Polarity governed selective amplification of through plane proton shuttling in proton exchange membrane fuel cells

Abstract

Graphene oxide (GO) anisotropically conducts protons with directional dominance of in plane ionic transport (σ IP) over the through plane (σ TP). In a typical H-O fuel cell, since the proton conduction occurs through the plane during its generation at the fuel electrode, it is indeed inevitable to selectively accelerate GO's σ TP for advancement towards a potential fuel cell membrane. We successfully achieved ∼7 times selective amplification of GO's σ TP by tuning the polarity of the dopant molecule in its nanoporous matrix. The coexistence of strongly non-polar and polar domains in the dopant demonstrated a synergistic effect towards σ TP with the former decreasing the number of water molecules coordinated to protons by ∼3 times, diminishing the effects of electroosmotic drag exerted on ionic movements, and the latter selectively accelerating σ TP across the catalytic layers by bridging the individual GO planes via extensive host guest H-bonding interactions. When they are decoupled, the dopant with mainly non-polar or polar features only marginally enhances the σ TP, revealing that polarity factors contribute to fuel cell relevant transport properties of GO membranes only when they coexist. Fuel cell polarization and kinetic analyses revealed that these multitask dopants increased the fuel cell performance metrics of the power and current densities by ∼3 times compared to the pure GO membranes, suggesting that the functional group factors of the dopants are of utmost importance in GO-based proton exchange membrane fuel cells.

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.…”

“…). − 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. Therefore, if the degree of anisotropy of proton conductivity could be decreased or even if the GO membrane could be transformed to a near-isotropic or isotropic proton conductor, the through-plane proton conductivity would be further enhanced for practical applications.…”

“…40−45 However, when a GO membrane is employed as the solid electrolyte in some practical electrochemical devices, 28,29,40,46−54 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 [42][43][44]55 or the functionalization with proton-conductive moieties. 46,47,52,56 However, although these strategies have increased the overall ionic conductivity, they have not changed the degree of anisotropy.…”

High Proton Conductivity of 3D Graphene Oxide Intercalated with Aromatic Sulfonic Acids

“…In a further approach, PEMFC efficiency has been demonstrated to be enhanced by incorporation of a dopant containing both hydrophilic and hydrophobic domains, as occurs in aromatic sulfonic acids (Scheme 1). Thus, in the presence of sulfonic acid functionality the water content of the hybrid is increased, with the presence of the hydrophobic aromatic group significantly contributing to the mechanical strength of the film [42] …”

“…Thus, in the presence of sulfonic acid functionality the water content of the hybrid is increased, with the presence of the hydrophobic aromatic group significantly contributing to the mechanical strength of the film. [42] In a new study, we now report an investigation of the proton conduction behavior of 3DGO hybrid materials following their respective intercalation with benzene sulfonic acid (BS), naphthalene sulfonic acid (NS), naphthalene disulfonic acid (DS) and pyrene sulfonic acid (PS).…”