Graphene oxide membranes for electrochemical energy storage and conversion

Graphene oxide (GO) membranes have recently attracted considerable attention for various applications involving filtration. In electrochemical systems, GO membranes serve as a separator or solid-state electrolyte; the roles which have been played for over four decades by the commercial ionic polymers such as Nafion. Owing to the versatility of GO membranes, they have shown an incredible potential for electrochemical energy storage and conversion. In lithium-sulphur batteries and similar electrochemical systems in which the electrode redox system is based on a conversion mechanism and subject to the so-called shuttle effect, a selective membrane can facilitate the transport of the electroactive species such as Li ions while blocking the release of the electroactive material (e.g., polysulphides) into the other half-cell. The same requirement is essential for the fabrication of redox flow batteries. Owing to the growing interest in flexible supercapacitors, there is a desire to replace liquid electrolytes with solid electrolytes, and GO membrane provides an appropriate scaffold for trapping gel electrolytes. The performance of Nafion in fuel cells has also been improved by the preparation of GO/Nafion membranes. All these different applications reveal the practical potential of GO membranes for the future energy storage and conversion.     

The choice of noble electrolyte for symmetric polyurethane-graphene composite supercapacitors

Simple, low cost, highly conducting, flexible, lightweight and porous electrodes are prepared using reduced graphene oxide (rGO) for energy storage device applications. Graphene oxide (GO) slurry is prepared using graphite powder through oxidation followed by solvothermal reduction. A simple dip and dry method to fabricate flexible electrodes by depositing GO on the skeleton of foams is reported. These electrodes are chemically reduced to enhance the conductivity and are used as an electrode material to facilitate large surface area and fast ionic diffusion. The state of the art of present work is all the devices studied under open air condition. The electrochemical studies demonstrate that the constructed supercapacitors exhibit a high specific capacitance of 69 F/g in 1 M NaOH electrolyte at 2 mVs⁻¹ scan rate which is significantly high. Also, the devices showed encouraging performance when constructed with different electrolytes, which helps to understand the electrolytic effect and to choose the best electrolyte for the high performance of supercapacitors.     

An overview of graphene in energy production and storage applications

Energy production and storage are both critical research domains where increasing demands for the improved performance of energy devices and the requirement for greener energy resources constitute immense research interest. Graphene has incurred intense interest since its freestanding form was isolated in 2004, and with the vast array of unique and highly desirable electrochemical properties it offers, comes the most promising prospects when implementation within areas of energy research is sought. We present a review of the current literature concerning the electrochemical application of graphene in energy storage/generation devices, starting with its use as a super-capacitor through to applications in batteries and fuel cells, depicting graphene's utilisation in this technologically important field.     

Polybenzimidazole composite membranes containing imidazole functionalized graphene oxide showing high proton conductivity and improved physicochemical properties

Polymer composite membranes are fabricated using poly[2,2'-(m-phenylene)-5,5′-bibenzimidazole] (PBI) as a polymer matrix and imidazole functionalized graphene oxide (ImGO) as a filler material for high temperature proton exchange membrane fuel cell applications. ImGO is prepared by the reaction of o-phenylenediamine with graphene oxide (GO). The compatibility of ImGO with PBI matrix is found to be better than that of GO, and as a result PBI composite membrane having ImGO exhibits improved physicochemical properties and larger proton conductivity compared with pure PBI and PBI composite membrane having GO. For example, PBI composite membrane having 0.5 wt% of ImGO shows enhanced tensile strength (219.2 MPa) with minimal decrease of elongation at break value (28.8%) compared with PBI composite membrane having 0.5 wt% of GO (215.5 MPa, 15.4%) and pure PBI membrane without any filler (181.0 MPa, 34.8%). The proton conductivity of this membrane, at 150 °C under anhydrous condition, is 77.52 mS cm^?1.     

A review of polymer electrolyte membranes for direct methanol fuel cells

This review describes the polymer electrolyte membranes (PEM) that are both under development and commercialized for direct methanol fuel cells (DMFC). Unlike the membranes for hydrogen fuelled PEM fuel cells, among which perfluorosulfonic acid based membranes show complete domination, the membranes for DMFC have numerous variations, each has its advantages and disadvantages. No single membrane is emerging as absolutely superior to others. This review outlines the prospects of the currently known membranes for DMFC. The membranes are evaluated according to various properties, including: methanol crossover, proton conductivity, durability, thermal stability and maximum power density. Hydrocarbon and composite fluorinated membranes currently show the most potential for low cost membranes with low methanol permeability and high durability. Some of these membranes are already beginning to impact the portable fuel cell market.     

N-spirocyclic ammonium-functionalized graphene oxide-based anion exchange membrane for fuel cells

Graphene oxide (GO) is a potential material in the electrode and membrane of polymer electrolyte membrane fuel cells due to its unique structure and various oxygen-containing functional groups. A class of three-layered GO/poly (phenylene oxide) for AEMs was prepared in this work. GO was functionalized with highly stable 6-azonia-spiro [5.5]undecane groups and used as a fast hydroxide conductor, named ASU-GO. Functionalized by N-spirocyclic cations, poly (phenylene oxide) (PIPPO) was then combined with ASU-GO and GO to fabricate the ASU-GO/PIPPO and GO/PIPPO. Notably, the maximum hydroxide conductivity of the ASU-GO/PIPPO was 73.7 mS cm⁻¹ at 80 °C, which was 3 times higher than that of the GO/PIPPO. The enhancement in hydroxide conductivity was due to the changes in the hydroxide transport mechanism and the poor stacked structure of the ASU-GO layer. Only 10.8% drops in hydroxide conductivity of ASU-GP/PIPPO after the alkaline test (1 M KOH at 80 °C for 700 h). Furthermore, the ASU-GO/PIPPO-50 membrane showed a maximum peak power density of 102 mW cm⁻², demonstrating the prepared membrane was promising in the AEM applications.     

Recent advances on graphene-based materials as cathode materials in lithium-sulfur batteries

Representing one of the advanced energy storage devices, lithium-sulfur batteries have a wide range of possible applications. While it serves as a promising energy storage system, it should adhere to multiple important criteria before a large-scale application can be realized. The novel uses of graphene can resolve the deficiencies of lithium-sulfur batteries. Graphene is an exceptional conductive material with excellent mechanical stability and is extremely flexible. Because of its large porosity value, it enables a high sulfur loading within its matrix and effectively encapsulates polysulfide. Graphene oxide, on the other hand, often exhibits a number of functional groups capable of chemically bonding to polysulfides, resulting in good capability for polysulfide entrapping. Physical containment of sulfur by graphene materials and its chemical interactions with sulfur can be further improved by designing 3D graphene-sulfur configurations through doping of functional groups or heteroatoms. This review article offers an insight into the strategies for achieving a high sulfur loading cathode with a long cycle life and high retention potential through sulfur containment and carbon host functionalization, for which this review focuses on the functionalization of graphene. Throughout these strategies, significant performance improvements can be achieved.     

Mixed-ceria reinforced acid functionalized graphene oxide-Nafion electrolyte membrane with enhanced proton conductivity and chemical durability for PEMFCs

Towards the next-gen energy solutions, Nafion, as a state-of-the-art polymer electrolyte to low temperature fuel cell (LTFC) application has been one of the most demanding hydrogen to clean energy conversion device ever achieved. However, the inherent issue of limiting chemical durability and restricted proton conductivity have always been a topic of concern with pure Nafion membranes. To tackle this, we report a mixed-ceria reinforced phosphorylated graphene oxide (sPGO)/Nafion membrane as a potential electrolyte to simultaneously improve the chemical durability and proton conductivity of bare Nafion by utilizing the redox property of ceria nanoparticles and acidic sites of sPGO for accelerated proton transfer. As a progressive method, the single-step phosphorylation of GO introduced short chain branching along with enhanced number of acidic sites in the Nafion matrix whereas incorporation of mixed-ceria nanoparticles improved the chemical durability of the membrane due to its superior radical scavenging property. As a result, mixed-ceria reinforced sPGO/Nafion (Ce-sPGO/NF) electrolyte membrane showed higher proton conductivity (1.2-times) and chemical durability (8.1-times) than bare Nafion. Furthermore, the polymer electrolyte membrane (PEM) was also showcased high enough thermomechanical and electrochemical stability at 80 °C and 100% relative humidity (RH).     

Neoteric advancements in polybenzimidazole based polymer electrolytes for high-temperature proton exchange membrane fuel cells - A versatile review

The world's dependence on hydrocarbon fuel to generate power has proven to be the primary source of energy production. The emission of dangerous toxic and effluent gases during the process of hydrocarbon extraction and utilisation poses a massive threat to the environment and human life. This has driven the research into green energy technology; polymer electrolyte membrane fuel cells (PEMFCs) is one of the future century's bright green and clean energy producers. The most critical factor in the fuel cell is the polymer electrolyte membrane (PEM), which is the heart of the fuel cell assembly. Recently, polybenzimidazole (PBI)-based high-temperature polymer electrolytes have attracted researchers because they have high chemical and thermal stability combined with fillers that can control proton mobility. There are several limitations to the usage of PBI in high-temperature PEMFCs and this review summarizes the various structural modification: phase inversion, semi-interpenetrating IPNs, branched blocks and physical modification methods like crosslinking, blending, doping, progress done by various researchers to tackle the drawbacks in PBI-based polymer electrolyte membranes.     

White graphene based composite proton exchange membrane: Improved durability and proton conductivity

Boron nitride, which is also known as “white graphene” may be an attractive filler for composite proton exchange membrane. Application of polymer electrolyte membranes in fuel cell as an electrolyte is gaining attention due to the requirement of clean energy. However, despite its attractive features it requires more consideration for complete commercialization. Herein we demonstrate the preparation of novel functionalized WHITE GRAPHENE (hexagonal boron nitride) and sulfonated poly ether sulfone (SPES) based polymer electrolyte membranes (PEM). Composite membranes have been characterized through thermal, mechanical, structural analysis. Membranes have been subjected to measure methanol permeability and proton conductivity at different temperatures for its use in DMFC. Composite membranes exhibit good physicochemical properties as well as high methanol crossover resistance. 0.5 wt % of FBN (SP-FBN-05) membrane is found to be adequate to get the better performance in DMFC.     

Self-assembled graphene film to enable highly conductive and corrosion resistant aluminum bipolar plates in fuel cells

We report in this paper a novel method to form protective graphene film on aluminum substrate, which is particularly applicable to bipolar plates in proton exchange membrane (PEM) fuel cells. By simply immersing an aluminum sheet in an aqueous solution of graphene oxide (GO), a layer of cross-linked GO gel forms on the aluminum sheet, taking advantage of dissociated aluminum ions as a cross-linker. Then the cross-linked GO is converted to graphene at 400 °C in hydrogen atmosphere. The chemistry of the self-assembled GO layer and its conversion to graphene film is revealed by FTIR and XPS. Under simulated fuel cell environment the graphene coated aluminum sheet shows a corrosion current density of <1 × 10^?6 A/cm², which is around four orders of magnitude lower than a bare aluminum sheet. Meanwhile, the graphene film on aluminum results in a much lower and more stable interfacial contact resistance (ICR) of <5 mΩ cm². These enable the graphene coated aluminum sheet to meet the U.S. DOE targets of 2020 for bipolar plates in terms of both the corrosion and electrical resistance. Thus the proposed method is very promising for protecting aluminum bipolar plates in PEM fuel cells.     

Enhanced transport properties of graphene‐based,thin Nafion® membrane for polymer electrolyte membrane fuel cells

A polymer electrolyte membrane fuel cell (PEMFC) is one of the promising renewable energy conversion systems; however, its performance is considerably limited by the sluggish transport properties and/or reaction kinetics of the catalyst layers, especially at a high current density. In this study, graphene‐based, thin Nafion® membranes are prepared using 0 to 4 wt% of graphene nanoflakes, and the effects of the graphene are examined for enhanced transport properties. The electrical conductivity and dielectric constant are drastically enhanced to 0.4 mS/cm and 26 at 4 wt% of graphene nanoflakes, respectively, while the thermal conductivity linearly increases to 3 W/m‐K. The proton conductivity also significantly increases with the aid of graphene nanoflakes at >2 wt% of graphene nanoflakes, and the enhancement doubles compared with those of the carbon‐black (CB)‐based and carbon nanotube (CNT)‐based, thin Nafion® membranes, perhaps due to unique graphene structures. Additionally, the quasi‐steady‐state water contact angle increases from 113° to ~130° with the addition of graphene nanoflakes, showing that a hydrophobic‐like water wetting change may be related to the significant proton conductivity enhancement. This work provides an optimal material design guideline for the transport‐enhanced cathode catalyst layer using graphene‐based materials for polymer electrolyte membrane fuel cell applications.     

Comparative methods for reduction and sulfonation of graphene oxide for fuel cell electrode applications

Graphene oxide (GO) was synthetized, reduced and further sulfonated for the preparation of electrodes. GO was obtained using modified Hummer's method from graphite flakes. The partial reduction of graphene oxide (rGO) was performed by chemical and thermal process and subsequently functionalized using sulfuric acid or aryl diazonium salt of sulfanilic acid as sulfonating agents to afford rGO-SO₃H. The influence of the reduction processes on the sulfonation reactions of rGO was evaluated through XRD, TGA, FT-IR and Raman techniques. Electrochemical properties of both rGO and sulfonated rGO materials as modified glassy carbon electrode were evaluated using a K₃FeCN₆ solution as a reference redox system. XRD confirmed the partial reduction of GO by two methods and FT-IR demonstrated that SO₃H groups were successfully grafted on GO. VC results confirmed that both reduction and sulfonated methods leads to a better material with superior electrochemical properties compared to GO and rGO. Thermally reduced GO and functionalized with sulfuric acid [rGO_T-SO₃H(1)] showed the best electron transfer activity compared to those chemically reduced or sulfonated with sulfanilic acid. The electrochemical properties observed for rGO_T-SO₃H(1) suggest than can be a suitable support material of nanoparticles for the preparation of electrodes for fuel cells applications.     

Evaluation of Quaternized polyvinyl alcohol/graphene oxide-based membrane towards improving the performance of air-breathing passive direct methanol fuel cells

Graphene oxide (GO) nanosheets are introduced to a Quaternized polyvinyl alcohol (QPVA) polymer matrix to obtain an anion exchange membranes (AEMs) for application of fuel cells. QPVA/GO nanocomposite membranes provide desirable properties such as low fuel uptake and permeability, excellent ionic conductivity, and cell performance, all of which are favorable for AEMs based on our previous works. Passive direct methanol fuel cells (DMFCs) are recognized as suitable technologies for use in portable devices. Nevertheless, the commercialization of DMFCs remains restricted due to a number of issues related to the conventional membrane; one of these issues is high fuel crossover problems due to high fuel uptake and permeability of Nafion membrane. This study aimed to expand the potential applications of QPVA/GO nanocomposite membranes in air-breathing passive DMFCs. The ionic conductivity, methanol uptakes (MUs), and permeabilities of self-synthesis QPVA/GO nanocomposites are examined to evaluate the ability to operate in methanol atmosphere. At 30°C, the ionic conductivity of the membranes reached 1.74 × 10⁻² S cm⁻¹. The MUs and permeabilities were as low as 35% and 7.6 × 10⁻⁷ cm² s⁻¹, respectively. The performance of air-breathing passive DMFCs bearing QPVA/GO nanocomposite membrane is much higher compared to conventional membranes. The maximum power density of air-breathing passive DMFCs was achieved 27.2 mW cm⁻² under the optimum condition of 2 M methanol + 4 M KOH at 70°C. Single-cells could be sustained for 1000 hours. This article is the first to optimize and highlight the performance air-breathing passive DMFCs by using a QPVA-based membrane.     

Highly durable phosphonated graphene oxide doped polyvinylidene fluoride (PVDF) composite membranes

The polyvinylidene fluoride (PVDF) drew great attention over time amongst the hydrocarbon polymer membranes because of its high C–F chemical bond strength. In this work is to increase the proton conductivity of the PVDF polymer by doping phosphonated graphene oxide to its structure and investigate the improvement of the membrane. Different amounts of phosphonated graphene oxide additive (0.5%, 1% and 1.5% w/w) were doped to PVDF polymer on the purpose of synthesizing proton exchange composite membranes. Characterization tests, i.e, water uptake, swelling properties, ion exchange capacity, and proton conductivity of the synthesized membranes were investigated. The electrochemical impedance analysis results of synthesized membranes vary between 0.0224 S cm^-1 for 0.5% graphene oxide doped PVDF (PVDF/0.5PGO) and 0.0867 S cm^-1 for PVDF/1.5GO membrane. The power density values of PVDF/1.5PGO and PVDF/0.5GO are 48 mW cm⁻² and 28 mW cm⁻² at 0.6 V and 100% relative humidity at 80 °C. The experimental results demonstrate the importance of phosphonated graphene oxide doping into the PVDF composite membrane.     

An overview of proton exchange membranes for fuel cells: Materials and manufacturing

Due to their efficient and cleaner operation nature, proton exchange membrane fuel cells are considered energy conversion devices for various applications including transportation. However, the high manufacturing cost of the fuel cell system components remains the main barrier to their general acceptance and commercialization. The main strategy for lowering the cost of fuel cells which is critical for their general acceptance as alternative energy sources in a variety of applications is to lower the cost of the electrolyte and catalyst. An electrolyte is one of the most important components in the fuel cell and a major contributor to the cost (>$500/m² for commercial Nafion® series). Nafion is widely used as an electrolyte in PEMs, but it has some limitations in addition to high costs such as low proton conductivity, high-temperature performance degradation, and high fuel crossover. Therefore, the development and manufacturing of low-cost and high-performance electrolyte membranes with higher conductivity (～0.1 S·cm ^?1) at a wider temperature range is a top priority in the scientific community. Recent years have seen extensive research on the preparation, modification, and properties of PEMs such as non-Nafion membranes (SPI, PBI, polystyrene, polyphosphazene, SPAEK, SPEEK, SPAS, SPEN), and their composites by incorporating functionalized CNTs, GO as fillers to overcome their drawbacks. This paper provides a comprehensive review of membrane materials and manufacturing with a focus on PEMs. In particular, the review brings out the basic mechanism involved in proton conduction, important requirements, historical background, contending technologies, types, advantages and disadvantages, current developments, future goals, and directions design aspects related to thermodynamic and electrochemical principles, system assessment parameters, and the prospects and outlook.     

Control Algorithm of Fuel Cell and Batteries for Distributed Generation System

This paper intends to propose a novel control algorithm for utilizing a polymer electrolyte membrane fuel cell (PEMFC) as a main power source and batteries as a complementary source, for hybrid power sources for distributed generation system, particularly for future electric vehicle applications. The control, which takes into account the slow dynamics of a fuel cell (FC) in order to avoid fuel (hydrogen and air) starvation problems, is obviously simpler than state machines used for hybrid source control. The control strategy lies in using an FC for supplying energy to battery and load at the dc bus. The structure is an FC current, battery current, and battery state-of-charge (SOC) cascade control. To validate the proposed principle, a hardware system is realized by analogical circuits for the FC current loop and numerical calculation (dSPACE) for the battery current and SOC loops. Experimental results with small-scale devices (a 500 W PEM FC and 33 Ah, 48 V lead-acid battery bank) illustrate the excellent control scheme during motor drive cycles.     

CVD grown graphene as catalyst for acid electrolytes

Chemical Vapor Deposition (CVD) process is utilized to grow and study behavior of porous, continuous-phase 3D graphene structures in acid electrolyte. Graphene layers that are produced by CVD process are tested for oxygen reduction reaction (ORR) activity by Rotating Disc Electrode (RDE) measurements in 0,1 M HClO₄ electrolyte. Raman spectroscopy measurements confirms multi-layer porous structure formation for more than 1 min grow on nickel foam. Multi-layer porous graphene has provided μA level current. When NH₃ is used for nitrogen (N)-doping, magnitude of the reduction current increases, but still low for practical usage of graphene in acid electrolytes as catalyst. N-doping is confirmed with XPS measurements showing all possible types of N-doping phases with 900 °C being better than 1000 °C doping. CVD grown continuous-phase graphene, by itself or N-doped, cannot provide enough electrocatalytic activity to be used in 0,1 M HClO₄ acid electrolyte or polymer electrolyte membrane (PEM) fuel cells for practical applications. Pt layer of 10 nm has been sputtered on to graphene (21.45 μg/cm² Pt loading) and has provided orders of magnitude increase in oxygen reduction current compare to bare graphene layers.     

Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study

Chitosan has been considered attractive in polymer electrolyte membrane fuel cells (PEMFCs) due to its excellent film forming and fuel barrier properties. Reflecting the limitation of its low proton conductivity, various materials were used to improve the proton conductivity of chitosan, through combination with inorganic materials like graphene oxide. We present an ideal molecular model for bio-nanocomposites and their mechanism of proton conductivity in PEMFCs. In this study, the diffusion behavior of hydronium ions in chitosan/graphene complex systems at various temperatures, concentrations and pH values were studied systematically using 3 ns long molecular dynamics (MD) simulations with an aim to provide the mechanisms of proton conductivity of chitosan/graphene composite at an atomistic scale. Various amounts of water content (10%, 20%, 30% and 40%), pH values (achieved by adjusting the protonation degree of amino groups of chitosan by 20%, 40%, 60%, 80% and 100%) and numbers of graphene sheets (1, 2, and 3) were considered during MD simulations at 4 temperatures (298 K, 320 K, 340 K and 360 K). Our results indicated that the chitosan system containing 40% water was the most suitable polymer electrolyte membrane and temperature was a key factor affecting diffusion proton. Adding graphene to the chitosan system and adjusting the pH values of chitosan were demonstrated to have a significant effect on improving the proton conductivity of the membrane.