Pochari Technologies has been searching for improved electrolytes for improved water electrolysis. During this investigation, this research effort naturally stumbled upon ionic liquids. Ionic liquids possess extraordinary properties especially in regard to their potential to perform catalysis, permit regeneration, while maintaining miscibility in water. Ionic liquids possess high electrochemical stability windows, up to 4.9 volts and 4.0 volts for the selected ionic liquids in our research. Imidazolium based ionic liquids have been previously investigated as electrolytes and demonstrated stellar results, particular in elevated Faradaic efficiency thanks to lower activation barriers in the HER. These selected ionic liquids are not alkaline no strongly acidic (mildly acidic), thus their corrosivity against metals is far less severe. Many ionic liquids are not miscible (soluble) is water, thus cannot be used as electrolytes for water electrolysis, certain ionic liquids that are miscible in water possess sufficiently high electrical conductivity to enable reasonable current densities, for example, one of the most common ionic liquids studied: 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-PF4) pH of 5 and fully soluble it water, has an electrical conductivity of 3.7 millisiemens at room temperature for 100% concentration, for 12% in H2O, it increases to 29 millisiemens/cm, equivalent to 6% wt KOH in water. Another water soluble ionic liquid is 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIM-PF4), with 14.1 millisiemens as a pure solution, and around 85 millisiemens as an aqueous solution of 50% wt. In 2005, Roberto F de Souza first discovered the possibility of using ionic liquids as an alternative electrolytes to replace potassium and sodium hydroxide. Although ionic liquids for water electrolysis electrolytes has reserve scant research attention, and effectively zero commercial interest, its untapped potential is truly enormous. Ionic liquids also possess the advantage of relatively little to no toxicity, enabling safe handling and environmental disposal.
The rational for using the imidazolium based ionic liquid electrolytes instead of the alkaline solutions are two fold.
#1 Near 100% Faradaic efficiency enabled by reduced activation barrier.
In the few studies of imidazolium based ionic liquid electrolytes for hydrogen the evolution reaction (HER), each test using various electrodes demonstrated a minimum of 90% efficiency, with results as high as 98%, compared to a maximum of 75% for conventional alkaline mediums. This higher efficiency does come at the price of current density using the 1-butyl-3-methylimidazolium tetrafluoroborate due to its lower electrical conductivity. Using 1-Ethyl-3-methylimidazolium tetrafluoroborate, we estimate current density of 140 milliamp/cm2 or more. Lower current density is less of a limitation for non-alkaline electrolysis since the electrolyte is less corrosive, enabling the use of less expensive materials, such as carbon steel. De Souza synthesizes a novel and obscure ionic liquid, 3-(triethylamine-N-yl)-propane-1-sultonate, with the same tetrafluoroborate anion and achieved current densities of 1.77 amps/cm2 permitted by its higher dissolved conductivity of 132 ms/cm. While this particular ionic liquid is not commercially available, it can be synthesized relatively easily. Pochari Technologies is currently studying 1-Ethyl-3-methylimidazolium tetrafluoroborate as the ideal electrolyte with potential current densities of 140 milliamp/cm2 with efficiencies over 90%. The advantage of this ionic liquid is commercial availability.
#2 Reduced corrosive stress:
Traditional alkaline electrolysis places significant corrosive stress on on the expensive electrode material. A typical alkaline electrolyzer’s cost structure is dominated by the electrode module, usually nickel anodes and cathodes, contributing up to 40% of the stack cost. Since KOH and NaOH strongly corrode metals, even highly corrosion resistance metals such as nickel, significant annual loss incourred, for large electrolyzers, many kgs of material are lost annually, requiring periodic overhaul depending on electrode thickness. Ionic liquids have been considered as corrosion inhibitors and have found not to be highly corrosive. “Previous studies on carbon steel pointed out limited corrosiveness of 1-butyl-3-methylimidazolium and 1-ethyl-3-methylimidazolium tetrafluoroborate, also in co-presence of amines and CO2; some imidazolium-based ILs are even considered for applications as corrosion inhibitors, due to their adsorption and consequent formation of a protective film on the steel surface”
List of available ionic liquids:
Cations:
1-Ethyl-3-methylimidazolium
1-Ethyl-2,3-dimethylimidazolium
1-Propyl-2,3-dimethylimidazolium
1-Butyl-3-methylimidazolium
1-Hexyl-3-methylimidazolium
1-Octyl-3-methylimidazolium
Trimethylbutylammonium
N-Ethylpyridinium
4-Methyl-N-butylpyridinium
Trihexyl(dodecyl)phosphonium
Anion:
Bis(trifluoromethylsulfonyl)imide
Tetrafluoroborate
Hexafluorophosphate
Ethylsulfate
Trifluoromethanesulfonate
Trifluoroacetate
Octylsulfate
Thiocyanate
Commercial availability:
1-Butyl-3 methylimidazolium tetrafluoroborate is available on commercial marketplaces for $60/kg. The anion (tetrafluoroborate) of these ionic liquids are known to very slowly decompose in aqueous solutions, so occasional addition of fresh ionic liquid is required, this is projected to add negligible operational cost.
Advanced Energy Systems 