Considerations for application of granular activated carbon as capacitive bioanode in bioelectrochemical systems

In the last decades, the research in Microbial Fuel Cells (MFCs) has expanded from electricity production and wastewater treatment to remediation technologies, chemicals production and low power applications. More recently, capacitors have been implemented to boost the power output of these systems when applied as wastewater treatment technology. Specifically, the use of granular capacitive materials (e.g. activated carbon granules) as bioanodes has opened up new opportunities for reactor designs and upscaling of the technology. One of the main features of these systems is that charge and discharge processes can be separated, which offers multiple advantages over more conventional reactor types. In this manuscript, we discuss several aspects to consider for the application of capacitive granules as bioanodes in MFCs and other bioelectrochemical systems, as well as the recent advances that have been made in applying these granules in various reactor systems. Similarly, we discuss the granule properties that are key to determine system operation and performance, and show that biofilm growth is highly dependent on the efficiency of discharge.</p

La0.8Sr0.2MnO3-d-based nanocomposite functional layers for improved cathode efficiency in SOFCs

Solid Oxide Fuel Cells (SOFC) are efficient electrochemical devices that are able to convert chemical energy in the form of fuels into electricity, but at low operating temperatures they are mainly limited by the high polarization resistance of the cathode. This way, optimizing the electrode microstructure and incorporating functional layers have proven to be useful to improve electrode properties. The present work proposes the incorporation of several nanocomposite functional layers by spray-pyrolisis at the La0.8Sr0.2MnO3-d (LSM) cathod, by combining different ionic conductors, i.e. LSM-Ce0.9Gd0.1O1.95 (CGO) and LSM-Bi1.5Y0.5O3 (BYO). These exhibited lower grain size compared to that of the homologous single phase; specially the LSM-CGO nanocomposite showed improved adherence to the electrolyte without the presence of any crack or delamination after annealing at 1000 ºC. The incorporation of functional layers also reduced the Area Specific Resistance (ASR) values of the cathode and the full cell with LSM-CGO as functional layer revealed higher power densities than the cell with no functional layer.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Nanostructured composites as active layer to boost cathode performance in Solid Oxide Fuel Cells

Since the efficiency of Solid Oxide Fuel Cells (SOFCs) is largely limited by the high polarization resistance of the cathode, several strategies have been proposed to enhance the electrochemical activity of such electrodes. Among them, optimizing the electrode microstructure by using different preparation methods, such as infiltration and spray-pyrolysis deposition, have rendered excellent and durable electrochemical performance. In addition, the tailoring of the electrode/electrolyte interface by incorporating active layers have proven to be particularly useful to improve electrode properties. The present work proposes alternative active layers based on nanocomposites by combining the properties of the La0.8Sr0.2MnO3-d (LSM) cathode and different ionic conductors with fluorite-type structure. Different nanocomposite layers were prepared by spray-pyrolysis deposition at 450 ºC for 30 min on Zr0.8Y0.16O1.92 (YSZ) electrolyte, i.e. LSM-Ce0.9Gd0.1O1.95 (CGO) and LSM-Bi1.5Y0.5O3 (BYO). Thereafter, the LSM was screen-printed on the YSZ pellet and sintered at 1000 ºC. The nanocomposite active layers were studied by different structural and microstructural techniques, such as XRD, SEM-EDX and HRTEM. The electrochemical properties of active layers were also investigated by impedance spectroscopy at different dc-bias and distribution of relaxation times. Similarly, fuel cell tests were performed in a NiO-YSZ anode supported cell. The nanocomposite layers were dense with a thickness of approximately 700 nm. Specially LSM-CGO layers showed improved adherence to the electrolyte without the presence of cracks, delamination or undesired reaction. Cathodes with active layer showed Area Specific Resistance (ASR) associated with a lower charge transfer resistance and a fast oxide ion transport at the electrode/electrolyte interface.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

A review on recent advances and trends in symmetrical electrodes for solid oxide cells.

Symmetrical solid oxide cells (SSOCs) with identical air and fuel electrodes have gained significant scientific interest in the last decade because they offer several advantages over conventional cell configurations. Among other features, simpler fabrication, better chemical and thermo-mechanical compatibility between cell layers, and electrode reversibility, make them attractive for electricity generation, H2 production and CO2 electroreduction. This review offers an overview of the most recent advances in the field of SSOCs, paying special attention to the relationship between electrode composition, crystal structure and properties. With that aim, symmetrical electrodes are classified in four groups according to their redox stability, i.e. single phases, composites, electrodes with exsolved metal particles and those that suffer a drastic phase transformation under reducing conditions, known in the literature as quasi-symmetrical electrodes. Furthermore, an outlook of other cell configurations with increased scientific interest are also discussed, i.e. symmetrical protonic fuel cells (H–SSOCs) and solid oxide electrolyzers for CO2 electroreduction. With this overview in mind, the authors would like to highlight the challenge ahead of finding electrode materials that optimally work under both oxidizing and reducing conditions in terms of redox stability and electrochemical properties, and further conclude on the future development of SSOCs

Unraveling the Influence of the Electrolyte on the Polarization Resistance of Nanostructured La0.6Sr0.4Co0.2Fe0.8O3-δ Cathodes

Large variations in the polarization resistance of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathodes are reported in the literature, which are usually related to different preparation methods, sintering temperatures, and resulting microstructures. However, the influence of the electrolyte on the electrochemical activity and the rate-limiting steps of LSCF remains unclear. In this work, LSCF nanostructured electrodes with identical microstructure are prepared by spray-pyrolysis deposition onto different electrolytes: Zr0.84Y0.16O1.92 (YSZ), Ce0.9Gd0.1O1.95 (CGO), La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM), and Bi1.5Y0.5O3-δ (BYO). The ionic conductivity of the electrolyte has a great influence on the electrochemical performance of LSCF due to the improved oxide ion transport at the electrode/electrolyte interface, as well as the extended ionic conduction paths for the electrochemical reactions on the electrode surface. In this way, the polarization resistance of LSCF decreases as the ionic conductivity of the electrolyte increases in the following order: YSZ > LSGM > CGO > BYO, with values ranging from 0.21 Ω cm2 for YSZ to 0.058 Ω cm2 for BYO at 700 °C. In addition, we demonstrate by distribution of relaxation times and equivalent circuit models that the same rate-limiting steps for the ORR occur regardless of the electrolyte. Furthermore, the influence of the current collector material on the electrochemical performance of LSCF electrodes is also analyzedThis work was funded by PID2021-126009OB-I00 and PID2019-110249RB-I00 (Ministerio de Ciencia, Innovación y Universidades) and UMA18-FEDERJA-033 (Junta de Andalucia, Spain/FEDER). J.Z.G. thanks the Ministerio de Ciencia, Innovación y Universidades for his FPU grant (FPU17/02621). L.C.J. would like to thank Plan Andaluz de Investigación, Desarrollo e Innovación (PAIDI 2020) for the research support (DOC 01168)

Combination of bioelectrochemical systems and electrochemical capacitors: Principles, analysis and opportunities

© 2019 The Authors Bioelectrochemical systems combine electrodes and reactions driven by microorganisms for many different applications. The conversion of organic material in wastewater into electricity occurs in microbial fuel cells (MFCs). The power densities produced by MFCs are still too low for application. One way of increasing their performance is to combine them with electrochemical capacitors, widely used for charge storage purposes. Capacitive MFCs, i.e. the combination of capacitors and MFCs, allow for energy harvesting and storage and have shown to result in improved power densities, which facilitates the up scaling and application of the technology. This manuscript summarizes the state-of-the-art of combining capacitors with MFCs, starting with the theory and working principle of electrochemical capacitors. We address how different electrochemical measurements can be used to determine (bio)electrochemical capacitance and show how the measurement data can be interpreted. In addition, we present examples of the combination of electrochemical capacitors, both internal and external, that have been used to enhance MFC performance. Finally, we discuss the most promising applications and the main existing challenges for capacitive MFCs

One step synthesis of nanocomposite electrodes for reversible electrochemical cells.

The irregular and seasonal disposition of renewable energy requires advanced devices for energy storage and conversion. Reversible electrochemical cells can address this approach by operating as both electrolyzer and fuel cell in an efficient and eco-friendly way. An important issue for increasing the performance of ceramic electrochemical cells is the sluggish oxygen reduction reaction kinetic at the air electrode [1]. It is well known that the efficiency of air electrodes may be improved by adding a second phase with high ionic conductivity, i.e. doped-CeO2 and Bi2O3, to obtain a composite electrode.[1] Moreover, they are usually employed to reduce the mechanical stress between electrode and electrolyte layers, originated by their different thermal expansion coefficients, thus enhancing the mechanical stability of the cell. Traditionally, composite electrodes are prepared by mechanically mixing pristine materials but, unfortunately, it is difficult to control the composition distribution and architecture with this method. In this work, different nanocomposite electrodes are successfully prepared by using both the freeze-drying powder precursor method and the spray-pyrolysis deposition, in a single-step synthesis, from precursor solutions containing all cations in stoichiometric amounts. For instance, La0.8Sr0.2MnO3-δ-Ce0.9Gd0.1O1.95 (LSM-CGO), La0.6Sr0.4Co0.2Fe0.8O3-δ-Ce0.9Gd0.1O1.95 (LSCF-CGO) and Sm0.5Sr0.5CoO3-δ-Ce0.9Sm0.1O1.95 (SSC-CSO). Both fluorite and perovskite-based phases are formed simultaneously, reducing drastically the preparation time, which is crucial for potential industrial application. The electrodes are composed of nanometric particles, providing high active area for electrochemical reactions. The intimate mixture of two immiscible phases hinder the cation diffusion and the grain growth rate. Very low polarization resistance values are obtained, i.e. 0.088 Ω cm2 at 700 °C for SSC-CSO.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Strategies to obtain efficient symmetrical electrodes for solid oxide cells.

There is an increase interest on finding alternative energy technologies that can cope with the current energy demand while mitigating the climate change. Among those technologies, Solid Oxide Cells (SOCs) are promising electrochemical devices that can efficiently convert fuels (e.g. hydrogen, methane) into electricity under fuel mode, as well as producing fuels from water and CO2 under electrolysis mode. In order to improve the durability of these devices, a new concept of symmetrical SOCs have been developed, where the same electrode material is used as both air and fuel electrode. However, the main challenge of symmetrical electrodes is that they need to operate efficiently under both oxidizing and reducing atmospheres. There are several strategies to synthesize high-performing symmetrical electrodes. In this work, we study three of them: i) preparation of novel materials based on Ti-doped Sr0.95FeO3-δ (SFT) and layered Pr0.5Ba0.5FeO3 (PBF) perovskites; ii) exsolution of Ni nanoparticles in Ni-doped PBF; and iii) tailoring of the electrode/electrolyte interface with a nanocomposite active layer. With that purpose, the electrode powders are prepared by the freeze-drying precursor method and screen-printed onto La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) electrolyte, while the nanocomposite active layer is deposited in one-step by the spray-pyrolysis technique onto the electrolyte. A thorough characterization of the materials is performed regarding their crystal structure, microstructure and electrochemical properties.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Sm0.5Sr0.5CoO3-δ - Ce0.9Sm0.1O1.95 nanocomposites for reversible electrochemical cells.

The generation of electricity through environmentally friendly sources of energy has been one of the main challenges of our society in the last decades. Nevertheless, the irregular and seasonal disposition of renewable energy requires devices for energy storage and conversion. Reversible electrochemical cells can address this approach by operating as an electrolyser, when an excess of electricity is available, and as a fuel cell, when the electricity is needed afterwards [1]. The efficiency of air electrodes with poor ionic conductivity may be improved by adding a second phase with high ionic conductivity, i.e. CeO2 and Bi2O3-based electrolytes. Many studies have shown that composite electrodes have higher efficiency than the single-phase ones due to the increased active area [2]. Traditionally, composite electrodes are prepared by mechanically mixing the pristine materials but, unfortunately, it is difficult to control the composition distribution with this method. In this work, Sm0.5Sr0.5CoO3-δ-Ce0.9Sm0.1O1.95 (SSC-CSO) nanocomposite cathodes are successfully prepared in a single process by using the freeze-drying precursor method, in a single-step synthesis, from a precursor solution containing all cations in stoichiometric amounts. SSC and CSO are formed simultaneously, reducing the preparation time, which is an important improvement for industrial application. Different percentages of SSC-CSO are investigated: 100-SSC, 80-SSC, 60-SSC and 50-SSC. The electrode is composed of nanometric particles, providing high active area for the electrochemical reactions. The CGO addition suppresses the grain growth of the nanocomposite cathodes, rendering lower particle size, from 0.53 to 0.32 nm of diameter for 100-SSC to 50-SSC, respectively. This is explained by the presence of CGO as secondary phase, which limits the cation diffusion and the grain growth rate. A low polarization resistance of 0.088 Ω cm2 is obtained at 700 °C for 50-SSC.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Ni-Doped PrBaFe2O5+δ As Symmetrical Electrode for Solid Oxide Cells.

The development of new redox stable electrode materials for Solid Oxide Cells (SOFs) have attracted great attention in recent years, since finding suitable compositions to operate efficiently in both fuel cell and electrolyzer modes is crucial for their wide commercialization. For this reason, research on new doping strategies and advanced deposition techniques for symmetrical electrode materials have significantly increased with the aim of simplifying cell fabrication and extending cell durability. In this sense, double perovskites are gaining great interest due to their excellent electrical properties and high redox and long-term stability in different atmospheres. In this study, the double-perovskite (PrBa)0.95Fe2O5+δ (PBF) is proposed as symmetrical electrode for SOCs. Two different strategies are implemented: on the one hand, Ni-doping in order to improve the electrical properties under reducing atmosphere and, on the other hand, the preparation of nanostructured electrodes in one-step by spray-pyrolysis deposition. A thorough characterization of the material is performed regarding its crystal structure, microstructure and electrochemical properties in both oxidizing and reducing atmospheres.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech