Electrolytic Alcohol Synthesis Demonstrated as a New Form of Energy Storage

A group of Japanese Scientists recently demonstrated a liquid flow-type electrolyzer that continuously produces an alcohol from a carboxylic acid. This electrolyzer was constructed by using a polymer electrolyte similar to those used PEM water electrolyzers. At the anode the chemical reaction is the same as that in a PEM water electrolyzer:

2H2O ==> O2 + 4H+ + 4e-

The hydrogen ions (protons) pass through the polymer electrolyte membrane (an form Nafion) to the other side of the cell while the electrons pass through the external circuit to the other cathode. Instead of the protons and electrons combining to for hydrogen gas as in a PEM electrolyzer the following reaction takes place:

HOOC-COOH + 4H+ + 4e- ==> HOOC-CH2OH + H2O

HOOC-COOH is a organic compound called oxalic acid. HOOC-CH2OH is another organic compound called glycolic acid. Is some technical sense which I do not understand it falls into the class of organic compounds called alcohols. However, you would definitely not want to eat or drink this compound. In order for this reaction to take place the cathode must be immersed in a solution containing Oxalic acid.

This electrolysis system can be run in a continuous flow mode. That is the fluid in a tank containing oxalic acid in solution can be pumped past the membrane electrode assembly (MEA) where the oxalic acid is converted to glycolic acid, afterwards flowing in to a second tank. The power rating of this device depends on the properties of the MEA but the energy storage capacity depends merely on the size of the tanks. This decoupling of power from energy storage capacity is the same property possessed by redox flow batteries such as vanadium flow batteries.

I have previously pointed out an article in which some MIT scientists discuss the potential of flow batteries as an economic means of long term energy storage. Long term energy storage (i.e. weeks or months) requires a low cost for the for the storage materials. The cost of vanadium ($112/gm-mol) is so high that thousands of energy storage cycles are required in order to being the storage costs down to a reasonable level. Vanadium flow batteries are capable of operating from many thousands of cycles, so that application niches are being found for this technology. However, long term energy storage application are not economically practical.

The chemical inputs to electrolysis system described above are water and oxalic acid. The cost of oxalic acid is $0.39/gm-mol which more than two orders of magnitude cheaper than vanadium. Therefore this electrolysis system has the potential for economical long term energy storage if other aspects of the system have the right performance characteristics.

It does not appear, however, that round trip energy efficiency (27%) and the current density (60 ma/cm2 compared to 600 ma/cm2 for commercial water electrolyzers) are adequate for practical applications. Another economic issue with this system compared to vanadium batteries is that the system is not reversible. In order to convert the stored energy back to electricity the glycolic acid has to be run through a separate fuel cell with specially optimized MEA.

Of course the electrolytic production of hydrogen from water has the same potential advantage of low cost of the chemical inputs and separation of energy storage capacity from power rating. However, hydrogen gas has a very low volumetric energy density and is further more extremely corrosive, thus requiring expensive materials for the storage tank.

The researchers estimate the storage capacity of the oxalic acid solution to be 107 Ah/liter while the storage capacity of the glycolic acid solution is 417 Ah/liter. Since theoretical voltage of the electrolysis process is 1.1V the maximum energy storage capacity of the 'charged' solution is 417×1.1 = 460 Wh/liter. This storage capacity is equal to the very high end of lithium ion batteries but is small compared to fossil fuels (the energy density of gasoline is 8760 Wh/liter.). Therefore using these solution as a transportable energy medium is probably not economically practical unless the glycolic and oxalic acid can be precipitated out of solution and shipped as pure substances.

Jan 21, 2018

rogerkb at energystoragenews dot com