Exploring Recent Developments in Graphene-Based Cathode Materials for Fuel Cell Applications: A Comprehensive Overview

Fuel cells are at the forefront of modern energy research, with graphene-based materials emerging as key enhancers of performance. This overview explores recent advancements in graphene-based cathode materials for fuel cell applications. Graphene's large surface area and excellent electrical conductivity and mechanical strength make it ideal for use in different solid oxide fuel cells (SOFCs) as well as proton exchange membrane fuel cells (PEMFCs). This review covers various forms of graphene, including graphene oxide (GO), reduced graphene oxide (rGO), and doped graphene, highlighting their unique attributes and catalytic contributions. It also examines the effects of structural modifications, doping, and functional group integrations on the electrochemical properties and durability of graphene-based cathodes. Additionally, we address the thermal stability challenges of graphene derivatives at high SOFC operating temperatures, suggesting potential solutions and future research directions. This analysis underscores the transformative potential of graphene-based materials in advancing fuel cell technology, aiming for more efficient, cost-effective, and durable energy systems.

Enhanced mixed proton and electron conductor at room temperature from chemically modified single-wall carbon nanotubes

Remarkably high mixed proton and electron conduction arising from oxidized single-wall carbon nanotubes at room temperature is demonstrated. The respective proton and electronic conductivities are 1.40 and 8.0 × 10-2 S cm-1 in the in-plane direction, and 3.1 × 10-2 and 1.1 × 10-3 S cm-1 in the out-of-plane direction, indicating their potential in a wide range of solid electrolyte membranes.

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

The development of efficient proton conductors that are capable of high power density, sufficient mechanical strength, and reduced gas permeability is challenging. Herein, we report the development of a series of aromatic sulfonic acid/graphene oxide hybrid membranes incorporating benzene sulfonic acid (BS), naphthalene sulfonic acid (NS), naphthalene disulfonic acid (DS) or pyrene sulfonic acid (PS) using a facile freeze dried method. For out-of-plane proton conductivity, the 3DGO-BS and 3DGO-NS yielded proton conductivities of 4.4×10-2  S cm-1 and 3.1×10-2  S cm-1 , respectively; this represents a two-times higher value than that which occurs for three dimensional graphene oxide (3DGO). Additionally, the respective prepared films as membranes in a proton exchange membrane fuel cell (PEMFC) show maximum power density of 98.76 mW cm-2 for 3DGO-NS while it is 92.75 mW cm-2 for 3DGO-BS which are close to double that obtained for 3DGO (50 mW cm-2 ).

Ion conduction switching between H+ and OH- induced by pH in graphene oxide

Ion conduction through graphene oxide (GO) nanosheets that is pH-switchable between H⁺ (in acid) and OH^- (in base) ions is demonstrated. This finding is the first observation of this type for ion conductive materials and demonstrates an example of stimuli-driven ion-conduction switching.

Branched Alkylamine-Reduced Graphene Oxide Hybrids as a Dual Proton-Electron Conductor and Organic-Only Water-Splitting Photocatalyst

We report multifunctionalities including the solid electrolytic property, electron conductivity (EnC), and photocatalytic water splitting (PWS) ability of organic-only hybrids obtained by intercalating short and branched-chain alkylamines including methylamine (MA), butylamine (BA), pentylamine (PA), and isomethylbytylamine (IMBA) in reduced graphene oxide (rGO). The alkylamine-rGO hybrids were synthesized by a facile solid-state reduction process. Within the series, IMBA-rGO exhibited high proton conductivity (PrC), EnC, and optimized PWS capacity. The PrC of IMBA-rGO was from 10^-4 to 10^-3 S cm^-1, which is only half an order less than that for pristine GO. The EnC was 1.25 μA/V. Though the PWS performances of MA-rGO, BA-rGO, and PA-rGO were comparatively lower, IMBA-rGO could generate about 1.5 times H₂ compared with that for R-TiO₂. The IR spectra indicate the association of IMBA and GO by chemical bonds. The Raman spectra show the transformation of GO's nonconductive sp³ carbon sites into electron-conductive sp² carbon centers. The thermogravimetric analysis show improved water adsorbing capacity of IMBA-rGO, which resulted in higher PrC. Doping of the nitrogen atom at the graphitic sp² system was confirmed from the presence of pyrrolic N in X-ray photoelectron spectroscopy spectra. The resultant N-type semiconducting behavior is majorly responsible for the PWS process. The powder X-ray diffraction analysis indicates a more flexible interlayer space in IMBA-rGO, which facilitates both the reformation of hydrogen bonds during proton conduction and water dynamics during photocatalysis. The material indicates the possibility of devising graphene-based organic-only multifunctional hybrids.

Proton Transfer at the Interaction Interface of Graphene Oxide

Proton transfer plays a crucial role in a variety of biological phenomena. The transformation of nanomaterials in the environment and biology makes probing the potential proton transfer between nanomaterials and biomolecules a crucial issue, but it still remains a significant challenge. Here, we report proton transfer at the interface of graphene oxide (GO) by studying the GO-induced vibrational changes of interfacial water and carboxyl-terminated self-assembled monolayer (SAM) with surface-enhanced infrared absorption spectroscopy. In addition to simply acting as a macromolecular buffer in solution, the GO sheet behaves as a two-dimensional hydrogen-bonded exchangeable proton pool to dissociate and transfer protons at the interface with a suitable Brønsted base pair, which may bear a significant potential toxic origin for biological systems with proton-coupled reactions.

Development of an All Solid State Battery Incorporating Graphene Oxide as Proton Conductor

Graphene oxide (GO) shows high proton conductivity (≈10-4 Scm-1), excellent mechanical stability, and electrical insulation property, which makes it an ideal candidate for use as a proton conducting solid state electrolyte. The prospects of using GO as single phase solid electrolyte in an all solid battery is presented herein. A battery with the cell configuration: Zn + ZnSO4•7H2O + graphite (anode) || GO (electrolyte) || MnO2 + graphite (cathode) is fabricated. Cyclic voltammetry confirms its rechargeable nature. The respective discharge capacity and power density of the cell are 360 μAh and 19.5 mW kg-1 at a constant current drain of 3 μA under the experimental conditions employed. GO based proton conductors are cleaner and cheaper than other solid electrolytes. The current study strongly suggests that GO can be used as a practical and beneficial component in solid state battery applications with low energy feedback.

Oxidation route dependent proton conductivities of oxidized fullerenes

Proton conductivities of oxidized fullerenes from different types of oxidizing agents were measured. Among all, NaOH treated fullerenes were showed higher proton conductivity.

Role of hydrophilic groups in acid intercalated graphene oxide as a superionic conductor

The role of hydrophilic groups in acid intercalated GO for proton conduction has been justified. The higher extents of adsorbed water due to the presence of hydrophilic groups are primarily responsible for offering enhanced proton conductivity.