Optimization By Experimental Design of the Synthesis of RuxIr1-XO2 Electrocatalysts for the Oxygen Evolution Reaction in a PEM-URFC
Authors: Lamy Claude , Guenot Benoît , Audichon Thomas , Baranton Stève , Coutanceau Christophe 
Published: February 2020

Introduction

The Oxygen Evolution Reaction (OER) is a crucial bottleneck for developing water electrolysis cells, such as Proton Exchange Membrane Electrolysis Cells (PEMEC) and/or Unitized Regenerative Fuel Cells (URFC), since the cell voltage is mainly limited by the slow kinetics of the oxygen electrode leading to relatively high energy consumption (5 kWh/Nm3 of H2). Many transition metal oxide electrocatalysts of the OER have been developed for many years [1-3], most of them based on ruthenium iridium mixed oxides RuxIr1-xO2 [2-5]. The electrocatalytic activity and the OER mechanism depend greatly on the crystalline size and structure of the catalytic materials which are determined by the synthesis method, such as the polyol method [6], the instant method [7] or the Pechini-Adams method [8].

In this communication we will present a new method, i.e. the hydrothermal synthesis [9], which allows to control the morphology of metal oxide nanoparticles, particularly to obtain crystalline nanoparticles of small size (~ 6 nm). This method, which requires an autoclave reactor to control the pressure up to 5 bars and the temperature up to 200°C, was mostly used until now for the preparation of ruthenium oxides for super capacitors [10]. The crystallographic structure and the average size of the nanoparticles obtained depend on 4 factors:

pH of the synthesis medium (pH)

Temperature of the hydrothermal treatment (TH)

Duration of the hydrothermal treatment (DH)

Temperature of the subsequent annealing treatment under air (AT)

which have been varied in 4 levels leading to 44 = 256 experiments for a complete experimental design. Using a fractional experimental design of the hydrothermal synthesis of ruthenium oxides reduces this number to 16 experiments and allows optimizing the electrocatalytic properties of the nanoparticles obtained. In particular 2 catalysts, e.g. one with an optimized structure leading to an OER high current density (135 mA cm-2 at 1.8 V vs. RHE), and the other one with an optimized particle size of 10 nm, led to experimental responses close to those predicted by the experimental design [11]. Mixed RuxIr1-xO2 electrocatalysts with a larger particle size are more convenient to use as an electronic conducting support, particularly stable at the high electrode potential encountered in the OER, and allowing the deposition of supported Pt nanoparticles for the Oxygen Reduction Reaction (ORR).

Experimental

Four electrocatalysts (RuO2, Ru0.75Ir0.25O2, Ru0.5Ir0.5O2, Ru0.25Ir0.75O2) for the OER were synthesized by the optimized hydrothermal method, and characterized by physicochemical methods (XRD, TEM, TGA) and electrochemical methods (CV, LSV). A bi-functional electrocatalyst (Pt/IrRuO2) was prepared by deposition of Pt nanoparticles on a Ru0.75Ir0.25O2 catalyst acting as a conducting support. Their electrocatalytic properties were investigated in a 3-electrode electrochemical cell thermostated at 25°C by recording the polarization curves, j(E), at a very slow sweep rate (1 mV s-1) from 1.2 V to 1.9 V vs. RHE for the OER and from 1.05 V to 0.45 V vs. RHE for the ORR in an Unitized Regenerative PEM FC (PEM-URFC).

Some conclusions

A bi-functional electrocatalyst (Pt/IrRuO2) is able to activate both the OER and the ORR.

However the electrode stability is questionable since platinum can be oxidized at high potential so that the reduction of oxygen needs higher overvoltages.

The potential of the positive electrode can be decreased by oxidizing, instead of water, another hydrogen-containing compound, such as methanol, in the presence of water, allowing a better durability of electrode materials and a decrease of the electric energy consumption.

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