Porous nickel MCFC cathode coated by potentiostatically deposited cobalt oxide: I. A structural and morphological study
Introduction
The interest of LiCoO2 as a candidate material for replacing the state-of-the-art molten carbonate fuel cell (MCFC) cathode, LixNi1−xO, is well described in the literature [1], [2], [3], [4], [5], [6]. This compound is less soluble than the nickel cathode in the MCFC conditions and has an electrocatalytical activity close to that of LixNi1−xO. However, its conductivity is lower than that of the usual cathode material, its mechanical resistance is lower and its cost is relatively high. In addition, problems in scale up of electrode area restrict its use. The thin layer technology is an interesting way of solving these practical problems by combining the properties of the LiCoO2 coating (low solubility) and the LixNi1−xO substrate (cheap, good conductivity and mechanical strength). The feasibility of coating the nickel cathode with well-controlled LiCoO2 thin layers has been analysed in recent papers [7], [8], [9], [10].
In a previous work, we have developed and optimised the electrochemical deposition of Co3O4 thin layers on dense nickel or nickel oxide substrates [11], [12]. This cheap and room-temperature technique allowed us to obtain thin and homogeneous layers of Co3O4 transformed into LiCoO2 after exposure to the molten carbonate melt. The use of dense nickel or nickel oxide substrates was a first necessary step to analyse thoroughly the structural and morphological features of the cobalt coatings; nevertheless, the situation can be significantly different in the case of porous electrodes. The present paper is dedicated to the elaboration and characterisation of cobalt oxide deposits on the porous nickel cathode before its oxidation and lithiation in situ in molten Li2CO3–Na2CO3, one of the candidate electrolytes for MCFC application, in the standard conditions. The structural properties of the deposits were characterised either directly or after an annealing treatment by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS). The morphological features were analysed by scanning electron microscopy (SEM) associated with energy-dispersive spectroscopy (EDS) and the pore size deposition by mercury porosimetry.
Section snippets
Electrochemical deposition
The samples were prepared by potentiostatic deposition of cobalt oxide films on porous Ni foils (6 mm × 25 mm × 0.5 mm). The deposition was performed in a four-compartment Tacussel glass cell with a 0.1 mol l−1 Co(II) solution prepared from Co(NO3)2·6H2O (Fluka, with a purity of 99.99% analytical-reagent grade chemical) in a 0.5 mol l−1 solution of NaNO3 (Merck) with a pH = 4. The solution was de-aerated during 30 min prior to electrochemical deposition and the pH fixed at 7.4 by addition of a 1 mol l−1
Results and discussion
Fig. 1 shows the XRD patterns of the cobalt oxide formed on the porous nickel foil obtained at 0.65 V during 24 h, before and after the thermal treatment (4 h at 500 °C in air). Initially, only the XRD peaks corresponding to metallic nickel were observed at 44.50°, 51.84°, and 76.37°(2θ). After the annealing treatment, apart of the XRD peaks of Ni, less intense peaks were detected: they correspond to NiO at 37.28°, 42.23°, and 62.97° (2θ) and to cubic Co3O4 compound that appear at 19.00°, 31.27°,
Conclusion
Cobalt oxide potentiostatic deposition was adapted to the protection of the porous MCFC nickel cathode. After an annealing treatment, provoking a relative loss of the cobalt content, experimental evidence was given on the formation of electrodeposited Co3O4 by XRD and Raman spectroscopy. Before the thermal treatment, XPS allowed the detection of CoOOH, which was transformed into Co3O4 after annealing. The morphology and porosity of Co3O4-coated nickel was relatively close to that of sole porous
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