Data underlying the manuscript: Preparation of biodegradable membrane utilizing chitosan and polyvinyl alcohol, and assessment of its performance after coating with graphene conductive ink
by: Sumit Maya Moreshwar Meshram
Experimental techniques:
Materials
Cellulose filter paper (diameter 7.5 cm, average pore size 1.5 mm, Ahlstrom, Helsinki, Finland) was used as a reinforcing substrate. Polyvinyl alcohol (PVA, molecular weight 146,000–186,000, 99+% hydrolyzed) and Chitosan (CS, high molecular weight) were obtained from Sigma-Aldrich. Conductive graphene ink was procured from FWG Limited, United Kingdom. PK booster compost tea was purchased from Biotabs (The Netherlands). All chemicals were analytically pure and used without further purification. Milli-Q water was used for all experiments.
Membrane synthesis
Membranes were prepared using the solution-casting method. Three types of membranes were fabricated: CS, PVA, and a 1:1 CS/PVA blend.
-	CS membrane: 2% (w/v) CS solution was prepared in 2% (v/v) aqueous acetic acid and stirred for 12 h at 1000 rpm. Filter paper (6 × 6 cm) was immersed for 2 min, dried (24 h at room temperature and 6 h at 60 °C), then neutralized with 2 M NaOH (5 min), washed, and crosslinked in 0.5 M H2SO4 (24 h).  
-	PVA membrane: A 10% (w/v) PVA solution was prepared by dissolving PVA at 80 °C with stirring at 500 rpm for 2 h. Filter paper (6 × 6 cm) was immersed for 2 min, dried (24 h at room temperature and 6 h at 60 °C), then treated with 10% H2O2 (1 h) and crosslinked with 10% H2SO4 (12 h).  
-	CS/PVA membrane: Equal volumes of 2% CS and 10% PVA solutions were mixed (1:1 v/v) and stirred for 2 h. Filter paper (6 × 6 cm) was immersed for 2 min, dried (24 h at room temperature and 6 h at 60 °C), then neutralized and crosslinked as for the CS membrane.  
-	Graphene ink coating: A 1:1 mixture of graphene ink and polymer solution (CS, PVA, or CS/PVA) was prepared and stirred for 24 h. Two layers of this mixture were manually applied to one side of each membrane using a paint brush.  
Performance studies
1) Swelling ratio and water uptake capacity  
- Membranes were dried at 30 °C (15 h) and weighed (Wdry).  
- After immersion in Milli-Q water for 24 h, wet weight (Wwet) and thickness (Twet) were  recorded.  
- Swelling ratio (%) = (Twet – Tdry) / Tdry × 100.  
- Water uptake (%) = (Wwet – Wdry) / Wdry × 100.  
2) Ion exchange capacity (IEC)  
- Membranes were saturated in 1 M H2SO4 for 24 h, washed, and immersed in 1 M NaCl for 24 h to exchange H⁺ with Na⁺.  
- The released H⁺ was titrated with 0.01 M NaOH using phenolphthalein as an indicator.  
- IEC (meq/g) = (VNaOH × MNaOH) / Wdry.  
3) Conductivity determination  
- Electrochemical impedance spectroscopy (EIS) was performed in a three-electrode setup (Autolab PGSTAT302N with FRA module).  
- Membrane area: 12.57 cm²; frequency range: 0.1–10⁵ Hz; amplitude: 0.01 V; measurements under 100% RH.  
- Proton conductivity (σ, S/cm) was calculated from σ = d / (Rs × S), where Rs is resistance from impedance spectra.  
4) Oxygen diffusivity determination  
- Measured in an H-type reactor (300 mL chambers) using a dissolved oxygen probe (Greisinger G1610).  
- The anode chamber was purged with N2 before measurements.  
- Oxygen mass transfer coefficient (cm/s) and diffusion coefficient (cm²/s) were calculated from concentration profiles.  

5) Linear sweep voltammetry (LSV)  
- Membrane separators (12.57 cm²) were tested in an H-type reactor with Pt anode (1 × 1 cm) and carbon felt cathode (1 × 2 cm).  
- Catholyte: 0.1 M phosphate buffer (pH 7) + 0.1 M potassium ferricyanide.  
- LSV was carried out from 0 to 2 V at 1 mV/s using Squidstat potentiostat.  

6) Degradation in compost tea  
- Compost tea prepared with 1:3 (v/v) compost:water, stored at 20 °C for 7 days, then filtered.  
- Membrane samples (2 × 1 cm) were dried, weighed (mi), and immersed for 5, 50, and 100 days.  
- After drying, final weights (mf) were measured.  
- Degradation (%) = (mi – mf) / mi × 100.  

7) Characterization techniques  
-  Contact angle : measured with optical tensiometer (Theta Lite, TL100-TL101).  
- SEM: morphology examined with JSM-IT800, including surface and cross-sections.  
- TGA: thermal stability assessed with Netzsch STA 409 C/CD (25–600 °C, 20 °C/min, under Ar).  
- Mechanical testing: tensile properties measured using Z010 Universal Testing Machine (Zwick, Germany).  
Key results
- CS membrane: highest water uptake (94.10%), negligible swelling ratio (2%), IEC = 1.82 meq/g, proton conductivity = 7.8 μS/cm.  
- PVA membrane: highest swelling ratio (150%), highest IEC (3.94 meq/g), proton conductivity = 31.8 μS/cm.  
- CS/PVA membrane: moderate swelling (111.7%), IEC = 1.50 meq/g, proton conductivity = 5.87 μS/cm.  
- After graphene ink coating:  
  - Water uptake and swelling reduced significantly.  
  - IEC decreased, but conductivity improved in CS/PVA (1.74 mS/cm).  
  - Oxygen diffusion coefficient lowest in coated CS/PVA (0.058×10⁻⁵ cm²/s).  
- Degradation in compost tea: all membranes showed slow mass loss (2–5% in 100 days).  
- SEM: CS and CS/PVA showed partial graphene coverage; PVA exhibited uniform dense coating.  
- TGA: all membranes stable below 100 °C, suitable for biobattery operating conditions.  
- Mechanical tests: CS/PVA showed highest tensile strength (3.73 ± 1.30 MPa), PVA showed highest elongation at break (11.4%).  
Data files
- Swelling ratio and water uptake data files  
- Ion exchange capacity (IEC) data files  
- Proton conductivity (EIS) spectra and data files  
- Oxygen diffusivity measurements and data files  
- LSV polarization curves and data files  
- Degradation study data files  
- Contact angle measurement data files  
- SEM image data files  
- TGA analysis data files  
- Mechanical properties data files
