Yaovi Holade

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.

Advances in Electrocatalysis for Energy Conversion and Synthesis of Organic Molecules

Ubiquitous electrochemistry is expected to play a major role for reliable energy supply as well as for production of sustainable fuels and chemicals. The fundamental understanding of organics-based electrocatalysis in alkaline media at the solid-liquid interface involves complex mechanisms and performance descriptors (from the electrolyte and reaction intermediates), which undermine the roads towards advance and breakthroughs. Here, we review and diagnose recently designed strategies for the electrochemical conversion of organics into electricity and/or higher-value chemicals. To tune the mysterious workings of nanocatalysts in electrochemical devices, we examine the guiding principles by which the performance of a particular electrode material is governed, thus highlighting various tactics for the development of synthesis methods for nanomaterials with specific properties. We end by examining the production of chemicals by using electrochemical methods, from selective oxidation to reduction reactions. The intricate relationship between electrode and selectivity encourages both of the communities of electrocatalysis and organic synthesis to move forward together toward the renaissance of electrosynthesis methods.

Surfactant- and Binder-Free Hierarchical Platinum Nanoarrays Directly Grown onto a Carbon Felt Electrode for Efficient Electrocatalysis

The future of fuel cells that convert chemical energy to electricity relies mostly on the efficiency of oxygen reduction reaction (ORR) due to its sluggish kinetics. By effectively bypassing the use of organic surfactants, the postsynthesis steps for immobilization onto electrodes, catalytic ink preparation using binders, and the common problem of nanoparticles (NPs) detachment from the supports involved in traditional methodologies, we demonstrate a versatile electrodeposition method for growing anisotropic microstructures directly onto a three-dimensional (3D) carbon felt electrode, using platinum NPs as the elementary building blocks. The as-synthesized materials were extensively characterized by integrating methods of physical (thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, inductively coupled plasma, and X-ray photoelectron spectroscopy) and electroanalytical (voltammetry, electrochemical impedance spectrometry) chemistry to examine the intricate relationship of material-to-performance and select the best-performing electrocatalyst to be applied in the model reaction of ORR for its practical integration into a microbial fuel cell (MFC). A tightly optimized procedure enables decorating an electrochemically activated carbon felt electrode by 40-60 nm ultrathin 3D-interconnected platinum nanoarrays leading to a hierarchical framework of ca. 500 nm. Half-cell reactions reveal that the highly rough metallic surface exhibits improved activity and stability toward ORR (Eonset ∼ 1.1 V vs reversible hydrogen electrode, p(HO2-) < 0.1%) and the hydrogen evolution reaction (-10 mA cm-2 for only 75 mV overpotential). Owing to its unique features, the developed material showed distinguished performance as an air-breathing cathode in a garden compost MFC, exhibiting better current and faster power generation than those of its equivalent classical double chamber. The enhanced performance of the material obtained herein is explained by the absence of any organic surfactants on the surface of the nanoarrays, the good metal-support interaction, particular morphology of the nanoarrays, and the reduced aggregation/detachment of particles. It promises a radical improvement in current surface reactions and paves a new way toward electrodes with regulated surface roughness, allowing for their successful application in heterogeneous catalysis.

Electrochemical Reactivity at Free and Supported Gold Nanocatalysts Surface

This chapter presents an overview on size, structure, morphology, composition as well as the effect of the support on the electrocatalytic properties of gold nanoparticles (AuNPs). It was found that the electrocatalytic properties of unsupported AuNPs strongly depend on their size and shape. Consequently, the electrocatalytic properties of AuNPs can be tuned. Furthermore, to design high-performance electrocatalysts with minimal precious metal content and cost, the direct immobilization of metal NPs onto carbon-based substrates during their synthesis constitutes another elegant alternative and has been thoroughly examined. These “easy-to-use” supports as scaffolds for AuNPs, namely carbon black, carbon paper, etc., offer beneficial contributions. Indeed, thanks to their high available surface area, good electronic conductivity and synergis‐ tic effect between the chemical species present on their surface and the loaded NPs, carbon-based supports enable maximizing the efficient utilization of the catalysts toward drastic enhancement in both activity and durability. We also examined different judicious combinations of (electro)analytical techniques for the unambiguous determi‐ nation of the reaction product(s) over the Au-based nanocatalysts, using glucose as model molecule given its importance in electrocatalysis. The performances of carbonsupported AuNPs as anode materials in direct glucose fuel cell in alkaline medium were

Irreversible Losses in Fuel Cells

Abstract The term “irreversible losses” is used to summarize the major effects that lead to a deviation of the observed cell voltage compared with the value calculated from thermodynamic theory. The aim of this chapter is to serve two purposes. First, an overview of the main loss mechanisms coming from electrode kinetics, ohmic resistance, and mass transport is provided. Second, this chapter demonstrates how the underlying physical model in our mind influences the method and result of calculations. Throughout the chapter, several modeling approaches are compared, and their implications on the model parameters are discussed. This is of great practical importance for the interpretation of experimental work and the comparison with literature data. The examples describe the performance of half cells, single cells, and fuel cell stacks.

Electrochemical Measurement Methods and Characterization on the Cell Level

Abstract A comprehensive approach is used in this chapter to explain the electrochemical system from two to three electrodes that require a reference electrode. Methods for cleaning glass electrochemical cells are described. Cyclic voltammetry used as an electrochemical basic characterization method is presented. Oxygen reduction is presented as an example of a reaction, and the main steps to assess the kinetics parameters are explained in detail. The electrochemical impedance spectroscopy is presented as a complementary tool to characterize the electrodes’ materials and the reactions.