Bamboo-graphite cathode for magnesium seawater battery in sustainable fisheries

Abstract

The integration of eco-friendly materials into seawater battery systems represents a sustainable approach to decentralized energy supply for small-scale fisheries. This study aimed to (1) synthesize nitrogen- and phosphorus-doped bamboo-derived graphite (BDG) through a two-step pyrolysis process, (2) fabricate a bamboo-graphite air cathode for magnesium seawater batteries (Mg-SWBs), (3) evaluate its electrochemical properties, and (4) demonstrate battery performance under laboratory seawater conditions by assessing discharge stability and voltage behavior. Bamboo (Dendrocalamus asper) was carbonized using ammonium dihydrogen phosphate (NH₄H₂PO₄) as a dual dopant, followed by cathode fabrication using stainless-steel mesh and polyvinyl acetate binder. Electrochemical performance was characterized using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Battery performance was evaluated by continuous discharge testing in natural seawater (salinity approximately 35 ppt) under laboratory conditions, where voltage and current were monitored every two hours over a 12 h period. The synthesized BDG exhibited an electrical conductivity of 7.8 × 10⁴ S cm⁻¹, representing approximately 95% of commercial graphite conductivity, with a mesoporous structure (average pore diameter ≈ 3.8 nm) favorable for oxygen reduction reactions. CV analysis demonstrated quasi-reversible redox behavior, while EIS measurements indicated low solution resistance (8.6 Ω) and moderate charge-transfer resistance (34 Ω), confirming efficient electron transport. Continuous discharge testing demonstrated a stable output voltage of 2.0 ± 0.05 V and a power density of 1.9 mW cm⁻² for approximately 10 h, followed by gradual voltage decline caused by magnesium anode passivation. A temporary voltage recovery after a resting period was attributed to chemical relaxation rather than self-recharging. Integration with a DC–DC boost converter increased the usable output voltage to approximately 3.5–3.8 V, enabling continuous operation of an LED lamp. These findings demonstrate that bamboo-derived graphite is a promising low-cost and renewable air-cathode material capable of maintaining stable battery performance under laboratory seawater conditions, supporting sustainable marine energy applications and blue economy initiatives.