Concentration Overpotential Research Articles

Fundamental study of gas species transport in the oxygen electrode of solid oxide fuel and electrolysis cells

A fundamental analysis of multicomponent gas transport models was performed in application to the oxygen electrodes of solid oxide cells. It is common practice to neglect the effect of pressure gradients within oxygen electrodes, even though a net molar flux at the electrolyte surface implies that a pressure gradient must exist. The influence of both Darcy velocity and Knudsen flux are considered in the context of ordinary (Fickian) diffusion, the dusty gas model, and the binary friction model. Comparisons between the models and different sets of assumptions are made via parametric studies on operating load, oxygen partial pressure, microstructural properties, and electrode thickness. Results show that the pressure gradient will have a significant impact on the oxygen concentration distribution and therefore the concentration overpotential. In electrolysis mode, pressure increases up to 1 atm are predicted, indicating that pressure at the electrode/electrolyte interface could contribute to electrode delamination. Additionally, it is found that Darcy's law is insufficient for calculating the pressure distribution without accounting for the flux due to Knudsen diffusion. Additionally, it is found that for the range of properties typical of oxygen electrodes, there is a negligibly small difference between the dusty gas model and binary friction model from a practical standpoint.

Experimental Characterization of a Novel Fluidized‐Bed Zn–Air Fuel Cell

AbstractZn–air fuel cells are promising energy storage devices for renewable energy and power sources, as they are cost‐effective and have high energy density. However, limited charge and discharge cycles and low round‐trip efficiency have long been obstacles to large‐scale market deployment. Herein, a new fluidized‐bed zinc–air fuel cell is designed, constructed, tested, and characterized. The integration of the fluidized bed in the zinc–air fuel cell leads to key advantages such as: erosion of the anode passivation layer, which plays a key role in the rapid voltage decay and keeping the products of reactions away from the proximity of the electrodes, reducing the concentration overpotential, due to the concentration gradient of electrode species in the diffusion boundary layer between the electrode–electrolyte interface and the electrolyte bulk.

An analytical model for parametric study on serpentine and interdigitated flow fields in redox flow batteries

Flow fields can enhance the peak power density of redox flow batteries through distributing the electrolyte and thus reactants uniformly in the porous electrode. However, due to the distinct mechanisms of serpentine and interdigitated flow fields for mass transfer enhancement, the effectiveness of different flow fields is highly dependent on design and operating parameters of the flow cell, such as electrode material, channel's dimension, applied flow rate, and applied current density. The significant computational costs induced by the 3-D or 2-D numerical model on the one hand, and the demand for a deep understanding with respect to the effects of the different flow fields on the flow cell under varying geometric and operating conditions on the other hand, call for an analytical approach. Therefore, the present study establishes an analytical model for quantifying the mass transfer and the electrochemical overpotentials in the flow cell with the serpentine or the interdigitated flow fields. The validity of the analytical model is confirmed by comparisons with experimental and 3-D numerical simulation results under varying electrode materials and channel dimensions. Parametric study results show that the properties changes affect the flow cell implemented with different flow fields paradoxically. First, for the flow cell utilizing electrodes corresponding to a relatively high specific surface area and a low permeability, the interdigitated flow field is more worthy of recommendation, while as the permeability of the electrodes increases, the serpentine flow field becomes more competitive. Second, widening channel's and rib's widths is an effective approach to decrease the concentration overpotential for the interdigitated flow field, while its effects on that with the serpentine flow field is opposite. These results are expected to provide a valuable guidance for the application of interdigitated and serpentine flow fields in flow cells.

A comparative study of electrochemical Brayton cycle and thermally regenerative electrochemical cycle

Both the emerging thermally regenerative electrochemical cycle (TREC) and electrochemical Brayton cycle (EBC) harvest low-grade heat by utilizing the temperature effect of electrode potential. Their key difference lies in the cycle configurations, the Stirling cycle and the Brayton cycle, respectively. Determining which cycle configuration performs better could indicate the direction of thermodynamics for further research. However, this remains an open question to be investigated. To address this, a performance comparison of EBC and TREC using KI/KI3 and K3Fe(CN)6/K4Fe(CN)6 solutions as electrolytes is conducted based on an improved system configuration and a two-dimensional model coupling mass transfer and electrode kinetics. Multi-objective optimization results show that there is no significant difference in performance between EBC and TREC. Under a given temperature difference of 50 °C, the power density and exergy efficiency of optimized EBC are respectively 5.28 W m−2 and 16.6%, compared with 5.93 W m−2 and 15.6% of optimized TREC. The corresponding temperature drops of heat sources are 14.49 °C and 12.26 °C, respectively, indicating that EBC is more suitable for heat sources with large temperature variations. The concentration overpotential is found to be the main contributor to entropy generation in both EBC and TREC besides heat transfer. Their power density and exergy efficiency can be further improved by approximately 50% and 40% by eliminating overpotentials, respectively. This study is expected to provide beneficial insights into the performance optimization and electrolyte improvement of EBC and TREC.

Numerical Analysis of Humidification Condition Effects on Protonic Ceramic Fuel Cell Performance Considering Concentration Overpotential

Protonic ceramic fuel cells (PCFCs) operating at an intermediate temperature range (400–600 °C) show high performance due to high proton conductivity in the electrolyte. Especially, because the water steam is generated at the oxygen side, so there is less dilution effect on fuel. Therefore, one of the main merits of PCFCs is that it can operate with extremely high fuel utilization. Furthermore, a higher fuel humidification rate leads to higher proton conductivity. However, under relatively high steam partial pressure, the concentration overpotential effect on cell performance may become non-negligible. In the present study, the performance of PCFC under several fuel humidification conditions is investigated by considering the influence of concentration overpotential by numerical analysis.

Flattening of Lithium Plating in Carbonate Electrolytes Enabled by All-In-One Separator.

The uncontrollable dendritic growth of metallic lithium during repeated cycling in carbonate electrolytes is a crucial obstacle hindering the practical use of Li-metal batteries (LMBs). Among numerous approaches proposed to mitigate the intrinsic constraints of Li metal, the design of a functional separator is an attractive approach to effectively suppress the growth of Li dendrites because direct contact with both the Li metal surface and the electrolyte is maintained. Here, a newly designed all-in-one separator containing bifunctional CaCO3 nanoparticles (CPP separator) is proposed to achieve the flattening of Li deposits on the Li electrode. Strong interactions between the highly polar CaCO3 nanoparticles and the polar solvent reduces the ionic radius of the Li+ -solvent complex, thus increasing the Li+ transference number and leading to a reduced concentration overpotential in the electrolyte-filled separator. Furthermore, the integration of CaCO3 nanoparticles into the separator induces the spontaneous formation of mechanically-strong and lithiophilic CaLi2 at the Li/separator interface, which effectively decreases the nucleation overpotential toward Li plating. As a result, the Li deposits exhibit dendrite-free planar morphologies, thus enabling excellent cycling performance in LMBs configured with a high-Ni cathode in a carbonate electrolyte under practical operating conditions.

Superaerophilic/superaerophobic cooperative electrode for efficient hydrogen evolution reaction via enhanced mass transfer.

Hydrogen evolution reaction (HER), as an effective method to produce green hydrogen, is greatly impeded by inefficient mass transfer, i.e., bubble adhesion on electrode, bubble dispersion in the vicinity of electrode, and poor dissolved H2 diffusion, which results in blocked electrocatalytic area and large H2 concentration overpotential. Here, we report a superaerophilic/superaerophobic (SAL/SAB) cooperative electrode to efficiently promote bubble transfer by asymmetric Laplace pressure and accelerate dissolved H2 diffusion through reducing diffusion distance. Benefiting from the enhanced mass transfer, the overpotential for the SAL/SAB cooperative electrode at -10 mA cm-2 is only -19 mV, compared to -61 mV on the flat Pt electrode. By optimizing H2SO4 concentration, the SAL/SAB cooperative electrode can achieve ultrahigh current density (-1867 mA cm-2) at an overpotential of -500 mV. We can envision that the SAL/SAB cooperative strategy is an effective method to improve HER efficiency and stimulate the understanding of various gas-involved processes.

Simulation of a thermo-electrochemical cell with graphite rod electrodes.

The rapid development of human society has resulted in the extensive release of waste heat. The thermo-electrochemical cell (TEC), a cutting-edge technology that converts low-grade waste heat into electricity, has garnered increasing attention. However, the complex interactions among various processes, such as fluid flow, electrochemical reactions and heat transfer, make it challenging to evaluate their effect on the overall performance of the TEC. Understanding the synergistic mechanisms and coupling effects of these processes is crucial for optimizing and implementing TECs in practical applications. In this paper, a mathematical model is developed by coupling electrochemical reactions and heat/mass transfer. The distributions of ion concentration, electrolyte velocity and temperature are analyzed under varying temperature differences and electrode distances. The results demonstrate a significant interaction between heat transfer and electrolyte flow. Higher temperatures not only improve the open circuit voltage, but also promote ion transport convection and hence enhance the current density. In addition, a higher concentration of ions or smaller electrode spacing exhibits an apparently improved performance of the TEC, due to the facilitated ion transport and reduced concentration overpotential. Notably, electrode spacing has a negligible effect on the maximum power density of the TEC under a constant heat flux, but it does enhance the current density due to the combined effect of heat and ion transfer. Overall, the proposed mathematical model provides deeper insight into the physical-chemical processes involved in TECs and offers valuable guidance for TEC design and practical applications.

Sulfur-deficient MoS2-carbon hollow nanospheres for synergistic trapping and electrocatalytic conversion of polysulfides.

Lithium-sulfur battery is one of the most promising candidates for next-generation energy storage systems, but the serious shuttle effect and sluggish reaction kinetics of polysulfides impair its practical applications. Herein, sulfur-deficient MoS2/carbon hollow nanospheres (MoS2-CHNs) are firstly synthesized by a NaCl-template pyrolysis and employed as sulfur host for lithium-sulfur batteries. TEM analysis reveals that carbon hollow nanospheres existing in the composite are the backbones that help to immobilize the ultrathin MoS2 nanosheets and ensure their large specific surface area. The MoS2 nanosheets consist of 5-10 layers of MoS2 with rich sulfur vacancies. The first principle calculation demonstrates that sulfur vacancy led to an intensively enhanced binding energy (-4.70eV) towards Li2S6 compared to the pristine MoS2 (-1.57eV). It suppressed the shuttle effect efficiently. The catalytic experiments reveal that MoS2-CHNs have a superior ability for the nucleation of Li2S and bidirectional electrocatalytic capability for the conversion of polysulfide. The large storage space inside MoS2-CHNs can work as a reservoir for intermediate polysulfides to substantially reduce the concentration overpotential. Due to this advantageous structural design of MoS2-CHNs electrode, its reversible capacity remains 1139 mAh g-1 after 100 cycles at 0.2C, and 600 mAh g-1 after 500 cycles at 5C with a sulfur loading of 5mgcm-2. Even though the sulfur loading increases to 10mgcm-2, the Li-S battery delivers a stable capacity of 978 mAh g-1 after 50 cycles at 0.2C. So the MoS2-CHNs demonstrate a promising application for high-energy Li-S batteries.

Design and Utilization of a Direct Methanol Fuel Cell.

This study introduces a step-by-step, summarized overview of direct methanol fuel cell (DMFC) fundamentals, thermodynamic-electrochemical principles, and system evaluation factors. In addition, a parametric investigation of a JENNY 600S DMFC is conducted to simulate cell performance behavior under varying operating conditions. The system is mathematically modeled and solved in MATLAB and accounts for multi-irreversibilities such as the activation and ohmic and concentration overpotentials. The performance of the modeled system was validated against theoretical and experimental results from the literature. The results indicated that increasing the fuel cell's operating temperature yields enhanced output cell voltages due to enhanced methanol oxidation reactions. Nevertheless, the maximum efficiency limits of the fuel cell tend to decrease with an increase in temperature. In addition, the model has also depicted that enhanced output cell voltages are associated with increased oxygen consumption, resulting in the lower exit flowrates of the reactants.

Towards Efficient Thermal Management within Intercalation Batteries through Electrolyte Convection

Ubiquitous in consumer electronics and emergent in transportation and stationary applications, lithium-ion batteries (LIB) are the state-of-the-art energy storage technology due to their energy density, roundtrip efficiency, and cycle life.1,2 While the past decade has seen a steady decline in battery price and concomitant increase in energy density due to a combination of materials development, manufacturing advances, and market scale,3,4 current LIBs are still unable to meet the often incongruous power and energy requirements of newer applications (e.g., fast charging of energy-dense batteries).5,6 In addition, these more extreme operating environments challenge battery longevity and safety, necessitating responsive balance-of-plant systems which include thermal management systems that control cell temperatures using heat transfer media. At elevated temperatures, accelerated solid-electrolyte interphase growth and component decomposition may lead to capacity/power fade,7,8 and, in the worst cases, thermal runaway and hazardous releases. Within the battery cell, temperature gradients lead to non-uniform electrode reaction distribution, and subsequently reduced cell performance and cycle life.8 Thus, typical LIB operating temperature ranges are constrained between 20 ℃ and 40 ℃, with minimal temperature differences across the cell. Most current thermal management systems rely on heat exchange through the surface or tab of the cell with a cooling media (air, liquid, phase-change materials, etc.).9,10 While generally sufficient under many of today’s applications (low C-rates), this approach can be challenged by newer applications, such as those that require high power input/output (EV fast charging, electric aviation), or need large battery formats (stationary storage systems).In this presentation, we will describe a novel concept of thermal management through forced convection of the electrolyte through the porous electrodes and separator. By leveraging battery simulation and dimensional analysis, we demonstrate that: (1) electrolyte convection provides efficient heat removal capability by carrying the generated heat out of the cell through the flowing medium; (2) the elimination of electrolyte concentration gradient by flow, and the resulting smaller ohmic resistance, concentration and activation overpotentials, help prevent cell temperature rise through reduced heat generation rate. Compared to current thermal management systems, this approach offers several important potential advantages, including (1) reduced internal temperature gradient, (2) rapid response time to temperature regulation, (3) simplifications to manufacturing, and ultimately, and (4) reduced system costs and improved battery safety.

Performance analysis of a metal-supported intermediate-temperature solid oxide electrolysis cell

Hydrogen as an energy carrier is critical for building a zero-carbon emission society. Solid oxide electrolysis cell (SOEC) is a feasible technology for hydrogen production with a high efficiency. Currently, the durability of SOEC systems still needs to be improved and technical issues need to be overcome. Reducing the working temperature is helpful for the lifetime. A good cell design to avoid delamination is also very important. In this study, the performance of a metal-supported intermediate-temperature SOEC is estimated using gadolinium doped ceria Gd0.1Ce0.9O2-δ (GDC) as the main electrolyte. First, a mathematical model is setup for the metal-supported SOEC. The effects of the porosity and tortuosity of the electrodes are analyzed. Subsequently, the influences of the working temperature, pressure, and steam concentration are estimated. Finally, the partial oxygen pressure inside the multi-layer electrolyte is determined and the risk of delamination is discussed. The results indicate that increasing the operation temperature can decrease the activation, concentration, and ohmic overpotentials simultaneously while increasing the pressure also can enhance the performance. Compared with the conventional design of Ceres Power, the new design using 10Sc1CeSZ as the barrier layer can increase the partial oxygen pressure of the GDC layer close to the cathode such that decomposition of GDC is avoided. Meanwhile, the partial oxygen pressure inside the multi-layer electrolyte close to the anode declines and the risk of delamination is reduced. Hence, the new design of the SOEC is beneficial for the durability of metal-supported SOEC.

Multiphase, multidimensional modeling of proton exchange membrane water electrolyzer

The water in a proton exchange membrane water electrolyzer (PEMWE) used for hydrogen production undergoes a multiphase flow, which makes it difficult to analyze the mass transfer phenomenon and predict the performance of the PEMWE. Therefore, the multiphase flow in the PEMWE requires a comprehensive study. This work theoretically investigates multiphase mass transfer along with the electrochemical reaction in a PEMWE using a three-dimensional computational modeling approach. First, the model was validated based on an in-house experiment, and a good agreement was achieved. Subsequently, the spatial distribution of species and the electrochemical process were investigated, and the liquid water saturation level was monitored. It was observed that the high current density results in low liquid saturation in the cathode. Oxygen is accumulated under the land owing to its longer diffusion path. Higher temperatures increase and decrease liquid saturation in the cathode and anode, respectively, because they weaken the capillary transport of water inside the porous layer. The concentration overpotential is significantly lower than the activation overpotential. Furthermore, it was found that the high porosity and low contact angle of the porous material aid the capillary transport of water inside the diffusion layer.

Understanding Bubble-Induced Overpotential Losses in Multiphase Flow Electrochemical Reactors

Green hydrogen production via water electrolysis can play a significant role in decarbonizing energy and multiple industrial processes. In this electrolysis process, water molecules are oxidized to produce oxygen in the anode, while protons are reduced to hydrogen in the cathode. Both of these electrochemical products are gaseous species that lead to bubble nucleation at the surface of electrodes. This bubble evolution phenomena results in substantial energy losses due to the blockage of ion conduction pathways, reduction of the available electrocatalytic area, and disruption of concentration gradients at the electrode–electrolyte interface. In this study, we implement a microfluidic water electrolyzer to elucidate the impacts of electrochemical reaction conditions and convective flows on bubble-induced overpotential losses. We show that high Reynolds (Re) number flows (i.e., Re > 20) mitigate the formation of large bubbles, resulting in minimal bubble-induced overpotential losses. For flows with smaller Re, periodic evolution of large bubbles leads to overpotential fluctuations on the order of ∼100 mV. Furthermore, to understand the impact of bubbles on concentration overpotentials, we use fluorescence microscopy and pH sensitive dyes to capture the spatiotemporal dynamics of pH gradients and correlate the strength and shape of these gradients to the applied potential and convective forces. We find that the presence of large bubbles at low Re can result in more severe concentration gradients that are affected by the hydrodynamic flows around the bubbles. The findings presented in this work provide insights into the effects of convective flows in mitigating bubble-induced overpotential losses.

Identification of Overpotentials in Vanadium Redox Flow Battery with Reference Electrodes and Determination of Apparent Electrochemical Rate Constants

Vanadium redox flow battery (VRFB) is an amateur technology that passed pilot stages in fulfilling the stationary applications at the energy storage market with a high share of renewable power generation. The commercial applications of VRFBs require the optimization of their internal losses and developing the optimal control strategies for in-grid operation. This calls for a deep understanding of the internal VRFB processes, which greatly benefit from both the mathematical modeling and experimental testing for the model validation and refinement.However, the available experimental data for VRFBs is usually limited by open-circuit voltage (OCV) and potential (OCP), charge/discharge voltage profiles at constant load current, full cell polarization curves at the fixed state of charge (SOC), and internal stack ohmic resistances. This also complicates the decoupling and accessing the voltage loss sources, with the prevailing ones usually classified as ohmic, concentration, and activation overpotentials. As such, Aaron et al. (1) showed that decoupling of overpotentials at positive and negative half-cell requires the half-cell potentials measurements with reference electrodes (RE) in addition to full cell data. Different configurations of RE incorporation (2–4) were suggested with a limited agreement with full cell experimental data. The alternative approach with extrapolation of parameters presented in the literature is also challenging due to their strong dependence on an experimental protocol (e.g., the electrode material and its pre-treatment procedure, a membrane, and a setup geometry). As an illustration, the comprehensive review Roznyatsovskaya et al. (5) highlights the four orders of magnitude difference in reported surface area-normalized rate constants. Therefore, the development of a self-consistent experimental setup is crucial for understanding and decoupling internal losses for flow batteries. Combined with VRFB modeling, this may extend accurate predictions of its operation at extreme SOCs and load currents, where voltage losses hamper the efficiency of the battery with the dominating ones usually attributed to concentration (mass-transport in solutions) and activation (sluggish kinetics of reactions on electrodes) overpotentials.Herein, we developed the experimental setup adopting a 5cm2 single flow cell equipped with reference electrodes at positive and negative sides. The experiments were performed in symmetric and full cell designs. The polarization curves were measured at different SOCs [10 – 90 %], electrolyte flow rates [5-30 ml min-1], and load currents. The additional effort was made to compare the results of the experiments in symmetric and full cell designs and extract the apparent kinetic rate constants for positive and negative half-cells. The latter was performed within the 0D modeling approach, which is based on general mass and charge conservation principles. The key feature of the model is the approach for the determination of the integral mass-transfer coefficient on the electrode-electrolyte interface that allows describing non-linear polarization behaviour at high current densities. The effective thermodynamically consistent single-step reaction mechanisms were considered at both half cells, thus decreasing the number of free kinetic parameters. Overall, the proposed approach demonstrated the growing impact of the membrane losses at high current densities. The extracted apparent rate constants under conditions of a real flow battery provide important insights for developing reliable and efficient VRFBs applicable for energy storage systems.1 D. Aaron, Z. Tang, A. B. Papandrew and T. A. Zawodzinski, J. Appl. Electrochem., 2011, 41, 1175–1182.2 H. Lim, J. S. Yi and D. Lee, J. Power Sources, 2019, 422, 65–72.3 S. Ressel, A. Laube, S. Fischer, A. Chica, T. Flower and T. Struckmann, J. Power Sources, 2017, 355, 199–205.4 M. Cecchetti, A. Casalegno and M. Zago, J. Power Sources, 2018, 400, 218–224.5 N. Roznyatovskaya, J. Noack, K. Pinkwart and J. Tübke, Curr. Opin. Electrochem., 2020, 19, 42–48.

Modeling Potential and Species Distribution in a Li||Bi Liquid Metal Battery Using the Finite Volume Method

To match the growing demand for grid scale energy storage in order to balance out renewable energies, the liquid metal battery (LMB) technology is highly promising. Key elements of these batteries are the liquid electrodes, which require LMBs operating at elevated temperatures. Hereby, the electrolyte can be liquid or solid state. Based on density differences, the battery is self-assembling since the components self-segregate when the system is heated. Additionally, many of the possible materials are earth-abundant and not expensive. This results in a very cost effective technology. Moreover, the molten electrodes feature superior kinetics and transport properties as well as high voltage efficiencies at high current densities. Otherwise, the liquidity has the disadvantage that high operating temperatures are necessary. Further, there is a risk of corrosion, self-discharge and short-circuiting. The latter makes the battery not applicable for portable applications [Kim, H. et al. Chem. Rev. 113 (3) (2013)]. Among all LMBs, the Li||Bi cell has been investigated to a large extend and therefore, most material properties are readily available. This is the reason why the present study focuses on this material pairing. A schematic of this cell during discharge is shown in figure 1a. Here, the anode metal Lithium is oxidized and the ion crosses the electrolyte layer. Then it alloys with the molten Bismuth. At charge, this process is reversed. A picture of a real cell can be found in figure 1b. Nevertheless, there are other very promising cell chemistries such as NaZn batteries [Xu, J. et al. J. Power Sources 332 (2016)].For theoretical investigations and numerical simulations, electromagnetic fields, electrochemistry and fluid dynamics need to be considered. Coupling of the corresponding fundamental equations and the different regions in the battery is challenging. Further, there are multiple fluid flow phenomena – like surface instabilities, Tayler instability, electro-vortex flow, thermal convection,solutal convection or Marangoni convection [Kelley, D. H. and Weier, T. Appl. Mech. Rev. 70 (2) (2018)] – that might occur when the battery is operated. To determine the operating voltage of a cell, overpotentials need to be investigated. Generally, there are three types of overpotentials: the ohmic loss, the mass transfer and the charge transfer overpotential. However, the latter can be neglected [Newhouse, J. M. Ph.D. thesis, MIT (2014)]. Determining the ohmic overpotential is quite straightforward using Ohm’s law, while the concentration overpotential requires the calculation of the species distribution in the electrolyte and the positive electrode.The present study will focus on the electrochemical behavior of an all liquid Li||Bi battery and neglects fluid flow. Hereby, the whole cell is modeled using the parent-child-mesh technique from Weber et al. [Weber, N. et al. Computers & Fluids 168 (2018)]. This implies that the species distribution in the electrolyte and the distribution of the dissolved species in the cathode are calculated locally and the potential and current density distribution are calculated globally. The equations to solve are coupled between the used meshes. To account for the potential distribution, it is necessary to consider the electrochemical double layer at the electrode-electrolyte interfaces. Macroscopically, the latter can be described as a potential jump at both interfaces in accordance to the Nernst equation. Weber et al. [Weber, N. et al. Electrochim. Acta 318 (2019)] introduced to model this jump using a source term. In the present study, the approach to use internal jump boundary conditions for modeling the potential jump is proposed. To the best knowledge of the authors, species distributions in the electrolyte have not been investigated previously. When there is no mixing of the electrolyte of LMBs, concentration gradients of each species might be significant [Vallet, C. E. et al. J. Electrochem. Soc. 130 (12) (1983)]. Based on the solution for the species distribution, the potential jump at each interface can be calculated. Further, the charge transfer overpotentials can be obtained.The equations are solved quasi-one-dimensional using the finite volume method and are implemented in the open source library OpenFOAM. To validate this new approach, a comparative study with Comsol is performed. Lastly, an application case is simulated, which confirms that concentration layers in the electrolyte are indeed present. This is, for the case of an Li+ ion in a LiCl-KCl eutectic molten salt, exemplarily shown in figure 1d. Further, as it can be seen in figure 1c, the simulation is able to solve the potential distribution in the cell with respect to the potential jump at the interface.Acknowledgment: This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 963599. Figure 1

(Invited) Engineering Challenges in Scaling-up Solar Water Splitting Devices

The last decade has witnessed significant progress in the development of solar water splitting devices, with solar-to-hydrogen (STH) efficiency as high as 30% already demonstrated. However, two major challenges remain. First, the high-efficiencies (> 15%) have only been achieved using devices based on expensive and non-scalable III-V semiconductors. On the other hand, low-cost metal-oxide based devices, mainly using BiVO4 as the absorber, have only achieved STH efficiency of < 10%. Due to stability limitations, many of these metal-oxide based devices are operated in near-neutral pH electrolytes, which presents an additional mass transport challenge. Second, the majority of the demonstrated devices are still at the laboratory scale. Reports on large-area devices start to emerge, but they typically show much lower efficiencies. This is best illustrated in a recent review:[1] even when III-V semiconductor-based devices are considered, there is no report of devices with a semiconductor absorber area larger than 10 cm2 and STH efficiency > 10%.In this talk, we will discuss the scale-up of our photoelectrochemical water splitting devices based on a complex metal oxide photoabsorber. Factors other than the semiconductor photoabsorber itself are found to be responsible for a total voltage loss of > 500 mV and therefore limit the overall performance of the large-area device.[2] To properly address this limitation, we quantify and break down the different loss mechanisms associated with the device scale-up and the practical operational conditions.[3] Concentration overpotential due to pH gradient is found to be a major contributor to the performance loss, and we show using multiphase multiphysics simulations and in-situ fluorescence measurements that careful control of natural and forced convection can overcome this limitation.[3-5] In addition, we also explore the possibility to achieve efficient product separation in devices with and without separators. The product crossover, optical and Ohmic losses are quantified using a combination of experiments and simulations, and the optimization of the device working parameters and/or separator properties to achieve the minimum overall loss will be discussed.[6,7] References J. H. Kim et al., Chem. Soc. Rev. 48, 2019, 1908I. Y. Ahmet et al., Sust. Energy Fuels 3, 2019, 2366F. F. Abdi et al. Sust. Energy Fuels 4, 2020, 2734K. Obata et al. Energy Environ. Sci. 13, 2020, 5104K. Obata & F. F. Abdi, Sust. Energy Fuels, 5, 2021, 3791K. Obata et al. Cell Rep. Phys. Sci. 2, 2021, 100358C. Özen et al. in revision

Electrolyte Effects on CO2 Electrochemical Reduction to CO.

ConspectusThe electrochemical reduction of CO2 (CO2RR) constitutes an alternative to fossil fuel-based technologies for the production of fuels and commodity chemicals. Yet the application of CO2RR electrolyzers is hampered by low energy and Faradaic efficiencies. Concomitant electrochemical reactions, like hydrogen evolution (HER), lower the selectivity, while the conversion of CO2 into (bi)carbonate through solution acid–base reactions induces an additional concentration overpotential. During CO2RR in aqueous media, the local pH becomes more alkaline than the bulk causing an additional consumption of CO2 by the homogeneous reactions. The latter effect, in combination with the low solubility of CO2 in aqueous electrolytes (33 mM), leads to a significant depletion in CO2 concentration at the electrode surface.The nature of the electrolyte, in terms of pH and cation identity, has recently emerged as an important factor to tune both the energy and Faradaic efficiency. In this Account, we summarize the recent advances in understanding electrolyte effects on CO2RR to CO in aqueous solutions, which is the first, and crucial, step to further reduced products. To compare literature findings in a meaningful way, we focus on results reported under well-defined mass transport conditions and using online analytical techniques. The discussion covers the molecular-level understanding of the effects of the proton donor, in terms of the suppression of the CO2 gradient vs enhancement of HER at a given mass transport rate and of the cation, which is crucial in enabling both CO2RR and HER. These mechanistic insights are then translated into possible implications for industrially relevant cell geometries and current densities.

Insight into the bubble-induced overpotential towards high-rate charging of Zn-air batteries

Tremendous efforts are made in designing effective electrocatalysts for rechargeable Zn-air batteries to lower overpotenitals during discharging and charging. When the charging current reaches a critical value, bubble evolution inevitably occurs due to the unreleased oxygen on the air electrode and the possible hydrogen generation on the Zn electrode, whereas the detailed effects on the electrochemical performance are usually overlooked. Herein, the bubble dynamics and the induced overpotential, including activation, ohmic, and concentration overpotentials, during high-rate charging are investigated based on a carbon-free electrode through in-situ observation, mathematical modelling, and numerical simulation. The bubble generation on the air electrode is observed with a three-step process: initial accumulation, transitional growth, and steady evolution. The modelling and simulation results demonstrate that the concentration overpotential is reduced by the bubble-induced convection-disturbance and sink-absorption, while the activation overpotential (4.29 mV at 20 mA cm−2) is an order of magnitude larger than the ohmic component (0.10 mV). Further, a formula to express the whole overpotential is also proposed by the incorporation of potential oscillation from hydrogen bubble evolution on the Zn electrodes. This work elucidates the fundamental relationship between bubble dynamics and electrochemical characteristics, which favors the development of high-performance rechargeable Zn-air batteries.

Prospect of modeling industrial scale flow batteries – From experimental data to accurate overpotential identification

A vast literature exists on modelling of small-scale single-cell experiments for flow batteries, but very few reports have been published on large stacks, consisting of tens of cells, each with an active area of hundred square centimeters. In this report, a large set of measurements taken on a kW-class vanadium test facility is used to develop an accurate ad-hoc physical model. Experimental data consist of polarization curves at a broad range of states of charge and electrolyte flow rates, as well as electrochemical impedance spectra. The model is capable to decouple three sources of voltage losses: activation, ohmic, and concentration overpotentials. In addition, a new numerical approach for identifying the main parameters of electrochemical kinetics and mass-transport has been proposed. To the best of our knowledge, this is the first time when a model for voltage losses analyses was developed and fitted to a data from large-scale flow battery, being validated with a sensitivity analysis study. Investigations showed that activation losses have a sophisticated nature in combination with mass-transport limitations and play an important role in a wide range of loading current densities. As a result, they must be included in a reliable model able to reflect a non-linear voltage behavior of large stacks. This work also highlights that, in the investigated case, activation losses are likely to be attributed to the positive electrode rather than to the negative one. The obtained results could facilitate the development of advanced simulation and design of large-scale flow battery stacks.