Rodrigo Parra

New ceramic electrodes allow reaching the target current density in bioelectrochemical systems

Whereas most of the studies conducted nowadays to boost electrode performance in bioelectrochemical systems (BES) are focused on carbonaceous scaffolds, in this study we demonstrate that ice-templated titanium-based ceramics (ITTC) can provide a new alternative for this purpose. We combined the chemistry of titanium suboxides (Ti4O7) with an ice-templating technique (ISISA) to produce electrically conducting and highly porous (88% porosity) 3D architectures. The ITTC platforms were characterized by strongly aligned macrochannels that provided a direct path for substrate supply under a flow-through configuration, while supporting the growth of electroactive Geobacter sulfurreducens biofilms. This new electrode material is demonstrated to outperform graphite when used as an anode in bioelectrochemical reactors, providing volumetric current densities of 9500 A m−3, equating to projected current densities of 128.7 A m−2 and maximum power densities of 1.9 kW m−3. The performance of the ITTC scaffolds levels that of any of the available materials on the current state of research. The presented alternative may lead to the start of a branch into the exploration of conducting ITTC materials in the growing area of bioelectrochemical technologies.

2D-ice templated titanium oxide films as advanced conducting platforms for electrical stimulation

Directional freezing has been widely employed to prepare highly ordered three-dimensional (3D) porous assemblies. However, in this scenario, there is one concept that has not been extensively explored: by applying directional freezing to a nanoparticle (NP) dispersion supported on a substrate, two-dimensionally (2D) patterned films may be produced. In this study, tunable 2D-patterning of TiO2-NP dispersions on alumina substrates is demonstrated. By imposing different temperature gradients throughout the ceramic dispersion coatings, both homogeneous (non-patterned) and highly aligned patterned topologies (consisting of parallel grooves) were produced. In the case of patterned films, the orientation of the grooves was modulated from those oriented along the freezing direction to those perpendicularly oriented to the temperature gradient. Thermally induced reduction of the prepared films led to electrically conducting titanium oxide Magneli phases. The measured resistances were strongly dependent on the orientation of the aligned patterns. To demonstrate the possibility of employing these structured films as platforms for electrical stimulation-related applications, a stimulating electronic circuit was developed and connected to the prepared films. Charge-balanced biphasic stimulus pulses with tunable current amplitudes and frequencies were successfully delivered through the conducting 2D-patterned assemblies.

Alizarin complexone: an interesting ligand for designing TiO2-hybrid nanostructures

Herein we report the characterization and photoelectrochemical behavior of nanocrystalline TiO2 films modified with some alizarin derivatives. Particularly, alizarin complexone (3-[N,N-bis(carboxymethyl)-aminomethyl]-1,2-dihydroxyanthraquinone) and its Fe(II) and Fe(III) metal complexes have been examined. Based on UV-Visible and Raman spectroscopic studies we proposed that AC forms a tridentate complex with titanium dioxide involving the 2-OH and (methylimino) diacetate groups, while Fe(II) and Fe(III) AC-complexes bind to titanium dioxide through the carbonyl (CO) and hydroxyl (OH) groups at the 1,9 positions of the alizarin framework. These findings allow the design of different nanostructures with diverse photoelectrochemical behavior. Analysis of the j–V curves reveals a sign reversal at −90 mV and −100 mV upon the excitation of AC@TiO2 and [FeIIIAC]@TiO2 photoelectrodes, respectively, whereas [FeIIAC]@TiO2 displays only anodic photocurrents. A description involving both electronic and geometric factors to account for these differences is proposed.

Métodos de medición de espesores de películas delgadas basadas en óxidos semiconductores

Transparent films based on Ti, Sn and Zn oxides are of great importance in electronic devices such as sensors, solar cells and conductive films, then the characterization techniques are highly relevant. The aim of this work is to identify the advantages and disadvantages of direct methods, such as profilometry, and indirect methods such as ellipsometry and spectrophotometry used to quantify film thickness. In this work, films were deposited by spray-pyrolysis on glass substrates at 425±C. Thicknesses varied between 150 and 300 nm. Thicknesses calculated by means of spectrophotometry and ellipsometry, led to differences below 10% and 20 %, respectively, with respect to the value measured by profilometry.

Layer-to-layer distance determines the performance of 3D bio-electrochemical lamellar anodes in microbial energy transduction processes

Microbial fuel cells (MFCs) harness the metabolic machinery of electro-active bacteria to transfer electrons from organic molecules to polarized anodes. In this context, increasingly higher anode surface areas have been pursued for maximizing MFC performance. In this study we prepared 3D layered Ti4O7 electrodes with different interlayer spacings (from 10 to 100 μm) but maintaining the same total void fraction (90%), so as to modify the electrode surface-to-volume ratios. This allowed us to test the hypothesis that there must be a limit in surface area per unit volume restricting the efficiency of 3D porous bio-electrochemical anodes. The lamellar scaffolds were evaluated in three-electrode cells cultured with G. sulfurreducens. Regardless of the electrode interlayer spacing or the biofilm developmental stage, the electron transfer rate was constant (0.11 pA per bacterium), with current scaling linearly with the size of the microbial population. However, maximum volumetric current densities (20 ± 0.8 kA m−3) were not obtained from electrodes with maximum surface-to-volume ratios (shorter interlayer distances), because bacterial biomass was not directly proportional to the surface area. This demonstrated that, by controlling the spacing between layers, it is possible to modulate the amount of bacteria per electrode unit volume, this ratio determining the final electrode performance. The limit obtained in surface area suggested that other effects, such as fluid dynamic constraints inside the “slit-shaped” pores, must be playing a critical role in anode performance.