The present critical review aims to portray the principles and theoretical foundations that have been used for the application of electrochemical impedance spectroscopy (EIS) to study electron-transfer mechanisms, mass transfer phenomena and distribution of the heterogeneous properties of microbial electrochemical systems (MXCs). Over the past eight years, the application of this method has allowed major breakthroughs, especially in the field of microbial fuel cells (MFCs); however, it is still most widely extended only to the calculation of internal resistances. The use and interpretation of EIS should greatly improve since the intrinsic knowledge of this field, and efforts and current trends in this field have already allowed its understanding based on rather meaningful physical properties and not only on fitting electrical analogues. From this perspective, the use, analysis and interpretation of EIS applied to the study of MXCs are critically examined. Together with the revision of more than 150 articles directly devoted to this topic, two examples of the correct and improved analysis of EIS data are extensively presented. The first one focuses on the use of graphical methods for improving EIS analysis and the other one concentrates on the elucidation of the constant phase element (CPE) parameters. CPEs have been introduced in equivalent circuit models, sometimes without solid justification or analysis; the effective capacitance has been obtained from CPE parameters, following an unsuitable theory for the case of microbial–electrochemical interfaces. The use of CPE is reviewed in terms of meaningful physical parameters, such as biofilm thickness. The use of a finite-diffusion element is reviewed throughout estimation of accurate values for obtaining the dimensionless numbers, Schmidt and Sherwood, in the context of a dioxygen-reducing-biocathode, under different flow-rate conditions. The use and analysis of EIS in this context are still emerging, but because of the promising potential of MXCs in renewable power generation, wastewater treatment and energy-positive biorefining, among other applications, it becomes necessary to boost our global capacities for the application of EIS—and especially its interpretation—so that we achieve a better understanding and optimization of these systems.

Sulfate-reducing bacteria (SRB) developed biocathodes efficient for reduction of acetic and butyric acids to alcohols and acetone via direct electron transfer reaching current densities of 160–210 A m⁻².

The electrode material is one of the key components of a bioelectrochemical system (BES), serving as substratum for the electroactive biofilm development and hence playing a pivotal role in regulating the type and rate of electron transfer processes and bioelectrochemical conversions. Microbial electrosynthesis (MES) requires biocompatible electrode materials with a large surface area, that can support the effective development of microbial biomass at high current densities to provide significant product titers. Carbon-based materials are the most employed and constitute the best available option to grow electroactive biofilms; yet, resolving which of their properties are truly relevant to achieve these goals remains elusive. The present study shows that biofilm coverage and the relative abundance of cell-bound polymeric filaments are directly proportional to the total charge consumed over time. The combination of high biofilm coverage and high relative abundance of the cell-bound polymeric filaments resulted in low charge transfer resistances, as determined by electrochemical impedance spectroscopy. Although a wide variety of physicochemical parameters of the supporting carbon electrode materials (electric conductivity, specific surface area, porosity, roughness, thermogravimetric mass spectrometry, etc.) were characterized in this study, the only one that showed a consistent correlation with the total charge consumed by the electroactive biofilms was the contact angle. This suggests that the hydrophilic moieties and surface tension are two fundamental parameters to consider for an effective design of microbial electrosynthesis biocathodes. Among the carbon electrode materials used in the present study, an activated carbon-based VITO CORE™ electrode was considered the most suitable electrode material to provide such desired features, while other electrodes exhibited roughnesses much higher than those amenable for the microbiological range. The superior wetting characteristics of the VITO CORE™ electrode seem highly reliant on the roughness provided by the manufacturing method (i.e., cold-rolling).

As a result of their unique physical, chemical and/or biological properties, the use of engineered nanoparticles (ENPs) is growing very rapidly. Iron oxide nanoparticles (IONPs) are of particular interest owing to their magnetic properties, and thus the development of suitable methods for their characterization is essential. Inductively coupled plasma-mass spectrometry (ICP-MS) operated in single-particle (SP) mode provides different types of relevant information, such as size distribution and particle number and mass concentrations. However, the use of SP-ICP-MS becomes less straightforward when the analyte signal is subject to spectral overlap. In the case of IONPs, characterization by means of SP-ICP-MS is hindered by the occurrence of ArO⁺ polyatomic ions with the same nominal mass-to-charge (m/z) ratio as the most abundant Fe isotope. In this work, different approaches relying on either chemical or physical (mass) resolution, to avoid this spectral interference otherwise jeopardizing accurate results, were assessed. In the case of chemical resolution, the performance of on-mass and mass-shift approaches was evaluated using different types of quadrupole-based ICP-MS instrumentation, including single-quadrupole (SQ) and tandem ICP-MS (ICP-MS/MS) units. Physical resolution was accomplished using a new generation of sector field (SF) ICP-MS instrumentation, capable of dealing with transient signals of extremely short duration (10–100 μs dwell time), even when operated at higher mass resolution (pseudo-resolution mode). Based on the figures-of-merit obtained for the different approaches evaluated, an on-mass approach using NH₃ as the reaction gas in SQ-ICP-MS, an on-mass approach using H₂ as the collision/reaction gas in ICP-MS/MS and pseudo-medium resolution in SF-ICP-MS were found to be the best-suited approaches for fast interference-free monitoring of the ion signals generated by IONPs in SP mode. While the use of chemical (H₂/on-mass) and physical (pseudo-medium) resolutions also provided accurate and precise results for custom-made Fe₃O₄ (magnetite) NPs of ≈50 nm, the use of NH₃ was less successful, as it leads to an extension of the SP pulse profile and a lower signal-to-background ratio. Finally, SF-ICP-MS operated in pseudo-medium resolution mode was used for the characterization of Fe₃O₄ NPs synthesized electrochemically (batch and continuous operation mode).

This study focuses on the effect of operational and physiochemical factors on a stable sulfate reducing bacteria biocathode and their effect on the electrochemical response thereof. Two carbon-based electrode materials i.e. VITO CORE™ and Paxitech® electrodes were evaluated. The electrochemical performance was assessed by changing both organic and inorganic components of the synthetic feed, which included carbon substrates (acetic and butyric acid), ionic strength (concentration of sodium chloride) and weak electrolytes acting as buffering agents (potassium dihydrogen phosphate and ammonium chloride). The concentration of sodium chloride (0.34 M) and acetate (0.1 M) plays a crucial role in determining the performance as well as the bioelectrochemical mechanism of the sulfate reducing bacteria (SRB) biocathode. Butyrate did not significantly influence the performance of the biocathode but showed a synergistic effect when given along with acetate. The developed biocathode was also successfully tested for dark fermentation based effluent treatment application. Concentrations higher than 10% of effluent were found to be detrimental to the electrochemical performance of the biocathode. This study essentially highlights the significance of optimization studies on bioelectrochemical system (BES) at lab scale before its potential upscaling for industrial wastewater processing.

The electrosynthesis of iron oxide nanoparticles offers a green route, with significant energy and environmental advantages. Yet, this is mostly restricted by the oxygen solubility in the electrolyte. Gas-diffusion electrodes (GDEs) can be used to overcome that limitation, but so far they not been explored for nanoparticle synthesis. Here, we develop a fast, environmentally-friendly, room temperature electrosynthesis route for iron oxide nanocrystals, which we term gas-diffusion electrocrystallization (GDEx). A GDE is used to generate oxidants and hydroxide in-situ, enabling the oxidative synthesis of a single iron salt (e.g., FeCl₂) into nanoparticles. Oxygen is reduced to reactive oxygen species, triggering the controlled oxidation of Fe²⁺ to Fe³⁺, forming Fe_3−xO_4−x (0 ≤ x ≤ 1). The stoichiometry and lattice parameter of the resulting oxides can be controlled and predictively modelled, resulting in highly-defective, strain-heavy nanoparticles. The size of the nanocrystals can be tuned from 5 nm to 20 nm, with a large saturation magnetization range (23 to 73 A m² kg⁻¹), as well as minimal coercivity (~1 kA m⁻¹). Using only air, NaCl, and FeCl₂, a biocompatible approach is achieved, besides a remarkable level of control over key parameters, with a view on minimizing the addition of chemicals for enhanced production and applications.