Enzyme-catalysed room temperature and atmospheric pressure synthesis of metal carbonate hydroxides for energy storage

For the first time, a generalised enzyme-catalysed room temperature and atmospheric pressure method for synthesising metal carbonate hydroxides is shown. The enzyme urease was used to catalyse the hydrolysis of urea at room temperature and atmospheric pressure. Product CO32- and OH- anions were separated from urease and used to precipitate low crystallinity metal carbonate hydroxides from solutions containing Ni2+, Co2+, Zn2+ and/or Cu2+ cations. Ni-Co carbonate hydroxides with different Ni2+/Co2+ molar ratios were evaluated as supercapacitor electrodes. An optimised Ni2+/Co2+ molar ratio of 3:1 yielded a specific capacitance of 1499 F g-1 @ 1 A g-1 current density. Combining this material with graphene oxide increased the specific capacitance to 1656 F g-1 @ 1 A g-1 current density. In turn, using this composite as the positive electrode in an asymmetric supercapacitor with activated carbon as the negative electrode yielded a high energy density of 45.8 Wh kg-1 at a power density of 899 W kg-1, and 70% capacitance retention after 10,000 cycles. Our enzyme-catalysed room temperature and atmospheric pressure method may be promising for industrial-scale production of nano-materials for energy conversion and storage.