Stereochemistry-Dependent Proton Conduction in Proton Exchange Membrane Fuel Cells

Abstract

Graphene oxide (GO) is impermeable to H2 and O2 fuels while permitting H(+) shuttling, making it a potential candidate for proton exchange membrane fuel cells (PEMFC), albeit with a large anisotropy in their proton transport having a dominant in plane (σIP) contribution over the through plane (σTP). If GO-based membranes are ever to succeed in PEMFC, it inevitably should have a dominant through-plane proton shuttling capability (σTP), as it is the direction in which proton gets transported in a real fuel-cell configuration. Here we show that anisotropy in proton conduction in GO-based fuel cell membranes can be brought down by selectively tuning the geometric arrangement of functional groups around the dopant molecules. The results show that cis isomer causes a selective amplification of through-plane proton transport, σTP, pointing to a very strong geometry angle in ionic conduction. Intercalation of cis isomer causes significant expansion of GO (001) planes involved in σTP transport due to their mutual H-bonding interaction and efficient bridging of individual GO planes, bringing down the activation energy required for σTP, suggesting the dominance of a Grotthuss-type mechanism. This isomer-governed amplification of through-plane proton shuttling resulted in the overall boosting of fuel-cell performance, and it underlines that geometrical factors should be given prime consideration while selecting dopant molecules for bringing down the anisotropy in proton conduction and enhancing the fuel-cell performance in GO-based PEMFC.

A Near-Isotropic Proton-Conducting Porous Graphene Oxide Membrane

“…Benefiting from the facile functionalization, high aspect ratio of GO nanosheets, and its highly selective, molecular/ionic sieves-like interlayers, the GO membrane can be well applied as an ideal separator for multiple important applications, such as gas separation, − water treatment, ,,− and electrochemical energy-storage or conversion devices (e.g., batteries, fuel cells, supercapacitors, solar cells, etc . ). − More notably, the high aspect ratio of nanosheets enables the GO membrane with typical anisotropic transportation properties, including thermal conductivity, electronic conductivity, , and water permeability. ,,− Accordingly, the ionic conductivities of a GO membrane, mainly proton conductivity and hydroxide ion conductivity, are also found to be anisotropic, favoring the in-plane direction rather than the through-plane. ,, The ratio of in-plane proton conductivity (σ ∥ ) over the through-plane one (σ ⊥ ), that is, the degree of anisotropy (σ ∥ /σ ⊥ ) is reported to range from tens to hundreds for the GO membranes. − However, when a GO membrane is employed as the solid electrolyte in some practical electrochemical devices, ,,,− its anisotropic proton-conducting properties are not favorable because the through-plane proton conductivity is more essential and meaningful than the in-plane counterpart. Some approaches have been proposed to enhance the proton conductivity of GO membranes via either doping small molecules − , or the functionalization with proton-conductive moieties. ,,, However, although these strategies have increased the overall ionic conductivity, they have not changed the degree of anisotropy.…”

Fabrication and characterization of sulfonated polybenzimidazole/sulfonated imidized graphene oxide hybrid membranes for high temperature proton exchange membrane fuel cells

“…76 Bound water and water-uptake account for slow dehydration, in addition, affected the proton conductivity. 78 On the contrary, Nafion 117 suffers poor proton conductivity at low humidity and/or high temperature and undergoes chemical degradation at high temperature. At low humidity, addition of three water molecules to sulfonated para-PBI caused to pass a proton from sulfonate group to imidazole ring through hopping mechanism.…”

“…41 Thus, sPBI/SIGO-15 could reveal higher proton conductivity than that of sPBI. 78 On the contrary, Nafion 117 suffers poor proton conductivity at low humidity and/or high temperature and undergoes chemical degradation at high temperature. 79 The proton conductivity of pristine sPBI was 0.168 mS cm −1 , at the similar conditions.…”

Sulfonated fluorinated multi-block copolymer hybrid containing sulfonated(poly ether ether ketone) and graphene oxide: A ternary hybrid membrane architecture for electrolyte applications in proton exchange membrane fuel cells

“…In this regard, perfluorosulfonic acid (PFSA) membranes, such as Nafion, are somewhat prosperous candidates because of their high proton conductivity and good thermochemical stability. However, they have several inevitable drawbacks, including high cost, high fuel permeability and low proton conductivity at high temperature ( > 80 °C) and low humidity ( < 30%) conditions, which hinder the widespread commercial usage of PFSA membranes [12][13][14][15][16] . Consequently, researchers are developing modified PFSA membranes or designing alternative PEMs.…”