Cellulose Microfibrils as a Pore Former in Electroless Co-Deposited Anodes for Solid Oxide Fuel Cells
Authors: Turnbull Rob James , Shearer Neil , Wilson Callum 
Published: February 2020

A study was conducted to investigate the feasibility of Cellulose Microfibrils (CMF) as a pore former in the manufacture of Solid Oxide Fuel Cell (SOFC) anodes using Electroless Co-Deposition (ECD). Previous work into the use of ECD to produce SOFC anodes has found that the lack of porosity restricted the maximum power density of the cell. Studies have also shown that an anode produced by ECD using rice starch as a pore former nearly doubled the power density of a cell. Therefore by increasing the anode porosity, the cell’s power density should also increase and lead to greater performance of SOFCs for the power generation market.

As the choice of pore former is closely related to the size and shape of pores produced, a whisker shaped pore former will produce a more cylindrical pore once removed. These cylindrical pores will increase the chances of producing an interconnected pore network compared with more spherical pores, therefore improving the gas diffusion throughout the anode.

Previous work by the author on using cellulose as a pore former has shown that a smaller pore former is able to produce a larger increase in porosity due to a greater inclusion within the coating. Samples were produced using four different cellulose powders as pore formers, ranging from 20μm to 200μm and varying morphologies. The result showed that the smaller cellulose powder had a greater impact on the porosity than the larger sized pore former. The smaller cellulose (20μm) increased the overall porosity by 218% and increased the total pore surface area by 2910%. This increase in porosity and surface area will allow for a greater density of reaction sites within the electrode increasing the overall power density of the cell. However the cellulose which produced the greatest change had a particulate morphology, with the fibrous cellulose being too large to have a significant inclusion within the coating to produce the desired microstructure. Therefore by using a cellulose pore former with similar size but with a cylindrical morphology will allow for a greater chance of producing an interconnected pore network required. Cellulose Microfibrils (CMF) typically has a size of ranging from 10μm to 50μm (average: 30μm) with a fibrous structure. This combined with its unique set of properties, such as being insolubility in water with a hygroscopic character and no melting point, makes it highly suitable for use with the ECD process.

Initial tests with CMF showed that a 10g/l bath loading of CMF produced an increase in Porosity by 200%, which was similar to the 20μm particulate cellulose previously used. Therefore button cells were produced using ECD with 10g/l, 5gl, and 2g/l bath loadings of CMF added to act as a pore former for the anode. A range was used to determine the optimum bath loading to produce the required microstructure for a SOFC anode. Standard lanthanum strontium manganite based cathodes were manufactured using the conventional applied using the standard method of screen printing and sintering. A cell was also produced using the same methods but without pore formers, to act as a comparison and determine any improvement produced by the addition of CMF as a pore former.

These cells were then characterized using Electrochemical Impedance Spectroscopy and a Scanning Electron Microscope with Energy Dispersive X-Ray Analysis capabilities. These were used to determine the power densities of the cells and the pore structure produced via a cross sectional analysis. A mercury porosimeter was used to determine the pore content and size in the ECD anodes.

Figure 1