Three steps in the anode reaction of the polymer electrolyte membrane fuel cell. Effect of CO

We studied the influence of CO poisoning of the anode in the polymer electrolyte membrane fuel cell (PEMFC) using electrochemical impedance spectroscopy (EIS). The anode impedance was found by first feeding neat hydrogen gas and next hydrogen with CO into one of the electrodes, keeping neat hydrogen gas on the other electrode as a reference. The electrodes were E-TEK Elat gas-diffusion electrodes with 0.5 mg Pt/cm², and the membrane was Nafion^® 117. The CO concentration was 103 ppm, and the total pressures were 1, 2.5 and 4 bar. Operating temperatures were kept constant, 30.0 °C or 50.0 ± 0.1 °C. Bias voltages of 0 and 0.05 V were used. Three steps were revealed in the reaction mechanism, the slow adsorption/diffusion step, the charge transfer step and the proton hydration step, confirming earlier results. Carbon monoxide affects the charge transfer step by blocking active sites and by affecting the surface polarisation. We further conclude that CO adsorbs to the porous carbon matrix, and reduces significantly the rate of surface diffusion of hydrogen to the surface.     

New strategy for reversal tolerant anode for automotive polymer electrolyte fuel cell

New approach for the reversal tolerant anode for polymer electrolyte membrane fuel cell is suggested by using the multifunctional IrRu alloy catalyst having concurrent superior activities towards hydrogen oxidation reaction and oxygen evolution reaction to mitigate the degradation of anode under the fuel starvation condition.     

Electrochemical pressure impedance spectroscopy for investigation of mass transfer in polymer electrolyte membrane fuel cells

Mass transfer phenomena in membrane fuel cells are complex and diversified because of the presence of complex transport pathways including porous media of very different pore sizes and possible formation of liquid water. Electrochemical impedance spectroscopy, although allowing valuable information on ohmic phenomena, charge transfer and mass transfer phenomena, may nevertheless appear insufficient below 1 Hz. Use of another variable, that is, back pressure, as an excitation variable for electrochemical pressure impedance spectroscopy is shown here a promising tool for investigations and diagnosis of fuel cells.     

Mechanical endurance of polymer electrolyte membrane and PEM fuel cell durability

The life of proton exchange membrane fuel cells (PEMFC) is currently limited by the mechanical endurance of polymer electrolyte membranes and membrane electrode assemblies (MEAs). In this paper, the authors report recent experimental and modeling work toward understanding the mechanisms of delayed mechanical failures of polymer electrolyte membranes and MEAs under relevant PEMFC operating conditions. Mechanical breach of membranes/MEAs in the form of pinholes and tears has been frequently observed after long‐term or accelerated testing of PEMFC cells/stacks. Catastrophic failure of cell/stack due to rapid gas crossover shortly follows the mechanical breach. Ex situ mechanical characterizations were performed on MEAs after being subjected to the accelerated chemical aging and relative humidity (RH) cycling tests. The results showed significant reduction of MEA ductility manifested as drastically reduced strain‐to‐failure of the chemically aged and RH‐cycled MEAs. Postmortem analysis revealed the formation and growth of mechanical defects such as cracks and crazing in the membranes and MEAs. A finite element model was used to estimate stress/strain states of an edge‐constrained MEA under rapid RH variations. Damage metrics for accelerated testing and life prediction of PEMFCs are discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2346–2357, 2006     

Water transport coefficient distribution through the membrane in a polymer electrolyte fuel cell

The net water transport coefficient through the membrane, defined as the ratio of the net water flux from the anode to cathode to the protonic flux, is used as a quantitative measure of water management in a polymer electrolyte fuel cell (PEFC). In this paper we report on experimental measurements of the net water transport coefficient distribution for the first time. This is accomplished by making simultaneous current and species distribution measurements along the flow channel of an instrumented PEFC via a multi-channel potentiostat and two micro gas chromatographs. The net water transport coefficient profile along the flow channels is then determined by a control-volume analysis under various anode and cathode inlet relative humidity (RH) at 80 °C and 2 atm. It is found that the local current density is dominated by the membrane hydration and that the gas RH has a large effect on water transport through the membrane. Very small or negative water transport coefficients are obtained, indicating strong water back diffusion through the 30 μm Gore-Select^® membrane used in this study.     

Pt and PtRh nanowire electrocatalysts for cyclohexane-fueled polymer electrolyte membrane fuel cell

Electrochemical dehydrogenative oxidation of cyclohexane to benzene is studied over Pt and Pt₁Rh₁ nanowire electrocatalysts fabricated by electrospinning method, which shows the higher catalytic activities in a polymer electrolyte membrane fuel cell anode than the conventional Pt nanoparticle catalysts such as carbon-supported Pt or Pt black. The improved performances over the Pt₁Rh₁ nanowire electrocatalyst can be rationalized by enhanced electrical property and pertinent interface formation with nanowire catalysts in the high Pt-loaded cyclohexane fuel cell system.     

Palladium-nickel alloys loaded on tungsten carbide as platinum-free anode electrocatalysts for polymer electrolyte membrane fuel cells

The strong interaction between PdNi alloys and WC makes PdNi/WC a novel Pt-free electrocatalyst for the anode hydrogen oxidation reaction of polymer electrolyte membrane fuel cells with activity and stability comparable to those of the conventional Pt/C catalysts.     

Assessment of transport-limited catalyst utilization for engineering of ultra-low Pt loading polymer electrolyte fuel cell anode

In this study, transport-limited catalyst utilization in polymer electrolyte fuel cell (PEFC) anodes is assessed via an agglomerate model with a broader view of designing ultra-low Pt loading, high performance anode. The model accounts for electrical and chemical potential-driven transport of electrons/protons and dissolved hydrogen, respectively and multi-component gas-phase transport in the catalyst layer. The model employs the kinetics of hydrogen oxidation reaction based on dual-pathway reversible reaction mechanism reported recently [J.X. Wang, T.E. springer, R.R. Adzic, J. Electrochem. Soc. 153 (2006) A1732]. The model predictions show that for conventional, randomly-structured catalyst transport limitations exist at two levels. At single-agglomerate level, the catalyst utilization is restricted by dissolved hydrogen diffusivity limiting the reaction to occur primarily in the outer shell of the agglomerate. At the catalyst layer level, the catalyst utilization is limited primarily by poor protonic conductivity. However, significant electronic potential gradients can exist in the catalyst layer thereby effectively reducing the available overpotential. Simulation results also show that by engineering the catalyst layer to overcome the transport limitations and, thereby, improving the effective catalyst utilization, high performance can be achieved in a PEFC anode at ultra-low Pt loading of 0.0225 mg/cm².     

Nuclear magnetic resonance of polymer electrolyte membrane fuel cells

In this review, the contribution of NMR spectroscopy to the development of the proton exchange membrane fuel cell (PEMFC) is discussed, with particular emphasis on its use in the characterization of structure and transport in proton exchange membranes (PEMs). Owing to copious amount of information available, results of the past decade will be the main focal point. In addition, its use as a screening tool for the PEM materials will be discussed. © 2010 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.201000010     

Fluorinated polyoxadiazole for high-temperature polymer electrolyte membrane fuel cells

For the first time a fluorinated polyoxadiazole doped with phosphoric acid as a proton-conducting membrane for operation at temperatures above 100 °C and low humidities for fuel cells has been reported. Fluorinated polyoxadiazole with remarkable chemical stability was synthesized. No changes in the molecular weight (about 200,000 g mol⁻¹) can be observed when the polymer is exposed for 19 days to mixtures of sulfuric acid and oleum. Protonated membranes with low doping level (0.34 mol of phosphoric acid per polyoxadiazole unit, 11.6 wt.% H₃PO₄) had proton conductivity at 120 °C and RH = 100% in the order of magnitude of 10⁻² S cm⁻¹. When experiments are conducted at lower external humidity, proton conductivity values drop an order of magnitude. However still a high value of proton conductivity (6 × 10⁻³ S cm⁻¹) was obtained at 150 °C and with relative humidity of 1%. In an effort to increase polymer doping, nanocomposite with sulfonated silica containing oligomeric fluorinated-based oxadiazole segments has also been prepared. With the addition of functionalized silica not only doping level but also water uptake increased. For the nanocomposite membranes prepared with the functionalized silica higher proton conductivity in all range of temperature up to 120 °C and RH = 100% (in the order of magnitude of 10⁻³ S cm⁻¹) was observed when compared to the plain membrane (in the order of magnitude of 10⁻⁵ S cm⁻¹).     

Electrochemical pressure impedance spectroscopy applied to the study of polymer electrolyte fuel cells

The development of novel in-situ diagnostic techniques allows new insight into the internal working of polymer electrolyte fuel cells (PEFCs) so that improved performance can be realised. Electrochemical impedance spectroscopy (EIS) is a widely used characterisation technique that takes advantage of the dynamic relationship between current and voltage to deconvolute critical mechanisms and sources of performance loss occurring with different time constants. Here, we apply electrochemical pressure impedance spectroscopy (EPIS) which examines the transfer function relating reactant gas pressure modulation to the electrical response of the fuel cell. A sinusoidally oscillating perturbation is applied to the cathode backpressure using a loudspeaker arrangement and the resulting voltage perturbation is monitored. It is shown that the technique can be used to separate the explicit effect of water management from reactant starvation when a PEFC is operated under different reactant humidification conditions.     

A novel reference electrode for application in alkaline polymer electrolyte membrane fuel cells

An in-situ fuel cell reference electrode (RE) consisting of a Pd-coated Pt wire exhibits stable potentials in alkaline polymer electrolyte membrane fuel cells (APEMFC). Results indicate that the overpotential and impedance of the anode are higher than those of the cathode in the cells tested (even at low currents); this is contrary to that found in proton-exchange membrane fuel cells (PEMFC) and it shows that caution is required when translating prior understating of PEMFC to APEMFC. The working hypothesis is that there is flooding-derived mass transport loss at the anode (where water is electro-generated).     

High performance nitrile copolymers for polymer electrolyte membrane fuel cells

This paper reports the fuel cells (DMFC and PEMFC) performance using sulfonated poly(arylene ether ether nitrile) (SPAEEN) copolymers containing sulfonic acid group arranged in structurally different ways. The membrane electrode assembly (MEA) fabricated from SPAEEN containing 60 mol% of angled naphthalenesulfonic acid group (m-SPAEEN-60) had superior performance over those derived from pendent naphthalenesulfonic acid group (p-SPAEEN) or sulfonated hydroquinone (HQ-SPAEEN) in H₂/air and/or DMFC conditions. For example, the current density of the MEA using m-SPAEEN-60 at 0.5 V and 2.0 M methanol was 250 mA/cm², whereas the current densities of the MEAs using p-SPAEEN-50 and HQ-SPAEEN-56 were 185 and 190 mA/cm², respectively. In addition, compared with the sulfonated polysulfone (BPSH-35) and Nafion membranes, the copolymer containing nitrile group showed the improved cell performance. For example, the power density of the MEA using m-SPAEEN-60 at 250 mA/cm² and 2.0 M methanol was 125 mW/cm², whereas the power densities of the MEAs using sulfonated polysulfone (BPSH-35) and Nafion were 115 and 113 mW/cm², respectively. m-SPAEEN-60 showed stable cell performance during extended operation (>100 h).     

Side chain crosslinking of aromatic polyethers for high temperature polymer electrolyte membrane fuel cell applications

Novel aromatic polymers bearing polar pyridine units in the main chain and side chain crosslinkable hydroxyl and propargyl groups have been successfully synthesized. The polymers have been investigated in terms of their critical properties related to their application in high temperature polymer electrolyte membrane fuel cells, such as doping ability, mechanical properties, and thermal stability. Crosslinked membranes were prepared by direct crosslinking of hydroxyl side chain groups with decafluorobiphenyl used for the first time as a crosslinking agent. However, further functionalization of hydroxyl groups to the propargyl derivative has also led to crosslinked polymers after thermal curing. Both types of crosslinked membranes exhibited higher glass transition temperatures as well as lower doping levels when doped in phosphoric acid compared with the non crosslinked analogs, confirming the formation of a successfully crosslinked network. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Dynamic behavior of water within a polymer electrolyte fuel cell membrane at low hydration levels

Protonic conduction across the membrane of a polymer electrolyte fuel cell is intimately related to the dynamic behavior of water present within the membrane. To further the understanding of water dynamics in these materials, quasielastic neutron scattering (QENS) has been used to investigate the picosecond dynamic behavior of water within a perfluorosulfonated ionomer (PFSI) membrane under increasing hydration levels from dry to saturation. Evaluation of the elastic incoherent structure factor (EISF) reveals an increase in the characteristic length-scale of confinement as the number of water molecules in the membrane increases, tending to an asymptotic value at saturation. The fraction of elastic incoherent scattering observed at high Q over all hydration levels is well fit by a simple model that assumes a single, nondiffusing hydronium ion per membrane sulfonic acid site. The quasielastic component of the fitted data indicates confined dynamic behavior for scattering vectors less than 0.7 A(-1). As such, the dynamic behavior was interpreted using continuous diffusion confined within a sphere at Q < 0.7 A(-1) and random unconstrained jump diffusion at Q > 0.7 A(-1). As the number of water molecules in the membrane increases, the characteristic residence times obtained from both models is reduced. The increased dynamical frequency is further reflected in the diffusion coefficients predicted by both models. Between low hydration (2 H2O/SO3H) and saturation (16 H2O/SO3H), the continuous spherical diffusion coefficient changes from 0.46 +/- 0.12 to 1.04 +/- 0.12 (10(-5) cm2/s) and jump diffusion indicates an increase from 1.21 +/- 0.03 to 2.14 +/- 0.08 (10(-5) cm2/s). Overall, the dynamic behavior of water has been quantified over different length scale regimes, the results of which may be rationalized on the basis of the formation of water clusters in the hydrophilic domain that expand toward an asymptotic upper limit with increased hydration.     

Fabrication and characterization of supported dual acidic ionic liquids for polymer electrolyte membrane fuel cell applications

In this study, we proposed an innovative and versatile method for preparation of highly stable and conductive supported ionic liquid (IL) membranes for proton exchange fuel cell applications. Novel covalently supported dual acidic IL membranes were prepared by radiation induced grafting of 4-vinyl pyridine (4-VP) onto poly(ethylene-co-tetrafluoroethylene) (ETFE) film followed by post-functionalization via sequential treatments with 1,4-butane sultone and sulfuric acid to introduce pyridinium alkyl sulfonate/hydrogen sulfate moieties. The advantage of our approach lies in grafting polymers with highly reactive functional groups suitable for efficient post-sulfonation. The membranes displayed better swelling and mechanical properties compared to Nafion 112 despite having more than 3 times higher ion exchange capacity (IEC). The proton conductivity reached superior values to Nafion above 80 °C. Particularly, the membrane with ion exchange capacity of 3.41 displayed a proton conductivity of 259 mScm⁻¹ at 95 °C. This desired conductivity value is attributed to the high IEC of the membranes as well as dissociation of the hydrophobic ETFE polymer and hydrophilic pyridinium alkyl sulfonate groups. Such appealing properties make the supported IL membranes promising for proton exchange membrane fuel cells (PEMFC).     

Effects of polymer electrolyte membrane's property on fuel cell performances

Platinum and/or metal‐oxide nanocrystals (d = 1 ‐ 2 nm) were highly dispersed in membranes such as a Nation® commercially available (denoted as Pt‐PEM or Pt‐oxide‐PEM) attempting to self‐humidify the PEMs and/or to suppress the short‐circuit reaction by a catalytic oxidation of the crossover hydrogen or methanol with oxygen on the Pt catalyst. High and stable performances under the suppressed crossover and lowered internal resistance are demonstrated at the H₂/O₂ fuel cells applied Pt‐PEM or Pt‐oxide‐PEM without any external humidification. An appreciable increase of the cathode potential due to the reduced methanol crossover was clearly demonstrated at a direct methanol fuel cell (DMFC) with Pt‐PEM. It also becomes clear that the development of new PEMs with lowered permeability against methanol is essential for DMFCs.     

Platinum dissolution and deposition in the polymer electrolyte membrane of a PEM fuel cell as studied by potential cycling

The behavior of platinum dissolution and deposition in the polymer electrolyte membrane of a membrane-electrode-assembly (MEA) for a proton-exchange membrane fuel cell (PEMFC) was studied using potential cycling experiment and high-resolution transmission electron microscopy (HRTEM). The electrochemically active surface area decreased depending on the cycle number and the upper potential limit. Platinum deposition was observed in the polymer electrolyte membrane near a cathode catalyst layer. Platinum deposition was accelerated by the presence of hydrogen transported through the membrane from an anode compartment. Platinum was transported across the membrane and deposited on the anode layer in the absence of hydrogen in the anode compartment. This deposition was also affected by the presence of oxygen in the cathode compartment.