Zinc Bromide Batteries are fairly simple to make. and are arguably the longest lived battery technology ever developed. The only real degradation process is the slow escape of bromine vapor. With careful selection of materials, ZnBr2 batteries will last as long as the container lasts, !00+ years certainly.
Their are literally 100's of patents on Zinc Bromide battery technology. There are many possible improvements in the Electrolyte, but to me, most of them would seem to have life issues. Please use google patents to read/download the US patent # 312,802.
Patent date 1885 , Not a misprint !!! 128 year old patent !!!
A source of Carbon Felt :Tillman Welding Blanket - Thermofelt 615 , I haven't tried the Tillman blanket Felt , but the description is exactly what I'm using, it might have a coating. A 3x3 ft blanket ~ $20-$30