Teko W. Napporn

Electrocatalytic reduction of dioxygen at platinum particles dispersed in a polyaniline film

Platinum based electrocatalysts for the oxygen reduction reaction were prepared by electrodeposition of Pt particles in a polyaniline film acting as a convenient matrix to achieve low Pt loadings (from 11 to 600 μg cm−2). Using a rotating disc electrode or a rotating ring disc electrode allowed us to separate the different contributions in the overall reaction : diffusion of molecular oxygen in the electrolyte solution, diffusion inside the PAni film, adsorption process, and electron transfer. Koutecky–Levich analysis made possible to evaluate the kinetics parameters (total number of exchanged electrons, limiting current density, Tafel slope and exchange current density). These parameters vary with the Pt loading, displaying a steep increase at around 200 μg.cm−2, above which the behavior of bulk platinum is found. At lower Pt loadings, the kinetics is controlled by a Temkin isotherm, and a large amount of hydrogen peroxide is formed.

Electrochemically induced surface modifications of mesoporous spinels (Co3O4−δ, MnCo2O4−δ, NiCo2O4−δ) as the origin of the OER activity and stability in alkaline medium

Co3O4−δ, MnCo2O4−δ, NiCo2O4−δ materials were synthesized using a nanocasting process consisting in replicating a SBA-15 hard template. Catalysts powders obtained were characterized using different physico-chemical techniques (X-ray scattering, transmission electron microscopy, N2 physisorption and X-ray photoelectron spectroscopy) in order to deeply characterize their morphostructural properties. Electrochemical measurements performed with cyclic voltammetry and electrochemical impedance spectroscopy techniques have shown that these catalysts were liable to surface modifications induced by the applied electrode potential. These surface structural modifications as well as their effect on the electroactivity of the catalyst towards the OER in alkaline medium are discussed. The activated NiCo2O4−δ material showed particularly excellent catalytic ability towards the OER in 0.1 M KOH electrolyte. In this material Co(IV) is found to be the active species in the catalyst composition for the OER. It exhibits an overpotential as low as 390 mV at a current density of 10 mA cm−2. This catalytic activity is especially high since the oxide loading is only of 0.074 mg cm−2. Furthermore, this anode catalyst showed high stability during an accelerated durability test of 1500 voltammetric cycles.

An electrochemical quartz crystal microbalance investigation of the adsorption and oxidation of CO on a platinum electrode

The adsorption and electrocatalytic oxidation of carbon monoxide on platinum was studied in both acid and alkaline supporting electrolytes using an electrochemical quartz crystal microbalance. Depending on the pH of the medium, a significant influence of CO on the electrode mass was observed, particularly in the potential range preceding the complete oxidation of CO. In acid medium coupling with solvent molecules was also evidenced and was found to be potential dependent. Furthermore, the mass responses depended strongly on the way the adsorption of CO was realised. In addition, the effect of the bulk concentration of CO was investigated in the case of perchloric acid medium.

Shape-dependent electrocatalytic activity of free gold nanoparticles toward glucose oxidation

The synthesis of shape and size-controlled free gold nanoparticles (AuNPs) was achieved by wet chemical methods. The UV–vis spectroscopy measurements and transmission electron microscopy characterizations confirmed the fine distribution in size and shape of the AuNPs. The zeta potential measurements permitted the evaluation of the stability of the AuNPs suspension. For the first time, the shape dependence on the electrocatalytic activity of these NPs is thoroughly investigated. The underpotential deposition (UPD) of lead reveals that their crystallographic facets are affected by their shape and growth process. Moreover, the glucose oxidation reaction strongly depends on the shape of AuNPs. Indeed, the gold nanocuboids (GNCs) and the spherical gold nanoparticles (GNSs) are significantly more active than the gold nanorods (GNRs) followed by the polyhedrons (GNPs). The oxidation process occurs at low potential for GNCs whereas the current densities are slightly higher for GNSs electrodes. Most importantly, the control of the shape and structure of nanomaterials is of high technological interest because of the strong correlation between these parameters and their optical, electrical and electrocatalytic properties.

Size‐Dependent Electrocatalytic Activity of Free Gold Nanoparticles for the Glucose Oxidation Reaction

Understanding the fundamental relationship between the size and the structure of electrode materials is essential to design catalysts and enhance their activity. Therefore, spherical gold nanoparticles (GNSs) with a mean diameter from 4 to 15 nm were synthesized. UV/Vis spectroscopy, transmission electron microscopy, and under-potential deposition of lead (UPDPb ) were used to determine the morphology, size, and surface crystallographic structure of the GNSs. The UPDPb revealed that their crystallographic facets are affected by their size and the growth process. The catalytic properties of these GNSs toward glucose electrooxidation were studied by cyclic voltammetry, taking into account the scan rate and temperature effects. The results clearly show the size-dependent electrocatalytic activity for glucose oxidation reactions that are controlled by diffusion. Small GNSs with an average size of 4.2 nm exhibited high catalytic activity. This drastic increase in activity results from the high specific area and reactivity of the surface electrons induced by their small size. The reaction mechanism was investigated by in situ Fourier transform infrared reflectance spectroscopy. Gluconolactone and gluconate were identified as the intermediate and the final reaction product, respectively, of the glucose electrooxidation.

Facile synthesis of highly active and durable PdM/C (M = Fe, Mn) nanocatalysts for the oxygen reduction reaction in an alkaline medium

The efficient design of highly active and durable materials towards the ultimate goal of improving kinetics of the oxygen reduction reaction (ORR), which allow enhanced performance in solid alkaline membrane fuel cells (SAMFCs), remains elusive. Seminal studies have shown that by alloying a noble metal such as palladium to a transition metal, it is possible to tune the electronic and/or bifunctional properties enabling substantial ORR performance to be achieved, thereby designing a costly catalyst. Herein, we address and discuss new findings from deeper ORR investigations at palladium-based nanostructures in an alkaline medium. We exploited and manipulated the straightforward and fast synthesis method, the so-called “Bromide Anion Exchange”, to prepare surfactant-free PdM/C (M = Fe, Mn) nanocatalysts exhibiting unprecedented activity and stability towards ORR. PdFe/C from bromide anion exchange (BAE) enables 40- and 4-fold enhancement in terms of exchange current density and kinetic current density and ca. 100 mV gains compared to the polyol microwave-assisted method. After 20 000 cycles of accelerated potential cycling test (APCT), our findings indicate that the present PdM/C bimetallics outperform, to the best of our knowledge, most of the data reported for ORR in alkaline media for Pd-based transition metals. The improved catalytic performances are assigned to the absence of any organic contaminants or protective ligands on their surface and their relatively heterogeneous character comprising nanoalloys and nanowire oxides.

Highly Selective Oxidation of Carbohydrates in an Efficient Electrochemical Energy Converter: Cogenerating Organic Electrosynthesis

The selective electrochemical conversion of highly functionalized organic molecules into electricity, heat, and added-value chemicals for fine chemistry requires the development of highly selective, durable, and low-cost catalysts. Here, we propose an approach to make catalysts that can convert carbohydrates into chemicals selectively and produce electrical power and recoverable heat. A 100% Faradaic yield was achieved for the selective oxidation of the anomeric carbon of glucose and its related carbohydrates (C1-position) without any function protection. Furthermore, the direct glucose fuel cell (DGFC) enables an open-circuit voltage of 1.1 V in 0.5 m NaOH to be reached, a record. The optimized DGFC delivers an outstanding output power Pmax =2 mW cm(-2) with the selective conversion of 0.3 m glucose, which is of great interest for cogeneration. The purified reaction product will serve as a raw material in various industries, which thereby reduces the cost of the whole sustainable process.

High impact of the reducing agent on palladium nanomaterials: new insights from X-ray photoelectron spectroscopy and oxygen reduction reaction

Palladium has exceptional affinity with hydrogen and the evolution of the surface of its nanomaterials prepared from chemical methods over time is still unclear. Here, the reducing agent effect on Pd nanomaterials and their long-term chemical stability were scrutinized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The subsequent impact on the catalytic properties was examined using the electrochemical oxygen reduction reaction (ORR). We have discovered that the nature of the reducing agent has noteworthy effects on the final composition of Pd nanomaterials prepared from chemical methods. The surface state of the nanomaterials prepared by using sodium borohydride as reducing agent (Pd/C–NaBH4) is radically different from those obtained from L-ascorbic acid (Pd/C–AA). In addition to pure metal, two oxides were identified: PdO and PdOx (x > 1). XRD analysis has upheld the presence of PdO only in Pd/C–NaBH4, thus underpinning the conclusion that NaBH4 has drastically changed the Pd structure. Furthermore, the reducing agent substantially affects the electrocatalytic properties. The ORR starts with enhanced kinetics (E > 1 V vs. RHE) by a 4-electron process, producing p(H2O2) < 0.5% associated with excellent durability over 5000 cycles. Both catalysts outperform all reported data for Pd electrocatalysts. The novelty of this work is combining ex/in situ XPS and XRD analyses together with ORR as a catalytic model. Overall, this work represents a clear development in our understanding of Pd affinity towards hydrogen and paves new ways for the successful synthesis of Pd-based nanomaterials free from hydrides and oxides, and having impressive catalytic activities.

Three dimensionally ordered mesoporous hydroxylated NixCo3−xO4 spinels for the oxygen evolution reaction: on the hydroxyl-induced surface restructuring effect

Surface restructuration upon potential cycling of three dimensionally ordered NixCo3−xO4 spinels for the oxygen evolution reaction (OER) in an alkaline medium is studied using structural, spectroscopic and electrochemical techniques. It was shown that the intrinsic activity of different catalysts depends on the incorporated amount of nickel and surprisingly correlates with the CoIII/CoIV peak potential. The electrochemical activity of the OER is amazingly improved upon potential cycling. It was observed that potential cycling induces an increase of active sites up to 45% on the most effective electrocatalyst. This unexpected increase in activity is very pronounced and becomes stable after 30 voltammetric cycles. Such a phenomenon is explained by the formation of a layered mixed nickel/cobalt oxyhydroxide active site whose oxidation potential is related to the nickel amount in the catalyst. The formation of this layer is promoted by the surface hydroxylation degree of non-cycled catalysts. In these catalysts, nickel modulates the electronic properties of the active site, which modifies the adsorption energies of key oxygenated intermediates. The synthesis route proposed herein allows an efficient way for obtaining high specific surface areas as well as highly hydroxylated surfaces, the latter being the key factor in the enhancement of the electrocatalytic activity of nickel cobaltites.

Chemistry for oncotheranostic gold nanoparticles

This review presents in a comprehensive ways the chemical methods used to functionalize gold nanoparticles with focus on anti-cancer applications. The review covers the parameters required for the synthesis gold nanoparticles with defined shapes and sizes, method for targeted delivery in tumours, and selected examples of anti-cancers compounds delivered with gold nanoparticles. A short survey of bioassays for oncology based on gold nanoparticles is also presented.