Pre-coating of LSCM perovskite with metal catalyst for scalable high performance anodes

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Abstract

In this work, a highly scalable technique is proposed as an alternative to the lab-scale impregnation method. LSCM–CGO powders were pre-coated with 5 wt% of Ni from nitrates. After appropriate mixing and adequate heat treatment, coated powders were then dispersed into organic based vehicles to form a screen-printable ink which was deposited and fired to form SOFC anode layers. Electrochemical tests show a considerable enhancement of the pre-coated anode performances under 50 ml/min wet H2 flow with polarization resistance decreased from about 0.60 Ω cm2 to 0.38 Ω cm2 at 900 °C and from 6.70 Ω cm2 to 1.37 Ω cm2 at 700 °C. This is most likely due to the pre-coating process resulting in nano-scaled Ni particles with two typical sizes; from 50 to 200 nm and from 10 to 40 nm. Converging indications suggest that the latter type of particle comes from solid state solution of Ni in LSCM phase under oxidizing conditions and exsolution as nanoparticles under reducing atmospheres.

Highlights

► Powders containing LSCM were pre-coated with Ni for use as anode for SOFCs. ► Nanoparticles, likely Ni, appears on the surface of LSCM in anodic conditions. ► Rp decreases by 38% at 900 °C and 80% at 700 °C for pre-coated LSCM based anodes. ► The small amount of Ni (5%) wouldn't cause any redox and thermal cycling issues. ► This method is highly scalable and extendable to different functional material.

Introduction

Solid Oxide Fuel Cells (SOFCs) and Solid Oxide Electrolysis Cells (SOECs) are highly efficient electrochemical devices for power generation and energy conversion. Cells consist of two porous electrodes (anode & cathode) separated by dense ionic conducting ceramic electrolyte. As well as presenting catalytic activity towards the desired electrode reaction, electrodes should be chemically and thermally compatible with the electrolyte. They should be both electronic and ionic conductors and should provide sufficient transport of gas by means of continuous interconnected porosity. Classically, electrodes are porous composite materials containing an ionic conductive phase and an electronic conductive phase presenting the desired catalytic activity. The electrode reactions take place at the Triple Phase Boundaries (TPB), the interface between the three phases: the ionic conductive; the electronic conductive and the gaseous phases in the pores. The larger the TPB the better the electrode performance [1].

Recently, much research has focused on improving the electrocatalytic performance of electrode materials by to maximizing the area of the active interfaces by either optimizing the microstructure or using nano-particles of catalyst materials dispersed on the surface of the electrode material. One successful technique has been the impregnation of porous electrodes by aqueous solutions containing the active elements [2]. Whilst good results have been reported at lab-scale, usually related to the increase of TPB density and/or the implementation of nano-catalysts [3], the technique is generally carried out manually using a syringe or a pipette. Thus it often results in inhomogeneous structures and can suffer from reproducibility issues. The industrialization of the impregnation method remains challenging, especially in systems involving multi porous layers and complex shapes [4].

In this work we propose the pre-coating of powders of electrode materials with active catalyst or suitable precursors as a scalable alternative to the impregnation method. The pre-coated powders are then used to prepare screen printable inks for electrode deposition. The perovskite (La0.75Sr0.25)0.97Cr0.5Mn0.5O3 (LSCM) was chosen as the main anode material. This material has demonstrated very good anodic performance and stability especially when using hydrocarbon fuels [5]. However, it exhibits poor ionic conductivity and low electronic conductivity in reducing conditions [6], Therefore a small amount of Ce0.9Gd0.1O2−δ (CGO) was added to the LSCM in order to provide some ionic conductivity and enhance the electronic conductivity in anodic conditions. This composition has been successfully integrated into a real world industrial design and although encouraging results were obtained they were not quite as good as the state of the art materials [4]. In this work, LSCM + 15 wt% CGO powders were pre-coated with Ni nitrate at a level equivalent to 5 wt% of metallic nickel. This was then used to prepare a screen printable ink. For comparison, another ink containing the same LSCM–CGO–Ni ratios was prepared but with Ni coming from submicron NiO powders.

Electrochemical tests show improved performances for both anodes where Ni was added. However, the pre-coated anodes show a far greater enhancement of performance, especially at intermediate temperatures, which is most likely due to the presence of nano-scaled particles of catalyst elements. This opens significant opportunities to improve performances of oxide based electrode materials in SOFCs and SOECs and as the amount of the metallic catalyst used is very low and percolation is not needed, there should be minimal detrimental effects on the mechanical and electrochemical stability of the oxide. Moreover, using the pre-coating method the application of simple and low cost processing techniques such as screen printing and tape casting can be maintained allowing for very efficiently scale up.

Section snippets

Experimental methods

Three anode compositions were screen printed, fired and tested under anode conditions as working electrodes in three electrodes – electrochemical half cell configuration [7]. The investigated compositions were: LSCM + 15 wt% CGO; LSCM + 15 wt% CGO + 5 wt% Ni (with Ni coming from submicron NiO powder) and LSCM + 15 wt% CGO + 5 wt% Ni (with Ni coming from Ni nitrate). The correspondent samples are respectively referred in this paper as (A), (B) and (C).

The components used for inks preparation

Pre-coating and ink formulation

In the initial trails to obtain the coated particles (ink C), Ni nitrate was dissolved in acetone to which LSCM–CGO powders were added during the ball milling stage, as described in the experimental section. However, before the evaporation of acetone was complete, the formation of precipitates on the walls and the bottom of the beaker was observed, the resulting ink looked rough and some granules were easily distinguishable. After sintering for 1 h at 1350 °C the printed layer was very uneven

Conclusion

LSCM–CGO powders were pre-coated with Ni nitrates prior to screen printing and sintering as SOFC anode layer. This concept was developed as a scalable alternative to the manual lab-scale impregnation technique. The pre-coating is likely to result in two typical sizes of Ni particles: 50–200 nm and 10–40 nm. The latter are thought to be involved in a solid state solution – exsolution process on the surface of the LSCM phase under oxidizing and reducing conditions respectively. This assumption,

Acknowledgements

S. Boulfrad, M. Cassidy and J.T.S. Irvine thank EPSRC, TSB for financial support for the research work performed at USTAN.

References (18)

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