Enhanced Four-Electron Selective Oxygen Reduction Reaction at Carbon-Nanotube-Supported Sulfonic-Acid-Functionalized...

In the present work, the oxygen reduction reaction (ORR) is explored in an acidic medium with two different catalytic supports (multi-walled carbon nanotubes (MWCNTs) and nitrogen-doped multi-walled carbon nanotubes (NMWCNTs)) and two different catalysts (copper phthalocyanine (CuPc) and sulfonic acid functionalized CuPc (CuPc-SO₃ ^- )). The composite, NMWCNTs-CuPc-SO₃ ^- exhibits high ORR activity (assessed based on the onset potential (0.57 V vs. reversible hydrogen electrode) and Tafel slope) in comparison to the other composites. Rotating ring disc electrode (RRDE) studies demonstrate a highly selective four-electron ORR (less than 2.5 % H₂ O₂ formation) at the NMWCNTs-CuPc-SO₃ ^- . The synergistic effect of the catalyst support (NMWCNTs) and sulfonic acid functionalization of the catalyst (in CuPc-SO₃ ^- ) increase the efficiency and selectivity of the ORR at the NMWCNTs-CuPc-SO₃ ^- . The catalyst activity of NMWCNTs-CuPc-SO₃ ^- has been compared with many reported materials and found to be better than several catalysts. NMWCNTs-CuPc-SO₃ ^- shows high tolerance for methanol and very small deviation in the onset potential (10 mV) between the linear sweep voltammetry responses recorded before and after 3000 cyclic voltammetry cycles, demonstrating exceptional durability. The high durability is attributed to the stabilization of CuPc-SO₃ ^- by the additional coordination with nitrogen (Cu-N_x ) present on the surface of NMWCNTs.