The effect of a vitamin B12 based catalyst on hydrogen peroxide oxidation reactions and the performance evaluation of a membraneless hydrogen peroxide fuel cell under physiological pH conditions

Abstract

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.