The plasma-enchanced chemical vapor deposition (PECVD) tool was used to grow graphene nanowalls (GNWs) and nitrogen-doped graphene nanowalls (NGNWs) on titanium substrate. Two identical GNW/ Ti electrodes were assembled in asymmetric electrical double layer capacitors (EDLC). Meanwhile, the asymmetric EDLC consisted of a positive GNW/Ti electrode and a negative NGNW/Ti electrode. The binder-free, vertically grown GNW and NGNW electrodes respectively provided good examples for extending the upper potential limit of a positive electrode of EDLCs from 0.1 to 1.5 V (vs Ag/AgNO3) as well as the lower potential limit of a negative electrode of EDLCs from −2.0 V to ca. −2.5 V in 1 M tetraethylammonium tetrafluoroborate/propylene carbonate (TEABF4)/ propylene carbonate (PC) compared to activated carbons. This designed asymmetric EDLC exhibits a cell voltage of 4 V, specific energy of 30 Wh kg−1 , and specific power of 4 kW kg−1 and ca. >92% coulombic efficiency after 10,000 charge/discharge cycles, reducing the number of EDLC connections for a larger module voltage application. This work brings up the future where we can combine EDLCs and LIBs in advanced applications.
