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.     

International Lead Award

The University of Duisburg-Essen and the Center for Fuel Cell Technology (ZBT Duisburg GmbH) have developed a compact multi-fuel steam reformer suitable for natural gas, propane and butane. Fuel processor prototypes based on this concept were built up in the power range from 2.5 to 12.5 kW thermal hydrogen power for different applications and different industrial partners. The fuel processor concept contains all the necessary elements, a prereformer step, a primary reformer, water gas shift reactors, a steam generator, internal heat exchangers, in order to achieve an optimised heat integration and an external burner for heat supply as well as a preferential oxidation step (PrOx) as CO purification. One of the built fuel processors is designed to deliver a thermal hydrogen power output of 2.5 kW according to a PEM fuel cell stack providing about 1 kW electrical power and achieves a thermal efficiency of about 75% (LHV basis after PrOx), while the CO content of the product gas is below 20 ppm. This steam reformer has been combined with a 1 kW PEM fuel cell. Recirculating the anodic offgas results in a significant efficiency increase for the fuel processor. The gross efficiency of the combined system was already clearly above 30% during the first tests. Further improvements are currently investigated and developed at the ZBT.     

Operation of PEM fuel cells at 120–150 °C to improve CO tolerance

Sulfonic acid modified perfluorocarbon polymer proton exchange membrane (PEM) fuel cells operated at elevated temperatures (120–150 °C) can greatly alleviate CO poisoning on anode catalysts. However, fuel cells with these PEMs operated at elevated temperature and atmospheric pressure typically experience low relative humidity (RH) and thus have increased membrane and electrode resistance. To operate PEM fuel cells at elevated temperature and high RH, work is needed to pressurize the anode and cathode reactant gases, thereby decreasing the efficiency of the PEM fuel cell system. A liquid-fed hydrocarbon-fuel processor can produce reformed gas at high pressure and high relative humidity without gas compression. If the anode is fed with this high-pressure, high-relative humidity stream, the water in the anode compartment will transport through the membrane and into the ambient pressure cathode structure, decreasing the cell resistance. This work studied the effect of anode pressurization on the cell resistance and performance using an ambient pressure cathode. The results show that high RH from anode pressurization at both 120 and 150 °C can decrease the membrane resistance and therefore increase the cell voltage. A cell running at 150 °C obtains a cell voltage of 0.43 V at 400 mA cm⁻² even with 1% CO in H₂. The results presented here provide a concept for the application of a coupled steam reformer and PEM fuel cell system that can operate at 150 °C with reformate and an atmospheric air cathode.     

Simulation of a low temperature water gas shift reactor using the heterogeneous model/application to a pem fuel cell

In the last few years, a renewed interest in the water gas shift (WGS) reaction at low temperature has arisen due to its application to fuel cells.In this work, a simulation of a fixed bed reactor for this reaction, which forms part of a hydrogen production–purification train for a 10 kW PEM fuel cell using ethanol as the raw material, was carried out. A commercial Cu/Zn/Ba/Al₂O₃ catalyst was employed and a one-dimensional heterogeneous model was applied for the simulation. The catalyst deactivation due to thermal factors (sintering) was taken into account in the model. Isothermal and adiabatic regimes were analyzed as well.Results of the simulation indicate that the pellet can be considered isothermal but temperature gradients in the film cannot be disregarded. On the other hand, concentration gradients in the film can be ignored but CO profiles are established inside the pellet. Adiabatic operation can be recommended because of its simplicity of operation and construction. The reactor volume is strongly sensitive to the CO outlet concentration at CO levels lower than 6000 ppm. For a 10 kW PEM fuel cell, using adequate pellet size and taking into account the catalyst deactivation, a reactor volume of 0.64 l would be enough to obtain an outlet CO concentration of about 7160 ppm. This concentration value can be handled by the next purification stage, COPROX.     

Direct formic acid fuel cells

The performance of formic acid fuel oxidation on a solid PEM fuel cell at 60 °C is reported. We find that formic acid is an excellent fuel for a fuel cell. A model cell, using a proprietary anode catalyst produced currents up to 134 mA/cm² and power outputs up to 48.8 mW/cm². Open circuit potentials (OCPs) are about 0.72 V. The fuel cell runs successfully over formic acid concentrations between 5 and 20 M with little crossover or degradation in performance. The anodic polarization potential of formic acid is approximately 0.1 V lower than that for methanol on a standard Pt/Ru catalyst. These results show that formic acid fuel cells are attractive alternatives for small portable fuel cell applications.     

Critical flow rate of anode fuel exhaust in a PEM fuel cell system

A manual purge line was added into the exterior fuel exhaust stream of a Ballard PEM stack in a Nexa™ power module. With the addition of manual exhaust purge, high levels of inert gases were intentionally added to the anode feed without changing normal operational procedures. A new method of determining the critical minimum flow rate in the anode exhaust stream was given by an anode mass balance. This type of operation makes dual use of membranes in the MEAs as both gas purifiers and as solid electrolytes. The PEM stack was successfully operated with up to ca. 7% nitrogen or carbon dioxide in the absence of a palladium-based hydrogen separator at ca. 200 W power level. Nitrogen in the anode stream was concentrated from 7.5% to 91.6%. The system maintained a fuel efficiency of 99% at a manual purge rate of 2.22 ml s⁻¹ and no auto purge. The fuel cell stack efficiency was 64% and the stack output efficiency was 75%. The overall system efficiency was 39%. After troublesome CO and H₂S poisons were removed, a hydrocarbon reformate containing high levels of CO₂ and H₂O was further used in the Nexa™ stack. The size and complexity of the fuel processing system may be reduced at a specified power level by using this operational method.     

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.     

Performance of direct methanol fuel cell using carbon nanotube-supported Pt–Ru anode catalyst with controlled composition

The performance of a single-cell direct methanol fuel cell (DMFC) using carbon nanotube-supported Pt–Ru (Pt–Ru/CNT) as an anode catalyst has been investigated. In this study, the Pt–Ru/CNT electrocatalyst was successfully synthesized using a modified polyol approach with a controlled composition very close to 20 wt.%Pt–10 wt.%Ru, and the anode was prepared by coating Pt–Ru/CNT electrocatalyst on a wet-proof carbon cloth substrate with a metal loading of about 4 mg cm⁻². A commercial gas diffusion electrode (GDE) with a platinum black loading of 4 mg cm⁻² obtained from E-TEK was employed as the cathode. The membrane electrode assembly (MEA) was fabricated using Nafion^® 117 membrane and the single-cell DMFC was assembled with graphite endplates as current collectors. Experiments were carried out at moderate low temperatures using 1 M CH₃OH aqueous solution and pure oxygen as reactants. Excellent cell performance was observed. The tested cell significantly outperformed a comparison cell using a commercial anode coated with carbon-supported Pt–Ru (Pt–Ru/C) electrocatalyst of similar composition and loading. High conductivity of carbon nanotube, good catalyst morphology and suitable catalyst composition of the prepared Pt–Ru/CNT electrocatalyst are considered to be some of the key factors leading to enhanced cell performance.     

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.     

AC impedance diagnosis of a 500 W PEM fuel cell stack: Part II: Individual cell impedance

AC impedance or electrochemical impedance spectroscopy (EIS) has been demonstrated to be a powerful technique for characterizing and evaluating fuel cells. In this work, as an extension of our previous study on the stack impedance of a 500 W PEM fuel cell, we report the AC impedance studies on individual cells of the same fuel cell stack. The EIS of the stack with an active area of 280 cm² was measured at currents from 10 to 210 A in steps of 20 A using the combination of a FuelCon test station, a TDI loadbank and a Solartron 1260 Frequency Response Analyzer. Measurement of the individual cell EIS was carried out with the help of a rotary switch unit made in our lab. Two methods (floating mode and grounded mode) were utilized for measuring the impedance spectroscopy of the individual cells. The results show that both methods are applicable to individual cells. The results also indicate a good agreement between the total Ohmic loss in the stack and the combined Ohmic losses of the individual cells.     

Vibration tests on a PEM fuel cell stack usable in transportation application

A fuel cell stack for use in transportation or in applications which involves the positioning of such systems in a location of high vibration and shock, is subjected to accelerated stress screening to ascertain the reliability of the stack, mechanical integrity and also to assess the mounting requirements. Such studies have not been well documented in open literature. In this paper, we have done the vibration test analysis on a 500 W Proton Electrolyte Membrane (PEM) fuel cell stack developed at our centre by simulating some of the vibration, shock and resonance in the stack and the likelihood of the stack to undergo in any application and evaluated the robustness of the stack. An experimental setup was designed for this purpose consisting of subjecting the PEM fuel cell stack to random and swept-sine excitations on a vibrating platform in three axes and measuring the mechanical response using accelerometers fixed at various locations in the stack. The fuel cell performance (pre-vibration test and post-vibration test) as obtained from polarization studies and the power–amperage curves does not show any significant damage effects, and a post-testing stack inspection showed a minor torque release. Further tests are recommended to study the dynamic life test. This study opens up further investigation which the authors propose to carry out in due course.     

Sulfonated poly(ether sulfone) for universal polymer electrolyte fuel cell operations

The performances of the proton exchange membrane fuel cell (PEMFC), direct formic acid fuel cell (DFAFC) and direct methanol fuel cell (DMFC) with sulfonated poly(ether sulfone) membrane are reported. Pt/C was coated on the membrane directly to fabricate a MEA for PEMFC operation. A single cell test was carried out using H₂/air as the fuel and oxidant. A current density of 730 mA cm⁻² at 0.60 V was obtained at 70 °C. Pt–Ru (anode) and Pt (cathode) were coated on the membrane for DMFC operations. It produced 83 mW cm⁻² maximum power density. The sulfonated poly(ether sulfone) membrane was also used for DFAFC operation under several different conditions. It showed good cell performances for several different kinds of polymer electrolyte fuel cell applications.     

Overview of the development of CO-tolerant anode electrocatalysts for proton-exchange membrane fuel cells

Poisoning of Pt anode electrocatalysts by carbon monoxide (CO) is deemed to be one of the most significant barriers to be overcome in the development of proton-exchange membrane fuel cell systems (PEMFCs). The use of CO-tolerant electrocatalysts serves as the most hopeful way to solve this problem. It is well established that Pt-based alloy systems of CO-tolerant electrocatalysts can substantially withstand the presence of CO in the fuel stream. Based on literature starting in 2000, a few efforts have still been conducted at developing a more CO-tolerant anode electrocatalyst than the traditional Pt/C or PtRu/C systems. This review introduces and discusses these efforts.Pt-based electrocatalysts, including PtSn/C, PtMo/C (atomic ratio = 5:1), PtRuMo/C (Mo = 10 wt.%), PtRu–H_xMoO₃/C and PtRu/(C nanotubes), appear to be poisoned by CO at the same, or a lower, level than traditional Pt/C or PtRu/C electrocatalysts. Platinum-free electrocatalysts, such as PdAu/C, have proven to be less strongly poisoned by CO than PtRu/C counterparts at temperatures of 60 °C.A greater tolerance to CO can be achieved by modifying the structure of the electrocatalyst. This involves the use of a composite or double-layer that is designed to make the CO react with one of the electrocatalyst in advance while the main hydrogen reacts at another layer with a traditional Pt/C electrocatalyst.     

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.     

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.     

Study of Ce-Pt/γ-Al2O3 for the selective oxidation of CO in H2 for application to PEFCs: Effect of gases

In order to supply pure hydrogen to proton exchange membrane (PEM) fuel cells and avoid CO poisoning, selective CO oxidation in H₂ was studied over Ce-Pt/γ-Al₂O₃. Adding the Ce promoted the CO conversion and selectivity of Pt/γ-Al₂O₃ with changing loading weights of Pt and Ce, oxygen concentration, residence time, and the composition of gases (H₂O, CO₂, and N₂). At 250 °C, adding H₂O to the feed gas enhanced the CO conversion due to the water–gas shift reaction. While, adding CO₂ to the feed gas suppressed the CO conversion due to the reversible water–gas shift reaction. In situ BET and XRD tests showed that well-dispersed metallic Pt particles (−2 nm) existed on the Ce oxide over the alumina support, which helps to supply oxygen to the Pt for a high activity of CO oxidation and selectivity.     

A 75-kW methanol reforming fuel cell system

A 75-kW methanol reforming fuel cell system, which consists of a fuel cell system and a methanol auto-thermal reforming fuel processor has been developed at Dalian Institute of Chemical Physics, Chinese Academy of Sciences (CAS). The core of the fuel cell system is a group of CO tolerant PEMFC stacks with a double layer composite structured anode. The fuel cell stacks show good CO tolerance even though 140 ppm CO was present in the reformate stream during transients. The auto-thermal reforming (ATR) fuel cell processor could adiabatically produce a suitable reformate without external energy consumption. The output of hydrogen-rich reformate was approximately 120 N m³ h⁻¹ with a H₂ content near 53% and the CO concentrations generally were under 30 ppm. The fuel cell system was integrated with the methanol reforming fuel processor and the peak power output of the fuel cell system exceeded 75 kW in testing. The hydrogen utilization approached 70% in the fuel cell system.     

Multi channel voltage control for fuel cells

Cell voltage control is a safe method to evaluate the operation of the single cells even though large efforts are being made to stabilise the behaviour of the fuel cells over the whole operating range. For fully developed systems only voltage control of single cells is necessary, whereas in test benches it is necessary to record data of the voltages during operation. Normally, a lot of cells are combined to a stack so that higher total voltages can be reached. Higher voltages make possible a high efficient transformation from direct current of the stack to alternate current of the public network. Sometimes more than 100 cells are connected together whereby the open circuit voltage of a single cell amounts to approximately 1 V. Therefore, the signal processing chips show a high supply maximum rating or the resolution of the values has to be reduced.This paper presents an economically priced multi channel voltage control for single cells of a PEM-fuel cell in combination with a microprocessor control and was developed at the Chair for Electric Power Networks and Renewable Energy Sources. The developed system can transfer the data of up to 32 single cells in serial connection. The resolution amounts to 10 bits per channel. The module has its own microprocessor, which is responsible for the intermediate storage of the collected data and the transfer to the RS-232 interface. Optionally, the module can be equipped with an LCD-display of 20 × 4 letters where different menus of the measured and calculated values can be indicated. The module is designed for a voltage of 1 V per channel and can be supplied with a direct voltage of between 6 and 24 V from the fuel cell stack.     

High voltage stability of nanostructured thin film catalysts for PEM fuel cells

This paper provides a comparative evaluation of electrocatalyst surface area stability in PEM fuel cells under accelerated durability testing. The two basic electrocatalyst types are conventional carbon-supported dispersed Pt catalysts (Pt/C), and nanostructured thin film (NSTF) catalysts. Both types of fuel cell electrocatalysts were exposed to continuous cycling between 0.6 and 1.2 V, at various temperatures between 65 and 95 °C, with H₂/N₂ on the anode and cathode, while periodic measurements of electrochemical surface area were recorded as a function of the number of cycles. The NSTF electrocatalyst surface areas were observed to be significantly more stable than the Pt/C electrocatalysts. A first order rate kinetic model was applied to the normalized surface area changes as a function of number of cycles and temperature, and two parameters extracted, viz. the minimum stable surface area, S_min, and the activation energy, E_a, for surface area loss in this voltage range. S_min was found to be 10% versus 66%, and E_a 23 kJ mole⁻¹ versus 52 kJ mole⁻¹, for Pt/C versus NSTF-Pt, respectively. The loss of surface area in both cases is primarily the result of Pt grain size increases, but the Pt/C XRD grain sizes increase significantly more than the NSTF grain sizes. In addition, substantial peak shifts occur in the Pt/C CVs, which ultimately end up aligning with the NSTF peak positions, which do not change substantially due to the voltage cycling. NSTF catalysts should be more robust against shut down/start-up, operation near OCV and local H₂ starvation effects.