P. Millet

Design and performance of a solid polymer electrolyte water electrolyzer

Abstract The development of medium size solid polymer electrolyte (SPE) water electrolyzers is of great practical interest for on-board production of oxygen in submarines and for energy storage purposes in space applications. While the SPE technology has largely demonstrated its potential on the laboratory scale, very little information is available in the literature on the technological characteristics and electrochemical performances of larger water electrolyzers. In this paper, we present part of the experience that we gained in the development of medium size electrolysis units. A 0.5 kW SPE water electrolyzer was designed and tested. Its performance is discussed with respect to technological design (fluids and current distribution) and operating conditions.

New solid polymer electrolyte composites for water electrolysis

A new procedure for the preparation of SPE-electrocatalyst composites has been developed. In this procedure, noble metal cationic species are chemically reduced within a solid polymer electrolyte. The metallic particles are not homogeneously distributed across the SPE thickness but predominate near its surfaces. The structure and the distribution of the precipitates, along the membrane thickness have been investigated by scanning electron microprobe analysis and transmission electron microscopy. Application to SPE water electrolysis has been performed using Nafion membranes and platinum-group electrocatalysts. The SPE-electrocatalyst composites prepared according to this new procedure present good electrochemical properties, low catalyst loadings, long-time stability and high energetic efficiencies.

Cobalt Clathrochelate Complexes as Hydrogen‐Producing Catalysts

Developing a hydrogen-based economy is one possible scenario to reach a sustainable energy development and also to put ourselves on a path to cut the carbon emissions for obvious climate issues. However, several inherent problems must be overcome, such as production, storage, transport, and efficiency. 3] We are involved in research towards developing metal complexes for the electrocatalysis of hydrogen production. The challenge is to replace the expensive and limited platinum metal with non-noble metal complexes. A bioinorganic approach to tackling this problem wherein metal complexes, based mainly on iron, are designed to mimic the catalytic site of the Fe-hydrogenases, in the hope of simulating their reactivity patterns, has attracted much attention. A general trend in the electrocatalytic activity of these complexes is high overpotential. More classical metal coordination complexes for electrocatalysis have also been reported to show interesting activities towards hydrogen evolution. Among these examples are the difluoroboryl annulated bis(glyoximato) cobalt derivatives studied by Espenson and Chao. In recent years several groups, including ourselves, have demonstrated that this family of complexes can perform electrocatalysis of proton reduction in organic media. Hexacoordinated cobalt complexes have also been reported to act as catalysts for proton reduction. However, no clear-cut mechanism is known for the catalytic activity of these complexes, which was evidenced only on a mercury electrode. In boron-capped tris(glyoximato) cobalt complexes, the metal ion is both coordinatively saturated and encapsulated by a single macrobicyclic ligand. These coordination compounds are classified as clathrochelate complexes, in which the metal ion is locked in a close-knit structure, inhibiting ligand exchange in the more labile oxidation states of the encapsulated metal ion, and, in turn, explaining why the chemical activity of this family of complexes has been particularly elusive. We have been interested in investigating the electrochemical activity of the boron-capped tris(glyoximato) cobalt complexes towards hydrogen evolution. Herein, we report on the synthesis and characterization of three clathrochelate Co complexes [1], [2], and [3] (Figure 1) together with their involvement in an electrocatalytic hydrogen-forming reaction in solution. X-ray crystallographic data for [1] and for a derivative Co complex [4] are also discussed.

Preparation of new solid polymer electrolyte composites for water electrolysis

Abstract A new simple procedure for the preparation of Solid Polymer Electrolytes (SPE)—electrocatalyst composites for water electrolysis-has been developed. In this new procedure (French patent, 1987), microparticles of noble metal electrocatalyst are precipitated simultaneously inside and outside near both surfaces of a perfluorinated ion-exchange membrane by chemical reduction of cationic precursor salts. The structure of the electrodes bonded onto the SPE has been investigated by scanning electron microprobe analysis and transmission electron microscopy. The effects of temperature and nature of anodic electrocatalyst on the cell voltage-current density relation are reported. Anodic overvoltage, cathodic overvoltage, ohmic drop within the SPE and roughness factor of the electrodes have been measured using an internal RHE reference electrode. The composites prepared according to this new procedure present low noble metal loadings ( −2 ), low cell voltage (1.75 V at 1 A cm −2 and 80°C) and long time stability (over 15,000 h of continuous electrolysis). Partial results concerning the use of such composites for H 2 /O 2 SPE fuel cell applications are also reported.

Solid polymer electrolyte water electrolysis: electrocatalysis and long-term stability

Abstract In order to enhance the performance and long-term stability of a water electrolysis pilot plant, IrO 2 /Ti electrodes were hot-pressed on both sides of a Nafion® membrane and tested. By comparison with Pt electrodes, we obtained much lower anodic overvoltages and a better long-term stability of the cell voltage due to a lower cathode sensitivity to poisoning. Purification of water using ion-exchange resins was also useful to prevent a poisoning of the membrane itself. In purified water, assemblies using Pt cathodes and IrO 2 anodes were found to be a good compromise between performance and stability.

Characterization of membrane-electrode assemblies for solid polymer electrolyte water electrolysis

The development of medium size SPE® (Solid Polymer Electrolyte) water electrolysers requires satisfactory membrane-electrode assemblies. Pt-Nafion®-Pt, Pt-Ir-Nafion®-Pt and Pt-Ru-Nafion®-Pt composites have been tested for such applications. Anodic overvoltage, cathodic over-voltage and ohmic drop across the SPE® have been measured in the temperature range of 20 to 80° C and in the current density range 0 to 1 A cm−2 Kinetic parameters for the oxygen and hydrogen evolution reactions have been measured and compared to values obtained in aqueous acid solutions. Electrode structure has been investigated, before and after 2 500 h of continuous electrolysis at 1 A cm−2, by scanning electron microscopy, in order to check electrode stability at high current density. The effect of membrane surface etching on the roughness factors of the electrodes has also been investigated. Life tests performed on bare membranes and SPE composites at various operating pressures are presented and discussed. Results on the poisoning effect of nickel ions added to the feed water of the cells are reported.

Chapter 2 – Water Electrolysis Technologies

The purpose of this chapter is to provide an overview of the different water electrolysis technologies. In the introduction section, the general characteristics of water electrolysis (thermodynamics, kinetics, efficiency) are described. Main electrolysis technologies used to produce hydrogen and oxygen of electrolytic grade are then described in the following sections. Alkaline water electrolysis is described in Section 2.2, proton-exchange membrane water electrolysis in Section 2.3 and high-temperature water electrolysis in Section 2.4 . For each technology, state-of-the-art performances are analyzed, limitations are identified and some perspectives are discussed.

A study of the hydrogen absorption and desorption reactions in palladium electrodes using the potential step method

Abstract The hydrogen absorption (HAR) and desorption (HDR) reactions in thin palladium electrodes are studied using the potential step method in order to analyse the mechanism of phase transformation. Transient current responses (i–ts) are recorded at the onset of the potential step for 47 μm thick Pd electrodes in 1 M H2SO4 at ambient temperature. A model based on a moving boundary mechanism is proposed to account for the experimental i–ts. The HAR shows interfacial kinetic limitations and only numerical solutions can be obtained. Such kinetic limitations do not exist for the HDR and a semi-analytical solution which satisfactorily fits the experimental data is proposed.

A solid polymer electrolyte-based ethanol gas sensor

Platinum-based membrane-electrode assemblies have been prepared to make electrochemical measurements of breath alcohol levels. Measurements are made by using a solid polymer electrolyte ethanol-air fuel cell. A diffusion membrane is placed in the feed compartment to limit the cell response by ethanol diffusion. The cell is found suitable for the realization of gaseous ethanol sensors. Reproducible cell responses are obtained over several hundreds of cycles. The cell current is proportional to the ethanol concentration in the feed compartment and to the thickness of the diffusion membrane. The transient response of the cell is simulated using Fickian phenomenological equations. Calculated results are in good agreement with experimental data.

Preparation of solid polymer electrolyte composites: investigation of the precipitation process

A model for the chemical reduction of platinum tetramine in perfluorinated Nafion® membranes using sodium borohydride as reducer is proposed. A Nernst-Planck equation is employed for the description of ion transport by diffusion and migration and a reaction term accounts for the in situ chemical reduction. Time-dependent concentrations of the diffusing species within the membrane are obtained numerically by an iterative technique until completion of the precipitation. The model assumes that mass transport is limited by diffusion and migration within the membrane and that the concentrations remain constant at the interfaces during the precipitation. The model shows the effect of (i) the reducer concentration in the solution, (ii) the number of precipitation cycles and (iii) the rate of chemical reduction. To check the validity of the model, metallic platinum concentration profiles across the membrane thickness are obtained by electron microprobe analysis. Values of the diffusion coefficients of the diffusing species within the membrane are obtained from conductivity and permeation measurements.