Carbon Nanocomposite Membrane Electrolytes for Direct Methanol Fuel Cells-A Concise Review

Abstract

A membrane electrolyte that restricts the methanol cross-over while retaining proton conductivity is essential for better electrochemical selectivity in direct methanol fuel cells (DMFCs). Extensive research carried out to explore numerous blends and composites for application as polymer electrolyte membranes (PEMs) revealed promising electrochemical selectivity in DMFCs of carbon nanomaterial-based polymer composites. The present review covers important literature on different carbon nanomaterial-based PEMs reported during the last decade. The review emphasises the proton conductivity and methanol permeability of nanocomposite membranes with carbon nanotubes, graphene oxide and fullerene as additives, assessing critically the impact of each type of filler on those properties.

Keywords: carbon nanotubes; direct methanol fuel cell; graphene oxide; proton exchange membranes.

Figure 1

Schematic diagram of a DMFC comprising the anode (Pt-Ru/C) and the cathode (Pt/C), separated by a polymeric proton exchange membrane (PEM).

Figure 2

Chemical structure of Nafion^®.

Figure 3

Chemical structures of (a) PEEK, (b) PES, (c) PEN and (d) cyclic PI.

Figure 4

Crosslinking of PVA using GA.

Figure 5

Percentage of GO, CNT and other carbon-based membrane electrolytes reported during last ten years. The data is generated by considering the references listed in Table 1.

Figure 6

Different functionalization approaches for CNTs using (a) carboxylic acid, (b) sulfonic acid (c) PSSA, (d) aminomethanesulfonic acid, (e) chitosan, (f) phosphotungstic acid (g) silica and (h) phosphonic acid groups.

Figure 7

Proton conduction mechanism in Nafion^®-ImCNTs composite membrane. Reproduced from [29] with permission from Elsevier, 2013.

Figure 8

Schematic representation of electrically aligned CNTs in SPEEK matrix. Reproduced from [41], with permission from American Chemical Society, 2015.

Figure 9

Different functionalization methods for GO.

Figure 10

Schematic illustration of proton and methanol transport in S-GO–SiO₂/Nafion^® composite membrane. Reproduced from [48], with permission from Royal Society of Chemistry, 2014.

Figure 11

SEM images of the GO-laminated Nafion^® membrane at different magnitudes. Reproduced from [72], with permission from Elsevier, 2013.

Figure 12

Selectivity of SPI and SPI-PSS-GO composite membranes compared with Nafion-117 membrane. Reproduced from [60], with permission from John Wiley and Sons, 2011.

Figure 13

Schematic representation of interaction between IL-GO and SPES matrix. Reproduced from [63], with permission from Elsevier, 2018.

Figure 14

SEM and TEM images of (A,B) SGO and (C,D) SGO/Fe₃O₄ nanosheets and cross-sectional SEM images of (E) PVA-GLA-SGO-Fe₃O₄ (F) PVA-GLA-SGO membranes. Reproduced from [93], with permission from American Chemical Society, 2015.

Figure 15

Photograph of a 12-μm-thick standalone GO laminate prepared by vacuum filtration of GO nanoplatelets suspension in DI water. Reproduced from [75], with permission from Elsevier, 2014.

Figure 16

(a) Schematic representation of sulfonation procedure for fullerene and (b) tensile strength of SPEEK-S-fullerene membranes after equilibrating in different concentration of methanol (0–8 M). Rreproduced from [76], with permission from Elsevier, 2016.

Figure 16

(a) Schematic representation of sulfonation procedure for fullerene and (b) tensile strength of SPEEK-S-fullerene membranes after equilibrating in different concentration of methanol (0–8 M). Rreproduced from [76], with permission from Elsevier, 2016.

Figure 17

(a) DMFC performance representing steady-state cell polarization for Nafion-117, pristine SPEEK and SPEEK-S-fullerene composite membranes at 60 °C and (b) oxidative stability for membranes. Reproduced from [76], with permission from Elsevier, 2016.

Figure 17

(a) DMFC performance representing steady-state cell polarization for Nafion-117, pristine SPEEK and SPEEK-S-fullerene composite membranes at 60 °C and (b) oxidative stability for membranes. Reproduced from [76], with permission from Elsevier, 2016.

Figure 18

DMFC polarization for (a) recast Nafion^® and (b) Nafion^®-FF (1 wt.%) at different methanol concentrations at 60 °C (anode: Pt-Ru/C, 2 mg cm⁻² and cathode: Pt/C, 2 mg cm⁻². Anode fuel: 2 M methanol 2 mL min⁻¹, cathode: oxygen 300 mL min⁻¹). Reproduced from [77], with permission from Elsevier, 2016.

Figure 19

AFM phase images for (a) pristine SPEEK, and (b) SPEEK-SCNF (1 wt.%) membranes. Reproduced with permission from [78], with permission from Elsevier, 2017.