I believe that reasonably pure materials are needed to make a nickel iron cell. I dont believe that the impure solution from wood ashes would be suitable.
One might consider ready made nickel iron cells, stored dry and sealed with the elctroylyte stores seperatly.

Or home made lead acid cells.
Lead sheet as used for roofing keeps indefinatly, as does sulphuric acid.
To make a lead acid cell, 2 lead plates are immersed in the acid which should have a specific gravity of about 1.25.
Capacity is maximised by having the greatest possible area of lead. This may be achieved by a spiral formation, it is vital that the plates dont touch, they should be seperated by some porous and acid resistant material such as fibre glass.

The polarity is arbitary when first manufactured, but one plate should be marked positive and one marked negative to avoid later confusion.
Pass current into the cell to charge, the initial capacity will be tiny, but it improves with each charge/discharge cycle.
The working life should be 20 years or more.