A Green Approach to Humidity Sensing: Paper-Based Charcoal-Graphite Composite for IoT Integration

Abstract

The widespread adoption of sensor technology for environmental monitoring is hindered by the high manufacturing costs and low environmental sustainability of traditional sensors. This study presents a sustainable humidity sensor fabricated from a paper substrate and graphite derived from natural charcoal, addressing the limitations of current sensors that generate non-biodegradable electronic waste and require complex network configurations. The sensor exhibits reliable humidity detection across 5-90% relative humidity, with rapid response times and ±5% accuracy, matching nanocellulose-based alternatives. Characterisation demonstrates high sensitivity, excellent bending durability, and suitability for flexible and wearable applications. Integration with IoT platforms is straightforward, requiring only basic microcontroller connections. Notably, the sensors are completely biodegradable, decomposing within a short period, and are fabricated from eco-friendly and cost-effective materials, offering a superior sustainability profile compared to conventional electronic sensors.

Introduction

This work presents a low-cost, eco-friendly paper-based humidity sensor made from charcoal-derived graphite, integrated with an ESP32 IoT system for real-time environmental monitoring.

The motivation stems from key issues in current environmental sensing technologies:

• High production cost

• Complex fabrication processes

• Electronic waste after disposal

• Limited accessibility for large-scale deployment

The proposed solution uses biodegradable cellulose paper combined with natural charcoal graphite, creating an inexpensive, sustainable alternative suitable for metropolitan, agricultural, and industrial monitoring.

Core Concept

Paper is:

• Flexible

• Biodegradable

• Made from cellulose (biocompatible and sustainable)

Graphite provides:

• Electrical conductivity

When graphite is deposited onto cellulose:

• The cellulose absorbs moisture

• Moisture alters ionic and electronic conduction

• Electrical resistance changes with humidity

This resistance variation enables humidity detection.

Materials and Fabrication

1. Graphite Source

• Extracted from locally available charcoal.

• Only low-resistance charcoal (≤50 Ω) used.

• Mechanically ground (no chemical treatment).

2. Conductive Ink Composition

• Charcoal-derived graphite powder

• Saline solution (NaCl + water) to enhance ionic conductivity

• Natural gum adhesive as binder

• Viscosity optimized (40–65 cP) for brush application

No advanced chemistry or expensive equipment required.

3. Substrate

• Standard A4 cellulose paper (80 gsm)

• Porous, hydrophilic, flexible

4. Fabrication Method

• Brush-based multi-layer coating (4–5 layers)

• Air-dried at room temperature

• Final curing for 24 hours

• No thermal annealing (energy-efficient)

This keeps fabrication simple, scalable, and environmentally friendly.

Sensor Mechanism

Humidity response is based on resistance modulation:

• Cellulose absorbs water

• Moisture affects ionic conduction and graphite percolation network

• Resistance changes with relative humidity (RH)

Initially modeled as:

R(RH) = R? × e^(−α·RH)

However, experimental data showed a monotonic increase in resistance with RH (40–90% range), likely due to restructuring of conductive pathways.

Measurement and IoT Integration

Hardware Setup

• ESP32 Dev Kit V1

  • 12-bit ADC

  • 3.3V operation

  • WiFi (IEEE 802.11 b/g/n)

• Voltage divider circuit with 10 kΩ reference resistor

• Resistance calculated via ADC readings

Reference Sensor

• DHT11 temperature–humidity sensor

• Placed 5 cm from graphite sensor

• Used for benchmarking

Cloud Platform

• Blynk IoT cloud

• HTTPS + TLS 1.2 secure transmission

• Data sent every 5–20 seconds

• Real-time dashboard with humidity/temperature display

Performance Results

Calibration Characteristics

• Resistance increases from ~1780 Ω (40% RH)

• To ~2100 Ω (90% RH)

• Smooth, quasi-linear relationship

• Moderate sensitivity (~tens of ohms per %RH)

Accuracy

• Approx. ±5% RH (within 40–90% RH)

• Under controlled temperature

• With periodic recalibration

Reproducibility

• Two samples (S1, S2) showed:

  • Parallel resistance–RH trends

  • Consistent response shape

  • Stable fabrication process

IoT Deployment Performance

• 24-hour logging test

• 95% correlation with DHT11

• No packet loss within 10 m WiFi range

• Reliable for distributed monitoring

Advantages of the Proposed Sensor

1. Extremely low cost

2. Biodegradable and eco-friendly

3. No complex chemical processing

4. No expensive microfabrication

5. Energy-efficient production

6. Suitable for large-scale deployment

7. Accessible to low-resource communities

Applications

• Smart agriculture

• Urban climate monitoring

• Food storage safety

• Industrial environmental tracking

• Disaster-response networks

• Wearable environmental sensing

Limitations

• Manual fabrication may introduce variability

• Binary resistance-based sensing (no multi-parameter analysis)

• Performance limited to 40–90% RH window

• Requires periodic recalibration

Conclusion

The charcoal-graphite sensor exhibits robust performance as a humidity sensing device, leveraging the hygroscopic properties of charcoal and conductive pathways of graphite for reliable detection across relative humidity levels from 20% to 90% RH. Experimental results confirm its high sensitivity through impedance changes driven by water molecule adsorption, following Grotthuss chain mechanisms that enhance proton conduction at elevated humidity, while maintaining low hysteresis and response times under ambient conditions. This low-cost, sustainable design advances paper-based flexible electronics for environmental monitoring, aligning with IoT integration needs in resource-constrained settings.

References

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Copyright

Copyright © 2026 N. Masanlungbou, Dr. K. G. Padmasine, Bharathi P. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.