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.     

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.     

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.     

Development of portable fuel cell arrays with printed-circuit technology

Portable hydrogen/oxygen fuel cell power sources were constructed using printed-circuit board (PCB) technology. Multiple iterations of miniature planar fuel cell devices were prototyped, demonstrating fast cycle innovation and dramatic power density improvements in <1 year of development. Several novel flow structure and gas routing designs were explored. Electrical interconnections for configurable voltage were wired on board by printed-circuit traces and vias. Fuel cell device voltages ranging from 1 V single cells to 16 V planar arrays were demonstrated, with power output ranging from <1 to >200 W. The lightweight laminate PCB technology allows the best prototypes to achieve >700 mW/cm² area power density and >400 mW/cm³ volumetric power density. PCB technology offers an intriguing platform for portable fuel cell development below 1 kW. Possibilities for on board diagnostics/control and further power density improvements are envisioned.     

Development of a PEFC under low humidified conditions

The life performance must be improved in order to commercialize polymer electrolyte fuel cells (PEFC). A decline of the cell voltage has been found to result from deterioration of the materials and a localization of reaction in the cell. We investigated the localization phenomenon, measuring the current density in the cell. The distribution of current density was measured by divided and isolated electrodes for a long period of operation. At the beginning of generation of electricity, a high current region is observed in the lower gas channel which is relatively humid. However, the high current region gradually moves to the upper dry channel in proportion to the voltage drop, which is remarkable under conditions of low humidity operation. This reaction seems to be reversible, since the PEFC can mostly recover the initial performance, once it is restarted. Improving the MEA and gas separators for low humidified conditions on the basis of this internal analysis, we operated a 20 cells PEFC stack of 0.4 kW for 5000 h and the stack showed −1.5 mV/1000 h of average voltage degradation.     

Fabrication and performance evaluation of 3-cell SOFC stack based on planar 10 cm × 10 cm anode-supported cells

This study reports the development of planar-type solid oxide fuel cell (SOFC) stacks based on an internal gas manifold and a cross-flow type design. A single-columned, 3-cell, SOFC stack is assembled using 10 cm × 10 cm anode-supported unit cells, metallic interconnects and glass-based compression-seal gaskets. The power-generating characteristics of the unit cell and stack are characterized as a function of temperature. The practical viability of the stack and stack components is investigated via long-term operation and thermal cycling tests. According to performance evaluation at 700 °C, the short stack produces about 100 W in total power at an average cell voltage of around 0.7 V. There are, however, some scale-up problems related to multi-cell stacking. This work addresses key issues in stack fabrication and performance improvement.     

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.     

Source of methane and methods to control its formation in single chamber microbial electrolysis cells

Methane production occurs during hydrogen gas generation in microbial electrolysis cells (MECs), particularly when single chamber systems are used which do not keep gases, generated at the cathode, separate from the anode. Few studies have examined the factors contributing to methane gas generation or the main pathway in MECs. It is shown here that methane generation is primarily associated with current generation and hydrogenotrophic methanogenesis and not substrate (acetate). Little methane gas was generated in the initial reaction time (<12 h) in a fed batch MEC when acetate concentrations were high. Most methane was produced at the end of a batch cycle when hydrogen and carbon dioxide gases were present at the greatest concentrations. Increasing the cycle time from 24 to 72 h resulted in complete consumption of hydrogen gas in the headspace (applied voltage of 0.7 V) with methane production. High applied voltages reduced methane production. Little methane (<4%) accumulated in the gas phase at an applied voltage of 0.6–0.9 V over a typical 24 h cycle. However, when the applied voltage was decreased to 0.4 V, there was a greater production of methane than hydrogen gas due to low current densities and long cycle times. The lack of significant hydrogen production from acetate was also supported by Coulombic efficiencies that were all around 90%, indicating electron flow was not altered by changes in methane production. These results demonstrate that methane production in single chamber MECs is primarily associated with current generation and hydrogen gas production, and not acetoclastic methanogenesis. Methane generation will therefore be difficult to control in mixed culture MECs that produce high concentrations of hydrogen gas. By keeping cycle times short, and using higher applied voltages (≥0.6 V), it is possible to reduce methane gas concentrations (<4%) but not eliminate methanogenesis in MECs.     

Screen-printed thin YSZ films used as electrolytes for solid oxide fuel cells

A thin yttria-stabilized zirconia (8 mol% YSZ) film was successfully fabricated on a NiO-YSZ anode substrate by a screen-printing technique. The scanning electron microscope (SEM) results suggested that the YSZ film thickness was about 31 μm after sintering at 1400 °C for 4 h in air. A 60 wt% La_0.7Sr_0.3MnO₃ + 40 wt% YSZ was screen-printed onto the YSZ film surface as cathode. A single cell was tested from 650 to 850 °C using hydrogen as fuel and ambient air as oxidant, which showed an open circuit voltage (OCV) of 1.02 V and a maximum power density of 1.30 W cm⁻² at 850 °C. The OCV was higher than 1.0 V, which suggested that the YSZ film was quite dense and that the fuel gas leakage through the YSZ film was negligible. Screen-printing can be a promising method for manufacturing YSZ films for solid oxide fuel cells (SOFCs).     

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.     

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.     

Calendar life performance of pouch lithium-ion cells

An accelerated method was used to determine the effect of temperature, end-of-charge voltage and the type of storage condition over the performance pouch lithium-ion cells. The cells were studied for 4.0 V and 4.2 V end-of-charge voltages (EOCV) both at 5 °C and 35 °C. The irreversible capacity loss of the cell was analyzed every month using a capacity measurement protocol. The results indicated that higher temperature and voltage accelerates the degradation of the cells. The open circuit voltage (OCV) decay of the cells stored under open circuit conditions was also analyzed. The reasons for the irreversible capacity loss, energy loss, OCV decay and the increase in the internal resistance of the cell are discussed in detail. The most detrimental storage condition and the most mild storage condition are identified and discussed in detail.     

Improvement of electricity generating performance and life expectancy of MCFC stack by applying Li/Na carbonate electrolyte: Test results and analysis of 0.44 m2/10 kW- and 1.03 m2/10 kW-class stack

Following the development of a 10 kW-class MCFC stack with a reactive area of 0.44 and 1.03 m², which applies a Li/Na carbonate electrolyte and a press stamping separator, many tests have now been carried out. In the installation tests, the observed cell voltages of the 0.44 m²/10 kW-class stack agreed with the voltage predicted from the test results of the 100 cm² bench scale cell. This agreement proves that the installing procedure of the bench scale cell can be applied to the 0.44 m²/10 kW-class stacks. The temperature distribution analysis model applied to the 100 kW-class stack was modified to calculate the temperature distribution of the 0.44 m²/10 kW-class stack. Taking the heat loss and the heat transfer effect of the stack holder into account, the calculated temperature was close to the measured temperature; this result proves that the modification was adequate for the temperature analysis model. In the high current density operating tests on the 0.44 m²/10 kW-class stack, an electrical power density of 2.46 kW/m² was recorded at an operating current density of 3000 A/m². In the endurance test on the 0.44 m²/10 kW-class stack, however, unexpected Ni shortening occurred during the operating period 2500–4500 h, which had been caused by a defective formation of the electrolyte matrix. The shortening seems to have been caused by the crack, which appeared in the electrolyte matrix. The voltage degradation rate of the 0.44 m²/10 kW-class stack was 0.52% over 1000 h, which proves that the matrix was inadequate for a long life expectancy of the MCFC stack. A final endurance test was carried out on the 1.03 m²/10 kW-class stack, of which the matrix had been revised. The fuel utilisation and the leakage of anode gas never changed during the 10,000 h operating test. This result suggests that no shortening occurred during the 10,000 h endurance test. The cell voltage degradation rate was around 0.2–0.3% over 1000 h in the 1.03 m²/10 kW-class stack. According to a comparison of the stack electricity generating performance of the 0.44 m² and the 1.03 m²/10 kW-class stack under the same operating conditions, the performance of the 1.03 m² stack was lower at the beginning of the endurance test, however, its performance exceeded the performance of the 0.44 m²/10 kW-class stack during the 10,000 h operating test. By carrying out the high current density operating test and the 10,000-hour endurance test using commercial sized 10 kW-class stacks, the stability of the MCFC stack with a Li/Na carbonate electrolyte and a press stamping separator has been proven.     

Assessment of an active-cooling micro-channel heat sink device,using electro-osmotic flow

Non-uniform heat flux generated by microchips causes “hot spots” in very small areas on the microchip surface. These hot spots are generated by the logic blocks in the microchip bay; however, memory blocks generate lower heat flux on contrast. The goal of this research is to design, fabricate, and test an active cooling micro-channel heat sink device that can operate under atmospheric pressure while achieving high-heat dissipation rate with a reduced chip-backside volume, particularly for spot cooling applications. An experimental setup was assembled and electro-osmotic flow (EOF) was used thus eliminating high pressure pumping system. A flow rate of 82 μL/min was achieved at 400 V of applied EOF voltage. An increase in the cooling fluid (buffer) temperature of 9.6 °C, 29.9 °C, 54.3 °C, and 80.1 °C was achieved for 0.4 W, 1.2 W, 2.1 W, and 4 W of heating powers, respectively. The substrate temperature at the middle of the microchannel was below 80.5 °C for all input power values. The maximum increase in the cooling fluid temperature due to the joule heating was 4.5 °C for 400 V of applied EOF voltage. Numerical calculations of temperatures and flow were conducted and the results were compared to experimental data. Nusselt number (Nu) for the 4 W case reached a maximum of 5.48 at the channel entrance and decreased to reach 4.56 for the rest of the channel. Nu number for EOF was about 10% higher when compared to the pressure driven flow. It was found that using a shorter channel length and an EOF voltage in the range of 400–600 V allows application of a heat flux in the order of 10⁴ W/m², applicable to spot cooling. For elevated voltages, the velocity due to EOF increased, leading to an increase in total heat transfer for a fixed duration of time; however, the joule heating also got elevated with increase in voltage.     

Development of a small DMFC bipolar plate stack for portable applications

The direct methanol fuel cell (DMFC) is regarded as a promising candidate in portable electronic power applications. Bipolar plate stacks were systematically studied by controlling the operating conditions, and by adjusting the stack structure design parameters, to develop more commercial DMFCs. The findings indicate that the peak power of the stack is influenced more strongly by the flow rate of air than by that of the methanol solution. Notably, the stack performance remains constant even as the channel depth is decreased from 1.0 to 0.6 mm, without loss of the performance in each cell. Furthermore, the specific power density of the stack was increased greatly from ∼60 to ∼100 W l⁻¹ for stacks of 10 and 18 cells, respectively. The current status of the work indicates that the power output of an 18-cell short stack reaches 33 W in air at 70 °C. The outer dimensions of this 18-cell short stack are only 80 mm × 80 mm × 51 mm, which are suitable for practical applications in 10–20 W DMFC portable systems.     

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 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.     

Dual-phase electrolytes for advanced fuel cells

Double-phase electrolyte (DPE) consisting of doped CeO₂/NiAl solid phase and NaOH liquid phase was used for fuel cells utilizing LiNiO₂ anode and Ag cathode at working temperatures over 450 °C. It was shown that the cells can produce a maximum output power of 716.2 mW cm⁻² at 590 °C even though utilized with relatively large thickness of electrolyte, from 0.8 to 1.2 mm. Most measurements of open circuit voltage (OCV) range between 1 and 1.2 V; a significantly higher OCV value of 1.254 V was also obtained. Liquid channel conductive mechanism of NaOH in DPE is proposed; both O²⁻ and H⁺ concur to conduct the current; the doped CeO₂ transports O²⁻ ions, whereas the molten second phase transports H⁺ protons. Moreover, SEM observations and EDS analysis suggest that Na⁺ and OH⁻ also contribute to enhance both OCV and output power of our cells. The addition of NiAl to the doped CeO₂ increases the mechanical strength and the output power of DPE; however the reasons of this latter effect are still to be further investigated. The results show that DPE is a promising electrolyte to manufacture fuel cells with advanced performances.     

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.     

Enhancement of the performance and reliability of CO poisoned PEM fuel cells

CO poisoning is a major issue when reformate is used as a fuel in PEM fuel cells. Normally, it is necessary to reduce the CO to very low levels (∼5 ppm) and to use CO tolerant catalysts, such as Pt–Ru alloys. As an alternative approach, we have studied the use of pulsed oxidation for the regeneration of CO poisoned cells. Results are presented for the regeneration of Pt and Pt–Ru anodes in a PEM fuel cell fed with CO concentrations as high as 10,000 ppm. The results show that periodic removal of CO from the catalyst surface by pulsed oxidation can increase the average cell potential and overall efficiency.Although use of pulsed techniques has been studied before, the careful control of each cell's voltage that this approach requires has limited its use in large fuel cell stacks. When uniform pulsing is done on a stack of fuel cells in series, the variations in voltage across the cells can limit the usefulness of this approach. A novel method that allows each cell in a stack to be separately pulsed under controlled conditions has been developed to overcome this problem. Weak or defective cells in a fuel cell stack can also be supplemented to enhance the power output and reliability of fuel cells. We present the results of experiments and calculations that quantify these benefits, specifically as they relate to PEM fuel cells operating on impure hydrogen produced by reforming fuels.