A Kinetic Study of the Electrochemical Vapor Deposition of Solid Oxide Electrolyte Films on Porous Substrates

The electrochemical vapor deposition (EVD) method is a very promising technique for making gas-tight dense solid electrolyte films on porous substrates, In this paper, theoretical and experimental studies on the kinetics of the deposition of dense yttria-stabilized zirconia films on porous ceramic substrates by the EVD method are presented, The more systematic theoretical analysis is based on a mode] which takes into account pore diffusion, bulk electrochemical transport, and surface charge-transfer reactions in the film growing process. The experimental work is focused on examining the effects of the oxygen partial pressure and substrate pore dimension on the EVD film growth rates. In accordance with the theoretical prediction, the pressure of oxygen source reactant (e,g., water vapor), the partial pressure of oxygen and substrate pore dimension are very important in affecting the rate-limiting step and film growth rate of the EVD process. In the present experimental conditions (e.g., low pressure of oxygen source reactant and small substrate pore-size/thickness ratio), the diffusion of the oxygen source reactant in the substrate pore is found to be the rate-limiting step for the EVD process, Solid oxide fuel cells (SOFC) are very attractive potential energy conversion systems due to their many advantages over the conventional energy conversion systems. The efficiency of the SOFC, which is theoretically much higher than other fuel combust ion-based energy conversion systems, depends on the ionic resistance of the solid oxide electrolyte and electrode materials. I t is generally known that reducing the thickness of the solid oxide electrolyte layer can effectively increase the SOFC efficiency (1). Among the several techniques available for making the electrolyte layers, electrochemical vapor depos i t ion has been demonst ra ted to be a very promising technique for making thin electrolyte layers (films) on porous substrates (2-4). The yttr ia-stabil ized zirconia (YSZ) films of 10-50 ~tm thick were deposi ted on porous a lumina substrates by the EVD teehique (2-6). The integration o f a ceramic membrane top layer with a coarse porous substrate makes it possible to deposi t much thinner (<2 ~m) layers. This kind of thin films also has potential applicat ions in membrane separat ion processes due to their high selectivity in oxygen permeation, In the EVD process for growing solid oxide electrolyte films, a porous substra te (which serves as an electrode when the SOFC is made) separates a mixture of two metal chlorides and an oxygen source reactant (e.g., a mixture of water and hydrogen, pure water or air). Init ially the two reactants interdiffuse in the substrate pore and react to form the corresponding solid oxide product. The solid product under certain condit ions (4, 7) can be deposi ted in the substrate pore near the end at th e metal chloride del ivery side and finally plug the pore part exposed to the metal chlor ide vapor, After this step no further direct reaction between the two reactants occurs. If the deposi ted solid oxide is mixed conducting, the metal oxide film may grow on the substrate surface exposed to the metal chloride vapor by the Wagner scaling process (8). In this process, oxygen is reduced at the oxygen/film interface. The oxygen ions transfer in the film and react with the metal chloride at the film/chloride interface to form the solid oxide, which is deposi ted to keep the EVD film growing. As such an EVD process is rather difficult to be experimental ly monitored, a theoretical s tudy on its film growth kinetics is very impor tant for unders tanding and controlling the process, In the current literature, very limited data were repor ted on the film growth kinetics of the EVD process. Carolan and Michaels (6) recently reported a theoretical analysis, with exper imenta l results, on the film growth kinetics in the EVD process. Only the electrochemical t ransport in the metal oxide film was considered in the analysis. Thus, following almost the identical procedure as employed for der iving thickness-t ime relations for the Wagner scaling process (8, 9), they found that the EVD film * Electrochemical Society Active Member. 3960 growth kinetics can be descr ibed by the conventional parabolic law (film thickness is proport ional to the squared root of time). This is consistent with some kinetic data of film thickness vs. t ime for growing YSZ film on a porous alumina substrate (6) and t in-doped indium oxide film on a dense YSZ substrate (10). Some other exper imenta l work on deposi t ing YSZ on porous substrates, however, shows that the deposi ted film thickness is a l inear function of t ime (2) and the composition of the doped oxide (yttria content) in the film is uniform across the EVD film (11). These results suggest that other mass-transfer step(s) may be the rate-limiting step. In addition, it is found that the exper imental ly determined film growth rates are much smaller than t h e predicted ones if electrochemical t ransport in the electrolyte film is the rate-l imiting step, More investigation on the EVD film growth kinetics is therefore needed in order to gain a better insight of the EVD process. In this paper we present a more systematic theoretical analysis on t h e EVD f i lm growth kinetics, evidenced by new exper imental results of growing YSZ films on porous substrates. As suggested by the theoretical analysis, the present exper imenta l s tudy is focused on investigating the film growth kinetics by examining the effects of the oxygen partial pressure and substrafe pore dimension on the YSZ film growth rate.