Despite the considerable in-plane
proton conductivity of graphene
oxide (GO) nanosheets, inadequate single-cell performance in a polymer
exchange membrane fuel cell (PEMFC) occurred on incorporating a vacuum-filtration-prepared
GO membrane between the electrodes. In particular, the proton transfer
between the electrodes in the PEMFC single cell is in the out-of-plane
direction of the GO membrane and was found to be significantly lower
than for the in-plane direction due to the presence of proton conduction
barriers arising from turbostratic stacking of the GO layers. Therefore,
the structural transformation of GO nanosheets into an ultrafast,
out-of-plane proton conductor is key to boosting GO-based PEMFC performance.
Here, we report the use of a freeze-dried route to three-dimensional
(3D) graphene oxide (3DGO) exhibiting a 3D interconnected network
and significant interlayer void space. The out-of-plane direction
proton conductivity of 3DGO was calculated to be 3.5 × 10–2 S cm–1 at 343 K and 100% relative
humidity (RH), which is about 175 times higher than that for the GO
membrane. The 3DGO was incorporated as a solid electrolyte in a PEMFC
single cell, and a maximum power density of 60.2 mW cm–2 was obtained at 30 °C. This high proton conductivity and PEMFC
performance of the 3DGO are correlated with the facile proton conduction
pathway and higher water uptake in the 3D porous architecture of 3DGO.
π‐π stacking between GO and benzene sulfonic acid (BS), naphthalene sulfonic acid (NS), pyrene sulfonic acid (PS), and naphthalene disulfonic acid (ND) results in the formation of respective GO‐BS, GO‐NS, GO‐PS and GO‐ND hybrid materials. The proton conductivities of these materials follow the trend as GO‐NS>GO‐BS>GO‐PS>GO>GO‐ND. GO‐NS, possessing the highest interlayer distance, exhibits the optimum proton conductivity. Evidently, GO‐sulfonic acid hybrids reveal excellent superionic conductivity.
Despite being generated from the same element and having some of the properties commonly shared, each type of carbon allotrope possessing divergent shape confers unique and distinguishable physicochemical properties, thereby making them attractive for a wide range of potential applications. Moreover, research progress has allowed for further tailoring the properties in a controlled way to design more fascinating and aesthetically pleasing architectures with outstanding materials properties. In fact, some chemically modified carbon allotropes and their products have shown significant promise to solve a number of major issues in polymer exchange membrane fuel cells (PEMFCs) and supercapacitors (SCs) such as efficient low Pt loaded oxygen reduction reaction (ORR) catalysts, carbon allotrope-based proton conductors for PEMFC electrolyte, carbon monoxide (CO) tolerant anode catalysts for PEMFC, and carbon allotropes-based SC electrodes. The tremendous progress made in carbon-based materials facilitates those technologies more realistically towards large-scale implementation in terms of increasing the stack power density and reducing cost. In this view, this review will provide a thorough insight for researchers into the use of carbon allotropes and relevant products for PEMFCs and SCs.
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