Alessandro Zana

On the structural composition and stability of Fe–N–C catalysts prepared by an intermediate acid leaching

The development of highly active and stable non-noble metal catalysts (NNMC) for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEM-FC) becomes of importance in order to enable cost reduction. In this work, we discuss the structural composition as derived from Fe-57 Mößbauer spectroscopy and X-ray diffraction, catalytic performance determined by a rotating (ring) disk electrode (RRDE) technique and stability evaluation of our Fe–N–C catalysts prepared by an intermediate acid leaching (IAL). The advantage of this IAL is given by a high density of active sites within the catalyst, as even without sulphur addition, an iron carbide formation and related disintegration of active sites are inhibited. In addition, our accelerated stress tests illustrate better stability of the sulphur-free IAL catalyst in comparison to the sulphur-added one.

Nanoparticles in a box: a concept to isolate, store and re-use colloidal surfactant-free precious metal nanoparticles

A concept is introduced that allows for the isolation, storage and re-use of surfactant-free precious metal nanoparticles (NPs) of catalytic relevance (Pt and Ru). “Surfactant-free NPs” well-defined in size (1–2 nm) are prepared in alkaline ethylene glycol. After synthesis these NPs are stabilized by surface bound CO, formed during synthesis by solvent oxidation, and OH−, added to the reaction mixture. We present a protocol that allows switching reversibly the stabilization between a “CO-protected” and “OH−-protected state”. Most importantly, “OH−-protected” Pt and Ru NPs exhibit remarkable resistance against sintering. These NPs can be isolated as solids, stored and “put into boxes” to be shipped. Thereafter they can be redispersed without changes in particle size or loss in catalytic activity. These results are expected to be of scientific and industrial relevance, as a methodology is introduced to handle “surfactant-free” catalytic nanoparticles like a normal solid chemical.

Benchmarking high surface area electrocatalysts in a gas diffusion electrode: measurement of oxygen reduction activities under realistic conditions

In this work, we introduce the application of gas diffusion electrodes (GDE) for benchmarking the electrocatalytic performance of high surface area fuel cell catalysts. It is demonstrated that GDEs offer several inherent advantages over the state-of-the-art technique, i.e. thin film rotating disk electrode (TF-RDE) measurements for fast fuel cell catalyst evaluation. The most critical advantage is reactant mass transport. While in RDE measurements the reactant mass transport is severely limited by the gas solubility of the reactant in the electrolyte, GDEs enable reactant transport rates similar to technical fuel cell devices. Hence, in contrast to TF-RDE measurements, performance data obtained from GDE measurements can be directly compared to membrane electrode assembly (MEA) tests. Therefore, the application of GDEs for the testing of fuel cell catalysts closes the gap between catalyst research in academia and real applications.

Accessing the Inaccessible: Analyzing the Oxygen Reduction Reaction in the Diffusion Limit

The oxygen reduction reaction (ORR) is one of the key processes in electrocatalysis. In this communication, the ORR is studied using a rotating disk electrode (RDE). In conventional work, this method limits the potential region where kinetic (mass transport free) reaction rates can be determined to a narrow range. Here, we applied a new approach, which allows us to analyze the ORR rates in the diffusion-limited potential region of high mass transport. Thus, for the first time, the effect of anion adsorption on the ORR can be studied at such potentials.

Levich Analysis and the Apparent Potential Dependency of the Levich B Factor

ABSTRACT Levich analysis is a frequently used method for extraction of diffusional properties of electrochemical active species. The analysis is based on plotting current versus the square root of the rotation rate of a rotating disc electrode. From the slope of this plot, the Levich B factor and the diffusional parameters may be obtained. In this article, we focus on issues related to the extraction of the B factor from experimental measurements. We show that the plot does not fully linearize the data: a prerequisite for the slope extraction. Furthermore, analyzing data at various potentials results in varying values of the B factor, contrary to predictions by theory. Thus, as an alternative, we introduce a ω½-normalized Levich plot. This plot linearized the data and the resulting B factor was potential-independent. The difference between the two methods is ascribed to the consideration that the electrolyte–electrode interface current may be limited. We conclude that the potential-dependent B factor based on the standard Levich analysis is an artifact and the resulting values are systematically underestimated in comparison to results from the ω½-normalized Levich analysis.