Improving Performance of Microbial Fuel Cells with Molecular Catalysts of Iron Azaphthalocyanine in Air-Breathing Cathode
Authors: Abe Hiroya , Nyangau Edwin Osebe , Nakayasu Yuta , Umetsu Masaki , Watanabe Masaru , Tada Chika 
Published: December 2024

Microbial fuel cells (MFCs) present a viable, sustainable methodology for the transformation of organic waste in wastewater into bioelectricity. Despite the promising potential of MFCs, challenges such as reduced power density and elevated initial expenditures have impeded their broader commercial adoption. Numerous studies have been directed toward advancing technologies that enhance MFC power densities. One notably effective strategy involves the reduction of overpotentials at the cathode. It is essential for optimal power generation that all electrons produced from the anode chamber are utilized in the cathode during the oxygen reduction reaction (ORR). The ORR can follow either a two-electron (2e−) or a four-electron (4e−) pathway. Particularly, the four-electron (4e−) transfer pathway is preferred, ultimately producing water. Platinum group metals (PGMs) are traditionally employed as standard electrocatalysts for the cathodic ORR, acclaimed for facilitating a four-electron transfer and minimizing hydrogen peroxide production. This enhancement in ORR efficacy has rendered PGMs as highly favorable electrocatalysts. Nevertheless, the cost of PGMs and their deactivation due to poisoning by various chemical interactions have limited their use in large-scale applications. Recent research has increasingly focused on developing non-platinum group metal (Non-PGM) catalysts, particularly based on iron transition metals or heteroatom-doped carbon alloys, synthesized through high-temperature pyrolysis to yield atomically dispersed iron on carbon supports.

Molecularly dispersed electrocatalysts constitute an emerging category of catalysts, synthesizable at temperatures below 200°C, compared with carbon-supported metal alloy catalysts using pyrolysis at temperatures above 800°C. The synthesis of molecularly dispersed electrocatalysts at lower temperatures provides a cost-efficient alternative, diminishing energy expenditures associated with heating and thereby facilitating mass production.

Iron phthalocyanine (FePc), characterized by its macrocyclic structure (FeN4), enables straightforward modification of the catalyst on carbon through a wet process and significantly augments ORR activity. The MN4 within FePc, comprising four ligands, exhibits a high oxygen affinity, thereby enhancing oxygen adsorption essential for ORR activity. Specifically, with iron as the central metal, it critically aids in electron transfer during the ORR, promoting the more efficient four-electron pathway.

In this study, a novel molecular electrocatalyst was developed through the introduction of iron azaphthalocyanine (FeAzPc), a derivative of iron phthalocyanine (FePc) [1]. This modification involved the replacement of the four benzene rings in FePc with pyridine rings, yielding FeAzPc. Previously, FeAzPc has exhibited superior performance among FePc analogs, particularly under alkaline conditions[2]. The focus of the current research was on the synthesis and characterization of FeAzPc adsorbed on carbon nanotubes (FeAzPc-CNT), which was then assessed as a cathodic catalyst for the oxygen reduction reaction (ORR) in a neutral medium. The catalytic activity of FeAzPc-CNT for the ORR was evaluated using a rotating ring-disk electrode (RRDE) setup, simulating the medium conditions typical of MFCs. Analytical techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM), were employed to elucidate the morphology of the synthesized electrocatalyst. Subsequently, this electrocatalyst was integrated into the air-breathing cathode of a two-chamber MFC, where its effectiveness was quantified by measuring voltage generation over time and chemical oxygen demand (COD) removal efficiency.

[1] EO Nyangau et al., Bioresource Technology Reports 23, 101565, 2023

[2] H. Abe et al., NPG Asia Materials 11 (1), 57, 2019