Creating Faradaic Carbon Nanotube Electrodes with Mild Chemical Oxidation

Abstract

The development of new materials to improve interfacial charge transfer characteristics will drastically improve energy storage, heterogeneous catalysis, and many other electrochemical applications. Here, we report a simple procedure that can harness the Faradaic nature of residual iron nanoparticle catalysts that endure within multi-walled carbon nanotubes (MWNT) post-synthesis, thereby alleviating the challenges associated with forging hybrid nanocomposite electrodes. Nonpurified MWNTs, undergo a chemical oxidation process in acidic conditions with KMnO 4 to partially "unzip" the MWNTs and expose the redox-active iron nanoparticles to the electrolyte. A stable redox peak associated with the Fe 2 + /3 + transition is achieved during the MWNT oxidation process yielding a~350 % increase in capacitance (> 300 F g À 1 ) relative to purified MWNT electrodes (70 F g À 1 ). While these materials solely may be pragmatic as energy storage electrodes, the integration of redox species within an inert carbon electrode will also provide new opportunities to accelerate heterogeneous charge transfer reactions.

Increasing Charge Transfer at the Liquid−Solid Interface Using Electrodes Modified with Redox Mediators

“…A new activation procedure has been developed by the group that permits large-scale modified CNTs. [37] These coin cells with two separate redox electrolytes had to be activated slowly to achieve a stable redox peak. Therefore, CV was conducted at 10 mV s À1 from 0.5 to 2 V for 15 cycles.…”

“…Redox reactions attributed to the presence of iron nanoparticles in M-CNT electrodes, in a previous paper, were shown to contribute 16 μWh of energy, which would account for 0.044% of the energy of M-CNT coin cells. [17,37] Therefore, this increase in energy density is predominantly associated with increased electrolytic conversion. Power density improvements are %141% with M-CNT coin cells with a maximum power density of 176 mW cm À2 for M-CNT coin cells and 73.0 mW cm À2 for C-CNT coin cells.…”

“…M-CNT electrode contained residual iron nanoparticles in the caps of the nanotubes from ferrocene/xylene catalysts commonly used to produce CNTs. [37] These CNTs were determined to contain 5.7 wt% iron and were in body-centered-cubic α-Fe, face-centered-cubic γ-Fe, and Fe 3 C formations. [17] CNTs are expensive carbon options (1À5$ gram À1 ) for RFB electrodes and improvements to charge transfer resistance should be applied for further investigation to other cheaper carbon allotropes.…”

“…[17] Chemical oxidation techniques were also used to partially open these M-CNT electrodes. [37] After activation, electrodes were wicked dried and soaked in redox electrolytes overnight. Nafion 212 membranes (0.05 mm, 0.92 meq g À1 , Beantown Chemical) were used for testing and were activated by refluxing in DI water, 3 v% hydrogen peroxide (30% Fisher), DI water, 0.5 M sulfuric acid (98% Fisher), DI water, and 1 M sodium hydroxide (99% Fisher), in this order for 1 h each at 90 C. Nafion was stored in 1 M NaOH solution until use.…”