Feasibility study of using microfluidic platforms for visualizing bubble flows in electrolyzer gas diffusion layers

397

347

Although polymer electrolyte water electrolyzers (PEWEs) have been used in small-scale (kW to tens of kW range) applications for several decades, PEWE technology for hydrogen production in energy applications (power-to-gas, power-to-fuel, etc.) requires significant improvements in the technology to address the challenges associated with cost, performance and durability. Systems with power of hundreds of kW or even MWs, corresponding to hydrogen production rates of around 10 to 20 kg/h, have started to appear in the past 5 years. The thin (∼0.2 mm) polymer electrolyte in the PEWE with low ohmic resistance, compared to the alkaline cell with liquid electrolyte, allows operation at high current densities of 1-3 A/cm 2 and high differential pressure. This article, after an introductory overview of the operating principles of PEWE and state-of-the-art, discusses the state of understanding of key phenomena determining and limiting performance, durability, and commercial readiness, identifies important 'gaps' in understanding and essential development needs to bring PEWE science & engineering forward to prosper in the energy market as one of its future backbone technologies. For this to be successful, science, engineering, and process development as well as business and market development need to go hand in hand. In 2015, the global primary energy consumption was 153 PWh, 1 corresponding to an average rate of energy conversion of 17 TW. About 30% (∼6 TW) of this is used for electricity generation, which yields around 2.8 TW of electrical power (24 PWh per year). Around two thirds of the electricity is generated from fossil fuels, hydropower contributes 16%, nuclear power 11%, and other renewables (such as solar and wind) only 6.7%.1 Solar (photovoltaics) and wind power have a combined installed capacity of about 660 GW (in 2015).2 Electricity supply based on a significant share of these "new renewables" is associated with large discrepancies between supply and demand, owing to the intermittent nature of these primary energy sources. Hence, solutions for the grid-scale storage of electricity need to be developed and implemented. The electrochemical splitting of water (electrolysis) is a clean and efficient process offering interesting prospects to store large amounts of excess electricity in form of chemical energy ('power-to-gas' concept). 3,4 The produced hydrogen and oxygen can be used to regenerate electricity in periods of low production and high demand or serve as clean transportation fuel for fuel cell electric vehicles. Therefore, water electrolysis is a key technology in future sustainable energy scenarios, since hydrogen as an energy 'vector', i.e., as a universal energy carrier, could promote the decarbonization of the energy economy, or even become its backbone in the context of a 'hydrogen economy'.5 Moreover, the produced hydrogen can be used to methanate CO 2 from suitable sources, such as biogas plants, to produce synthetic natural gas (SNG), which can be injected and stored in the natural gas network. 6...

Section: Transport Processes (2-phase)mentioning

confidence: 99%

Critical Review—Identifying Critical Gaps for Polymer Electrolyte Water Electrolysis Development

Babic

Suermann

Büchi

et al. 2017

397

347

“…7 This two-phase flow was visualized by visible light, neutron and X-ray imaging with focus of the PTL/flowfield interface and the flow regime in the channels. [20][21][22][23] Based on operando in-plane neutron radiography data, Seweryn et al 24 reported a gas/water ratio in the PTL independent of current density, verified up to 2.5 A · cm −2 . However, in none of these studies flow properties were correlated with electrochemical performance.…”

Section: F974mentioning

confidence: 99%

Influence of Operating Conditions and Material Properties on the Mass Transport Losses of Polymer Electrolyte Water Electrolysis

Suermann

Takanohashi

Lamibrac

et al. 2017

110

Three different porous transport layer (PTL) structures, based on titanium sintered powders, were characterized using X-ray tomographic microscopy to determine key geometric properties such as porosity, pore and particle size distributions as well as effective transport properties. The mass transport through the PTL contributes to the voltage losses in the polymer electrolyte water electrolysis cell. Therefore, influence of the PTL structure on the mass transport overpotential is investigated as function of current densities (≤ 4 A · cm −2 ), operating pressures (1-100 bar) and temperatures (40-60 • C), respectively. A decrease of transport losses was observed with increasing pressure and temperature for all investigated PTLs. At around 100 bar balanced pressure, the transport losses for all PTLs converge to about 40 mV per applied A · cm −2 , suggesting that other parts of the cell such as the catalyst layer or their interface contribute to these remaining losses. The performance loss, induced by the different PTL structures, shows a stronger correlation with geometric parameters such as pore and particle size distributions than transport properties like effective diffusivity and permeability. The finest materials with d 50 pore and particle diameters of 40-48 and 68 μm, respectively, are performing better than the coarsest material with diameters roughly twice the sizes. Polymer electrolyte water electrolysis (PEWE) is a promising technology to convert (surplus) electric energy into chemical energy in form of hydrogen to avoid curtailment of the fluctuating renewable electricity sources 1 and thus reducing the need of fossil fuels. 2 Depending on the use of hydrogen, the gas has to be provided at pressure levels up to 1000 bar, i.e. for the use in mobility applications.3 Commercial electrolyzers are operated at pressures in the order of 30 bar (hydrogen and optionally oxygen) 4 to reduce downstream compression and drying efforts. 5For energy applications, high efficiency of the electrolysis conversion process is a key property. As in any electrochemical cell, also with PEWE, losses/overpotentials incur when a current is passed. Ohmic and kinetic overpotentials, related to the movement of protons in the polymer electrolyte and the finite rates of the electrochemical reactions are reasonably well understood for PEWE. [6][7][8] In most cases, at least at current densities above about 1 A · cm −2 , losses in addition to ohmic and kinetic sources are observed and related to mass transport resistances in the porous structures of the catalyst and porous transport layers (PTLs), though their nature and origin in PEWE are not well understood.6 Nevertheless, as long as a sufficient water supply is guaranteed, no transport limitations occur in the form of a turning point in the current/voltage characteristics (i/E-curves), as demonstrated by Lewinski et al. 9 up to 19 A · cm −2 . In polymer electrolyte fuel cells (PEFC), a technology similar to PEWE, the effect of mass transport in the micro-porous structures of the el...

“…Comparisons of the numerical modelling predictions and previous lab-on-chip work 14,15 was also presented. To demonstrate the usefulness of this model, pressure variations during bubble during propagation were calculated.…”

mentioning

confidence: 99%

Three-Dimensional Computational Fluid Dynamics Modelling of Oxygen Bubble Transport in Polymer Electrolyte Membrane Electrolyzer Porous Transport Layers

Arbabi

Montazeri

Abouatallah

et al. 2016

Self Cite

A three-dimensional (3D), two-phase numerical model was developed and presented as a useful tool for investigating oxygen bubble propagation in porous transport layers (PTLs) (otherwise known as gas diffusion layers (GDLs)) of polymer electrolyte membrane (PEM) electrolyzers. The volume-of-fluid (VoF) technique was employed to simulate the liquid-gas interface movement through liquid-saturated porous media designed to be representative of PEM electrolyzer PTLs. The circulation of the liquid within the channel and the porous domain was included in the model. Bubble propagation patterns and bulk saturations for porous material representations of commonly used PTLs were determined as a function of time leading up to the moment of breakthrough. Previously conducted experimental microfluidic investigations were used for model validation, and it was found that the numerical results were in good agreement with the numerical predictions. The validated model was used to calculate pressure variations in bubbles during propagation, and the highest threshold capillary pressure corresponding to a critical throat was introduced as a means to measure the efficacy of oxygen bubble removal. The information obtained from the developed numerical tool can be used for designing and evaluating PTL microstructures for next generation electrolyzer materials.