Open-circuit dissolution of platinum from the cathode in polymer electrolyte membrane water electrolysers

Platinum is the state-of-the-art catalyst for hydrogen evolution in polymer electrolyte membrane (PEM) water electrolysers; however, its stability has only been characterized to a limited extent in situ. This study measures platinum dissolving from the cathode during intermittent operation of a 3-electrode PEM electrolyser cell, using a differential pulse voltammetry technique that provided detection limits for platinum of less than 2 ng L−1. Water samples were periodically taken during on-off current cycling, and during periods of open-circuit voltage (OCV) platinum dissolution was detected when the cathode potential rose above 0.85 V NHE due to diffusion of oxygen from the anode. This reached a maximum dissolution rate at the highest cathode potential of 1.02 V NHE, and gradually decayed over a 90 h period. The average total amount of platinum dissolved per 90 h OCV period was estimated to be 152 ng cm−2 or 0.005% of the initial electrode catalyst mass. The dissolution mechanism was predicted to be the same as that occurring in PEM fuel cell cathodes, although being kinetically hindered in PEM electrolysers by the slow diffusion of oxygen from the anode to the cathode

Acoustic time-of-flight imaging of polymer electrolyte membrane water electrolysers to probe internal structure and flow characteristics

Acoustic time-of-flight (AToF) imaging has been demonstrated as a low-cost, rapid, non-destructive, operando tool to characterize processes in the flow channels and liquid-gas diffusion layer (LGDL) of polymer electrolyte membrane water electrolysers (PEMWEs). An array of 64 piezoelectric sensors was used, with all sensors emitting input pulses and detecting the acoustic wave reflected by the sample (pulse-echo mode). The shape and intensity of this reflected waveform depends on the ratio of reflection and transmission at phase interfaces and is strongly affected by resonant scattering of acoustic waves by gas bubbles. This AToF imaging technique was deployed to produce reflection intensity maps of the anode flow-field and LGDL; by measuring the AToF response for current densities ranging from 0.00 A cm−2 to 2.00 A cm−2, a close correlation was found between the acoustic attenuation in the flow-field and the production and removal of oxygen gas through the flow channels. Furthermore, a close link between the AToF response and water thickness in the LGDL was demonstrated, as supported by literature data. The application of the AToF technique has been established as a novel way of investigating PEMWE operation and as an alternative to more complex imaging techniques such as neutron imaging

Diagnosing Stagnant Gas Bubbles in a Polymer Electrolyte Membrane Water Electrolyser using Acoustic Emission

The use of acoustic emission as a low-cost, non-destructive, and operando diagnostic tool has been demonstrated for a range of electrochemical energy conversion and storage devices, including polymer electrolyte membrane water electrolysers (PEMWEs) and fuel cells. In this work, an abrupt change in acoustic regime is observed during operation of a PEMWE as the current density is increased from 0.5 to 1.0 A cm^{-2}. This regime change is marked by a sudden drop in the number of acoustic hits, while hit duration, amplitude, and energy increase significantly. It is found that the change in acoustic regime coincides with a significant extension of the stagnant bubble region in the flow channels of the PEMWE, observed with high-speed optical imaging. These results demonstrate that acoustic emission can be used effectively as an operando diagnostic tool to monitor bubble formation (two-phase flow conditions) in PEMWEs, facilitating rapid testing or prototyping, and contributing to operational safety

Recent advances in acoustic diagnostics for electrochemical power systems

Over the last decade, acoustic methods, such as acoustic emission and ultrasonic testing, have been increasingly deployed for process diagnostics and health monitoring of electrochemical power devices including batteries, fuel cells, and water electrolysers. These acoustic are non-invasive, highly sensitive, and low cost, while also providing a high level of spatial and temporal resolution, and practicality. The application of these tools in electrochemical devices is based on identifying changes in acoustic signals due to physical, structural, and electrochemical properties change within the material which are then correlated to critical processes and the health status of the devices. This review discusses recent progress in the use of acoustic methods for process and health-monitoring of major electrochemical energy conversion and storage devices. First, the fundamental concepts and principles of acoustic emission and ultrasonic testing are introduced, followed by a discussion of the range of electrochemical energy conversion and storage systems, and how acoustic techniques are being used to study relevant materials and devices. Conclusions and future perspectives highlighting some of the unique challenges and potential commercial and academic applications of the devices are also discussed. It is expected that, with further developments, acoustic techniques will form a key part of the suite of diagnostic techniques routinely used to monitor electrochemical devices across various processes including fabrication, on-board maintenance, post-mortem examination and second life or recycle decision support to aid the deployment of these devices in increasingly demanding applications

Identifying Defects in Li-Ion Cells Using Ultrasound Acoustic Measurements

Identification of the state-of-health (SoH) of Li-ion cells is a vital tool to protect operating battery packs against accelerated degradation and failure. This is becoming increasingly important as the energy and power densities demanded by batteries and the economic costs of packs increase. Here, ultrasonic time-of-flight analysis is performed to demonstrate the technique as a tool for the identification of a range of defects and SoH in Li-ion cells. Analysis of large, purpose-built defects across multiple length scales is performed in pouch cells. The technique is then demonstrated to detect a microscale defect in a commercial cell, which is validated by examining the acoustic transmission signal through the cell. The location and scale of the defects are confirmed using X-ray computed tomography, which also provides information pertaining to the layered structure of the cells. The demonstration of this technique as a methodology for obtaining direct, non-destructive, depth-resolved measurements of the condition of electrode layers highlights the potential application of acoustic methods in real-time diagnostics for SoH monitoring and manufacturing processes

Identifying Defects in Li-Ion Cells Using Ultrasound Acoustic Measurements

Identification of the state-of-health (SoH) of Li-ion cells is a vital tool to protect operating battery packs against accelerated degradation and failure. This is becoming increasingly important as the energy and power densities demanded by batteries and the economic costs of packs increase. Here, ultrasonic time-of-flight analysis is performed to demonstrate the technique as a tool for the identification of a range of defects and SoH in Li-ion cells. Analysis of large, purpose-built defects across multiple length scales is performed in pouch cells. The technique is then demonstrated to detect a microscale defect in a commercial cell, which is validated by examining the acoustic transmission signal through the cell. The location and scale of the defects are confirmed using X-ray computed tomography, which also provides information pertaining to the layered structure of the cells. The demonstration of this technique as a methodology for obtaining direct, non-destructive, depth-resolved measurements of the condition of electrode layers highlights the potential application of acoustic methods in real-time diagnostics for SoH monitoring and manufacturing processes

Two-dimensional model of low-pressure PEM electrolyser: Two-phase flow regime, electrochemical modelling and experimental validation

International audienceBased on proton conduction of polymeric electrolyte membrane (PEM) technology, the PolymerElectrolyte Membrane Water Electrolyser (PEMWE) offers an interesting solution forefficient hydrogen production. During the electrolysis of water in PEMWE, water is split intooxygen, protons and electrons at the anode and a water-gas two-phase flow results. The aimof this study is to investigate the link between the two-phase flow at the anode side and cellperformance under low-pressure conditions. We have developed a two-dimensional stationaryPEMWE model that takes into account electrochemical reaction, heat transfer, masstransfer (bubble flow) and charge balance through theMembrane Electrodes Assembly (MEA).In order to take into account the changing electrical behaviour, our model combines twoscales of descriptions: at microscale within anodic active layer and MEA scale. The watermanagement at both scales is strongly linked to the Not Coalesced Bubble regime (NCBregime) or the Coalesced Bubble regime (CB regime). Therefore, water content close to activesurface areas depends on two-phase flow regimes. Our simulation results demonstrate thatthe coalesced phenomenon is associated with improvement of mass transfer, a decrease inohmic resistance and an enhancement of the PEMWE efficiency. At low and medium currentdensity values, themodel has been validated using two separate experiment electrolysis cells

Operando flow regime diagnosis using acoustic emission in a polymer electrolyte membrane water electrolyser

Polymer electrolyte membrane water electrolysers (PEMWE) are a key technology for producing clean (‘green’) hydrogen for decarbonisation of the transport sector and grid stabilisation utilising increasing levels of renewable energy. In this work, acoustic emission analysis is used as a non-destructive, operando diagnostic tool to provide information about the relative number and size of gas bubbles generated locally within a PEMWE, providing effective characterisation of the local flow conditions. An optically transparent single-channel PEMWE is used to investigate the relationship between the acoustic signals obtained and the two-phase flow conditions inside the cell. The number of acoustic hits, their frequency, and average peak amplitude is reported for several flow rates and current densities. Using high-speed imaging, the average bubble number and size in the flow cannels is compared to the acoustic signal. Results show good correlation between the number of acoustic ‘hits’ and the number of bubbles passing through the flow channel. The size of bubbles is also shown to affect the average frequency of the hits. Consequently, the transition between bubbly and slug flow regime can be identified by acoustic emission analysis, paving the way for a simple, low-cost, non-destructive means of mapping flow inside commercial-scale PEMWEs

Mass transport in polymer electrolyte membrane water electrolyser liquid gas diffusion layers A combined neutron imaging and X ray computed tomography study

The increasing use of intermittent renewable energy sources calls for novel approaches to large scale energy conversion and storage. Hydrogen can be readily stored and produced from renewable sources using polymer electrolyte membrane water electrolysers PEMWEs . Mass transport of water and product gas in the liquid gas diffusion layer LGDL is critical for PEMWE performance, particularly at high current densities. In this work, neutron radiography is deployed to measure the spatial distribution of water within three different LGDLs, while X ray micro computed tomography XCT is used to characterize the microstructure of the LGDL materials. The combination of these two techniques yields valuable insight into water transport within the LGDL. Significant local water heterogeneity is observed and a link between flow field geometry location and LGDL mass transport is identified. It is further shown that the pore volume in these LGDLs is significantly under utilized, pointing the way towards design optimisation of LGDL materials and architecture