Quaternized PVA/Graphene Oxide Composite Membrane Enhances Ethanol Barrier and Ionic Conductivity for Passive Alkaline DEFCs

Abstract

Passive alkaline direct ethanol fuel cells (DEFCs) promise sustainable power for portable devices, yet ethanol crossover remains a key challenge. We report a cross‑linked quaternized poly(vinyl alcohol)/graphene oxide (QPVA/GO) composite membrane that markedly reduces ethanol permeability while boosting ionic conductivity. With 15 wt % GO and 1 M KOH doping, the membrane shows ethanol permeability of 1.49 × 10⁻⁷ cm² s⁻¹ (30 °C) and 3.65 × 10⁻⁷ cm² s⁻¹ (60 °C), and ionic conductivity of 1.74 × 10⁻² S cm⁻¹ (30 °C) and 6.24 × 10⁻² S cm⁻¹ (60 °C). In a single‑cell passive alkaline‑DEFC, the maximum power density reaches 9.1 mW cm⁻² at 30 °C and 11.4 mW cm⁻² at 60 °C—surpassing commercial Nafion 117/KOH (7.68 mW cm⁻² at 30 °C). These results confirm the composite’s potential as an advanced anion‑exchange membrane for portable fuel‑cell applications.

Introduction

Fuel cells convert chemical energy directly into electricity with high efficiency (≈60 %) and no combustion by‑products. The direct ethanol fuel cell (DEFC) is attractive for portable power because it uses a readily storable, low‑toxic fuel and a simple cell architecture. Passive DEFCs—those that rely on capillary action for fuel delivery—offer further advantages: no pumps or blowers, lightweight design, and minimal power consumption. However, two issues limit their commercial viability: sluggish anode kinetics and high ethanol crossover from anode to cathode, which lowers voltage and poisons cathode catalysts.

Conventional membranes (e.g., Nafion®) have high proton conductivity but poor ethanol barrier, high cost, and environmentally harmful manufacturing. Alkaline‑DEFCs alleviate some of these problems by operating in a basic medium, where ethanol oxidation is faster, non‑platinum catalysts (Ag, Ni) can be used, and hydroxide transport opposes ethanol diffusion. Yet commercial anion‑exchange membranes (AEMs) still suffer from low conductivity and degradation above 60 °C.

Poly(vinyl alcohol) (PVA) is an attractive, inexpensive polymer with excellent chemical resistance and mechanical stability. Quaternization with glycidyltrimethyl‑ammonium chloride (GTMAC) introduces permanent cationic sites, enhancing ionic conductivity (up to 2.1 × 10⁻² S cm⁻¹ at 60 °C) and reducing water uptake. Incorporating graphene oxide (GO) as a nanofiller further improves thermal stability, mechanical strength, and, crucially, creates a hydrophobic barrier that reduces ethanol crossover. Here we synthesize a cross‑linked QPVA/GO composite membrane, evaluate its physicochemical properties, and test its performance in a passive alkaline‑DEFC.

Methodology

Materials

All reagents (PVA, GTMAC, GO precursors, KOH, GA, solvents) were analytical grade and used without further purification.

Synthesis of Quaternized PVA

PVA was dissolved in deionized water at 90 °C, cooled to 65 °C, then reacted with GTMAC and KOH (1:1 mol ratio) for 4 h. Yellow precipitate of QPVA was collected by ethanol precipitation and dried under vacuum.

Synthesis of Graphene Oxide

GO was prepared by the modified Hummers method (graphite, NaNO₃, H₂SO₄, KMnO₄, H₂O₂) followed by washing with dilute HCl and DI water.

Preparation of QPVA/GO Composite Membranes

8 wt % QPVA was dissolved at 75 °C, mixed with GO solutions (5–20 wt % relative to QPVA), and cross‑linked with 10 wt % GA. The solution was cast to 0.015 mm thickness, dried, annealed at 100 °C, and then soaked in 1 M KOH at 80 °C for 24 h. Excess KOH was rinsed, and membranes were stored in DI water.

Characterization

Fourier‑transform infrared (FTIR) spectroscopy, X‑ray diffraction (XRD), field‑emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), thermogravimetric analysis (TGA), water/ethanol uptake, ion‑exchange capacity (IEC), swelling ratio, oxidative stability (Fenton test), ionic conductivity (four‑probe impedance), and ethanol permeability (diffusion cell) were performed.

Fuel‑Cell Testing

A single‑cell passive alkaline‑DEFC was assembled with a 4 cm² active area, Pt–Ru anode (4 mg cm⁻²), Pt cathode (4 mg cm⁻²), and the QPVA/GO membrane. 2 M KOH + 2 M ethanol served as fuel; air was supplied to the cathode. Polarization curves were recorded at 30 °C and 60 °C. A durability test ran for 1000 h at 0.3 V (60 °C) with refueling every 12 h.

Results and Discussion

Structure and Morphology

FTIR confirmed successful quaternization (C–N peak at 968 cm⁻¹) and cross‑linking (C–O–C peak at 1115 cm⁻¹). GO introduced characteristic peaks (epoxy at 1351 cm⁻¹, carboxyl at 1415 cm⁻¹, aromatic C=C at 1656 cm⁻¹). XRD showed a diminished GO peak (10.92°) after exfoliation, indicating uniform dispersion. FESEM/TEM/AFM revealed smooth, dense membranes with uniformly distributed GO sheets (~0.87 nm thick). TGA demonstrated enhanced thermal stability: residual mass of 22.2 wt % for the 15 wt % GO composite vs. 7.7 wt % for QPVA alone.

Swelling, Uptake, and Oxidative Stability

Alkaline uptake increased from 105 % (PVA) to 136 % (QPVA) but decreased to 45 % at 20 wt % GO, reflecting a “blocking” effect of GO’s hydrophobic domains. Water uptake dropped from 145 % to 46 % with GO, while ethanol uptake fell by ~35 % at 20 wt % GO. In‑plane swelling reduced from 18 % to 7.5 %, and through‑plane swelling from 60 % to 32 % with GO. Fenton tests showed >91 % weight retention after 24 h, indicating excellent oxidative resistance.

Ionic Conductivity and Activation Energy

At 30 °C, ionic conductivity rose from 4.7 × 10⁻³ S cm⁻¹ (QPVA) to 1.76 × 10⁻² S cm⁻¹ (15 wt % GO). Conductivity increased further with temperature, reaching 4.6 × 10⁻² S cm⁻¹ at 60 °C (15 wt % GO). Arrhenius analysis gave an activation energy of 18.1 kJ mol⁻¹ for the 15 wt % GO composite, the lowest among tested compositions, reflecting efficient anion transport pathways.

Ethanol Permeability

Permeability dropped from 8.7 × 10⁻⁷ cm² s⁻¹ (pristine QPVA) to 1.32 × 10⁻⁷ cm² s⁻¹ (20 wt % GO) at 30 °C. Temperature elevation increased permeability slightly (up to 2.5 × 10⁻⁷ cm² s⁻¹ at 60 °C for 15 wt % GO) due to increased chain mobility, but the relative barrier remained superior to commercial Nafion and other PVA composites.

Selectivity and Fuel‑Cell Performance

The selectivity factor (σ/P) peaked at 1.33 × 10⁴ S cm⁻¹ for the 15 wt % GO composite. In single‑cell tests, the membrane achieved an open‑circuit voltage of 0.61 V (30 °C) and 0.78 V (60 °C). Maximum power densities were 9.1 mW cm⁻² (30 °C) and 11.4 mW cm⁻² (60 °C), outperforming both QPVA (5.88 mW cm⁻²) and Nafion 117/KOH (7.68 mW cm⁻²). During a 1000‑h durability run at 60 °C, the cell maintained 84.7 mA cm⁻² (≈3 % loss), with only a 32.8 % drop in peak power after cycling—attributable to modest catalyst degradation rather than membrane failure.

Conclusion

We have fabricated a robust cross‑linked QPVA/GO composite membrane that delivers high ionic conductivity (4.6 × 10⁻² S cm⁻¹ at 60 °C) and a remarkably low ethanol permeability (1.32 × 10⁻⁷ cm² s⁻¹). These properties translate into superior passive alkaline‑DEFC performance, with maximum power densities of 9.1 mW cm⁻² (30 °C) and 11.4 mW cm⁻² (60 °C). The membrane’s mechanical resilience, thermal stability, and oxidative resistance make it a promising candidate for next‑generation portable fuel cells.

Abbreviations

DEFC

Direct ethanol fuel cell

GO

Graphene oxide

KPVA

Quaternized poly(vinyl alcohol)

AEM

Anion‑exchange membrane

IEC

Ion‑exchange capacity

FTIR

Fourier‑transform infrared spectroscopy

XRD

X‑ray diffraction

FESEM

Field‑emission scanning electron microscopy

AFM

Atomic force microscopy

TGA

Thermogravimetric analysis

KOH

Potassium hydroxide