A polymer electrolyte fuel cell life test: 3 years of continuous operation

Implementation of polymer electrolyte fuel cells (PEMFCs) for stationary power applications requires the demonstration of reliable fuel cell stack life. One of the most critical components in the stack and that most likely to ultimately dictate stack life is the membrane electrode assembly (MEA). This publication reports the results of a 26,300 h single cell life test operated with a commercial MEA at conditions relevant to stationary fuel cell applications. In this experiment, the ultimate MEA life was dictated by failure of the membrane. In addition, the performance degradation rate of the cell was determined to be between 4 and 6 μV h⁻¹, at the operating current density of 800 mA cm⁻². AC impedance analysis and DC electrochemical tests (cyclic voltammetry and polarization curves) were performed as diagnostics during and on completion the test, to understand materials changes occurring during the test. Post mortem analyses of the fuel cell components were also performed.     

Characterisation of proton exchange membrane fuel cell (PEMFC) failures via electrochemical impedance spectroscopy

Two PEMFC failure modes (dehydration and flooding) were investigated using in situ electrochemical impedance spectroscopy (EIS) on a four-cell stack under load. The EIS measurements were made at different temperatures (70 and 80 °C), covering the current density range 0.1–1.0 A cm⁻², and the frequency range 0.1–2 × 10⁵ Hz. Dehydration and flooding effects were observed in the frequency ranges 0.5–10⁵ and 0.5–10² Hz, respectively.We propose that impedance measurements at separate frequency ranges (or narrow bands thereof) can be used to distinguish between flooding and dehydration events. Similar approaches may be used to diagnose other important PEMFC failures.     

Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method

A proton exchange membrane fuel cell (PEMFC) electrode having a modified morphology of conventional Teflon (PTFE) bonded electrodes was studied using the AC impedance method. The electrode differs from other types of electrodes in the presence of a thin catalyst-supporting layer between the gas diffusion backing and the catalyst layer. The thickness and composition of the supporting layer were optimized on the basis of the information from AC impedance measurements. The optimal thickness of the supporting layer and its PTFE content turned out to be approximately 3.5 mg cm⁻² and 30 wt.%, respectively. The catalyst layer was cast on top of the supporting layer, from solution that has the proper ratio of ionomer Nafion and Pt/C catalyst. The optimal amount of the ionomer in the catalyst layer was approximately 0.8 mg cm⁻² when Pt loading was kept at 0.4 mg cm⁻². These values are rationalized in terms of the catalyst active area and the transport of the involved species for the electrode reaction.     

Control of miniature proton exchange membrane fuel cells based on fuzzy logic

A control strategy is presented in this paper which is suitable for miniature hydrogen/air proton-exchange membrane (PEM) fuel cells. The control approach is based on process modelling using fuzzy logic and tested using a PEM stack consisting of 15 cells with parallel channels on the cathode side and a meander-shaped flow-field on the anode side. The active area per cell is 8 cm². Commercially available materials are used for the bipolar plates, gas diffusion layers and the membrane-electrode assembly (MEA). It is concluded from a simple water balance model that water management at different temperatures can be achieved by controlling the air stoichiometry. This is achieved by varying the fan voltage for the air supply of the PEM stack. A control strategy of the Takagi Sugeno Kang (TSK) type, based on fuzzy logic, is presented. The TSK-type controller offers the advantage that the system output can be computed in an efficient way: the rule consequents of the controller combine the system variables in linear equations. It is shown experimentally that drying out of the membrane at high temperatures can be monitored by measuring the ac impedance of the fuel cell stack at a frequency of 1 kHz. Flooding of single cells leads to an abrupt drop of the corresponding single-cell voltage. Therefore, the fuzzy rule base consists of the ac impedance at 1 kHz and all single-cell voltages. The parameters of the fuzzy rule base are determined by plotting characteristic diagrams of the fuel cell stack at constant temperatures. The fuel cell stack can be controlled at T=60 °C up to a power level of 7.5 W. The fuel cell stack is controlled successfully even when the external electric load changes. At T=65 °C, a maximum power level of 8 W is found. A decrease of the maximum power level is observed for higher temperatures.     

Effect of open circuit voltage on performance and degradation of high temperature PBI–H3PO4 fuel cells

The impact of open circuit voltage (OCV) on the performance and degradation of polybenzimidazole–phosphoric acid (PBI–H₃PO₄) fuel cells operated at 180 °C was investigated. The OCV showed an initial quick increase in the first few minutes, followed by a much slower increase, and peaked after about 35 min. It then started an exponential and monotonous decline. Along with the decline of OCV, the performance of the fuel cell also declined. Operating the fuel cell with a load of 0.2 A cm⁻² could temporarily boost the OCV and the fuel cell performance, but it could not recover the lost performance permanently. Electrochemical impedance spectroscopy (EIS) indicated significant loss of catalyst activity and increase in mass transport resistance due to the relatively high potential at OCV. X-ray diffraction (XRD) measurement showed that the cathode Pt crystallite size increased by as much as 430% after a total of 244.5 h of exposure to OCV.     

Research on electrochemical impedance spectroscope behavior of fuel cell stack under different reactant relative humidity

The reactant relative humidity (RH) is a key parameter to keep a suitable water content in the large active area fuel cell. Electrochemical impedance spectroscopy (EIS) is one of the effective methods to identify the water state within the membrane. In this work, the EIS behavior of fuel cell stack under different reactant RH and current density is investigated. Both the whole stack and individual cell impedances are experimentally measured. The internal reactions of the stack and individual cells with different current densities and different reactants RH can be distinguished by impedances. Based on the experiment results, the low frequency impedance has a greater variation than high frequency impedance when the reactant RH changes. And the impedance is more sensitive to the reactant RH under low current density. With the current density increases, the internal self-humidification can be realized to obtain a good performance for large area fuel cells.     

Development and demonstration of a higher temperature PEM fuel cell stack

Research and development was conducted on a proton exchange membrane (PEM) fuel cell stack to demonstrate the capabilities of Ionomem Corporation's composite membrane to operate at 120 °C and ambient pressure for on-site electrical power generation with useful waste heat. The membrane was a composite of polytetrafluoroethylene (PTFE), Nafion^®, and phosphotungstic acid. Studies were first performed on the membrane, cathode catalyst layer, and gas diffusion layer to improve performance in 25 cm², subscale cells. This technology was then scaled-up to a commercial 300 cm² size and evaluated in multi-cell stacks. The resulting stack obtained a performance near that of the subscale cells, 0.60 V at 400 mA cm⁻² at near 120 °C and ambient pressure with hydrogen and air reactants containing water at 35% relative humidity. The water used for cooling the stack resulted in available waste heat at 116 °C. The performance of the stack was verified. This was the first successful test of a higher-temperature, PEM, fuel-cell stack that did not use phosphoric acid electrolyte.     

Dynamic behaviour of SOFC short stacks

Electrical output behaviour obtained on solid oxide fuel cell stacks, based on planar anode supported cells (50 or 100 cm² active area) and metallic interconnects, is reported. Stacks (1–12 cells) have been operated with cathode air and anode hydrogen flows between 750 and 800 °C operating temperature. At first polarisation, an activation phase (increase in power density) is typically observed, ascribed to the cathode but not clarified. Activation may extend over days or weeks. The materials are fairly resistant to thermal cycling. A 1-cell stack cycled five times in 4 days at heating/cooling rates of 100–300 K h⁻¹, showed no accelerated degradation. In a 5-cell stack, open circuit voltage (OCV) of all cells remained constant after three full cycles (800–25 °C). Power output is little affected by air flow but markedly influenced by small fuel flow variation. Fuel utilisation reached 88% in one 5-cell stack test. Performance homogeneity between cells lay at ±4–8% for three different 5- or 6-cell stacks, but was poor for a 12-cell stack with respect to the border cells. Degradation of a 1-cell stack operated for 5500 h showed clear dependence on operating conditions (cell voltage, fuel conversion), believed to be related to anode reoxidation (Ni). A 6-cell stack (50 cm² cells) delivering 100 W_el at 790 °C (1 kW_el L⁻¹ or 0.34 W cm⁻²) went through a fuel supply interruption and a thermal cycle, with one out of the six cells slightly underperforming after these events. This cell was eventually responsible (hot spot) for stack failure.     

Behavioral pattern of a monopolar passive direct methanol fuel cell stack

A passive, air-breathing, monopolar, liquid feed direct methanol fuel cell (DMFC) stack consisting of six unit cells with no external pump, fan or auxiliary devices to feed the reactants has been designed and fabricated for its possible employment as a portable power source. The configurations of the stack of monopolar passive feed DMFCs are different from those of bipolar active feed DMFCs and therefore its operational characteristics completely vary from the active ones. Our present investigation primarily focuses on understanding the unique behavioral patterns of monopolar stack under the influence of certain operating conditions, such as temperature, methanol concentration and reactants feeding methods. With passive reactants supply, the temperature of the stack and open circuit voltage (OCV) undergo changes over time due to a decrease in concentration of methanol in the reservoir as the reaction proceeds. Variations in performance and temperature of the stack are mainly influenced by the concentration of methanol. Continuous operation of the passive stack is influenced by the supply of methanol rather than air supply or water accumulation at the cathode. The monopolar stack made up of six unit cells exhibits a total power of 1000 mW (37 mW cm⁻²) with 4 M methanol under ambient conditions.     

Performance of a solid oxide fuel cell utilizing hydrogen sulfide as fuel

The electrochemical performance of a hydrogen sulfide solid oxide fuel cell having the configuration H₂S, Pt/(ZrO₂)_0.92(Y₂O₃)_0.08/Pt, air has been examined at atmospheric pressure and 750–800°C, using both pure and 5% H₂S anode feed streams. The performance of the cell is higher when using diluted H₂S feed compared with pure H₂S feed: current densities up to 100 mA cm⁻² and power densities up to 15.4 mW cm⁻² have been achieved using diluted H₂S gas (5%) at 800°C. However, the platinum anode degrades over time in H₂S stream due to the formation of PtS. Electrochemical oxidation of H₂S on the Pt anode significantly accelerated its degradation. Polarization and impedance spectroscopy measurements show that at low current density (i) electrochemical reaction is the major cause of polarization in the fuel cell. Ohmic loss due to the resistance of the electrolyte material and the electrical connecting wire is a major part of cell polarization at high i.     

Measurement of ohmic voltage losses in individual cells of a PEMFC stack

The ohmic voltage loss in a fuel cell can be determined with the current interruption method. The method was utilized to measure the ohmic voltage loss in an individual cell of a fuel cell stack. This was achieved by producing voltage transients and monitoring them with a digital oscilloscope connected in parallel with the individual cell. In this study, the method was applied to a small polymer electrolyte membrane fuel cell (PEMFC) stack in which different air supply levels were employed on the cathode side. In the case of higher air-feed rate, the results revealed an increase of ohmic losses in the middle of the stack by up to 21% at 400 mA cm⁻², compared to the unit cell with the lowest ohmic loss. This probably resulted from the decrease of membrane conductivity because of drying. Comparison to individual cell voltages showed that the decrease of conductivity would not be observed if only the individual cell voltages alone were measured. The total ohmic loss in the stack was measured using the same method to verify the reliability of the measurement system. The results indicate a good agreement between the total ohmic loss and the combined ohmic losses in the individual cells.     

AC impedance technique in PEM fuel cell diagnosis—A review

Because the AC impedance technique, also known as electrochemical impedance spectroscopy (EIS), is being utilized by more and more researchers in proton exchange membrane (PEM) fuel cell studies, the technique has developed into a primary tool in such research. In this paper the recent work on PEM fuel cells using the AC impedance technique is reviewed. Both in situ and ex situ impedance measurements are discussed, with primary focus on the in situ measurements. Within the domain of in situ studies, various methods for measuring the impedance of a PEM fuel cell are examined, and typical impedance spectra in several common scenarios are presented. Representative applications of the AC impedance technique in PEM fuel cell research are also discussed. Finally, the necessity of a time domain rapid AC impedance technique is briefly discussed.     

Temperature behavior and impedance fundamentals of supercapacitors

Electrochemical impedance spectroscopy (EIS) is one of the most important analytical tools for characterization of electrochemical double-layer capacitors (EDLC). As an example, we have characterized a commercial capacitor (BCAP0350 Maxwell Technologies) by EIS, and we will discuss the typical performance of an EDLC compared to an ideal capacitor.EIS was used to determine internal resistance and capacitance of the same capacitor as a function of temperature and as a function of time during constant voltage tests. In addition, the effect of the electrolyte on the temperature behavior was investigated. While the capacitance is a very weak function of temperature, the ESR increases significantly with reduced temperature. Temperature effects are much more pronounced for propylene carbonate (PC) than for acetonitrile (AN)-based electrolytes. From the data obtained at various temperatures and voltages, we could determine acceleration factors for the degradation. On the basis of an Arrhenius plot of the leakage current measured during load life tests at capacitor voltages between 2.5 V and 3.0 V and temperatures between −40 °C and +70 °C, we determined acceleration factors for capacitor degradation of about 2 for a temperature increase of 10 °C and also a factor of about 2 for a potential increase of 0.1 V.     

Characterization of a high performing passive direct formic acid fuel cell

Small fuel cells are considered likely replacements for batteries in portable power applications. In this paper, the performance of a passive air breathing direct formic acid fuel cell (DFAFC) at room temperature is reported. The passive fuel cell, with a palladium anode catalyst, produces an excellent cell performance at 30 °C. It produced a high open cell potential of 0.9 V with ambient air. It produced current densities of 139 and 336 mA cm⁻² at 0.72 and 0.53 V, respectively. Its maximum power density was 177 mW cm⁻² at 0.53 V. Our passive air breathing fuel cell runs successfully with formic acid concentration up to 10 and 12 M with little degradation in performance. In this paper, its constant voltage test at 0.72 V is also demonstrated using 10 M formic acid. Additionally, a reference electrode was used to determine distinct anode and cathode electrode performances for our passive air breathing DFAFC.     

Development of an equivalent circuit model of a fuel cell to evaluate the effects of inverter ripple current

In this paper, an impedance model of the proton exchange membrane fuel cell stack (PEMFCS) is proposed. The proposed study employs an equivalent circuit of the PEMFCS derived by the frequency response analysis (FRA) technique. An equivalent circuit for the fuel cell stack is developed to evaluate the effects of ripple currents generated by the power-conditioning unit. The calculated results are then verified by means of experiments on two commercially available fuel cells: Avista Labs SR-12 (500 W) and Ballard Nexa (1.2 kW) PEMFC system. The relationship between ripple current and fuel cell performance, such as power loss and fuel consumption is investigated. Experimental results show that the ripple current can contribute up to a 10% reduction in the available output power.     

Uniformity analysis at MEA and stack Levels for a Nexa PEM fuel cell system

The Nexa™ power module is evaluated at membrane-electrode-assembly (MEA) and stack levels. The I–V Curves of the Nexa™ PEM fuel cell system is measured using periodic current interruption to maintain isothermal stack temperature. The uniformity analysis is mainly performed on the load of 800 W for all MEAs in 10 individual Nexa™ stacks. Statistical data show that the MEA voltage without an external load averages 224 mV higher than that with a load of 800 W. The MEA voltage difference is especially pronounced around the two cells at the air compressor side. The average difference is 8.8% and the highest difference is 13.1% between the minimum MEA voltage in the stack and the mean value. This voltage difference reveals a possibility to increase the product power capability and cut the cost per kilowatts by improving the weak performance electrodes or MEAs in the stack.     

A 28-W portable direct borohydride–hydrogen peroxide fuel-cell stack

A 28-W direct borohydride–hydrogen peroxide fuel-cell stack operating at 25 °C is reported for contemporary portable applications. The fuel cell operates with the peak power-density of ca. 50 mW cm⁻² at 1 V. This performance is superior to the anticipated power-density of 9 mW cm⁻² for a methanol–hydrogen peroxide fuel cell. Taking the fuel efficiency of the sodium borohydride–hydrogen peroxide fuel cell as 24.5%, its specific energy is ca. 2 kWh kg⁻¹. High power-densities can be achieved in the sodium borohydride system because of its ability to provide a high concentration of reactants to the fuel cell.     

Assessing cell polarity reversal degradation phenomena in PEM fuel cells by electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) is identified as one of the most promising in-situ diagnostics tools available for assessing fuel cell ageing and degradation. In this work, the degradation phenomena caused by cell polarity reversal due to fuel starvation of an open cathode 16 membrane electrode assembly (MEA) – low power (PEM) fuel cell (15 W nominal power) – is reported using EIS as a base technique. Measuring the potential of individual cells, while the fuel cell is on load, was found instrumental in assessing the “state of health” of cells at fixed current. Location of affected cells, those farthest away from hydrogen entry in the stack, was revealed by very low or even negative potential values. EIS spectra were taken at selected break-in periods during fuel cell functioning. The analysis of impedance data was made using an a priori equivalent circuit describing the transfer function of the system in question – equivalent circuit elements were evaluated by a complex non-linear least square (CNLS) fitting algorithm, and by calculating and analyzing the corresponding distribution of relaxation times (DRT). Results and interpretation of cell polarity reversal due to hydrogen starvation were complemented with ex-situ MEA cross section analysis, using scanning electron microscopy. Electrode thickness reduction and delamination of catalyst layers were observed as a result of reactions taking place during hydrogen starvation. Carbon corrosion and membrane degradation by fluoride depletion are discussed.     

Operational characteristics of a 50 W DMFC stack

The characteristics of a 50 W direct methanol fuel cell (DMFC) stack were investigated under various operating conditions in order to understand the behavior of the stack. The operating variables included the methanol concentration, the flow rate and the flow direction of the reactants (methanol and air) in the stack. The temperature of the stack was autonomously increased in proportion to the magnitude of the electric load, but it decreased with an increase in the flow rates of the reactants. Although the operation of the stack was initiated at room temperature, under a certain condition the internal temperature of the stack was higher than 80 °C. A uniform distribution of the reactants to all the cells was a key factor in determining the performance of the stack. With the supply of 2 M methanol, a maximum power of the stack was found to be 54 W (85 mW cm⁻²) in air and 98 W (154 mW cm⁻²) in oxygen. Further, the system with counter-flow reactants produced a power output that was 20% higher than that of co-flow system. A post-load behavior of the stack was also studied by varying the electric load at various operating conditions.     

Dependence of polarization in anode-supported solid oxide fuel cells on various cell parameters

The performance of solid oxide fuel cells (SOFCs) is affected by various polarization losses, namely, ohmic polarization, activation polarization and concentration polarization. Under given operating conditions, these polarization losses are largely dependent on cell materials, electrode microstructures, and cell geometric parameters. Solid oxide fuel cells (SOFC) with yttria-stabilized zirconia (YSZ) electrolyte, Ni–YSZ anode support, Ni–YSZ anode interlayer, strontium doped lanthanum manganate (LSM)–YSZ cathode interlayer, and LSM current collector, were fabricated. The effect of various parameters on cell performance was evaluated. The parameters investigated were: (1) YSZ electrolyte thickness, (2) cathode interlayer thickness, (3) anode support thickness, and (4) anode support porosity. Cells were tested over a range of temperatures between 600 and 800 °C with hydrogen as fuel, and air as oxidant. Ohmic contribution was determined using the current interruption technique. The effect of these cell parameters on ohmic polarization and on cell performance was experimentally measured. Dependence of cell performance on various parameters was rationalized on the basis of a simple analytical model. Based on the results of the cell parameter study, a cell with optimized parameters was fabricated and tested. The corresponding maximum power density at 800 °C was ∼1.8 W cm⁻².