Influence of oxygen on the cathode in HT-PEM fuel cells

In this paper, we have shown that an increase in the pressure of oxygen on the cathode side in HT-PEM fuel cells leads first to an increase and then a decrease in the potential of the cell and again at even higher pressures to an increase. The most likely explanation for this peculiar phenomenon is a competition between oxygen and phosphoric acid on the surface of the platinum catalyst in the three phase boundary. An increase in the oxygen pressure will in this way remove phosphoric acid from the platinum giving more space for oxygen but at higher pressure creating mass transport problems due to lack of phosphoric acid whereas at even higher pressures oxygen can orient itself in a different way on the platinum surface giving more space to the acid.     

Design and demonstration of an ethanol fuel processor for HT-PEM fuel cell applications

This work describes the development of a compact ethanol fuel processor for small scale high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) systems with 200–500 W electrical power output. Promising markets for reformer fuel cell systems based on ethanol are mobile or portable leisure and security power supply applications as well as small scale stationary off grid power supply and backup power. Main components of the fuel processor to be developed were the reformer reactor, the shift converter, a catalytic burner and heat exchangers. Development focused in particular on the homogeneous evaporation of the liquid reactants ethanol and water for the reformer and burner and on the development of an efficient and autarkic start-up method, respectively. Theoretical as well as experimental work has been carried out for all main components separately including for example catalyst screening and evaporator performance tests in a first project period. Afterwards all components have been assembled to a complete fuel processor which has been qualified with various operation parameter set-ups. A theoretically defined basic operation point could practically be confirmed. The overall start-up time to receive reformate gas with appropriate quality to feed an HT-PEMFC (x_CO < 2%) takes around 30 min. At steady state operation the hydrogen power output is around 900 W with H₂ and CO fractions of 41.2% and 1.5%, respectively.     

Characterization methodology for anode starvation in HT-PEM fuel cells

Degradation caused by fuel starvation may be an important reason for limited fuel cell lifetimes. In this work, we present an analytical characterization of the high temperature polymer exchange membrane fuel cell (HT-PEM FC) behavior under cycled anode starvation and subsequent regeneration conditions to investigate the impact of degradation due to H₂ starvation. Two membrane electrode assemblies (MEAs) with an active area of 21 cm² were operated of up to 550 min, which included up to 14 starvation/regeneration cycles. Overall cell voltage as well as current density distribution (S⁺⁺ unit) were measured simultaneously each minute during FC operation. The cyclicity of experiments was used to check the long term durability of the HT-PEM FC. After FC operation, micro-computed tomography (μ-CT) was applied to evaluate the influence of starvation on anode and cathode catalyst layer thicknesses.During starvation, cell voltage and current density distribution over the active area of the MEA significantly differed from nominal conditions. A significant drop in cell voltage from 0.6 to 0.1 V occurred after approx. 20 min for the first starvation step, and after 10 min for all subsequent starvation steps. By contrast, the voltage response is immediately stable at 0.6 V during every regeneration step. During each starvation, the local current density reached up to 0.3 A∙point⁻¹ at the area near the gas inlet (9 cm²) while near the outlet it drops to 0.01 A∙point⁻¹. The deviation from a balanced current density distribution occurred after 10 min for the first starvation step, and after ca. 2 min for the subsequent starvation steps. Hence, compared to the voltage drop, the deviation from a balanced current density distribution always starts earlier. This indicates that the local current density distribution is more sensitive to local changes in the MEA than overal cell voltage drop. This finding may help to prevent undesirable influences of the starvation process.The μ-CT images showed that H₂ starvation lead to thickness decrease of ca. 20–30% in both anode and cathode catalyst layers compared to a fresh MEA. Despite of the 14 starvation steps and the thinning of the catalyst layers the MEA presents stable cell voltage during regeneration.     

Experimental investigation of a liquid cooled high temperature proton exchange membrane (HT-PEM) fuel cell coupled to a sodium alanate tank

In high temperature proton exchange membrane (HT-PEM) fuel cells, waste heat at approximately 160 °C is produced, which can be used for thermal integration of solid state hydrogen storage systems. In the present study, an HT-PEM fuel cell stack (400 W) with direct liquid cooling is characterized and coupled to a separately characterized sodium alanate storage tank (300 g material). The coupled system is studied in steady state for 20 min operation and all relevant heat flows are determined. Even though heat losses at that specific power and temperature level cannot be completely avoided, it is demonstrated that the amount of heat transferred from the fuel cell stack to the cooling liquid circuit is sufficient to desorb the necessary amount of hydrogen from the storage tank. Furthermore, it is shown that the reaction rate of the sodium alanate at 160 °C and 1.7 bar is adequate to provide the hydrogen to the fuel cell stack. Based on these experimental investigations, a set of recommendations is given for the future design and layout of similar coupled systems.     

Electrochemical characterization of a polybenzimidazole-based high temperature proton exchange membrane unit cell

This work constitutes detailed EIS (Electrochemical Impedance Spectroscopy) measurements on a PBI-based HT-PEM unit cell. By means of EIS the fuel cell is characterized in several modes of operation by varying the current density, temperature and the stoichiometry of the reactant gases. Using Equivalent Circuit (EC) modeling key parameters, such as the membrane resistance, charge transfer resistance and gas transfer resistance are identified, however the physical interpretation of the parameters derived from EC’s are doubtful as discussed in this paper. The EC model proposed, which is a modified Randles circuit, provides a reasonably good fit at all the conditions tested. The measurements reveal that the cell temperature is an important parameter, which influences the cell performance significantly, especially the charge transfer resistance proved to be very temperature dependent. The transport of oxygen to the Oxygen Reduction Reaction (ORR) likewise has a substantial effect on the impedance spectra, results showed that the gas transfer resistance has an exponential-like dependency on the air stoichiometry. Based on the present results and results found in recent publications it is still not clear what exactly causes the distinctive low frequency loop occurring at oxygen starvation. Contrary to the oxygen transport, the transport of hydrogen to the Hydrogen Oxidation Reaction (HOR), in the stoichiometry range investigated in this study, shows no measurable change in the impedance data. Generally, this work is expected to provide a basis for future development of impedance-based fuel cell diagnostic systems for HT-PEM fuel cell.     

Performance of a poly(2,5-benzimidazole)-based polymer electrolyte membrane fuel cell

The performance of an ABPBI-based High Temperature H₂/O₂ PEMFC system was studied under different experimental conditions. Increasing the temperature from 130 to 170 °C improved the cell performance, even though further increase was not beneficial for the system. Humidification of the H₂ stream ameliorated this behaviour, even though operating above 170 °C is not advisable in terms of cell performance. A significant electrolyte dehydration seems to negatively affect the fuel cell performance, especially in the case of the anode. In the presence of 2% vol. CO in the H₂ stream, the temperature exerted a positive effect on the cell performance, reducing the strong adsorption of this poison on the platinum sites. Moreover, humidification of the H₂ + CO stream increased the maximum power densities of the cell, further alleviating the CO poisoning effects. Actual CO–O₂ fuel cell results confirmed the significant beneficial effect of the relative humidity on the kinetics of the CO oxidation process.     

Boundary model-based reference control of blower cooled high temperature polymer electrolyte membrane fuel cells

Fuel cells have, by design, a limited effective life time, which depends on how they are operated. The general consent is that operation of the fuel cell at the extreme of the operational range, or operation of the fuel cell without sufficient reactants (a.k.a. starvation), will lower the effective life time of a fuel cell significantly. On air cooled HTPEMFCs, the blower, which supplies the fuel cell with oxygen for the chemical process, also functions as the cooling system. This makes the blower bi-functional and as a result a higher supply of oxygen is often available, hence changes in the fuel cell output can be optimised by the knowledge of how much oxygen is supplied to the fuel cell at any given time, without reducing the effective life time of a fuel cell by starvation.     

Direct measurement of current density under the land and channel in a PEM fuel cell with serpentine flow fields

One of the most common types of flow field designs used in proton exchange membrane (PEM) fuel cells is the serpentine flow field. It is used for its simplicity of design, its effectiveness in distributing reactants and its water removal capabilities. The knowledge about where current density is higher, under the land or the channel, is critical for flow field design and optimization. Yet, no direct measurement data are available for serpentine flow fields. In this study a fuel cell with a single channel serpentine flow field is used to separately measure the current density under the land and channel, which is either catalyzed or insulated on the cathode. In this manner, a systematic study is conducted under a wide variety of conditions and a series of comparisons are made between land and channel current density. The results show that under most operating conditions, current density is higher under the land than that under the channel. However, at low voltage, a rapid drop off in current density occurs under the land due to concentration losses. The mechanisms for the direct measurement results and general guidelines for serpentine flow field design and optimizations are provided.     

Optimization of PEM fuel cell flow field via local current density measurement

The serpentine flow field is the leading type of flow field used today in proton exchange membrane (PEM) fuel cells and for this reason optimization of serpentine flow field design is extremely important. In this study, a unique technique developed in house is utilized to separately measure current density under the land and channel on a variety of serpentine flow field geometries. Each flow field is tested under a wide variety of operating conditions thereby providing guidance for the optimum design geometry. Experimental results show that generally flow fields with both thinner lands and thinner channels provide better overall performance. However, the optimal flow field designs are highly dependent on fuel cell operating parameters.     

Branched poly(ether ether ketone) based anion exchange membrane for H2/O2 fuel cell

High-performance anion exchange membranes (AEMs) are in need for practical application of AEM fuel cells. Novel branched poly(ether ether ketone) (BPEEK) based AEMs were prepared by the copolymerization of phloroglucinol, methylhydroquinone and 4,4′-difluorobenzophenone and following functionalization. The effects of the branched polymer structures and functional groups on the membrane's properties were investigated. The swelling ratios of all the membranes were kept below 15% at room temperature and had good dimensional stability at elevated temperatures. The branching degree has almost no effect on the dimensional change, but plays a great role in tuning the nanophase separation structure. The cyclic ammonium functionalized membrane showed a lower conductivity but a much better stability than imidazolium one. The BPEEK-3-Pip-53 membrane with the branching degree of 3% and piperidine functionalization degree of 53% showed the best performances. The ionic conductivity was 43 mS cm⁻¹ at 60 °C. The ionic conductivity in 1 M KOH at 60 °C after 336 h was 75% of its initial value (25% loss of conductivity), and the IEC was 83% of its initial value (17% loss of IEC), suggesting good alkaline stability. The peak energy density (60 °C) of the single H₂/O₂ fuel cell with BPEEK-3-Pip-53 membrane reached 133 mW cm⁻² at 260 mA cm⁻².     

Effects of humidification temperatures on local current characteristics in a PEM fuel cell

It is well known that water plays a very important role in the performance of proton exchange membrane (PEM) fuel cells. Non-uniform water content in the membrane leads to non-uniform ionic resistance, and non-uniform liquid water fraction in the porous electrode causes varied mass transfer resistances. The objective of this work is to study the effects of different anode and cathode humidification temperatures on local current densities of a PEM fuel cell with a co-flow serpentine flow field. The method used is the current distribution measurement gasket technique [H. Sun, G.S. Zhang, L.J. Guo, H. Liu, J. Power Sources 158 (2006) 326–332]. The experimental results show that both air and the hydrogen need to be humidified to ensure optimal cell performance, and too high or too low humidification temperature can cause severe non-uniform distribution of local current density. From the experimental results of local current density distributions, the local membrane hydration, the optimal humidification temperature, and if flooding occurs can be obtained. Such detailed local measurement results could be very valuable in fuel cell design and operation optimizations.     

Behavior of a unit proton exchange membrane fuel cell in a stack under fuel starvation

Durability is an important issue in proton exchange membrane fuel cells (PEMFCs) currently. Fuel starvation could be one of the reasons for PEMFC degradation. In this research, the fuel starvation conditions of a unit cell in a stack are simulated experimentally. Cell voltage, current distribution and localized interfacial potentials are detected in situ to explore their behaviors under different hydrogen stoichiometries. Results show that the localized fuel starvation occurs in different sections at anode under different hydrogen stoichiometries when the given hydrogen is inadequate. This could be attributed to the “vacuum effect” that withdraws fuel from the manifold into anode. Behaviors of current distribution show that the current will redistribute and the position of the lowest current shifts close to the anode inlet with decreasing hydrogen stoichiometry, which indicates that the position of the localized fuel starvation would move towards the inlet of the cell. It is useful to understand the real position of the degradation of MEA.     

Current density measurements in direct methanol fuel cells

The current density in the fuel cell is the direct consequence of reactions taking place over the active surface area. Thus, measurement of its distribution will lead to identification of the location and nature of reactions and will give opportunity to improve the overall efficiency of fuel cells. Within this study, the current density distribution in a direct methanol fuel cell was analyzed by segmenting the current collector into nine sections. Besides, the effect of the different operating parameters such as molarity, flow rate and reactant gas on the current density distribution was analyzed.     

Current distribution in polymer electrolyte membrane fuel cell with active water management

Air delivery is typically the greatest parasitic power loss in polymer electrolyte membrane fuel cell (PEMFC) systems. We here present a detailed study of an active water management system for PEMFCs, which uses a hydrophilic, porous cathode flow field, and an electroosmotic (EO) pump for water removal. This active pumping of liquid water allows for stable operation with relatively low air flow rates and low air pressure and parallel cathode channel architectures. We characterize in-plane transport issues and power distributions using a three by three segmented PEMFC design. Our transient and steady state data provide insight into the dynamics and spatial distribution of flooding and flood-recovery processes. Segment-specific polarization curves reveal that the combination of a wick and an EO pump can effect a steady state, uniform current distribution for a parallel channel cathode flow field, even at low air stoichiometries (α_air = 1.5). The segmented cell measurements also reveal the mechanisms and dynamics associated with EO pump based recovery from catastrophic flooding. For most operating regimes, the EO pump requires less than 1% of the fuel cell power to recover from near-catastrophic flooding, prevent flooding, and extend the current density operation range.     

A vibrational spectroscopic and modeling study of poly(2,5-benzimidazole) (ABPBI) – Phosphoric acid interactions in high temperature PEFC membranes

This paper reports a FT-ATR-IR spectroscopic study on proton conducting poly(2,5-benzimidazole) (ABPBI) membranes doped with orthophosphoric acid. The analysis of the vibrational profiles is a good diagnostic tool to help understand the interactions occurring between the phosphoric acid and the polymer membranes. The experimental data show evidence that an acid-base proton exchange reaction has occurred between the imidazole moieties in the polymer chain and phosphoric acid to produce dihydrogen phosphate ions and protonated imidazolium cations in ABPBIⁿ⁺. Vibrational modes associated with the dihydrogen phosphate ions are evident in the FT-IR spectra at lower doping levels and then become partially masked by the large amount of free phosphoric acid at high acid concentrations. Several bands in the FT-IR and FT-Raman spectra attributed to mixed modes containing varying contributions from NH bending motions exhibit high frequency shifts upon protonation the imidazole moieties. The correlatively assigned experimental vibrational bands were compared with calculated normal modes for small molecule models. The optimized geometry of the benzimidazolium dimer suggests that protonation of ABPBI results in a perturbation of the extended conjugated π system and allows rotation of the benzimidazole monomer units along the polymer chain. The results described here provide insight into the roles of phosphoric acid and ABPBI in the conduction mechanism of polybenzimidazole systems.     

An alkaline direct ethylene glycol fuel cell with an alkali-doped polybenzimidazole membrane

An alkaline direct ethylene glycol fuel cell (DEGFC) with an alkali-doped polybenzimidazole membrane (APM) is developed and tested. It is demonstrated that the use of APMs enables the present fuel cell to operate at high temperatures. The fuel cell results in the peak power densities of 80 mW cm⁻² at 60 °C and 112 mW cm⁻² at 90 °C, respectively. The power output at 60 °C is found to be 67% higher than that by DEGFCs with proton exchange membranes, which is mainly attributed to the superior electrochemical kinetics of both ethylene glycol oxidation and oxygen reduction reactions in alkaline media.     

Preliminary design of a small-scale system for the conversion of biogas to electricity by HT-PEM fuel cell

In this work a novel concept for the decentralized conversion of biogas to electricity is introduced. It consists of five segments: gas supply, gas treatment, gas reforming, gas usage and post-combustion. The system was designed in a regional project called GREEN-FC. The project is dealing with a design study for the conversion of 1 m³ h⁻¹ biogas to electricity, based on equilibrium calculations for steam reforming and water–gas shift reaction in combination with CFD simulations. The simulation results revealed that the system converts methane fully and delivers a maximum yield of hydrogen with a low concentration of carbon monoxide, thus making it suitable for a high-temperature polymer–electrolyte membrane (HT-PEM) fuel cell. The calculated electrical efficiency of the novel process is approximately 40%. Another important result of this work is the modular prototype design, because the individual components of the prototype can be replaced. For example alternative reactors that convert biogas into hydrogen and other technologies that use hydrogen can be included.     

Surrogate model for proton exchange membrane fuel cell (PEMFC)

The main goal of this work is to realize a PEMFC model that can be used efficiently for the global modelling of the fuel cell system. The modelling method proposed in the paper is an approach from an empirical point of view that allows a PEMFC model of “black-box” class to be developed. Moving least squares (MLS) have therefore been employed to approximate the cell voltage characteristics V, using an experimental dataset measured in determinate conditions. The MLS approach appears to present a good balance of response surface accuracy, smoothness, robustness, and ease of use. This kind of numerical model offers good perspectives for the systems identification, the simulation of the systems, the design and the optimization of process control, etc. The results prove that the method is suitable for predicting and describing the fuel cell behaviour in all the points of the approximation domain. The proposed model can be included in a numerical application to optimize the operation of an existing fuel cell system.     

Prediction of local current distribution in polymer electrolyte membrane fuel cell with artificial neural network

In the development process of a fuel cell, understanding the local current distribution is essentially required to achieve better performance and durability. Therefore, many developers apply a segmented fuel cell to observe current distribution under various operating conditions. With the application, experimental data is collected. This study suggests a utilization method for this collected data to develop a local current prediction model. The details of this neural network-based prediction model are introduced, including the pretreatment of the data. In the pretreatment process, current residual values are used for better prediction performance. As a result, the model predicted local current values with a 2.98% error. With the model, the effects of pressure, temperature, cathode relative humidity, and cathode flow rate on local current distribution trends are analyzed. Since the non-uniform current distribution of a fuel cell often leads to low performances or fast local degradation, the optimal operating condition to achieve current uniformity is acquired with an additional model. This model is developed by switching inputs and outputs of the local current prediction model. With the model application, the uniform current distribution is achieved with a standard deviation of 0.039 A/cm² under the current load at 1 Acm^?2.     

Analysis and optimization of high temperature proton exchange membrane (HT-PEM) fuel cell based on surrogate model

The stoichiometric ratio and flow channel geometry play a vital role in the performance of high temperature proton exchange membrane (HT-PEM) fuel cells. Because of the high cost of experiments or simulations, most analyses and optimization of the stoichiometric ratio and flow channel geometry are limited to several points in the entire design domain. In this study, an analysis and optimization method for HT-PEM fuel cells based on the surrogate model was proposed. Surrogate models were constructed using some of the available budgets of samples to analyze and optimize the entire design domain. With this method, it was indicated that the effect of the cathode stoichiometric ratio is more significant to the cell performance than the anode stoichiometric ratio and there are significant nonlinear interactions among the flow channel geometry parameters. At the fixed operating voltage, the flow channel geometry with the maximum current density and that with the maximum real power were obtained. Compared with the base design, the designs obtained by the surrogate model improve the current density and real power by 10.54% and 3.93%, respectively. Thus, this analysis and optimization method is demonstrated to be helpful and deserves attention in future research.