Microstructure characterization and electrical conductivity of electroless nano Ni coated 8YSZ cermets
Introduction
Solid oxide fuel cells (SOFCs) have received much attention because of their ability to convert chemical energy to electrical energy directly [1] with a variety of fuels [2], [3]. Due to the rapid development of materials science and technology SOFCs will be commercialized very soon. Recent efforts focus on decreasing the operating temperature to an intermediate temperature (IT) range or less. This can be done by decreasing the ohmic loss of the electrolyte, decreasing the polarization losses at the electrodes [4], [5] and by exploring the better performance SOFC materials.
Ni-8 mol% yttria stabilized zirconia (Ni-8YSZ) is widely used as anode material in SOFC [1], [6]. A SOFC anode requires a very long triple phase boundary (TPB) where the electrochemical reactions take place to provide better performance. The TPB consists of the simultaneous existence of an ionically conductive part, an electronically conductive part and the pores which provide gas/fuel [7], [8], [9]. The TPB can be lengthened by introducing nano pores, but incorporation of only nano pores increases the gas diffusion resistance, thereby increases the polarization resistances [10]. Moreover, the porosity cannot be increased to a very high level because of the limitation of mechanical strength as well as electrical conductivity. Proper microstructure control, especially nano/submicron microstructure, is another way to improve the anode performance [11].
The conventional way to fabricate the anode cermet is the mechanical mixing of NiO and ceramic powders. This process requires a high amount of Ni to achieve conductivity percolation [12]. Moreover, the Ni grains usually grow larger during sintering, thereby reducing the length of the TPB. To improve the performance of the anode cermet, some other methods such as the mechanofusion method, co-precipitation method, heat decomposable aqueous salt solution route, etc., [11], [13], [14] have been performed and obtained some good results. However, from an economic perspective, these processes are too costly, and therefore, not attractive.
One method to prepare the Ni-cermet is the electroless deposition process in which metallic Ni is coated on ceramic powder particles [15], [16], [17]. Spot type nano metal particles can be homogeneously deposited on the surface of substrate powders by this technique.
In the present study, 8YSZ nano powders (< 70 nm) were coated with metallic Ni using the electroless coating process to be used for the SOFC anode. The commonly used reducing agent sodium phosphinate monohydrate was used in this process without using any sensitizing agent. The objective of this work was to achieve uniform, homogeneous and nano scale Ni and YSZ composite powders. A comprehensive microstructural study has been performed to investigate the nature of Ni coating on the nano powders and as well as in the sintered body. In addition, the electrical conductivity of sintered bodies was investigated at some elevated temperatures.
Section snippets
Experimental procedure
Nano 8YSZ powders (TZ-8Y, Tosoh—Japan) were used as substrate powders to coat metallic Ni. The electroless bath, which is commonly available, was used to synthesize the Ni-8YSZ cermet. It consisted of sodium hypophosphite monohydrate (NaH2PO2·H2O) as a reducing agent, sodium acetate trihydrate (C2H3O2Na·3H2O) as a complexing agent and nickel sulphate hexahydrate (NiSO4·6H2O) as the source of Ni2+ ions. Fixed compositions of all the reagents were used. The composition of the bath was NaH2PO2·H2O
Results and discussion
Electroless deposition of Ni coatings using hypophosphite is a complicated process. In the present deposition system, hypophosphite participates at least following three different reactions: Ni2+ reduction by the following reactionNi2+ + 2H2PO2− + 2H2O → Ni + 2H2PO3− + H2 + 2H+,the catalytic oxidation in the reaction with waterH2PO2− + H2O → H2PO3− + H2;and the reaction results in phosphorous (P) co-deposition4H2PO2− + H+ + H2O → 3H2PO3− + P + 2.5 H2.P appears in the coating according to the above reactions. Reaction 3 is
Conclusions
Using the electroless deposition method, Ni-8YSZ nano composite powders have been prepared to be used in IT SOFCs. In the composite powder, the coated Ni showed spot type morphology. There was the appearance of a Ni3P phase in the cermets with 41% to 51% Ni. The cermets with 41% and 47% Ni showed the transformation of c-zirconia to m-zirconia. However, the cermet with 36% and 41% Ni did not show appreciable transformation of stabilized zirconia. The prepared cermets showed conductivity
Acknowledgement
This work was supported by NRL project of the Korean Ministry of Science and Technology.
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