Nanoscopic and Macro-Porous Carbon Nano-foam Electrodes with Improved Mass Transport for Vanadium Redox Flow Batteries

Abstract

Although free-standing sheets of multiwalled carbon nanotubes (MWCNT) can provide interesting electrochemical and physical properties as electrodes for redox flow batteries, the full potential of this class of materials has not been accessible as of yet. The conventional fabrication methods produce sheets with micro-porous and meso-porous structures, which significantly resist mass transport of the electrolyte during high-current flow-cell operation. Herein, we developed a method to fabricate high performance macro-porous carbon nano-foam free standing sheets (Puffy Fibers, PF), by implementing a freeze-drying step into our low cost and scalable surface-engineered tape-casting (SETC) fabrication method, and we show the improvement in the performance attained as compared with a MWCNT sheet lacking any macro pores (Tape-cast, TC). We attribute the higher performance attained by our in-lab fabricated PF papers to the presence of macro pores which provided channels that acted as pathways for electrolytic transport within the bulk of the electrode. Moreover, we propose an electrolytic transport mechanism to relate ion diffusivity to different pore sizes to explain the different modes of charge transfer in the negative and the positive electrolytes. Overall, the PF papers had a high wettability, high porosity, and a large surface area, resulting in improved electrochemical and flow-cell performances.

Non‐Solvent Induced Phase Separation Enables Designer Redox Flow Battery Electrodes

“…[ 32–35 ] Advanced numerical simulations on porous media have demonstrated that streamwise‐oriented fibers and pores can lead to increased permeabilities and improved dispersion efficiency; [ 36 ] furthermore, material sets possessing wide variability in pore sizes with high specific surface area induce higher dispersion and reaction rates, improving overall performance. [ 37–41 ] Collectively, these prior works constitute important advances in the electrochemical science and engineering of porous electrodes. However, translation beyond the lab‐scale remains a concern, as constraints inherent to existent large‐scale carbon fiber manufacturing processes necessitate the introduction of additional and often complex post‐treatments to produce electrodes with suitable performance characteristics, which results in increased production costs.…”

“…[32][33][34][35] Advanced numerical simulations on porous media have demonstrated that streamwise-oriented fibers and pores can lead to increased permeabilities and improved dispersion efficiency; [36] furthermore, material sets possessing wide variability in pore sizes with high specific surface area induce higher dispersion and reaction rates, improving overall performance. [37][38][39][40][41] Collectively, these prior works constitute important advances in the electrochemical science and engineering of porous electrodes. However, translation beyond the Figure 1.…”

Recent Development of Carbon-based Electrode for Vanadium Redox Flow Battery

“…[13][14][15] The large surface area introduced by chemical treatment and the deposition of carbon nanomaterials are also enhanced reaction points for the vanadium species on the electrodes. [22][23][24][25][26][27][28][29] In recent research, the optimal combination of thermally activated and nonactivated carbon felts in VO 2+ /VO 2 + and V 2+ /V 3+ redox couples showed the most efficient configuration. 31 The electron transfer was found to occur by an inner-sphere process in VO 2+ /VO 2 + redox couple, whereas redox reaction of V 2+ /V 3+ progressed an outer-sphere electron transfer.…”

“…15 With some of metal and metal oxide additions, the chemical stability of the electrode surface remains poor, and the introduction is not inexpensive. Carbon-based nanostructured materials include carbon nanofibers (CNFs), 22 carbon nanotubes (CNTs), [23][24][25][26] and graphene (graphene oxide, nanowalls), 27,28 carbon nanosheets and nanorods. 29 These modification, deposition and growth techniques with carbon nanomaterials introduce enhanced wettability, a much larger surface area, and higher conductivity for vanadium RFBs.…”

Electrochemical Analysis of Architecturally Enhanced LiFe0.5Mn0.5PO4 Multiwalled Carbon Nanotube Composite

“…e secondary particles formed from agglomeration of the primary particles were random in size and ranged between 100 and 200 nm indicating that the crystals of the phosphor-olivine LiFe 0.5 Mn 0.5 PO 4 grow very well and have interparticle boundaries that have an effect in the chemistry of the material and its reactivity due to its porous nature, whereas the LiFe 0.5 Mn 0.5 PO 4 -MWCNT composite cathode shown in Figure 3(e) revealed nanoclusters of long-stranded carbon nanotubes which facilitate the movement of electrons during extraction and insertion of lithium within 3D framework between nanotubes and adjacent LiFe 0.5 Mn 0.5 PO 4 particles. e porous nanostructure of the LiFe 0.5 Mn 0.5 PO 4 is lamented by the carbon nanotubes, providing a larger electrode surface area, reducing the energy loss due to both activation and concentration of polarizations at the electrode surface [32]. e synthesized LiFe 0.5 Mn 0.5 PO 4 particles were subsequently attached to the ends and walls on the nanotubes.…”