Ferric Iron Reduction in Extreme Acidophiles

Biochemical processes are a key element of natural cycles occurring in the environment and enabling life on earth. With regard to microbially catalyzed iron transformation, research predominantly has focused on iron oxidation in acidophiles, whereas iron reduction played a minor role. Microbial conversion of ferric to ferrous iron has however become more relevant in recent years. While there are several reviews on neutrophilic iron reducers, this article summarizes the research on extreme acidophilic iron reducers. After the first reports of dissimilatory iron reduction by acidophilic, chemolithoautotrophic Acidithiobacillus strains and heterotrophic Acidiphilium species, many other prokaryotes were shown to reduce iron as part of their metabolism. Still, little is known about the exact mechanisms of iron reduction in extreme acidophiles. Initially, hypotheses and postulations for the occurring mechanisms relied on observations of growth behavior or predictions based on the genome. By comparing genomes of well-studied neutrophilic with acidophilic iron reducers (e.g., Ferroglobus placidus and Sulfolobus spp.), it became clear that the electron transport for iron reduction proceeds differently in acidophiles. Moreover, transcriptomic investigations indicated an enzymatically-mediated process in Acidithiobacillus ferrooxidans using respiratory chain components of the iron oxidation in reverse. Depending on the strain of At. ferrooxidans, further mechanisms were postulated, e.g., indirect iron reduction by hydrogen sulfide, which may form by disproportionation of elemental sulfur. Alternative scenarios include Hip, a high potential iron-sulfur protein, and further cytochromes. Apart from the anaerobic iron reduction mechanisms, sulfur-oxidizing acidithiobacilli have been shown to mediate iron reduction at low pH (< 1.3) under aerobic conditions. This presumably non-enzymatic process may be attributed to intermediates formed during sulfur/tetrathionate and/or hydrogen oxidation and has already been successfully applied for the reductive bioleaching of laterites. The aim of this review is to provide an up-to-date overview on ferric iron reduction by acidophiles. The importance of this process in anaerobic habitats will be demonstrated as well as its potential for application.

Exsolution in La and Ni co-doped strontium titanate: a suitable anode for running SOFCs on ammonia as alternative fuel

Ammonia exhibits interesting features as fuel to feed Solid Oxide Fuel Cell. Herein, Ni and La co-doped strontium titanate was synthetized using wet chemistry route. Ni nanoparticles emerged via exsolution in reducing environment to decorate the surface. X-Ray Diffraction measurements exhibits perovskite structure was also preserved after the exsolution, as expected. H2 – Temperature Programmed Reduction highlights the great resistance of titanates in anode operation condition. Ammonia conversion in nitrogen and hydrogen were investigated by catalytic tests. It begins to decompose at 560°C and the full yield was achieved at 720°C. Electrochemical measurements were recorded at 800°C using 10% of ammonia in Ar. They were analysed though the model of equivalent circuit and two processes were attributed. Results certify Ni exsolution strongly enhances the hydrogen oxidation and the total polarisation resistance in ammonia approaches to the one in hydrogen.

PEM Single Cells under Differential Conditions: Full Factorial Parameterization of the ORR and HOR Kinetics and Loss Analysis

The anode and cathode kinetics are parameterized based on differential cell measurements. Systematic parameter variations are evaluated to disentangle the dependencies of the electrochemical impedance spectroscopy (EIS) signatures in H2/H2 mode. We introduce a new CO recovery protocol for both electrodes that enables to accurately characterize the hydrogen oxidation reaction (HOR) kinetics. Then, we demonstrate that a compact Tafel kinetics law captures the oxygen reduction reaction (ORR) kinetics for a full factorial grid of conditions, covering a wide range of relative humidities (rH), temperatures, oxygen partial pressures and current densities. This yields the characteristic activation energy and effective reaction order, and we reconcile models that make different assumptions regarding the rH dependency. Moreover, we analyze O2 transport contributions by steady-state and transient limiting current techniques and heliox measurements. Although the rising uncertainty of loss corrections at high current densities makes it impossible to unambiguously identify an intrinsic potential-dependent change of the Tafel slope, our data support that such effect needs not be considered for steady-state cathodic half-cell potentials above 0.8 V.

Grain-boundary-rich Pt nanoparticle assembly for catalytic hydrogen sensing at room temperature

We report a facile method of synthesizing grain-boundary(GB)-rich platinum nanoparticle assembly. GBs are formed between platinum nanoparticles during their random collision and attachment in solution driven by water electrolysis. The GB-rich nanoparticle assembly exhibits ~400-fold higher catalytic hydrogen oxidation rate than platinum nanoparticles before assembly, enabling catalytic hydrogen sensing at room temperature without external heating. Our sensor also demonstrates fast response/recovery (~7 s at >1% H2), nearly no signal variation during a 280-hour-long stability test, and high selectivity toward hydrogen over 36 interference gases. Furthermore, this sensor can be easily fabricated from commercial thermometers at a low cost (< $5 per unit). Theoretical calculation results reveal that the high performance of GB-rich platinum nanoparticle assembly arises from tensile strain at the GBs.