Organic hydrogenations
are key steps in the production of numerous
valuable chemicals. Their requirement for high temperature, pressure,
and compressed hydrogen has motivated the interest to develop safer
electrocatalytic hydrogenation (ECH) routes in benign aqueous electrolytes.
However, faradaic efficiencies in organic ECH tend to be greatly limited
by competition with the hydrogen evolution reaction and low reactant
solubility, which hinders the implementation of these more sustainable
hydrogenation routes. Using the hydrogenation of adiponitrile to hexamethylenediamine
(HMDA), a monomer used in the production of nylon-6,6, as a case study,
we investigate the effect of reactant concentration, temperature,
pH, and organic cosolvents on the ECH of nitrile groups with Raney
nickel electrodes. Higher reactant concentrations, alkaline electrolytes,
and mild temperature (40 °C) are key conditions that enhance
the hydrogenation of organic substrates against hydrogen evolution.
A maximum faradaic efficiency of 92% toward HMDA was obtained in aqueous
electrolytes at −60 mA cm–2. The addition
of an organic cosolvent is subsequently studied to evaluate the effect
of enhanced reactant solubility, achieving a 95% faradaic efficiency
at the same current density with 30% methanol by volume in water.
The insights gained from this study are relevant for the design of
energy efficient organic ECH and can help accelerate the implementation
of sustainable chemical manufacturing.