K. Boniface Kokoh

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.

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.

Reversible Electrocatalytic Activity of Carbon-Supported PtxNi1−x in Hydrogen Reactions

Hydrogen oxidation and evolution reactions (HOR and HER) are studied on Ptx Ni1-x /C materials synthesized by the bromide anion exchange method. Physicochemical characterization shows that this surfactant-free method enables the preparation of well-dispersed and effective catalysts for the processes involved in the anode of H2 /O2 fuel cells (HOR) and the cathode of water electrolyzers (HER). The Pt-based materials are modified with different Ni contents to decrease the amount of costly precious metal in the electrode materials. These modified Pt-based materials are found to be electroactive for both reactions without additional overpotential. Kinetic parameters such as the Tafel slope, exchange (j0 ) and kinetic current densities, and the rate-determining steps of the reaction mechanisms are determined for each Pt-Ni catalyst and compared to those obtained at the Pt/C surface in alkaline medium. The high j0 values that are obtained indicate a probable contribution of the surface structure of the catalysts due to their roughness and the presence of oxygenated Ni species even at low potentials.

Tools and Electrochemical In Situ and On-Line Characterization Techniques for Nanomaterials

In the last century, progress in electrochemistry and electrocatalysis was very spectacular due to the remarkable evolution in surface science, chemistry, and physics. Electrochemical study of perfect smooth or bulk materials was the usual way to understand the interaction between the surfaces of such materials with their close environment. Therefore, any modification of the surface structure or composition provides change in the material behavior and the nature of the adsorbed species or near. Usually, the modification of smooth surface consists in the increase of its roughness factor through the deposition of sublayer or layer of metal particles. The deposition can be done on a well-defined surface (model electrode with a known crystallographic structure) [1]. Then, surface modification becomes a way of creating new active sites to enhance the reactivity of molecules. The development of nanoscale materials has changed the approach of studying and identifying active sites in heterogeneous catalysis, and particularly in electrocatalysis. Indeed, electrocatalysis uses the surface of a material, which is submitted to an electrode potential, as the reaction site. Therefore, the material structure, morphology and its composition are the key parameters to control the electrochemical process [2]. The nature of the active site depends on these parameters. Furthermore, the assessment of the nature of the active site before, during, and after the electrocatalytic reaction becomes a huge challenge. Thereby, electrochemical tools like cyclic voltammetry, underpotential deposition of a monolayer of a species [3–5], the specific adsorption of species or molecule, and CO stripping [6] were the first approaches. It is the basic measurement of the electrons flow through the surface per unit of time during the reaction at the surface. Therefore, the electric current per area unit represents the charge transfer reaction that occurs at a metal-solution interface. Since the middle of the last century, an increase in the development of several in situ spectroscopic techniques was observed due to the need of understanding the structure of the interface between electrodes and solutions. Indeed, coupling the electrochemistry measurements to other techniques such as Fourier Transform Infrared Spectroscopy (FTIRS), X-Ray Diffraction (XRD) [7, 8], Transmission Electron Microscopy (TEM) [9], Scanning Tunneling Microscopy (STM) [10], Surface-Enhanced Raman Scattering (SERS) [11] becomes a suitable approach to assess in real time at the electrified interface electrode-solution; some relevant data on electrocatalysts structure, morphology, composition, and stability of materials; and on the reaction intermediates and products. The identification of the surface state in addition to that of adsorbed species, intermediates, and products of the reaction process have permitted to determine a mechanistic pathway which is essential for enhancing the material performance and selectivity. It appears obvious that the identification of surface state of a nanomaterial under realistic electrochemical reaction conditions represents a noble scientific breakthrough. In the present chapter, for the first time some techniques coupled with electrochemistry able to characterize nanomaterials as electrodes will be extensively addressed. This chapter will also show the progress in in situ electrochemical approaches. One motivated approach is to be able to characterize electrochemically and experimentally the surface of the nanoparticle. Therefore, in the first part of the chapter, an example of a pure electrochemical tool, which permits to probe the nanoelectrocatalyst surface, is discussed. Although the progress in nanotechnology increases rapidly, various tools have been developed in electrochemistry for understanding the reaction pathway, intermediates, and products formation.

Preparation and Electrochemical Properties of NiCo2O4 Nanospinels Supported on Graphene Derivatives as Earth-Abundant Oxygen Bifunctional Catalysts

This work reports on the facile synthesis and characterisation of a non-precious-metal bifunctional catalyst for oxygen reduction and evolution reactions (ORR and OER). A few-layer reduced graphene oxide-supported NiCo2 O4 catalyst is prepared using a rapid and easy two-step method of synthesis. It consists of the solvothermal poyl(vinylpyrrolidone)-assisted assembly of metal complexes onto few-layer graphene followed by a calcination step aiming at converting metal complexes into the spinel phase. Using this synthesis approach, the most active material demonstrates an outstanding activity towards the OER and ORR, making it one of the best bifunctional catalysts of these reactions ever reported. This composite catalyst exhibits improved bifunctional behaviour with a low reversibility criterion of 746 mV. The ORR process follows a four-electron pathway and the hydroxyl selectivity is higher than those with pure reduced graphene oxide or NiCo2 O4 materials, showing the synergistic effect between the two phases. Moreover, the high activity of this composite catalyst is confirmed by comparing its performance with those obtained on other cobaltite catalysts prepared using a different synthesis method, or those obtained using a different graphene-based support.