Charge Transport Limitations Research Articles

High power density redox-mediated Shewanella microbial flow fuel cells

Microbial fuel cells utilize exoelectrogenic microorganisms to directly convert organic matter into electricity, offering a compelling approach for simultaneous power generation and wastewater treatment. However, conventional microbial fuel cells typically require thick biofilms for sufficient metabolic electron production rate, which inevitably compromises mass and charge transport, posing a fundamental tradeoff that limits the achievable power density (<1 mW cm−2). Herein, we report a concept for redox-mediated microbial flow fuel cells that utilizes artificial redox mediators in a flowing medium to efficiently transfer metabolic electrons from planktonic bacteria to electrodes. This approach effectively overcomes mass and charge transport limitations, substantially reducing internal resistance. The biofilm-free microbial flow fuel cell thus breaks the inherent tradeoff in dense biofilms, resulting in a maximum current density surpassing 40 mA cm−2 and a highest power density exceeding 10 mW cm−2, approximately one order of magnitude higher than those of state-of-the-art microbial fuel cells.

Photovoltaic limitations of FAPbI3 nanocrystal solar cells associated with ligand washing processes

The processing of perovskite nanocrystals (PNCs) solar cells (SCs) usually incorporates ligand washing (LW) procedures. We show that a LW step of FAI in EtAc assists in the removal of the oleic acid/oleylamine ligands and improves the JSC and FF values of FAPbI3 based SCs. Although by increasing the exposure time of EtAc the removal of oleic acid/oleylamine ligands is more effective, sintering/necking of the FAPbI3 results to NC agglomerations and formation of defects/traps are observed within the active layer by the reported photoluminescence studies. Despite identifying the balanced in LW procedure exposure time, limitations on the performance of FAPbI3 PNCs SCs are reported. To identify the photovoltaic limitations impedance spectroscopy studies are presented and show that in the case of FAPbI3 PNC SCs an intermediate frequency response manifests. The observed impedance spectroscopy intermediate frequency response correlates with a parasitic resistance effect, indicating charge transport limitations and charge recombination losses even after the applied LW processing procedure.

Alkaline Water Electrolysis – Investigation of the Charge Transport Limitations across the Separator Under Low KOH Concentration

Currently, the water electrolysis is employed when high purity hydrogen is required, or excess of cheap electricity is available. With a broader future perspective, an important benefit of this technology represents fact, that it can be powered from renewable energy sources. The hydrogen produced from renewables is, thus, “green” CO2 neutral, playing an important role as energy vector in the so called “hydrogen economy”. Nowadays, the hydrogen economy represents strategy for transition of our economy towards the renewable energy sources, with water electrolysis (denoted as Power-to-X technology) being a crucial part of this scheme.Nowadays, the alkaline water electrolysis technology (AWE) operating at elevated temperatures (70 – 100 °C) with KOH electrolyte (25 – 30 wt.% KOH) dominates in the field of industrial hydrogen production by the water electrolysis. The alkaline environment is advantageous since it does not require platinum-group catalysts. On the other hand, highly concentrated caustic solution at elevated temperatures used as a liquid electrolyte limits the flexibility of the process operation. Currently several alternative technologies offering higher production intensity (proton-exchange membrane water electrolysis) and higher efficiency (high temperature solid-oxides steam-electrolysis) are developed and approach industrial application state. There are also promising innovations of the AWE process developed, e.g. application of an anion-selective membrane (AM) as the separator alternative to the diaphragm used in the conventional AWE. The AMs of interest exhibit improvement, contrary to diaphragm, of anionic conductivity with decreasing KOH concentration and operating temperature. Therefore, application of such AM allows to reduce KOH concentration in a liquid electrolyte (down to 5 – 15 wt.%) and operating temperature, thus, solve the above-mentioned problems of conventional AWE in connection with renewables. Moreover, the AM is dense, therefore might represent more effective barrier for produced gasses than diaphragm. However, one has to keep on mind increasing solubility of gasses with decreasing KOH concentration followed by potentially enhanced H2 cross-over. Another non-negligible benefit of lower KOH concentration and operating temperature is lower electrolyte conductivity in the manifolds contributing to reduced parasitic (bypass) current. However, systematic research aimed at evaluation of real application potential and searching for optimal conditions of the AM at diluted KOH concentrations is so far missing.In the presented contribution, study on a behaviour of two types of the separators in the laboratory scale AWE stack in bipolar zero-gap configuration is presented. The separators used are: a) a homogeneous AM based on chlormethylated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene copolymer functionalized by 1,4-diazabicyclo-[2.2.2]octane (PSEBS-CM-DABCO) and b) a commercial composite diaphragm Zirfon™ Perl. The load curves were recorded to assess optimal performance of the stack with both separators at KOH concentration range of 1 – 15 wt.% and operating temperature range of 25 – 40 °C. The effect of different electrolyte flow rate through the Ni-foam electrodes was investigated as well. Additionally, 1-dimensional stationary mathematical model of single AWE (both with diaphragm and AM) cell corresponding to the studied laboratory set-up was developed to clarify the differences in the stack load curves recorded for the two studied separators. The Nernst-Planck equation is used to describe migration and diffusion transport in the cell, while convective flow of electrolyte in the porous electrode is considered in material balance. The gas evolution is neglected in the present model. A macrohomogeneous continuum theory is employed for description of the porous electrodes. The AM-solution interface is characterized by Donnan equilibrium. At all studied experimental conditions, the stack with AM outperforms the one with diaphragm. This fact is even more pronounced at KOH concentration below 5 wt.% due to occurring transport limitation, which is more significant in the case of the diaphragm, as demonstrated both by modelling and experimentally. The presented work clearly identifies interval of operating conditions, in which AM represents an important advantage over porous separator and opposite. Thus, it gives important hints for further development of this technology.Project has received funding from the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 862509).

Molecular Catalysis of Energy Relevance in Metal–Organic Frameworks: From Higher Coordination Sphere to System Effects

The modularity and synthetic flexibility of metal-organic frameworks (MOFs) have provoked analogies with enzymes, and even the term MOFzymes has been coined. In this review, we focus on molecular catalysis of energy relevance in MOFs, more specifically water oxidation, oxygen and carbon dioxide reduction, as well as hydrogen evolution in context of the MOF-enzyme analogy. Similar to enzymes, catalyst encapsulation in MOFs leads to structural stabilization under turnover conditions, while catalyst motifs that are synthetically out of reach in a homogeneous solution phase may be attainable as secondary building units in MOFs. Exploring the unique synthetic possibilities in MOFs, specific groups in the second and third coordination sphere around the catalytic active site have been incorporated to facilitate catalysis. A key difference between enzymes and MOFs is the fact that active site concentrations in the latter are often considerably higher, leading to charge and mass transport limitations in MOFs that are more severe than those in enzymes. High catalyst concentrations also put a limit on the distance between catalysts, and thus the available space for higher coordination sphere engineering. As transport is important for MOF-borne catalysis, a system perspective is chosen to highlight concepts that address the issue. A detailed section on transport and light-driven reactivity sets the stage for a concise review of the currently available literature on utilizing principles from Nature and system design for the preparation of catalytic MOF-based materials.

Bulk Embedding of Ferroelectric Nanodomains in CuBi2O4 Photocathodes Enables Boosted Photoelectrochemical Hydrogen Generation

AbstractIt is widely accepted that metal oxide‐based photoelectrodes (MOPs) hold great promise for future solar hydrogen generation but are facing awkward challenge arising from their low intrinsic carrier mobility. The highly polarized nature of the predominantly ionic metal‐oxygen bond always leads to the formation of small polarons that are responsible for the localized trapping of photo‐generated carriers. Present study explores the reduction of carriers transport barrier via bulk embedding of ferroelectric nanodomains (FNDs) in MOPs that results in a new performance benchmark for the CuBi2O4 photocathode. By embedding laser‐generated sub‐10 nm BaTiO3 nanocrystals in the bulk of CuBi2O4 photocathode, numerous FNDs are created that can lead to two times enhancement of the carrier mobility, which is proposed to originate from the overlaying of the internal electric fields and effective electrons transport channel at the heterointerfaces of BaTiO3/CuBi2O4. Such strategy leads to the CuBi2O4 photocathode with the photocurrent density of up to 3.21 mA cm−2 at 0.6 VRHE, as well as a pronounced absorbed photon‐to‐current efficiency up to 80% at 400 nm. The universal feature of present technology is further verified by laser embedding of SrTiO3 FNDs, providing an effective route for addressing the charge transport limitations in MOPs.

Unveiling Charge Transport and Degradation Mechanisms of Aqueous Zn|α‐MoO3 Batteries in Conventional Concentration and Water‐in‐Salt Electrolytes: A Multi‐Modal In Situ and Operando Study

AbstractHerein charge storage and transport properties are elucidated and cell degradation mechanisms of rechargeable aqueous Zn|α‐MoO3 batteries in three electrolyte systems (3 m ZnSO4, 3 m ZnCl2, and 30 m ZnCl2 [12.5 m] water‐in‐salt (WIS)) are distinguished by a combination of in situ X‐ray diffraction (XRD), in situ X‐ray absorption spectroscopy (XAS), operando optoelectrochemistry, and operando energy dispersive X‐ray diffraction (EDXRD). In conventional concentration 3 m electrolytes, in situ XRD and XAS, as well as ex situ scanning transmission electron microscopy data collectively support Zn2+ as the primary charge carrier. In addition, these systems are susceptible to cathode dissolution, Zn corrosion coupled with the hydrogen evolution reaction, and the resultant formation of basic zinc salt phases. The multi‐modal in situ and operando experimental analyses validate facile H+ intercalation and extraction in concentrated 30 m ZnCl2 WIS electrolyte. Via operando EDXRD, reaction front and charge transport limitation during discharge and charge in the viscous WIS electrolyte are spatially tracked. This work provides new insight into the stability and degradation mechanisms of aqueous zinc batteries during static storage and upon dynamic cycling, and highlights the utility of in situ and operando techniques in understanding the superior stability of WIS electrolytes.

Simple Formalisms for the Concept of Heterogeneity in the Porous Electrodes of Lithium-Ion Batteries

The presence of inhomogeneity in the porous electrodes of lithium-ion cells can significantly affect the active material utilization, energy/power density, and ageing dynamics of the cell, among others. 1-4 In Li-ion cells, inhomogeneity is caused by an uneven spatial distribution of the active material, carbon, binder, and electrolyte, or porous electrode micro/nanoscale properties such as the contact resistances, porosity and tortuosity. The existing literature suggests that the heterogeneity could inadvertently be introduced to the electrode with improper combination of design parameters and manufacturing steps, e.g. slurry formulation, thickness, mixing, and drying.5 Important insights into the local microstructure of the porous electrodes have been provided by FIB-SEM,6 X-ray tomography,7 or full simulation of electrode microstructure.8 Little attention has been paid, however, to develop formalisms to quantify the electrode heterogeneity and its correlation to the design parameters, manufacturing steps, and the battery performance.In this presentation, the electrochemical and microstructural characteristics of a large group of NMC111 and NMC622 porous electrodes with different design parameters will be presented and discussed. The experimental data were obtained by preparing porous electrodes of different thickness, porosity, formulation (NMC loading and carbon/binder ratio), and surface chemistry. We spotlight the sensitivity of the energy and power density of the Li|NMC cells to the microstructural indexes including tortuosity, porosity, and effective conductivities. The experimental data were analysed with the help of modeling and simulations to introduce a simplified formalism which expresses the concept of heterogeneity in the porous electrodes of lithium-ion batteries. References H. Hamed, S. Yari, J. D'Haen, FU. Renner, N. Reddy, A. Hardy, M. Safari, “Demystifying Charge Transport Limitations in the Porous Electrodes of Lithium‐Ion Batteries,” Advanced Energy Materials (2020)10 (47), 2002492.S. Yari, H. Hamed, J. D’Haen, MK. Van Bael, FU. Renner, A. Hardy, M. Safari, “Constructive versus Destructive Heterogeneity in Porous Electrodes of Lithium-Ion Batteries,” ACS Applied Energy Materials (2020) 3 (12), 11820-11829. J. Harris and P. Lu, “Effects of Inhomogeneities—nanoscale to mesoscale—on the durability of Li-Ion batteries,” J. Phys. Chem. C, (2013) 117(13). M. Forouzan, B. A. Mazzeo, and D. R. Wheeler, “modeling the effects of electrode microstructural heterogeneities on Li-Ion battery performance and lifetime,” J. Electrochem. Soc., (2018) 165(10). Jaiser, N. Sanchez Salach, M. Baunach, P. Scharfer, and W. Schabel, “Impact of drying conditions and wet film properties on adhesion and film solidification of lithium-ion battery anodes,” Dry. Technol., (2017) 35(15). Kehrwald, P. R. Shearing, N. P. Brandon, P. K. Sinha, and S. J. Harris, “local tortuosity inhomogeneities in a lithium battery composite electrode,” J. Electrochem. Soc., (2011) 158(12). Ebner, D. W. Chung, R. E. García, and V. Wood, “tortuosity anisotropy in lithium-ion battery electrodes,” Adv. Energy Mater., (2014) 4(5). Liu, V. Battaglia, and P. P. Mukherjee, “mesoscale elucidation of the influence of mixing sequence in electrode processing,” Langmuir, (2014) 30(50).

Charge Transport Limitations to the Power Performance of LiNi0.5Mn0.3Co0.2O2 Composite Electrodes with Carbon Nanotubes

The power performance of EV-designed LiNi0.5Mn0.3Co0.2O2 (NMC 532)/PVdF/CB-CNT based electrodes with 25 mg.cm−2 active mass loading, 3.4 g.cm−3 density, 23.5% porosity and 96:1.8:2.2 wt% composition, were measured at 0 °C, 22 °C and 40 °C. The CB/CNTs ratio is 100:0 or 50:50 v%. The polarization resistance and the discharge capacity are improved by using the CB-CNTs blend especially at 0 °C and 40 °C. The shape factor of the CNTs favor the electronic wiring of the active mass at 0 °C by forming conductive bridges over the resistive grain boundaries at the surface of NMC clusters. At 40 °C, the CNTs favor both the electronic and the ionic wirings of the active mass. At this temperature, the swelling of the CB/PVdF domains could be responsible for the degradation of the electrons transfer and ions transport through the electrode. This detrimental phenomenon is mitigated with the CB/CNTs blend likely due to the CNTs shape factor.

(Invited) Probing Heterogeneous Transformations in NMC Cathodes with Microbeam Diffraction

Ideally, the redox activity in Li-ion batteries, which is associated with storage and release of Li ions, is distributed homogeneously throughout the electrode, thereby minimizing detrimental processes while maximizing battery performance. However, it appears that every electrode material displays inhomogeneous redox activity which is expected to play a dominant role in battery performance parameters such as cycle life, efficiency and rate performance.1 For example, the occurrence of localized redox activity in layered electrodes can accelerate degradation reactions through local strain and/or decomposition1-2. Further, inhomogeneous reactions render estimating the true state of charge challenging and can introduce history effects3 that make optimization of battery performance via a battery management system problematic. Inhomogeneous redox reactions in Li-ion battery electrodes can occur on different length scales, starting at the nm length in individual electrode particles progressing up to the dimensions of complete composite electrodes (tens of μm) and can have various origins. Most directly, inhomogeneous reactions in electrodes stem from charge transport limitations, generally determined by the electrode morphology (including electrode thickness, porosity and tortuosity).4-6 A second origin for inhomogeneous reactions can be a difference in insertion potential, through mixing electrode materials5 or due to a distribution of nanoparticle sizes.7 Lastly, the nature of the phase transition can also give rise to inhomogeneous reactions. For example, the particle-by-particle transformation mechanism in LiFePO4 has been associated with a hysteresis effect8 and has been suggested to give rise to a memory effect that can influence the observed potential.3 Despite the above, limited research has been performed on this subject to date and many questions remain, mainly due to the difficulty to probe these heterogeneities in realistic battery geometries and during cycling conditions.Here we present a study of the phase transformation of ten's of individual NMC crystallites under operando cycling conditions in pouch cells using microbeam diffraction (ID11, ESRF, France). Due to the small, bright and sub-micron sized synchrotron beam diffraction rings observed with powder diffraction fall apart in individual spots each representing individual NMC crystallites in the positive electrode. In this way the phase transition behaviour through time of ten's of individual NMC crystallites is monitored at the same time while varying the electrochemical conditions.9,10 Figure 1 shows snapshots of the (108) and (110) reflection of individual NMC111 crystallites during a full C/4 charge-discharge cycle, demonstrated the anticipated continuous shift. For each individual crystallite the evolution of the lattice parameters provide insight in the distribution of the composition and local potential of each crystallite, as well as the individual transformation rate. In addition the average transformation rate can be translated in the active particle fraction and average local current density. By studying this for the series NMC111, NMC622 and NMC811, we gain insight in the heterogeneous redox activity and the impact of composition, relevant for both rate performance and cycle life of this class of intercalation cathodes. Figure 1. Following the phase transformation of single NCM111 crystallites through the (108) and (110) reflection during C/4 charge and discharge. Zhang et al. Journal of Materials Chemistry A 2019, 7 (41), 23628-23661. Lin et al. Nature Communications 2014, 5 (1), 3529. Sasaki et al. Nature Materials 2013, 12 (6), 569-575. Strobridge et al. Chemistry of Materials 2015, 27 (7), 2374-2386. Sasaki et al. Advanced Science 2015, 2 (7), 1500083. Zhang et al. Advanced Energy Materials 2015, 15, 1500498. Van der Ven et al. Electrochemistry Communications 2009, 11 (4), 881-884. Dreyer et al. Nature Materials 2010, 9 (5), 448-453.van Hulzen et al. Font. Energy Res. 2018, 6 (59). 5.Zhang et al. Nature Commun. 2015, 6 Figure 1

Porous Electrodes: Demystifying Charge Transport Limitations in the Porous Electrodes of Lithium‐Ion Batteries (Adv. Energy Mater. 47/2020)

Porous Electrodes: Demystifying Charge Transport Limitations in the Porous Electrodes of Lithium‐Ion Batteries (Adv. Energy Mater. 47/2020)

Demystifying Charge Transport Limitations in the Porous Electrodes of Lithium‐Ion Batteries

AbstractA possible strategy to give a simultaneous boost to the energy and power attributes of the current generation of lithium‐ion batteries is developing thick porous electrodes with a high loading of active material alongside optimal percolation networks for the ions and electrons. However high the insertion capacity and kinetics of the single particle lithium‐insertion materials, the energy and power density of the cell can be capped by the ionic and electronic transport limitations in the porous electrode. In this work, a physical picture grounded in experiment and theory is proposed to spotlight and quantify the pivotal role of the micro‐scale porosity and active‐material loading in determining the tortuosity, effective transport properties, and performance limitations of porous electrodes. The outcome is a phenomenological picture coupled with a theoretical framework for the deconvolution of the relative shares of the electronic and ionic transport limitations over short and long ranges regarding the performance limitation of lithium‐ion batteries.

In-situ and synchronous generation of oxygen vacancies and FeOx OECs on BiVO4 for ultrafast electron transfer and excellent photoelectrochemical performance

Efficient separation and transport of charge is a critical issue in solar-driven water oxidation. Herein, we developed a novel, facile, controllable method based on zero-valent iron (ZVI) reduction to in-situ and synchronous generate oxygen vacancies (Ov) and FeOx oxygen evolution co-catalysts (OECs) on BiVO4 for ultrafast electron transfer and excellent photoelectrochemical (PEC) water oxidation. In this method, a moderate and controllable ZVI reduction was the critical step to ensure whole penetration of Ov from BiVO4 to FeOx layer. This is because a special galvanic cell is formed between ZVI and BiVO4, making it easy to capture oxygen atom from BiVO4 and obtain a FeOx layer (5 nm) outside simultaneously in oxygen-free annealing. The Ov can extend light absorption by narrowing bandgap and significantly improve electron mobility (8.6 × 10−7 cm2 s−1) by reducing the trap-assisted recombination, which is 6.1-fold of BiVO4. Meanwhile, electron lifetime increases from 11.6 to 115.3 ms. Ultrathin FeOx layer provides more sites and dramatically reduces OER over-potential of 210 mV, resulting in fast hole-to-oxygen kinetics. A photocurrent of 3.13 mA·cm−2 at 1.23 VRHE is achieved for PEC water oxidation, which is 4.6-fold of pristine BiVO4. This work provides a new path to overcome charge transport limitations and achieve enhanced solar water oxidation.

(Invited) Charge Transport Limitations to the Power Performance of LiNi0.5Mn0.3Co0.2O2 Composite Electrodes with Carbon Nanotubes

Electrode power performance is tied with electrode design that influences particularly the electronic wiring of the active mass and the speed of lithium salt diffusion in the pores. Improved capacity delivery, especially at 0 and 40°C was seen for NMC532 electrodes containing carbon nanotubes (CNT) comparatively to carbon black (CB). At low temperature CNT significantly increase the electrode conductivity as seen from the decrease of the polarization resistance. Furthermore, the analysis of the current density dependence of the discharge capacity with the Penetration Depth Model (PDM) reveal that the CNT improve the ionic transport at high temperature because they would minimize the effect of the PVdF swelling by the electrolyte and the resulting increase of the electrode tortuosity.

Understanding capacity fading mechanism of thick electrodes for lithium-ion rechargeable batteries

The use of thick electrodes with high-loading density of active material is one of the most practical strategies to increase the volumetric/specific energy density of lithium-ion battery, while taking advantage of the current electrode chemistry. However, their use is accompanied by serious deterioration of electrochemical performance, especially exhibiting poor capacity retention with low power capability. Here, the degradation behavior of the LiNi0·6Co0·2Mn0·2O2, one of widely adopted cathodes, is comparatively investigated under high loading levels as high as 28 mg cm−2 over the extended cycling. It is revealed that the charge transport limitation is cumulatively dominated by the lithium ionic diffusion rather than the electronic conduction in the thick electrode. More importantly, as the cycle proceeds, the thick electrode gets exposed to a serious reaction inhomogeneity because of the negative feedback between the accumulated ion transport limitation and locally increasing resistance. It leads to the generation of current hot spot in the electrode and the corresponding local material degradation, which further inhibit the charge transport, resulting in unavoidable capacity fading. This finding proposes that rational electrode architecture detouring the hot spot generation needs to be considered with respect to the ion transport and the electrode material degradation toward the high-loading electrodes.

(Invited) Charge Transport Limitations to the Power Performance of LiNi0.5Mn0.3Co0.2O2 Composite Electrodes

Automotive applications of Lithium ion batteries demand two key characteristics: mileage (energy) and torque (power). Energy demand can be simple achieved by increasing electrode mass loadings and/or density. However, power suffers due to the increased electrode thickness and/or density increases the path length for ions transport, which is the key limiting factor for proper functioning at high current densities [1]. This study deals with how the different electrode parameters affect electrochemical performance especially at high current densities.The interplay between electrode microstructure, electrochemical performance and lithium salt diffusion must be further studied to improve battery engineering. Herein, the electrochemical performance of NMC532-based positive electrodes for EV application were analyzed in relationship with their microstructure and their transport properties.The NMC532 electrodes were designed with varying degrees of loading, mass percentages of electrode components and calendaring to evaluate electrode microstructure effects against electrode performance.The electrode microstructures were acquired numerically in 3D at different scales by X-ray tomography and FIB-SEM tomography, which allowed their fine analysis through the quantification of several parameters such as the intra- and interconnectivity between the various phases, and the porosity distribution in size and tortuosity. The electrodes electronic transport properties were measured by using broadband dielectric spectroscopy from 40 Hz to 10 GHz [2]. The electrode ionic conductivities were evaluated by performing numerical simulations using the Fast Fourier Transform (FFT) method. Such simulations are directly computed on the grid represented by the voxels in the 3D numerized FIB-SEM volumes [3].The electrochemical performance were measured in half-cells with LP30 electrolyte at various temperatures between 0 and 40°C. The results were analyzed with the penetration depth model from Gallagher et.al. [4]. This one proposes to relate the capacity at high C-rates (typically C or higher) with the electrode microstructure parameters, the electrolyte transport properties and the applied current density.The decrease in specific capacity observed with the increase in current density as well as in electrode thickness, the decrease of its porosity and the temperature decrease is in agreement with the existing literature. Our results show more particularly the significant influence of the NMC532 conductivity and its modulation with the lithiation state. Moreover, we have been able to find a good agreement between the experimental results and the predictions of the penetration depth model. But, it is necessary to take into account precisely the different electrode porosities (open or closed, micrometric or nanometric) which participate with variable efficiencies in ion transport. We found finally that increased CB-PVdF content increases tortuosity due to any existing interactions between the electrolyte species and these additives, in agreement with our Pulsed Field Gradient Spin - Echo NMR (PFG-SE NMR) recent study [5].Hence, this work provides a better understanding of how these composite electrodes perform depending on their microstructure and the active material intrinsic conductivity. Acknowledgments Financial funding from the ANR program no. ANR-15-CE05-0001-01 is acknowledged.

(Invited) Impact of Mesoscale Morphology on Electrode Performance through Multi-Physics Simulations

While lithium-ion battery electrodes are manufactured in large-scale, high-speed, roll-to-roll manufacturing processes, it is the morphology of the particle and conductive binder structures that greatly affect macroscale properties and can vary significantly as a function of manufacturing conditions. This relationship is particularly true in the extremely fast (dis)charge environments relevant for electrification of our vehicle fleet, where the short time scales probe ever smaller length scales and increase the impact of ionic and charge transport limitations. To explore the process-structure-property relationship, we employ mesoscale finite element simulations of the three-phase particle-conductive binder-electrolyte composite of NMC cathodes. Particle and conductive binder domain mesostructures are derived from both x-ray computed tomography imaging and simulated using discrete element methods. The Conformal Decomposition Finite Element Method is applied to create conformal three-phase finite element discretizations of these mesostructures. Effective properties including electrical conductivity, tortuosity, and mechanical modulus are calculated and compared to experimentally measured values. Coupled electrochemical-mechanical simulations of cathode discharge are performed and validated against half-cell experimental data. We focus on how the conductive binder domain morphology affects electrode performance. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Embedding laser generated nanocrystals in BiVO4 photoanode for efficient photoelectrochemical water splitting

Addressing the intrinsic charge transport limitation of metal oxides has been of significance for pursuing viable PEC water splitting photoelectrodes. Growing a photoelectrode with conductive nanoobjects embedded in the matrix is promising for enhanced charge transport but remains a challenge technically. We herein show a strategy of embedding laser generated nanocrystals in BiVO4 photoanode matrix, which achieves photocurrent densities of up to 5.15 mA cm−2 at 1.23 VRHE (from original 4.01 mA cm−2) for a single photoanode configuration, and 6.22 mA cm−2 at 1.23 VRHE for a dual configuration. The enhanced performance by such embedding is found universal owing to the typical features of laser synthesis and processing of colloids (LSPC) for producing ligand free nanocrystals in desired solvents. This study provides an alternative to address the slow bulk charge transport that bothers most metal oxides, and thus is significant for boosting their PEC water splitting performance.

The non-ohmic nature of intercalation materials and the consequences for charge transport limitations

Accurate modeling of the internal battery resistance is imperative in predicting the state of charge and state of health. A mathematical model has been developed that, in addition to the ionic transport, introduces an accurate description of the electronic transport in the porous semiconducting LiFePO4 electrodes. The model is based on the fundamental principles of electrochemistry, electrochemical kinetics, and semiconductor physics, combining them in an efficient model. This framework provides for the non-ohmic nature of semiconductor electrode materials and their current dependent conductivity. The model is validated by comparison with experimental data of Li-ion concentration profiles. It is demonstrated that the mass transport of the electrons, typically simplified or considered negligible in calculation models, have a significant influence on the electrode kinetics and therefore on the current dependent internal resistance of the battery. The accurate description of the internal resistance and the related heat production under various cycling conditions allows the design of safer battery electrode architectures. Additionally, the model allows optimization of the electrode components for various loading regime, increasing the effective energy density leading to decreasing demand for materials and costs. The present model, its principles, and methods are generally applicable and can be used for the description of the wide range of energy storage materials and systems where combined ion and electron transport takes place.

Hierarchical 3D electrodes for electrochemical energy storage

The discovery and development of electrode materials promise superior energy or power density. However, good performance is typically achieved only in ultrathin electrodes with low mass loadings (≤1 mg cm−2) and is difficult to realize in commercial electrodes with higher mass loadings (>10 mg cm−2). To realize the full potential of these electrode materials, new electrode architectures are required that can allow more efficient charge transport beyond the limits of traditional electrodes. In this Review, we summarize the design and synthesis of 3D electrodes to address charge transport limitations in thick electrodes. Specifically, we discuss the role of charge transport in electrochemical systems and focus on the design of 3D porous structures with a continuous conductive network for electron transport and a fully interconnected hierarchical porosity for ion transport. We also discuss the application of 3D porous architectures as conductive scaffolds for various electrode materials to enable composite electrodes with an unprecedented combination of energy and power densities and then conclude with a perspective on future opportunities and challenges. 3D electrodes with interconnected and interpenetrating pathways enable efficient electron and ion transport. In this Review, the design and synthesis of such 3D electrodes are discussed, along with their ability to address charge transport limitations at high areal mass loading and to enable composite electrodes with an unprecedented combination of energy and power densities in electrochemical energy storage devices.

Unconventional Relation between Charge Transport and Photocurrent via Boosting Small Polaron Hopping for Photoelectrochemical Water Splitting

Doping in semiconductor photoelectrodes controls defect formation and carrier transport that critically determine the device performance. Here we report an unconventional carrier transport relation that is tuned by extrinsic molybdenum (Mo) doping in BiVO4 photoanodes. Using the single-crystalline thin film approach, we identify that Mo doping significantly condenses the optimization regime between carrier transport and photon collection. For Mo-doped BiVO4 films, an unprecedentedly thin layer (50 nm), less than one-third of the pristine BiVO4 thickness, delivers larger photocurrents by overcoming the charge transport limitation, representing a regime not covered in conventional models. We provide direct evidence that Mo doping improves electron transport by boosting not only the donor density but also the electron mobility in the form of a small polaron, with the latter applying substantial impact on the photoelectrochemical performance. Density functional theory calculations reveal that fully ionized Mo...