Fe-based catalytic sites for the reduction of oxygen in acidic medium have been identified by 57Fe Mössbauer spectroscopy of Fe/N/C catalysts containing 0.03 to 1.55 wt% Fe, which were prepared by impregnation of iron acetate on carbon black followed by heat-treatment in NH3 at 950°C. Four different Fe-species were detected at all iron concentrations: three doublets assigned to molecular FeN4-like sites with their ferrous ion in low (D1), medium (D2) or high spin state (D3), and two other doublets assigned to a single Fe-species (D4 and D5) consisting of surface oxidized nitride nanoparticles (FexN, with x≤2.1). A fifth Fe-species appears only in those catalysts with Fe-contents ≥ 0.27 wt%. It is characterized by a very broad singlet, which has been assigned to incomplete FeN4-like sites that quickly dissolve in contact with an acid. Among the five Fe-species identified in these catalysts, only D1 and D3 display catalytic activity for the oxygen reduction reaction (ORR) in the acid medium, with D3 featuring a composite structure with a protonated neighbour basic nitrogen and being by far the most active species, with an estimated turn over frequency for the ORR of 11.4 e− site−1 s−1 at 0.8V vs RHE. Moreover, all D1 sites and between 1/2 to 2/3 of the D3 sites are acid-resistant. A scheme for the mechanism of site formation upon heat-treatment is also proposed. This identification of the ORR-active sites in these catalysts is of crucial importance to design strategies to improve the catalytic activity and stability of these materials.

The high cost of proton-exchange-membrane fuel cells would be considerably reduced if platinumbased catalysts were replaced by iron-based substitutes, which have recently demonstrated comparable activity for oxygen reduction, but whose cause of activity decay in acidic medium has been elusive. Here, we reveal that the activity of Fe/N/C-catalysts prepared through a pyrolysis in NH3 is mostly imparted by acid-resistant FeN4-sites whose turnover frequency for the O2 reduction can be regulated by fine chemical changes of the catalyst surface. We show that surface N-groups protonate at pH 1 and subsequently bind anions. This results in decreased activity for the O2 reduction. The anions can be removed chemically or thermally, which restores the activity of acid-resistant FeN4-sites. These results are interpreted as an increased turnover frequency of FeN4-sites when specific surface N-groups protonate. These unprecedented findings provide new perspective for stabilizing the most active Fe/N/C-catalysts known to date.

With the ability to engineer ferroelectricity in HfO2 thin films, manufacturable and highly scaled MFM capacitors and MFIS-FETs can be implemented into a CMOS-environment. NVM properties of the resulting devices are discussed and contrasted to existing perovskite based FRAM

Epitaxial growth of SrTiO₃ on silicon by molecular beam epitaxy has opened up the route to the integration of functional complex oxides on a silicon platform. Chief among them is ferroelectric functionality using perovskite oxides such as BaTiO₃. However, it has remained a challenge to achieve ferroelectricity in epitaxial BaTiO₃ films with a polarization pointing perpendicular to the silicon substrate without a conducting bottom electrode. Here, we demonstrate ferroelectricity in such stacks. Synchrotron X-ray diffraction and high-resolution scanning transmission electron microscopy reveal the presence of crystalline domains with the long axis of the tetragonal structure oriented perpendicular to the substrate. Using piezoforce microscopy, polar domains can be written and read and are reversibly switched with a phase change of 180°. Open, saturated hysteresis loops are recorded. Thus, ferroelectric switching of 8- to 40-nm-thick BaTiO₃ films in metal-ferroelectric-semiconductor structures is realized, and field-effect devices using this epitaxial oxide stack can be envisaged.

In situ X-ray absorption spectroscopy along with electrochemical measurements (CV and RDE) and previously published EQCN data provide further understanding of the nature of chloride poisoning on different faces/ sites of carbon supported platinum clusters (1-2 nm) in acidic medium (HClO 4 ). Chloride is shown to adsorb in 3-fold sites on the Pt(111) faces at the investigated Clconcentrations (10 -3 and 10 -2 M). Atop chloride was found to be present within a narrow potential range (0.4-0.7 V RHE) when compressed adlayers of Clare formed on the Pt(111) faces forcing some Clto exist in atop/bridged sites. The interplay of anionic (Cl -, Br -, OH -, and HSO 4 -) adsorption on the different surfaces of Pt are also considered. For example O/OH can easily displace atop chloride on the edges/corners but not the Clat the Pt(111) sites, and therefore Cldramatically raises the overpotential for water activation at the Pt(111) sites. Chloride also drastically alters the ORR causing an increase of the overpotential by ∼85 mV for every 10-fold increase in chloride concentration with a total 150-200 mV increase in the overpotential at large concentrations at the Pt(111) sites. Finally Clions cannot displace the bisulfate overlayer on the Pt(111) faces after it is formed at lower potentials; however, once the bisulfate adsorption is disturbed at higher potentials, the bisulfate cannot displace the Cladsorption. These relative anion adsorption preferences can help to explain the different dependencies of the important ORR on anion adsorption, and suggests that the effect of Clpoisoning might be quite dependent on the Pt particle size.

Static domain structures and polarization dynamics of silicon doped HfO2 are explored. The evolution of ferroelectricity as a function of Si-doping level driving the transition from paraelectricity via ferroelectricity to antiferroelectricity is investigated. Ferroelectric and antiferroelectric properties can be observed locally on the pristine, poled and electroded surfaces, providing conclusive evidence to intrinsic ferroic behavior.

A scanning probe microscopy approach for mapping local irreversible electrochemical processes based on detection of bias-induced frequency shifts of cantilevers in contact with the electrochemically active surface is demonstrated. Using Li ion conductive glass ceramic as a model, we demonstrate near unity transference numbers for ionic transport and establish detection limits for current-based and strain-based detection. The tip-induced electrochemical process is shown to be a first-order transformation and nucleation potential is close to the Li-metal reduction potential. Spatial variability of the nucleation bias is explored and linked to the local phase composition. These studies both provide insight into nanoscale ionic phenomena in practical Li-ion electrolyte and also open pathways for probing irreversible electrochemical, bias-induced, and thermal transformations in nanoscale systems.

Local electrochemical phenomena on the surfaces of the LaAlO(3)-SrTiO(3) heterostructure are explored using unipolar and bipolar dynamic electrochemical strain microscopy (D-ESM). The D-ESM suggests the presence of at least two distinct electrochemical processes, including fast reversible low-voltage process and slow high-voltage process. The latter process is associated with static surface deformations in the sub-nanometer regime. These behaviors are compared with Kelvin probe force microscopy hysteresis data. The possible origins of observed phenomena are discussed, and these studies suggest that charge-writing behavior in LAO-STO includes a strong surface/bulk electrochemical component and is more complicated than simple screening by surface adsorbates.