Boniface Kokoh

Efficient electrolyzer for CO2 splitting in neutral water using earth-abundant materials

Significance Electrochemical CO2-to-CO conversion is one important option for storing intermittent, renewable electricity into chemical bonds so as to produce fuels and to use CO2 as a feedstock for chemicals. The setup of an electrolyzer, associating cheap and abundant materials able to split CO2 into CO and O2, in environmentally friendly conditions (neutral pH, ambient temperature) with a high selectivity and stability, and a 50% energy conversion efficiency is reported. The results open the way to solar energy driving of the CO2 /CO + 1/2 O2 splitting by associating the electrochemical cell with a light-to-electricity conversion device, and more generally with surplus electricity from renewable intermittent sources. Low-cost, efficient CO2-to-CO+O2 electrochemical splitting is a key step for liquid-fuel production for renewable energy storage and use of CO2 as a feedstock for chemicals. Heterogeneous catalysts for cathodic CO2-to-CO associated with an O2-evolving anodic reaction in high-energy-efficiency cells are not yet available. An iron porphyrin immobilized into a conductive Nafion/carbon powder layer is a stable cathode producing CO in pH neutral water with 90% faradaic efficiency. It is coupled with a water oxidation phosphate cobalt oxide anode in a home-made electrolyzer by means of a Nafion membrane. Current densities of approximately 1 mA/cm2 over 30-h electrolysis are achieved at a 2.5-V cell voltage, splitting CO2 and H2O into CO and O2 with a 50% energy efficiency. Remarkably, CO2 reduction outweighs the concurrent water reduction. The setup does not prevent high-efficiency proton transport through the Nafion membrane separator: The ohmic drop loss is only 0.1 V and the pH remains stable. These results demonstrate the possibility to set up an efficient, low-voltage, electrochemical cell that converts CO2 into CO and O2 by associating a cathodic-supported molecular catalyst based on an abundant transition metal with a cheap, easy-to-prepare anodic catalyst oxidizing water into O2.

The oxidation of formaldehyde on high overvoltage DSA type electrodes

Neste trabalho a oxidacao eletroquimica do formaldeido e estudada sobre eletrodos dimensionalmente estaveis preparados por decomposicao termica de percursores (correspondentes cloretos). Foram utilizados como eletrodos de trabalho: Ti/Ir0.3Ti0.7O2, Ti/Ru0.3Ti0.7O2 e Ti/Ir0.2Ru0.2Ti0.6O2. As eletrolises foram realizadas galvanostaticamente em uma celula do tipo filtro prensa em solucoes 0,5 mol L-1 H2SO4 com concentracao inicial de formaldeido de 100 mmol L-1. A concentracao de formaldeido decresce rapidamente com o tempo de eletrolise sendo que o eletrodo ternario (Ir + Ru + Ti) e o que apresenta maior atividade catalitica. O ânodo contendo somente Ir, apesar de maior carga superficial e o de menor atividade eletrocatalitica. Para a oxidacao de acido formico, formado pela oxidacao de formaldeido, a presenca de Ir na composicao do ânodo nao favorece o processo, sendo o ânodo contendo somente Ru o mais efetivo para este processo.

Selective TEMPO‐Catalyzed Chemicals vs. Electrochemical Oxidation of Carbohydrate Derivatives

TEMPO‐catalyzed electrochemical oxidation of carbohydrate derivatives was performed and compared with chemical oxidation, which requires the use of co‐oxidants. Allyl‐protected derivatives could be readily oxidized by both methods. Electrochemical selective oxidation of primary positions has been adapted to unprotected mono‐ and oligosaccharides.

Decorated nanotube buckypaper as electrocatalyst for glucose fuel cells

We present novel metallic/bimetallic (Pt, Au-Pt) nanoparticle-decorated carbon nanotubes and bilirubin oxidase-decorated carbon nanotubes deposited on nano-tube buckypaper as promising supported electro-catalytic systems and as electrode material respectively for mixed-reactant biofuel cell applications at neutral pH. We found that the novel enzyme-decorated carbon nanotubes on nanotube buckypaper material is a promising cathode for glucose biofuel cells. It exhibited a high tolerance and catalytic activity resulting in higher current densities compared to carbon black based electrodes.

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.