Low temperature operation and influence parameters on the cold start ability of portable PEMFCs

The start up behaviour of PEM fuel cells below 0 °C is one of the most challenging tasks to be solved before commercialisation. The automotive industry started to develop solutions to reduce the start up time of fuel cell systems in the middle of the nineties. The strategies varied from catalytic combustion of hydrogen on the electrode catalyst to fuel starvation or external stack heating via cooling loops to increase the stack temperature.Beside the automotive sector the cold start ability is as well important for portable PEMFC applications for outdoor use. But here the cold start issue is even more complicated, as the fuel cell system should be operated as passive as possible.Below 0 °C freezing of water inside the PEMFC could form ice layers in the electrode and in the gas diffusion layer. Therefore the cell reaction is limited or even inhibited. Product water during the start up builds additional barriers and leads to a strong decay of the output power at isothermal operating conditions.In order to find out which operational and hardware parameters affect this decay, potentiostatic experiments on single cells were performed at isothermal conditions. These experiments comprise investigations of the influence of membrane thickness and different GDL types as well as the effect of gas flow rates and humidification levels of the membrane. As pre stage to physical based models, empirical based prediction models are used to gain a better understanding of the main influence parameters during cold start. The results are analysed using the statistical software Cornerstone 4.0.The experience of single cell investigations are compared to start up behaviour of portable fuel cell stacks which are operated in a climate chamber at different ambient temperatures below 0 °C. Additional flow sharing problems in the fuel cell stack could be seen during cold start up experiments.     

Experimental and theoretical investigations on a proton exchange membrane fuel cell starting up at subzero temperatures

Ice/frost formation in a proton exchange membrane fuel cell (PEMFC) operating under subzero temperatures can lead to its shutdown during start up. Isothermal potentiostatic and galvanostatic tests were performed on 220 cm² single cells under a wide range of operating conditions in order to investigate the “cold start” behaviour. Different parameters have been investigated: the initial water contained in the membrane, the operating voltage, the cell temperature and current. An optimal wetting level of the fuel cell (FC) core for which cumulated heat generated by the electrochemical reaction is maximal, has been observed. Water management analysis from the membrane coupled with cell resistance measurement allowed to formulate a phenomenological interpretation of the overall FC performance evolution. FC starving is not only due to ice formation in the cathode layer pores, thus hindering oxygen transport. It is also due to ice formation in active reaction sites increasing the electrical resistance of the cell. Both factors dramatically reduce FC performance under load. The relative balance of each effect has been assessed. After each shutdown and start up at subfreezing temperature, a polarization curve at rated conditions was carried out to quantify the FC performance degradation. Performances reduce less than 1% per cold start at rated current density.     

Quantitative analysis on cold start process of a PEMFC stack with intake manifold

To systematically explore the low-temperature operating characteristics of polymer electrolyte membrane fuel cell (PEMFC) stack, a three-dimensional PEMFC stack model with intake manifold is developed in this study. The characteristics of different cold start modes in the stack are compared and analyzed. The distribution and transmission characteristics of water, ice, and heat in each cell of the stack are analyzed in detail. The location of water accumulation in each cell of the stack is also explored. Finally, finite difference sensitivity is calculated for the cumulated charge transfer density to quantify the effects of operating parameters on the cold start process at low temperature. And how these parameters affect the operation of the PEMFC stack at low temperature is investigated. The results show that inconsistency exists in stack operation due to the position particularity of the intermediate cell. Irreversible heat is the main heat source for the cold start of the stack, and the cathode catalyst layer is the main heat-generating component. The heat production proportion of cathode catalyst layer can reach 90%, which decreases with the increment of current density and the running time, especially for the edge cell. The initial ionomer water content is most sensitive to the cold start process of the stack, followed by the porosity of cathode catalyst layer. These parameters are sensitive to the cold start process mainly because of the change in volumetric exchange current density and oxygen concentration.     

Numerical study of cold start performance of proton exchange membrane fuel cell with coolant circulation

It has been well recognized that cold start is one of the key issues of proton exchange membrane fuel cell (PEMFC) used as the engine of vehicles. Coolant circulation is usually launched synchronously with the fuel cell during cold start to avoid sudden large temperature variation, which greatly increases the cell thermal mass, lowers the heating rate, and worsens the cell performance. Considering the flow and heat transfer of coolant circulation, a three-dimensional, transient, multi-disciplinary model for cold start is built up. The numerical results agree reasonably well with experimental data, indicating that the model can be used for the investigation of PEMFC cold start processes. The analysis of circulation parameter effects shows that increasing the coolant flow rate or coolant tank capacity has little influence on the cell voltage, but will increase the non-uniformity of temperature distribution along flow direction. At lower start-up temperature, this non-uniformity is more obvious. With higher coolant flow rate, although the distribution of current density becomes more evenly, the ice formation amount increases and its distribution and location are greatly affected.     

A review of low-temperature proton exchange membrane fuel cell degradation caused by repeated freezing start

Proton exchange membrane fuel cell (PEMFC) has advantages of zero emission, fast response and high-power density. There are still obstacles such as manufacturing cost, life span, infrastructure construction and subzero temperature star-up restricting commercialization of PEMFC. The low-temperature start-up is one of them that needs to be solved in the field of fuel cell vehicle. This paper presents research progresses involving PEMFC degradation caused by the low-temperature start-up. Degradation phenomena and mechanism under component-level caused by repeated freezing start, influencing factors and mitigation strategies are summarized and reviewed. Conclusions are made that frequent ice freezing and melting causes the membrane electrode assembly damaged irreversibly, the quality of cold start and low temperature influence the degradation strongly and purge after shutdown, better materials and optimal fuel cell structure design are helpful to reduce the impact of cold start on fuel cell performances. It is suggested that future work should be focused on optimizing strategies of the shutdown purge, promoting the quality of cold start, enhancing properties of the materials, improving internal structure design of stack and developing low-temperature attenuation models.     

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.     

Portable proton exchange membrane fuel-cell systems for outdoor applications

A hydrogen fuelled, 30 W proton exchange membrane fuel-cell (PEMFC) system is presented that is able to operate at an ambient temperature between −20 and 40 °C. The system, which comprises the fuel-cell stack, pumps, humidifier, valves and blowers is fully characterized in a climatic chamber under various ambient temperatures. Successful cold start-up and stable operation at −20 °C are reported as well as the system behaviour during long-term at 40 °C. A simple thermal model of the stack is developed and validated, and accounts for heat losses by radiation and convection. Condensation of steam is addressed as well as reaction gas depletion. The stack is regarded as a uniform heat source. The electrochemical reaction is not resolved. General design rules for the cold start-up of a portable fuel-cell stack are deduced by the thermal model and are taken into consideration for the design. The model is used for a comparison between active-assisted cold start-up procedures with a passive cold start-up from temperatures below 0 °C. It is found that a passive cold start-up may not be the most efficient strategy. Additionally, the influence of different stack concepts on the start-up behaviour is analysed by the thermal model. Three power classes of PEMFC stacks are compared: a Ballard Mk902 module for automotive applications with 85 kW, the forerunner stack Ballard Mk5 (5 kW) for medium power applications, and the developed OutdoorFC stack (30 W), for portable applications.     

Cold start characteristics of proton exchange membrane fuel cells

In this study, the effects of the start-up temperature, load condition and flow arrangement on the cold start characteristics and performance of a proton exchange membrane fuel cell (PEMFC) are investigated through in-situ experiments with the simultaneous measurements of the current and temperature distributions. Rather than the commonly recognized cold start failure mode due to the ice blockage in cathode catalyst layer (CL), another failure mode due to the ice blockage in flow channel and gas diffusion layer (GDL) leading to significantly high pressure drop through cathode flow field is observed at a start-up temperature just below the lowest successful start-up temperature. Three ice formation mechanisms are proposed, corresponding to the ice formations in cathode CL, GDL and flow channel. The general distributions of current densities and temperatures during the constant current cold start processes are similar to the constant voltage cold start processes, except that the temperatures at the end of the constant current cold start processes are more evenly distributed over the active reaction area because of the increased heat generation rates. The cold start characteristics are mainly dominated by the cathode flow, and changing the flow arrangement has unimportant impact on the cold start performance.     

Parametric studies of a double-cell stack of PEMFC using Grafoil™ flow-field plates

A parametric study of a double-cell stack of a proton exchange membrane fuel cell (PEMFC) using Grafoil™ flow-field plates is performed. A self-made membrane–electrode assembly (MEA) is used to integrate the PEMFC. Emphasis is placed on the effect of the transport parameters such as cell temperature, pressure and humidity of the reaction side, and flow-field geometry on the performance of the stack. Potential–current and power–current curves are presented. At a fixed dew point of the incoming reactants, say T_dp=30 °C, increasing the cell operating temperature past a threshold value of about 50 °C reduces the cell performance due to membrane dehydration. At a fixed cell operating temperature, a high flow back-pressure increases the cell performance through enhancing the reaction on both electrodes of the fuel cell. Moreover, the cell performance for the pressurised cathode side is better than that for the pressurised anode side due to the favourable back-diffusion of water in the membrane. Finally, empirical correlations are developed to describe the electrode process of the PEMFC stack under various operating conditions.     

Behaviour of a PEMFC supplying a low voltage static converter

This paper presents the behaviour of a proton exchange membrane fuel cell (PEMFC) connected to a static dc–dc converter. Two different models of the PEM fuel cell are obtained from static measurements and impedance spectroscopy. The paper points out the necessity to use different models according to the type of study performed on the system. The comparison of models at high semiconductors switching frequency (25 kHz) is illustrated. Various experimental results obtained on a 500 W PEMFC test bench are compared with simulation ones to illustrate the accuracy of the proposed models.     

Surface modified Nafion® membrane by ion beam bombardment for fuel cell applications

The interfacial structure between an electrolyte membrane and an electrode catalyst layer plays an important role in determining performance of proton exchange membrane fuel cell (PEMFC) since the electrochemical reactions produce electricity occur on the interfaces that are in contact with hydrogen or oxygen gas, so-called three phase boundaries. To improve performance of the PEMFC by enlarging effective area of the interfaces, surface of Nafion^® 115 membrane was roughened by Ar⁺ ion beam bombardment before being coated with a catalyst ink to form the electrode layer. With increasing ion dose density from 0 to 1 × 10¹⁷ ions cm⁻², roughness and hydrophobicity of the membrane surface increased, which could be favored for a high-performance PEMFC. In fuel cell tests, the single cell using Nafion^® membrane bombarded at an ion dose density of 10¹⁶ ions cm⁻² exhibited maximum power density of 0.62 W cm⁻², which was two times higher than that of the single cell employing untreated Nafion^® 115 membrane, i.e. 0.30 W cm⁻².     

A new approach to empirical electrical modelling of a fuel cell,an electrolyser or a regenerative fuel cell

In terms of fuel cell steady-state performance modelling, many electrical models have been developed either from a theoretical point of view or from an empirical point of view. The model described in this article is from the empirical point of view approach. This model enables to simulate both fuel cells and electrolysers V–J curves (cell voltage versus current density) in typical conditions. This model is particularly adapted to regenerative fuel cell (RFC) simulation. It is a four degree-of-freedom model and it is convergent near zero current. It depends on the stack temperature and the oxygen partial pressure. The regions where mass transfer limitations occur have not been modelled, because they are usually avoided for efficiency or thermal reasons. The parameters have been fitted with a 4 kW_e proton exchange membrane fuel cell (PEMFC) and a 3.6 kW_e electrolyser. The electrical equations and the experimental data are well correlated.     

Cold start analysis of polymer electrolyte membrane fuel cells

Cold start is critical to the commercialization of polymer electrolyte membrane fuel cell (PEMFC) for practical applications such as backup power and automotive applications. In this study, various numerically simulated PEMFC cold start processes are analyzed. The success of the cold start process depends on the competition between how fast the cell is heated up to the freezing point temperature and how fast ice is formed and built up in the pores of the cathode catalyst layer (CL) blocking oxygen transport to the reaction sites; the success of the cold start process thus depends on the product water (i) that is absorbed into the ionomer in the CL and membrane, (ii) that is taken away in vapour form by the gas flows (can be neglected), and (iii) that is frozen into ice in the CL pores. It is found that the membrane thickness and the ionomer volume fraction in the CL play pivotal roles in reducing the amount of ice formation. A thicker membrane leads to a larger water capacity but a slower water absorption process, and increasing the ionomer volume fraction in the CL enlarges the ionomer water capacity and enhances the membrane water absorption. Starting the cell under the potentiostatic condition is confirmed to be superior to the galvanostatic condition. Heating up the external surfaces and the inlet air enhances the temperature increment of the cell. However, the external heating methods have negligible improvement in reducing the amount of ice formation. Even though heating the inlet air is more effective in increasing the cell temperature than heating the outer surfaces, the heat capacity of the inlet air is low.     

The Gaston Planté Medal Presentation Address

The paper shows and discusses a procedure of parameter estimation applied to the evaluation of some operating parameters of a proton-exchange membrane fuel cell (PEMFC). First, a brief literature review about the main parameters (exchange current density, cell resistance, internal current density and limiting current density) has been done. Then the analytical model adopted to describe the polarization curve has been discussed. Based on this model, a parameter analysis has been done, and it has been shown that three parameters of the cell polarization curve model can be simultaneously estimated: the cathode exchange current density, the cell resistance and the internal current density. To evaluate these parameters both a set of our measurements on a PEM single cell (active area of 25 cm² and Nafion 115 membrane) and data from other authors has been considered.The cell has been fed with pure hydrogen and air, the cell temperature has been varied from 50 °C to 80 °C, and accordingly the reactants have been introduced in the cell humidified at the same temperature. The parameters have been estimated in each operating conditions of the cell, and their behavior, as a function of the cell operating temperature, has been discussed.     

Performance of planar free-breathing PEMFC at temperatures below freezing

For practical fuel cell applications it is vital to know how the fuel cell operates in varying ambient conditions, especially when passive control methods are used. In this contribution, the effect of subzero temperatures with constant current density and cold-start behavior of planar free-breathing PEMFC were studied in a temperature chamber. The temperature levels used in constant current measurements varied between 0 and −27.5 °C. The cell maintained stable operation without irreversible performance losses at higher current densities as the heat generation was high enough to prevent the product water from freezing inside the cell. However, ice formation on the outer part of the cathode side of the cell was observed. At low temperature and current density level, the freezing of product water inside the cell led into irreversible performance loss. The cold-start measurements showed that the cell is capable of starting operation at −5 °C without irreversible performance losses when the cell is initially dry. The cell was capable of starting operation also at −10 °C when the starting procedure was slow enough, but a slight irreversible performance loss was encountered.     

Effects of various operating and initial conditions on cold start performance of polymer electrolyte membrane fuel cells

Cold start is a challenging and important issue that hinders the commercialization of polymer electrolyte membrane fuel cell (PEMFC). In this study, a three-dimensional multiphase model has been developed to simulate the cold start processes in a PEMFC. Numerical simulations have been conducted for a single PEMFC starting at various operating and initial conditions, which are cell voltages, initial water contents and distributions, anode inlet relative humidity (RH), surrounding heat transfer coefficients, and cell temperatures. It is found that the heating-up time can be significantly reduced by decreasing the cell voltage and effective purge is critical for PEMFC cold start. The largest heating source at high cell voltages is the activational heat, and it becomes the ohmic heat at low cell voltages. The water freezing in the membrane is not observed when the cell is producing current due to the heat generation and the slow water diffusion into the membrane at subzero temperatures, and it is only observed after the cold start is failed, further confirming the importance of purge. Humidification of the supplied hydrogen has negligible effect on the cold start performance since only small amounts of water vapour can be taken by the gas streams at subzero temperatures. The surrounding heat transfer coefficients have significant influence on the heating-up time, indicating the importance of cell insulation or heating. The rate of cell heating up is reduced when the startup temperature is lowered due to the more sluggish electrochemical reaction kinetics.     

Maximum power cold start mode of proton exchange membrane fuel cell

Successful and fast cold start is important for proton exchange membrane (PEM) fuel cell in vehicular applications in addition to the desired maximum power in any case. In this study, the maximum power cold start mode is investigated in details and compared with other cold start modes based on a multiphase stack model. It is found that for the maximum power cold start mode, the current density is generally kept at high levels, and the performance improvement caused by the membrane hydration and temperature increment may not be observable. Therefore, before the melting point, the performance drops continuously. The maximum power cold start mode could better balance the heat generation and ice formation, leading to improved cold start survivability than that in the constant voltage and constant current modes, with a fast start-up generally guaranteed. Once the survivability can be ensured, the initial water content needs to be higher for fast cold start, suggesting that over purging should be avoided. The maximum power mode is suggested to be optimal for PEM fuel cell cold start based on the modeling results.     

The role of ambient conditions on the performance of a planar,air-breathing hydrogen PEM fuel cell

This paper presents experimental data on the effects of varying ambient temperature (10–40 °C) and relative humidity (20–80%) on the operation of a free-breathing fuel cell operated on dry-hydrogen in dead ended mode. We visualize the natural convection plume around the cathode using shadowgraphy, measure the gas diffusion layer (GDL) surface temperature and accumulation of water at the cathode, as well as obtain polarization curves and impedance spectra. The average free-convection air speed was 9.1 cm s⁻¹ and 11.2 cm s⁻¹ in horizontal and vertical cell orientations, respectively. We identified three regions of operation characterized by increasing current density: partial membrane hydration, full membrane hydration with GDL flooding, and membrane dry-out. The membrane transitions from the fully hydrated state to a dry out regime at a GDL temperature of approximately 60 °C, irrespective of the ambient temperature and humidity conditions. The cell exhibits strong hysteresis and the dry membrane regime cannot be captured by a sweeping polarization scan without complete removal of accumulated water after each measurement point. Maximum power density of 356 mW cm⁻² was measured at an ambient temperature of 20 °C and relative humidity of 40%.     

Investigation on sub-zero start-up of polymer electrolyte membrane fuel cell using un-assisted cold start strategy

The technical barriers for commercialization of polymer electrolyte membrane fuel cell (PEMFC) are the startup ability and survivability at sub-zero temperatures. Ice formation causes cold start fail and volume change damages the cell components leading to performance decay. Many strategies are used to assist successful cold start and to reduce the performance decay. But, unassisted cold start is very crucial and needs attention. Here, an experimental protocol is reported for successful unassisted cold start using low temperature gas purging at various temperatures (-5,-8,-10,-15, and -20 °C) as well as to recover temporary performance decay. The stability of the membrane electrode assembly is also studied in freeze/thaw and sequential cold start cycles. At temperature −10 °C, there is small performance decay after the 6th freeze/thaw cycle. However, the subsequent cold start cycle shows significant performance decay after the 6th cycle. Changes in microstructures and loss of hydrophobicity in the gas diffusion layer are attributed to the performance decay in both freeze/thaw and sequential cold start cycles. The effect of cold start temperature on the performance of a PEMFC in subsequent freeze/thaw cycles is also studied. It shows that depending upon the start-up temperature, the preferential ice formation can affect the performance decay characteristics.     

Durability investigation of carbon nanotube as catalyst support for proton exchange membrane fuel cell

Electrochemical surface oxidation of carbon black Vulcan XC-72 and multiwalled carbon nanotube (MWNT) has been compared following potentiostatic treatments up to 168 h under condition simulating PEMFC cathode environment (60 °C, N₂ purged 0.5 M H₂SO₄, and a constant potential of 0.9 V). The subsequent electrochemical characterization at different treatment time intervals suggests that MWNT is electrochemically more stable than Vulcan XC-72 with less surface oxide formation and 30% lower corrosion current under the investigated condition. As a result of high corrosion resistance, MWNT shows lower loss of Pt surface area and oxygen reduction reaction activity when used as fuel cell catalyst support.