EMI started working on the invention by preparing a group of carbon precursor powders.   The precursor powders were compressed under pressure to form a monolithic disk followed by carbonization to produce porous carbon disk with over 850 m2/gram surface area, average nano pore size (1 – 2 nm), and a narrow pore size distribution.

Activated carbon/carbon composite (AC/C), prepared by binding activated carbon powder with a polymer precursor followed by carbonization, were developed in 1990s by Japan’s NEC and Johnson Controls.  Due to weight loss and shrinkage of the polymer precursor, micro-cracks or micro-tunnels were developed during the carbonization process, yielding low electrolyte diffusion resistance and low electric series resistance (ESR) when AC/C was used as electrodes in electrochemical capacitors.

AC/C did not reach the market.  The polymer precursor after carbonization became dense carbon which blocks the pores of activated carbon, resulting low specific capacitance.

AC/C composite was one of early efforts of making binderless activated carbon electrodes.  The more recent efforts include carbon aerogel and carbon nano-tubes grown perpendicular on conductive substrates.

By using similar methodology of AC/C composite, EMI’s polymer precursor binds activated carbon into a monolith followed by carbonization to produce nanoporous carbon composite (NCC).   Different from AC/C composite, NCC possesses both high specific capacitance and low ESR.


APPLICATIONS OF NCC

EMI’s Nano-porous carbon composites (NCC) are used as monolithic porous carbon electrodes providing following merits in comparison with conventional carbon electrodes:
  • Lower cost electrochemical capacitors with higher power density and two to four times decrease of equivalent series resistance (ESR)
  • Capacitor/battery hybrid, such as PbC, nickel carbon battery, and ultrabattery, with longer cycle life and lower cost
  • Capacitive desalination devices to remove salt from brackish water for new industrial and residential water-reuse
  • Plus many other potential applications in NiMH, Li-Ion and other energy storage devices                                                                                                                                               


ELECTRODES FOR ELECTROCHEMICAL CAPACITORS

When NCC is used as electrodes for electrochemical capacitors, the specific capacitance is comparable to the conventional electrochemical capacitors using activated carbon powder electrodes while ESR, particularly ESR(1 kHz) is significantly lower.

ESR of NCC does not decrease with electrode thickness as fast as that of the conventional activated carbon powder electrodes.


ANODE FOR PbC

Lead acid batteries are common car and E-bike batteries.  PbC is prepared when anode or at least one of the anodes of the lead acid battery is replaced by activated carbon electrode for the purpose of significantly increasing charging/discharging rate and increasing the cycle life by a factor of 4 – 5 in deep cycles and over 10 times in shallow cycles.  One of the PbC applications is for micro hybrid electric vehicles. 

So far, PbC have not reached the market because of a number of obstacles caused by conventional activated carbon electrodes:
  • Metallic current collector protected by a conductive polymer coating is required in strong acid electrolyte
  • High bulk and contact resistance of conductive polymer coating
  • Difficulty in making defect free and tightly sealed conductive polymer coating with less than 50 micron thickness
  • Low resistance requires activated carbon powder based anode thinner than 0.5 mm, this limits the thickness of cathode to not more than 0.3 mm, leading to high production cost and shortened life of cathode

EMI’s NCC overcomes above obstacles:
  • Bulk electrical resistance of NCC (0.1Ω cm) is 20 times lower than conventional activated carbon electrode
  • NCC functions as both carbon electrode and current collector, eliminating metallic current collector
  • Over 2 mm thick NCC electrode matches the energy capacity of conventional lead acid battery cathode
  • ½ to 1/4 dc cell resistance compared to the literature value for activated carbon powder electrode as anode
  • 80 – 85% energy efficiency in comparison with 70 – 75% energy efficiency of lead acid batteries