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Summary: A three‐dimensional model was developed for an air‐breathing micro direct methanol fuel cell (μDMFC) with an extended anode catalyst region on the cell channels. The model was evaluated against experimental studies for a μDMFC under several anode distribution conditions, and the results showed a close agreement. The model was employed to study if catalysts coated on the fluid‐flow channel walls could enhance the power generation performance. Further, the effects of the anode catalyst loading of channel walls on the overall cell and individual electrode performances were examined. The modeling results indicated that the fuel cell with anodes both on the proton‐exchange membrane and on channel walls did not show superior performance to the fuel cell with anode catalysts only on the membrane since the overall power generation was mainly limited by the kinetics of the methanol electrode reaction but not the methanol transfer. The modeling results also demonstrated that increasing the anode catalyst loading on channel walls decreased the cathode potential due to an increase in the ohmic loss for the fuel cell with anode catalysts both on the membrane and on channel walls. Reducing channel dimensions decreased the ionic resistance and increased the methanol concentration at the anode and the methanol crossover flux to the cathode. [ABSTRACT FROM AUTHOR]

The design approach of catalysed flow field channel walls as an additional surface reaction area for the micro fuel cells was demonstrated in an air-breathing micro iron-oxygen redox fuel cell (μIOFC). The μIOFC with a graphite channel improved the maximum power density by 280% when compared to the μIOFC with inactive channel walls for the redox couple reaction. In the micro redox anode fuel cell with a graphite channel, the performance is limited by the mass transfer of the redox species but not the anode reaction rate. The proton transfer to the cathode is also a limiting factor determining the μIOFC performance, and the anode layer on the ion exchange membrane should be eliminated to reduce the resistance of the proton transfer [ABSTRACT FROM AUTHOR]

An economical and novel half-cell approach for quick and precise measurement of oxygen reduction catalysts in various practical electrode formats, including GDE and bonded GDE/membrane layers is demonstrated. Fuel cell current densities (~1 A/cm²) were achieved with the key design considerations clearly outlined, to highlight the challenges in developing such a test platform. Constant current polarizations with IR drop measurement at each step are found to provide a simple, reproducible method of measuring catalyst performance. An optimal electrolyte concentration of 1.0 M HClO4 balances the requirement of non-limiting H+ transport at high currents whilst minimizing the effect of electrolyte impurities, which reduce Pt activity. The half-cell used here can accurately provide the ORR activity of commercial products showing good agreement with measurements made in fuel cell hardware, but at a significantly lower testing cost. This half-cell approach can be used to characterize various types of GDEs and CCMs with different catalyst layers. [ABSTRACT FROM AUTHOR]

The high cost and limited supply of platinum for Pt-based catalysts in proton exchange membrane fuel cells (PEMFCs) have driven intensive research into the use of non-noble metal catalysts in recent years. As the most promising non-noble metal catalysts for PEMFC oxygen reduction reactions (ORR), metal/N/C class of catalysts has been extensively explored. Earlier efforts (1964–2004) were mainly focused on the exploration of various synthesis routes and the investigation of active site mechanisms. During recent years (2005–2010), great progress in the development of these types of non-noble metal catalysts in real PEMFC environments has been achieved both in terms of catalytic activity and stability. From 2011 to present, several new synthetic approaches have been explored to produce highly dense catalytically active sites decorated within micropores using rationally designed zeolite imidazolate frameworks (ZIFs) and porous organic polymers (POPs). Currently, the most active non-noble metal catalysts are derived using this method and are able to deliver a kinetic volumetric current density of 450 A/cm 3 at 0.8 V under fuel cell operating conditions. These results are superior to the US DOE 2020 target of 300 A/cm 3 . In terms of fuel cell maximum power density, the best non-noble metal catalysts for cathodes can achieve results as high as 0.98 W/cm 2 and 0.41 W/cm 2 with feeds of pure O 2 and air respectively. In terms of stability, some non-noble metal catalysts have remained stable for over 1000 h with only minor degradation under PEMFC conditions. Nonetheless, activity and stability still remain major challenges for non-noble metal catalysts when compared to Pt-based ones in PEMFCs. Improvements in the structure of both catalysts and catalyst layers are urgently needed to realize the activity targets established for automobile fuel cell applications as well as the US DOE Hydrogen and Fuel Cell (H&FC) program. In the long term and the sustainable commercialization of fuel cells, replacing Pt-based catalysts with non-noble metal catalysts is, in the present authors' opinion, the most sustainable solution. Therefore, further intensive research into fundamental studies is critical to uncovering the workings of active site mechanisms. Once controllable design and synthesize of non-noble metal catalysts with high active site densities and utilization can be achieved, the goal of cost-effective, non-noble metal catalysts in automobile fuel cells can become reality. [ABSTRACT FROM AUTHOR]

The microporous layer (MPL) is a key cathodic component in proton exchange membrane fuel cells owing to its beneficial influence on two-phase mass transfer. However, its performance is highly dependent on material properties such as morphology, porous structure, and electrical resistance. To improve water management and performance, electrochemically exfoliated graphene (EGN) microsheets are considered as an alternative to the conventional carbon black (CB) MPLs. The EGN-based MPLs decrease the kinetic overpotential and the Ohmic potential loss, whereas the addition of CB to form a composite EGN+CB MPL improves the mass-transport limiting current density drastically. This is reflected by increases of approximately 30 and 70 % in peak power densities at 100 % relative humidity (RH) compared with those for CB- and EGN-only MPLs, respectively. The composite EGN+CB MPL also retains the superior performance at a cathode RH of 20 %, whereas the CB MPL shows significant performance loss. [ABSTRACT FROM AUTHOR]

Water management remains a significant challenge for the Proton Exchange Membrane Fuel Cell (PEMFC) with respect to performance, lifetime and operational flexibility. In recent years, microporous layers (MPL) have been widely used on the cathode side of the PEMFC in order to improve fuel cell performance and water management capabilities. Many modeling and experimental studies have with limited success attempted to analyze the underlying mechanisms that are responsible for the performance improvement due to the MPL. In this study, porous inserts along with various in-situ experimental techniques are used to investigate the MPLs. It was observed that the anode pressure drop increased when a cathode MPL was present, indicating water cross-over from the cathode towards the anode side. Further testing identified that the MPL improved cell performance due to the reduction of water saturation in the cathode catalyst layer, which resulted in enhanced oxygen diffusion. The influence of the MPL on the anode side was also studied with the aid of porous inserts and other techniques, and it was observed that the anode MPL improves cell voltage stability and reduces water accumulation in the anode catalyst layer. The present investigation provides further important information on the critical role of the MPL in the PEMFC. [ABSTRACT FROM AUTHOR]

The industrial anthraquinone auto-oxidation process produces most of the world's supply of hydrogen peroxide. For applications that require small amounts of H2O2 or have economically difficult transportation means, an alternate, on-site H2O2 production method is needed. Advanced drinking water purification technologies use neutral-pH H2O2 in combination with UV treatment to reach the desired water purity targets. To produce neutral H2O2 on-site and on-demand for drinking water purification, the electroreduction of oxygen at the cathode of a proton exchange membrane (PEM) fuel cell operated in either electrolysis (power consuming) or fuel cell (power generating) mode could be a possible solution. The work presented here focuses on the H2/O2 fuel cell mode to produce H2O2. The fuel cell reactor is operated with a continuous flow of carrier water through the cathode to remove the product H2O2. The impact of the cobalt-carbon composite cathode catalyst loading, Teflon content in the cathode gas diffusion layer, and cathode carrier water flowrate on the production of H2O2 are examined. H2O2 production rates of up to 200 μmol h−1 cmgeometric −2 are achieved using a continuous flow of carrier water operating at 30 % current efficiency. Operation times of more than 24 h have shown consistent H2O2 and power production, with no degradation of the cobalt catalyst. [ABSTRACT FROM AUTHOR]

Due to improved kinetics and reduced CO poisoning effects, higher temperature alcohol fuel cells have been shown to have higher activity and increased performance output when compared to lower temperature alcohol fuel cells. In this work, the Direct Alcohol Phosphoric Acid Fuel Cell (DAPAFC) with methanol and ethanol as reactant fuels, and Pt and PtRu as catalysts is studied. The electrolyte/separator consists of Phosphoric Acid in a Silicon Carbide (SiC) matrix and replaces conventional polymer electrolyte or PBI membranes. Comprehensive studies are conducted to demonstrate the performance effects of the Gas Diffusion Layer (GDL), Micro-porous Layer (MPL) and such parameters as the stream pressures, higher temperature operation (120-180°C), and the thickness of the SiC separator, etc. Results show that at the same operating conditions, the cell performance is comparable to that of a vapour fed Nafion membrane based fuel cell. Structure improvement of the Phosphoric Acid Electrode Assembly (PAEA) and reactant vapor composition can significantly improve durability. This type of fuel cell demonstrates performance improvement and stability in a higher temperature range than is possible with the conventional PEMFC. [ABSTRACT FROM AUTHOR]

Abstract: Five different compositions of brownmillerite materials, Ce- and La-doped, and undoped Ba2In2O5 were synthesized via the solid-state reaction and the glycine-nitrate combustion process. Properties of the materials were characterized using XRD, HR-TEM, and TGA/DSC techniques. Conductivity measurements by AC impedance spectroscopy, performed in air and hydrogen-containing atmospheres, in the temperature range from 100°C to 500°C, were correlated to the properties of the materials and revealed the effects of dopant, microstructure, temperature and atmosphere on the total electrical conductivity of the materials. All compositions showed low conductivity in air. In hydrogen-containing atmospheres, while Ce- and La-doped Ba2In2O5 showed low conductivity (4×10−6–3×10−6 Scm−1 at 500°C), undoped Ba2In2O5 demonstrated a surprisingly high conductivity (between 0.02Scm−1 and 0.7Scm−1 in the temperature range from 300°C to 500°C), especially samples produced by the glycine-nitrate process. This finding creates potential opportunity for using Ba2In2O5 as a proton-conductive material in intermediate temperature fuel cells. [Copyright &y& Elsevier]

A one-dimensional model has been developed to aid in the rapid screening of three-dimensional (3D) anode configurations for a direct methanol fuel cell. The model was fitted to experimental data for a single electrode structure having 1, 2, or 4 mg cm-2, and the resulting parameters were used in the model to qualitatively predict the performance, current distribution, and methanol concentration distribution of a multilayered anode structure having one, two, or four layers and a total catalyst loading of 4.0 mg cm-2 carbon-supported (Vulcan XC-72) 40 wt % Pt-Ru. This model can be used to provide insight into the conditions for which crossover can be completely eliminated. [ABSTRACT FROM AUTHOR]

dvancements in the performance of the direct methanol redox fuel cell (DMRFC) were made through anolyte/catholyte composition and cell temperature studies. Catholytes prepared with different iron salts were considered for use in the DMRFC in order to improve the catholyte charge density (i.e., iron salt solubility) and fuel cell performance. Following an initial screening of different iron salts, catholytes prepared with FeNH(SO), Fe(ClO) or Fe(NO) were selected and evaluated using electrolyte conductivity measurements, cyclic voltammetry and fuel cell testing. Solubility limits at 25 °C were observed to be much higher for the Fe(ClO) (>2.5 M) and Fe(NO) (>3 M) salts than FeNH(SO) (~1 M). The Fe(ClO) catholyte was identified as a suitable candidate due to its high electrochemical activity, electrochemical reversibility, observed half-cell potential (0.83 V vs. SHE at 90 °C) and solubility. DMRFC testing at 90 °C demonstrated a substantial improvement in the non-optimized power density for the perchlorate system (79 mW cm) relative to that obtained for the sulfate system (25 mW cm). Separate fuel cell tests showed that increasing the cell temperature to 90 °C and increasing the methanol concentration in the anolyte to 16.7 M (i.e., equimolar HO/CHOH) yield significant DMRFC performance improvements. Stable DMRFC performance was demonstrated in short-term durability tests. [ABSTRACT FROM AUTHOR]

Abstract: The control of a direct methanol fuel cell (DMFC) operating under variable power conditions is important in the development of a commercially applicable device. Fuel cells are conventionally designed for a maximum power output. However variable load cycles can result in fuel cell operation under sub-optimal conditions. In this paper, a simple method of power management using a physical guard is presented. The guard can be used on the anode or cathode electrode, in the membraneless gap or in any combination. This design selectively deactivates specific active regions of the electrode assembly and enables the DMFC to operate at a constant voltage and current density at different absolute power conditions. The guard also serves to control excessive crossover during shutdown and low power operation. [Copyright &y& Elsevier]

An approach to direct liquid fuel cells (DLFCs) has been investigated where the air cathode is replaced with a redox couple cathode. This different configuration, a direct liquid redox fuel cell (DLRFC), reduces the effect of fuel crossover, eliminates cathode flooding, allows the use of a carbon-based three-dimensional electrode for the cathode, and increases cell design flexibility. Furthermore, the use of a carbon-based cathode significantly reduces the fuel cell's total platinum group metal content. Contrary to conventional DLFCs, high fuel concentrations can be employed in this type of configuration without depolarizing the cathode because the carbon-based cathode is selective to the redox couple only. Cyclic voltammetry (CV), differential scanning calorimetry (DSC), conductivity measurements, and fuel cell tests were used to characterize the system. CV and DSC have confirmed the electrochernical stability of methanol and the redox couple (Fe2+/Fe3+) up to 70°C and in the potential window of 0-1.2 V vs a standard hydrogen electrode. DLRFCs at 70°C employing the Fe2+/Fe3+ redox couple at the cathode delivered peak power densities 5 and 3 times greater than DLFC equivalents (ambient pressure air) for acidic anolytes containing 16.7 M CH3OH and 18 M HCOOH, respectively. [ABSTRACT FROM AUTHOR]

Abstract: A significant amount of work has been done in the field of the transition metal (TM) macrocyclic-catalyzed oxygen reduction reaction (ORR), but not as a function of temperature and substitution in an acidic electrolyte to simulate the environment of an operating PEM fuel cell. The electrocatalytic activity of several iron-based phthalocyanine macrocycles for the ORR were evaluated using cyclic voltammetric (CV) and rotating disc electrode (RDE) techniques. The activities of these substituted iron phthalocyanines were compared to those of the unsubstituted species in the temperature range of 20–80°C. The activity was also evaluated in the presence of hydrogen peroxide, both a possible by-product of the ORR and potential de-activator of iron phthalocyanines. The kinetics and corresponding parameters such as overall ORR electron transfer numbers, reaction rate constants, Tafel slopes, electron transfer numbers in the rate-determining step, and electron transfer coefficients were all measured in the temperature range of 20–80°C. A mechanism for the different FePc-catalyzed ORR''s was suggested based on the experimental results. The effect of substitution and temperature on ORR kinetics was also studied in this paper. [Copyright &y& Elsevier]

Abstract: A conventional membrane electrode assembly (MEA) for a direct methanol fuel cell (DMFC) consists of a polymer electrolyte membrane (PEM) compressed between an anode and cathode electrode. Limitations with this conventional design include: cost, fuel crossover, membrane degradation or contamination, ohmic losses and reduced active triple phase boundary (TPB) sites for catalyst located away from the electrode/membrane interface. In this work, ex situ and in situ characterization of a novel electrode assembly based on a membraneless architecture and advanced 3D anodes was investigated. The approach was shown to be fuel independent and scaleable to a conventional bi-polar fuel cell arrangement. The membraneless configuration exhibits comparable performance to a conventional ambient (25°C, 1atm) air-breathing DMFC. However, it has the additional advantages of a simplified design, the elimination of the membrane (a significant component expense) and enhanced fuel and catalyst utilization through the extension of the active catalyst zone. [Copyright &y& Elsevier]

Abstract: Direct fuel cells (DFCs) are receiving increased interest for portable power applications. Cell and stack architecture is a vital technical issue for portable DFCs. The architecture of a DFC not only has to meet particular application requirements such as a compact size and easy handling, but also has to ensure desired performance, reliability and fabrication costs. In this paper, the most recent advances related to portable DFCs and their architecture are reviewed. The current status of system architecture, stack/unit cell architecture, flow-field designs and MEA morphology strategies along with analysis are surveyed. In addition, promising methods of passive fuel delivery are also presented. [Copyright &y& Elsevier]

A major challenge to full commercialization of the polymer electrolyte fuel cell (PEFC) is the cost of materials and performance. A focus for research and development is to reduce material costs while maintaining or improving performance under practical operating conditions. The challenge to reach optimal performance in commercial fuel cells requires optimization of electrochemical activity over the entire active area. This is necessary because in a practical fuel cell as reactants pass along the flow field channels between inlet and outlet the composition and other parameters will change as the reactant is consumed and products are formed. [Copyright &y& Elsevier]

Discusses the emergence of fuel cells as an electrochemical solution to global warming in the 21st century. Impact of air quality issues; Cost projections for manufacturing fuel cells; Future directions and challenges for fuel cells.

Abstract Cathode microporous layers (MPLs) play an important role in the performance of proton exchange membrane (PEM) fuel cells through two-phase flow and interfacial effects. In this study, alternative graphene-based MPLs are developed and characterized by ex-situ methods and fuel cell polarization experiments at 100% and 20% cathode relative humidity (RH). The studied MPLs include: graphene (GN), reduced graphene oxide (RGO), graphite (GR) and their composites with conventional carbon black (CB). GN and RGO promote interfacial contact and adhesion to the catalyst layer (CL). These MPLs show improved kinetic and ohmic polarization compared to CB, but limit oxygen gas transport at 100% RH due to excessive water retention. The addition of CB to create composite MPLs, however, mitigates water retention near the CL by capillary suction. Hence, maximum power densities improve by 68% and 22% compared to the original GN and RGO MPLs, and is also 28% higher for the GN + CB MPL than for the CB MPL alone. At 20% RH, the graphene-composite MPLs demonstrate remarkable performance preservation (1.4 mV h−1 voltage decrease for GN + CB vs. 464 mV h−1 for CB at 1000 mA cm−2). Through extensive physical and performance evaluations, this study provides a comprehensive understanding of how graphene materials behave in MPLs under varied humidity. Highlights • Investigated graphite, reduced graphene oxide and graphene based microporous layers. • Graphene layer promotes water retention at the catalyst layer interface. • This benefits low humidity operation, but causes flooding at high humidity. • Addition of CB increases water permeability, alleviating flooding for graphene. • Layer has superb overall performance incl. performance preservation at low humidity. [ABSTRACT FROM AUTHOR]

Abstract: In this paper the operational and architectural flexibilities of a membraneless direct liquid fuel cell were demonstrated under aerobic and anaerobic configurations at 60°C and 1atm. The aerobic membraneless direct methanol fuel cell (DMFC) was fed an anolyte solution of 1M CH3OH/0.5M H2SO4 and an air oxidant. The anaerobic membraneless direct methanol redox fuel cell (DMRFC) was fed an anolyte solution of 1M CH3OH/0.1M HClO4 and a catholyte solution of 2M Fe(ClO4)3 and 0.22M Fe(ClO4)2 oxidant. For both cases the membraneless architecture performed significantly better than for the conventional PEM architecture with Nafion® 117. The maximum power density for the membraneless and Nafion® 117 based DMFC was 52mW⋅cm−2 and 41mW⋅cm−2 respectively. The maximum power density for the membraneless and Nafion® 117 based DMRFC was 46mW⋅cm−2 and 34mW⋅cm−2 respectively. In addition, anaerobic operation using a Fe2+/Fe3+ catholyte gave similar performance to that for air as an oxidant. Both membraneless and anaerobic operation can result in significant cost reduction with improved operational flexibility. [Copyright &y& Elsevier]

Abstract: In the present work, instantaneous gas flow rates in each of two parallel channels of gas–liquid two-phase flow systems were investigated through measurements of the pressure drop across the entrance region. Liquid flow rates in two branches were pre-determined through liquid injection independently into each channel. Experiments were conducted in two different manners, i.e., the gas flow rate was varied in both ascending and descending paths. Flow hysteresis was observed in both gas flow rate distributions and the overall pressure drop of two-phase flow systems. Effects of liquid flow rates on gas flow distributions were examined experimentally. The presence of flow hysteresis was found to be associated with different flow patterns at different combinations of gas and liquid flow rates and flow instability conditions. A new and simple method was developed to predict gas flow distributions based on flow regime-specific pressure drop models for different experimental approaches and flow patterns. In particular, two different two-phase pressure drop models were used for slug flow and annular flow, separately. Good agreement was achieved between theoretical predictions and our experimental data. The developed new method can be potentially applied to predict gas flow distributions in parallel channels for fuel cells. [Copyright &y& Elsevier]

Abstract: Two-phase flow in horizontal parallel channels has been experimentally investigated under fuel cell related operating conditions. Pronounced hysteresis is observed in the pressure drop versus flow characteristic curve when starting from either flooded or dry conditions. When gas is introduced into channels initially filled with water (flooded initial condition), both gas and liquid tend to flow predominantly in one channel at low gas or liquid flow velocities. As the gas flow velocity increases, even distribution of gas and liquid flow in both channels is observed, accompanied with a sudden decrease in the pressure drop. On the other hand, even gas and liquid flow distribution between both channels is found at comparatively lower gas flow velocities when starting with dry-gas flow conditions with liquid introduced into channels filled with gas (stratified flow regime). The flow regimes of this system are visualized in plots of the pressure drop against gas and liquid flow velocities. However, this phenomenon tends to vanish at high gas and liquid flow velocities, suggesting that high gas and liquid flow velocities are required to ensure even flow distribution in parallel channels. The hysteresis points appear at the same level of the pressure drop, reflecting intrinsic characteristics of the parallel channels used in this study. These results have important implications for PEM fuel cell operational strategies. In order to avoid reactant mal-distribution in parallel flow channels in the flow field in the two-phase flow regime, fuel cells should be operated at sufficiently high gas flow velocities. [Copyright &y& Elsevier]

Abstract: This paper reviews the literature on the preparation aspect of non-noble electrocatalysts for PEM fuel cell reactions, especially focusing on cathode electrocatalyst preparation methods. Various effective synthesis methods for two kinds of promising catalysts such as transition metal chalcogenides, and heat-treated nitrogen containing complexes (macrocyclic complexes) are discussed. Though some remarkable progress has been made in catalyst preparation techniques, neither of these catalysts has reached the level of a Pt based catalyst in terms of catalytic activity, durability and chemical/electrochemical stability. In order to make non-noble electrocatalysts commercially feasible, cost-effective and innovative, catalyst synthesis methods are needed for new catalyst discovery and catalyst performance optimization. [Copyright &y& Elsevier]

Abstract: A model dealing with the anode catalyst contamination induced by fuel impurities has been developed. This model can be used to describe the transient and steady-state performance losses. Several characteristics such as performance loss, contamination transient time constant and recovery process have also been introduced into the model. The obtained equations can be used to simulate and estimate the chemical and electrochemical reaction rate constants, and make some prediction about the severity of the contamination and the performance recoverability. [Copyright &y& Elsevier]

The effects of electroless Pt layers (<80 μg Pt cm−2) in the membrane subsurface on hydrogen crossover, ORR kinetics, and overall fuel cell performance are closely investigated. The electroless Pt layer in the membrane subsurface reduces gas crossover for a 17 μm thick membrane in an inversely proportional relationship to the loading, yielding up to >65% reduction. High electroless Pt layer loading reduces proton conductivity and results in less optimal ORR kinetic performance. Given the relationship between kinetic performance, exchange current density, and proton concentration, the impact of electroless Pt layer loading on proton concentration is determined. This correlation sheds light on the relationship between proton concentration, proton conductivity, and Pt loading, and can be useful as a probe for proton concentration diagnostic examinations. Hydrogen fuel cell tests reveal that with an additional membrane Pt loading of <20 μg Pt cm−2, the performance of GDE-based MEAs at low humidity are improved in both polarization and long-term humidity cycling tests, while the performance under wet conditions is still nearly identical to the baseline. This suggests that the addition of electroless Pt in the membrane subsurface at such loadings enhances the operational flexibility of hydrogen fuel cells and reduces hydrogen crossover effects. • Deposition of ≤80 μg Pt cm−2 electroless Pt layer in the membrane subsurface. • Reduction of hydrogen crossover by >65% in hydrogen fuel cell. • Enhanced operational flexibility for H 2 /O 2 polymer electrolyte membrane fuel cell. • Delineation of electroless Pt layer role on proton conductivity and humidity level. [ABSTRACT FROM AUTHOR]

A novel rotating disk electrode (RDE) tip capable of characterizing catalyst coated substrates and compatible with commercial RDE systems is presented. The RDE tip, which uses a top-oriented Ti-based current collector, is demonstrated with Pt foil and commercial catalyst coated membranes (CCM). The specific activity of the platinum foil for oxygen reduction reaction is about 1.55 mA kinetic cm Pt −2 in 0.1 M HClO 4 at room temperature (20°C), which is in agreement with the measurements reported for polycrystalline Pt. The Ti-based current collector leads to a very small (≈ 5.6%) loss in the electrochemical surface area (ECSA). Hydrodynamic calculations and oxygen reduction measurements confirm that even at the fastest rotation speeds used in this type of measurement (i.e., 1600 rpm), the laminar flow regime (Re ≈ 56) is maintained, allowing one to apply classical RDE analysis. Repeatable and high quality cyclic voltammograms (CVs), oxygen reduction curves, and mass and specific activities for commercial CCMs were obtained in 0.1 M HClO 4 at room temperature (20°C). The activities determined from these experiments are in agreement with those of the H 2 /O 2 fuel cell measurements under equivalent operating conditions. Degradation of the CCM’s catalyst during a 30,000 potential cycling accelerated degradation test (ADT) is performed using a 0.4 mg Pt cm −2 commercial CCM and shows an ECSA loss of ≈ 0.0010 m 2 g Pt −1 cycle −1 and a mass activity loss of ≈ 1.8 × 10 −6 A mg Pt −1 cycle −1 . Details of the design, implementation, and results of this economical and novel approach for characterization of CCMs are presented and discussed. [ABSTRACT FROM AUTHOR]

Abstract: In the present study, gas–liquid two-phase flow in a parallel square minichannel system oriented horizontally and at an incline is studied under operating conditions relevant to fuel cell operations. Flow mal-distribution in parallel channels occurs at low gas and liquid flow rates. In general, high superficial gas velocities are required to ensure even flow distribution, and the minimum gas flow rates required to achieve even distribution depend on the liquid flow rates, channel orientation and experimental procedures. As the inclination angle is increased, a higher gas flow rate is required to ensure even gas–liquid flow distribution while flow channels inclined downward seems to help in improving the even flow distribution. The presence of flow hysteresis phenomena indicate that multiple flow distributions exist at the same given flow conditions when the gas flow rates are varied in ascending and descending manners. Flow mal-distribution and flow hysteresis are directly linked with flow stability. More specifically, the actual gas and liquid distribution in parallel channels is determined by the stability of mathematical solutions of mass and momentum balance equations and also the flow history. For the first time, the present work investigates flow distributions in fuel cell flow fields by accounting for two-phase flow conditions. In addition, a novel approach is introduced to ensure flow distributions and their stability through contour construction of isobars where unstable flow region can be identified, which can be used in the design of parallel channel flow fields, especially for fuel cells. [Copyright &y& Elsevier]

Abstract: A passive (air breathing) planar three-cell direct methanol fuel cell was designed, fabricated and tested. The design concepts for plates, current collectors, seals and membrane electrode assemblies are discussed in this paper. Testing of single cells and stacks with parallel and serial connections were carried out. The results show that a reliable areal power density of 8.6mWcm−2 can be achieved at ambient temperature with passive operation. Stacks with a serial connection of the single cells gave significantly higher performance than a parallel connection. It was also identified that high electrical resistance was the dominant factor in reducing performance. The major causes of the high resistance were the stainless steel hardware used and poor contact between the electrode and current collector. Future work will involve material and design studies of current collectors and seals to minimize the electrical resistance, and architectural design studies to effectively utilize the heat generated to raise operation temperature. [Copyright &y& Elsevier]

Abstract: Diagnosis of stack performance is of importance to proton exchange membrane (PEM) fuel cell research. This paper presents the diagnostic testing results of a 500W Ballard Mark V PEM fuel cell stack with an active area of 280cm2 by electrochemical impedance spectroscopy (EIS). The EIS was measured using a combination of a FuelCon test station, a TDI loadbank, and a Solartron 1260 Impedance/Gain-Phase Analyzer operating in the galvanostatic mode. The method described in this work can obtain the impedance spectra of fuel cells with a larger geometric surface area and power, which are normally difficult to measure due to the limitations on commercial load banks operating at high currents. By using this method, the effects of temperature, flow rate, and humidity on the stack impedance spectra were examined. The results of the electrochemical impedance analysis show that with increasing temperature, the charge transfer resistance decreases due to the slow oxygen reduction reaction (ORR) process at low temperature. If the stack is operated at a fixed air flow rate, a low frequency arc appears and grows with increasing current due to the shortage of air. The anode humidification cut-off does not affect the spectra compared to the cut-off for cathode humidification. [Copyright &y& Elsevier]

Abstract: There are several compelling technological and commercial reasons for operating H2/air PEM fuel cells at temperatures above 100°C. Rates of electrochemical kinetics are enhanced, water management and cooling is simplified, useful waste heat can be recovered, and lower quality reformed hydrogen may be used as the fuel. This review paper provides a concise review of high temperature PEM fuel cells (HT-PEMFCs) from the perspective of HT-specific materials, designs, and testing/diagnostics. The review describes the motivation for HT-PEMFC development, the technology gaps, and recent advances. HT-membrane development accounts for ∼90% of the published research in the field of HT-PEMFCs. Despite this, the status of membrane development for high temperature/low humidity operation is less than satisfactory. A weakness in the development of HT-PEMFC technology is the deficiency in HT-specific fuel cell architectures, test station designs, and testing protocols, and an understanding of the underlying fundamental principles behind these areas. The development of HT-specific PEMFC designs is of key importance that may help mitigate issues of membrane dehydration and MEA degradation. [Copyright &y& Elsevier]

Abstract: One of the predominant failure modes of polymer electrolyte membrane (PEM) fuel cells is the degradation of the PEM, especially when the fuel cell is used for transportation applications. Numerous studies have been carried out regarding this issue but many aspects of the degradation are not yet understood. This paper reviews the available literature regarding membrane degradation, and attempts to classify the degradation modes into three categories: mechanical, thermal and chemical/electrochemical. The factors that contribute to each mode are discussed, along with detailed mechanisms for some degradations. Some possible mitigation strategies are also explored. [Copyright &y& Elsevier]

Abstract: Liquid-fed direct methanol fuel cells (DMFCs) are one of the most promising candidates for portable power electronics and automotive applications due to their potentially high-energy density, simple storage, and distribution of the fuel. The concentration of methanol in the fuel circulation loop of a DMFC system is an important operating parameter, because it determines the electrical performance and efficiency of the system. The methanol concentration in the circulating fuel stream is usually measured continuously with a suitable sensor for the purpose of maintaining optimal power and efficiency in the DMFC system. Various methods of sensing methanol concentration have been proposed over the past decade. This paper reviews these methanol concentration sensors for DMFCs, which are generally classified into two groups: electrochemical and physical. The construction and operating principles of each sensor, as well as its advantages and disadvantages, are described. The sensorless methods for controlling the methanol concentration are introduced briefly. Finally, the perspective on the future of methanol concentration sensors is discussed. [Copyright &y& Elsevier]

Abstract: In this paper, more than 100 articles related to anode catalysts for the direct methanol fuel cell (DMFC) are reviewed, mainly focusing on the three most active areas: (1) progress in preparation methods of Pt–Ru catalysts with respect to activity improvement and utilization optimization; (2) preparation of novel carbon materials as catalyst supports to create a highly dispersed and stably supported catalysts; (3) exploration of new catalysts with a low noble metal content and non-noble metal elements through fast activity down-selection methods such as combinatorial methods. Suggested research and development (R&D) directions for new DMFC anode catalysis are also discussed. [Copyright &y& Elsevier]

This paper provides an overview of several operating conditions which can have a significant effect on the durability of polymer electrolyte fuel cells (PEFCs) and direct methanol fuel cells (DMFC), including: low reactant flows, high and low humidification levels, and high and low temperatures. The possible effects of these conditions, along with possible mitigating strategies, are discussed. Data from various tests are presented demonstrating lifetimes from 1000 h to greater than 13,000 h for various conditions and applications. [Copyright &y& Elsevier]