Effects of self-humidification on the dynamic behavior of polymer electrolyte fuel cells

The dynamic behavior of polymer electrolyte fuel cells was investigated experimentally at sudden load change conditions. The present study mainly focused on the variation of membrane hydration due to self-humidification. Steady-state results for various temperatures and humidities were used as the basic data for the analysis of dynamic behavior. Electrochemical impedance spectroscopy (EIS) showed that the ohmic resistance was reduced with the increase of humidity and current while the total polarization resistance including the mass transfer effect showed various trends according to cell temperature. The dynamic behavior of the cells was measured with time. The current increment just after an abrupt voltage reduction jumped to a certain level and then increased gradually, showing a logarithmic-shape curve. The stabilization time to steady-state was determined by using the curve-fitted lines representing the variation of the current increment at each operating condition. The stabilization time showed various trends according to cell temperature, humidity, and voltage range.     

Performance and stability characteristics of MEAs with carbon-supported Pt and Pt1Ni1 nanoparticles as cathode catalysts in PEM fuel cell

Polarization curves of membrane electrode assemblies (MEAs) containing carbon-supported platinum (Pt/C) and platinum-nickel alloy (Pt₁Ni₁/C) as cathode catalysts were obtained for durability test as a function of time over 1100 h at constant current. Charge transfer resistance was measured using electrochemical impedance spectroscopy and postmortem analysis such as X-ray diffraction and high-resolution transmission electron microscopy was conducted in order to elucidate the degradation factors of each MEA. Our results demonstrate that the reduced performance of MEAs containing Pt₁Ni₁/C as a cathode catalyst was due to decreased oxygen reduction reaction caused by the corrosion of Ni, whereas that of MEAs containing Pt/C was because of reduced electrochemical surface area induced by increased Pt particle size.     

Electrochemical impedance analysis with transmission line model for accelerated carbon corrosion in polymer electrolyte membrane fuel cells

The effects of varying the applied voltage and relative humidity of feed gases in degradation tests of polymer electrolyte membrane fuel cells (PEMFCs) were analyzed using electrochemical impedance spectroscopy (EIS). A transmission line model that considers the proton-transport resistance in the cathode catalyst layer was used to analyze impedance spectra obtained from degraded PEMFCs. As the applied cell voltage was increased from 1.3 to 1.5 V to induce accelerated degradation, the cell performance decayed significantly due to increased charge- and proton-transfer resistance. The PEMFC degradation was more pronounce at higher relative humidity (RH), i.e. 100% RH, as compared with that observed under 50% RH. Furthermore, changes in the charge transfer resistance of the electrode accompanied changes in the ionic conductivity in the PEMFC catalyst layer. Although the initial ionic and charge-transfer resistances in the catalyst layer were lower under higher RH conditions, the impedance results indicated that the performance degradation was more significant at higher water contents in the electrode due to the consequential carbon corrosion, especially when higher voltages, i.e. 1.5 V, were applied to the PEMFC single cell.     

Nonlinear frequency response analysis of dehydration phenomena in polymer electrolyte membrane fuel cells

Dehydration phenomena in a PEM fuel cell were investigated by nonlinear frequency response analysis (NFRA) in a differential H₂/H₂ cell. The linear H_1,0 spectra, which are equal to classic EIS spectra, showed not only an increase of the membrane resistance but also an increase of the anode reaction resistance, caused by dehydration leading to the decrease of the protonic conductivity of the polymer network in the catalyst layer. With this, active sites with long protonic pathes to the membrane become inactive. In order to further clarify this effect, modelling work was used. Therefore, proton transport was incorporated into an existing model of a differential H₂/H₂ cell. Finally, the key features of NFRA spectra under dehydration and CO poisoning are compared in order to discuss the suitability of NFRA for unambiguous diagnosis of PEMFC. It can be seen that while the linear spectrum is not sufficient to distinguish between both cases, the second order frequency response functions can be used for discrimination.     

Catalytic performance of Pt film with dendritic structure for PEFC

A Pt catalyst film with dendritic microcrystalline structure has been prepared by reducing PtO₂ deposited by reactive sputtering. It is to be employed as the cathode catalyst of a polymer electrolyte fuel cell (PEFC). Despite using no support material, this dendritic Pt film exhibits a very low density of 3.3 g cm⁻³. When applying the dendritic Pt as the cathode catalyst layer for a single fuel cell, the dendritic Pt provided higher performance, larger electrochemical surface area (ECA) and improved diffusion characteristics compared to a conventional sputtered Pt film. The activity based on unit ECA of the dendritic Pt was higher than that of Pt/C.     

Polymer electrolytes for dye-sensitized solar cells prepared by photopolymerization of PEG-based oligomers

We report on the preparation and characterization of novel polymer electrolyte membranes for quasi-solid dye-sensitized solar cells. New methacrylic–acrylic gel-polymer electrolytes were prepared by photo-polymerization of mono/di-functional monomers. The crosslinked films were self standing, transparent and flexible. They were swelled by an iodine–iodide solution, obtaining a stable gel, where the polymeric network acts as a cage to retain the liquid, preventing its evaporation. Such a system combines the cohesive property of a solid with the high ionic conductivity of a liquid. The evaluation of the structural and physical-chemical characteristics of the polymer, combined with the electrical characterization of the membranes by means of the electrochemical impedance spectroscopy, allowed us to investigate the structure/property relationship of the material. The electric characterizations of the solar harvester based on the gel-polymer electrolyte showed a maximum photovoltaic conversion efficiency of 4.41%. Moreover, a significant improvement in the durability of the device was demonstrated with respect to the liquid electrolyte-based counterpart.     

Experimental observation of the effect of cleansing agents on the performance of polymer electrolyte fuel cells

Regular maintenance/cleaning of fuel pipeline and system hardware is an essential requirement in fuel cell operation to prevent contamination. An experimental and analytical study is performed to aid the selection of appropriate cleansers to be used as cleaning agents in polymer electrolyte fuel cell (PEFC). Screening tests for several cleansers are carried out during the injection of samples into the PEFC cathode inlet. One proper agent (naphtha) has shown a fully recoverable and minimal effect on the fuel cell performance and as such is determined as the best cleansing agent. Unlike other samples, naphtha does not contain any metallic components such as sodium or potassium in its composition. Furthermore, PEFC can still operate at ～0.4 V at constant current (1 A/cm²) even with a considerable flow rate (250 μl/min) of the selected cleanser. Detailed analytical analysis of this cleanser is provided by curve fitting the electrochemical impedance spectroscopy data, and evaluation of binary gas diffusion coefficients. It is indicated that performance loss during naphtha exposure is mainly due to the adsorption of contaminants on active Pt sites and an increase in mass transfer resistance.     

Carbon xerogels as Pt catalyst supports for polymer electrolyte membrane fuel-cell applications

Carbon xerogels prepared by the resorcinol-formaldehyde (RF) sol-gel method with ambient-pressure drying were explored as Pt catalyst supports for polymer electrolyte membrane (PEM) fuel cells. Carbon xerogel samples without Pt catalyst (CX) were characterized by the N₂ sorption method (BET, BJH, others), and carbon xerogel samples with supported Pt catalyst (Pt/CX) were characterized by thermogravimetry (TGA), powder X-ray diffraction (XRD), electron microscopy (SEM, TEM) and ex situ cyclic voltammetry for thin-film electrode samples supported on glassy carbon and studied in a sulfuric acid electrolyte. Experiments on Pt/CX were made in comparison with commercially obtained samples of Pt catalyst supported on a Vulcan XC-72R carbon black support (Pt/XC-72R). CX samples had high BET surface area with a relatively narrow pore size distribution with a peak pore size near 14 nm. Pt contents for both Pt/CX and Pt/XC-72R were near 20 wt % as determined by TGA. Pt catalyst particles on Pt/CX had a mean diameter near 3.3 nm, slightly larger than for Pt/XC-72R which was near 2.8 nm. Electrochemically active surface areas (ESA) for Pt as determined by ex situ CV measurements of H adsorption/desorption were similar for Pt/XC-72R and Pt/CX but those from CO stripping were slightly higher for Pt/XC-72R than for Pt/CX. Membrane-electrode assemblies (MEAs) were fabricated from both Pt/CX and Pt/XC-72R on Nafion 117 membranes using the decal transfer method, and MEA characteristics and single-cell performance were evaluated via in situ cyclic voltammetry, polarization curve, and current-interrupt and high-frequency impedance methods. In situ CV yielded ESA values for Pt/XC-72R MEAs that were similar to those obtained by ex situ CV in sulfuric acid, but those for Pt/CX MEAs were smaller (by 13-17%), suggesting that access of Nafion electrolyte to Pt particles in Pt/CX electrodes is diminished relative to that for Pt/XC-72R electrodes. Polarization curve analysis at low current density (0.9 V cell voltage) reveals slightly higher intrinsic catalyst activity for the Pt/CX catalyst which may reflect the fact that Pt particle size in these catalysts is slightly higher. Cell performance at higher current densities is slightly lower for Pt/CX than the Pt/XC-72R sample, however after normalization for Pt loading, performance is slightly higher for Pt/CX, particularly in H₂/O₂ and at lower cell temperatures (50 °C). This latter finding may reflect a possible lower mass-transfer resistance in the Pt/CX sample.     

Degradation mechanism of electrocatalyst during long-term operation of PEMFC

Long-term operation of a polymer electrolyte membrane fuel cell (PEMFC) was carried out in constant-current (CC) and open-circuit-voltage (OCV) modes. The main factors causing electrocatalyst deactivation were found to be Pt sintering and dissolution. In Pt sintering, growth in particle size occurred mostly during the initial stage of operation (40 h). Pt dissolution occurred mostly at the cathode, rather than the anode, due to chemical oxidation of Pt to PtO by residual oxygen present in the cathode layer, resulting in a gradual decrease in cell performance during long-term operation. After the dissolution of PtO in water, Pt²⁺ was formed, which migrated from the cathode to the membrane phase, and was re-deposited as Pt crystal upon reduction by crossover hydrogen, as was confirmed by transmission electron microscopy (TEM) after long-term operation. Under normal operating conditions, there exists a balance at the cathode between chemical oxidation by oxygen and electrochemical reduction by input electrons. Therefore, Pt dissolution at the cathode is accelerated by an imbalance of these reactions under OCV conditions or by a high O₂ concentration in the feed.     

Dissolution and migration of platinum after long-term operation of a polymer electrolyte fuel cell under various conditions

The distribution patterns of Pt crystals that have moved from electrodes to the membrane phase of membrane electrode assembly (MEA) are monitored using transmission electron microscopy (TEM) after long-term operation (>1000 h) of a polymer electrolyte membrane fuel cell (PEMFC) at various operating and feed conditions. The dissolution of cathode Pt and subsequent migration to the membrane is readily observed when residual oxygen concentrations inside the cathode are kept high under low current density conditions. Dissolution of anodic Pt can also be observed under constant-current operation when hydrogen feed is kept low to induce a hydrogen shortage on the Pt surface. It is postulated that the Pt at the both electrodes is dissolved by chemical oxidation to PtO in the presence of residual oxygen. The Pt ions that are dissolved in water migrate to the membrane phase and undergo repeated oxidation/dissolution and reduction/deposition by crossover of oxygen and hydrogen, respectively. As a result, the distribution patterns and crystal sizes of the migrated Pt are strongly dependent on the relative concentrations of the crossover oxygen and hydrogen. The final position of the deposited Pt band is located at the point where crossover oxygen becomes depleted, typically between 1 and 10 μm from the cathode–membrane interface. Higher concentrations of oxygen and hydrogen in the membrane yield sharper and narrower Pt bands with large Pt aggregates, whereas lower concentrations yield wider distribution bands with smaller Pt crystals.     

High pressure polymer electrolyte water electrolysis: Test bench development and electrochemical analysis

A test bench for a polymer electrolyte water electrolysis (PEWE) cell for high pressure operation of up to 100 bar in differential and balanced pressure mode is described. Important aspects referring to the design, safety and operability of the test bench and the design of a small scale electrolysis cell are described. The electrolyzer cell comprises a special compression mechanism which allows accommodating porous transport layers of different thickness and setting of the compression pressure independent of the clamping pressure. In order to analyze the electrochemical results with respect to the overpotentials, a power source with integrated high frequency resistance (HFR) as well as electrochemical impedance spectroscopy (EIS) measurement capabilities is implemented. The versatility of the test environment is demonstrated by comparing the DC, HFR and EIS data as a function of operating pressure, temperature (up to 70 °C) and current density (up to 4 A/cm²). With respect to pressurized operation of PEWE cells, only the differential pressure mode (hydrogen pressurized) shows the expected isothermal compression behavior, for balanced pressure operation a different characteristic is observed.     

Effects of operating conditions on durability of polymer electrolyte membrane fuel cell Pt cathode catalyst layer

In this study, we investigated the effects of humidity and oxygen reduction on the degradation of the catalyst of a polymer electrolyte membrane fuel cell (PEMFC) in a voltage cycling test. To elucidate the effect of humidity on the voltage cycling corrosion of a carbon-supported Pt catalyst with 3 nm Pt particles, voltage cycling tests based on 10,000 cycles were conducted using 100% relative humidity (RH) hydrogen as anode gas and nitrogen of varying humidities as cathode gas. The degradation rate of an electrochemical surface area (ECSA) was almost 50% under 189% RH nitrogen atmosphere and the Pt average particle diameter after 10,000 cycles under these conditions was about 2.3 times that of a particle of fresh catalyst because of the agglomeration of Pt particles.The oxygen reduction reaction (ORR) that facilitated Pt catalyst agglomeration when oxygen was employed as the cathode gas also demonstrated that Pt agglomeration was prominent in higher concentrations of oxygen. The ECSA degradation figure in 100% RH oxygen was similar to that in 189% RH nitrogen. It was concluded that liquid water, which was dropped under a supersaturated condition or generated by ORR, accelerated Pt agglomeration. In this paper, we suggest that the Pt agglomeration degradation occurs in a flooding area in a cell plane.     

Ionomer content effects on the electrocatalyst layer with in-situ grown Pt nanowires in PEMFCs

The effects of ionomer contents were investigated in composite electrodes with in-situ grown single crystal Pt nanowires (Pt-NWs) for PEMFCs, including the amount in the carbon matrix and impregnated on the surface of the electrocatalyst layer. The electrocatalyst layer was prepared by growing Pt-NWs directly on the carbon matrix with a simple one-step wet chemical approach at room temperature. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), polarization curve tests, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) were employed to evaluate the ionomer effects. The experimental results showed that the ionomer in the carbon matrix had an influence on the ionic conductivity and aggregation and distribution of the Pt-NWs, and the ionomer impregnated on the surface of the electrocatalyst layer affected the mass transport and ionic conductivity. The performance of the MEA was improved by optimizing the ionomer contents.     

Sulfonation of carbon-nanotube supported platinum catalysts for polymer electrolyte fuel cells

Sulfonic acid groups were grafted onto the surface of carbon-nanotube supported platinum (Pt/CNT) catalysts to increase platinum utilization in polymer electrolyte fuel cells (PEFCs) by both thermal decomposition of ammonium sulfate and in situ radical polymerization of 4-styrenesulfonate. The resultant sulfonated Pt/CNT catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectrometry, thermal gravimetric analysis (TGA) and electrochemical methods. The electrodes with the Pt/CNT catalysts sulfonated by the in situ radical polymerization of 4-styrenesulfonate exhibited better performance than did those with the unsulfonated counterparts, mainly because of the easier access with protons and well dispersed distribution of the sulfonated Pt/CNT catalysts, indicating that sulfonation is an efficient approach to improve performance and reduce cost for the Pt/CNT-based PEFCs. The electrodes with the Pt/CNT catalysts sulfonated by the thermal decomposition of ammonium sulfate, however, did not yield the expected performance as in the case of carbon black supported platinum (Pt/C) catalysts, probably due to the significant agglomeration of platinum particles on the CNT surface at high temperatures, indicating that the Pt/CNT catalysts are more sensitive to temperature than the Pt/C catalysts.     

Direct liquid-feed fuel cells: Thermodynamic and environmental concerns

The present paper briefly reviews the different direct liquid-feed fuel cells that have been regarded through the open literature. It especially focuses on thermodynamic-energetic data and toxicological–ecological hazards of the chemicals used as liquid fuels. The analysis of those two databases shows that borohydride, ethanol and 2-propanol would be the most adequate liquid fuels for the polymer electrolyte membrane fuel cell-type systems, even if they are inferior to hydrogen. All the fuels and also all the by-products stem from their decomposition are more or less harmful towards health and environment. More particularly, hydrazine should be avoided because it and its by-product are very dangerous. It is to note that the present paper does not intend to review and to compare the performances of those fuel cells because of great differences in the efforts devoted to each of them.     

Synthesis and electrochemical performance of La0.6Ca0.4Fe1−xNixO3 (x = 0.1, 0.2, 0.3) material for solid oxide fuel cell cathode

Polycrystalline samples of La_0.6Ca_0.4Fe_1−xNi_xO₃ (x = 0.1, 0.2, 0.3) (LCFN) are prepared by liquid mix method. The structure of the polycrystalline powders is analyzed with X-ray powder diffraction data. The XRD patterns are indexed as the orthoferrite similar to that of LaFeO₃ having a single phase with orthorhombic perovskite structure (Pnma). The morphological characterization is performed by scanning electron microscopy (SEM) obtaining a mean particle size less than 300 nm.Polarization resistance is studied using two different electrolytes: Y-stabilized zirconia (YSZ) and Sm-doped ceria (SDC). Electrochemical impedance spectroscopy (EIS) measurements of LCFN/YSZ/LCFN and LCFN/SDC/LCFN test cells are carried out. These electrochemical experiments are performed at equilibrium from 850 °C to room temperature, under both zero dc current intensity and air. The best value of area specific resistance (ASR) obtained is 0.88 Ω cm², corresponding to the La_0.6Ca_0.4Fe_0.9Ni_0.1O₃ material using SDC as electrolyte. The dc four-probe measurement indicates that La_0.6Ca_0.4Fe_0.9Ni_0.1O₃ exhibits fairly high electrical conductivity, over 300 S cm⁻¹ at T > 500 °C.     

Synthesis of highly ordered three-dimensional nanostructures and the influence of the temperature on their application as solid oxide fuel cells cathodes

The creation of nanostructured materials with three-dimensional periodicity has been identified as a potential interesting field for increasing the overall performance of solid oxide fuel cells (SOFCs). In this work, we have investigated the formation of Pr_0.6Sr_0.4Fe_0.8Co_0.2O₃ nanotubes with different diameter sizes employing polymeric membranes as templates. The samples were characterized by X-ray diffraction and field emission scanning electron microscopy. The polarization resistance of the materials was measured by electrochemical impedance spectroscopy (EIS). A study of the influence of the temperature on the nanostructure has also been carried out, demonstrating that the sintering process affects to the electrochemical performance of the cathode. The study shows that the nanotubes with higher diameter size present a better performance at high temperatures than those with diameter sizes smaller than 100 nm. The ASR (area specific resistance) value of the sample synthesized with a pore diameter size of 0.8 μm is as good as 0.12 Ω cm², allowing it use as cathode in solid oxide fuel cells (SOFCs).     

X-ray absorption analysis of nitrogen contribution to oxygen reduction reaction in carbon alloy cathode catalysts for polymer electrolyte fuel cells

The electronic structure of nitrogen introduced into various carbon-based cathode catalysts for the polymer electrolyte fuel cell (PEFC) is investigated using X-ray absorption spectroscopy (XAS). The profile of π* peaks at the pre-edge of N 1s XAS spectra is used to determine the chemical state of nitrogen, which can be an indicator of oxygen reduction reaction (ORR) activity. It is found that catalysts with a relatively larger amount of graphite-like nitrogen exhibit a higher ORR activity than those with a relatively larger amount of pyridine-like nitrogen. We propose that effective doping with graphite-like nitrogen is a practical guideline for the synthesis of active carbon alloy catalysts.     

Electrochemical durability of gas diffusion layer under simulated proton exchange membrane fuel cell conditions

An effective ex-situ method for characterizing electrochemical durability of a gas diffusion layer (GDL) under simulated polymer electrolyte membrane fuel cell (PEMFC) conditions is reported in this article. Electrochemical oxidation of the GDLs are studied following potentiostatic treatments up to 96 h holding at potentials from 1.0 to 1.4 V (vs.SCE) in 0.5 mol L⁻¹ H₂SO₄. From the analysis of morphology, resistance, gas permeability and contact angle, the characteristics of the fresh GDL and the oxidized GDLs are compared. It is found that the maximum power densities of the fuel cells with the oxidized GDLs hold at 1.2 and 1.4 V (vs.SCE) for 96 h decreased 178 and 486 mW cm⁻², respectively. The electrochemical impedance spectra measured at 1500 mA cm⁻² are also presented and they reveal that the ohmic resistance, charge-transfer and mass-transfer resistances of the fuel cell changed significantly due to corrosion at high potential.