High Hydrogen Utilization Research Articles

Nonlinear model predictive control of PEMFC anode hydrogen circulation system based on dynamic coupling analysis

Anode hydrogen circulation system with purge valve has become one of the most effective ways for proton exchange membrane fuel cell (PEMFC) to alleviate performance degradation caused by nitrogen permeation and improve the hydrogen utilization rate. However, nonlinearity and multivariable coupling of the system make it difficult for PEMFC to achieve pressure and hydrogen supply synchronous tracking under complex operating conditions. To solve the problem, a novel nonlinear model predictive control scheme based on coupling analysis is proposed in this paper. Firstly, coupling characteristics of the hydrogen circulation system and the control pairing of the multivariable system are analyzed based on the relative gain array method. Then a novel control scheme based on an adaptive model predictive controller and a nonlinear model predictive controller is designed to keep the anode pressure stable and sufficient hydrogen supply, which use nonlinear observers to estimate the internal states online. Finally, the proposed controllers are implemented in control experiments of a 50 kW PEMFC anode hydrogen circulation system. The results demonstrate that the proposed control approach has great dynamic performance, anti-disturbance ability and can maintain a high hydrogen utilization rate under purge operation, current disturbance and uncertain intake pressure.

A New Methodological Approach on the Characterization of Optimal Charging Rates at the Hydrogen Plasma Smelting Reduction Process Part 2: Results.

To meet the target for anthropogenic greenhouse gas (GHG) reduction, the European steel industry is obliged to reduce its emissions. A possible pathway to reach this requirement is through developments of new technologies for a GHG-free steel production. One of these processes is the hydrogen plasma smelting reduction (HPSR) developed since 1992 at the Chair of Ferrous Metallurgy at the Montanuniversitaet Leoben in Austria. Based on the already available publication of the methodology in this work, potential process parameters were investigated that influence the reduction kinetics during continuous charging to improve the process further. Preliminary tests with different charging rates and plasma gas compositions were carried out to investigate the impacts on the individual steps of the reduction process. In the main experiments, the obtained parameters were used to determine the effect of the pre-reduction degree on the kinetics and the hydrogen conversion. Finally, the preliminary and main trials were statistically evaluated using the program MODDE® 13 Pro to identify the significant influences on reduction time, oxygen removal rate, and hydrogen conversion. High hydrogen utilization degrees could be achieved with high iron ore feeding rates and low hydrogen concentrations in the plasma gas composition. The subsequent low reduction degree and thus a high proportion of oxide melt leads to a high oxygen removal rate in the post-reduction phase and, consequently, short process times. Calculations of the reduction constant showed an average value of 1.13 × 10−5 kg oxygen/m2 s Pa, which is seven times higher than the value given in literature.

Numerical study on water transfer characteristics under joint effect of placement orientation and flow channel size for PEMFC with dead-ended anode

Proton exchange membrane fuel cell (PEMFC) with dead-ended anode is of outstanding advantages of high hydrogen utilization and simple hydrogen system. Because of its unique operation mode, the effect of gravity on water and gas transportation in PEMFC with dead-ended anode cannot be ignored. A two-phase model of PEMFC with dead-ended anode was developed, and the effects of placement orientation and cross section sizes of flow channel on water transport characteristics were analyzed. The results show that the flow channel with smaller cross section (0.5mm × 0.5 mm) showed better initial performance, but its voltage drop rate was faster due to liquid water was easier to be accumulated in the channel. Conversely, the liquid water volume fraction was smaller with a larger flow channel size. Moreover, the volume fraction of liquid water at the end of the channel was significantly greater than that in other areas when it was placed vertically. According to the results, the flow channel placed vertically combined with a larger cross section size (2mm × 2 mm) could effectively transferred liquid water to the end of the flow channel. Thus, it can effectively reduce the damage to PEMFC caused by water accumulation.

Numerical Simulation on Pressure Dynamic Response Characteristics of Hydrogen Systems for Fuel Cell Vehicles

A proton exchange membrane fuel cell (PEMFC) is known as one of the most promising energy sources for electric vehicles. A hydrogen system is required to provide hydrogen to the stack in time to meet the flow and pressure requirements according to the power requirements. In this study, a 1-D model of a hydrogen system, including the fuel cell stack, was established. Two modes, one with and one without a proportion integration differentiation (PID) control strategy, were applied to analyze the pressure characteristics and performance of the PEMFC. The results showed that the established model could be well verified with experimental data. The anode pressure fluctuation with a PID control strategy was more stable, which reduced the damage to the fuel cell stack caused by sudden changes of anode pressure. In addition, the performance of the stack with the PID control mode was slightly improved. There was an inflection point for hydrogen utilization; the hydrogen utilization rate was higher under the mode without PID control when the current density was greater than 0.4 A/cm2. What is more, a hierarchical control strategy was proposed, which made the pressure difference between the anode and cathode meet the stack working requirements, and, more importantly, maintained the high hydrogen utilization of the hydrogen system.

Experimental study of key operating parameters effects on the characteristics of proton exchange membrane fuel cell with anode recirculation

Water management is essential to ensure high performance and stable operation of the proton exchange membrane fuel cell (PEMFC). However, little is known about water management in the hydrogen circulation anode (HCA) mode which can achieve high hydrogen utilization. In this work, the effects of hydrogen circulation rate, a key parameter of HCA mode, on water management and cell performance are experimentally investigated. In particular, transparent fuel cell and printed circuit board segmented fuel cell technologies are used to study the water distribution and the local current density distribution, respectively. Furthermore, the electrochemical properties and PEMFC stability at different hydrogen circulation rates are discussed in detail. The experimental results indicate that increasing the hydrogen circulation rate reduces liquid water in the cathode and anode flow channels, which can effectively alleviate the adverse effects of flooding on overall performance and cell stability. However, subsequent drying of the membrane and catalyst layer results in an increase in ohmic resistance and a decline in cell performance, especially at low current densities. The current density distribution shows that severe local flooding is likely to occur in the hydrogen and air outlet regions at a low hydrogen circulation rate. For a high hydrogen circulation rate, membrane drying will occur in some areas, especially near the hydrogen inlet. To achieve uniform current density distribution and high fuel cell performance, the hydrogen circulation rate should be increased with the load current.

Tandem Photocatalysis Protocol for Hydrogen Generation/Olefin Hydrogenation Using Pd-g-C3N4-Imine/TiO2 Nanoparticles.

An unprecedented visible-light-driven photocatalytic system consisting of Pd nanoparticles stabilized on g-C3N4-imine-functionalized TiO2 nanoparticles was discovered for photoassisted hydrogen generation followed by olefin hydrogenation under mild conditions. The structural integrity of the as-synthesized photocatalyst was corroborated by Fourier transform infrared spectroscopy, X-ray powder diffraction, energy-dispersive X-ray spectroscopy, inductively coupled plasma atomic emission spectroscopy, X-ray photoelectron spectroscopy, ultraviolet-diffuse reflectance spectroscopy, Brunauer-Emmett-Teller measurements, and thermogravimetric analysis (TGA). Transmission electron microscopy and high-resolution scanning electron microscopy revealed the nanoscopic nature of the catalyst. The photocatalyst promoted several different transformations in a one-pot reaction sequence: hydrogen evolution through photocatalytic acceptorless formation of benzimidazoles as important therapeutic agents followed by visible-light-driven photocatalytic reduction of olefins with a high hydrogen utilization efficiency of up to 92% under mild conditions. A significant volume of H2 was produced under blue light-emitting diode (LED) irradiation during the selective formation of benzimidazole, while the selectivity reduced significantly under a Xe lamp or in the dark. The in situ-generated H2 could be activated by the as-prepared Pd-C3N4-imine/TiO2 photocatalyst to effectively hydrogenate olefins under mild conditions at appropriate time exposed to blue LED irradiation. The light-dependent photocatalytic performance of the title catalyst was assessed using action spectra by calculating the apparent quantum efficiency (AQE), which exhibited the maximum AQEs at 410 and 550 nm, at which the highest performance for styrene hydrogenation was obtained. The improved photoredox activity of the title nanohybrid could be caused by the synergistic effects of the heterojunction of carbon nitride-Pd on TiO2 nanoparticles evidenced by photoluminescence spectra and catalytic reactions. The catalyst proved to be air-stable, robust, recyclable, and very active in the absence of any undesirable additives and reducing agents. Thus, this work presents a new protocol for improving the photocatalytic properties of semiconducting materials for various photocatalytic applications under environmentally friendly conditions.

Bifunctional Pt–Mo catalyst for in situ hydrogenation of methyl stearate into alkanes using formic acid as a hydrogen donor

In situ hydrogenation of methyl stearate on Pt–Mo/AC has a high hydrogen utilization rate due to the strong interaction between Pt and β-Mo2C.

Operation of PEMFC Anodes in Dead-Ended Vs. Flow-through Modes

Low system complexity and high hydrogen utilization are important attributes for effective proton exchange membrane fuel cells (PEMFCs). Operating a PEMFC with a dead-ended anode (DEA) mode configuration is a simple and effective way to solve these two challenges1, 2. In comparison with the hydrogen flow-through and recirculation mode, in DEA mode the hydrogen supplied to the anode compartment is controlled by pressure regulation instead of a mass flow controller, where a pressure regulator is installed at the anode inlet and a normally closed solenoid valve blocks the outlet. Over time, accumulated water and nitrogen diffused from the cathode are removed by forced gas convection through intermittent opening of the solenoid valve.The anode and cathode gas diffusion media (GDM) play a role in PEMFC water management and have a significant impact on PEMFC performance. We compare an asymmetric GDM pairing with Freudenberg GDM (H24C3 at anode, H23C2 at cathode) to a symmetric GDM pairing frequently used in the open literature containing SGL 29BC at both the anode and cathode, to highlight the impact of the GDM water management on fuel cell operation in flow-through mode and DEA mode. We have previously shown in open-cathode fuel cells that an asymmetric GDM pairing featuring higher porosity in the anode GDM than the cathode significantly improves hydration and power production3, 4.The results at 25% relative humidity (RH) are shown in Figure 1. Consistent with our prior results5, in regular flow-through mode, Figure 1A shows the PEMFC with an asymmetric Freudenberg GDM pairing has significantly higher current densities vs. the PEMFC with symmetric SGL 29BC GDM at an operating cell voltage ≤ 0.60 V. The advantage of using asymmetric GDM observed in flow-through mode is maintained when the PEMFCs are operated in DEA mode (i.e. open symbols). There is only a marginal decrease in cell voltages during operation in DEA mode compared to flow-through mode probably due to the use of dry hydrogen on the anode side during testing in DEA mode.Figure 1B shows that the cell voltage decay is significantly different for a PEMFC containing the symmetric SGL 29BC GDM compared one with asymmetric Freudenberg GDM when tested in DEA mode configuration. The time between purge events is much greater for the asymmetric Freudenberg GDM pairing. The anode is purged with 99.999% H2 when the cell voltage drops by 100 mV. The PEMFC tested with the asymmetric Freudenberg GDM pairing dwells longer at steady-state prior to voltage decay, and its periods of voltage decay are more gradual. The PEMFC containing the symmetric SGL 29BC pairing has a mean purge interval of about 2min 33s and a total of fourteen purge events are necessary over 80 minutes operation, compared to a mean purge interval of about 18min 29s and a total of four purge events are necessary for the PEMFC containing the asymmetric Freudenberg pairing.A longer purge duration is desirable in a fuel cell system because it increases H2 utilization and reduces valve wear. The results show that the asymmetric GDM pairing improves water management in PEMFCs and improves both power density and system level performance.Acknowledgements:The authors are grateful to the Office of Naval Research for support of this research. References I.-S. Han, J. Jeong and H. K. Shin, Int. J. Hydrogen Energy, 38, 11996 (2013).K. Nikiforow, H. Karimäki, T. M. Keränen and J. Ihonen, J. Power Sources, 238, 336 (2013).R. W. Atkinson, M. W. Hazard, J. A. Rodgers, R. O. Stroman and B. D. Gould, J. Electrochem. Soc., 166, F926 (2019).R. W. Atkinson, J. A. Rodgers, M. W. Hazard, R. O. Stroman and B. D. Gould, J. Electrochem. Soc., 165, F1002 (2018).R. W. Atkinson, Y. Garsany, B. D. Gould, K. E. Swider-Lyons and I. V. Zenyuk, ACS Appl. Energy Mater., 1, 191 (2018) Figure 1. (A) Comparison of I-V polarization curves measured for a PEMFC containing a typical symmetric SGL 29BC GDM pairing on the anode and cathode side and a PEMFC containing an asymmetric Freudenberg GDM pairing . Polarization curves are measured in DEA mode and flow-through mode at 25 % RHinlet cathode. In all cases the cell temperature is 65 °C, in H2|air, with air supplied to the cathode a stoichiometric ratio of 2 at atmospheric pressure. During DEA mode, dry H2 is supplied to the anode at 2 psi. (B) Time evolution of the cell voltage obtained at a current density of 1200 mA cm-2 for an H2 inlet pressure of 2 psi and 25 % RHinlet cathode. Figure 1

A transient Pseudo-3D model of the PEM fuel cell for the analysis of dead-ended anode and anode bleeding operation modes

The high energy density of hydrogen is very advantageous for automotive applications, however the high cost and low durability of proton exchange membrane (PEM) fuel cells limit their commercial use. The anode-bleeding operation mode offers a very high hydrogen utilization and cost reduction by eliminating the components necessary for the recovery of hydrogen in the flow-through mode and avoids the carbon corrosion reaction, which causes degradation of the catalyst layer when the bleeding rate is set to zero in the dead-ended mode. A two-phase, non-isothermal, transient and pseudo-three-dimensional model is developed here to study the cell performance and carbon corrosion during dead-ended and anode bleeding operation modes. The model is validated against the experimental data from the literature and used to investigate the effects of the geometric and operation parameters on the voltage transient during dead-ended anode (DEA) operation mode. Results demonstrate that a lower load-current density, higher anode pressure, lower relative humidity at the cathode inlet, higher stoichiometric flow in the cathode, higher cell temperature, and shorter, deeper and wider channels can improve the cell performance under the DEA operation. The bleeding rate is optimized to sustain a stable transient cell voltage without carbon corrosion in the cathode catalyst layer while the hydrogen utilization is more than 99%.

Metal hydride reactor for dual use: Hydrogen storage and cold production

In case a metal hydride system is used for gaseous hydrogen storage application, the overall efficiency of the storage system can be significantly improved when the corresponding cooling or heating effect is additionally integrated into the application. In this study, it is analyzed how the efficiency of the gaseous hydrogen storage as well as the temperature level of the cold production can be adjusted using a simple, modular, air-cooled metal hydride reactor. For the study, a Hydralloy C5-graphite composite has been selected as reference material, and as geometry a simple tubular shape has been considered. In the first part of the manuscript, an analytical approach is presented analyzing the three different regimes in which a metal hydride reactor can be operated: to store hydrogen (Regime I), produce cold (Regime III), or to couple both options as “dual use” (Regime II), depending on the mass flow rates of hydrogen and air. In the second part of the manuscript, results of a detailed numerical study are presented and evaluated with respect to the outlet air temperature and the hydrogen utilization factor of the hydride reactor. It is shown that in Regime I, a high utilization factor and in Regime III, low outlet air temperature levels can be reached. Furthermore, in the “dual use” regime, the reactor can produce a cooling effect at different temperature levels and still show a high hydrogen utilization factor. In the last part of the manuscript, the effects of a modular reactor system as well as the operation of the reactor in pressure control mode are discussed to extend the basic description of the problem.

CO2 Enrichment in Anode Loop and Correlation with CO Poisoning of Low Pt Anodes in PEM Fuel Cells

AbstractIn automotive fuel cell systems anode fuel re‐circulation is often used to achieve high hydrogen utilization rates which reduces the hydrogen consumption of the fuel cell car, as well as it is an appropriate way to control hydrogen emissions. During operation hydrogen is consumed, while residual gases increase in the previously mentioned hydrogen loop. Besides nitrogen, we have found that CO2 accumulates in the anode loop and concentrations between 150–350 ppm were measured for varying current densities. We attribute this finding to CO2 crossover from the cathode to the anode and subsequent enrichment in the anode loop. To study the effect of this relatively small CO2‐concentration on the cell performance, tests were conducted with a proton exchange membrane (PEM) 45 cm2 single test cell with contaminated hydrogen/air feed. The data clearly indicate that electrochemical reduction of CO2 to CO takes place which has a significant impact on the cell performance due to blocked catalyst sites by CO affecting the current density of the hydrogen oxidation reaction (HOR). The measurements with hydrogen containing CO2 were matched with hydrogen plus CO measurements to quantify the impact and to determine a “CO‐equivalent concentration” for CO2. Consequences for the operation strategy of fuel cell systems are given.

The experimental analysis of a dead-end H2/O2 PEM fuel cell stack with cascade type design

Proton exchange membrane fuel cells (PEMFCs) with a dead-ended anode and cathode can reach high hydrogen and oxygen utilization by a relatively simple system. Nevertheless, the accumulation of the water in the anode and cathode channels can lead to a local fuel starvation deteriorating the performance and the durability of PEMFCs. In this study, a novel design for a polymer electrolyte membrane (PEM) fuel-cell stack was presented which could achieve higher fuel utilization without using hydrogen and oxygen recirculation devices such as hydrogen pumps or ejectors that consume parasitic power and require additional control schemes. The basic concept of the innovatively proposed design was to divide the cells of a stack into several stages by conducting the outlet gas of each stage to a separator and reentering it into the next stage; thereby, a multistage anode and cathode system was prepared. In this relatively ingenious design, a higher gaseous flow rate was maintained at the cell outlet, even under dead-end conditions resulted in a reduced purge-gas emission by avoiding the accumulation of liquid water in the cells. The results revealed that proposed design had the same polarization curve as the open-end mode, leading to an enhanced PEMFC performance.

Design, manufacturing, assembling and testing of a transparent PEM fuel cell for investigation of water management and contact resistance at dead-end mode

PEM fuel cell can operate at two different modes. At first mode the outlet of gas flow field is open and at the second mode the outlet of flow field is closed. The second mode is known as dead-end PEMFC. Proton exchange membrane fuel cells (PEMFCs) with dead-ended anode and cathode can obtain high hydrogen and oxygen utilization by a comparatively simple system. Nevertheless, the accumulation of the water in anode and cathode channels can lead to a local fuel starvation degrading the performance and durability of PEMFCs. There are different methods for investigation of water management such as: neutron radiography, gas chromatography, capturing by use of X-ray and capturing by use of infrared ray. Due to high cost and many hazards these methods at most cases cannot be used. According to the above mentioned problem we recommend a transparent PEMFC as a simplest, cheapest and the most suitable method for investigation of water management. Designing and manufacturing this type of PEMFC require special techniques. In this paper at first an optimal flow field is numerically designed and according to numerical results a transparent PEMFC is designed, manufactured and tested. Furthermore the performance of PEMFC at dead-end mode and open-end mode is studied. The applied design with a higher efficiency could have a same polarization curve as open-end mode. The results showed that by setting the purge interval time on 5 s and then opening purge valve for 1 s, there isn't any degradation on PEMFC performance but for purge interval of 10 s gradual performance degradation is recorded.

(Invited) Modeling and Control of Pemfcs with Dead-Ended Anodes

Nitrogen, oxygen and water crossover and their accumulation in the anode channels of low temperature Proton Exchange Membrane Fuel Cells present system level challenges for the design and operation of devices with high hydrogen utilization. The low flow rates enable operation under self-humidified anode conditions further reducing the system weight, cost, and complexity which is especially important for small portable applications. However, accumulated inert gasses and liquid water need to be purged before severe starvation conditions lead to carbon corrosion, inevitably wasting some Hydrogen. Frequent purging wastes fuel and dries the membrane leading to physical degradation and membrane delamination observed in post mortem Scanning Electron Microscopy images. Model based control methods provide an automated way to schedule the anode purges and study the tradeoff between degradation and fuel efficiency over the life of a fuel cell stack [1]. In addition the models can be used for design purposes such as optimizing the anode channel dimensions to balance the tradeoff between power density and interval between purges. The transport of gas species in the anode and cathode channels can be predicted by partial differential equations, specifically the 1-D the multi-component Stefan-Maxwell equation along the length of the channel. The distribution of membrane hydration level and temperature must also be considered as they impact the crossover rate. The model accurately predict the nitrogen accumulation and resulting recoverable drop in terminal voltage over time between purges when operating with a Dead Ended Anode (DEA). The model has been validated by simultaneous Neutron Imaging (NI) and Gas Chromatography (GC) sampling from within the anode channel [4]. Neutron imaging provides a unique measurement of the liquid water distribution within the cell and can be performed on relevant materials and channel designs without the need for modified hardware with different thermal properties such as transparent plates for optical imaging. The two dimensional images provide information about both the membrane electrode assembly water content, and channel liquid water [5]. This allows us to validate model predictions of the two phase (liquid water) transition point along the length of the channels. Dead Ended Anode (DEA) operation represents the extreme case of operation with high hydrogen utilization. There are several practical advantages to studying this limiting behavior of the system. Foremost is the direct measurement of membrane permeation rates since the only sources of nitrogen and oxygen in the anode channel is crossover. The introduction of oxygen into the anode gas stream from membrane crossover is the primary mechanism for cathode carbon corrosion. The oxygen present in a hydrogen depleted region creates a voltage reversal and hence large cathode potential in the adjacent region of the cell similar to the one observed during startup conditions when the air is pushed out of the channels by hydrogen. A quaternary system for the anode channels using the Stefan-Maxwell framework was augmented with a model of carbon corrosion [3]. The resulting model of cathode active site loss predicts the slow non-recoverable voltage drop over 1000 hours of operation. The modeled corrosion rate matched the decrease in cathode catalyst layer thickness observed in post mortem SEM images [2]. Finally the augmented model has been used to study the optimum purge scheduling considering both corrosion and fuel efficiency [1]. The physical channel dimensions provide a rough estimate of feasible target purge duration (volume) and interval between purges. The minimum purge duration pushes the nitrogen blanketed region to the end of the channel, fully recovering the terminal voltage, whereas the maximum duration completely clears the channel (anything beyond this needlessly wastes fuel). The analysis and methodologies presented herein can be used for the design and control of fuel cell systems operating with high hydrogen utilization to prolong stack life and achieve high efficiency. [1] J. Chen, J. Siegel, A. Stefanopoulou, and J. Waldecker, IJHE, 38, (2013). [2] T. Matsuura, J. Chen, J. Siegel, and A. Stefanopoulou, IJHE, 38 (2013). [3] J. Chen, J. B. Siegel, T. Matsuura, and A. G. Stefanopoulou, J. Electrochem. Soc., 158, 9, (2011). [4] J. B. Siegel, S. V. Bohac, A. G. Stefanopoulou, and S. Yesilyurt, J. Electrochem. Soc., 157, 7 (2010). [5] J. B. Siegel, D. A. McKay, A. G. Stefanopoulou, D. S. Hussey, and D. L. Jacobson, J. Electrochem. Soc. 155, 11 (2008). Figure 1

Carbon corrosion and performance degradation mechanism in a proton exchange membrane fuel cell with dead-ended anode and cathode

This paper presents the work on a PEMFC (proton exchange membrane fuel cell) with DEAC (dead-ended anode and cathode) to achieve high hydrogen and oxygen utilization. Gas purging is an effective way to remove the excess water and maintain normal operation of fuel cell while the voltage drops to the set value chosen arbitrarily due to accumulation of generated water. However, serious carbon corrosion has been observed in MEA (membrane electrode assembly) under this purging model. Voltage degradation process during one purging cycle can be divided into three stages: dehydration of membrane induced ohmic loss, quasi-equilibrium state, and flooding induced concentration polarization. To improve the lifespan of PEMFC, gas purging should be conducted before water flooding. The operation time till the quasi-equilibrium state is suggested as the purging duration in the subsequent research. The results show that with the introduction of time regulator purging strategy, the decay rate of the carbon corrosion is greatly reduced. Moreover, it has also been found that the cell performance and purge cycle duration increase with increasing operating pressure, whereas they decrease with the increase in current density.

Operation characteristics and carbon corrosion of PEMFC (Proton exchange membrane fuel cell) with dead-ended anode for high hydrogen utilization

PEMFC (Proton exchange membrane fuel cell) operated under dead-ended anode mode is an effective and simple device for high hydrogen utilization. Herein, the operation characteristics of PEMFC with dead-ended anode are systematically investigated under various operating temperature, anode inlet pressure, and cathode stoichiometry. In each operating parameter, the solenoid valve located at the anode outlet is closed all the time and the hydrogen utilization reaches 100%. It has been found that fuel cell with dead-ended anode can operate continuously for 26 h with slight decrease in output voltage under 600 mA cm−2 at 65 °C, 80% of RH (relative humidity) with hydrogen inlet pressure of 0.4 bar gauge and oxygen stoichiometry of 4.0. After accumulated 150 h operation under dead-ended anode mode, the degradation of according MEA (membrane electrode assembly) has been analyzed using CV (cyclic voltammetry) and cross-sectional morphology. The results show that the accumulation of water induced corrosion of catalyst layer is responsible for the decrease in ECSA (electrochemical surface area) of catalyst layer and the according degradation of cell performance. Moreover, the degradation of catalyst layer became much more serious at the vicinity of cathode outlet than the other regions.

Analysis of the behavior and degradation in proton exchange membrane fuel cells with a dead-ended anode

Proton exchange membrane fuel cells (PEMFCs) with a dead-ended anode (DEA) can obtain high hydrogen utilization by a comparatively simple system. Nevertheless, the accumulation of the nitrogen and the water in the anode channels can lead to a local fuel starvation, which degrades the performance and durability of PEMFCs. In this paper, the behaviors of PEMFCs with a DEA are explored experimentally by detecting the current distribution and the local potentials. The results indicate that the current distribution is uneven during the DEA operation. The local current firstly decreases at the region near the anode outlet, and then extends to the inlet region along the channels with time. The complete fuel starvation near the anode outlet leads to a high local potential and carbon corrosion on the cathode side. The SEM images of the cathode electrode reveal that the significant thickness reduction and the collapse of the electrode's porous structure happen in the cathode catalyst layer, leading to the irreversible decline of the performance. The comparison of the experiments with different oxidants and fuels reveals that the nitrogen crossover from cathode to anode is the dominant factor on the performance decline under the DEA operations.

Bioreduction of para-chloronitrobenzene in drinking water using a continuous stirred hydrogen-based hollow fiber membrane biofilm reactor

para-Chloronitrobenzene ( p-CNB) is particularly harmful and persistent in the environment and is one of the priority pollutants. A feasible degradation pathway for p-CNB is bioreduction under anaerobic conditions. Bioreduction of p-CNB using a hydrogen-based hollow fiber membrane biofilm reactor (HFMBfR) was investigated in the present study. The experiment results revealed that p-CNB was firstly reduced to para-chloraniline ( p-CAN) as an intermediate and then reduced to aniline that involves nitro reduction and reductive dechlorination with H 2 as the electron donor. The HFMBfR had reduced p-CNB to a major extent with a maximum removal percentage of 99.3% at an influent p-CNB concentration of 2 mg/L and a hydraulic residence time of 4.8 h, which corresponded to a p-CNB flux of 0.058 g/m 2 d. The H 2 availability, p-CNB loading, and the presence of competing electron acceptors affected the p-CNB reduction. Flux analysis indicated that the reduction of p-CNB and p-CAN could consume fewer electrons than that of nitrate and sulfate. The HFMBfR had high average hydrogen utilization efficiencies at different steady states in this experiment, with a maximum efficiency at 98.2%.

Correlating Nitrogen Accumulation With Temporal Fuel Cell Performance

The permeability or crossover characteristics of a typical perfluorosulfonic acid base type membrane are used for the temporal and spatial estimations of nitrogen concentration along the anode channels of a polymer electrolyte membrane fuel cell stack. The predicted nitrogen accumulation is then used to estimate the impact of local fuel starvation on stack voltage through the notion of apparent current density. Despite the simplifying assumptions on the water accumulation and membrane hydration levels, the calibrated model predicts reasonably well the response of a 20-cell stack with a dead-ended anode. Specifically, the predicted voltage decay and the estimated gas composition at the anode outlet are experimentally validated using the stack-averaged voltage and a mass spectrometer. This work shows that the crossover of nitrogen and its accumulation in the anode can cause a considerable decay in stack voltage and should be taken into account under high hydrogen utilization conditions.

A Novel Transfer Hydrogenation with High Hydrogen Utilization for the Hydrogenation of Halogenated Nitrobenzene without Hydrodehalogenation

In the novel catalytic transfer hydrogenation, 12 active hydrogen (H*) can be transferred from water/ethanol system as the efficient hydrogen donor and used directly for the hydrogenation of halogenated nitrobenzene over Ru–Fe/C catalyst, obtaining o-chloroaniline (o-CAN) with 98.0% selectivity at 99.8% conversion without hydrodehalogenation. Most other halogenated nitrobenzenes were reduced with selectivity >98%, and conversion >99.0%. Moreover, the Ru–Fe/C catalyst was still active after reaction for 480 h.