Stefano P. Trasatti

Parameters characterization and optimization of activated carbon (AC) cathodes for microbial fuel cell application.

Activated carbon (AC) is employed as a cost-effective catalyst for cathodic oxygen reduction in microbial fuel cells (MFC). The fabrication protocols of AC-based cathodes are conducted at different applied pressures (175-3500 psi) and treatment temperatures (25-343°C). The effects of those parameters along with changes in the surface morphology and chemistry on the cathode performances are comprehensively examined. The cathodes are tested in a three-electrode setup and explored in single chamber membraneless MFCs (SCMFCs). The results show that the best performance of the AC-based cathode is achieved when a pressure of 1400 psi is applied followed by heat treatment of 150-200°C for 1h. The influence of the applied pressure and the temperature of the heat treatment on the electrodes and SCMFCs is demonstrated as the result of the variation in the transfer resistance, the surface morphology and surface chemistry of the AC-based cathodes tested.

Electrochemistry and environment: the role of electrocatalysis

Abstract After an introductory discussion of the conceptual meaning of electrocatalysis and its technological significance, the impact of electrochemistry on the environment is illustrated with two major examples: Hg cells in chlor-alkali technology and electric cars. The problems of fossil fuel exhaustion and pollution call for a resort to renewable energy sources. Electrochemistry can help to alleviate environmental issues with appropriate technological strategies involving processes and products. In this context, research on electrocatalysis has been focused on a number of major targets which are discussed and illustrated by means of examples: intermittent water electrolysis; destruction of organic pollutants; ozone, hydrogen peroxide and chlorine dioxide electrosynthesis; cathodic conversion of CO 2 ; anodic oxidation of SO 2 ; cathodic destruction of NO x ; replacement of chromates; desulfurization of natural gas, etc.

Hydrogen permeation into aluminium AA1060 as a result of corrosion in an alkaline medium. Influence of anions in solution and of temperature

Aluminium AA1060 corroding in 0.01 mol dm−3 KOH solution is permeated by hydrogen whose diffusion can be monitored by recording the hydrogen ionization current on the other face of the metal sample. The effect of small concentrations of nitrite, sulphide and cyanide on the corrosion rate of Al has been investigated using such an experimental arrangement. Results have shown that corrosion is enhanced in the sequence OH− < NO2− < S2− < CN− as a consequence of double layer effects on hydrogen discharge operated by specifically adsorbed anions. The influence of temperature has also been investigated. The apparent activation energy of the steady-state permeation current is shown to result from contributions of (i) hydrogen diffusivity, (ii) corrosion process and (iii) electrocatalysis. The advantage of monitoring corrosion processes in real time by recording the hydrogen permeation current is stressed.

PTFE effect on the electrocatalysis of the oxygen reduction reaction in membraneless microbial fuel cells.

Influence of PTFE in the external Gas Diffusion Layer (GDL) of open-air cathodes applied to membraneless microbial fuel cells (MFCs) is investigated in this work. Electrochemical measurements on cathodes with different PTFE contents (200%, 100%, 80% and 60%) were carried out to characterize cathodic oxygen reduction reaction, to study the reaction kinetics. It is demonstrated that ORR is not under diffusion-limiting conditions in the tested systems. Based on cyclic voltammetry, an increase of the cathodic electrochemical active area took place with the decrease of PTFE content. This was not directly related to MFC productivity, but to the cathode wettability and the biocathode development. Low electrodic interface resistances (from 1 to 1.5 Ω at the start, to near 0.1 Ω at day 61) indicated a negligible ohmic drop. A decrease of the Tafel slopes from 120 to 80 mV during productive periods of MFCs followed the biological activity in the whole MFC system. A high PTFE content in the cathode showed a detrimental effect on the MFC productivity, acting as an inhibitor of ORR electrocatalysis in the triple contact zone.

Evaluation of material properties and design requirements for biodegradable magnesium stents

Magnesium represents a very attractive material for biodegradable stents since the process of its natural and gradual dissolution into the human body by a corrosion process would prevent restenosis risks and would allow the progressive transmission of the mechanical load to the surrounding tissues after several months of service. The objective of the present work is to develop a frame of mechanical and microstructural data about several commercially available Mg alloys in view of their use for biodegradable stents. The AZ31, AZ61, AZ80, ZM21, ZK61 and WE43 alloys in the form of extruded bars were thus investigated to compare their mechanical properties and corrosion resistance. Further high-temperature characterization was carried out by compression tests at high temperature (temperature range: 260-450°C, strain rate range: 5•10-4 ÷ 3•10-2 s-1) in order to assess the optimal processing window for stent precursors manufacturing (small tubes 1÷2 mm in diameter) by hot extrusion. The experimental results made available by this investigation were adopted to support the development of a finite element (FE) framework combining a shape optimization procedure and a detailed model for Mg alloy mechanical and corrosion damage behavior.

Single-chamber microbial fuel cells as on-line shock-sensors for volatile fatty acids in anaerobic digesters

In anaerobic digesters (AD), volatile fatty acids (VFAs) concentration is a critical operative parameter, which is usually manually monitored to prevent inhibition of microbial consortia. An on-line VFAs monitoring system as early-warning for increasing concentrations would be of great help for operators. Here, air-cathode membraneless microbial fuel cells (MFCs) were investigated as potential biosensors, whose electrical signal instantaneously moves from its steady value with the accumulation of VFAs in the anodic solution. MFCs were operated equipping four lab-scale ADs with carbon-based electrodes. Reactors were filled with the digestate from a full-scale AD and fed in batch with four kinds of feedstock (cheese whey, kitchen waste, citrus pulp and fishery waste). The MFC signal initially increased in parallel to VFAs production, then tended to a steady value for VFAs concentrations above 1000mgAcL-1. Peak concentrations of tVFAs (2500-4500mgAcL-1) and MFCs potentials were negatively correlated (r=0.916, p<0.05), regardless of the type of substrate. Inhibition of the MFC system occurred when VFAs increased fast above 4000mgAcL-1. Polarization curves of electrodes stressed that electroactive bacteria on bioanodes were strongly subjected to inhibition. The inhibition of electroactivity on bioanode trended like typical shock-sensors, opening to direct application as early-warning monitoring system in full-scale ADs.

Hybrid Coatings Based on Conducting Polymers and Polysiloxane Chains for Corrosion Protection of Al Alloys

It was previously demonstrated that the use of a pyrrole-based silane (PySi) for surface treatment of Al alloys provides both active and barrier protection due to the deposition of a hybrid coating, containing polypyrrole and polysiloxane chains. To further explore these features, a wider range of Al substrates and different silane-based formulations in terms of silane molecule, solvent nature, water amount and pH, were investigated. Also, some tests were carried out by using aniline-based silane (AniSi). Structural/morphological characterization of the coatings, as well as the investigation of PySi solutions by diverse spectroscopic techniques, in addition to corrosion tests in NaCl, strongly support the very promising protection performance of the hybrid film. This is indicated as well from the preliminary results obtained with the AniSi-based approach. Thus, typical silane-based treatments with principally barrier action can gain in active properties if the silane compound contains monomers of conducting polymers as a funtional group.

Detection of sigma phase in 22% Cr duplex stainless steel by electrochemical methods

Duplex stainless steels (DSS) may undergo various structural transformations as a function of temperature, leading to formation of secondary phases when exposed to temperatures in the range 600–1000°C for prolonged periods of time. Among these phases, apart from carbides and nitrides, sigma is the most prominent intermetallic compound, since it is known to adversely affect mechanical properties and corrosion resistance of DSS as a consequence of Cr and Mo depletion in the boundary zones of ferrite grains. As a result of sigma precipitation, DSS become susceptible to localized corrosion via a mechanism resembling sensitization in austenitic stainless steels. Electrochemical methods, known to successfully detect sensitization in austenitic SS, have been used to identify reliable test conditions with appropriate sensitivity also for DSS. A modified electrochemical potentiokinetic reactivation method was developed to investigate 22% Cr DSS sensitivity under different thermal cycles. Modifications were designed by closely analysing DSS anodic behaviour changes with respect to the operating testing conditions. All findings were confirmed by SEM, optical microscopy and XRD.

Electrochemical approach to repassivation kinetics of Al alloys: gaining insight into environmentally assisted cracking

Abstract This work provides a broad overview of the systematic experimental studies conducted in our group to understand the factors governing the transition to a more occluded corrosion front during the repassivation of Al alloys, manifested by the appearance of an inflection during anodic polarization into the active region in NaCl solutions. The collected data and the derived empirical relationships enabled us to discern the thermodynamic and kinetic aspects controlling the transition to more occluded local damage. From the thermodynamic standpoint, the potential at the inflection, namely, the pit transition potential Eptp, can be considered as the thermodynamic driving force of Al dissolution in the acidified pit-like solution in contact with a freshly created surface. The associated current density iptp and the steepness of the potential decrease with current below Eptp are determined by interfacial electrochemical kinetics. More importantly, all these properties are influenced by electrochemical and metallurgical events, including residual stresses. These findings point to the electrochemical approach in combination with external mechanical load as a promising tool for investigating environmentally assisted cracking, in particular, crack nucleation and non-steady crack tip processes.

Anodic films containing zirconia nanoparticles for corrosion protection of AA1050 aluminum alloy

In this paper, production of anodic aluminum oxide (AAO) is based on a parameter-optimized literature scheme. Highly ordered tubular structures are achieved as a starting point for subsequent modification steps. These steps include sealing of AAO pores. Sealing is here attempted via the spontaneous adsorption of ZrO2 nanoparticles on preformed AAO. Nanoparticles are synthetized by several different methods, in particular, the innovative water-free microwave-assisted synthesis. These novel water-free nanoparticles are, here for the first time, used for AAO sealing and corrosion protection. Increase in corrosion resistance is tested by electrochemical methods.