Current Density Region Research Articles

Huge Baltic project adds PE, later PP

To improve performance of membrane electrode assembly (MEA) at large current density region, efficient mass transfer at the cathode is desired, for which a feasible strategy is to lower catalyst layer thickness by constructing high loading Pt-alloy catalysts on carbon. But the high loading may induce unwanted particle aggregation. In this work, H-PtNi/C with 33% (mass) Pt loading on carbon and monodisperse distribution of 3.55 nm PtNi nanoparticles, was prepared by a bimodal-pore route. In electrocatalytic oxygen reduction reaction (ORR), H-PtNi/C displays an activity inferior to the low Pt loading catalyst L-PtNi/C (13.3% (mass)) in the half-cell. While in H2-O2 MEA, H-PtNi/C delivers the peak power density of 1.51 W·cm−2 and the mass transfer limiting current density of 4.4 A·cm−2, being 21% and 16% higher than those of L-PtNi/C (1.25 W·cm−2, 3.8 A·cm−2) respectively, which can be ascribed to enhanced mass transfer brought by the thinner catalyst layer in the former. In addition, the same method can be used to prepare PtFe alloy catalyst with a high-Pt loading of 36% (mass). This work may lead to a range of catalyst materials for the large current density applications, such as fuel cell vehicles.

On the mechanism of carrier recombination in downsized blue micro-LEDs

The mechanism of carrier recombination in downsized μ-LED chips from 100 × 100 to 10 × 10 μm2 on emission performance was systemically investigated. All photolithography processes for defining the μ-LED pattern were achieved by using a laser direct writing technique. This maskless technology achieved the glass-mask-free process, which not only can improve the exposure accuracy but also save the development time. The multi-functional SiO2 film as a passivation layer successfully reduced the leakage current density of μ-LED chips compared with the μ-LED chips without passivation layer. As decreasing the chip size to 10 × 10 μm2, the smallest chip size exhibited the highest ideality factor, which indicated the main carrier recombination at the high-defect-density zone in μ-LED chip leading to the decreased emission performance. The blue-shift phenomenon in the electroluminescence spectrum with decreasing the μ-LED chip size was due to the carrier screening effect and the band filling effect. The 10 × 10 μm2 μ-LED chip exhibited high EQE values in the high current density region with a less efficiency droop, and the max-EQE value was 18.8%. The luminance of 96 × 48 μ-LED array with the chip size of 20 × 20 μm2 exhibited a high value of 516 nits at the voltage of 3 V.

Laser Structured Gas Diffusion Layers for Improved Water Transport and Fuel Cell Performance

Polymer electrolyte fuel cells are promising energy converters for future green energy systems. For wide commercial use, still cost reductions by increasing the power density are required. Structurally optimized gas diffusion layers (GDLs), allowing for cell operation at high current densities with reduced mass transport losses, are an essential component for this development. Here we demonstrate that GDLs modified with perforations can improve water transport in the high current density regions and increase fuel cell performance. Laser perforation prior to GDL hydrophobization circumvents previously observed flooding problems and allows for stable operation. The advanced pattern, connecting channel and land regions drains pores, hence decreasing the liquid water fraction, especially close to the GDL and catalyst layer interface. Operando dynamic X-ray tomographic microscopy (XTM) is employed to understand water transport in the modified structure, and the competition with random break-through of water clusters to the channel is analyzed. The detailed and time-resolved insights allow us to propose guidelines for future advanced patterning geometries, suggesting denser perforations and methods to counteract the observed high frequency resistance (HFR) increase. Furthermore, this approach can act as model system for next generation GDLs and lead to a more rational design of transport layers.

Operation characteristics of open-cathode proton exchange membrane fuel cell with different cathode flow fields

Open-cathode proton exchange membrane fuel cell (PEMFC) is considered as a promising power source for unmanned aerial vehicles and portable applications due to its simple structure and low parasitic power. In this study, the operating condition effects of the open-cathode PEMFC with different cathode flow fields (parallel, pin-type, and metal foam) are investigated experimentally. Polarization curve and electrochemical impedance spectroscopy (EIS) are tested to characterize the cell performance and electrochemical impedance, respectively. The results show that the cell with the metal foam flow field has better performance than that with the other flow fields in the baseline conditions. Although increasing the fan voltage could provide adequate reactant gas and remove excess water, too large air flow rate could reduce membrane water content and decrease temperature that is bad for the cell performance. Increasing the operating temperature would improve the cell performance, but the increase of ohmic resistance caused by higher water evaporation rate cannot be neglected. The cell performance does not increase significantly with increasing the anode backpressure in the low current density region, but decreases with increasing the anode backpressure in the high current density region.

Effects of Condensed Water on Oxygen Transport Resistance Components Separated By the Limiting Current Analysis in PEFC

To increase the power output of PEFC, management of generated water is extremely important. In previous study, using a heat conduction and water vapor diffusion model, appropriate operating conditions were investigated to reduce oxygen transport resistance (1). However, it clarified only total oxygen transport from catalyst layer (CL) to channel without considering the effect of condensed water. In this study, the oxygen transport resistances in CL and in the other parts were evaluated quantitatively under flooding conditions. The water distribution in the cathode side micro-porous layer (MPL) and CL were observed by using a freezing method and a cryo-SEM (2).In this study, active area of the cell was 1.8 cm2. The anode and cathode flow fields were parallel straight. Two kinds of separators with 0.3 mm or 1.0 mm width rib and channel were employed (we call as 0.3 mm separator and 1.0 mm separator respectively). The anode side was supplied with hydrogen, and the cathode side was supplied with a mixed gas of oxygen and nitrogen. The gas flow rates were 100 sccm for the anode and 2000 sccm for the cathode. All experiments were conducted with 81%RH. We used gas diffusion layer (GDL) with MPL: 28 BC (made by SGL Co., Ltd.). The catalyst coated membrane was produced by Japan Gore-Tex Inc.The oxygen transport resistances were measured by using the limiting current method (3). Until now, decomposition of oxygen transport resistance is limited under dry condition because it is on the assumption that total oxygen transport resistance is constant. Then, this study introduced the separation of oxygen transport corresponding to inside and outside CL under flooding condition. Inside the CL, it was assumed that the water production rate dominates the water accumulation, which is proportional to current densities. Outside the CL, it was assumed that the product of pressure and current density determined the component of oxygen transport resistance. Based on these novel assumption, division of oxygen transport resistance was determined by a least square error calculation of actual total oxygen transport resistances and theoretical values. this experiment was conducted with different pressure (100 kPa, 140 kPa, 180 kPa), and different oxygen concentration (1%, 2%, 4%, 6%, 8%, 12%, 16%, 20%, 24 %). The cell temperature was set at 35°C and 80°C.Figure 1 shows the results of oxygen transport resistance in each inside and outside CL. I Lim is limiting current density. R P and R NP are pressure-dependent or independent oxygen transport resistances, respectively. These are corresponded to the oxygen transport resistance outside and inside the CL. In Figure 2, actual total oxygen transport resistances (R T) are shown as dots, and theoretical values which are obtained from the results of Figure 1 are shown as line. Both results are in good agreement, and the effectiveness of the oxygen transport resistance decomposition was confirmed.From the results at Figure 1, focusing on the temperature condition (1.0 mm separator), R P increases from low current density region at low temperature. But, R NP shows the similar behavior which is suppressed at low current density and increases from high current density region. These results indicate that high temperature is good at water discharge outside CL in case of present GDL.To clarify the reason why R NP increases in high current density region, effect of the difference in rib and channel widths was investigated. In case of 0.3 mm separator, R NP is relatively constant in high current density region. This result suggests that wider rib and channel increase the oxygen transport resistance in the CLFigure 3 shows the water distribution in the vicinity of MPLs at just before limiting current condition. This experiment was conducted with 80°C, and 24% oxygen concentration. No ice is observed in the MPL and CL for 0.3 mm separator (Figure 3 (a)). On the other hand, ice layer was observed at the MPL/CL interface for 1.0 mm separator (Figure 3 (b)). This is related to the result that R NP increases in high current density region in case of 1.0 mm separator (Figure 2). When wider channel separator is employed, the contact of MPL and CL becomes worse. As a result, water is accumulated at the MPL/CL interface and the water prevents oxygen transport to the CL under the channel. These results suggest that a cell structure that improves the MPL/CL interface contact is important to suppress oxygen transport resistance.Reference Kitami, et al., ECS Transactions, 92(8), 213-221 (2019).Y. Aoyama, et al., Electrochem. Commun., 41, 72-75 (2014)David A. Caulk, et al., Electrochem. Soc., 157(8), B1237-B1244 (2010) Figure 1

Pd@Pt Core-Shell Structured Catalyst Supported on Mesoporous Carbon for Achieving a High Cell Performance

1. Introduction An extremely high performance is required for polymer electrolyte fuel cells (PEFCs) installed in fuel cell electric vehicles after 2030 as shown in Figure 1 [1] and a high oxygen accessibility [2] in addition to a high ORR activity and a durability is crucial for Pt-based cathode catalysts in PEFCs. In order to achieve the cell performance, it is of great importance to suppress cell voltage drop especially in a high current density (HCD) region and we consider that carbon support for the Pt-based catalysts would play an important role. In this study, mesoporous carbon (MPC) with a large specific surface area was selected for the support of Pd@Pt core-shell structured catalyst and cell performance of the catalyst supported on the MPC was compared with the catalyst supported on a conventional carbon support of Ketjen Black (KB). 2. Experimental KB-600JD (SBET: 1,335 m2/g) and MPC (CNovel MH-18, SBET 1,278 m2/g, TOYO TANSO [3]) were used as carbon supports for synthesis of Pd@Pt core-shell structured catalyst. The Pd@Pt core-shell structured catalyst was synthesized by a direct displacement reaction method [4]. The metal loadings of the catalyst were 20 wt.% for Pt and 30 wt.% for Pd. CV was recorded at 25oC in Ar-saturated 0.1 M HClO4 and ORR activity was evaluated by RDE method performed in O2-saturated 0.1 M HClO4 at 25oC with a rotation speed of 1,600 rpm and a positive potential scan rate of 10 mV/s. Fuel cell performance was evaluated by using a single cell with an active area of 1 cm2 (5 serpentine flow paths). Nafion® DE2020 was used as an ionomer for the catalyst ink preparation (I/C was set to 0.83) and Nafion® NRE 211 (25 µm in thickness) was used as a membrane. The Pt loading in cathode catalyst layer was set to 0.1 mg/cm2, and H2 and air were supplied to anode and cathode by 418 NmL/min. and 988 NmL/min., respectively (utilization was 5% at current density of 3.0 A/cm2). The cell temperature was set to 80oC and humidified by 95% RH. 3. Results and Discussion Figure 1 shows cell performances of a reference Pt catalyst supported on KB-300J (TEC10E50E, mean diameter: 2.8 nm, metal loading: 48 wt.%, TKK) and Pd@Pt core-shell structured catalyst supported on KB-600JD evaluated with 1.0 atm at gas outlet. Although the cell performance of the Pt/Pd/KB-600JD catalyst overcame the performance of the reference Pt/KB-300J catalyst, cell voltage significantly dropped in HCD region (> 2.0 A/cm2) compared to cell voltage required in 2030. In order to suppress the voltage drop, gas diffusion layer (GDL) was altered to a high current use (22BB, SGL Carbon Inc.) from a stational use (28BC, SGL Carbon Inc.) and the cell performances are overwritten on Figure 1. It is clear that the voltage drop in HCD region was well suppressed by using the GDL 22BB, which arose from a three-fold higher gas permeability of the GDL 22BB compared to the GDL BC28.Next, CNovel MH-18 was used as a carbon support and pore size distribution is demonstrated in Figure 2. The KB-600JD possesses mesopores broadly ranging from 2 to 20 nm in diameter, while the CNovel MH-18 has mesopores with a narrower size distribution centered at 4 nm in diameter. Figures 3 and 4 summarize cell performances using the Pt/KB-300J, the Pt/Pd/KB-600JD and the Pt/Pd/CNovel MH-18 cathode catalysts. When the cell performance was evaluated with 1.0 atm at gas outlet, the Pt/Pd/KB-600JD catalyst exhibited a superior cell performance to that of the Pt/Pd/CNovel MH-18 catalyst. On the contrary, when the cell performance was evaluated with 1.5 atm at gas outlet, the performance using the Pt/Pd/CNovel MH-18 catalyst was much improved and exhibited a higher cell voltage in a low current density region (0.01-1.0 A/cm2) as shown in Figure 5. Since more than 60% of the Pd@Pd NPs were deposited at interior part of the CNovel MH-18 MPC support, it is considered that the cell performance using the Pt/Pd/CNovel MH-18 catalyst exhibited the significant improvement under the pressurized condition at 1.5 atm and the higher cell voltage in the low current density region due to suppression of a direct ionomer adsorption onto the catalyst surface.This study was partly supported by NEDO, Japan.

Evaluation of the Efficiency of a High Temperature Proton Exchange Membrane Water Electrolysis System

In recent years, a significant interest has been growing for the development of high temperature (HT) electrolytes for proton exchange membrane water electrolysis (PEMWE), promising a lower reversible potential and improved reaction kinetics and ionic conductivity.In this study, the energy and exergy analysis developed for PEMWE has been extended to evaluate the performance of HT-PEMWE, with the model aiming to utilise the energy in the most efficient manner and also take into account potential heat losses, which is particularly important considering that these become more pronounced with higher temperature differences.The model shows that the stack operates in autothermic mode over a considerable range of current density. Thus heating inputs to the stack and feed water become progressively unnecessary as polarization losses make up for these heating requirements. For the considered model, a current density above 1.44 A/cm2 would allow surplus heat to be utilised for secondary applications whilst not recovering any heat would result in an exergy efficiency loss of 4.2 % points. The effect of accounting for pipework and stack heat losses is most evident in the lower region of current density. This however diminishes as the losses start constituting a minor fraction of the total power share.The exergy efficiency for HT has been calculated to surpass that for low temperature (LT) at 0.54 A/cm2. This improvement is attributed to a lower electrical power input for the same production of hydrogen and the heat management strategy incorporated within the model. The maximum improvement reaches 3.8 % points at 3 A/cm2. Taking into account exergy favours higher temperature differences – a benefit which outweighs the fact that a greater quantity of thermal power is recovered in the LT system (due to higher polarization losses). This finding also shows the suitability of adopting exergy efficiency as the performance indicator for PEMWE systems.Given the novelty in opting for HT operation, this study offers further understanding on its performance and evaluates the cost borne with opting for more demanding operating conditions from the perspective of energy and exergy efficiency.

Examination on Water Management Method in the Same Electrode in PEFC

Recently, Polymer Electrolyte Fuel Cells (PEFCs) are beginning to be used in various fields as a clean energy source because they don’t emit carbon dioxide during operating. However, PEFCs have some problems such as a cost reduction and durability etc. Generally, PEFCs need a humidification system to avoid dryness of membrane during operating, but the amount of energy supplied to the humidifier is large, and reduces a system efficiency, and the cost of the humidification system is also necessary. And, the steam in the cathode gas becomes supersaturation in the vicinity of gas outlet, and thereafter it is condensed because the steam is generated by the cell reaction and is multiplied toward the gas outlet. Because the use of humidifier increases the amount of condensation and causes the water management issues such as the flooding phenomenon and the plugging phenomenon in the vicinity of gas outlet, these issues make the cell voltage unstable. Our laboratory has proposed the Self Water Management Separator (SWMS) with a bypass channel to solve these issues. The SWMS moves excess water from gas outlet to gas inlet through the bypass channel where filled water absorption material. Excess water was absorbed by a capillary phenomenon of water absorption material, and it moved from gas outlet to gas inlet by a concentration difference of water. The previous study clarified that the bypass position depended on the operating condition, and the optimum bypass position under non-humidifying and non-heating conditions (w/o humidifier and heater) was at gas inlet. Therefore, this study aims to select the optimum water absorption material for SWMS. The specification demanded for the water absorption material is a water supply and drainage ability and a handling ability. We prepared four water absorption material such as polyvinyl alcohol sponge (PVA), diatomite powder, cellulose fiber insulation (CFI), and EM diatomite grain. Here, cellulose insulation is a wood fiber insulation material. The SWMS was installed in only cathode side and a single serpentine was applied to both separators as a gas channel. The cell performance was evaluated by IV performance and Cole-Cole plots of AC impedance method under the standard condition and a durability test condition. Here, the standard condition and the durability test condition mean 0.3A/cm2 of the current density and Uf/Uox=70%/40% of the fuel/oxidant gas utilization, and 0.2A/cm2 and 45%/25%, respectively. The cell temperature of both conditions was 30°C. As a result, the resistance polarizations of the cell with SWMS on all water absorption material were equivalent or smaller than that of the cell w/o bypass channel as shown in Fig.1. Especially, the initial performance of the cell where PVA or CFI was filled in the bypass channel was better than that of others in a high current density region. The activation polarization of the cell with PVA was the smallest, and it has hardly changed on the durability test. Therefore, because PVA uniforms the humid condition in the cell, consequently the activation and resistance polarizations are improved by the increases of three phase interface and the ion conductivity. However, although the IV performance of the cell with CFI was almost the same as that of PVA, the resistance and activation polarizations were a little large compared with others and the diffusion polarization began to grow up. Because CFI has a high water supply and drainage ability, it deprived the membrane and three phase interface of necessary water, consequently it deteriorated the cell performance. Therefore, we decided that PVA is the most suitable as the water absorption material of the bypass channel from viewpoints of cell performance and handling ability. Figure 1

Soft X-Ray Imaging of Polymer Electrolyte Fuel Cells Using Different Support Materials for Catalyst Layers

The polymer electrolyte fuel cell (PEFC) is a highly efficient and environmentally friendly energy device that has been developed for fuel cell vehicles and other applications due to its low operating temperature and compact size. The PEFC is assembled by putting a membrane electrode assembly (MEA), which consists of the polymer electrolyte membrane with the catalyst layers and the gas diffusion layers on both sides, between separators. Pt nanoparticle catalysts supported on carbon black (Pt/C) are usually used for catalyst layers because of their large surface area and high electrical conductivity. However, carbon supports are oxidized and corroded during operation, resulting in performance degradation. Therefore, research has been conducted using Pt supported on tin oxide (Pt/Nb-SnO2), which has high durability and a porous structure similar to that of carbon supports, as an alternative material for the catalyst layers [1]. Tin oxide supports have material properties such as wettability that differ greatly from carbon supports, and the effect of these material properties on liquid water behavior during power generation has not been clarified. It is essential to understand the liquid water transport in PEFCs for the improvement of performance, because excessive liquid water accumulation (flooding) at the cathode hinders the transport of the reaction gas and causes concentration overvoltage.In this study, in order to clarify the effect of the catalyst layer support material on the liquid water transport, we made catalyst layers using carbon and tin oxide supports respectively, and investigated the liquid water behavior in the MEAs under fuel cell operation using high-resolution soft X-ray radiography.PEFCs with a reaction area of 5×8 mm2 were constructed using carbon support catalyst layers and tin oxide support catalyst layers, respectively. The same components were used except for the catalyst layer. The PEFCs were set in a soft X-ray imaging system (TUX-9000D, Mars-Tohken X-ray Inspection). This system can observe PEFCs and take images at a spatial resolution of about 1.0 µm. Hydrogen was supplied to the anode and air to the cathode, and the power generation tests were conducted in the high current density region where flooding is likely to occur. The voltage drop during power generation of the PEFC with Pt/SnO2 catalyst layer was larger than that of the PEFC with Pt/C catalyst layer. In-situ observation of the liquid water behavior in the PEFC was performed by taking images in the MEA cross-sectional direction at the same time as power generation. The images were taken at the position where both the area under the rib and the area under the channel could be observed. The soft X-ray radiograph of the MEA before power generation is shown in Figure 1 (a). The generated water during the power generation was identified by image processing that subtracted the image taken after operation from the image taken under open-circuit voltage conditions. Figure 1 (b) and (c) show the soft X-ray radiographs of liquid water after 20 minutes of power generation under the current density of 0.8 A/cm2. After the cell startup, the amount of water retention increases on the cathode side of the PEFC and the difference in the amount of water retention under the rib and under the channel was observed. It was also shown that the distribution of liquid water in the porous material was different between the carbon support and tin oxide support catalyst layers attributed to support materials and structures of the catalyst layers properties.AcknowledgmentsThe catalyst with Tin oxide support material used in this study was provided by NEDO ECCEED'30. The authors would like to acknowledge their assistance in assembling PEFCs for visualization study.[1] Y. Chino et al, J Electrochem Soc., 162-7 (2015), F736. Figure 1

Effect of Micro-Patterned Membrane PEFC Dependent on Operating Current Density

1. IntroductionPolymer electrolyte fuel cells (PEFCs) have been widely recognized as a key technology for a future hydrogen-based society, because of their several practical applications such as in fuel cell vehicles, owing to their good operating capability at low temperatures.It is well known that the mass transport and the chemical reactions at the cathode side of PEFCs limit their performance. Heinzmann et al. used electrochemical impedance spectroscopy (EIS) and distribution of relaxation time (DRT) analysis [1] to show that the dominant processes that occur are cathode gas diffusion and oxygen reduction reaction (ORR).One method to improve the ORR is to increase the surface available for the chemical reaction. Jeon et al. fabricated a micropatterned membrane and applied it to a membrane electrode assembly (MEA) [2]. They confirmed that the use of a micropatterned membrane improved the performance of the PEFC.However, the mechanism of this improvement is yet to be clarified in detail. Micropatterns on the membrane should increase the active reaction area (Fig.1(a)). However, patterns on the membrane surface increase the length of the path from the top to the bottom of the catalyst layer, which should hamper oxygen diffusion (Fig.1(b)). We previously performed DRT analysis with micro-patterned MEA to investigate the mechanism of improvement in PEFC performance using a micro-patterned MEA [3]. In this study, a wider range of EIS was performed, and both the positive and negative effects of using a micro-patterned MEA are discussed herein.2. ExperimentA cathode micro-patterned MEA was fabricated, and EIS and DRT analyses were performed. The catalyst ink was prepared using Pt/C (TEC10V50E, Tanaka Kikinzoku Kogyo K.K.), isopropyl alcohol, water, and a 10% ionomer solution. This ink was sprayed onto a 5.0 mm line-patterned membrane (NRE-212, Sigma-Aldrich Co. LLC) to fabricate the MEA. The catalyst loading was adjusted to 0.4 mg/cm2. The cell was assembled using a commercial gas diffusion layer (Sigracet®︎ 29BC, SGL Carbon GmbH). The operation was carried out galvanostatically with a relative humidity of 80%, gas flow rate of 200 ccm, and oxygen partial pressure of 0.2. EIS was measured at 0.02 to 1.00 A, with frequency ranging between 0.1 Hz and 1 MHz. The acquired data were then subjected to DRT analysis using DRT tools by Wan et al. [4]. Noisy data points were excluded before application because of the high sensitivity of DRT analysis to noise.3.Results and discussionFig.2 shows the polarization curve results. Solid lines with circle markers and broken lines with ‘x’ markers indicate the patterned and flat samples, respectively. In the lower current region, where the ORR is dominant, the patterned sample outperforms the conventional flat sample. On the other hand, in the higher current region, where gas diffusion is dominant, the patterned sample suffers steep drops and the flat becomes relatively better.Fig.3 shows the DRT analysis graph. Fig.3(a) stands for patterned sample result and Fig3.(b) for flat sample. The peaks of DRT indicate the polarization resistance at its time dependency. In the high current density region, the peaks of the patterned and the flat samples are significantly different. In particular, at around 101 to102 Hz, an independent peak appears at 101 Hz for the patterned sample as the current density increases. This shows the polarization resistance of gas diffusion, which indicates that the gas diffusion in the patterned sample is significantly worse than that in the flat sample.The micro-patterned MEA has a negative effect on cell performance because of the longer oxygen diffusion path involved. It can be assumed that they might not perform as well as conventional flat MEA under certain conditions that favor good oxygen transportation, such as high current or low oxygen partial pressure operation. Therefore, it is important to consider the physical properties (catalyst layer porosity, ionomer content, etc.) and operating conditions in order to maximize the potential of the micro-patterned MEA.4. ConclusionWe performed a wide range of galvanostatic EIS and DRT analyses of the data from the operation of a cathode micro-patterned PEFC. The micro-patterned membrane is shown to have both a positive and a negative effect. Comprehensive optimization of parameters such as micropattern geometry, catalyst composition, and operating conditions, are required to take full advantage of the surface-patterning method.

Modified New Microporous Carbon Layer Structure for Improved PEM Fuel Cell Performance with Low-Pt Catalyst Loadings

Three types of commercial carbon black with different porosity and hydrophilicity were investigated at various conditions in a new architecture of microporous carbon layers where they are deposited directly on the low loading cathode catalyst layers (0.1 mgPt cm−2) of membrane electrode assemblies (MEAs) used in H2/O2 fuel cells. The benefits of this modified microporous layer (MPL) include reduced interfacial gaps present between the catalyst layer and the conventional MPL-coated gas diffusion layer of the MEAs, reduced water accumulation, and therefore, higher power densities. Acetylene Black and Vulcan XC72R based modified MPLs with mass loadings in the range of 0–1.0 mg cm−2 show improved fuel cell performance in the higher current density regions due to their lower porosity and higher hydrophobicity. The MPL optimum loading under wet and dry conditions is found to be around 0.8 mg cm−2 (∼20 μm), at which a power gain of up to ∼37% is possible as a result of the improved interlayer contact and mass transport under wet conditions as well as enhanced membrane and cathode catalyst layer hydration under dry conditions. Detailed performance characteristics and longer-term studies performed under different humidification levels were also examined and are discussed in this paper.

Spectroelectrochemical Detection of Water Dissociation in Bipolar Membranes

The potentials at which water dissociation occurs in bipolar membranes (BPM) and the relationship between water dissociation and current-voltage curve characteristics are explored using a novel spectroelectrochemical approach in which an anion exchange membrane is doped with a pH indicator. Using this method, we visually detect a pH change in the BPM resulting from OH- formed during the water dissociation reaction. The color change is measured with a UV/vis spectrometer, while electrochemical characterization of the BPM is performed simultaneously. Additional measurements were performed on BPMs with varying anion and cation exchange membrane layer thickness. Our measurements provide direct evidence of water dissociation occurring within a BPM at cross-membrane potentials below 0.5 V, within the first limiting current density region. We also show that the effects of changing bulk anion and cation exchange layer thickness is highly dependent on the permselectivity of these layers.

The Activity Enhancement Effect of Ionic Liquids on Oxygen Reduction Reaction Catalysts: From Rotating Disk Electrode to Membrane Electrode Assembly

Ionic liquids (ILs) have been explored as a surface modification strategy to promote the oxygen reduction reaction (ORR) on Pt/C and their chemical structures were identified to have strong influence on the ORR activities. To better understand the roles of anion and cation of ILs on the catalytic reaction, two cations ([MTBD]+ and [bmim]+) were paired with three anions ([TFSI]−, [beti]−, and [C4F9SO3]−) to form various IL structures. By systematically varying the IL combinations and studying their effects on the electrochemical behaviors, such as electrochemical surface area and specific ORR activities, it was found that cation structure had a higher influence than anion, and the impact of the [MTBD]+ series was stronger than the [bmim]+ series. In addition to the investigation in the half-cell, studies were also extended to the membrane electrode assembly (MEA). Considerable performance enhancements were demonstrated in both the kinetic region and high current density region with the aid of IL. This work suggests that IL modification can provide a complementary approach to improve the performance of proton exchange membrane fuel cells.

Elastocaloric cooler for waste heat recovery from proton exchange membrane fuel cells

In addition to generate electricity, proton exchange membrane fuel cells (PEMFCs) also produce a considerable quantity of waste heat, which may affect the cell normal operation if not removed immediately. To remove and harvest the waste heat, a novel combined system that couples an elastocaloric cooler (ECC) to a PEMFC is proposed, where the ECC harnesses the waste heat from the PEMFC for cooling purposes. Including thermodynamic and electrochemical irreversible losses, mathematical expressions for efficiency and power output of the combined system are obtained. The operating current density region of the PEMFC allowing the ECC to work is determined and the optimum operation regions for power output and efficiency of the combined system are specified. Moreover, numerical calculations indicate that the equivalent maximum power density and its corresponding efficiency of the combined system can be, respectively, increased by 163.2 % and 64.2 % in comparison with that of the single PEMFC. Exhaustive parametric studies are further undertaken to reveal how the combined system performance is associated with some key design parameters and operation conditions, such as operating temperature, operating pressure, proton exchange membrane thickness, environment temperature, cross-sectional area ratio, length ratio and thermodynamic losses related parameters. The results may offer some theoretical bases for designing and running such a real combined system and open up a new avenue to harvest waste heat from PEMFCs.

Is Hydroxide Just Hydroxide? Unidentical CO2 Hydration Conditions during Hydrogen Evolution and Carbon Dioxide Reduction in Zero-Gap Gas Diffusion Electrode Reactors

The implementation of gas diffusion electrodes is a prerequisite to achieving industrially relevant reaction rates in gas-phase electrochemical CO2 reduction (CO2RR). In the state-of-the-art anion exchange membrane flow electrolyzers, however, there is a substantial loss of reactants due to a nonelectrochemical CO2 consumption at the cathode and the transport of its products to the anode. Our detailed analysis of CO2 crossover in a zero-gap CO2-to-CO flow electrolyzer showed a change in the chemical nature of the transported ionic species through the membrane. With the increasing reaction rate, a continuous shift from HCO3– to CO32– conduction was found to be similar to pure carbonate conduction in the high current density region (>100 mA cm–2). As competing hydrogen evolution takes over the cathodic reaction in a CO2-rich environment, hydroxide conduction becomes more pronounced. This reveals an alteration in the chemical CO2 consumption, the so-called CO2 hydration (CO2 + OH– ↔ HCO3– + OH– ↔ CO32–), implying an unidentical environment for the hydroxide ions generated in CO2RR and hydrogen evolution reaction under a CO2 atmosphere. Our work draws attention to the incomplete description of CO2 hydration at the confined cathode/membrane interface in membrane electrode assembly-type zero-gap CO2 electrolyzers.

DRT Analysis of Solid Oxide Electrolysis Cells: Polarization Resistance of Fuel Electrodes

Introduction Solid oxide electrolysis cell (SOEC), which enables steam electrolysis by reversely operating solid oxide fuel cell (SOFC), is capable of highly efficient hydrogen production. While SOFC and SOEC are using similar materials and cell structures, electrode reactions in SOEC operation have to be studied in more details. Here in this study, we aim to establish experimental evaluation method for the development of solid oxide cells with high performance and durability by characterizing the electrode reactions through electrochemical impedance measurements and subsequent analysis of distribution of relaxation times (DRT). Such analysis is made to reveal similarities and differences of electrode reactions between SOFC and SOEC. Experimental Three types of cells were prepared with different fuel electrodes: Ni-ScSZ cermet fuel electrode; Ni-GDC co-impregnated fuel electrode; and Rh-GDC co-impregnated fuel electrode. Ni-cermet is widely used as the fuel electrode for SOFC. (Ni, Rh)-GDC co-impregnated fuel electrodes, made by impregnating the catalysts (Ni, Rh) and the Gd-doped ceria on the composite of La-Sr-Ti oxide (LST) and GDC, exhibit high durability at high water vapor partial pressure and against redox cycling (1,2).The electrochemical impedance under the open-circuit condition and at given current densities was measured in the frequency range between 0.1 Hz and 1 MHz with a signal amplitude of 0.02 A/cm2. Negative bias current density means a measurement in the SOEC mode, while positive one means a measurement in the SOFC mode. Measurements were mainly performed for the cells with the Ni-ScSZ cermet fuel electrode at low and high current density up to ± 1.2 A/cm2, in every 0.2 A/cm2. In addition, the cells with (Ni, Rh)-GDC co-impregnated fuel electrodes were also characterized up to high current density. Impedance measurements were made by using an impedance analyzer (Solartron 1255WB). 50%-humidified hydrogen was supplied to the fuel electrode. The operating temperature was 800℃. Results and discussion Figure 1 shows typical DRT peaks of the cell using the Ni-ScSZ cermet fuel electrode. A few DRT peaks are distinguished, which can be used to analyze individual electrode processes and their dependencies. Figure 2 shows the polarization resistance of the cell with the Ni-GDC and Rh-GDC co-impregnated fuel electrodes. Whilst the cell with the Ni-cermet fuel electrode exhibited an increase in polarization resistance in the SOEC mode, the Ni-GDC co-impregnated fuel electrode cell exhibited identical polarization resistance within a wide current density region. A similar trend was also found for the Rh-GDC co-impregnated fuel electrode cell. It was suggested that electrode reactions in the SOEC mode could be more complicated, compared to those in the SOFC mode. Various factors could affect polarization resistance, including the difference in the microstructure of these fuel electrodes, and catalytic activity of GDC. Detailed DRT analysis of the polarization resistance for various fuel electrodes will be presented. References Futamura, A. Muramoto, Y. Tachikawa, J. Matsuda, S. M. Lyth, Y, Shiratori, S. Taniguchi, and K. Sasaki, International J. Hydrogen Energy, 44 (16), 8502 (2019). S. P. Jiang, Mater. Sci. Eng. A, 418, 199 (2006). Figure 1

Numerical modeling and analysis of the temperature effect on the performance of an alkaline water electrolysis system

Alkaline water electrolysis (AWE) is the most mature and reliable technology for producing green hydrogen among other types of water electrolysis. Although AWE has been developed over recent decades, both the capital and operational costs of AWE are still so high that hydrogen produced from AWE is economically less competitive compared with other hydrogen production technologies such as methane steam reforming. To reduce the operational cost of AWE hydrogen production, the AWE operations need to be optimized. In this study, to capture the temperature effect on the AWE system performance, AWE system model developed in our previous work has been simulated by varying the operating temperature. The simulation results present that higher operating temperature increases the system efficiency only in the relatively high current density region. The electrolyte flow rate required to maintain the temperature gradient throughout the AWE stack has been modeled as a polynomial equation based on the temperature and current density. When the temperature gradient throughout the AWE stack is reduced, the system efficiency is substantially increased. The model developed in this study can be used to find the optimal configuration and operating condition of the AWE system.

Local Two-Phase Flow and Performance in Polymer Electrolyte Water Electrolysis Cells

Over the past decade, investigations[1-3] have been carried out to better understand the two-phase flow characteristics within a polymer electrolyte water electrolyzer (PEWE) and optimize the porous transport layer (PTL) morphology to improve the system performance and efficiency. Zhang, et al.[3] used a high-speed camera and transparent cell to study the strong influence of temperature and current density on bubble growth rate. Ito, et al. [4] showed that PTL porosity has a minor effect on cell performance while pore diameter greatly influences performance. They categorized the type of two-phase flow experienced by an electrolyzer by using liquid/gas superficial velocity plots. Furthermore, they showed that bubble detachment diameter strongly influenced the two-phase flow regime at any operating point. In a similar direction, Majasan, et al.[5] showed that longer flow path length promotes gas accumulation and results in annular flow at high current density, negatively impacting cell performance. Immerz et al.[6] also suggested that increased local ionic resistance down-the-channel may be attributed to large bubble accumulation. Therefore, the impacts of two-phase flow characteristics on mass transport as well as ohmic losses need to be better understood.Although the serpentine flow-field is very useful to study transport phenomena within a PEWE, it is rarely used as in commercial and large-scale PEWE system; the parallel channel-land configuration is a more conventional flow-field architecture. Through this work we hope to understand how architectural features in the flow field impact local performance to inform next-generation design. Furthermore, this work focuses on characterizing the two-phase flow and bubble accumulation occurring within the PEWE and understanding its relationship with performance. Initial experimental results revealed that the parallel flow-field outperforms the triple serpentine flow-field at high (50 ml/min) flowrates as seen in Figure 1a. This may be caused by the shorter channel lengths that do not exacerbate bubble accumulation near the outlet. Preliminary calculations of the superficial gas/liquid velocities have supported this conclusion (Figure 1c). However, at lower flowrates the parallel flow-field was observed to underperform and exhibit severe mass transport limitations. We believe that the fluid velocity in the channels at these low flowrates (6 ml/min) was insufficient to facilitate proper bubble detachment at the channel/PTL interface. This led to the formation of a large and spread out region of mass transport limited current density as seen in figure 1b. In conclusion, insights into local two-phase flow characteristics and their impact on performance in a PEWE cell will be described in this presentation[1] Lopata, J., et al., Effects of the Transport/Catalyst Layer Interface and Catalyst Loading on Mass and Charge Transport Phenomena in Polymer Electrolyte Membrane Water Electrolysis Devices. Journal of The Electrochemical Society, 2020. 167(6): p. 064507 DOI: 10.1149/1945-7111/ab7f87.[2] Lopata, J.S., et al., Considering Two-Phase Flow in Three-Dimensional Computational Fluid Dynamics Simulations of Proton Exchange Membrane Water Electrolysis Devices. ECS Transactions, 2020. 98(9): p. 653-662 DOI: 10.1149/09809.0653ecst.[3] Li, Y., et al., In-situ investigation of bubble dynamics and two-phase flow in proton exchange membrane electrolyzer cells. International Journal of Hydrogen Energy, 2018. 43(24): p. 11223-11233 DOI: https://doi.org/10.1016/j.ijhydene.2018.05.006.[4] Ito, H., et al., Experimental study on porous current collectors of PEM electrolyzers. International Journal of Hydrogen Energy, 2012. 37(9): p. 7418-7428 DOI: https://doi.org/10.1016/j.ijhydene.2012.01.095.[5] Majasan, J.O., et al., Two-phase flow behaviour and performance of polymer electrolyte membrane electrolysers: Electrochemical and optical characterisation. International Journal of Hydrogen Energy, 2018. 43(33): p. 15659-15672 DOI: https://doi.org/10.1016/j.ijhydene.2018.07.003.[6] Immerz, C., et al., Local Current Density and Electrochemical Impedance Measurements within 50 cm Single-Channel PEM Electrolysis Cell. Journal of The Electrochemical Society, 2018. 165(16): p. F1292-F1299 DOI: 10.1149/2.0411816jes. Figure 1

Ru2P nanofibers for high-performance anion exchange membrane water electrolyzer

The development of advanced electrodes with highly active electrocatalysts and appropriate structures is essential for sustainable hydrogen production via water electrolysis. Moreover, there is significant demand for an affordable electrocatalyst that exhibits comparable activity to that of Pt. Herein, we report Ru2P nanofibers (NFs) as an efficient electrode material for a high-performance anion exchange membrane water electrolyzer (AEMWE). The electrospinning method enables the formation of a porous catalyst layer that comprises tangled NFs, which exhibit a three-dimensional structure with abundant empty space. In a half cell test, the Ru2P NFs exhibit a high catalytic activity for the hydrogen evolution reaction, which is comparable to the activity of a commercial Pt/C. In a single cell test, an AEMWE with Ru2P NFs demonstrates a higher performance than that with a commercial Pt/C, especially in the high current density region; this is attributed to the structural advantage of the porous catalyst layer, which enhances the mass transfer of the reactant, as well as the product.

Resistance Separation of Polymer Electrolyte Membrane Fuel Cell by Polarization Curve and Electrochemical Impedance Spectroscopy

The separation of resistances during their measurement is important because it helps to identify contributors in polymer electrolyte membrane (PEM) fuel cell performance. The major methodologies for separating the resistances are electrochemical impedance spectroscopy (EIS) and polarization curves. In addition, an equivalent circuit was selected for EIS analysis. Although the equivalent circuit of PEM fuel cells has been extensively studied, less attention has been paid to the separation of resistances, including protonic resistance in the cathode catalyst layer (CCL). In this study, polarization curve and EIS analyses were conducted to separate resistances considering the charge transfer resistance, mass transport resistance, high frequency resistance, and protonic resistance in the CCL. A general solution was mathematically derived using the recursion formula. Consequently, resistances were separated and analyzed with respect to variations in relative humidity in the entire current density region. In the case of ohmic resistance, high frequency resistance was almost constant in the main operating load range (0.038–0.050 Ω cm2), while protonic resistance in the CCL exhibited sensitivity (0.025–0.082 Ω cm2) owing to oxygen diffusion and water content.