Multimethod Approach to the Low-Overpotential Region of Micro- to Macro-Scale Working Electrodes of Sub-10 nm Gold Nanoparticles in the CO2 Reduction Reaction.

The electrochemical carbon dioxide reduction reaction (CO2RR) over carbon-supported gold nanoparticles (AuNP) was investigated using a broad variety of (electro)analytical methods, including linear sweep voltammetry with a rotating disk electrode (LSV-RDE), sample-generation tip-collection mode of scanning electrochemical microscopy (SG/TC-SECM), as well as full cell tests with highly sensitive online gas chromatography (GC). In contrast to most other studies, this work focuses on the low-overpotential region (0 to -0.4 V vs RHE) where initial product formation is already detected and addresses micro- to macro-sized electrodes. The sub-10 nm AuNPs supported on three different carbon supports (CNTs and carbon blacks) were pretreated in H2/Ar to remove the stabilizer used during AuNP synthesis. LSV-RDE points toward different CO2RR mechanisms at the samples, additionally confirmed by the SG/TC-SECM and full cell tests with online GC. Besides H2 and CO, the AuNP supported on carbon nanotubes showed significant evolution of H2CO in contrast to the other two samples, which was additionally confirmed by accumulating the product during chronoamperometric RDE experiments followed by mass spectroscopic analysis. Surface analysis indicated a complete removal of residual thiolate stabilizer molecules exclusively at the AuNPs supported on carbon nanotubes, which may result in a change in the adsorption geometry or reaction mechanism at this sample. The results demonstrate the effectiveness of the combination of these multiple methods to investigate the CO2RR in the low-overpotential region.

Characterization of sulfur/carbon copolymer cathodes for Li-S batteries: a combined experimental and ab initio Raman spectroscopy study.

Optimization of lithium-sulfur batteries highly depends on exploring and characterizing new cathode materials. Sulfur/carbon copolymers have recently attracted much attention as an alternative class of cathodes to replace crystalline sulfur. In particular, poly(sulfur-n-1,3-diisopropenylbenzene) (S/DIB) has been under considerable experimental and theoretical investigations, promising a good performance in mitigating the so-called shuttle effect. Here, combining ab initio Raman spectroscopy simulations with experimental measurements, we show that S/DIB copolymers containing short and long sulfur chains are distinguishable based on their Raman activity in 400-500 cm-1. This frequency range corresponds to S-S stretching vibrations and is only observed in the Raman spectra of those copolymers with longer sulfur chains. The results reported in this study have direct applications in identification and characterization of general sulfur/carbon copolymers with different sulfur contents.

Quantifying the removal of stabilizing thiolates from gold nanoparticles on different carbon supports and the effect on their electrochemical properties.

Gold nanoparticles <10 nm in size are typically prepared using stabilizing agents, e.g. thiolates. Often standard recipes from literature are used to presumably remove these stabilisers to liberate the surface, e.g. for catalytic or electrocatalytic applications, however the success of these procedures is often not verified. In this work, thiolate-stabilised AuNPs of ca. 2 nm in size were synthesized and supported onto three different carbon supports, resulting in loadings from 15 to 25 wt% Au. These materials were post treated using three different methods in varying gas atmospheres to remove the stabilizing agent and to liberate the surface for electrochemical applications. Using thermogravimetry - mass spectroscopy (TG-MS), the amount of removed stabilizer was determined to be up to 95%. Identical location scanning transmission electron microscopy (il-(S)TEM) measurments revealed moderate particle growth but a stable support during the treatments, the latter was also confirmed by Raman spectroscopy. All treatments significantly improved the electrochemically accessible gold surface. In general, the results presented here point out the importance of quantitatively verifying the success of any catalyst post treatment with the aim of stabilizer removal.

Anomalous circular bulk photovoltaic effect in BiFeO3 thin films with stripe-domain pattern.

Multiferroic bismuth ferrite, BiFeO3, offers a vast landscape to study the interplay between different ferrroic orders. Another aspect which is equally exciting, and yet underutilized, is the possibility of large-scale ordering of domains. Along with symmetry-driven bulk photovoltaic effect, BiFeO3 presents opportunities to conceptualize novel light-based devices. In this work, we investigate the evolution of the bulk photovoltaic effect in BiFeO3 thin films with stripe-domain pattern as the polarization of light is modulated from linear to elliptical to circular. The open-circuit voltages under circularly polarized light exceed ± 25 V. The anomalous character of the effect arises from the contradiction with the analytical assessment involving tensorial analysis. The assessment highlights the need for a domain-specific interaction of light which is further analyzed with spatially-resolved Raman measurements. Appropriate positioning of electrodes allows observation of a switch-like photovoltaic effect, i.e., ON and OFF state, by changing the helicity of circularly polarized light.

In Situ Characterization of Ni and Ni/Fe Thin Film Electrodes for Oxygen Evolution in Alkaline Media by a Raman-Coupled Scanning Electrochemical Microscope Setup.

We present a spectroelectrochemical setup, in which Raman microscopy is combined with scanning electrochemical microscopy (SECM) in order to provide both spectroscopic and electrochemical information on the very same location of an electrode at the same time. The setup is applied to a subject of high academic and practical interest, namely, the oxygen evolution reaction at Ni and Ni/Fe electrodes. It comprises a transparent substrate electrode, onto which Ni and Ni/Fe thin films are deposited. An ultramicroelectrode (UME) is placed closely above the substrate to obtain electrochemical information, while a Raman microscope probes the same sample spot from below. To obtain information on oxygen evolution activity and structural changes, increasingly positive potentials from 0.1 up to 0.7 V vs Hg|HgO|1 M KOH were applied to the Ni/Fe-electrodes in 0.1 M KOH solution. Evolved oxygen is detected by reduction at a Pt UME, allowing for the determination of onset potentials, while the substrate current, which is recorded in parallel, is due to both overlapping oxygen evolution and the oxidation of Ni(OH)2 to NiOOH. An optimum of 15% Fe in Ni/Fe films with respect to oxygen evolution activity was determined. At the same time, the potential-dependent formation of γ-NiOOH characterized by the Raman double band at 475 and 557 cm-1 allows for the conclusion that a certain amount of disorder introduced by Fe atoms is necessary to obtain high oxygen evolution reaction (OER) activity.