Novel Slightly Reduced Graphene Oxide Based Proton Exchange Membrane with Constructed Long-Range Ionic Nanochannels via Self-Assembling of Nafion

Advances in the use of cellulose-based proton exchange membranes in fuel cell technology: A review

Fuel cells are electrochemical, ecologically friendly appliances that transform chemical energy into electricity in a clean, simple, and effective manner. With the advancement of technology in the field of computer science, electronic downsizing, and the ongoing need for mobility, the demand for portable energy sources such as fuel cells has considerably increased. The proton exchange membrane, which is designed to be a good conductor for protons while isolating electrons to move from the anode to the cathode, imprinting them an external circuit, and thus creating electricity, is at the heart of such an energy source. Perfluorosulfonic acid-based (NAFION) membranes, first introduced over 50 years ago, are still the state of the art in the field of fuel cell proton exchange membranes today. However, because of the numerous drawbacks connected with the usage of NAFION membranes, the scientific community has shifted its focus to producing new generation membranes based on natural materials, such as cellulose. Therefore, we believe that a review of the most recent studies on the use of cellulose as a material for proton exchange membranes in fuel cells may be very much appreciated by the scientific community.

Oxygen Absorption in Electrocatalyst Layers Detected by Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) is able to track the local electrochemical activity of an electrolyte-immersed substrate employing an ultra-micro-electrode (UME) in micrometer-scale spatial resolution. In this study, SECM is employed to investigate the presence of oxygen in the electrocatalyst layers of polymer electrolyte membrane fuel cells and electrolyzers. Approach curves on electrocatalyst layers with the tip potential set for oxygen reduction reveal that a significant amount of oxygen is absorbed in the catalyst layer. We confirm that the coexistence of Nafion ionomer and carbon black leads to oxygen confinement. It is suggested that this oxygen is confined within the hydrophobic parts of the self-assembled Nafion on the graphitic surfaces of the carbon black.

Exclusively Proton Conductive Membranes Based on Reduced Graphene Oxide Polymer Composites

Proton exchange membranes are at the heart of various technologies utilizing electrochemical storage of intermittent energy sources and powering electrical devices. Current state of the art membranes are based on perfluorosulfonic acid, introduced more than a half century ago. Low specificity to protons accompanied by permeance by other species is one of the main impediments for various promising applications in green technologies in an energy sustainable economy. Here we present composite membranes that are exclusively proton selective and do not allow crossover of any ionic or molecular species other than protons. Membranes have high proton conductivity and exceptional mechanical and chemical stability and thus may significantly improve performance of hydrogen-based technologies such as electrolyzers, various kinds of fuel cells, and flow batteries in the future.

Supramolecular Calix[n]arenes-Intercalated Graphene Oxide Membranes for Efficient Proton Conduction

Graphene oxide (GO) membranes with 2D interlaminar channels have triggered intensive interest as ion conductors. Incorporating abundant ion-conducting sites into GO interlayers is recognized as an effective strategy to facilitate ion conduction. Herein, we designed supramolecular compounds, para-sulphonato-calix[n]arenes (p-SC[n]As), as versatile intercalators to acquire highly conductive and robust GO membranes. The SC[n]A with ultrahigh ionic exchange capacity (IEC_w, 5.37 mmol g^-1) imparts sufficient proton donors, and its rigid framework imparts strong support of adjacent nanosheets. We designed three kinds of SC[n]As with the same IEC_w but different sizes as intercalators, endowing the GO/SC[n]A membranes with increasing ion concentration and d-spacing in the order of GO/SC[4]A < GO/SC[6]A < GO/SC[8]A. Therefore, the interlayers of GO/SC[8]A membranes afforded higher density of proton donors and could accommodate more water molecules to construct more continuous H-bond networks for proton transfer. Accordingly, the proton conductivities exhibited the same increasing trend, up to 327.0 mS cm^-1 of GO/SC[8]A-30% at 80 °C, 100% RH, which was 2.80 times higher than that of the GO membrane. Moreover, the GO/SC[n]A membranes remained stable in wet state, along with a 66% elevation in mechanical performance compared to the GO membrane.

Quaternized cellulose and graphene oxide crosslinked polyphenylene oxide based anion exchange membrane

Anion exchange membrane fuel cells (AEMFCs) have captivated vast interest due to non-platinum group metal catalysts and fuel flexibility. One of the major shortcomings of AEMFCs, however, is the lack of a stable and high anion conducting membrane. This study introduces a new strategy for fabrication of high conducting anion exchange membrane (AEM) using a hybrid nanocomposite of graphene oxide (GO), cellulose, and poly(phenylene oxide) (PPO), which are functionalized with 1,4-diazabicyclo[2.2.2]octane. The compositional ratio of GO/cellulose/PPO was optimized with respect to ionic conductivity, water uptake, swelling ratio, and mechanical properties. The membrane at GO/cellulose/PPO weight ratio of 1/1/100 displayed an impressive hydroxyl conductivity of ∼114 mS/cm at 25 °C and ∼215 mS/cm at 80 °C, which is considerably higher than the highest value reported. Further, the hybrid composite membranes were mechanically stable even when operating at high temperature (80 °C). The result indicates that the introduction of quaternized GO and cellulose into a polymer matrix is a promising approach for designing high performance AEMs.

Amorphous-to-crystalline transformation toward controllable synthesis of fibrous covalent organic frameworks enabling promotion of proton transport

Reversible imine exchange is adopted to transform amorphous polyazomethine into fibrous crystalline covalent organic frameworks that are able to immobilize Nafion for enhanced proton transport.

A graphene oxide polymer brush based cross-linked nanocomposite proton exchange membrane for direct methanol fuel cells

Functional polymer brush modified graphene oxide (FPGO) with functional linear polysiloxane brushes was synthesized via surface precipitation polymerization (sol-gel) and chemical modification. Then, FPGO was covalently cross-linked to the sulfonated polysulfone (SPSU) matrix to obtain novel SPSU/FPGO cross-linked nanocomposite membranes. Meanwhile, SPSU/GO composite membranes and a pristine SPSU membrane were fabricated as control groups. Reduced agglomeration of the inorganic filler and better interfacial interaction, which are benefit to increase diffusion resistance of methanol and to generate continuous channels for fast proton transportation at elevated temperature, were observed in SPSU/FPGO cross-linked membranes. Moreover, the enhanced membrane stability (thermal, oxidative and dimensional stability) and good mechanical performance also guaranteed their proton conducting durability. It is noteworthy that the SPSU/FPGO-1 cross-linked membrane possesses the best comprehensive properties among all the prepared membranes and Nafion®117, it acquires the highest proton conductivity of 0.462 S cm-1 at 90 °C under hydrated conditions together with a low methanol permeability of 1.71 × 10-6 cm2 s-1 at 30 °C. The resulting high membrane selectivity displays the great potential of the SPSU/FPGO cross-linked membrane for DMFCs application.