Dual Compartment Electrochemical Harvester

• Dual compartment electrochemical harvester is a battery-less, water-activated transducer that couples two series-connected galvanic cells to generate sufficient voltage and current.
• It employs a robust design with magnesium-anode and aluminum-cathode electrodes, CNF mats, and polyethylene separators to enable voltage doubling and mitigate high-impedance startup challenges.
• Its integration with boost converters and supercapacitors supports direct LTE-M cloud communication, making it ideal for event-driven, remote sensor applications.

A dual compartment electrochemical harvester is a self-contained chemical-to-electrical transducer that couples two identical galvanic cells in series within a single enclosure to generate sufficient voltage and current for powering demanding loads in intermittently energized, battery-less applications. This architecture leverages the voltage summation of series-connected cells while maintaining the current capability of a single cell, optimizing the design for both high-impedance startup and high-peak-power wireless transmission requirements. Its use in power-autonomous sensing platforms, such as long-range LTE-M water leak detectors, obviates the need for batteries and intermediate gateways, enabling direct event-driven connectivity in resource-constrained environments (Nepal et al., 25 Jan 2026).

1. System Architecture and Physical Implementation

The harvester comprises two spatially separated but electronically connected cells, each with the following configuration:

• Anode: Magnesium foil (99.9 %, 50 µm)
• Cathode: Aluminum foil (99.5 %, 50 µm)
• Electrolyte: High-surface-area carbon-nanofiber (CNF) mat impregnated with 0.5 M NaCl
• Separator: Polyethylene microporous film (1.0 mm electrode spacing)
• Compartment volume: 2.5 mL per cell
• Series wiring for output voltage doubling

The two compartments feature identical electrode stacks and electrochemistries, electrically isolated except for the series interconnect, housed in a robust polymeric enclosure to ensure water ingress triggers synchronous activation. Each CNF mat serves as both ionic conductor and water-distribution matrix, while the microporous separator blocks direct electronic conduction. Series arrangement boosts the open-circuit voltage (OCV) to ≈2.7 V immediately after wetting, with the internal resistance scaling linearly with compartment number (Nepal et al., 25 Jan 2026).

2. Electrochemical Reactions and Energy Transduction

Upon exposure to water, the Mg anode undergoes oxidation with concurrent water reduction at the Al cathode. The half-cell and net reactions per compartment are:

• Anode (oxidation):

$\mathrm{Mg (s) \to Mg^{2+} + 2e^-}$

• Cathode (reduction):

$$2\,\mathrm{H}_2\mathrm{O} (l) + 2e^- \to 2\,\mathrm{OH}^- + \mathrm{H}_2(g)$$

• Net reaction:

$$\mathrm{Mg (s) + 2\,H_2O (l) \to Mg(OH)_2 (s) + H_2 (g)}$$

The two cells in series yield an aggregate reaction but with doubled cell voltage, as the chemical processes in each compartment proceed independently but their electrical outputs sum. Charge transport proceeds from anodic Mg through the ionic matrix and separator to cathodic Al, generating an electrical potential compatible with low-voltage boost conversion circuits.

3. Electrical Model and Performance Metrics

Each compartment acts as a Thevenin equivalent comprising $V_\text{cell}$ and $R_\text{cell}$; the overall harvester is then characterized by:

• $V_{\rm oc} = 2\,V_{\rm cell}$
• $R_{\rm int} = 2\,R_{\rm cell}$

The maximum power output, when loaded by $R_L=R_\text{int}$, is:

$$P_{\max} = \frac{V_{\rm oc}^2}{4\,R_{\rm int}} = \frac{V_{\rm cell}^2}{2\,R_{\rm cell}}$$

Empirical measurements show initial OCV ≈2.7 V, decaying to ≈1.6 V after 30 minutes, with peak short-circuit current (SCC) ≈450 mA (leveling to 150 mA). Typical compartment active area is 2 cm², yielding early-time power density $$P_{\rm sc}/A_{\rm total} \approx 0.30\,\mathrm{mW/cm^2}$$ (Nepal et al., 25 Jan 2026). The energy harvested during a 23 min event is approximately 20 J.

Performance is constrained by the match between the application load and the internal resistance, as well as time-varying OCV due to reactant consumption and passivation effects.

4. System Integration and Power Management

The dual compartment harvester interfaces with a high-efficiency boost converter (ME2108: $V_{\text{in,start}}=0.9$ V, $\eta\approx 80\%$ at 50 mA, $\approx 73\%$ at 250 mA), charging a 1.5 F supercapacitor (5 V rated). A TLV431 precision shunt and P-channel MOSFET gate load connection, isolating high power wireless modules (e.g., Nordic Thingy:91 for LTE-M), until the system accumulates sufficient stored energy ($V_{\text{on}}=4.87$ V, $V_{\text{off}}=3.67$ V). This architecture enables the emission of up to eight LTE-M cloud beacons per wetting event, with average energy per transmission about 2.2 J (Nepal et al., 25 Jan 2026).

5. Challenges: Electrochemistry, Management, and Longevity

Design constraints are dominated by:

• Electrode passivation: The Mg anode forms a Mg(OH)₂ layer with use, raising $R_{\text{cell}}$ and reducing current density.
• Mitigation strategies: Electrode pre-roughening, CNF salt matrices promoting uneven wetting, and single-use paradigms.
• Cell balancing: Identical geometry/materials plus resistor ladders ensure voltage sharing and synchronized triggering.
• Ionic isolation: Both compartments are sealed, as only the common water ingress channel is shared to eliminate cross-talk.
• High peak current provisions: Energy storage and load gating mitigate voltage sags from high-current wireless bursts.

This suggests that the dual-compartment structure is especially suited to single-use, event-driven, high-impedance startup followed by brief high-power intervals, as in environmental event sensors.

6. Comparison with Related Approaches

Single-compartment harvesters provide OCV ≈1.3 V, inadequate for typical low-noise boost conversion or direct digital modem operation. The dual-compartment design achieves OCV ≈2.7 V with similar SCC, thus supporting both low-threshold startup and high-current pulse loads. Scalability to more compartments will increase output voltage but also raise internal resistance and physical footprint. Compared to BLE/LoRa, LTE-M deployment enabled by this platform eliminates the need for local gateway relay, supporting direct-to-cloud communication at 250 mA bursts per transmission (Nepal et al., 25 Jan 2026).

7. Broader Context and Implications

The dual-compartment electrochemical harvester demonstrates a balance of energy density, system simplicity, and direct integrability with modern IoT and cellular protocols. The architecture is especially relevant in applications where temporary wetting events must be autonomously reported over long-range wireless (e.g., infrastructure leak detection). Efficiency measurements indicate 0.75 (converter peak) × 0.60 (modem radio) ≈ 0.45 overall chemical-to-cloud efficiency for the system, allowing reliable operation in battery-free, maintenance-minimized deployments. Widespread implementation could extend to additional event-powered sensing or emergency signaling scenarios, a plausible implication being further reduction in long-term environmental e-waste and operational costs by eliminating battery dependencies (Nepal et al., 25 Jan 2026).