Inlet Gas Humidity Research Articles

Experimental dissection of oxygen transport resistance in the components of a polymer electrolyte membrane fuel cell

Abstract Oxygen transport resistance is a major obstacle for obtaining high performance in a polymer electrolyte membrane fuel cell (PEMFC). To distinguish the major components that inhibit oxygen transport, an experimental method is established to dissect the oxygen transport resistance of the components of the PEMFC, such as the substrate, micro-porous layer (MPL), catalyst layer, and ionomer film. The Knudsen numbers are calculated to determine the types of diffusion mechanisms at each layer by measuring the pore sizes with either mercury porosimetry or BET analysis. At the under-saturated condition where condensation is mostly absent, the molecular diffusion resistance is dissected by changing the type of inert gas, and ionomer film permeation is separated by varying the inlet gas humidity. Moreover, the presence of the MPL and the variability of the substrate thickness allow the oxygen transport resistance at each component of a PEMFC to be dissected. At a low relative humidity of 50% and lower, an ionomer film had the largest resistance, while the contribution of the MPL was largest for the other humidification conditions.

Effect of electrode design and operating condition on performance of hydrogen alkaline membrane fuel cell

Hydrogen alkaline anion exchange membrane (AAEM) fuel cells are fabricated and tested, by measuring the output current densities at various cell voltages with a step change of 0.1V, under various ionomer contents in catalyst layer (volume fraction, 0.07, 0.21 and 0.50), inlet gas relative humidities (100%, 86% and 70%), anode back pressures (1.0, 1.01 and 1.02atm) and with/without insertion of micro porous layer (MPL). The corresponding theoretical analysis is also carried out, to clarify the factors that determine these experimental results. The experimental results show that high ionomer content, inlet gas humidity, slight anode pressurization and insertion of micro porous layer (MPL) are favorable. Since insufficient water supply (not oxygen) in cathode contributes to the major performance losses and low limiting current density, it is expected that properly increasing the water amount in cathode could effectively improve the membrane hydration, but not to causes considerable negative effect to cell performance.

Two-phase flow and droplet behavior in microchannels of PEM fuel cell

Water emerges in microchannels of proton exchange membrane fuel cells forming different two-phase flow patterns. In the current study, the two-phase flow in cathode side of a single-serpentine microchannel of proton exchange membrane fuel cell is quantified and characterized for different operating conditions as anode/cathode stoichiometry (flow rate), anode/cathode relative humidity and temperature of inlet gases. The two-phase flow is studied by direct visualization and image processing. The detected two-phase flow patterns are small/medium/large droplets, film, annular, slug and plug. An empirical correlation is proposed for average dimensionless diameter of droplets. The numbers of channel passes which involve in two-phase flow formation are derived. The distributions of each two-phase flow pattern in the channel are calculated. The effect of change of operating conditions on two-phase flow distribution is surveyed. The average local distributions of different two-phase flow patterns are studied for each channel pass, elbows and middle region. The dominant flow patterns are determined locally and overall. The droplets are categorized based on their diameters into three categories as small, medium and large droplets, then the average dimensionless droplet diameter is calculated. The distributions of three droplet categories in the channel are studied, and the dominant categories are identified for each case.

Relation Between Current Density Distribution in Land-Channel Direction and Presence of Liquid Water in Proton Exchange Membrane Fuel Cell Flow Channel

Since most of the PEMFCs have land-channel geometry in their flow field, the distance between the flow channel to the catalyst layer is not uniform. The distance is longer from the flow channel to the catalyst layer under the land area than to the catalyst layer under the channel area. This difference causes the non-uniform current density in land-channel direction, and the distribution largely depends on the operating conditions. As the inlet gas humidity in the operating condition decreases, the trend in the current density distribution changes from high local current under the channel to relatively uniform, and to higher local current under the land (Figure 1). This trend is due to the combined effects of (1) non-uniform oxygen transport resistance, (2) non-uniform liquid water transport distance causing the local flooding under the land area at wet condition, and (3) the tendency of faster membrane dryout under the channel area at dry condition. In order to investigate the distributions of various parameters in the land-channel direction, a small-scale segmented cell with about 300-micron resolution was developed by the authors’ group, and successfully measured the current and high-frequency resistance distribution in the land-channel direction. Distribution of oxygen transport resistance from the flow channel to the catalyst layer was also measured using limiting current technique. In this study a new segmented cell is developed. Using the transparent cathode plate the behavior of liquid water in cathode flow channel can be observed, and simultaneously the current density distribution is measured in land-channel direction. With this cell the relation between the current density distribution in the land-channel direction and the presence of liquid water is investigated. First, it is studied that at what gas humidity and at what overall current density the liquid water starts emerging from the gas diffusion layer surface to the flow channel. This provides us the information about the dependency of the liquid water emergence location on the current density distribution is studied. Then, how the presence of liquid water in the flow channel changes the current distribution is studied. Figure 1

Acceleration of Liquid Water Removal from Cathode Electrode of PEFC by Combination of Channel Hydrophilization and Diffusion Medium Perforation

Water flooding at cathode catalyst layer (CL) is a critical issue for high power density and long-term operation of polymer electrolyte fuel cells (PEFCs). Especially, at high current densities, excessive water produced by oxygen reduction reaction (ORR) on cathode side is rapidly condensed and accumulated inside porous electrode. When open pores in CL and gas diffusion layer (GDL) are filled with liquid water, oxygen cannot be sufficiently fed to reaction sites. To alleviate this issue, it is necessary to design the optimum channel/GDL structure for accelerating through-plane water removal. Turhan et al. [1] investigated the through-plane liquid storage, transport and flooding mechanism as a function of channel wall wettability with the use of high-resolution neutron imaging. Results revealed that hydrophilization of cathode channel forms liquid film layers around channel walls, which is difficult to purge. However, hydrophilic channel effectively enhances liquid water suction from under-land locations into gas channels. Nishida et al. [2] also demonstrated that the through-plane water transport from GDL to channel is encouraged by channel hydrophilization, and the voltage drop due to flooding is reduced. On the other hand, several researchers presented a concept of perforated GDL structure to secure sufficient water passages through porous electrode. Gerteisen et al. [3] developed the customized GDL which is structured with water transport pathways by laser perforation, and revealed that this modified GDL improves the limiting current density of 8-22%. The perforated GDL structure beneficially enhances in-plane water discharge from porous media toward large penetration holes. However, the laser perforations may act as water pooling locations under high-current and high-humidity conditions because of the removal of PTFE coating, leading to performance degradation [4].This study proposes the novel channel/GDL combinational structure of channel hydrophilization and GDL perforation as shown in Fig. 1, in order to promote water removal through cathode electrode.Liquid water accumulated at the CL is discharged into the large penetration hole. Subsequently, the water droplets gathered in the hole gradually grow up and reach the hydrophilic channel. When these droplets are attached to the channel sidewall, they are immediately spread out on the hydrophilic surface and liquid films are moved upward along the sidewall. This water suction through the penetration hole effectively alleviates water flooding near the cathode CL. In this experiment, the liquid droplet behavior inside the cathode channel of an operating PEFC is firstly observed based on a cross-sectional visualization technique, and the effect of hydrophilic treatment of cathode channel on the enhancement of through-plane water transport and the performance improvement is investigated. Secondly, the impact of combinational structure of channel hydrophilization and GDL perforation on the reduction of water flooding is demonstrated under high-current and high-humidity conditions. The GDL perforation is carried by two different methods of electric discharge machining (EDM) and manual micro-drilling technique. The EDM method has the disadvantage of removing the PTFE coating of GDL, resulting in local water flooding. In contrast, the micro-drilling technique does not damage the hydrophobic treatment of GDL.Fig. 2 shows the changes of cell voltage during startup for two different channel/GDL structures. The red line denotes the result for the untreated channel/GDL structure; the blue line denotes the combinational structure with hydrophilized channel and perforated GDL. The effective electrode area of the cell is 2.88 cm2. The width, depth and total length of the gas channel are 1.0, 1.0 and 88.5 mm. The contact angle of water of the hydrophilized channel is 17 deg. In the modified cell, 35 penetration holes (Hole diameter: 0.3 mm) are installed into the cathode electrode by the micro-drilling method. The fuel cell operation is performed at 0.73 A/cm2 for 1000 s. The inlet gas temperature and humidity are 45 deg. C and 80% RH, respectively. As shown in the figure, in the case of the untreated structure, the cell voltage drastically drops after starting due to water flooding. On the other hand, the modified channel/GDL structure gradually recovers the voltage after t=150 s, because liquid water is removed from the cathode CL.

Numerical study on the effects of gas humidity on proton-exchange membrane fuel cell performance

To examine the effects of gas humidity on proton-exchange membrane fuel cells (PEMFCs), three-dimensional multi-phase non-isothermal fuel cell model simulations were performed. Various inlet gas humidity conditions were independently imposed at the inlets of the anode and cathode flow channels. This numerical simulations revealed that fuel cell performance is significantly affected by both anode and cathode humidification. The ohmic loss is influenced by both anode and cathode inlet gas humidity, whereas the concentration loss is mainly affected by the cathode inlet gas humidity. The gas humidity in the flow channel can be controlled by adjusting the bipolar plate temperature. Additional simulations assessed different bipolar temperature distributions. When the temperature of the bipolar plate was increased in the direction of gas flow, the concentration loss decreased, owing to the reduced condensation of liquid water in the catalyst layer. Consequently, fuel cell performance increased, especially in the high current density region.

Analysis of the Influence of Temperature and Gas Humidity on the Performance Stability of Polymer Electrolyte Membrane Fuel Cells

Adequate water management represents one of the main challenges in the design and operation of polymer electrolyte membrane fuel cells. In this work, the influence of inlet gas humidification on cell performance is investigated by in-situ current density measurements obtained using the segmented cell approach. Particular attention is paid to the combined effect of cell temperature and relative humidity of the anode and cathode feed streams. When operated at 80°C and low humidity conditions, the cell is seen to undergo a severe voltage decline that is not observed at 60°C. The analysis shows that the variation with temperature of the water uptake rate of the gaseous streams plays a key role in determining the observed differences in performance stability. In the case of 60°C operation, the water uptake rate of the cathode stream at 50% inlet relative humidity is roughly 30% of its value at 80°C at the same humidification level, resulting in a significantly lower drying capacity. A simple balance of water model, able to explain the observed cell behavior, is finally presented and discussed.

Degradation Study of the Cells Operated Under Low or High Humidity Conditions at Cathode and/or Anode

Water management represents one of the main challenges for the design and operation of polymer electrolyte membrane fuel cells (PEMFC), the amount of water in the cell affects the performances and degradation processes. Commonly, the main goal is try to reduce the humidification in order to simplifying the humidification system and enhancing the performance stability of the cell. However, insufficient water content can lead to increased degradation and performance loss. These negative effects depend strongly on water distribution [1] in the cell and can therefore be highly non-uniform [2]. The relation between low humidification and degradation is complex and not yet fully understood. To bring more insight into this, the differences in the local conditions of the cell must be taken into account when examining performance and degradation. Thus, in this work, particular attention is paid to the local analysis of performance and degradation of cells operated under low or high humidity conditions at cathode and/or anode. In order to study locally the degradation influence of inlet gas humidification on cell performance all the experiments were performaned with the DLR- PCB segmented cell current density measurements, other global effect were analyzed using in-situ diagnostic tools, such as cyclic voltammetry. The changes observed locally in the current densities distributions were investigated locally by post-mortem ex-situ investigations by XPS and SEM/EDX to determine changes in the chemical composition of the different layers during the experiment. [1] D. G. Sanchez and P. L. Garcia-Ybarra, Int. J. Hydrogen Energy, 37, 7279 (2012). [2] D. G. Sanchez, D. G. Diaz, R. Hiesgen, I. Wehl and K. A. Friedrich, J. Electroanal. Chem., 649, 219 (2010).

Water Management in an Alkaline-Exchange-Membrane Fuel Cell

Among the existing fuel cell types, alkaline exchange membrane fuel cells (AEMFC) have intriguing features as compared to proton exchange membrane fuel cells. For example, their advantage is the high current density and mainly the possibility of using non-noble catalysts due to faster ORR kinetics in the alkaline media than in the acidic media. However, CO2, reacting with hydroxide ions to form carbonate ions, has a harmful effect on the air-breathing AEMFC by reducing the number of hydroxide available for the hydrogen oxidation reaction at the anode. Thus, AEMFC typically operates on pure oxygen, which dramatically increases its cost. Herein, we developed a first electrochemical model that can predict the response of AEMFC according to variations of operating parameters, such as the CO2 concentration in the oxidant flow and the relative humidity of inlet gases. The result in Figure 1 shows that current density significantly decays with higher CO2 concentration mainly due to hydroxide depletion by carbonate formation at the cathode. Other possible reactions are also accounted for in the model, such as the reverse water-gas shift reaction (RWGS) and the participation of carbonate ions in the hydrogen oxidation reaction. Water management and mass transport characteristics of an AEMFC assembly are discussed in detailed. With experimental verification, our model can be used to demonstrate different strategies for reducing harmful effects of CO2 poisoning, significantly increasing the possibility of AEMFC commercialization. Figure 1

Effect of Ionomer Content and Relative Humidity on IT-PEMFC Performances at 120 oc

Due to its higher operating temperature (100 -120oC), intermediate temperature PEMFC (IT-PEMFC) is expected to provide improved heat and water management, CO tolerance, and electrode reaction kinetics. The previous researches regarding IT–PEMFCs have been focused on the material developments: proton exchange membranes and electrocatalysts, to achieve high performance and durability at elevated temperatures. Maintaining hydration of membrane electrode assemblies (MEAs) to provide sufficient conductivity is one of the key issues in IT-PEMFC operation. When pressurized air feed is supplied with low humidity and high temperature, the sulfonated polymer membranes can be easily dried and, as a result, cell performances can be decreased severely. Therefore, the optimization of catalyst layer would be also very important, in order to achieve acceptable performances under unique operation conditions of IT-PEMFCs. However, there have been only reports regarding catalyst optimization of IT-PEMFC (1) (2). For conventional PEMFCs (~ 80oC), various groups have reported the effect of ionomer content (3), catalyst loading (4) and electrode thickness (5); but these results cannot be directly utilized to IT-PEMFCs that have much more severe drying problem. In this study, the effect of ionomer content and relative humidity (RH) of inlet gas on single cell performance are examined under various RHs(10 ~ 35%). For this, MEAs were fabricated with ionomer content from 20 to 40 wt%. The cell performances were characterized through constant current operation and i-V polarization. In addition, impedance analysis was carried out in order to monitor the variation of ohmic resistances under various operating conditions.

Transport membrane condenser for water and heat recovery from gaseous streams: Performance evaluation

Using a tubular ceramic membrane as the transport membrane condenser for simultaneous water and heat recovery from gaseous streams is experimentally investigated in the current study. The effects of several important operational parameters (e.g. gas flow rate, coolant flow rate, transmembrane pressure and inlet gas temperature) on the process performance in terms of mass and heat transfer across the membrane are systematically studied. It is found that mass and heat transfer rates can be enhanced by increasing the gas flow rate, coolant water flow rate and the temperature of the inlet gas stream. To improve the water and heat recovery, a low gas flow rate but a high coolant flow rate should be maintained. Increasing the transmembrane pressure difference decreases the mass and heat transfer mainly due to the reduced inlet gas humidity, enthalpy and flow rate. However, water and heat recovery does not change significantly with the change in transmembrane pressure. 20–60% water recovery and 33–85% heat recovery are achievable when using cold water as the coolant. The mass transfer mechanism in membrane condensation is complex and needs further exploration. These findings offer significant implications in using transport membrane condensers for water and heat recovery from gas streams with high moisture.

On active disturbance rejection in temperature regulation of the proton exchange membrane fuel cells

Operating a Proton Exchange Membrane fuel cell (PEMFC) system to maintain the stack temperature stable is one of the key issues in PEMFC's normal electrochemical reaction process. Its temperature characteristic is easily affected by inlet gas humidity, external disturbances, and electrical load changes and so on. Because of the complexity and nonlinearity of the reaction process, it is hard to build a model totally consistent with the real characteristic of the process. If model uncertainty, external disturbances, parameters changes can be regarded as “total disturbance”, which is then estimated and compensated, the accurate model is no longer required and the control design can be greatly simplified to meet the practical needs. Based on this idea, an active disturbance rejection control (ADRC) with a switching law is proposed for the problem of precise temperature regulation in PEMFC. Results of the work show that the proposed control system allows the PEMFC to operate successfully at the temperature of 343 K point in the presence of two different disturbances.

Effect of the Inlet Gas Humidification on PEMFC Behavior and Current Density Distribution

Water management represents one of the main challenges in the design and operation of PEMFCs. The influence of inlet gas humidification on cell performance is analyzed using in-situ diagnostic tools, such as cyclic voltammetry and segmented cell current density measurements, supported by post-mortem ex-situ investigations. Particular attention is paid to the effect of low humidity conditions in both cathode and anode, under which the cell is observed to suffer severe voltage decline. A simple one-dimensional water balance model is proposed to contribute to the understanding of the various operation regimes observed in PEMFCs under medium-to-low humidification conditions.

Effect of the Inlet Gas Humidification on PEMFC Behavior and Current Density Distribution

Water management represents one of the main challenges in the design and operation of PEMFCs. The influence of inlet gas humidification on cell performances is analyzed in general and especially the effect of low humidity conditions in both cathode and anode is studied. Hereby in situ diagnostic tools like EIS and current density measurements are used.Dry cathode air supply and a fully humidified anode gas stream lead to a periodic transition between low- and high-current cell operations. A review of the main variables involved in this phenomenology is given. Furthermore, this oscillatory behavior is used as a tool to study degradation processes. Figure1 Overall cell current density oscillations (symbols) and current density distributions at the low current (lower inset) and high current (upper inset) states of a single PEM cell operated with dry cathode air supply (RHc=5%) and a fully humidified anode gas stream (RHa=100%); Cell voltage= 600mV, Flow H2= 209 ml·min-1, Flow Air= 664 ml·min-1, Pressure= 1.5 bar,Tc=80°C

Maximum power efficiency operation and generalized predictive control of PEM (proton exchange membrane) fuel cell

Operating a proton exchange membrane fuel cell (PEMFC) system to produce power at the maximum power efficiency is one of the key issues in PEMFC's wide-spread applications. However, power density exhibits complex behavior and nonlinear dynamics with respect to the output cell voltage, fuel cell temperature, anode and cathode pressure, inlet gas humidity, and so on. In this paper, the distribution of power density in the domain of the output cell voltage and fuel cell temperature is delineated. By this delineation, the quadratic polynomial fitting was used to approximate the power density curve in local interval and estimate the maximum power efficiency point. Generalized predictive control (GPC) is presented to overcome the problem of time-varying dynamics of PEMFC in real time via applying a forgetting factor recursive least square (FFRLS) method. Based on the approximation and generalized predictive control strategy, maximum power efficiency operation of PEMFC is applied. The results of this work can contribute to the operation of PEMFC at the maximum power point, which guarantees the plant generating maximum power at the lowest consumption of hydrogen.

Measurements of Water Vapor and Current Distributions and Prediction of Water Crossover in PEMFC under Low-Humidity Conditions

In order to alleviate membrane dryout in proton exchange membrane fuel cells (PEMFCs) under low-humidity conditions, it is necessary to understand water transport between anode and cathode. In this study, the water vapor distribution within the anode gas channel of a low-humidity PEMFC was quantitatively evaluated by using humidity test paper (HTP), and the effects of inlet gas humidification and micro-porous layer (MPL) addition were investigated. In addition, the distribution of the hydrogen flow rate was calculated based on the current distribution measurement, and the local water crossover in the PEM was numerically predicted. It was noted that the MPL addition on the cathode side promotes the back-diffusion of product water at the low-humidity anode inlet of the co-flow serpentine flow field.

Quantitative estimation of water vapor distribution in low-humidity proton exchange membrane fuel cell using humidity test paper

To prevent performance degradation due to membrane dryout in proton exchange membrane fuel cells (PEMFCs) under low-humidity conditions, fundamental understanding of water transport and reaction mechanism on anode side in operating fuel cell is required. In this study, the water vapor distribution along the anode flow channel of a low-humidity PEMFC was quantitatively evaluated by using humidity test paper (HTP), and the effects of inlet gas humidification and flow configuration on the water distribution in the anode were investigated. HTP is a test paper for detecting water vapor in the range of 20–90% RH, and makes it possible to directly visualize the water distribution in the gas flow channel of the transparent PEMFC during operation. Furthermore, the locally resolved current density was also measured using segmented cell structure concept. It was found that the high humidification of the anode inlet gas increases the water vapor concentration in the anode channel and prevents the membrane dehydration. In addition, the counter-flow configuration effectively improves the water management near the anode inlet region of the low-humidity PEMFC and enhances the proton conductivity in the electrolyte membrane.

Removal of Elemental Mercury by Gas Phase Surface Discharge Plasma

The removal of elemental mercury (Hg0) with the reactive species produced by gas phase surface discharge plasma was studied. The effects of several operational parameters, such as applied voltage, inlet Hg0 concentration and gas humidity, were investigated. The removal of Hg0 was significantly promoted by an increase in the applied voltage of the reactor system. Hg0 oxidation efficiency decreases with increasing inlet Hg0 concentration. Furthermore, with the addition of H2O, Hg0 oxidation is remarkably restrained.

Electrochemical hydrogen separation and compression using polybenzimidazole (PBI) fuel cell technology

Simultaneous hydrogen separation and compression using high-temperature (>100 °C) PEM fuel cell technology (PBI) was demonstrated for pure hydrogen, nitrogen/hydrogen mixtures and reformate feed gas mixtures containing various amounts of CO, CO 2 and CH 4. Gas purity measurements of the separated gas were performed and significant reductions in impurities were achieved. The effects of hydrogen concentration, inlet gas humidification, operating temperature and air bleed on the cell performance were studied and hydrogen diffusion through the polybenzimidazole membrane was measured. The hydrogen separation required relatively low energy consumption and demonstrated good dynamic response. Simultaneous separation and compression of hydrogen up to 0.65 barg was demonstrated.

Effect of primary parameters on the performance of PEM fuel cell

A one-dimensional, steady-state and isothermal model for a proton exchange membrane (PEM) fuel cell has been developed to investigate the effects of various parameters such as the molar fraction of nitrogen gas, relative humidity, temperature, pressure, membrane thickness, anode and cathode stoichiometric flow ratio and the distribution of oxygen in the cathode catalyst while water transfer in membrane is produced by diffusion, pressure gradient and electro-osmotic drag. The most critical problems to overcome in the proton exchange membrane (PEM) fuel cell technology are the water and thermal management. The results show that the cell performance increases as operating pressure and temperature are increased. The performance of cell can decrease by decreasing the relative humidity of inlet gases and increasing the membrane thickness. Increasing the anode and cathode stoichiometric flow ratio can also improve the cell performance. As the oxygen concentration becomes zero in about 8 percent depth of cathode catalyst layer, the thickness of cathode catalyst layer can be reduced 92 percent without any potential loss in output voltage. The cathode activation loss also becomes high by increasing the molar fraction of nitrogen gas. The modeling results agree very well with experimental results.