7 research outputs found
Mass transport in PEM water electrolysers: A review
While hydrogen generation by alkaline water electrolysis is a well-established, mature technology and currently the lowest capital cost electrolyser option; polymer electrolyte membrane water electrolysers (PEMWEs) have made major advances in terms of cost, efficiency, and durability, and the installed capacity is growing rapidly. This makes the technology a promising candidate for large-scale hydrogen production, and especially for energy storage in conjunction with renewable energy sources – an application for which PEMWEs offer inherent advantages over alkaline electrolysis. Improvements in PEMWE technology have led to increasingly high operational current densities, which requires adequate mass transport strategies to ensure sufficient supply of reactant and removal of products. This review discusses the current knowledge related to mass transport and its characterisation/diagnosis for PEMWEs, considering the flow channels, liquid-gas diffusion layer, and polymer electrolyte membrane in particular
Towards Improved Understanding of Mass Transport in Polymer Electrolyte Membrane Water Electrolysers
The advent of a global societal and governmental movement to curb climate change has put low-carbon technologies at the centre stage of public interest and scientific efforts. In the wake of rising concerns around the carbon footprint of personal mobility and the energy sector, the concept of a ‘Hydrogen Economy’ has experienced yet another rapid spur of popularity. Polymer electrolyte membrane water electrolysers (PEMWEs) are a promising candidate for large-scale hydrogen production, and improvements in the technology have led to increasingly high operational current densities exceeding 2 A cm-2, which requires adequate mass transport strategies to ensure sufficient supply of reactant and removal of products. Optimization and diagnosis of mass transport processes in PEMWEs has long been neglected despite its significance, but the amount of scientific literature has recently increased sharply. This thesis uses existing diagnostic tools to gather new insights into the processes within PEMWE flow channels and liquid-gas diffusion layers, aims at providing new low-cost diagnostic tools to accelerate the investigation of mass transport processes, and consequently deduces novel approaches to the design of PEMWEs components, cells, and stacks. Neutron and X-ray imaging are used to demonstrate the effect of liquid-gas diffusion layer microstructure on the water-gas distribution in a PEMWE, revealing significant inhomogeneity across the active area. Due to cost and accessibility issues around radiation imaging, acoustic methods are explored as alternative diagnostic tools. Acoustic emission is successfully demonstrated as an operando technique to monitor two-phase flow in the flow channels, detecting the transition from bubbly to slug flow. Bubbly flow is observed at the onset of electrochemical activity and low current densities, with a high number of small bubbles, while at higher current densities these small bubbles coalesce and form larger slug bubbles. Lastly, acoustic time-of-flight imaging is used to monitor the water-gas distribution in the liquid-gas diffusion layer and the flow channels, with results being consistent with expectations and previous results obtained via neutron imaging
Influence of Bubbles on the Energy Conversion Efficiency of Electrochemical Reactors
Bubbles are known to influence energy and mass transfer in gas evolving
electrodes. However, we lack a detailed understanding on the intricate
dependencies between bubble evolution processes and electrochemical phenomena.
This review discusses our current knowledge on the effects of bubbles on
electrochemical systems with the aim to identify opportunities and motivate
future research in this area. We first provide a base background on the physics
of bubble evolution as it relates to electrochemical processes. Then we outline
how bubbles affect energy efficiency of electrode processes, detailing the
bubble-induced impacts on activation, ohmic and concentration overpotentials.
Lastly, we describe different strategies to mitigate losses and how to exploit
bubbles to enhance electrochemical reactions.Comment: Joule(2020
On the Mass Transport Phenomena in Proton Exchange Membrane Electrolyzers
Proton exchange membrane (PEM) water electrolysis is a technology designed to produce H2 using only water and electricity as inputs; it has gained increased attention in industry and academia due to its advantages over incumbent H2 generation processes (of which the most widely used are steam reforming and coal gasification) namely, low temperature, carbon-neutral and intermittent operation. PEM electrolysis can be instrumental for creating a hydrogen economy, although still much research needs to be carried out before widespread industrial adoption is achieved. PEM water electrolyzers suffer energy losses associated with the chemical reactions and the transport of charge
and mass; of these phenomena, mass transport in PEM electrolyzers is the least understood subject, given the complex nature of the interaction of multiphase flows (mainly consisting of liquid water and evolved gases) through micrometric pores.
The subject of multiphase flow in water electrolysis and its relationship with the mass transport phenomena in PEM water electrolysis has been a prevalent subject in the literature. Despite numerous attempts at pinpointing the relationship between mass transport overpotential and the operating parameters, there is no clear consensus about which transport mechanisms dominate, nor about how the component design of PEM electrolyzers affects the mass transport. While the effect of temperature and current density on mass transport losses has been extensively studied and is well understood, there are significantly fewer studies that focus on the effect of water flow and pressure. Both
water flow and pressure have a direct effect on mechanisms such as bubble nucleation and two-phase flows that occur in the porous structures within a PEM electrolyzer (electrodes and porous transport layers, PTLs).
In this work, I studied the effect of water flow and pressure on the mass
transport phenomena in PEM electrolyzers. Chapters 1 and 2 provide an introduction to the topic as well as a description of the materials and experimental setups used. Chapter 3 of this thesis depicts the visualization and modeling of bubble nucleation in an operating PEM electrolyzer. I discovered that bubble detachment radii are largely independent of water flow and I identified two types of bubbles: bubbles that detach after reaching a critical size, and bubbles that fill up the pores of a PTL before detaching. Chapter 3 consists of the measurements I carried out regarding the transport of evolved gas through the water-filled pores of a PTL, where I observed that water flow severely impedes
the gas transport through the pores and that such impediment is related to a shear stress exerted by the water flow on the pores. Chapter 5 shows the measuring of mass transport losses using electrochemical impedance spectroscopy (EIS) on an operating PEM electrolyzer; the results indicate that pressure and water flow affect the diffusion of gas in the electrode and that the mass transport overpotential depends on design parameters of the PEM electrolyzer, such as electrode thickness and hydrophobicity.
Overall, I derived a theoretical framework based on the assumption that
the evolved gas in a PEM electrolyzer permeates through the PTL after diffusing from the active sites to the bubble nucleation sites. Such framework, constructed on the basis of the models regarding gas transport in porous media, can be used to explain the mass transport loses in a PEM electrolyzer that arise from operating with increased water flows and pressures. The model I derived can be used in future work as a guideline to optimize the components of a PEM electrolyzer, in particular regarding the hydrophobicity and pore size distribution of PTLs as well as the composition of the catalyst ink to produce the electrodes. Moreover, this work can also be used to further understand the mass transport losses and optimize the operation of PEM electrolyzers to decrease the energy consumption of H2 generation
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Oxide-Encapsulated Electrocatalysts for Solar Fuels Production
As the cost of solar energy continues to drop, the major hurdle limiting the widespread use of intermittent renewable solar energy is the lack of efficient and cost-effective energy storage. Electrochemical technologies, such as electrolyzers, photoelectrochemical cells, and fuel cells, have the potential to compensate for solar energy intermittency on a large scale, by converting excess solar energy into storable solar fuels, such as hydrogen (H2), which can be converted back to electrical energy at a later time. However, improvements in the efficiency and lifetime of these technologies, in particular the electrocatalysts, are necessary for their commercialization. During operation, efficiency losses result from energetic penalties (overpotentials) associated with several processes occurring at or near the electrocatalyst/electrolyte (ohmic resistance, kinetic barriers, and mass transport limitations). These losses can be further exacerbated due to electrocatalyst durability issues such as dissolution, agglomeration, detachment, and poisoning. A major challenge in electrocatalysis field is developing methods to mitigate these losses without adversely affecting the electrocatalytic stability, selectivity, and/or activity.
One promising solution is an oxide-encapsulated electrocatalyst architecture, which has been shown to improve electrocatalyst durability and provide mechanisms for controlling reaction pathways. Previous studies on oxide-encapsulated electrocatalysts, in which metal catalysts are fully or partially covered by ultrathin layers of permeable oxide films, have mostly focused on supported nanoparticles because of their high electrochemically active surface area per catalyst loading. However, these nanoparticle-based architectures tend to have poorly defined and/or non-uniform structures which make it difficult to understand and elucidate structure-property-relationships. This dissertation investigates well-defined oxide-coated electrocatalysts, which serve as model platforms for gaining a fundamental understanding of kinetic and transport phenomena that underlie their operation. This dissertation presents three studies which highlight the versatile functionalities of oxide-encapsulated electrocatalysts to improve the electrocatalyst stability, selectivity, and activity in different electrochemical systems. This dissertation demonstrates the ability of room temperature synthesized silicon oxide (SiOx)-encapsulated Pt electrocatalysts to: i) stabilize nanoparticles and improve electron transfer, ii) mitigate catalyst poisoning and control reaction pathways through selective transport, and iii) alter reaction energetics associated with catalysis at the buried interface.
First, this dissertation establishes the ability of room temperature synthesized SiOx coatings to stabilize nanoparticle electrocatalysts by mitigating electrocatalyst migration, coalescence, and detachment on metal-insulator-semiconductor (MIS) photoelectrodes for solar-driven water splitting. Metallic Pt nanoparticles are inherently unstable on the insulating support due to poor physical adhesion and electronic coupling between Pt and SiO2. To overcome this issue, a room temperature UV ozone synthesis process was used to deposit 2-10 nm thick SiOx overlayers on top of electrodeposited Pt nanoparticles to stabilize Pt on the electrode surface. The photoelectrodes containing oxide-encapsulated electrocatalysts exhibit superior durability and electron transfer (ohmic) properties compared to the photoelectrode that lacked the SiOx encapsulation. While this study demonstrates that the oxide-encapsulated electrocatalyst architecture improves the stability of electrocatalytic nanoparticles deposited on insulating materials, it does not elucidate how reactants and products transport through the SiOx barrier to reach the Pt surface.
In order to gain a better understanding of kinetic and transport phenomena that govern performance of oxide-encapsulated electrocatalysts, the following studies investigate model electrodes consisting of continuous SiOx overlayers of uniform thickness deposited onto smooth Pt thin films. This planar electrode geometry allows for simple and unambiguous characterization of structure-property relationships. The next study systematically evaluates the influence of SiOx thickness on the HER performance to understand species transport through SiOx. Through detailed characterization and electroanalytical tests, it is shown that proton and H2 transport occur primarily through the SiOx coating such that the HER occurs at the buried Pt|SiOx interface. Importantly, the SiOx nanomembranes were found to exhibit high selectivity for proton and H2 transport compared to Cu2+, a model HER poison. Leveraging this property, it is shown that SiOx–encapsulation can enable poison-resistant operation of Pt HER electrocatalysts. This oxide-encapsulated architecture offers a promising approach to enhancing electrocatalyst stability while incorporating advanced catalytic functionalities such as poison resistance or tunable reaction selectivity.
The final study demonstrates ability of SiOx overlayers to alter reaction energetics associated with catalysis at the buried interface. Carbon monoxide (CO), methanol, and ethanol oxidation reactions are studied for their relevance in direct alcohol fuel cell applications. Oxide-supported catalysts have been shown to enhance alcohol oxidation by promoting CO oxidation at metal/oxide interfacial regions through the so-called bifunctional mechanism, in which hydroxyls on the oxide facilitate the removal of adsorbed CO−intermediates from active sites. A key advantage of the oxide-encapsulated electrocatalyst design compared to oxide–supported nanoparticles is that the former maximizes the density of metal/oxide interfacial sites. This study shows that the SiOx overlayer provides proximal hydroxyls, in the form of silanol groups, which can enhance CO and alcohol oxidation through unique interactions at the buried Pt|SiOx interface. Overall, this dissertation highlights the potential of using oxide-encapsulated electrocatalysts for stable, selective, and efficient electrochemical production and use of solar fuels
Development of Novel Thin Membrane Electrode Assemblies (MEAs) for High-Efficiency Energy Storage
Hydrogen is a ‘zero-emission’ energy carrier, which could be an important part of environment-friendly solutions to the global energy crisis via energy storage without producing greenhouse gases. The proton exchange membrane electrolyzer cell (PEMEC) is one of the most practical and energy efficient methods for producing high purity hydrogen from renewable sources, such as wind, hydro and solar energy. Since the wide commercialization of PEMECs is still hindered by their performance, cost and durability, superior performance PEMECs with low-cost and high-efficiency are strongly desired. The membrane electrode assembly (MEA), which consists of liquid/gas diffusion layers (LGDLs), catalyst layers (CLs) and membrane, is the core component of the PEMECs. LGDLs play an important role in enhancing the performance of PEMECs. They are expected to transport electrons, heat, and reactants/products simultaneously with minimum electrical, thermal, interfacial, and fluidic losses. CLs are mainly formed by noble metals or their oxides, which has great impact on PEMEC performance, durability and cost. The objective of this research is to develop novel MEAs coupled with the titanium-based thin/tunable LGDLs (TT-LGDLs) that has the well-controlled pore morphologies. The main achievements of this research include: (a) The TT-LGDLs can achieve superior performance due to the remarkably reduced ohmic and activation losses, and the effects of pore morphologies have been identified. (b) The gold electroplating is a promising method for the PEMEC performance enhancement by surface modifications. (c) The microporous layers (MPLs) offer some improved PEMEC performance under specific conditions, but may not be required for optimum TT-LGDLs. (d) The novel GDEs with ultra-low Pt catalyst loadings have been developed, which has obtained an acceptable performance with a significantly improved catalyst mass activity. (e) The theoretical analysis is adopted to study the true electrochemical reaction mechanism in PEMECs, and a model is developed, which is used to simulate the PEMEC performance and optimize the parameters of the electrodes. The novel thin MEAs developed in this research point out a promising direction for future MEA development, and can be a guide for the high-efficiency and large scale energy storage
Fundamental Studies of Electrochemical Reactions and Microfluidics in Proton Exchange Membrane Electrolyzer Cells
In electrochemical energy devices, including fuel cells, electrolyzers and batteries, the electrochemical reactions occur only on triple phase boundaries (TPBs). The boundaries provide the conductors for electros and protons, the catalysts for electrochemical reactions and the effective pathways for transport of reactants and products. The interfaces have a critical impact on the overall performance and cost of the devices in which they are incorporated, and therefore could be a key feature to optimize in order to turn a prototype into a commercially viable product. For electrolysis of water, proton exchange membrane electrolyzer cells (PEMECs) have several advantages compared to other electrolysis processes, including greater energy efficiency, higher product purity, and a more compact design. In addition, the integration of renewable energy sources with water electrolysis is very attractive because it can be accomplished with high efficiency, flexibility, and sustainability. However, there is a lack in fundamental understanding of rapid and microscale electrochemical reactions and microfluidics in PEMECs. This research investigates the multiscale behaviors of electrochemical reactions and microfluidics in a PEMEC by coupling an innovative design of the PEMEC with a high-speed and microscale visualization system (HMVS). The results of the investigation are used to aid in revealing the electrochemical reaction mechanisms and the microfluidics behavior including bubble generation, growth and detachment, which all together play a very important role in the optimization of the design of PEMECs. The effects of operating parameters such as current density, temperature and pressure on the electrochemical reactions and the microfluidics are determined and analyzed by mathematical models of PEMECs, which also match the experimental results. Improved understanding of the electrochemical reactions and microfluidics in PEMECs can not only help to optimize their designs, but can also help advance many other applications in energy, environment and defense research fields