Serpentine Flow Channel Research Articles

Numerical investigation into the effect of serpentine flow channel with a variable cross‐section on the performance of proton exchange membrane fuel cell

Numerical investigation into the effect of serpentine flow channel with a variable cross‐section on the performance of proton exchange membrane fuel cell

Measurements of Temperature and Relative Humidity Dependencies of Oxygen Reduction Reaction Rate in PEFC

IntroductionFor widespread use of polymer electrolyte fuel cell (PEFC), it is necessary to reduce the amount of platinum by optimizing the cathode catalyst layer (CCL) structure. To establish the model of the CCL,[1] the intrinsic oxygen reduction reaction (ORR) rate which does not include mass transfer resistance in the through-plane direction of the CCL should be formulated. Neglecting the backward reaction, the ORR rate per unit mass of Pt, r mc, is proportional to oxygen partial pressure, p O [Pa], as follows: r mc = k mc p O [mol/(kg·s)][1] (1) k mc = k mc0 + exp(–E/RT – E c/b c) (2)where k mc is the reaction rate constant per unit mass of Pt [mol/(Pa·kg·s)], E c is the cathode electromotive force (emf) [V], E is the activation energy [J/mol], T is temperature [K] and b c is the Tafel slope [V].The objective of this study is to develop a measurement method of temperature and relative humidity dependencies of the intrinsic ORR rate constant using platinum-sputtered catalyst.ExperimentalA Japan Automobile Research Institute (JARI) standard cell whose active area was 50 mm × 50 mm was used in experiments. The membrane-anode assembly (MAA, Eiwa Corp.) was composed of a proton exchange membrane (PEM, DuPont Nafion® NR-212), an 8.3‒9.8 μm thick anode which was made of Pt/Vulcan® XC-72 catalyst (Pt/C weight ratio: 1) with a Nafion® ionomer (ionomer/carbon weight ratio: 0.74). Pt thin-layer cathode (ρs = 0.054 mg/cm2) was prepared by sputtering (AOV, SPAD-4240UM, Ar atmosphere, 5 Pa, 100 ℃, for 50 minutes) on the microporous layer (MPL) on a carbon paper (SIGRACET-GDL 29 BC, 235‒238 μm thick) to eliminate mass transfer resistance in the through-plane direction.[2] The MAA and Pt thin-layer cathode were stacked as shown in Fig. 1. A carbon current collector had a single serpentine flow channel, the width and depth of which were 1.0 mm and the width of the ribs between the channels was 1.0 mm. H2 and O2 were humidified at 75 oC. H2 and O2 flow rates were 600 and 300 cm3/min, respectively. The cell was operated at 80 oC. The pressure at the cell outlet was 1 atm.Results and DiscussionFig. 2 shows the polarization curves at different temperatures. The ORR rate constant per geometric area k gc (= ρs k mc) on the sputtered catalyst calculated from Fig. 2 is plotted with the ORR rate constant on the supported catalyst in Fig. 3. As shown in Fig. 2, current density at a fixed E cm , cathode emf vs. RHE at PEM–catalyst boundary, increased with raising temperature. The observed activation energy was 45 kJ/mol in case of the sputtered catalyst while the observed activation energy was 9–26 kJ/mol in case of the supported catalyst. This is because temperature dependency of mass transfer resistance is weaker than that of ORR, so temperature dependency of current density is not remarkable in case of supported catalyst, which has mass transfer resistance in the through-plane direction in CCL. In fact, the apparent activation energy on Pt-supported catalyst of 20–29 kJ/mol was reported.[3][4] Fig. 3 also shows that the observed activation energy on Pt-sputtered catalyst is almost independent of E cm . That indicates the effects of temperature and E cm on ORR rate constant was appropriately separated by using Pt-sputtered catalyst.Fig. 4 shows the experimental results at different RH. The current density at a fixed E cm increased by raising RH although the ORR rate constant should not depend on relative humidity. Since Pt-sputtered catalyst whose CCL thickness can be regarded as 0 does not have proton transfer resistance, this result means there are other factors affected by relative humidity. For example, as shown in Fig. 5, ECSA in working cells depends on relative humidity. It was also reported in literature.[5] In addition, RH dependency of current per ECSA in Fig. 6 was not remarkable. In fact, the current density at E cm = 0.5 V is 1.66 times higher at RH = 0.81 than 0.53 while the ratio is 1.22 in case of the current per ECSA.ConclusionsTemperature and relative humidity dependencies of ORR rate constant, k gc, were measured using platinum-sputtered catalyst. Experimental results at different temperatures showed k gc on the sputtered catalyst did not include through-plane mass transfer resistance and the effects of temperature and cathode emf on k gc was appropriately separated. Also, experimental results at different RH showed current density at a fixed E cm increased by raising RH. RH dependency of ORR rate was about 70 % reduced by eliminating the effect of ECSA. The RH dependency of ORR rate is mostly attributed to ECSA. Figure 1

Numerical modeling of anode-bleeding PEM fuel cells: Effects of operating conditions and flow field design

Operating the PEM fuel cell in the dead-ended anode mode reduces the overall cost and complexity of the system but causes a voltage loss and carbon corrosion in the cathode catalyst layer due to hydrogen starvation in the anode. Whereas allowing an ultra-low flowrate at the anode outlet offers a very high utilization of hydrogen and achieves a stable voltage transient. Here, a time-dependent pseudo-three-dimensional, two-phase, and non-isothermal model is developed to study the optimum bleeding rate, which maximizes the hydrogen utilization, achieves a stable cell voltage and avoids carbon corrosion, which is commonly observed when the bleed rate is set to zero, i.e. the dead-ended mode. The model is validated against the experimental data by comparing the polarization curves and cell voltage transients during the dead-ended anode operation of small experimental cells with serpentine and straight anode flow channels. Moreover, the effects of operating conditions on cell performance during the anode bleeding operation mode are investigated. Results demonstrate that the hydrogen utilization exceeds 99% in the anode-bleeding mode without hydrogen starvation, and the cell performance improves significantly for higher anode pressure, lower cell temperature, and lower relative humidity at the cathode inlet. Lastly, it is found that serpentine channels in the anode improve the uniformity of the distribution of hydrogen compared to straight and interdigitated channels in the anode-bleeding mode while the cathode flow field consists of serpentine channels.

Development of an Open-Source Solver for Polymer Electrolyte Fuel Cells

As a clean energy conversion device, the fuel cell has attracted much attention over the past decade. Among the different types of fuel cells, the polymer electrolyte fuel cell (PEFC) shows promise for automotive, backup power, and portable device applications, due to its low operating temperature, high power density, quick dynamic response, cold start capacity and other advantageous features 1, 2. Various numerical and experimental studies of PEFCs have been conducted 3, 4. Experimental equipment has developed substantially; however, it remains limited with respect to conducting in-situ measurements for PEFCs. As an alternative, numerical approaches offer useful tools for obtaining local internal data. The utility of the latter is increasing in the light of the increasing capacity of supercomputers.Commercial computational fluid dynamics (CFD) software has been widely used, and provides relatively straightforward solvers for fuel cell applications 5-7. However, there are several drawbacks concerning model development and application, e.g., non-transparency, licensing limitations, parallelization, etc. Open-source libraries are much more flexible with respect to the transparency of source code, model development, free utilization between multiple institutes and large-scale parallelization.A model, which is capable of reproducing all major physico-chemical hydrodynamic processes within a PEFC, is developed and implemented within the open-source CFD source library, OpenFOAM. All major transport phenomena, including two-phase fluid flow, inter-phase heat and mass transfer, and ionic and electronic transfer, are taken into account. An Eulerian-Eulerian formulation is applied for the two-phase flow in the gas channels, a Leverett J-function is utilized for liquid water transfer in the porous regions, and a Butler-Volmer relationship employed to describe the electrochemical reaction. A two-potential model is used to describe proton and electron transfer. A multiple region technique is applied for domain decomposition in the various components, including air/fuel passages, membranes, and electrodes.Figure 1. Numerical and experimental results. (a)-(d). in-house Designed. 8; (e). Design of Schneider et al. 9.The model is applied to an in-house-designed PEFC prototype,8 which features three serpentine flow channels, as is shown in Figure 1 (a). The comparison of polarization curves is shown in Figure 1 (b). The present model predicts overall performance with good agreement. Figure 1 (c) and (d) display the current density in the membrane and liquid water saturation in the cathodic catalyst electrode, respectively. In general, the current density decreases from the inlet to the outlet, whereas the liquid water saturation increases. However, both current density and liquid water saturation show higher values under the lands. Liquid water is prone to accumulate in these regions, which increases the local electric conductivity, and therefore the local current densities.Validation of the model is also facilitated by means of a comparison of the local current density with experimental measurements,9 as shown in Figure 1 (e). It can be seen that these values increase from under-land to under-channel regions, a feature which is captured both by the model and the experimental data. This contrasts with those shown in Figure 1 (a)-(d), where the gases are 90% humidified.Further investigations will focus on the comparison of polarization curves under a wide variety of operating conditions, varying such parameters as temperature, relative humidity, etc. The local current density distribution measured with the S++ device will also be used to provide additional validation for the present model. M. Andersson, S. B. Beale, M. Espinoza, Z. Wu and W. Lehnert, Applied Energy, 2016, 180, 757 - 778.G. Zhang and K. Jiao, Journal of Power Sources, 2018, 391, 120 - 133.A. Z. Weber, R. L. Borup, R. M. Darling, P. K. Das, T. J. Dursch, W. Gu, D. Harvey, A. Kusoglu, S. Litster, M. M. Mench, R. Mukundan, J. P. Owejan, J. G. Pharoah, M. Secanell and I. V. Zenyuk, Journal of The Electrochemical Society, 2014, 161, F1254-F1299.H.-W. Wu, Applied Energy, 2016, 165, 81 - 106.S. Chen, Z. Xia, X. Zhang and Y. Wu, Energy Procedia, 2019, 158, 1678 - 1684.G. Zhang and K. Jiao, Energy Conversion and Management, 2018, 176, 409 - 421.L. Fan, G. Zhang and K. Jiao, Energy Conversion and Management, 2017, 150, 763 - 774.Y. Shi, H. Janßen and W. Lehnert, Energies, 2019, 12.I. A. Schneider, S. von Dahlen, A. Wokaun and G. G. Scherer, Journal of The Electrochemical Society, 2010, 157, B338-B341. Figure 1

A comparison on the dynamical performance of a proton exchange membrane fuel cell (PEMFC) with traditional serpentine and an open pore cellular foam material flow channel

Minimising the pressure drop in flow channels, ensuring high efficiency and utilisation of open pore cellular foam (OPCF) material in place of a traditional serpentine channel are investigated in this work. The paper establishes novel mathematical model that takes into account the effect of pressure drop in the flow channel and compares the dynamics of a porous flow channel with those of the traditional serpentine flow channel. The performance of a Polymer Electrolyte Membrane fuel cell with porous foam flow channel is analysed under static and transient conditions. The fuel cell mass transport equations are used in the model that also takes into account the effect of varying the current on the stack temperature. The membrane water content and IV-curves are analysed and simulation results are presented based on the mathematical models of the proposed system using the MATLAB®/Simulink® environments. The effect of varying pore diameter, porosity, and the flow velocity on pressure drop are also investigated using sensitivity analysis. Due to the lower pressure drop provided by the uniform distribution of reactants in OPCF channel, an improvement of approximately 55% is observed in current density when compared with that of the serpentine channel under the same operating conditions. The investigation further concluded that a higher pore diameter can have a lower drop in pressure provided the flow velocity of the reactant does not exceed 6 m/s.

Numerical investigation of water dynamics in a novel wettability gradient anode flow channel for proton exchange membrane fuel cells

The effective removal and transport of water in flow channels play an important role in the water management of proton exchange membrane fuel cells (PEMFCs). In this paper, a novel design of anode serpentine flow channel with the wettability gradient wall is discussed and numerically investigated by utilizing the volume-of-fluid (VOF) method. The effects of the contact angle and the wettability gradient of channel walls, as well as hydrogen flow velocity and water droplet size, on the droplet dynamic behavior are studied. The results indicate that compared with the conventional flow channel, the water droplet can be more effectively removed from the turning part in the wettability gradient flow channel. And the water removal ability in the turning part is improved with the increase of the wettability gradient. Moreover, the wettability gradient flow channel can also improve the water removal performance for the cases with different hydrogen flow velocities and water droplet sizes. This study provides ideas for guiding the design of flow channel to effectively enhance anode water management.

Experimental investigation on PEM fuel cell using serpentine with tapered flow channels

The performance comparison of various flow fields for practical application can be better understood when loading the cell at a lower voltage region for an operational duration of more than an hour. In this paper, the performance of serpentine and serpentine with tapered channels are compared by loading the Polymer Electrolyte Membrane (PEM) fuel cell at a constant voltage of 0.5 V for an operational duration of 5 h. Purging with nitrogen at the inlets is done to recover the drop in current over the period of operation and flush out water. The presence of tapered flow fields in a serpentine flow channel shows an improvement of 15% in the current obtained due to better reactant gas transport in the gas diffusion layer. The performance of both flow fields were studied with polarization and power density curve obtained after 5 h of testing. Further, to confirm the performance improvement at lower voltage region, impedance study is done and obtained Nyquist plot confirms the better transport phenomenon of reactant gases to catalyst site and better removal of water in Gas Diffusion Layer (GDL).

Performance evaluation of an air breathing–direct methanol fuel cell with different cathode current collectors with liquid electrolyte layer

AbstractDevelopment of possible technologies for the generation of green energy is the focus of research worldwide. Fuel cells are a promising option in this regard. Out of the various categories of fuel cells, air breathing–direct methanol fuel cells (AB‐DMFCs) are finding a prominent place as an electric power source for charging of compact electronic devices. In AB‐DMFC, the design and construction of the cathode current collector is very crucial, since the cathode current collector performs important functions in the fuel cell operation such as collecting the electrons, facilitating the flow of oxygen for the cathode reaction and evacuating the water bubbles formed at the cathode side. An ideal current collector improves the cell performance multifold. The present paper deals about the performance of an AB‐DMFC fitted with three different cathode current collector designs, viz., a wire mesh current collector (WMCC) made of SS316L, WMCC with a supporting plate (SP) and a perforated current collector (PCC) with an open ratio of 45.40%. A single serpentine flow channel engraved on a graphite plate was used as the anode flow channel plate. Experimental investigations are carried out to examine the impact of the cathode current collector design on the performance of AB‐DMFC and also by varying the operating parameters of methanol flow rate and concentration. The next stage of the experiments are carried out with the addition of a liquid electrolyte layer (LE), which is kept between two half Membrane Electrode Assemblies. Experimental results reveal that the LE layer enhances the fuel cell performance compared to conventional AB‐DMFC.

Optimum hydrogen flowrates and membrane-electrode clamping pressure in hydrogen fuel cells with dual-serpentine flow channels

Hydrogen fuel cells have been designed and fabricated with an aim to investigate effect of cell clamping pressure and hydrogen flowrates on the performance of fuel cells. Fuel cells with active area 1.9 cm × 1.6 cm were fabricated with aluminum anode, cathode and other accessories. Membrane Electrode Assembly (MEA) was made up of nafion 212 (50 µm) membrane sandwiched between two gas diffusion electrodes (GDE) on either side of nafion membrane. Anode and cathode GDE had carbon cloth with 0.25 mg/cm2 and 0.50 mg/cm2 Pt loading, respectively. Double serpentine flow channels were used for the flow of hydrogen and oxygen at anode and cathode. Hydrogen was humidified with an external humidifier. Cells were fabricated with two clamping pressures, 5 kg/cm2 and 25 kg/cm2 both at 80 °C. Hydrogen and oxygen flowrates were varied from 10 sccm to 70 sccm. The polarization plots indicate that the cell with clamping pressure of 25 kg/cm2 and with a flowrate 20 sccm have higher power output (350 mW/cm2) compared to other flowrates thereby implying an optimum flowrate for a given design.

A numerical study of electrode thickness and porosity effects in all vanadium redox flow batteries

Vanadium redox flow battery (VRFB) is one of the promising technologies suitable for large-scale energy storage in power grids due to high design flexibility, low maintenance cost and long-life cycle. Vanadium redox flow cell consists of two porous electrodes with serpentine flow channels and electrolyte solutions which is separated by an ion-exchange membrane. The temperature has been set to 298 K for the electrolytes which is composed of 1500 mol/m³ initial vanadium concentration with 4000 mol/m³ initial H2SO4 concentration. We developed a three-dimensional model to scrutinize the complexities of fluid dynamics and electrochemical reactions when considering different electrode thickness sizes, electrode porosity and electrolyte flow rates. In this study, a three-dimensional numerical simulation have been performed in order to investigate the effect of electrode thickness and electrode porosity on the performance of VRFB. The impact of electrolyte solution flow rate on the VRFB electrical characteristics and efficiencies are also numerically investigated. The results show that the cell voltage increases with increasing the electrolyte flow rate and electrode porosity during discharging process of VRFB. Increasing the initial vanadium concentration, the VRFB cell voltage is significantly increased due to reduced overpotential in the porous electrodes. The maximum power-based efficiency of 96.8% is calculated with the electrode thickness of 1 mm at 10 ml/min, while the power-based efficiency of 96.4% is calculated with the electrode thickness of 4 mm at 50 ml/min. This work gives comprehensive insights on electrode configurations for VRFBs.

Performance Analysis of a PEM Fuel Cell Stack Having 150 cm2 Active Layer by Using Design of Experiments (DOE)

In this study, the effects of operating parameters on power density of a 3-cell PEMFC (Polymer Electrolyte Membrane Fuel Cell) stack with serpentine flow channels having 150 cm2 total active layer have been examined experimentally. Desing Expert, which is the experimental design program (trial version) was used, and the data obtained as a result of the experiments were analyzed by entering this program. A total of 25 experiments were carried out according to the design created with the data entered into the program within the specified operating conditions range. The independent variables were entered which are cell temperature, humidification temperature, H2 flow rate and O2 flow rate, and the response is the power density. In this study, the hydrophobic cell stack which has the highest cell performance of which was previous studies results was used. In the optimization study, keeping the power density and maximum H2 flow to a minimum, the most suitable values are cell temperature 57.826°C, humidification temperature 56.151°C, O2 flow 1.587 L/min. Finally 432.398 mW/cm2 power density value was obtained under these operating conditions.

Computational analysis of the impact of a micro porous layer (MPL) on the characteristics of a high temperature PEMFC

High-temperature PEM fuel cells (HT-PEMFCs) are of current interest due to their favorable characteristics in relation to PEM fuel cells operating at a low temperature (LT-PEMFCs). The benefits concern poisoning of the CO catalyst, water management, thermal management and humidity issues. The major challenge facing these fuel cells is to have the membrane performing well at high temperature. This manuscript reveals the impact of an added micro porous layer (MPL) at both the cathode and anode electrodes by using a three-dimensional (3-D) numerical single-phase model. The bipolar plates have serpentine flow channels for the reactants. The most important result is that the MPL, known as the water management layer (WML) in LT-PEMFCs, improves the performance of an HT-PEMFC in terms of enhanced mass and heat transfer. The difference in the polarization curve and the three overpotentials (activation, ohmic, concentration) between two cases, i.e., with and without MPL, prove that the HT-PEMFC performance improvement is most significant for the ohmic overpotential (about 130% decrease at high current densities). In addition, a more uniform temperature distribution is obtained within the fuel cell components, especially in the membrane electrode assembly (MEA). This improved the performance of the membrane and accordingly a slight improvement of the fuel cell performance. The result shows a difference (at the point of maximum power) between the curves of with and without MPL of about 35%.

Experimental analysis of PEM fuel cell performance using lung channel design bipolar plate

ABSTRACTPerformance of a polymer electrolyte membrane fuel cell is appreciably improved by flow-field geometry. In recent literature, it has been observed that naturally inspired flow channel designs enhance the fuel-cell performance. The relevant examples are plant leaves and human lung structure. The high performance of these designs is uniform reactant distribution and better water management. In this study, the lung channel geometry of bipolar plate is used in the fuel cell for the supply of reactants. A fuel cell of 49 cm2 area with Nafion-117 membrane is assembled by incorporating the lung channel bipolar plate and tested on the programmable fuel-cell test station. The influence of various operating parameters like temperature, pressure, anode (A), and cathode (K) humidification temperatures and the back pressure on the performance of fuel cell is analyzed experimentally. From these experimental results, it is observed that the fuel-cell generates 25.62 W at 42.7 A current when it is operated at a temperature of 60°C, 1-bar pressure, A and K humidification temperatures are at 60°C and 50°C, respectively. The experimental results of lung channel are compared with that of triple serpentine flow channel. The fuel cell with lung channel generates around 34% more power than triple serpentine flow field.

Influence of corner structure of fuel cell serpentine channel on water removal

Proton exchange membrane fuel cell (PEMFC), as a representative of fuel cell technology, has attracted much attention for its huge advantages in transportation application. Water management has a significant impact on the lifetime and performance of PEMFC. This study numerically investigates the movement of a water droplet in a single serpentine flow channel of PEMFC with different U-turn designs. The transport characteristics of the water droplet are obtained under both low and high airflow speeds, respectively. It is found that the droplet can pass through the U-turn for large fillet radius, while it is stuck in the corners of the U-turn for small fillet radius. The water droplet is transported as a whole without breakup under low airflow speed, but it is split into small droplets under high airflow speed at the U-turn. Hydrophobic channel surface promotes water removal at the U-turn of the channel, but its effect is weakened under high airflow speed.

Electrode Perforation Induced Performance Increase in Vanadium Redox Flow Batteries

In the pursuit of providing reliable energy to an increasingly energy dependent society, the development of electrochemical devices that can meet various demand output conditions is not only important, but necessary. The Redox Flow Battery (RFB) is one option that can provide for this demand by offering the ability to modularize energy and power at a high standard of safety over a long product lifetime. While the RFB has already been successfully implemented at the commercial scale, there exists a need to research further methods towards cost/kWh reduction and power density increase. A conventional cell stack for a RFB consists of end plates that sandwich a set of current collectors, bipolar plates, electrodes, and an exchange membrane arranged in a symmetrical cell.[1] In this study we explore the potential of electrode perforation towards increases in battery performance using cell reactive resistivity as a performance indicator. The perforation method in this study has been chosen for its commercial feasibility and ease of implementation; it has been achieved through the use of a scalable punch press to bore a matrix of holes with variable diameter dimensions of 1-2 mm and a range of hole densities from ~3 holes/cm2 to ~6 holes/cm2. Three sets of serpentine bipolar plates were also prepared with varying turn densities of 4, 7, and 11. These bipolar plates were created keeping effective channel diameter consistent while varying depth and width. The channel to ridge ratios in all three plates were kept at 1:1 and the effect of electrode perforation was explored. The test cells were assembled with the aforementioned bipolar plates and SGL 39AA carbon paper electrodes activated at 600°C for 30 min. Nafion NR211 was used as the membrane material. The cell was assembled with 0.5 mm silicon spacers and 3 sheets of activated SGL 39AA for both the positive and negative electrodes. The electrolyte for the experiment was a 1.5M Vanadium solution prepared with VOSO4-nH2O, 20%w/w H2SO4, and DI water. An averaged improvement in reactive resistivity of 20.8% from 0.262 Ohms*cm2 to 0.207 Ohms*cm2 was demonstrated for a set of three serpentine flow fields via the mechanical perforation of 1 mm diameter holes at a density of ~3 holes/cm2. It is noted that reactive resistivity was separated out of ASR resistivity using a LCR meter to measure frequency independent bulk effects which is referred to as conductive resistance.[2] Reactive resistivity was further broken down into mass transport, charge transfer, and diffusion resistivities through the analysis of relaxation times during cell charge and discharge. Through this, the effect of perforated electrodes for charge transfer resistivity and mass transfer resistivity are shown. (Figure 1) It is realized that electrode perforation via 1 mm bores improves all subsections of reactive resistivity, and it is hypothesized that electrode perforation allows for increased reactant mass transport to the membrane side electrode partition, which has been shown to exhibit lower electrode volumetric utilization than the bipolar plate side electrode partition.[3] Is it also observed that the difference in performances of the 4, 7, 11 turn bipolar plates, which were significant and proportional to channel depth when evaluated with non-perforated electrodes showed a significant decrease in variance with the introduction of perforated electrodes. It can thus be extrapolated that the electrode modification method yields a higher apparent consequence to cell performance than the variation of channel density for the same active electrode area. In conclusion, it is shown that the introduction of 1 mm diameter perforations at ~3 holes/cm2 in carbon paper electrodes is effective in improving performance in a vanadium redox flow cell with serpentine flow channels. The over perforation of electrodes at ~6 holes/cm2 was noted to have the decreased performance associated with an extreme reduction in useable electrode surface area. Pressure drop analysis also reveals that the introduction of perforated electrodes reduces the pump pressure needed to operate at the same nominal flow rate. It is thus realized that the introduction of perforations in electrode material can improve both system efficiency and cell efficiency in the VRFB. [1] N. Tokuda et al.: “Development of a Redox Flow Battery System”, SEI Technical Review, No. 151 (2000) [2] J. Ross Madonald.: “Simplified impedance/frequency response results for intrinsically conducting solids and liquids”, The Journal of Chemical Physics 61, 3977 (1974); doi: 10.1063/1.1681691 [3] Q. Liu et al.: “In Situ potential disctribution measurement in an all-vanadium flow battery”, Chem. Commun. 49, 6292-6294 (2013); doi: 10.1039/C3CC42092B Figure 1

Investigation of Membrane Chemical Degradation As a Function of Catalyst Platinum Loading

Membrane chemical degradation is one of many factors that can impact fuel cell durability. The fuel cell’s lifetime heavily depends on the membrane and its ability to maintain chemical and mechanical integrity. Studies indicate degradation is due to the formation of hydroxyl radicals that attack the polymer structure resulting in membrane thinning, and the release of fluoride and sulfate ions1. This consequently causes mechanical failures to transpire, e.g. cracks, tears, perforations and pinhole formation, leading to an increase in membrane degradation rate. Previous research employed supported and non-supported platinum (Pt) catalysts and PtCo alloy catalysts in their studies. Their findings showed a lower membrane degradation rate when using the alloy catalyst at low RH indicating that catalyst activity may be an important factor in membrane degradation.2 In this study, we investigated how varying the amount of Pt-supported catalyst affects membrane degradation. In order to expedite degradation, we used the DOE’s accelerated stress test (AST) for chemical stability to induce the production of free radicals.3 Free radicals are known to degrade membranes and other membrane electrode assembly (MEA) components. The focus of this research is to probe membrane degradation for MEAs with ultra-low Pt loadings (i.e. ≤ 0.1 mgPt cm-2) at each electrode to contribute to existing knowledge of membrane degradation mechanisms. Twenty-five cm2 catalyst coated membranes (CCMs) were made by spraying catalyst inks on to Nafion® membranes (NR211, Ion Power, Inc.) using a Sono-Tek Exactacoat spray coater. The catalyst inks were comprised of 50% Pt/C (TKK) catalyst mixed with 5% ionomer solution (Dupont D521). Fuel Cell Technologies current collection endplates were used to assemble the cell, which utilized SGL’s Sigracet 25 BC gas diffusion layers and two graphite plates with triple serpentine flow channels sealed by Furon® gaskets. The AST test conditions require the fuel cell to be held at open circuit voltage while at 90oC, 30% relative humidity and a back pressure of 150kPa. H2 and air flows were set at 10 stoics at 0.2 A/cm2. We evaluated the fuel cell performance every 12 hours using various diagnostic tests to probe the membrane degradation. Polarization (VI) curves were measured to analyze overall fuel cell performance; electrochemical impedance spectroscopy (EIS) provided useful information on the membrane resistance, electrode kinetics and transport properties, and cyclic voltammetry allowed tracking electrochemical active surface area (ECSA). We also assessed the hydrogen permeation through the membrane using linear sweep voltammetry. In addition, a performance recovery technique similar to Zhang et al. was applied to the fuel cell to mitigate impacts of recoverable catalyst poisoning4. We also collected the effluent water for further scrutiny of membrane degradation by probing for fluoride and sulfate ions using Ion Chromatography (IC). Fluoride emission rates (FER) and sulfate emission rates (SER) were used to gauge the condition of the MEA during its lifetime. Figure 1A shows VI curves in 24 hour increments for a fuel cell exposed to AST conditions with 0.1 mgPtcm-2 at each electrode. The results show recoverable voltage losses within the first 48 hours. This indicates that the performance losses were due to catalyst poisoning rather than membrane pinhole formation. IC results for FER are shown in Figure 1B. Although FER increased during AST conditions, fluoride loss is not impactful on performance in the initial 48 hours of testing. After 72 hours, the voltage losses are irreversible which suggest the onset of membrane failure. SER results also increased during recovery, and degradation is also evident by irreversible open circuit voltage losses and a lower ECSA. EIS results are consistent with the polarization curve data in that the charge transfer resistances remained constant over the initial 48 hours after recovery. The ECSA gradually decays during the testing with only partial surface area restoration after the recovery protocol was applied. H2 crossover results showed a constant increase during fuel cell testing further signifying membrane failure. This talk will summarize the results of these tests at various catalyst loadings and illustrate the influence of catalyst loading on membrane degradation. Acknowledgements: The authors wish to acknowledge the financial support of the LANL African American Partnership Program and NNSA Minority Serving Institutes Program Managers: David Canty, Jennifer Kline and Dave Rude.

Impact of Porosity Gradients within Catalyst Layer and MPL of a PEM Fuel Cell on the Water Management and Performance: A Neutron Radiography Investigation

The influence of porosity modifications of either microporous layer (MPL) or the catalyst layer of a low temperature Polymer Electrolyte Membrane Fuel Cell (PEMFC) on the performance of the cell has been investigated in many studies. However, we have been focusing in our study on the interactive impact of this morphological modification of both cathode MPL and catalyst layer in the water distribution and transport inside the cell and consequently also on the performance of the cell. In this study, three types of cathode MPLs with different porosity distributions and three type of membrane electrode assemblies (MEA) with different porosity of the catalyst layer are manufactured. Five one-cell fuel cells are investigated using combinations of those in-house made layers. To modify the porosity of the cathode MPL polymer particle in two sizes of 30 µm and 1.5 µm from company Chemisnow® are introduced into the carbon slurry, which are then evaporated out of the MPL during the sintering process. For the catalyst layers, 0.5 µm Polystyrene particles are mixed in the electrode slurry with variable weight fractions and then washed out of the dried MEA. A more detailed description of the production procedure of these layers will be demonstrated in the presentation. Performance of fuel cells using these five combination of cathode MPL and electrode are measured using a Fuel Cell with 25 cm² active area and 3 serpentine channel flow fields. Neutron Radiography is used to investigate the water distribution dynamics and water content of the fuel cell. For this purpose, a specially designed single cell with 8 cm² active area and 3 serpentine channel flow fields is used. The measurements are obtained at two different humidity values on both cathode and anode inlet gases of RH=70% and RH=120% and different current densities. With an exposure time of 10 s, images with 12.4 µm pixel size are achieved. The quantitative analysis of the water thickness in the cathode GDL and cathode electrode of 5 cells at RH=120% for an increasing current density is shown at an exemplary radiogram for water distribution inside the cell with our reference cathode MPL and electrode (without porosity modification). Also the Current (ICell) and Voltage (VCell) evolution of the same cell is shown in figure 1. All cells have reached a water content plateau even before reaching the highest current density of 1A/cm². The cell with a porous catalyst layer and a double layer cathode MPL shows the highest water content on the cathode side of the cell, whereas the cell with a highly porous catalyst layer and a porous MPL shows the lowest water content of all cells, comparable to the water thickness observed for the cell without modified layers. Further results of both water content and performance dependent on porosity changes will be provided in the presentation. Figure 1

Computational Fluid Dynamic Analysis on PEM Fuel Cell Performance Using Bio Channel

A 3-D computational model was developed to examine the proton exchange membrane fuel cell (PEMFC) performance using Bio inspired (Bio channel) flow channel design bipolar plate. The model was developed using ANSYS FLUENT-15.0 software and simulations were carried out at 100 % humidity conditions. The parameters such as pressure distribution, hydrogen and oxygen concentrations and proton conductivity were briefly presented. The simulation results of bio channel are presented in the form of polarization curves. The results of a Bio channel compare with the conventional flow channel and observed that the bio channel gives a less pressure drop, uniform distribution of reactants and high cell voltage at a particular current density. From the observation from the polarization data, the bio channel performance was 20% higher than triple serpentine flow channel.

Influence of Lung Channel Design Bipolar Plate on Performance of PEMFC Using Computational Fluid Dynamic Analysis

A 3-D computational model was developed to examine the proton exchange membrane fuel cell (PEMFC) performance using Lung channel design bipolar plate. The model was developed using ANSYS FLUENT-15.0 software and simulations were carried out at 100 % humidity conditions. The parameters such as pressure distribution, hydrogen and oxygen concentrations and proton conductivity were briefly presented. The simulation results of Lung channel are presented in the form of polarization curves. The results of a Lung channel compare with the conventional flow channel and observed that the Lung channel gives a less pressure drop, uniform distribution of reactants and high cell voltage at a particular current density. From the observation from the polarization data, the Lung channel performance was 17% higher than triple serpentine flow channel. Keywords: Humidity conditions, Simulation, Lung channel, Polarization curves, Current density.

Three-dimensional electrochemical Bunsen reaction characteristics in the sulfur–iodine water splitting cycle for hydrogen production

Hydrogen production from the sulfur–iodine water splitting cycle integrated with solar or nuclear energy has been proposed as a promising technique. Bunsen reaction is one of the three main steps in the cycle and electrochemical method has been applied to this reaction. In present work, a three-dimensional numerical study of the electrochemical Bunsen reaction was conducted. A three-dimensional, steady state, laminar and isothermal mathematical model of electrolytic cell was developed and verified by experiments. The spatial maldistribution of species concentration was found between electrodes and proton exchange membrane (PEM). The electric power drives most H2SO4 and I2 to the anode and cathode surface, respectively, while the proton attraction contributes to HI enrichment on the surface of PEM. At the high inlet H2SO4 concentration of 50 wt%, the transformation of flow channel from single serpentine to single entry & double serpentine with the same inlet flow rate cannot solve the insufficient problem of SO2. But the increase of the overall inlet flow rate in the double entry & double serpentine flow channel make SO2 sufficient for anode reaction. Further decreasing the inlet H2SO4 concentration to 40 wt% and 30 wt% make the initial SO2 sufficient for overall reactions. The single serpentine channel gives the highest SO2 conversion rate, followed by the single entry & double serpentine and double entry & double serpentine flow channels. The single serpentine flow channel at the H2SO4 inlet concentration of 40 wt% is found optimal for achieving a high electrochemical Bunsen reaction performance.