Performance Of Electrolysis Cells Research Articles

Electrolysis Cell Performance and Durability - a Contradiction in Terms?

In the current stage the focus of PEM water electrolysis cells development is on improving durability and reliability. Over the next decade growing attention has to be paid to reduce capital and operating cost as well as to increase flexible operation in order to fulfil the needs of the costumers, e.g. reducing the systems total cost of ownership (TCO). In most cases, however, achieving the desired cost reduction and higher flexibility contradicts durability and reliability expectations as well as performance requirements. TCO reductions can be achieved by changing the operating conditions, e.g. increasing temperature, current density, operating pressure or load flexibility, but also by e.g. reducing the catalyst loading or membrane thickness. With respect to the operating conditions, it seems problematic as the different parameters also act as stressors for certain degradation phenomena in the system. On the other hand, reducing the so far oversized materials used, increases the vulnerability of the components with regard to degradation and thus limits the durability and reliability of the system. This contribution discusses this described interplay between performance and durability in more detail. First, stressors (e.g. high compartment pressures) are attributed to certain degradation effects (e.g. voltage increase) appearing in the cell. Concerning such a general analysis, there is much information available about the response of the electrochemical cell to certain stressors and the relations are well known. However, only less can be said about the origin of this measureable effects. But to achieve the required performance while maintaining or even increasing the durability, a deeper understanding of the underlying degradation phenomenon (e.g. loss of ionic conductivity) is required. It is shown on the example of the membrane as critical component in electrolysis cells that standard electrochemical characterization methods, if evaluated properly, can already provide important information about the origin of such measured effects. Starting from this, new materials can be developed or adequate mitigation strategies can be implemented to suppress degradation. It is also shown that additionally applied comprehensive modelling of electrolysis cells can support these efforts and can potentially be used for optimized cell design, considering performance and durability requirements.

Modeling of Two-Phase Flow during the Electrolysis of Water in an Electrochemical Reactor in Serpentine Array

Currently the characterization of the reaction environment within any filter-press type reactor through computational modeling is very important in the design to obtain an effective flow dispersion and therefore a uniform current and potential distribution [1]. During two-phase electrolysis such as chlorine production, water electrolysis, alumina reduction, electrocoagulation, and many other electrochemical processes appears gas release on the electrodes, H2 or O2 (bubbles), which imply a quite important electrical properties and electrochemical processes disturbance [2]. The bubbles are motion sources for the flow cells, and then hydrodynamic properties are strongly coupled with species transport and electrical performances, hence, the bubbles presence modifies the electrolysis cell performance and principally the hydrodynamics and current density distribution [3]. Therefore, this works deals with the two-phase modeling (H2O-H2) flow inside of a pre-pilot-scale continuous reactor with a stack of six cells in a serpentine array by solving the Reynolds-averaged Navier-Stokes (RANS) equations with κ-ε turbulence model (Euler-Euler approach). The hydrogen evolution reaction comes from the electrolysis of water. The electrodes were plates of aluminum, typically used in electrocoagulation processes. In order to perform a more complete flow pattern characterization, residence time distribution (RTD) studies were carried out to validate the CFD simulations in biphasic flow. The RTD simulations were calculated solving the averaged diffusion-convection equation. Close agreement between experimental and theoretical RTD simulations were attained. Finally, simulations of distribution of current and potential density distributions inside the pre-pilot-scale continuous reactor were carried out using the Bruggeman relation, observing the effect of bubble production on the distribution of potential along the electrodes.

Assessment of a Solid Oxide Electrolysis Cell Performance Applying a Prediction Tool with Correlation Equations

A performance prediction tool with correlation equations was developed for a performance assessment of a solid oxide electrolysis cell (SOEC). For the purpose, several sets of experiments with various gas content were carried out, showing the SOEC performance. Based on the numerical model developed with correlation equations, reaction factors of hydrogen, steam and oxygen were determined, depending on the resistance of the hydrogen and oxygen electrodes. For determining the reaction factors of an SOEC, those of solid oxide fuel cells were taken into the considerations and also the developed performance prediction tool was applied for the verification. It was found that the suitable reaction factors for the present studied SOEC was 0 for hydrogen, -0.5 for steam of the hydrogen electrode and 0 for oxygen of the oxygen electrode indicating that the partial pressure of the reactant dominates the rate determination rather than that of the reaction product.

Intermediate-temperature electrolysis of energy grass Miscanthus sinensis for sustainable hydrogen production

Biohydrogen produced from the electrolysis of biomass is promising because the onset voltages are less than 1.0 V and comparable to those of water and alcohol-water electrolysis. The present study focuses on Miscanthus sinensis as a model grass because of its abundance and ease of cultivation in Japan. The electrochemical performance and hydrogen formation properties of electrolysis cells using grass as a biohydrogen source were evaluated at intermediate temperature to achieve electrolysis. The components, such as holocellulose, cellulose, lignin, and extractives, were separated from Miscanthus sinensis to understand the reactions of Miscanthus sinensis in the electrolysis cell. The relatively high resistivity and low current-voltage performance of an electrolysis cell using lignin were responsible for degradation of the electrolysis properties compared to those with pure cellulose or holocellulose as biohydrogen resources. Biohydrogen was formed according to Faraday’s law and evolved continuously at 0.1 A cm−2 for 3,000 seconds.

Two Phase Flow Modeling and Characterization of Oxygen Bubbles in PEM Water Electrolysis Cells

The formation, growth, and stability of oxygen bubbles inside the porous transport layer (PTL) pores of a polymer electrolyte membrane water electrolysis (PEMWE) cell have been modeled. In a PEMWE cell, O2 bubbles form and grow inside the anode catalyst layer (ACL) and PTL pores. The most critical impact of the O2 bubbles on the overall performance is the screening of the ACL surface. Semi-empirical relations for the current density-surface coverage for gas evolving electrodes have previously been developed [1] and were used for our PEMWE modeling [2]. This screening effect has been evaluated as an overpotential term, and the results were used to predict the effects of the catalyst and PTL properties on the electrolysis cell performance. The fluctuations in the cell current density were used to identify different types of bubbles and their detachment frequency from the anode electrode. In this work, the anode catalyst surface has been described as a smooth, planar and horizontal plane. The PTL pores are described as identical straight hydrophilic cylinders with 1-mm height. Such a structure resembles tunable PTL structures such as those synthesized by Kang et al. [3]. Stable O2 bubble nuclei form mainly at the intersection of the anode catalyst layer and the PTL pore wall [3,4]. Our modeling results show the effects of three different types of bubbles in a PEMWE cell: i) nucleation-driven, ii) drag-driven, and iii) buoyancy-driven bubbles. Figure 1 shows the relative lifetime and overpotential effects of the O2 bubbles in a PEMWE with a PTL consisting of 11 μm dia. pores operated at a fixed temperature of 80°C and a balanced pressure of 1 bar [5]. Comparing the current density fluctuations in chronoamperometry experiments with the bubble detachment frequencies obtained through modeling provides a distribution of different types of bubbles in the electrolysis cell. These frequencies vary according to the PTL structure and the PEMWE cell operating conditions. Understanding of bubble growth and detachment mechanisms are of significant importance for reduction of mass transport-related losses and enhancement of PEMWE performance.

Two Phase Flow Modeling and Characterization of Oxygen Bubbles in PEM Water Electrolysis Cells

The formation, growth, and stability of oxygen bubbles inside the porous transport layer (PTL) pores of a polymer electrolyte membrane water electrolysis (PEMWE) cell have been modeled. In a PEMWE cell, O2 bubbles form and grow inside the anode catalyst layer (ACL) and PTL pores. The most critical impact of the O2 bubbles on the overall performance is the screening of the ACL surface. Semi-empirical relations for the current density-surface coverage for gas evolving electrodes have previously been developed [1] and were used for our PEMWE modeling [2]. This screening effect has been evaluated as an overpotential term, and the results were used to predict the effects of the catalyst and PTL properties on the electrolysis cell performance. The fluctuations in the cell current density were used to identify different types of bubbles and their detachment frequency from the anode electrode. In this work, the anode catalyst surface has been described as a smooth, planar and horizontal plane. The PTL pores are described as identical straight hydrophilic cylinders with 1-mm height. Such a structure resembles tunable PTL structures such as those synthesized by Kang et al. [3]. Stable O2 bubble nuclei form mainly at the intersection of the anode catalyst layer and the PTL pore wall [3,4]. Our modeling results show the effects of three different types of bubbles in a PEMWE cell: i) nucleation-driven, ii) drag-driven, and iii) buoyancy-driven bubbles. Figure 1 shows the relative lifetime and overpotential effects of the O2 bubbles in a PEMWE with a PTL consisting of 11 μm dia. pores operated at a fixed temperature of 80°C and a balanced pressure of 1 bar [5]. Comparing the current density fluctuations in chronoamperometry experiments with the bubble detachment frequencies obtained through modeling provides a distribution of different types of bubbles in the electrolysis cell. These frequencies vary according to the PTL structure and the PEMWE cell operating conditions. Understanding of bubble growth and detachment mechanisms are of significant importance for reduction of mass transport-related losses and enhancement of PEMWE performance.

Model of oxygen bubbles and performance impact in the porous transport layer of PEM water electrolysis cells

The formation, growth, and stability of oxygen bubbles inside the porous transport layer (PTL) pores of the anode of a polymer electrolyte membrane water electrolysis (PEMWE) cell was investigated by 1-dimensional mathematical modeling inside the PTL and into the flow channel. The effect of oxygen bubbles on the electrolysis overpotential has been evaluated, and the results were used to predict the effects of catalyst and PTL structures on the electrolysis cell performance. The results show how surface oxygen bubbles block active catalyst sites and decrease the electrochemically active surface area (ECSA). The effects of catalyst layer wettability, PTL wettability and pore size, and the operating conditions on the growth and detachment of bubbles from the catalyst surface were modeled. The model accounts for the growth and stability of three types of bubbles in a PEMWE cell: nucleation-driven, drag-driven, and buoyancy-driven bubbles. Nucleation-driven bubbles show the smallest overpotential: 28 mV at the reference operating conditions and PEMWE properties, compared to 35 mV for the drag-driven bubbles and 43 mV for the buoyancy-driven bubbles. Catalyst wettability is found to be the most influential material property concerning bubble overpotential for all three types of bubbles. PTL wettability and the cell pressure are the next most important factors in this regard. The modeling results can be used for improved PTL, catalyst layer, and flow field design for PEMWE cells.

Microbial electrolysis cell performance using non-buffered and low conductivity wastewaters

Microbial electrolysis cells (MEC) are a novel technology aiming at producing hydrogen from wastewater. MEC performance gives successful results in lab-scale experiments with well buffered media and synthetically-increased conductivity, thus preventing operational problems and reducing the internal resistance of the cell. This is especially important in two-chamber configuration where membranes cause potential losses associated to the pH gradients across them. However, domestic and many industrial wastewaters have a limited buffer capacity and low conductivity. In this study, the performance of an MEC with a culture medium more like a real wastewater, in terms of buffer capacity and conductivity, was assessed in both single-chamber and two-chamber configurations and compared to that of a well-buffered cell. Single-chamber MEC tests demonstrated that the lack of buffer affected both the overpotentials of the anode and the cathode, although the overpotential of the latter was significantly higher. In two-chamber configuration, the non-buffered cell failed as a result of a high pH drop in the anode, which harmed the anodic biofilm. The conductivity increase from low (4mS/cm) to high values (13mS/cm) did not improve significantly any configuration tested.

Neutron Radiography of PEM Water Electrolysis Cells

In the past neutron radiography has been successfully applied to investigate the water transport behavior in polymer electrolyte fuel cells and its influence on overall cell performance [1-2]. The same technique can be also used to visualize the gas and water transport inside working polymer electrolyte membrane (PEM) electrolyzers. While it is obvious that under operating conditions, a two phase flow of water and its gaseous splitting products occurs inside the porous layers and the flow channels, it is not clear to what extent this can influence overall electrolysis cell performance. In theory gas bubble formation can block the catalyst surface on the anode, decreasing the overall efficiency In addition, the resulting inhomogeneity of the current density over the active area can lead to a local overheating and membrane damage. Strong attenuation of the neutron beam inside liquid water, as compared to typical electrolyzer materials like steel and titanium, makes neutron radiography a perfect technique to investigate these phenomena. Neutron radiograms of PEM electrolysis cells (1 cm2 active area) were obtained at varying working parameters. Our experimental setup is designed to provide maximal imaging capabilities at simulated local conditions which can occur in real scale devices. Special attention will be put to the water – gas transport perpendicular to the porous layer. Figure 1 shows the typical patterns with gradients of water thickness being little dependent on the operating conditions. This presentation is intended to provide the fundamentals about both the experimental possibilities as well as the water/gas transport behavior during PEM electrolysis cell operation, stimulating the development of numerical models describing this complex system.

Steam Electrolysis by Proton-Conducting Solid Oxide Electrolysis Cells (SOECs) with Chemically Stable BaZrO3-Based Electrolytes

Solid oxide electrolysis cells (SOECs) are expedient to solve the site-specific and intermittent problems for current renewable energies, such as solar, wind and tidal power, as SOECs can convert these renewable energies into chemical energy in the form of H2 that is clean and easily to be transported. Most of the studies on SOECs are focused on oxygen-ion conducting SOECs, using mainly yttria-stabilized zirconia (YSZ) as electrolyte. However, problems such as the dilution of produced H2, high temperature operation, and oxidation of Ni-based fuel electrode occur, which the use of proton-conducting SOECs can circumvent. In addition, the high conductivity of proton-conducting oxides with respect to YSZ enables proton-conducting SOECs operating at intermediate temperatures [1]. The few works reporting on proton-conducting SOECs show the use of BaCeO3-based electrolytes, which has been demonstrated to be chemically unstable in H2O. Although proton-conducting BaZrO3-based materials are the most promising electrolyte candidates for proton-conducting SOECs due to the excellent chemical stability and high bulk conductivity [2], their processing difficulties due to poor sinterability and high-resistive grain boundaries prevented the deployment in SOEC devices. In this report, we pioneeringly used BaZrO3-based electrolytes for SOECs, demonstrating that tailoring the electrolyte and electrode materials results in further improving the electrolysis cell performance. Anode supported BaZr0.9Y0.1O3-δ (BZY) electrolyte films were fabricated using an ionic diffusion method [3]. Both thermodynamic calculation and experimental results suggest that BZY has an excellent chemical stability in H2O, which is critical for practical applications. A current density of 119 mA cm-2 was obtained at 600°C, with an applied voltage of 1.65 V for the electrolysis cell, which is comparable or even higher than that for proton-conducting SOECs with BaCeO3-based electrolyte at similar conditions, while BZY electrolyte offers much better chemical stability. This cell also operated at 600 oC for more than 80 h without any obvious degradation, while the BaCeO3-cells are reported to only last tenths of minutes or a few hours [4]. To improve the cell performance, BaZr0. 7Y0. 2Pr0.1O3-δ (BZPY10), which is a more conductive proton-conducting electrolyte material, wass used for the SOEC. With La0.8Sr0.2MnO3-δ (LSM)-BZPY10 composite air electrode, the SOEC with BZPY10 electrolyte reached a current density of 576 mA cm-2 with an applied voltage of 1.6 V at 600 oC. This cell performance is much improved compared with the above discussed BZY cell. Further cell performance improvement was achieved by tailoring the air electrode material with BaZr0. 5Y0. 2Pr0. 3O3-δ (BZPY30), which is a mixed protonic-electronic conductor rather than a pure protonic conductor [5]. By coupling BZPY30 with LSM, the triple phase boundary (TPB) is much improved due to the mixed conducting behavior of BZPY30, which is beneficial to the reaction. As a result, the cell with BZPY30-LSM air electrode produced a current density of 1007 mA cm-2 with an applied voltage of 1.6 V at 600 oC. An obvious cell performance improvement is obtained with the tailored electrode. Reference s L. Bi, S. Boulfrad and E. Traversa, Chem. Soc. Rev., 2014, 43, 8195-8300.D. Pergolesi, E. Fabbri, A. D'Epifanio, E. Di Bartolomeo, A. Tebano, S. Sanna, S. Licoccia, G. Balestrino and E. Traversa, Nature Mater., 2010, 9, 846-852.L. Bi, E. Fabbri, Z. Q. Sun and E. Traversa, Energy Environ. Sci., 2011, 4, 409-412.L. Bi, S.P. Shafi and E. Traversa, J. Mater. Chem. A, 2015, DOI: 10.1039/c4ta07202b.E. Fabbri, L. Bi, D. Pergolesi and E. Traversa, Energy Environ. Sci., 2011, 4, 4984-4993.

Impact of volatile fatty acids on microbial electrolysis cell performance

This study investigated the performance of microbial electrolysis cells (MECs) fed with three common fermentation products: acetate, butyrate, and propionate. Each substrate was fed to the reactor for three consecutive-batch cycles. The results showed high current densities for acetate, but low current densities for butyrate and propionate (maximum values were 6.0±0.28, 2.5±0.06, 1.6±0.14A/m2, respectively). Acetate also showed a higher coulombic efficiency of 87±5.7% compared to 72±2.0 and 51±6.4% for butyrate and propionate, respectively. This paper also revealed that acetate could be easily oxidized by anode respiring bacteria in MEC, while butyrate and propionate could not be oxidized to the same degree. The utilization rate of the substrates in MEC followed the order: acetate>butyrate>propionate. The ratio of suspended biomass to attached biomass was approximately 1:4 for all the three substrates.

Influence of the Anode Gas Flow Rate and Composition on Solid Oxide Electrolysis Cell Performance

Electrochemical Impedance Spectroscopy (EIS) is a powerful tool to get a deeper insight on the performance of high-temperature electrolysis cells. Indeed, when used in particular conditions, this technique allows distinguishing different phenomena contributing to electrochemical reactions and degradation mechanisms. Besides, the two-electrode configuration allows applying EIS to any kind of single cell. In this study, EIS and chronopotentiometry were combined in order to analyze the influence of the anode gas flow rate and composition on the electrochemical performance of a commercial cathode-supported cell, at different temperatures and current densities.

Liquid | liquid biphasic electrochemistry in ultra-turrax dispersed acetonitrile | aqueous electrolyte systems

Unstable acetonitrile | aqueous emulsions generated in situ with ultra-turrax agitation are investigated for applications in dual-phase electrochemistry. Three modes of operation for liquid | liquid aqueous–organic electrochemical processes are demonstrated with no intentionally added electrolyte in the organic phase based on (i) the formation of a water-soluble product in the aqueous phase in the presence of the organic phase, (ii) the formation of a product and ion transfer at the liquid | liquid–electrode triple phase boundary, and (iii) the formation of a water-insoluble product in the aqueous phase which then transfers into the organic phase. A three-electrode electrolysis cell with ultra-turrax agitator is employed and characterised for acetonitrile | aqueous 2 M NaCl two phase electrolyte. Three redox systems are employed in order to quantify the electrolysis cell performance. The one-electron reduction of Ru(NH 3) 6 3+ in the aqueous phase is employed to determine the rate of mass transport towards the electrode surface and the effect of the presence of the acetonitrile phase. The one-electron oxidation of n-butylferrocene in acetonitrile is employed to study triple phase boundary processes. Finally, the one-electron reduction of cobalticenium cations in the aqueous phase is employed to demonstrate the product transfer from the electrode surface into the organic phase. Potential applications in biphasic electrosynthesis are discussed.

Effect of mass and charge transport speed and direction in porous anodes on microbial electrolysis cell performance

The use of porous electrodes like graphite felt as anode material has the potential of achieving high volumetric current densities. High volumetric current densities, however, may also lead to mass transport limitations within these porous materials. Therefore, in this study we investigated the mass and charge transport limitations by increasing the speed of the forced flow and changing the flow direction through the porous anode. Increase of the flow speed led to a decrease in current density when the flow was directed towards the membrane caused by an increase in anode resistance. Current density increased at higher flow speed when the flow was directed away from the membrane. This was caused by a decrease in transport resistance of ions through the membrane which increased the buffering effect of the system. Furthermore, the increase in flow speed led to an increase of the coulombic efficiency by 306%.

Increase of Electrolysis Cell Performance by Addition of PVDF and Graphite Powder on MEA for Regenerative Fuel Cells

For the regenerative fuel cell (RFC), water electrolysis cell performance using membrane electrode assembly (MEA) in polymer electrolyte fuel cell (PEMFC) were investigated. A part of Nafion had been secondary sprayed on the surface of catalytic layer and variation of cell performance was diminished. The conformation of stability, improvement of mechanical and electrical properties was accomplished by addition of PVDF, graphite and RuO2. With the addition of graphite power and RuO2, the voltage was decreased from 3.6V to 2.5V and 2.2V. The improvement of the mechanical properties was obtained by addition of PVDF. The electrolysis cell manufactured with MEA electrode was showed less decomposition voltage of 1.3V than with Nafion electrode at 10A of applied current. The stability of MEA was confirmed from 30 days of cell operation

Testing the silicon carbide side-wall lining for aluminum electrolysis cells

Results of testing the side-wall lining made up from nitride-bonded silicon-carbide plates for an aluminum electrolysis cell equipped with pre-fired anodes are reported. Technical and economic characteristics of the performance of electrolysis cells equipped with the newly-designed lining are superior to those of standard electrolysis cells equipped with the carbon-block refractory lining.

Performance of electrolysis cells with proton and oxide-ion conducting electrolyte for reducing nitrogen oxide

A steam electrolysis cell was constructed with a proton conductor SrZr 0.9Yb 0.1O 3− α as an electrolyte. The steam electrolysis cell efficiently reduced nitrogen oxide (NO) on the cathode, using hydrogen produced by steam electrolysis as a reducing agent at around 460 °C. When a Pt/Ba/Al 2O 3 catalyst was placed on the cathode, NO was reduced even under an O 2-rich atmosphere. An electrolysis cell was also constructed with an oxide-ion conductor 8 Y–ZrO 2 as an electrolyte. The cell could reduce NO on the cathode, not only electrochemically electrolyzing NO directly but also chemically using hydrogen produced by steam electrolysis. However, the cell with the oxide-ion conductor could not reduce NO in an atmosphere containing O 2.

High temperature electrolysis cell performance characterization

Electrochemical cells employing yttria-stabilized zirconia solid electrolyte were evaluated in the electrolysis of steam at 800–1050°C. Performance levels of 1.23 V at 300 mA cm −2 were obtained with current efficiencies approaching 100%. A limiting current was approached when hydrogen content in the steam exceeded approximately 90% (650 mA cm −2; 1000°C). Voltage-current behavior prior to diffusion control folllows a Tafeltype slope of approximately 250 mV per decade. Polarization in this region has a temperature dependence corresponding to approximately d ln 1/d(1/ T) = 30,000/R which permits prediction of cell performance above the 1050°C maximum testing temperature.