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In this study, a new cathodic catalyst including hemin catalyzing the H2O2 reduction reaction (HPRR) is suggested to improve the performance of flow-type hydrogen peroxide fuel cells (HPFCs). To develop the hemin-based catalyst showing excellent HPRR reactivity, buckypaper (BP) and polyethyleneimine (PEI) are used, and it is proved that the final [BP/hemin]/PEI/hemin catalyst has excellent catalytic activity in terms of current and onset potential for the HPRR. To evaluate the effect of the catalyst on the performance of HPFC, BP/hemin and BP/PEI/hemin catalysts as the control groups are fabricated and compared. When [BP/hemin]/PEI/hemin is used, the amount of immobilized hemin and the reactivity of the HPRR increase due to the interstitial pores of the carbon nanotube network. Furthermore, its onset potential increases due to the formation of the coordinate bond between the iron core within hemin and the axial ligand within PEI. For preparing membraneless HPFCs, flexible materials such as hemin-modified BP, carbon cloth and polyethylene terephthalate are used, and its open circuit potential and maximum power density are 0.171 ± 0.012 V and 90.7 ± 6.88 μW cm−2 when 0.1 M H2O2 is injected. These results are far better than those of other previous studies. [ABSTRACT FROM AUTHOR]

Summary: A facile and inexpensive method of fabricating a myoglobin‐mimic nanostructure is introduced by evaluating the influence of temperature conditions on the axial coordination between the Fe core of hemin and amine of polyethyleneimine (PEI). Through the high‐temperature (100°C) synthesis condition, more hemin molecules are strongly attached to the carbon nanotube and PEI composite owing to the amide bond formation, whereas the energy distribution of hemin is deformed, and the electrical connection is improved by the coordination of axial ligands when the catalyst is synthesized on a lower temperature (25°C). Benefiting from the high concentration of axial ligands, the onset potential is positively shifted by 0.258 V, and the highest current density (155.43 μA cm−2) is observed with 10 mM H2O2 under physiological conditions. These phenomena occur because of the different hydrogen peroxide reduction reaction (HPRR) mechanisms and the overpotential stemming from the effect of the axial ligand, which induces the lowest catalytic and charge transfer resistance for HPRR at 51 and 820 Ω cm−2, respectively. In the polarization curves measured using a 3D printed membraneless flow‐type fuel cell, the maximum power density reaches 129.0 μW cm−2 with 0.340 V of open‐circuit voltage, respectively, which offers the best performance among the reported studies for the membraneless hydrogen peroxide fuel cells driving under physiological conditions so far. [ABSTRACT FROM AUTHOR]

Summary: Hollow cobalt nitrogen‐doped carbon (H‐CoNC) is suggested for use in the membraneless hydrogen peroxide fuel cell (HPFC) and enzymatic biofuel cell (EBC) as anodic catalyst boosting hydrogen peroxide oxidation reaction (HPOR). For fabricating H‐CoNCs, a facile spray pyrolysis‐assisted process is used, and such produced H‐CoNCs show a porous and hollow‐shell structure, while they include a large amount of isolated Co atoms and coordinate bonds with Co and nitrogen (CoN4). This structure promotes mass transfer to the active site and excellent catalytic activity for HPOR. With these benefits of H‐CoNCs, the current density of the bioanode consisting of H‐CoNC, carbon nanotube, and glucose oxidase (H‐CoNC/CNT/GOx) observed at 0.3 V under 150 mM glucose is 315.5 μA cm−2, which is 2.1 times higher than that of a conventionally synthesized catalyst using Co‐doped carbon nanoparticles (CoNC‐NPs) (CoNC‐NPs/CNT/GOx, 146.2 μA cm−2). With this superior catalytic activity for HPOR, maximum power density (MPD) of membraneless EBC using H‐CoNC/CNT/GOx is 33.8 ± 4.52 μW cm−2, which is 52% higher than that of CoNP‐NPs/CNT/GOx. In addition, a membraneless HPFC using H‐CoNC/CNT demonstrates 4.87 times higher MPD (231.3 ± 11.3 μW cm−2) than that using CoNC‐NPs/CNT, proving that H‐CoNC improves the performance of fuel cells by its excellent catalytic activity. [ABSTRACT FROM AUTHOR]

A new anodic catalyst containing carbon nanotubes (CNTs) and vitamin B12 (CNT/VitB12) and its electrochemical activation are introduced for promoting the hydrogen peroxide oxidation reaction (HPOR) and the performance of a membraneless hydrogen peroxide fuel cell (HPFC). Through activation, the anodic current density of the activated catalyst (CNT/VitB12-Co2+) (271.0 μA cm−2) is 2.28 times better than that of the non-activated catalyst (CNT/VitB12-Co3+) (118.8 μA cm−2) at a low potential (0.4 V) when only 3 mM H2O2 is injected. This is due to the two-step HPOR mechanism of CNT/VitB12 where Co2+ ions are initially converted into Co3+ ions by the reaction with H2O2 (step 1) and electrons are generated when Co3+ ions return to Co2+ ions (step 2). Such produced electrons play a role in increasing the anodic current density. Based on that, it is confirmed that electrochemical activation controlling the oxidation state of Co ions is a key factor to enhance the catalytic activity of CNT/VitB12. Regarding stability, desirable CNT/VitB12-Co2+ preserves its initial current for 2 h in an amperometric response test. The onset potentials for CNT/Vit B12 and CNT/hemin, which are used as cathodic catalysts, are 0.1 and 0.35 V vs. Ag/AgCl, respectively, indicating that when the two catalysts and H2O2 fuel are used, the open circuit voltage (OCV) is 0.25 V. When the membraneless HPFC is operated, the OCV and maximum power density are 0.233 ± 0.005 V and 53.8 ± 0.4 μW cm−2 with 0.1 M H2O2 fuel. The similarity between the difference in onset potential of the catalysts and the OCV of the membraneless HPFC is evidence that the catalytic activity of the catalysts is the dominant parameter to determine the performance of membraneless HPFCs. The suggested membraneless HPFC shows an excellent performance despite a low fuel concentration and membraneless structure. [ABSTRACT FROM AUTHOR]