Anode Water Research Articles

Investigation of the Effect of Initial Anode Structures on the Durability of Water Electrolysis Cells

Introduction: Water electrolysis is an essential technology for efficiently combining renewable energy with hydrogen energy. Long-term durability against voltage fluctuations is necessary to produce hydrogen by water electrolysis directly using renewable energy. In our previous study, potential fluctuation protocols have been developed based on actual wind power voltage fluctuations (1). In our recent work, the durability of water electrolysis cells has been evaluated for 320 days (2). Reversible and irreversible performance losses were present. As an irreversible performance loss, delamination of IrO2 anode layer was found, which was most likely due to increase in the internal pressure derived by stagnation of generated gases, as similarly seen in the durability test up to 160 days (1). However, regarding to changes in pore structures, the trend was different from our previous study. That was assumed that the change in initial pore structures of anode layers derived by different ionomer/catalyst ratio, rather than the change in the length of the durability test, made different trends.Therefore, in the first part of this study, the durability test condition was aligned with that of the previous study (1), equivalent to 160 days, and the effect of the initial pore structure on durability was examined. In the second part, for the purpose of developing more durable anode pore structure, addition of a hydrophobic material was considered since partially hydrophobic surface probably promotes releasing of gas bubbles to eliminate gas stagnation. Experimental: A water electrolysis cell was made by spray printing 0.3 mg Pt/cm2 using 46.3% Pt/KB for the cathode and 0.5 mg IrO2/cm2 using commercial IrO2 (Tokuriki Honten) for the anode on Nafion117 membrane. The amount of Nafion ionomer in the anode was kept to 15% in this study. The resulting membrane electrode assembly (MEA) was placed in a holder which can flow water in the both anode and cathode with porous transport layers of titanium mesh and carbon paper for the anode and cathode, respectively. After initial electrochemical measurements of AC impedance and I-V performance were taken, a durability test of 80 sets corresponding to 160 days was performed using a potential fluctuation protocol developed in our laboratory (1). The potential was applied to the anode while the cathode was maintained at 0 V. Electrochemical measurements were similarly taken at the end of each set. In addition to electrochemical properties, anode structural properties were evaluated from 3D reconstructed images made by hundreds of FIB-SEM images. For the second part of this study, MEAs with an increased hydrophobic property were prepared by adding 10-30 wt% of polypyrrole against the mass of IrO2 to the anode. Results and discussion: The current density at 2.0 V was monitored during the durability test equivalent to 160 days. Both MEAs with 33% (1) and the 15% Nafion ionomer in the anode showed a similar trend of the current decrease and recovery even though initial performance was higher for MEA with 15% Nafion ionomer. However, the irreversible current loss was larger for MEA in this study. This irreversible current loss is most likely caused by the stagnation of the generated gases based on our previous studies (1), (2). Since gas stagnation is deeply related to the pore structure of the anode layer, 3D reconstruction of the anode was performed using several hundred FIB-SEM images, and comparison to our previous study (1) was done. As a result, when pore size distributions based on 3D reconstructed images were analyzed as shown in Figure 1, a denser anode structure was found with 15% Nafion ionomer. In other words, MEA with 15% Nafion ionomer has high initial IV performance but low durability, while 33% Nafion has low initial IV performance but high durability, indicating that there is a trade-off between the two.Optimizing the amount of Nafion ionomer and the mixing method is one way to achieve both high performance and high durability. However, in the second part of this study, addition of a hydrophobic material was rather considered since partially hydrophobic surface promotes releasing of gas bubbles to eliminate gas stagnation. Polypyrrole was chosen because of its conductivity as well as it hydrophobicity. Unlike the addition of an insulator, PTFE, there was almost no change in initial IV performance up to 30% addition, but a significant improvement in hydrophobicity was observed. The durability of MEA with the anode containing 30% polypyrrole has been evaluated.

Limiting Current Investigations for PEM Water Electrolysis in the Presence of Metal Cations

Pretreatment and purification of water is necessary to use it as a feed for the oxygen evolution reaction (OER) at the anode of PEM water electrolysis (PEMWE). Fresh water typically contains several multivalent cations such as Ca2+ and K+. In addition, iron cations may accumulate in PEMWE over a long period of time due to corrosion of system components and fittings of the PEMWE. The presence of these multivalent cations in the feed water causes higher ohmic resistance of the cell and higher iR-free voltages, resulting in a performance loss.[1] Furthermore, the cations can be transported by the water crossover stream from anode to cathode and by diffusion transport.[2] In other words, even traces of metal cations drastically influence the efficiency and durability of PEMWE stacks. Despite this knowledge, contamination with multivalent cations is one to the main reason for the breakdown of PEMWE stacks.[3]Very interestingly, simulations of cationic contamination for PEM fuel cells (PEMFCs)[4] have shown that the distribution of metal cations in the through-plane direction depends on the proton current through the membrane. The cation concentration profile indicates an accumulation of metal cations close to the cathode side and leads to an increase of the proton transport resistance. In addition, the simulations indicate that in case of a complete substitution of protons by metal cations, the proton transport resistance approaches infinity. Finally, the proton current cannot increase further and a limiting current is reached.[4] The transfer/adoption of these simulations from PEMFC to PEMWE allows to clarify the behavior of the limiting current in dependence of the nature of metal cations such as Ca2+ and K+ and their concentrations, which is still poorly understood to date.In this work, we investigated if the concept of limiting protonic current from simulations for the PEMFC can be (partially) transferred to PEMWE on a laboratory scale. For this purpose, we evaluated the effect of trace metal cations in the anode feed water on the performance of a PEMWE single cell and quantified the cation concentration in the membrane using spectroscopic techniques such as micro X-ray fluorescence (µ-XRF) and energy-dispersive X-ray (EDX). Electrochemical measurements were carried out in an in-house PEMWE test bench equipped with a single cell of 5 cm2 geometric electrode area and potentiostat with booster. Commercially available catalyst coated membranes with a loading of 0.3 mg cm-2 geo of Pt/C and 1 mg cm-2 geo of IrOx were used. The measurements were carried out at atmospheric pressure and a cell temperature of 80°C. First, a conditioning procedure was performed using highly purified feed water (>20 MΩ cm at room temperature) until a stable cell performance had been achieved. Cation contamination experiments were conducted by introducing various concentrations of metal sulfates (1-100 µmol L-1 cation concentration) into the anode feed water. Performance evaluation was carried out by measuring the polarization curves and electrochemical impedance spectroscopy (EIS) to determine the Tafel slope, overvoltage, and high frequency resistance (HFR). µ-XRF and EDX spectroscopy techniques were used to detect the cation concentration profile along the membrane.We observed a significant rise of the cell voltage by adding cations to the anode water feed within few hours. More precisely, a limiting current density of 1 A cm-2 geo after 16 hours of exposure to 100 µmol L-1 K+-ions in the anode feed water was determined using the Koutecký-Levich equation. This result is in line with our first simulations. The switch back to purified feed water allows us to monitor the dynamics of the performance recovery process obtained from EIS data. Additional measurements of the polarization curves and HFR were used to distinguish between reversible and irreversible degradation processes due to the cation contamination. Very interestingly, a partial reversibility was observed after the K+-contamination, as the limiting current increased from 1 to 2 A cm-2 geo after 16 hours recovery with purified water feed.Altogether, this study evaluated the transferability of limiting protonic current concept from simulations in PEMFC to PEMWE to uncover the impact of trace metal cations in the anode feed water on the performance of PEMWE.[1] C. Immerz, M. Singer, F. Hasché, B. Bensmann, M. Suermann, R. Hanke-Rauschenbach, M. Oezaslan, Meet. Abstr. 2020, MA2020-02, 2456.[2] M. Friedrichs-Schucht, F. Hasché, M. Oezaslan, ECS Trans. 2023, 111, 3.[3] N. Danilovic, K. E. Ayers, C. Capuano, J. N. Renner, L. Wiles, M. Pertoso, ECS Trans. 2016, 75, 395.[4] B. L. Kienitz, H. Baskaran, T. A. Zawodzinski, Electrochim. Acta 2009, 54, 1671.

Strategies for gas management in the PEM water electrolyzer anode

Proton Exchange Membrane Water Electrolyzers (PEMWE) are viewed as a promising pathway to generate green hydrogen. The need to reduce the cost of the produced hydrogen motivates efforts to increase the operating current density, which requires better management of the oxygen bubbles released within the PEMWE's flooded anode. Prior research has shown that the buildup of gas bubbles within the PEMWE anode increases mass transport resistance and reduces performance. This study employs a 10 cm2 single-cell PEMWE operating at over 2.5 A/cm2 with a transparent single-serpentine anode to visualize bubble production and transport under varying operating conditions such as the cell's orientation with respect to gravity, anode water supply rate, and the effect of adding surfactant to the anode water supply. These strategies are studied to optimize the management of oxygen bubbles in the anode channels of a PEMWE operating at 80 °C. It was found that the Anode Top channel orientation with respect to gravity was optimal in terms of assisting with bubble evacuation and improved performance. Increasing the anode water supply rate also facilitated bubble detachment and alleviated mass transport losses. Lastly, the addition of surfactant decreased the water contact angle from 83° to 57° which was found to aid bubble detachment from the porous transport layer and improve current stability.

The Effect of Cation Contamination on the Water Transport for PEM Water Electrolysis

Large-scale hydrogen production by PEM water electrolysis (PEMWE) in the energy sector requires a deep understanding of the system behaviour in failure cases and possible recovery procedures. This includes the purification of the anolyte feed water because the contamination with ions can cause the PEMWE stack to breakdown.[1] For instance, the presence of iron trace cations or other metal cations in the feed water leads to a performance loss due to higher ohmic resistance of the cell and higher iR-free voltages.[2] [3] Moreover, cation contamination might influence the water transport in a PEMWE cell. In principle, the transfer of the water molecules to the cathode is proportional to the proton flux from the cathode to the anode. The proportionality factor is defined as water transport coefficient and is also temperature dependent.[4] In case of the mono- and multivalent cations as trace elements in the feed water, the water loading of the membrane, which is expressed by water molecules per sulfonic acid group of the membrane, λ, decreases over time and leads to successive increase in the ohmic resistance. [5] However, the influence of cation contamination on the water transport in PEMWE is poorly understood to date.In this work, we investigated the effect of cation contamination in the anode feed water on the water crossover for PEMWE. Cation contamination experiments were carried out at a constant current density by adding different concentrations such as K2SO4, Na2SO4, etc. to the anode feed water. All electrochemical measurements were carried out in an in-house test bench equipped with a single cell of 5 cm2 geometric electrode area and potentiostat with booster. A commercially available catalyst coated perfluorosulfonic acid membranes (loading of 0.3 mg cm-2 geo Pt/C and 1 mgcm-2 geo IrOx) was used. The anode and cathode compartments were kept at atmospheric pressure and cell temperature of 80°C. To determine the water crossover from anode to cathode at different current densities, the cathode exhaust was cooled in a heat exchanger and measured by a gravimetric method. [4] Firstly, a conditioning procedure was performed with purified feed water until steady cell performance was achieved. The cell performance was evaluated using polarization curves, electrochemical impedance spectroscopy (EIS) and water crossover measurements. The last two are of particular interest, because they provide information about membrane humidification. The Tafel slope and mass transport overvoltage were established from the polarization curves and high frequency resistance (HFR). We observed a significant rise of the cell voltage by adding cations to the anode water feed within few hours. The analysis of Tafel slope and EIS data also reveal an increase of ohmic and mass transport resistances. In these experiments, we were able to correlate the HFR results with the water crossover as a function of the nature of the cation and its concentrations. Our data shows that the specific water crossover per current density decreases as a consequence of cation contamination. The switch back to purified feed water allows us to monitor the dynamics of the performance recovery process obtained from EIS and water crossover measurements. Additional measurements of the polarization curves and HFR were used to distinguish between reversible and irreversible degradation processes due to the cation contamination.Altogether, we present the effect of cation contamination on the water transport processes and provide deeper insights into the mass transport and performance losses for PEMWE.

Water Management in Anion Exchange Membrane Fuel Cells Via Distribution of Relaxation Times Analysis

In response to growing concerns regarding air pollution and energy scarcity, anion exchange membrane fuel cells (AEMFCs) have emerged as a promising technology for power generation due to their high efficiency, zero emissions, and the abundance of hydrogen resources. Within AEMFCs, water serves as a reactant, being consumed at the cathode, and generated at the anode, which can lead to an imbalance in the water content within the fuel cell. Maintaining an appropriate water distribution is essential for efficient charge transport and electrochemical reaction kinetics. Therefore, the development of a comprehensive water management strategy is crucial to ensure the stable operation and durability of AEMFCs.Characterizing the water condition of the anode and cathode in different operation conditions (normal, drying, or flooding) is a critical step that provides data foundation for devising effective water management control strategies. Electrochemical impedance spectroscopy (EIS) offers a means to investigate different resistances (kinetics, mass transport, and ohmic losses) over varying time scales during operation to assess water transport. The conventional interpretation of EIS data typically relies on equivalent circuit models, necessitating prior assumptions and initial component selections. In this study, we employed the distribution of relaxation times (DRT) methodology, known for its powerful ability to separate components, to conduct a more precise analysis of polarization processes. Initially, electrochemical impedance dynamics related to hydroxide transport, hydrogen oxidation reactions, and oxygen reduction reactions were effectively extracted. Subsequently, DRT was employed to investigate the loss and variation trends of each polarization process under conditions of anode and cathode water flooding and drying conditions, respectively. Furthermore, to gain a deeper understanding of the effects of water content on mass transport and electrochemical reaction kinetics, a three-dimensional multi-physics model for AEMFCs was developed. This model incorporated water phase transition, heat and mass transfer, liquid water and dissolver water transport, and electrochemical reactions. These endeavors represent a comprehensive and systematic approach to guide the utilization of the distribution of relaxation times in fuel cells.

The Effect of PTFE Dip Coating on Enhancing the Durability and Performance of Anion Exchange Membrane Electrolyzers

Anion exchange membrane electrolyzers (AEMELs) are widely regarded as one of the most promising technologies to enable low-carbon hydrogen production. AEMELs can combine the advantages the traditional alkaline electrolyzer (AEL) and proton exchange membrane electrolyzer (PEMEL) – allowing for the use of PGM free electrodes, high operating current density, environmentally friendly materials, and compact design. Although significant progress has been made in the field of AEMELs, there is still a gap in performance compared to PEMELs that can be overcome through electrode design.One of the most critical aspects affecting the overall performance of the AEMEC is the water management. It has a major influence on cell behavior, and specifically mass transfer at the anode side of the cell. To improve AEMEL anode water dynamics, our team modified its porous transport layer (PTLs) through the addition of controlled amounts of hydrophobic polytetrafluoroethylene (PTFE). This study investigated the effect of PTFE loading and coating method on the performance of AEMELs. Ni-based PTLs were used. Experimental results revealed that a low amount of PTFE not only enhanced the performance, but it also enhanced the catalyst layer attachment. On the other hand, higher PTFE content led to a reduction in water availability in the electrode, leading to increased mass transfer overpotentials. In summary, increasing the hydrophobicity of Ni-PTLs appears to improve AEMEC performance, with the best performing electrodes having a PTFE loading of ~1-2% (equivalent to 5-10 wt.% on carbon GDLs).

High‐Performance Zinc Anode Enabled by Zincophilic and Hydrophobic ZnO@Nitrogen‐Doped Carbon/MXene Interface

Aqueous zinc‐ion batteries (AZIBs) are one of the most promising systems for large‐scale energy storage, but their zinc metal anodes suffer from unsatisfactory stability and reversibility due to the uncontrollable Zn dendrite growth and undesirable side reactions. Herein, a ZnO‐anchored nitrogen‐doped carbon/Ti3C2Tx MXene composite (ZnO@NC/MXene) is developed as a protective layer onto the zinc anode, which establishes a zincophilic and hydrophobic interface. In the ZnO@NC/MXene layer, the nitrogen sites efficiently enhances the adsorption of Zn2+, the ZnO provides homogenous nucleation sites for Zn2+ deposition, and the highly conductive MXene ensures even electric field distribution, synergistically inhibiting the zinc dendrites. Additionally, the hydrophobic ZnO@NC/MXene layer suppresses side reactions by limiting contact between the Zn anode and active water. Therefore, the Zn electrode modified by the ZnO@NC/MXene layer shows remarkable stability with a cycle life of over 2600 h in Zn||Zn symmetric cell and outstanding reversibility with an average coulombic efficiency of 99.37% for over 1000 cycles in Zn||Cu asymmetric cell. Coupled with V2O5 cathode, the full cell reveals excellent cycle stability of exceeding 1000 cycles at 4 A g‐1. These results indicate the potential of the zincophilic and hydrophobic ZnO@NC/MXene as a promising interface layer for protecting Zn anode in AZIBs.

Low humidification effect on performance of proton exchange membrane fuel cells under high current density conditions

To enhance water management in proton exchange membrane fuel cells, the influence of drying and flooding on performance in different voltage ranges is investigated by experiment. The current density attenuates as the voltage decreases in the concentration polarization range. Through the analysis of the performance curves under different conditions, the performance attenuation is due to the high current and overpotential at low voltage, which leads to the temperature rising and drying up. At high flow rates, the current attenuates at low voltages, indicating that the current density attenuation is not because of flooding and insufficient reactants. According to calculations, the anode water content is low due to more protons and water being transferred from anode toward cathode at the high current, which causes the current density to attenuate. From the current ratio, the cell can generate an additional current density of 3.2% at 0.1 V voltage, which can evaluate the degree of carbon corrosion and water electrolyzation due to the insufficient supply of reactants. In addition, the voltage undershoot of switching to high current is great, and the fuel cell has a high demand for water. Finally, reducing temperature and increasing humidification can alleviate the attenuation of current density.

Failure mechanism of dielectric breakdown in electrochemical hydrogen pumps

Electrochemical hydrogen pump (EHP) cells, employing ultra-thin (This article refers specifically to films with a thickness of 12μm) proton exchange membranes (PEMs), are used in different fields such as hydrogen purification, hydrogen compression, and the rapid activation of hydrogen fuel cells. However, under certain extreme conditions, the voltage of an EHP cell with an ultra-thin proton exchange membrane can rise rapidly and trigger dielectric breakdown of the PEM, resulting in a PEM perforation and cell failure due to an internal short circuit. In order to investigate the failure mechanism, this paper comprehensively analyzes the electrochemical failure process leading to the breakdown of EHP cells. This investigation is accomplished by monitoring characteristic parameters such as voltage and resistance. The research outcomes indicate that the failure process in EHP cells can be divided into three stages: (I) Normal operation stage, (II) Carbon corrosion stage, and (III) Dielectric breakdown stage. In stage-I, the main process is the EHP reaction itself, the cell is undamaged and the cell voltage is about −0.22 V. In stage-II, the localized water shortage at the anode leads to a rapid increase in the anode ohmic overpotential, and the cell voltage is around −1.9 V, which reaches the carbon corrosion threshold. As a result, the anode catalyst layer undergoes a severe carbon corrosion reaction, generating CO and CO2 gases, and the cell performance decreases but the PEM is not seriously damaged. As substantial amount of anode water is consumed, the cell enters into stage-III. At this stage, the anode overpotential further increases, and the cell voltage reaches above −14 V, resulting in dielectric breakdown of the PEM, internal short circuit of the cell, and the cell cannot work normally. This study can enhance the understanding of the electrochemical failure process in EHP cells, thereby providing a basis for improving their durability and reliability.

Visualization study of improving anode water management by variable pressure hydrogen supply in proton exchange membrane fuel cell

To alleviate the prolonged closure of anodic outlet, which causes water to gradually accumulate in the porous layers that hinder hydrogen transfer in dead-ended anode mode. This study proposes an anode variable-pressure hydrogen supply strategy. The hydrogen supply pressure is suddenly reduced for a period before triggering purging, creating a pressure difference between the membrane assembly electrode and flow channels to induce water discharging into the flow channels, then directly purging out of the cell. Optical visualization is utilized to observe and analyze the effect of this strategy on water distribution and quantified through voltage, power, purging pressure change, and water coverage rate. Experiments indicate that the sudden pressure change in the variable pressure hydrogen supply mode effectively promotes water discharge from the porous layers, provides higher water coverage within the entire flow field, and promotes the fusion of neighboring droplets to form larger ones for better water management. Compared to normal mode, the proposed strategy achieves 3.4% performance improvement at 0.32A/cm2. Furthermore, a higher abrupt pressure difference favors porous layers drainage, but the optimal low-pressure duration requires a trade-off between drainage facilitating hydrogen transport and recovery of the membrane water content.

Operando X-ray photoelectron spectroscopy cell for water electrolysis: A complete picture of iridium electronic structure during oxygen evolution reaction

Operando investigations are crucial for understanding various catalytical processes. We present a newly designed cell for operando X-ray Photoelectron Spectroscopy water electrolysis. All measurements are done on a laboratory X-ray source, which makes the cell easily accessible to a wide audience. We demonstrate the cell operation on a magnetron-sputtered iridium catalyst for the anode of a Proton Exchange Membrane Water Electrolyzer. The main challenges consist of the anode water intake through the membrane from the cathode and the electrical contact. We can oxidize metallic Ir0 into a stable IrIV and reduce it back to metal, which agrees well with the ex-situ measurements. Furthermore, the operando tracking of the Ir 4f and O 1s spectra during the oxidation uncovers the presence of intermediates of the Oxygen Evolution Reaction on Ir, and thus allows the improvement of the understanding of its mechanism. The cell can be used to further study other catalysts for low-temperature water electrolyzers, both noble and non-noble metal-based.

Pressure and Impedance Based in Situ Diagnostic Methods for Water Management in Proton Exchange Membrane Fuel Cells Applications

Recent advancements in proton exchange membrane fuel cells (PEMFCs) have led to significant developments in low-carbon transport technology, enabling the generation of clean, efficient electricity with minimal noise and toxic emissions for heavy-duty applications. However, effective water management remains a critical challenge in PEMFCs due to the intricate phase changes and liquid-gas transport processes within key components, which significantly impact cell performance and longevity in industrial applications. Given the rapid dynamics of water activities and material responses within cells, in situ diagnostic techniques for water management in PEMFCs are essential for optimizing cell performance and extending the lifespan of fuel cell stack and system.Water, electrochemically produced on the cathode side, can cause potential flooding in key components including the catalyst layer (CL) and gas diffusion layers (GDLs), impeding air/oxygen supply to the cathode side. To characterize the performance loss induced by water flooding, Electrochemical Impedance Spectroscopy (EIS) is commonly employed as a fuel cell diagnostic tool. EIS records the contributions of different polarization processes to the global impedance with individual time constants during operation. However, the interpretation of complex EIS spectra can be very challenging, as polarization processes may overlap in frequency and be difficult to identify. To address this issue, Distribution of Relaxation Times (DRT) analysis is implemented as a powerful tool that allows for a clear separation and quantification of the polarization processes showing as peaks in a frequency domain [1]. Thus, we utilize EIS-based DRT analysis as an in-situ diagnostic tool to detect cathode flooding during cell operation. To validate this approach, we have conducted tests on a 300 cm2 low temperature PEMFC, varying one operation parameter at a time and evaluating its effects on cathode flooding by studying the frequency and height evolution of DRT peaks.Nevertheless, water flooding is not confined to the cathode side, as water can also permeate through the proton exchange membrane (PEM) and reach the anode side, resulting in potential fuel starvation on the cathode's counterpart and subsequent degradation of carbon-based materials. To address the sensitivity limitation of the EIS-based DRT method in detecting anode flooding, we employ an in-house developed pressure drop indicator (PDI) tool, as a non-invasive and real-time approach for diagnosing water flooding in the anode. The PDI tool assesses the pressure drop across anode channels, which is a function of the liquid water contents in bipolar plates considering the pressure reduction mainly caused by the frictional drag [2]. The effectiveness of the PDI tool is validated through cell testing at different water flooding levels by varying relative humidity and pressure conditions. This technique offers timely and sensitive detection of anode water flooding in PEMFCs, which is crucial for alarming irreversible material degradation in the electrode and facilitating appropriate follow-up actions.To conclude, we have developed and employed in situ PDI and EIS-based DRT methods within our water management framework for diagnosing water flooding in both the anode and cathode of PEMFCs in industrial applications. The combination of these techniques offers real-time and accurate assessments on water management conditions, which is crucial for optimizing overall performance and longevity of PEMFC stacks and systems.[1] Heinzmann, M., Weber, A., Ivers-Tiffée, E. (2018). Advanced impedance study of polymer electrolyte membrane single cells by means of distribution of relaxation times. Journal of Power Sources, 402, 24-33.[2] Barbir, F., Gorgun, H., & Wang, X. (2005). Relationship between pressure drop and cell resistance as a diagnostic tool for PEM fuel cells. Journal of Power Sources, 141(1), 96-101. Figure 1

Evaluation of Anode Water Electrolyzed with Anion Exchange Membrane for Cleaning EUV Semiconductor

Electrically nonconducting UPW was electrolyzed without electrolyte through anion exchange membrane for evaluating applicability to EUV semiconductor cleaning. Produced anode water held positive ORP up to 900 mV, which is very oxidative. ORP, pH, and conductivity measurements showed delicately complementary each other for understanding anode water. Correlation of concurrent ORP decrease and conductivity increase in ultra-pure anode water domain was observed first time. The oxidative OH° was formed as the major species in anode water, causing positive ORP during ORP measurement. H+ and OH− ions, and OH° radical coexisted in anode water at amphoteric nonequilibrium, while pH was less than 6. It was concluded that OH°, as a strong oxidant, transformed itself to OH− by ORP measurement. OH° radical would oxidize selectively and then remove nano-contaminants. Anode water is considered to fulfill the requirement of EUV semiconductor cleaning where no oxygen species should be required because of likely oxide layer formation during cleaning, and it will even remove the native oxide developed unintentionally before cleaning.

3D structured liquid/gas diffusion layers with flow enhanced microchannels for proton exchange membrane electrolyzers

Porous transport layers (PTLs) work as key water/gas transport components in proton exchange membrane electrolyzer cells. In PTLs, the gas accumulation under the lands of bipolar plate (BP) has been widely recognized, which decreases the local water saturation and blocks some active areas. Especially for some 2D structured PTLs, the absence of in-plane transport ability raises transport concerns at high current densities. In this study, a wet etching method is introduced to fabricate 3D-structured PTLs, named flow enhanced liquid/gas diffusion layer (FELGDL) for promoting multiphase transport under BP lands. In-situ performance evaluation validates a 330-mV voltage drop and a 7.9% efficiency improvement at 6 A/cm2 compared with the 2D-structured LGDL, which are mainly attributed to the improved reactant supply to the pores under BP lands. Additionally, the mass transport limitation is extended from 6.3 to 9.0 A/cm2 with an anode water flow rate of 20 mL/min. By the in-situ visualization method, the enhanced in-plane mass transport under BP lands is directly visualized. The successful fabrication and performance validation of FELGDL opened a new direction to high-performance 3D structured PTL manufacturing.

Developing Reversal-Tolerant Anode for PEFCs by Introducing Iridium-Based Layered Structure

On the anode side of the polymer electrolyte fuel cell (PEFC), hydrogen fuel can be blocked from reaching the catalyst layer due to anode water flooding which can occur at high humidity, low current density, and rapid load changing conditions [1-3]. This can cause the cell voltage reversal phenomena where anode potential increases relative to the cathode potential resulting in severe carbon corrosion, Platinum catalyst dissolution and agglomeration, and then causing performance loss and eventually cell failure. Currently, control based strategies such as voltage monitoring or flushing excess water have been suggested to overcome this issue [4,5], but unfortunately this increases the complexity and cost of the overall system. In order to greatly reduce system cost, more passive, material-based based solutions such asdevelopment of reversal tolerant anodes (RTAs) are necessary. RTAs include a water electrolysis catalyst such as Ir, Ru, IrO2, RuO2, etc, that can preferentially promote and sustain water electrolysis reaction during cell reversal in order to delay or even prevent the anode from reaching voltages higher than ~1.7 V, [6] resulting in a passive and hence cost-effective solution for the fuel starvation problem.The standard Pt/C+IrO2 RTA catalyst was first studied to investigate its cell reversal characteristics, and several current shortcomings were identified. We found that the RTA underwent significant carbon-based performance degradation even during the water electrolysis stage, and the oxygen evolution reaction (OER) catalyst, IrO2, was found to be deactivated after every subsequent cell reversal experiments. Furthermore, a comparative study of the standard Pt/C+IrO2 RTA with other structures such as Pt/C+Ir/C and Ir(IrOx)/Pt/C hybrid catalyst revealed a new cause of performance degradationbased on Platinum and Iridium particle alloying. Therefore, it is important to manage both the carbon-based degradation and alloying-based degradation in order to make RTAs feasible, and thus tackle the fuel starvation problem.Therefore introduction of layered structure was introduced in RTA catalyst, as explained in Fig. 1. In this study two types of layered RTA catalysts were explored towards addressing these problems. Layered RTA catalysts, LRC1, was made by spray-printing IrO2 (0.24 mgIrO2/cm2) first and then Pt/C (0.3 mgPt/cm2) on the Nafion 212 membrane as shown in Fig.1 (b). In LRC2 case, Cvulcan layer was spray-printed between IrO2 and Pt/C layers as described in Fig.1 (c). Compared to the standard RTA, LRC1 showed improved OER activity and higher performance after the fuel starvation durability test. The LRC2 structure was able to suppress alloying based degradation almost completely. Our findings contribute greatly to the understanding of the fuel starvation based degradation phenomena, and will help in designing better RTAs with improved performance and durability. Fig. 1 Schematic diagram showing (a) standard mixed RTA: Pt/C+IrO2, (b) LRC1: Pt/C -> IrO2, (c) LRC2: Pt/C -> Cvulcan -> IrO2 References [1] Hong, B. K., Mandal, P., Oh, J.-G. & Litster, S. On the impact of water activity on reversal tolerant fuel cell anode performance and durability. J. Power Sources 328, 280–288 (2016).[2] Knights, S. D., Colbow, K. M., St-Pierre, J. & Wilkinson, D. P. Aging mechanisms and lifetime of PEFC and DMFC. J. power sources 127, 127–134 (2004).[3] O’Rourke, J., Ramani, M. & Arcak, M. In situ detection of anode flooding of a PEM fuel cell. Int. J. Hydrog. Energy 34, 6765–6770 (2009). [4] Kurnia, J. C., Sasmito, A. P. & Shamim, T. Advances in proton exchange membrane fuel cell with dead-end anode operation: A review. Appl. Energy 252, 113416 (2019).[5] Chen, Y.-S., Yang, C.-W. & Lee, J.-Y. Implementation and evaluation for anode purging of a fuel cell based on nitrogen concentration. Appl. Energy 113, 1519–1524 (2014).[6] Ralph, Thomas R., Sarah Hudson, and David P. Wilkinson. "Electrocatalyst stability in PEMFCs and the role of fuel starvation and cell reversal tolerant anodes." ECS Transactions1.8 (2006): 67. Figure 1

Investigation of water effects on hydrogen dissolution, diffusion, and reaction in high temperature proton exchange membrane fuel cell

High-temperature proton exchange membrane fuel cell with phosphoric acid-doped polybenzimidazole membrane is typically utilized with a methanol steam reformer due to its high CO tolerance. However, methanol reformate commonly contains 20 % water, and its effect on the high temperature fuel cell is unclear. A comprehensive 3D multiphase non-isothermal model with an enhanced electrochemical kinetics model considering the water influence mechanism is developed. The water influence mechanism includes the proton conductivity, hydrogen dissolution on the pore-ionomer interface, hydrogen diffusion inside the ionomer, and hydrogen surface reaction on the platinum. The sensitivity analysis found that hydrogen dissolution on the pore-ionomer and hydrogen surface reaction on the platinum significantly affect cell performance. The influences of water on ionomer hydrogen concentration, platinum surface hydrogen concentration, and surface reaction rates are investigated under the voltage range of 0.9–0.4 V. The increase of water content can improve the net adsorption rate and benefit electrochemical reaction. Besides, the anode water is better for the uniformity of surface reaction rate distributions while not affecting the hydrogen distributions inside the ionomer and on the platinum surface. Additionally, the positive influence of water on CO poisoning is investigated in terms of surface reaction. The water's beneficial effect on decreasing platinum loading is also discussed. The results show that 20 % water can reduce the platinum loading by 20 %.

Pressure-alternating gas impedance: A new pathway for estimating recirculation flow rate of ejectors to avoid fuel cell degradation

An ejector-based gas recirculation system (EBGRS) has emerged as a mainstream configuration for hydrogen subsystems of proton exchange membrane (PEM) fuel cells. However, this configuration poses challenges in regulating anode water and hydrogen concentration owing to the passive characteristics of the ejector. To avoid anode flooding and hydrogen starvation and improve the lifetime of PEM fuel cells, precise closed-loop control of anode purge is essential. Such control relies on the feedback of the ejected recirculation flow rate (ERFR). However, existing methods to measure or estimate ERFR, including sensor measurement, model-based estimation, and pressure-drop-based estimation, do not meet the needs of vehicle applications. To address this bottleneck, this study proposes a new pathway for estimating ERFR, which is named the gas impedance method. Two gas impedance variables are defined and a theoretical model of EBGRS is proposed. By systematically analyzing the modeling methods, the lumped parameter method is adopted to model the recirculation pipeline. The flow rate–pressure characteristics of the ejector are locally linearized to couple with the recirculation pipeline model. The analytical expressions of the two gas impedance variables are derived from the model. Moreover, the analytical model is validated via numerical simulations and experiments. Validation results demonstrate that the analytical model effectively describes the frequency response of gas impedance and its correlation with ERFR for various operating conditions and frequencies, paving the path for estimating ERFR accurately using gas impedance. Future research will focus on improving the accuracy of the analytical model and fully exploiting the potential of gas impedance in estimating ERFR.

Water Confinement by a Zn2+-Conductive Aqueous/Inorganic Hybrid Electrolyte for High-Voltage Zinc-Ion Batteries

Aqueous zinc-ion batteries (ZIBs) have received extensive attention owing to the intrinsic safety and high specific energy. However, the practical performance of the ZIBs is impacted by the water-induced complicated interfacial reactions, including the corrosion/dendrites of a Zn anode and the dissolution/degradation of a cathode. Herein, we developed a type of inorganic Zn2+-interlayered montmorillonite-based aqueous/inorganic hybrid electrolyte (ZM-30% HE) that not only realizes low water content but also shows a strong water confinement effect, which leads to a significantly weakened water activity and water solvation effect. Therefore, the side reactions between the Zn anode and free water molecules are effectively suppressed and the diffusion/insertion of zinc ions into the cathode is substantially facilitated. At the Zn anode side, ZM-30% HE can stabilize the symmetric cell for 2000 h at 1 mA cm–2. Moreover, low water content enables the application of the intercalation-type Zn-ion cathode. Based on the ZM-30% HE and Mn4[Fe(CN)6]2.84·11.8H2O cathode, record-high voltage (1.70 V vs Zn2+/Zn) and voltage efficiency (92%) among the reported Zn metal cells with hybrid electrolytes and Zn2+-storage cathodes were achieved. Meanwhile, the dissolution/degradation problem of the cathode in the traditional liquid electrolyte was solved using the HE. The Zn/ZM-30% HE/Mn4[Fe(CN)6]2.84·11.8H2O full cell shows stable cycling up to 400 cycles even at a low current density of 100 mA g–1. The development of the hybrid electrolyte with unique water confinement effect provides a solution to boost the performance of the current ZIBs.

Numerical analysis of improved water management of open-cathode proton exchange membrane fuel cells with a dead-ended anode by pulsating flow

Anode water management is critical for the efficient operation of proton exchange membrane fuel cells with a dead-ended anode. To clarify the mass transfer phenomenon in the anode flow channel under the dead-ended anode mode, and reveal the influence mechanism of pulsating flow on water management, a three-dimensional, two-phase, non-isothermal transient model is established in this study. The water content and species distribution in different layers are analyzed, and the internal relationship between water transport behavior and output performance of the proton exchange membrane fuel cell under different operating conditions is explored. The simulation results show that the output performance of the proton exchange membrane fuel cell in dead-ended anode mode is directly related to the gas diffusion layer's water saturation and the hydrogen mass transfer. Furthermore, pulsating flow can effectively suppress the back diffusion of water, and improve the mass transfer rate of hydrogen. Consequently, the water management and the operational stability of the proton exchange membrane fuel cell are significantly improved. The research results of this paper have important guiding significance for improving the water and gas management of fuel cells.

Investigation of Water and Heat Transfer Mechanism in PEMFCs Based on a Two-Phase Non-Isothermal Model

In a proton exchange membrane fuel cell (PEMFC) system, proper management of water and heat transport is essential to improve its overall performance and durability. To comprehensively investigate the internal processes of PEMFCs, an improved two-phase non-isothermal model based on heat and water transfer mechanisms inside the fuel cell is developed. The results show that the model proposed in this work can predict the fuel cell’s performance accurately and is capable of exploring water and heat transfer phenomena inside fuel cells. Additionally, the water and heat transfer of cathodes and anodes under different relative humidity and temperatures are studied. It can be concluded that when the PEMFC operates under a constant voltage, the anode water content gradually increases, while the cathode water content gradually decreases. The maximum water content occurs at the interface between cathode catalyst layer and cathode gas diffusion layer, while the minimum value is attained at the interface between anode catalyst layer and anode gas diffusion layer. When the fuel cell operates at 0.75 V, although the water content of CCL is the highest, no back-diffusion of dissolved water occurs.