaluation in proton exchange membrane fuel cell

This study deals with the synthesis and characterization of V-doped, TiO2 supported Pt catalyst (Pt/V-TiO2) for oxygen reduction reaction (ORR) and its in-situ performance investigation in proton exchange membrane (PEM) fuel cell. Pt/V-TiO2 nanocomposite catalyst is prepared via a facile sol-gel and microwave assisted, modified chemical reduction route and its performance is compared with the undoped TiO2 supported catalyst, Pt/TiO2 prepared in an identical way. The prepared Pt/V-TiO2 and Pt/TiO2 catalysts are employed as cathode catalyst in PEM fuel cell and compared with standard Pt/C catalyst. Their comparative studies are conducted with physical and electrochemical techniques. In-situ electrochemical characterization studies show improved ORR catalytic activity of Pt/V-TiO2 compared to Pt/TiO2. Furthermore, both Pt/TiO2 and Pt/V-TiO2 are more stable than Pt/C when subjected to 6000 voltammetric cycles in the range of 0.2–1.2 V vs. standard hydrogen electrode in operating fuel cell conditions, losing only <20% of its electrochemical surface area as compared to 50% loss exhibited by Pt/C. This study thus demonstrates Pt/V-TiO2 nanocomposite material as a potential cathode catalyst for PEM fuel cell with immense scope for further investigation.

Ex situ measurement and modelling of crack propagation in fuel cell membranes under mechanical fatigue loading

Fatigue-induced membrane fracture due to dynamic stresses is an important lifetime-limiting failure mode in automotive fuel cell applications. Here, a series of ex situ experiments are first conducted to measure the rate of crack growth in Nafion NRE211 membranes for a range of stress, temperature (23–70 °C), and relative humidity (50–90%) conditions relevant to automotive fuel cell operation. The crack growth rate is found to be ∼1–10 nm per load cycle and strongly depends on the stress intensity: the rate increases by an order of magnitude for a mere 10–30% increase in stress, which suggests that improved stress uniformity and avoidance of high stress points is important for durability. Moreover, the sensitivity to applied stress doubles from room conditions to fuel cell conditions, where the temperature has 2–3x stronger impact on the fracture propagation than the relative humidity. Microstructural analysis indicates that plastic deformation (60% localized thinning) at the crack tip accompanies crack growth. A semi-analytical model based on Paris law is then developed to simulate crack growth as a function of cyclic loading. The model incorporates elastic-viscoplastic mechanical behaviour of ionomer membranes and provides crack growth predictions in agreement with ex situ data up to 100% strain.

Comparative analysis of ceramic-carbonate nanocomposite fuel cells using composite GDC/NLC electrolyte with different perovskite structured cathode materials

A comparative analysis of perovskite structured cathode materials, La0.65Sr0.35MnO3 (LSM), La0.8Sr0.2CoO3 (LSC), La0.6Sr0.4FeO3 (LSF) and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF), was performed for a ceramic-carbonate nanocomposite fuel cell using composite electrolyte consisting of Gd0.1Ce0.9O1.95 (GDC) and a eutectic mixture of Na2CO3 and Li2CO3. The compatibility of these nanocomposite electrode powder materials was investigated under air, CO2 and air/CO2 atmospheres at 550 °C. Microscopy measurements together with energy dispersive X-ray spectroscopy (EDS) elementary analysis revealed few spots with higher counts of manganese relative to lanthanum and strontium under pure CO2 atmosphere. Furthermore, electrochemical impedance (EIS) analysis showed that LSC had the lowest resistance to oxygen reduction reaction (ORR) (14.12 Ω∙cm2) followed by LSF (15.23 Ω∙cm2), LSCF (19.38 Ω∙cm2) and LSM (>300 Ω∙cm2). In addition, low frequency EIS measurements (down to 50 μHz) revealed two additional semi-circles at frequencies around 1 Hz. These semicircles can yield additional information about electrochemical reactions in the device. Finally, a fuel cell was fabricated using GDC/NLC nanocomposite electrolyte and its composite with NiO and LSCF as anode and cathode, respectively. The cell produced an excellent power density of 1.06 W/cm2 at 550 °C under fuel cell conditions.

Degradation mechanism of La0.6Sr0.4Co0.2Fe0.8O3-δ/Gd0.1Ce0.9O2-δ composite electrode operated under solid oxide electrolysis and fuel cell conditions

A set of long-term tests (t≥1000h) have been carried out in fuel cell and electrolysis modes on typical Ni-YSZ//YSZ//LSCF-CGO cells. The degradation rates were found to be higher in electrolysis than in fuel cell operation. Post-test analyses have revealed that Sr diffusion and formation of SrZrO3 at YSZ/CGO interface occur mainly during electrolysis operation, whereas the process is very limited in fuel cell mode. As a consequence, LSCF destabilization is found to be not involved in the degradation of cell performances during fuel cell operation while it could explain the highest degradation rates recorded in electrolysis mode. An in-house multi-scale model has been used to interpret the role of the cell operating mode on the LSCF demixing mechanism. The simulations have shown that the electrolysis operation leads to a strong depletion of oxygen vacancies in LSCF material (while the fuel cell condition results in an increase in the concentration of oxygen vacancies). It has been proposed that the depletion in oxygen vacancies under electrolysis polarization could drive the Sr release from the structure, and in turn, could explain the experimental results. Based on this proposition, a possible mechanism for the LSCF destabilization and SrZrO3 formation is detailed.

(Invited) Electroreduction of Oxygen on Nitrogen-Doped Carbide-Derived Carbon Materials in Alkaline Media

The oxygen reduction reaction (ORR) is one of the most important electrochemical reactions, used in metal-air type batteries and different types of fuel cells. The traditionally used catalyst, platinum, is expensive and scarcely available and thus many new catalysts have emerged, among which are heteroatom-doped carbon materials. Various carbon nanostructures have been widely used as base materials for heteroatom doping due to their high electrical conductivity, porosity and stability in electrochemical conditions. Owing to their porous structure, reactants and products are easily transferred to and from the active sites. Among these materials, carbide-derived carbon (CDC), which is a carbon material produced by removing metal atoms from a carbide lattice, stands out as an exceptional way to gain a base material with tunable properties. The specific surface area of CDC materials is typically in the range of 1000–2000 m2 g−1, the pore size is easily tunable via selection of different starting carbides and synthesis temperature with reproducible results in large scales. Previously, we have studied ORR electrocatalysis on undoped CDC materials in alkaline media. These materials catalyzed a 2e‒ reduction of O2.1 In this work, we demonstrate a pyrolysis method to synthesize nitrogen-doped carbide-derived carbon catalysts using titanium carbide as a base material and dicyandiamide (DCDA) as a nitrogen source. The CDC materials are obtained using chlorination to remove the titanium and then pyrolyzed along with DCDA at 800 °C for 2 h in a tube furnace to substitute nitrogen into the carbon lattice.2 The physical characteristics of the catalyst materials were studied using N2 physisorption, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy, which revealed that the materials had been successfully nitrogen doped, had a very high specific surface area and were highly amorphous. SEM revealed highly porous materials consisting of particles of various sizes (Figure 1a). These catalysts were then tested using the rotating disk electrode (RDE) method in alkaline solution toward the ORR. The mediocre activity of the CDC materials was increased by the nitrogen-doping procedure, which further increased the already high specific surface area of the materials and produced nitrogen-containing groups on the surface of the catalysts. The catalysts produced via the nitrogen doping of carbide-derived carbon are superior to the catalyst synthesized by chlorinating titanium carbonitride, but still not as active as 20% Pt/C (Figure 1b). However, the process for CDC synthesis is already used commercially and adding nitrogen doping to it would be just one more step. The N-doped CDC catalysts also showed excellent stability over 1000 potential cycles, which is an important property for fuel cell applications. Overall, the nitrogen-doped CDC catalysts are promising cathode materials for alkaline membrane fuel cell as they are stable and highly active toward ORR. Work is in progress to test these materials in fuel cell conditions. References I. Kruusenberg, J. Leis, M. Arulepp, and K. Tammeveski, J. Solid State Electrochem., 14, 1269 (2010). S. Ratso, I. Kruusenberg, M. Käärik, M. Kook, R. Saar, M. Pärs, J. Leis, and K. Tammeveski, Carbon, 113, 159 (2017). Figure 1

Cu-Ni-Mo-P/C Electrocatalyst for the Co-Generation of Energy and Fine Chemicals from Glycerol Electrooxidation

CuNiMoP/C synthesized using a mixture of sodium hypophosphite and formaldehyde as reducing agents in a single electroless bath has been used as anode side catalysts for the oxidation of glycerol. The copper to nickel ratio in the electroless deposits was 9. The ratio of NaOH/glycerol charged to the anode side of an unstirred divided electrochemical reactor at the beginning of the electro-oxidation was 4. The cathode (platinum) was depolarized with oxygen during each oxidation. The anode and cathode compartments were separated by an anion exchange membrane. Using voltammetry and high pressure liquid chromatography, we show that changing the potential applied at a CuNiMoP/C anode affects both glycerol conversion and product yield/selectivity. The applied anode potentials studied ranged from 0.5 V - 1.3 V (versus Ag/AgCl in satd KCl). The major liquid products obtained were glyceraldehyde, dihydroxyacetone, oxalic acid, lactic acid and tartronic acid at room temperature. Glycerol conversions ranged from 20% to 30% for six to eight hour operations. Electrochemical characterization of the CuNiMoP/C gave a Tafel slope of 130 mV/decade, and an exchange current density of 1.8 mA/cm2. Electrochemical impedance spectroscopy was carried out between 25 0C and 80 0C to study the effect of temperature on impedances, since the higher temperature approximate actual liquid fuel cell conditions. Some preliminary results on the CuNiMoP/C stability while in use, determined from 72 hours uninterrupted use of the electro-catalysts for glycerol electro-oxidation, will also be presented. These results have important implications for portable applications with a direct alkaline glycerol fuel cell. Figure 1

A direct ascorbate fuel cell with an anion exchange membrane

Abstract Ascorbic Acid (Vitamin C) is investigated as a renewable alternative fuel for alkaline direct liquid fuel cells (DLFCs). The environmentally- and biologically-friendly compound, l -ascorbic acid (AA) has been modeled and studied experimentally under acidic fuel cell conditions. In this work, we demonstrate that ascorbic acid is a more efficient fuel in alkaline media than in acidic media. An operating direct ascorbate fuel cell is constructed with the combination of l -ascorbic acid and KOH as the anode fuel, air or oxygen as the oxidant, a polymer anion exchange membrane, metal or carbon black anode materials and metal cathode catalyst. Operation of the fuel cell at 60 °C using 1 M AA and 1 M KOH as the anode fuel and electrolyte, respectively, and oxygen gas at the cathode, produces a maximum power density of 73 mW cm-2, maximum current density of 497 mA cm−2 and an open circuit voltage of 0.90 V. This performance is significantly greater than that of an ascorbic acid fuel cell with a cation exchange membrane, and it is competitive with alkaline DLFCs fueled by alcohols.

Impact of the patterned membrane morphology on PEMFC performances of ultra-low platinum loaded MEAs

Two types of patterns were achieved on the surface of NR211 membranes: holes and knolls. These patterns were produced on only one side of the membrane at the cathode. For equivalent specific surfaces, these two membrane patterning morphologies were tested in fuel cell conditions with very low platinum loading (30 μg/cm2). Catalyst loading was achieved by magnetron plasma deposition either on the microporous carbon electrode or directly on the patterned membrane. The fuel cell performances in dry conditions were found to be highly dependent on the morphology of the membrane surface. Results showed that the knoll morphology gave better fuel cell performances than the hole morphology. For the knoll morphology the current density increased by a factor of 1.78 at 0.7 V versus a pristine membrane, whereas the hole morphology appeared in some cases to deteriorate the fuel cell performances, despite an increase in the specific surface by a factor of 1.87 versus a pristine membrane. The concepts of top level (extreme surface of the patterned membranes) and bottom level (the pattern base) were introduced to highlight the phenomena of micro water management on the areas of the micro-patterns which act on the local conductivity of the protonic membranes. These results suggest that it is necessary to choose the morphology of the patterns very carefully before simply increasing the specific surface of protonic membranes.

Progress in the morphology control and stability of shaped Pt alloy nanocrystal catalysts for fuel cells

With the increasing demand in new clean energy conversion and storage technologies in recent years, metal alloy nanocrystals have drawn considerable interests as high performance electrocatalysts for proton-exchange-membrane fuel cells (PEFMCs), lithium air batteries and (photo)electrochemical electrolyzers. In particular, PEMFCs are considered to be the ideal clean energy techniques for automobiles due to their high energy density and environment-friendly. Through continuous efforts from both academy and industry for decades, spring in the commercialization of PEMFCs is now coming marked by the first mass-produced hydrogen fuel cell car released by Toyota in 2014. Still, one of the most challenging tasks towards the commercialization of PEMFCs is to develop highly active, stable and low cost electrocatalysts for the sluggish oxygen reduction reaction (ORR) at the cathode, for which Pt is the best pure metal catalyst yet expensive for large scale applications. Alloying of Pt with non-noble 3D transition metal M (M=Fe, Co, Ni, etc.) are known to improve the ORR catalytic activity, while recent breakthrough results further demonstrat that certain Pt alloy facets particularly Pt3Ni {111} surface processes the potential to greatly increase the ORR activity by 90-fold compared to the state-of-the-art commercial Pt catalyst. Inspired by the exceptional ORR activity reported on the extended Pt-skin {111} surface, numerus studies have been devoted to the synthesis of shaped Pt alloy nanocrystals with {111}-oriented surfaces, e.g., nanooctahedra enclosed by six {111} surfaces. These particles are perceived to be the realistic “dream-ORR-catalyst” and are expected to bring in the 90-fold enhancement over the commercial Pt catalysts. Significant progress in the synthesis of shaped-controlled platinum alloy nanocrystals has been made in recent years, resulting in several families of highly active shaped Pt alloy electrocatalysts including nanooctahedra, icosahedra, nanoframe and nanocage catalysts. Meanwhile, their ORR activity improvements over commercial Pt catalyst have also increased greatly from 3–4 fold reported in 2010 to more than 30-fold reported recently. Despite these progresses, the long-term stability of the shaped Pt alloy nanocrystals particularly under realistic fuel cell condition is still the most challenging issue. In this review, we summarize recent progresses in the morphology control, catalytic activity and durability of shaped Pt-alloy nanocrystals for application in ORR electrocatlaysts in PEMFCs. We first introduce the syntheses of shaped Pt alloy nanocrystals such as octahedra, icosahedra, nanoframes and nanocages and their ORR activity improvement over commercial Pt catalyst. We then review recent efforts on a deeper understanding of the relationship among the element-specific growth, structure and catalytic stability of shaped Pt alloy catalysts. Emphasis will be placed on atomic insights into the structural evolutions of shaped Pt nanocrystals across their full life cycle (from growth to application in electrocatalysis) gained by advanced high-resolution scanning transmission electron microscopy combined with X-ray energy dispersive spectroscopy and electron energy loss spectroscopy. Finally, we summarize several approaches reported recently to improve the catalytic stability of the shaped Pt alloy nanocrystals, for instance, by rational design of tri-metallic shaped alloy nanocrystals and new nanostructures. These results are helpful for designing next-generation shaped Pt alloy nanocatalysts with both high activity and high durability in realistic PEFMCs and may also provide useful insights into metal alloy nanocatalysts for other catalytic reactions.

The European Young Chemists Award 2016

The future is in good hands! The European Young Chemist Award for 2016 was awarded at the 6th EuCheMS Chemistry Congress in Seville. Reflections on the participants and their chemistry from the Convener of the event, Professor Bruno Pignataro, are given in the Guest Editorial.

Control over fuel cell performance through modulation of pore accessibility: investigation and modeling of carbon nanotubes effects on oxygen reduction at N-graphene-based nanocomposite

The lack of performance of graphene-based electrocatalysts for oxygen reduction (ORR) is a major concern for fuel cells which can be mastered using nanocomposites. This work is highlighted by the optimization of nitrogen(N)-doped graphene/carbon nanotubes (CNTs) nanocomposite’s ORR performance examined by galvanostatic measurements in realistically approached glucose half-cells. Obtained results mark an essential step for the development of nanocarbon-based cathodes, as we specifically evaluate the electrode performance under real fuel cell conditions. The 2D simulations exclusively represent an important approach for understanding the catalytic efficiency of the nanocomposite with unique structure. The kinetics features extracted from simulations are consistent with the experimentally determined kinetics. The morphology analysis reveals a 3D porous structure. The results demonstrate that the incorporation of CNTs implements mesoscale channels for improved mass transport and leads to efficient 4-electron transfer and enhanced overall catalytic activity in pH-neutral media. The nanocomposite shows increased specific surface area of 142 m2 g−1, positively shifted ORR onset potential of 67 mV and higher open circuit potential of 268 mV versus Ag/AgCl compared to N-graphene (11 m2 g−1, −17, 220 mV). The findings are supported by 2D simulations giving qualitative evidence to the significant role of CNTs for achieving better accessibility of pores, i.e. enabling improved transfer of oxygen and OH−, and providing more reaction sites in the nanocomposite. The nanocomposite demonstrates better ORR performance than constituent components regarding potential application in miniaturized single-compartment glucose-based fuel cells.

Mechanistic Insights into the Oxygen Reduction Reaction on Metal–N–C Electrocatalysts under Fuel Cell Conditions

AbstractThree different transition metal‐C‐N catalysts are tested under a range of fuel cell conditions. It is found that common features of the polarisation curve can be explained by a change in electrocatalytic mechanism. Utilising a simple model to quantify the change in mechanisms, iR‐free results of the fuel cell experiments are fit and found to be represented by a common set of parameters. The change in mechanism is assumed to be a switch from four‐electron reduction of oxygen to water to a two‐electron reduction to hydrogen peroxide followed by disproportionation of the hydrogen peroxide to water and oxygen. The data is used to estimate a mass specific exchange current density towards the oxygen reduction reaction (ORR) to water in the range 10−11–10−13 A g−1 depending on the catalyst. For the reduction of oxygen to hydrogen peroxide, the mass specific exchange current density is estimated to be in the range 10−2–10−3 A g−1. Utilising the electrokinetic model, it is shown how the mass transport losses can be extracted from the polarisation curve. For all three catalyst layers studied, these mass transport losses reach about 100 mV at a current density of 1 A cm−2. Finally a discussion of the performance and site density requirements of the non‐precious metal catalysts are provided, and it is estimated that the activity towards the ORR needs to be increased by an order of magnitude, and the site density by two/three orders of magnitude to compete with platinum as an ORR electrocatalyst.

Enduring Anion-Exchange Membrane Fuel Cells

Alkaline anion exchange membrane fuel cells have become a topic of substantial interest in recent years, opening up a new electrochemical environment for hydrogen fuel cells. Facile kinetics for the oxygen reduction reaction open the promise of non-PGM or even non-metal AEMFCs, and radical stability. The most challenging aspect for the field, as defined by the 2016 DOE AMFC III Workshop, is membrane and ionomer stability in alkaline conditions at elevated operating temperatures. Few papers report endurance data, and best practices for in situ fuel cell conditioning and electrochemical characterization are still being developed by the community. Here, we report that HMT-PMBI exhibits membrane and ionomer stability in situ as AEMFCs in relevant conditions for device operation. These fuel cells demonstrate re-equilibration from extensive carbonation and complete re-conditioning in a shut-down / start-up cycle. We further report operation in typically challenging conditions, e.g. increased temperature and reduced humidity. Finally, we report on our attempts to define best-practices for electrochemical characterization.

Oxygen Electrocatalysis on Dealloyed Pt Nanocatalysts

We review the fundamental principles, the preparation and catalytic performance of dealloyed Pt core–shell electrocatalysts for the electroreduction of molecular oxygen. This reaction is key to the efficiency of all fuel cell cathodes, as the oxygen electrocatalysis exhibits much larger kinetic overpotentials compared to typical fuel cell anode reactions. We discuss structural surface lattice strain in metal overlayers and show that they serve as models for nanostructured core–shell catalysts. We address preparation pathways with particular emphasis on the dealloying routes. Trends in reactivity of different dealloyed Pt core–shell catalysts are compared with a focus on the dealloyed Pt–Ni alloy system. Size effects are discussed. Practical catalytic performance data in automotive fuel cells and under automotive fuel cell conditions is provided and contrasted to other state-of-art catalyst concepts. This review concludes that dealloyed Pt core–shell cathode catalysts are currently the most attractive commercialization candidate for automotive applications.

Size-Controlled Synthesis of Sub-10 nm PtNi3 Alloy Nanoparticles and their Unusual Volcano-Shaped Size Effect on ORR Electrocatalysis.

Dealloyed Pt bimetallic core-shell catalysts derived from low-Pt bimetallic alloy nanoparticles (e.g, PtNi3 ) have recently shown unprecedented activity and stability on the cathodic oxygen reduction reaction (ORR) under realistic fuel cell conditions and become today's catalyst of choice for commercialization of automobile fuel cells. A critical step toward this breakthrough is to control their particle size below a critical value (≈10 nm) to suppress nanoporosity formation and hence reduce significant base metal (e.g., Ni) leaching under the corrosive ORR condition. Fine size control of the sub-10 nm PtNi3 nanoparticles and understanding their size dependent ORR electrocatalysis are crucial to further improve their ORR activity and stability yet still remain unexplored. A robust synthetic approach is presented here for size-controlled PtNi3 nanoparticles between 3 and 10 nm while keeping a constant particle composition and their size-selected growth mechanism is studied comprehensively. This enables us to address their size-dependent ORR activities and stabilities for the first time. Contrary to the previously established monotonic increase of ORR specific activity and stability with increasing particle size on Pt and Pt-rich bimetallic nanoparticles, the Pt-poor PtNi3 nanoparticles exhibit an unusual "volcano-shaped" size dependence, showing the highest ORR activity and stability at the particle sizes between 6 and 8 nm due to their highest Ni retention during long-term catalyst aging. The results of this study provide important practical guidelines for the size selection of the low Pt bimetallic ORR electrocatalysts with further improved durably high activity.