Environmentally Benign Energy Harvesting Biophotovoltaic Cells with Carbon Shell-Encased Fe₃O₄ Nanoparticles in CNTs

Carbon nanotubes (CNTs) comprising carbon-shell-confined iron oxide (Fe 3 O 4 ) nanoparticles (Fe 3 O 4 /CNT) are synthesized through the chemical vapor deposition method, employing a catalytic tip-growth mechanism. The rationale behind the formation of Fe 3 O 4 /CNT nanostructures from the catalytic growth of Fe 3 O 4 nanoparticles is investigated using diverse morphological and structural characterization techniques, and the impacts of these nanostructures’ properties on green energy generation in the biophotovoltaic (BPV) system are assessed using a range of electroanalytical methods. The substantial oxidation current witnessed from the cyclic voltammograms of processed electrodes discloses the existence of electroactive proteins in microalgae. The dominance of water photolysis reaction on BPV energy generation compared to the metabolism of organic substrates is substantiated from the scrutinized voltammograms. The extracellular polymeric substances secreted by Chlorella vulgaris cells effectively bind with Fe 3 O 4 nanoparticles via hydrogen bonding, electrostatic, and van der Waals interactions, which presumably improves the kinetics of the involved electrochemical reactions. Furthermore, the integration of carbon shell-encased Fe 3 O 4 nanoparticles in CNTs capitalizes on the strengths of each component-offering protection and stability through the aligned porous carbon walls, enhanced electrical conductivity via both the CNTs and proximity between the Fe 3 O 4 core and carbon shell, and increased catalytic activity from the Fe 3 O 4 nanoparticles, augmenting the maximum BPV power generation. The high BPV operational stability scrutinized for Fe 3 O 4 /CNT is corroborated to the protective graphitic layer-encased Fe 3 O 4 nanoparticles, limiting the aggregation and disintegration of electroactive sites and structural deformation of tubular nanostructures in stringent BPV conditions. The excellent BPV performance exhibited by the Fe 3 O 4 /CNT establishes it as one of the most efficient anode materials reported to date, reflecting its substantial potential in steering the advancement of next-generation sustainable energy conversion technologies.