Pt, Au and Pd Based Binary and Ternary Catalysts for the Oxygen Reduction Reaction

Challenges and opportunities in modelling of proton exchange membrane fuel cells (PEMFC)

Carbon-supported metal single-atom catalysts (M-SACs) are promising oxygen reduction reaction (ORR) catalysts. Their ORR activity and selectivity are significantly affected by the heteroatoms that coordinate the central metal atoms. Previous reports found that oxygen-coordinated M-SACs promoted a 2e- ORR rather than the 4e- ORR that is more desirable for fuel cells. Herein, we report for the first time that oxygen-coordinated M-SACs are capable of promoting the 4e- ORR in acid media. We prepared a Cr(acac)-NC catalyst with the central Cr atom coordinated by two O atoms. The Cr(acac)-NC catalyst not only exhibits excellent ORR activity and stability in acid media, but also delivers high proton exchange membrane fuel cell (PEMFC) performance comparable to N-coordinated M-SACs. Density functional theory (DFT) calculations reveal that Cr-O2 moieties located on the zigzag edge of the carbon support are the ORR-active sites.

Today, the development of active and stable catalysts still represents a challenge to be overcome in the research field of low-temperature fuel cells.[...]

Green hydrogen offers a sustainable alternative to fossil fuels, with proton exchange membrane fuel cells (PEMFCs) and water electrolysers (PEMWEs) playing key roles in its production and use1,2. However, the oxygen reduction reaction (ORR) at the cathode of PEMFCs and the oxygen evolution reaction (OER) at the anode of PEMWEs face challenges due to the need for electrocatalysts that withstand harsh conditions3. Platinum-based catalysts for ORR and iridium-based catalysts for OER, while effective, are scarce and expensive, requiring strategies to reduce their usage without sacrificing performance. One approach to enhance their catalytic activity is alloying them with transition elements like copper and nickel, which optimizes reactant adsorption through global lattice strain4,5. However, the instability of such alloying elements under operating conditions is a challenge. In contrast, microstrain (or local lattice distortion) has emerged as effective method for boosting the intrinsic activity and stability of catalysts in electrocatalytic processes such as ORR6,7. The introduction of structural defects ‘by design’, such as stacking faults, twins, grain boundaries, or dislocation arrays, induces diverse catalytic site and local configurations, including highly active ones tailored for a given electrocatalytic reaction7.In this study, we explore surface-distorted platinum-nickel nanoparticles supported on carbon with smaller size than the original particles first reported by Chattot et al8. for ORR (Figure 1a). Testing these catalysts in different electrocatalytic set-ups (liquid cell and PEMFC single cells) has confirmed their desirable high electrocatalytic activities. The causes for such activity enhancement and sustainability were investigated using a broad portfolio of ex situ and operando characterization techniques. Notably, operando wide angle X-ray scattering (WAXS) allowed tracking of the microstructure (crystallite size, lattice strain and microstrain) of these catalysts during electrochemical operation. Additionally, the fate of the alloying species in the harsh electrochemical environment is investigated by electrochemical on-line inductively coupled plasma mass spectrometry (ICP-MS). Finally, the concept of microstrain is extended to OER on unsupported hollow iridium-copper aerogels originally proposed by Shi et al.9 (Figure 1b). Abe JO et al. Hydrogen energy, economy and storage: Review and recommendation. Int J Hydrogen Energy. 2019;44(29):15072-15086. Sapountzi FM et al. Electrocatalysts for the generation of hydrogen, oxygen and synthesis gas. Prog Energy Combust Sci. 2017;58:1-35. Feng Qet al. A review of proton exchange membrane water electrolysis on degradation mechanisms and mitigation strategies. J Power Sources. 2017;366:33-55. Luo M et al. Strain-controlled electrocatalysis on multimetallic nanomaterials. Nat Rev Mater. 2017;2. Yang X et al. Strain Engineering in Electrocatalysts: Fundamentals, Progress, and Perspectives. Adv Energy Mater. 2022;12(5). Dubau L et al. Defects do Catalysis: CO Monolayer Oxidation and Oxygen Reduction Reaction on Hollow PtNi/C Nanoparticles. ACS Catal. 2016;6(7):4673-4684. Chattot R et al. Surface distortion as a unifying concept and descriptor in oxygen reduction reaction electrocatalysis. Nat Mater. 2018;17(9):827-833. Chattot R et al. Beyond strain and ligand effects: Microstrain-induced enhancement of the oxygen reduction reaction kinetics on various PtNi/C nanostructures. ACS Catal. 2017;7(1):398-408. Shi Q et al. Nanovoid Incorporated IrxCu Metallic Aerogels for Oxygen Evolution Reaction Catalysis. ACS Energy Lett. 2018;3(9):2038-2044. Figure 1

Proton exchange membrane fuel cells (PEMFC) technology is already in commercialization phase for automotive applications. However, the biggest challenges for its successful mass commercialization are cost and durability. Current PEMFC technology can meet the lifetime targets but the cost to achieve it can be very high due to material’s cost and system level mitigation complexities. Understanding of the membrane electrode assemblies (MEA) degradations using relevant durability accelerated tests at early stages of development is essential. PEMFCs for automotive applications goes through thousands of cycles during normal operation of the vehicle over the expected life span of 10 years. These cycles include a combination of load or voltage cycles, start and stop cycles, humidity cycles and temperature cycles. Study have shown that start-stop cycling accounts for 44% of operation mode while load cycling and idling for 28% each (1). Under these cycles, fuel cells suffer from severe degradations of its main components; cathode, anode, membrane and bipolar plates. The PEMFC cathode can degrade during start-stop operation via carbon corrosion (2). Cathode degradation can be mitigated by using stable materials or by applying system mitigations. System mitigation by a voltage limiting control (VLC) can be one of the effective measures (3). The VLC can, however, lead to voltage spikes (high positive potentials) at the anode during fuel cell start-up and shut-down. Moreover, during fuel (H2) starvation conditions, the potential at the anode can give rise to high positive potentials leading to anode catalyst layer degradation. During idling operation mode, PEMFC is at or near open circuit conditions which is known to accelerate proton exchange membrane (PEM) decomposition especially under low humidification (4). Peroxy-radicals originating from hydrogen peroxide produced at the anode catalyst surface cause the PEM degradation (5). Pt band formation in the membrane is also known to promote peroxide formation leading to membrane decomposition (6-7). Several accelerated protocols are currently being used to evaluate the durability of PEMFC. At Nissan, our durability protocols are focused on understanding the material degradation under relevant automotive cycles and correlating single cell durability to the stack level. Start-stop accelerated protocol to evaluate the cathode support durability consist of potential cycling from 1.0 V to 1.5 V using triangular waveform for 1000 cycles with diagnostics conducted intermittently after certain number of cycles. Cathode catalyst durability is evaluated using load cycling accelerated protocol wherein the potential is cycled between 0.6 V to 0.95 V to simulate peak load and OCV/idle using a square wave profile with 3 second hold at each potential. The upper potential is known to have significant impact on catalyst degradation (8). Load cycling is performed for 10,000 cycles with diagnostics conducted intermittently. Anode degradation under cell reversal condition is evaluated using a protocol designed to simulate high voltage spikes at the anode. Cell reversal cycling is performed by applying a load of 0.2 A/cm2 for 1 sec while flowing N2 at the anode and air at the cathode while measuring the cell voltage under the applied load for a total of 200 cycles. Membrane durability is evaluated using humidity cycling (mechanical durability) and potential hold at open circuit (chemical durability). Membrane’s chemical durability is evaluated at 90 °C and 30% RH under hydrogen and oxygen by operating MEA at open circuit. The target of OCV hold test is 500 hrs while maintaining potential above 0.8 V. Mechanical durability of membrane is evaluated by cycling RH to simulate dry condition (0%) and wet condition (100%) for target of 20,000 cycles. All these accelerated tests are performed at ambient pressure and summarized in the Table 1 below. Acknowledgments: The author would like to gratefully acknowledge Nissan Technical Center North America Fuel Cell Research team members (Dr. Amod Kumar, Dr. Ramesh Yadav and Dr. Cenk Gumeci) and Nissan Motor Co. Ltd Japan Fuel Cell research team members (Dr. Atsushi Ohma, Dr. Yoshihisa Furuya) for technical discussions and data.

The effective utilization of clean energy and finding alternatives to fossil resources are highly important to ensure the sustainability of human society and are always among the major goals of both chemistry and material science research. Advanced electrochemical devices, such as fuel cells, water electrolysers and metal-air batteries, represent the most promising strategies for clean-energy utilization. In an electrochemical device, the redox reactions are spatially separated by a membrane, allowing direct extraction/transfer of electrons at an electrode-electrolyte interface, which leads to higher intrinsic energy conversion efficiencies, milder process conditions, easy product separation and excellent design features for coupling to renewable energy infrastructure. The performance of such electrochemical processes is fundamentally determined by the physicochemical properties of the electrochemical interfaces, encompassing both the electrocatalyst and the structure of the adjacent electrochemical double layer. Specifically, electrocatalysts play key roles in electrochemical reactions and often limit the performance of entire systems due to their insufficient activity, low durability or high cost. Ideally, the rate, efficiency, and selectivity of the above electrochemical reactions can be substantially improved by developing high-performance electrocatalyst. One of the central tasks for chemists and material scientists is to design and fabricate the high-efficient efficiency but low-cost electrocatalysts systems. The current promising electrochemical reactions mainly focus on the realization of the reversible conversion between chemical and electricity energy, e.g., the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and hydrogen evolution reaction (HER). Coupling of the above electrochemical reactions provide a solid foundation for various essential electrochemical devices, such as direct hydrogen fuel cells (HOR + ORR); electrolysers (OER + HER); rechargeable zinc (Zn)-air battery (ORR + OER). Therefore, this thesis aims to design and synthesize high-performance electrocatalysts for HER, ORR and OER based on earth-abundant materials with proper hierarchical 2D or 3D nanostructures. Combined with the advanced characterization techniques and density functional theory (DFT) calculations, the relationship between the electrochemical activity and active sites of these earth-abundant electrocatalysts were detailedly explored and confirmed. Furthermore, to emphasize the hierarchical 2D or 3D nanostructures, the actual performance of these electrocatalysts was all evaluated in practical devices including Zn-air battery and proton exchange membrane fuel cell (PEMFC), specifically as follows: (1) The vast majority of the reported HER electrocatalysts performs poorly under alkaline conditions due to the sluggish water dissociation kinetics. In the first work, a hybridization catalyst construction concept is presented to dramatically enhance the alkaline HER activities of catalysts based on 2D transition metal dichalcogenides (TMDs) (MoS2 and WS2). A series of ultrathin 2D-hybrids are synthesized via facile controllable growth of 3d metal (Ni, Co, Fe, Mn) hydroxides on the monolayer 2D-TMD nanosheets. The resultant Ni(OH)2 and Co(OH)2 hybridized ultrathin MoS2 and WS2 nanosheet catalysts exhibit significantly enhanced alkaline HER activity and stability compared to their bare counterparts. The combined theoretical and experimental studies confirm that the formation of the heterostructured boundaries by suitable hybridization of the TMD and 3d metal hydroxides is responsible for the improved alkaline HER activities because of the enhanced water dissociation step and lowers the corresponding kinetic energy barrier by the hybridized 3d metal hydroxides. (2) Nitrogen-coordinated iron atoms on carbon matrix (Fe-N-C) materials are the most active Pt-group-metal-free ORR catalysts but still suffering their low stability and relatively lower activity compared to platinum-based materials. In the second work, Fe and Ni dual sites atomically dispersed in hierarchically ordered macroporous carbon support (Fe-Ni/N-HOMC) was designed and successfully prepared. Isolated atomic Fe- N4 and Ni-N4 active sites were confirmed via various characterizations. The ORR activity and stability of Fe-Ni/N-HOMC in both acid and alkaline electrolyte were much higher than commercial Pt/C and the mono-Fe doping counterpart, which was among the state-of-the-art ORR electrocatalysts. In addition, this 3D ordered interconnected macroporous structure with abundant mesopores and micropores could greatly increase the accessible ORR active site and also enhance the mass transport during the ORR process. When employed as cathodes for PEMFC, we found the excellent ORR activity of Fe-Ni/N-HOMC was completely translated to the cathode in the fuel cell. (3) High-performance bifunctional electrocatalysts with ORR and OER activity is the key to developing efficient rechargeable Zn-air batteries. In the third work, a high-performance bifunctional electrocatalysts for both OER and ORR were synthesized via further hybridizing as-prepared Fe-Ni/N-HOMC with NiFe layer double hydroxides (LDHs). Layered double hydroxides (LDHs) have been reported to be promising OER electrocatalysts with ultrahigh OER performances. The as-synthesized new composites exhibited almost the same ORR activity as Fe-Ni/N-HOMC, revealing that hybridization of NiFe-LDHs would not deteriorate the initial ORR activity. Moreover, the remarkable enhancement of OER activity was observed after the hybridization, which was attributed to the strong coupling of uniformly dispersed small NiFe-LDH nanoparticles with the carbon substrate. The prototype Zn-air battery was assembled using these new composites, which displayed the ultralow voltage gap and long-term stability. (4) Compared with Fe-N-C or Co-N-C based ORR electrocatalysts, the Cu-nitrogen-carbon composites were attracted little attention. However, the natural multicopper oxidases (MCOs) enzymes, such as laccase, can serve as efficient ORR catalyst with almost no overpotential. Inspired by their tris-copper centers in MCO, one novel Cu-nitrogen-carbon composite (Cu SAs/N-CS) with atomic Cu coordination sites were synthesized via the pyrolysis of the Cu-involved metal-organic-framework. The copper contents in Cu SAs/N-CS reaches as high as 3.17 wt.%, and the average distances of adjacent copper sites was around only 3.1 Å. Due to the synergetic effect of abundant single atomic copper active sites with closer distance and ultrathin carbon nanosheet structure, Cu SAs/N-CS exhibited superior ORR activity exceeding commercial Pt/C catalyst, methanol tolerance, and long-term stability in both alkaline and neutral electrolyte. In summary, four kinds of new composites were successfully designed and prepared as high-performance electrocatalysts for HER, ORR and OER. Multi-dimensional heterostructures, atomic metal coordination sites and 3D hierarchically porous structure were designed and observed, which contributed greatly to improve activities of these composites. This thesis suggests several new viewpoints in the design of electrocatalysts based on earth-abundant materials: (i) offering new strategies for the preparation of novel 2D and 3D heterostructures as electrocatalysts; (ii) expanding methods for the synthesis of atomic metal coordination sites and evaluating their activities for ORR; (iii) evaluating the practical performances of achieved electrocatalysts in proton exchange membrane fuel cell and Zn-air battery; (iv) attempting to explain reaction mechanisms of some electrocatalysts by DFT calculation.

In the recent years, by increasing the consumption of fossil fuels, the carbon dioxide level increases, which affects human health and climate change. One of the important technologies to solve these problems is fuel cell vehicles (FCVs), and, due to low operating temperature and high efficiency, the proton exchange membrane fuel cells (PEMFCs) have been more considered among various types of FCVs. Computational fluid dynamics (CFD) and numerical modeling have a great potential to study the detailed physical phenomena in the fuel cells such as mass, heat and energy transport, electrode kinetics, and potential fields and can reveal which factors in a real system are most important from a sensitivity point of view We reviewed several existing works relevant to CFD simulation and mathematical modeling of PEMFCs, which were performed by different software and codes like ANSYS Fluent, COMSOL, lattice Boltzmann, MATLAB codes, and OpenFOAM. The key factors that affect performance and failure modes of PEMFCs were discussed based on these simulations. One of these factors is water management that can be improved by using appropriate flow field configuration. The various configurations were reviewed. In addition, the effect of humidification of inlet reactants on the performance of PEMFCs was discussed.

Imaging PEMFC performance heterogeneity by sensing external magnetic field

This paper is the outcome of a small scale fuel cell project. Fuel cell is an electrochemical device that converts energy from chemical reaction to electrical work. Proton Exchange Membrane Fuel Cell (PEMFC) is one of the different types of fuel cell, which is more efficient, having low operational temperature and fast start up capability results in high energy density. In this study, a mathematical model of 1.2 W PEMFC is developed and simulated using MATLAB software. This model describes the PEMFC behaviour under steady-state condition. This mathematical modeling of PEMFC determines the polarization curve, power generated, and the efficiency of the fuel cell. Simulation results were validated by comparing with experimental results obtained from the test of a single PEMFC with a 3 V motor. The performance of experimental PEMFC is little lower compared to simulated PEMFC, however both results were found in good agreement. Experiments on hydrogen flow rate also been conducted to obtain the amount of hydrogen consumed to produce electrical work on PEMFC.

Due to its chemical stability in acidic environments and high ionic conductivity, Nafion has proven to be an essential material in proton exchange membrane fuel cell (PEMFC) designs. Nafion functions as both a membrane – separating the two electrodes – and as a conductive binder in each catalyst layer (CL). Nano-thin films of Nafion ionomer allow protons to reach carbon-supported Pt catalysts. In addition to protons, oxygen and water also move through the nano-thin Nafion ionomer to and from reactive Pt sites. These physical transport processes directly impact PEMFC performance. Therefore, measuring and determining ways to improve upon each of these processes is critical in the development of high performance PEMFCs.In regards to thick Nafion membranes, conductivity and diffusion measurements can be readily performed in most laboratories. Nano-thin Nafion films on the other hand prove more challenging to study due to experimental limitations. Nevertheless, neutron reflectometry (NR) has been reported in literature as a capable technique to provide simultaneous quantitative structural and composition profiles for thin-film Nafion [1-5]. Results for Nafion at native silicon oxide interfaces show complex multi-layered structures forming distinct water-rich and water-poor regions [1-3]. Despite these advancements in our understanding of thin-film Nafion, it is still unclear how these observed structures may impact PEMFC performance. This is because silicon does not interface with Nafion in PEMFC CLs. Instead, carbon and Pt are the primary and secondary materials that interface with the thin-film ionomer. In the latter case, thin-film Nafion at Pt interfaces has been briefly studied in literature and shows significantly less structure than Nafion at silicon oxide interfaces [4,5]. Beyond the limited number of studies for Pt-Nafion interfaces, carbon-Nafion interface studies are even more scarce in literature [5]. This is likely because carbon black (i.e. the electron conductor in PEMFC CLs) is too rough to be used with NR. Therefore, to confidently assume the structure of Nafion at carbon black interfaces, trends from NR work on other carbon-based samples need to be compiled and compared, while considering their bonding structures and surface chemistries.In this work, we extend the understanding of thin-film Nafion at carbon interfaces by testing four different carbon-based substrates. Using thin-film Nafion (< 100 nm) deposited on each carbon sample, NR was performed in both dry and humidified environments. Fitting the data to these experiments, results suggest a variety of polymer structures at the carbon-Nafion interface. These include complex multi-layered structures, similar to those at silicon oxide interfaces, as well as simple homogenous films with little-to-no interfacial structure. During this presentation, the implications of these results on species transport in PEMFC CLs will be discussed. Additionally, PEMFC performance predictions that incorporate learnings from this study will be made using an extended model from our earlier work [6].[1] S.C. DeCaluwe, A.M. Baker, P. Bhargava, J.E. Fischer, and J.A. Dura, “Structure-property Relationships at Nafion Thin-film Interfaces: Thickness Effects on Hydration and Anisotropic Ion Transport,” Nano Energy, vol. 46, pp. 91–100, 2018.[2] J.A. Dura, V.S. Murthi, M. Hartman, S.K. Satija, and C.F. Majkrzak, “Multilamellar Interface Structures in Nafion,” Macromolecules, vol. 42, no. 13, pp. 4769–4774, 2009.[3] U.N. Shrivastava, H. Fritzsche, and K. Karan, “Interfacial and Bulk Water in Ultrathin Films of Nafion, 3M PFSA, and 3M PFIA Ionomers on a Polycrystalline Platinum Surface,” Macromolecules, vol. 51, no. 23, pp. 9839–9849, 2018.[4] V.S. Murthi, J. Dura, S. Satija, and C. Majkrzak, “Water Uptake and Interfacial Structural Changes of Thin Film Nafion Membranes Measured by Neutron Reflectometry for PEM Fuel Cells,” ECS Transactions, vol. 16, no. 2, 2019.[5] D.L. Wood, J. Chlistunoff, J. Majewski, and R.L. Borup, “Nafion Structural Phenomena at Platinum and Carbon Interfaces,” Journal of the American Chemical Society, vol. 131, no. 50, pp. 18096–18104, 2009.[6] C.R. Randall and S.C. DeCaluwe, “Physically Based Modeling of PEMFC Cathode Catalyst Layers: Effective Microstructure and Ionomer Structure–Property Relationship Impacts,” Journal of Electrochemical Energy Conversion and Storage, vol. 17, no. 4, Jan. 2020.

Pt-based intermetallic compound catalysts for the oxygen reduction reaction: From problems to recent developments

We review our recent work on dealloyed nanoparticle electrocatalysts and address their synthesis, structural characterization and surface catalytic performance in low-temperature Polymer Electrolyte Membrane fuel cells (PEMFCs). The active form of the catalyst is obtained by voltammetric dealloying of non-noble metal rich Pt alloy precursors. In the dealloying process, the less noble precursor component, here Cu, is selectively removed from the surface of the precursor alloy particles and hence a Pt enriched particle shell is formed. Single fuel cell tests showed that, when used on the cathode of PEMFCs, dealloyed Pt catalysts show reactivities for the oxygen reduction reaction (ORR) which are up to 6 times higher than those of conventional pure Pt fuel cell catalysts. Similarly, the stability of dealloyed nanoparticle catalysts is superior to that of pure Pt particles. X-ray based structural and compositional studies suggested a core—shell particle structure as the active form of the catalyst consisting of a Pt enriched particle shell surrounding a Pt alloy core. At the present time, this catalyst system constitutes one of the most active fuel cell catalyst system reported in the literature. I. THE CONCEPT OF DEALLOYED NANOPARTICLE CATALYSTS Despite much recent focus on the development of advanced Li ion batteries for use as power source for short-range inner city transportation 255 Bereitgestellt von | Technische Universitat Berlin Angemeldet Heruntergeladen am | 14.04.15 16:12 Vol. 25. No. 4. 2009 Dealloyed Core-Shell Fuel Cell Eleclrocatalysts applications, low temperature fuel cells continue to be the solution of choice for medium and long range transportation technologies based on their gravimetric power density as well as the gravimetric energy density of commonly used fuels. Wider use of low-temperature fuel cell technology is hampered by performance, cost, and durability issues associated with materials and components of a single fuel cell. Figure 1 displays a cross section of the layered structure of a low temperature PEMFC showing the anode (left) and cathode (right) gas diffusion layers (GDLs), which sandwich the anode and cathode catalyst layers and the proton exchange membrane. Figure 1 also schematically shows the molecular as well as electrical pathways of hydrogen fuel molecules, oxygen molecules, protons as well as of electrons. The overall performance of a PEMFC in terms of its practical cell voltage is limited by kinetic, ohmic, and mass transport processes for low, medium and high current densities, respectively. Of these, the kinetic surface catalytic reactions cause the most severe fuel cell voltage losses. * Anode fl( / Cathode ·>' '..·· Electrical | O2+*H* + 4e -> 2 H2O Energy Fig. 1: Cross sectional SEM micrograph through a membrane electrode assembly sandwiched between gas diffusion layers. Reaction processes at anode and cathode, mass and charge flows are indicated (from ref) In particular, the electrocatalytic Oxygen Reduction Reaction (ORR) at the cathode according to O2 + 4H + +4e->2H 2 O E°=+1.23 V/RHE (1) represents the key challenge to improved PEMFCs. In acidic media, Pt catalysts offer the highest catalytic activities which made first unsupported, later high surface area carbon-supported Pt particles the ORR catalyst of 256 Bereitgestellt von | Technische Universitat Berlin Angemeldet Heruntergeladen am | 14.04.15 16:12 Peter Strasser Reviews in Chemical Engineering choice· . Figure 2 illustrates the associative (via O2H) as well as the dissociative (via O) pathways of reaction (1) from molecular oxygen to water. Key for the catalysis is the chemisorption energy of the adsorbed oxygenated intermediates, such as Pt-O2H, Pt-O, and Pt-OH. On pure Pt, atomic oxygen is bonded too strongly and requires considerable overpotentials to react to Pt-OH. As a result of this, Pt is covered by (hydr)oxide adsorbates near the equilibrium potential of reaction (1). There is a consensus within the fuel cell catalysis community that a moderate reduction of the Pt-O chemisorption energy would result in significant ORR activity increases. H2O2 Hydrogen peroxide 0=0 Oxygen adsorption Oxygenated OH intemediates

The energy industry is set to be revolutionized by polymer electrolyte membrane fuel cells (PEMFCs). Commercial PEMFC stationary units for industrial / home use are already on sale in Japan, and PEMFC vehicles will be commercialized in various countries in 2015. Platinum-decorated carbon black electrocatalyst powder is a crucial aspect of PEMFC technology. However, Pt is expensive, and limits PEMFC durability due to aggregation, dissolution, ripening, and carbon corrosion. Pt-free catalysts are therefore desirable for next-generation fuel cells. Non-precious, Pt-free catalysts for the electrochemical oxygen reduction reaction (ORR) in acid media have been the subject of intense research for the past few decades. One of the most popular materials in this field are pyrolysed mixtures of Fe/C/N-containing precursors, subjected to various heating, acid washing, and milling procedures. The mechanism for the ORR in these catalysts is not well understood, and is still debated. There are two main camps; those who see Fe as part of the catalytic center; and those who see Fe as a generator of active sites, whilst nitrogen would play the active role in the ORR. The history of this field is well-summarized in a recent review.1 Resolution of this debate is hampered by the complicated chemical structure in these materials; the possible combinations of Fe, N, and C atoms that could be ORR active are many. Therefore our approach is to simplify the system by removing Fe from the equation, and clarifying the fundamental catalytic activity of nitrogen-doped carbons. Initially we synthesized carbon nitride, and pyrolysed carbon nitrides supported on carbon black and carbon nanotubes. These materials had some inherent ORR activity, but Fe contamination was an issue,2,3,4 as with most materials approaches to this problem. We therefore developed synthesis of a truly metal-free nitrogen-doped graphene foam (GFN), which showed high inherent 4-electron oxygen reduction in acid medium.5,6 Here we synthesize GFN by combustion of nitrogen-containing sodium alkoxide, followed by washing, 1000˚C heat treatment in N2 and H2, and graphitization at 1400˚C. This is a 3D carbon with micron-scale pores encapsulated by thin defective graphene walls with a thickness of around 2 nm, with a surface area of > 700 m2/g and a nitrogen content of around 0.5 at.%. (Fig. 1). The material was confirmed to be Fe-free by ICP analysis. Linear sweep voltammograms (LSVs) of GFN (Fig. 2) show relatively high activity for an entrirely metal free catalyst (maxinum current density, -4 mA/cm2, mass activity (at 0.6 V), 2.79 A/gcat). The onset potential (at -10 µA/cm2) is around 0.85 V, similar to many Fe-containing catalysts. The electron transfer cooeficient (calculated from the ring electrode) is 3.6, incdicating majority 4-electron transfer. We conclude that Fe-free active sites may contribute significantly to the 4-electron ORR in Fe/C/N-based catalysts (although this does not mean that Fe-centers have no activity).Figure 1. (a) Schematic showing some PEMFC degradation mechanisms; (b) nitrogen-doped graphene schematic; (c) SEM image of GFN; (d) XPS N1s spectrum; linear sweep voltammograms for GFNin acid, at various rotation speed; (e) disc electrode, (f) ring electrode. The International Institute for Carbon-Neutral Energy Research (WPI-I2CNER) is supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan. 1. J-. P. Dodelet, Electrocatalysis in Fuel Cells, Springer (2013):271 2. Lyth, S. M., et al. J. Phys. Chem. C113.47 (2009): 20148 3. Lyth, S. M., et al. J. Electrochem. Soc. 158.2 (2011): B194 4. Lyth, S. M., et al. J. Nanosci. & Nanotech. 12.6 (2012): 4887 5. Lyth, S. M., et al. e-J. Surf. Sci. & Nanotech. 10 (2012): 296. Liu, J, et al. J. Electrochem. Soc. 161.4 (2014):F544-F550

Performance analysis of PEM fuel cell in mobile application under real traffic and environmental conditions

The regulation of reactants pressure and supply are prerequisite for the performance and prolong life of Proton Exchange Membrane Fuel Cell (PEMFC). The depleted supply of reactants may deteriorate the operation of PEMFC. Moreover, the large deviation of reactants pressure can cause severe damage to the stack. The main aim of this paper is to control the pressure of hydrogen at anode and oxygen at cathode side by maintaining hydrogen and oxygen flow rates at optimum levels during dynamic load conditions. Sliding Mode Controller (SMC) is employed to control the reactants pressure at set point under dynamic load change. The results reveal that SMC employed in PEMFC produce better response than the PI controller reported earlier.