Reduced Methanol Crossover Research Articles

Vapor-feed direct methanol fuel cells using pure methanol

This work optimizes the performance of the direct methanol fuel cell (DMFC) to increase its efficiency and strengthen its validity in portable power generation. Specifically, this work focuses on optimizing vapor-feed supply techniques and incorporating water management layers (WMLs) to analyze their effect on methanol crossover. The significance of the vapor-feed supply technique is to enhance the reaction kinetics of the methanol oxidation reaction (MOR) and enable the use of pure methanol (MeOH) as a fuel. Pure methanol is the ideal fuel for the DMFC as it has the highest possible energy density compared to dilute concentrations. However, use of pure methanol is hindered by methanol crossover, which is regarded as the largest technical barrier to commercializing DMFCs. This study measured methanol crossover through a CO2 sensor attached to the cathode outlet and added hydrophobic WMLs to the cathode to alleviate the methanol crossover. The hydrophobic WMLs increased the mass transfer resistance to generate a pressure gradient that encourages water backflow for use in both the proton exchange membrane (PEM) and anode reactions. The influence of vapor flow rate and fuel concentration will also be explored to show their impact on performance and methanol crossover. Likewise, long-term consumption and durability tests were conducted with and without a WML to dictate the WML’s superior fuel efficiency, total efficiency, energy density, and reduced methanol crossover using pure methanol. The addition of the WML increased the energy density of the vapor feed DMFC, using pure methanol, from 705.9 Wh kgMeOH-1 to 867.7 Wh kgMeOH-1 and lowered the crossover current density by 14.8 % when discharged at a constant 200 mA cm−2.

Managing Fuel Crossover in Direct Methanol Fuel Cells By Optimizing the Anode Microporous Layer

Direct methanol fuel cells (DMFCs) have been considered a promising candidate as a replacement for small scale and portable power applications, however, there are technical barriers that have prevented widespread use of DMFCs in commercial applications. Of these technical barriers, unreacted methanol crossing over from the anode to the cathode has been identified as a primary limiting factor of DMFCs as this phenomenon results in lowered efficiency and reduced open circuit voltages (OCVs) on the cathode, and ultimately limits the OCV of the cell itself.1 In this work, the authors used an anode design proposal provided by (Metzger et al, 2021, JEECS-20-1185) to create a strongly hydrophilic microporous layer (MPL) on the anode gas diffusion layer (GDL) to reduce methanol crossover by increasing the capillary pressure in the pores of the MPL required for the methanol to crossover prior to reacting.2 The authors fabricated three membrane electrode assemblies. (MEAs) MEA 1 was fabricated using a traditional hydrophobic MPL on the anode GDL, whereas MEAs 2 and 3 were fabricated using customized hydrophilic MPLs. The GDL for MEA 2 was heat treated at 165 F for one hour and the GDL for MEA 3 was dried in atmospheric conditions for 24 hours. A peak power density of 71 mW/cm2 was observed using 1M methanol solution with MEA 1. Using this approach, the authors were able to achieve a peak power density of 68.1 mW/cm2 and 70.1 mW/cm2 with MEAs 2 and 3, respectively, using 3M methanol solution. Additionally, the authors achieved a peak power density of 32 mW/cm2 and 30.7 mW/cm2 using 7.5M methanol solution. This study provides a framework for creating low-cost hydrophilic MPLs that can be utilized on the anode GDL to reduce fuel crossover in DMFCs. Figure 1

Polyaniline decorated graphene oxide on sulfonated poly(ether ether ketone) membrane for direct methanol fuel cells application

AbstractIn direct methanol fuel cells (DMFC), methanol crossover is a major issue which has reduced the performance of polymer electrolyte membrane (PEM) for energy generation. In this study, graphene oxide (GO) and conductive polyaniline decorated GO (PANI‐GO) were used as additives in fabrication of sulfonated poly(ether ether ketone) (SPEEK) nanocomposite PEM membrane to reduce methanol crossover. PANI‐GO was synthesized by in situ polymerization method and the formation of PANI coated GO nanostructures was confirmed by surface morphology and crystallinity analysis. The membrane morphology and topography analysis confirmed that GO and PANI‐GO were well dispersed on the surface of SPEEK membrane. 0.1 wt% PANI‐GO modified SPEEK nanocomposite membrane exhibited the highest water uptake and ion exchange capacity of 40% and 1.74 meq g−1, respectively. The oxidative stability of the nanocomposite membranes also improved. Lower methanol permeability of 4.33 × 10−7 cm−2S−1 was noticed for 0.1 wt% PANI‐GO modified SPEEK membrane. PANI‐GO modified SPEEK membrane enhanced the proton conductivity, which was due to the existence of acidic and hydrophilic group present in PANI and GO. PANI‐GO modified SPEEK membrane held higher selectivity of 1.94 × 104 S cm−3 s−1. Overall, these studies revealed that PANI‐GO modified SPEEK membrane is a potential material for DMFC applications.

Investigation of thermo-mechanical behavior, proton transfer and methanol permeation of polymer electrolyte membrane in low sulfonated state modified with thermally stable surface functionalized graphene oxide nanosheets

Sulfonated polyether ether ketone is currently under investigation to replace the expensive Nafion® as polymer electrolyte membrane. Sulfonic acid group is hydrophilic in nature and plays a key role in proton transfer in polymer electrolyte membranes. The higher the degree of sulfonation the higher proton conductivity in sPEEK based polymer exchange membrane is achieved; however, high degree of sulfonation causes severe dimensional instability due to higher water uptake which in turn affects membrane fuel retention capabilities as well as thermo-mechanical strength. Graphene oxide nanosheets were functionalized with aryl dizonium salt of p-Aminobenzene sulfonic acid for modification of sPEEK in low sulfonated state (DS = 53%) to improve its proton conductivity and suppress fuel crossover. The functionalization was carried out to compensate for the possible dilution of proton exchangeable sites by fillers. The resulting membranes were characterized in terms of morphology, thermo-mechanical behavior, methanol permeability, water uptake, and proton conductivities. The results proved that sulfonated GO not only improved thermo-mechanical behavior of the membrane but also dramatically increased proton conductivity (47 mS cm−1) than pristine sPEEK (25 mS cm−1). The composite membranes showed highly reduced methanol crossover compared with pristine sPEEK. The electrochemical selectivity increased from 9.5 × 104 Scm−3s (pristine sPEEK) to 26.9 × 104 Scm−3s.

Nafion® reinforced with polyacrylonitrile/ZrO2 nanofibers for direct methanol fuel cell application

AbstractNafion® membrane blended with polyacrylonitrile nanofibers decorated with ZrO2 was successfully fabricated. The composite membrane showed improved proton conductivity, swelling ratio, thermal and mechanical stability, reduced methanol crossover, and enhanced fuel cell efficiency. The nanocomposite membranes achieved a reduced methanol crossover of 5.465 × 10−8 cm2 S−1 compared to 9.118 × 10−7 cm2 S−1 of recast Nafion® membrane using a 5 M methanol solution at 80°C. The composite membrane also showed an ion conductivity of 1.84 compared to 0.25 S cm−1 recast Nafion® at 25°C. The composite membranes showed a peak power density of 68.7 mW·cm−2 at 25°C, these results show a promising composite membrane for fuel cell application.

Enhancement of proton/methanol selectivity via the in-situ cross-linking of sulfonated poly (p-phenylene-co-aryl ether ketone) and graphene oxide (GO) nanosheets

It has been reported that introducing graphene oxide (GO) or functionalized GO into proton exchange membranes (PEMs) can inhibit methanol diffusion by increasing the complexity of the methanol diffusion pathway in the hydrophilic region. In fact, methanol permeation also occurs in the hydrophobic area, which is very important, but rarely studied. In this study, we propose a facile and effective in-situ cross-linking method to prepare GO/sulfonated poly (p-phenylene-co-aryl ether ketone) crosslinked PEMs. By using the sulfonic acid group in parent polymer as a solid acid catalyst, the –COOH groups in GO undergo the Friedel-Craft acylation with the nucleophilic phenyl rings in the hydrophobic segments under vacuum heating, and the construction of covalent cross-linking network between GO and hydrophobic region is achieved successfully. The obtained crosslinked membranes show obviously reduced methanol crossover and greatly enhanced selectivity. The highest power density of the passive DMFC is obtained by using the 2 wt% GO crosslinked membrane and its value (28.1 mW cm-2) is higher than the original parent polymer (24.4 mW cm-2). These results demonstrate that the hydrophobic phase is closely relevant to the transport of methanol molecules in the membrane, which provides a new idea for the construction of methanol-blocking PEMs.

Methanol crossover reduction and power enhancement of methanol fuel cells with polyvinyl alcohol coated Nafion membranes

This paper presents the effect of polyvinyl alcohol (PVA) coated Nafion membranes on their water uptake, swelling and proton conductivity for various PVA coating thicknesses. These studies show that the optimum coating thickness of PVA on Nafion is 2 µm. Methanol permeation studies show that 2 µm thick PVA coating forms a barrier for methanol and significantly reduces methanol permeation through the membranes. Further, passive methanol fuel cells are tested with 2 µm thick PVA coat on Nafion as proton exchange membranes and their polarization plots show a significant enhancement in power as compared to the methanol fuel cells with pristine Nafion due to reduction in methanol crossover.

Effect of Calcium Hydroxy Phosphate as a Crystallinity Enhancer in Nafion Membrane for Direct Methanol Fuel Cell

Nafion is a widely used membrane in direct methanol fuel cell (DMFC). Many advantages of Nafion including good chemical stability, good thermal resistance, and high proton conductivity. However, Nafion has high methanol crossover which remarkably reduces the performance of DMFC and results in waste of fuel. Therefore, in this work, calcium hydroxy phosphate (CHP) was incorporated into Nafion to increase its crystallinity. The properties of Nafion/CHP such as crystallinity, water uptake, ion exchange capacity, methanol crossover, and solubility were studied. SEM/EDS with elemental mapping was used to observe the dispersion of CHP within the membrane. It was clearly shown that by incorporating 1.0 wt% and 2.5 wt% of CHP into the membrane, methanol crossover at 30 C was reduced from 125.6 x 10-6 cm2s-1 to 2.58 x 10-6 cm2s-1 and 65.58 x 10-6 cm2s-1. The reduction of methanol crossover of CHP/Nafion composite membrane at higher weight was lesser than that of the composite at lower weight due to the agglomeration of CHP.

Preparation of sulfonated polysulfone/sulfonated titanium dioxide hybrid membranes for DMFC applications

ABSTRACTSulfonated titanium dioxide (STiO2) was prepared by the reaction of TiO2 with 1,3‐propanesultone. Novel STiO2 incorporated sulfonated poly(aryl ether sulfone) (SPAES) nanocomposite proton exchange membranes (PEMs) were made by solution casting. Fourier transform infrared, X‐ray photoelectron spectroscopy, and proton nuclear magnetic resonance indicated the successful preparation of STiO2 and SPAES. The thermogravimetric analysis and oxidative stability testing results implied that SPAES/STiO2 membranes had better stability than pristine SPAES membrane. Meanwhile, the scanning electron microscopy spectra exhibited that the introduction of sulfonated groups on the surface of TiO2 significantly improved its dispersibility in SPAES matrix. More specifically, SPAES membrane incorporated with 2%STiO2 exhibited higher proton conductivity (60 mS cm−1), lower methanol permeability (2.1 × 10−7 cm2 s−1) and better proton selectivity (28.0 × 104 S s cm−3) than that of pure SPAES and SPAES/1%TiO2 membrane. The SPAES‐1%STiO2 membrane showed better performance in direct methanol fuel cell (DMFC) test than SPAES and Nafion 117 due to the reduction of methanol crossover. From these results, it is evident that SPAES/STiO2 nanocomposite PEMs have great potential for applications in DMFCs. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48938.

Improving the performance of direct methanol fuel cells by implementing multilayer membranes blended with cellulose nanocrystals

Methanol crossover through the proton exchange membranes of a direct methanol fuel cell (DMFC) significantly affects its performance and efficiency. Low methanol permeability and high proton conductivity of the membrane is desired for optimum performance. In this work, a multilayer (ML) membrane configuration prepared by a simple pressing technique is employed with and without the incorporation of sprayed cellulose nanocrystals (CNC) to achieve enhanced membrane properties. Assembled multilayer electrolytes show 19% enhanced maximum power density, while the addition of 1.5 wt% CNC (wt % of total final membrane) further improves the performance, giving a 38% better performance compared to standard Nafion N115. Methanol flux density and electrochemical impedance measurements attribute these improvement to the ~20% enhancement in the proton conductivity for the multilayer membrane which is enhanced further by an 11% reduction in methanol crossover when the cellulose nanocrystals are added.

Covalent grafting of polystyrene sulfonic acid on graphene oxide nanoplatelets to form a composite membrane electrolyte with sulfonated poly(ether ether ketone) for direct methanol fuel cells

Covalent grafting of polystyrene sulfonic acid (PSSA) on graphene oxide nanoplatelets (GONP) via in situ radical polymerization was conducted. PSSA-g-GONP is used as an additive to form a nanocomposite membrane with sulfonated poly(ether ether ketone) (sPEEK) for its application in direct methanol fuel cells (DMFCs), with better physicochemical properties and reduced methanol crossover compared to that of pristine sPEEK. The optimized sPEEK/PSSA-g-GONP (0.15 wt %) nanocomposite membrane exhibits a DMFC peak power density of 170 mW cm-2, which is 50 % greater than the 110 mW cm-2 for pristine sPEEK and one-fold greater than that of 80 mW cm-2 observed for commercial Nafion® 117 due to its high electrochemical selectivity. Furthermore, the nanocomposite membrane shows better durability up to 100 h compared with sPEEK and Nafion® 117 electrolyte membranes, demonstrating its potential in diverse membrane applications.

Nanocomposite membrane electrolyte of polyaminobenzene sulfonic acid grafted single walled carbon nanotubes with sulfonated polyether ether ketone for direct methanol fuel cell

Nanocomposite membranes of sulfonated polyether ether ketone (sPEEK) are prepared with polyaminobenzene sulfonic acid grafted single-walled carbon nanotubes copolymer (PABS-SWCNT) and its zwitterion interactions are studied. The nanocomposite membranes are prepared through solution cast technique using PABS-SWCNT as additive in different weight % (0.1, 0.15, and 0.2) ratio. The additive and nanocomposite membranes are characterized for its surface morphology, composition, thermal and physico-chemical properties. The nanocomposite membrane comprising optimized content of PABS-SWCNT (0.15 wt %) shows improved proton conductivity and reduced methanol crossover resulting in enhanced DMFC peak power density of 150 mW cm−2 in comparison to 110 mW cm−2 for sPEEK and 80 mW cm−2 for Nafion® 117 respectively. The improved durability till 100 h for sPEEK/PABS-SWCNT (0.15 wt %) compared to sPEEK and Nafion-117 confirms its viability in DMFC application.

Improved Anode Two-Phase Mass Transfer Management of Direct Methanol Fuel Cell by the Application of Graphene Aerogel

The main purpose of this paper is to utilize the intrinsic hydrophobicity and microstructure of graphene aerogel to construct a novel anode two-phase mass transfer structure for passive micro direct methanol fuel cell. Compared with its traditional counterpart composed of carbon black and poly(tetrafluoroethylene), the anode gas diffusion layer based on graphene aerogel holds some greatly improved microstructures, which contributes to an excellent gas–liquid mass transfer management. During discharge, smaller, more evenly distributed bubbles of CO2 fill in the pores of the three-dimensional structure of graphene aerogel and form a convection in the opposite direction of methanol transport, increasing the mass transfer resistance of methanol without causing a loss of effective reaction area. A methanol crossover test system is specifically designed to verify the effect of CO2 on reducing methanol crossover under operation. As a result, an increase of more than 20% in the output power density and a signific...

A facile synthesis of graphene nanoribbon-quantum dot hybrids and their application for composite electrolyte membrane in direct methanol fuel cells

In this work, we report the concept of synthesizing graphene nanoribbon-graphene quantum dot (GNR-GQD) hybrids with even grafting of GQD on GNR sheet in a facile single step process under ultrasonication in chlorosulfonic acid. Further, diazotization route is followed for the preparation of 4-benzenediazonium sulfonate precursor to sulfonate both GNR and GQD to form sulfonated graphene nanoribbons-sulfonated graphene quantum dots (sGNR-sGQD) nanohybrids. Synergistic and structure dependent effect of nanohybrids is seen via its dispersion in sulfonated poly(ether ether ketone) (sPEEK) to form nanocomposite membrane. sPEEK/sGNR-sGQD (1.5 wt %) nanocomposite membrane shows remarkable direct methanol fuel cell (DMFC) performance compared to pristine sPEEK and Nafion 117 with 40% increment in peak power density along with higher durability up to 100 h due to better physicochemical properties like water uptake, ion exchange capacity, proton conductivity, and reduced methanol crossover to suggest its potential which includes diverse membrane applications.

Highly fluorescent carbon quantum dots-Nafion as proton selective hybrid membrane for direct methanol fuel cells

Herein, we explored the carbon quantum dots as potential inorganic filler to realize a Nafion hybrid membrane with much reduction in methanol crossover for direct methanol fuel cell and is first ever reported in the literature. Highly fluorescent carbon quantum dots is synthesized by simple hydrothermal process using Gelatin as source material and the formation of carbon quantum dots is confirmed by UV–Visible and Photoluminescence spectroscopy studies. The added carbon quantum dots filler promotes the ionic conductivity primarily via hydrogen bonding formation between plenty of hydrophilic groups exist in carbon quantum dots and sulfonic acid moieties in the Nafion matrix. Remarkably, the optimized Nafion-carbon quantum dots hybrid membrane shows about 33% reduction in methanol crossover and about 30% higher proton conductivity in relation to the pristine Nafion membrane. While utilizing in direct methanol fuel cell, a peak power density of 113 mW cm−2 is realized with optimized Nafion-carbon quantum dots hybrid membrane at 70 °C and ambient pressure which is about 75% higher in performance than that of pristine Nafion membrane operated under identical conditions.

PtCo/rGO nano-anode catalyst: enhanced power density with reduced methanol crossover in direct methanol fuel cell

The higher methanol utilization efficiency in direct methanol fuel cell (DMFC) is one of the key factors that determine the performance of DMFC. Herein, we have synthesized bimetallic PtCo nano-particles (with optimized Pt:Co ratio) decorated reduced graphene oxide (rGO) nano-composite as anode catalyst. The electrochemical response of optimized PtCo (1:9)/rGO catalyst revealed efficient oxidation of 5 M methanol in half-cell configuration with ~ 60% Faradaic efficiency. A current density of 463.5 mA/cm2 and a power density of 136.8 mW/cm2 were achieved using PtCo (1:9)/rGO anode catalyst in a complete DMFC set-up at 100 °C with 5 M methanol supply which is ~ three times greater as compared to commercial Pt/C (48.03 mW/cm2). The low activation energy of 9.88 kJ/mol indicates the faster methanol oxidation reduction (MOR) kinetics of PtCo (1:9)/rGO anode catalyst. Furthermore, the higher methanol utilization and open-circuit voltage in complete DMFC using PtCo (1:9)/rGO as compared to commercial Pt/C indicate the reduced methanol crossover. The excellent catalytic behavior of PtCo (1:9)/rGO towards MOR and high methanol utilization warrant its potential application as anode catalyst in DMFC.

A three-dimensional non-isothermal model for a membraneless direct methanol redox fuel cell

In the membraneless direct methanol redox fuel cell (DMRFC), three-dimensional electrodes contribute to the reduction of methanol crossover and the open separator design lowers the system cost and extends its service life. In order to better understand the mechanisms of this configuration and further optimize its performance, the development of a three-dimensional numerical model is reported in this work. The governing equations of the multi-physics field are solved based on computational fluid dynamics methodology, and the influence of the CO2 gas is taken into consideration through the effective diffusivities. The numerical results are in good agreement with experimental data, and the deviation observed for cases of large current density may be related to the single-phase assumption made. The three-dimensional electrode is found to be effective in controlling methanol crossover in its multi-layer structure, while it also increases the flow resistance for the discharging products. It is found that the current density distribution is affected by both the electronic conductivity and the concentration of reactants, and the temperature rise can be primarily attributed to the current density distribution. The sensitivity and reliability of the model are analyzed through the investigation of the effects of cell parameters, including porosity values of gas diffusion layers and catalyst layers, methanol concentration and CO2 volume fraction, on the polarization characteristics.

Enhanced Water Management and Fuel Efficiency of a Fully Passive Direct Methanol Fuel Cell With Super-Hydrophilic/ -Hydrophobic Cathode Porous Flow-Field

Water management is a critical issue for a direct methanol fuel cell (DMFC). This study focuses primarily on the use of a super-hydrophilic or super-hydrophobic cathode porous flow field to improve the water management of a passive air-breathing DMFC. The flow field layer was made of an in-house copper-fiber sintered felt (CFSF) which owns good stability and conductivity. Results indicate that the super-hydrophilic flow field performs better at a lower methanol concentration since it facilitates water removal when the water balance coefficient (WBC) is high. In the case of high-concentration operation, the use of a super-hydrophobic pattern is more able to reduce methanol crossover (MCO) and increase fuel efficiency since it helps maintain a lower WBC due to its ability in enhancing water back flow from the cathode to the anode. The effects of methanol concentration and the porosity of the CFSF are also discussed in this work. The cell based on the super-hydrophobic pattern with a porosity of 60% attains the best performance with a maximum power density of 18.4 mW cm−2 and a maximum limiting current density of 140 mA cm−2 at 4 M.

Modeling of passive vapor feed alkaline membrane direct methanol fuel cell

A two-dimensional model for passive vapor feed alkaline anion membrane direct methanol fuel cell (AAEM-DMFC) is developed to investigate the effects of open ratio, carbon dioxide (CO2) exit length, micro-porous layer (MPL) and porous layer’s hydrophobicity. The results show that decreasing the open ratio and enlarging the CO2 exit can relieve the methanol crossover. The fuel utilization efficiency can be significantly improved with the help of methanol recovery. The back-flow water direction shifts toward anode at low current density. The anode MPL (AMPL) can effectively mitigate the methanol intake from the tank and methanol crossover, however, the cathode MPL (CMPL) enhances the methanol intake and crossover at low current density. Moreover, the minimum methanol crossover can be achieved under an appropriate AMPL contact angle at high current density. Increasing the ACL hydrophobicity and vapor transport layer (VTL) hydrophilicity contributes to reducing methanol crossover. Overall, good water management and low methanol crossover can be achieved by optimizing porous layer structure, open ratio and carbon dioxide (CO2) exit length. The simulation results presented in this study can provide further guidance on the optimization of passive vapor feed AAEM-DMFC.

Enhanced performance of sulfonated poly (ether ether ketone) membranes by blending fully aromatic polyamide for practical application in direct methanol fuel cells (DMFCs)

The high dimensional swelling has been considered as one of the most important issues for obstructing sulfonated poly (ether ether ketone) (SPEEK) membrane with high IEC value as an excellent performance proton exchange membrane (PEM) for direct methanol fuel cell (DMFC) application. Here, we present a facile way to overcome the issue via blending the fully aromatic polyamide (fa-PA) into SPEEK membrane. The Influence of the fa-PA on the key properties of the fabricated fa-PA/SPEEK composite membranes (fa-PASPs) was systematically investigated. As the promising dimensional stability of the fa-PA, the fa-PASPs display excellent dimensional stability both in water and methanol aqueous solution. In addition, the fa-PASPs exhibit uniform and smooth morphology due to the same fully aromatic polyamide backbones as SPEEK and strong hydrogen bonding interactions between fa-PA and SPEEK with significantly reduced methanol crossover. The remarkable cell performance at high methanol concentration up to neat methanol was significantly obtained, suggesting that the prepared fa-PASPs are suitable for using as PEMs in direct methanol fuel cells application.