The present invention relates to methods of preparing porous structures for retaining electrolyte in fuel cells or in secondary electrochemical cells. The structures are of electrically insulating ceramics with sufficient porosity and pore size to retain molten electrolyte between the electrodes of the cell.
A molten carbonate fuel cell typically operates at high temperatures of about 900-1000 K to convert chemical energy to d.c. electricity. Fuels such as H.sub.2 and CO or methanol and oxidant gases such as O.sub.2 and CO.sub.2 react together during this conversion. Typical reactions are as follows.
At the anode: EQU H.sub.2 +CO.sub.3.sup.= .fwdarw.CO.sub.2 +H.sub.2 O+2e.sup.- EQU CO+CO.sub.3.sup.= .fwdarw.2CO.sub.2 +2e.sup.-
At the cathode: EQU 2e+CO.sub.2 +1/2O.sub.2 .fwdarw.CO.sub.3.sup.=
Such molten carbonate fuel cells have been suggested as stacks of repeating elements. Each element contains an anode, a cathode and an electrolyte structure separating the two. Anode structures can include porous sintered nickel, possibly alloyed with chromium or cobalt for strength. Cathodes of similar structure contain nickel oxide formed by reaction with the cell oxidants. Suitable means of current collection and an electrically conductive separator plate between the anode of one cell and the cathode of the next cell in the stack are incorporated. Fuel cells of this type are more fully described in the assignee's copending application Ser. No. 107,741, filed Dec. 27, 1979 by Singh and Dusek entitled "Porous Electrolyte Retainer for Molten Carbonate Fuel Cell".
The electrolyte structure disposed between the electrodes can include an electrolyte of a mixture of alkali metal carbonates such as Li.sub.2 CO.sub.3, Na.sub.2 CO.sub.3 and K.sub.2 CO.sub.3. Various mixtures and eutectic compositions of these materials well-known in the art can be employed to reduce melting points. For example, a mixture of 62 mole % LiCO.sub.3 and 38 mole % K.sub.2 CO.sub.3 has a melting point of about 750 K. Various other suggested electrolyte mixtures are given in U.S. Pat. No. 4,115,632, cited below in the Prior Art Statement.
Lithium aluminate structures and materials have been of particular interest in forming a porous substrate or matrix for molten alkali metal carbonates used as electrolytes in fuel cells. One method used in preparing lithium aluminate reacts alkali metal carbonates with alumina at temperatures of about 900 K. Since alumina and lithium carbonate are of substantially different densities, a homogeneous powdered mixture of these reactants is difficult to achieve. Methods of this type also are sensitive to the presence of aluminum hydroxide or water of hydration in the starting material. The presence of water may cause agglomeration, poor mixing of the powders and incomplete reaction to lithium aluminate. Aluminum hydroxide reacts too slowly with the carbonate at 900 K, but at higher temperatures undesirable particle growth occurs. Other methods have included various alkali metal compounds other than lithium hydroxide or lithium carbonate in the reaction mixture. Unfortunately materials such as potassium hydroxide have been found to encourage particle growth and decrease surface area of the lithium aluminate structure.