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Abstract

Transition metal oxides have been proposed as a possible replacement of Pt for the electrocatalytic oxygen reduction reaction (ORR), however the low intrinsic conductivity makes their application in electrocatalysis challenging. In this work, we demonstrate that atomic layer deposition (ALD) is capable to overcome this problem by coating conductive carbon nanotubes substrates with a conformal Mn₃O₄ layer of just few‐nm in thickness. The deposition parameters have been optimized in terms of thickness and crystallite sizes in order to produce a material exhibiting catalytic efficiency close to the one of carbon‐supported Pt particles and low polarization costs. The current densities recorded in linear sweep voltammetry prove that the Mn₃O₄ coating leads to a substantial increase of the catalytic efficiency, compared to uncoated carbon nanotubes, and was also higher than other manganese‐based catalysts reported so far. The sample prepared from only 50 ALD cycles (e.g. coating thickness of ^~2 nm) shows the best compromise between catalytic efficiency, with an onset potential at 0.867 V (vs. RHE), and good conductivity of the electrode materials minimizing polarization. Indeed, the Tafel plots exhibit a similar slope than Pt/C demonstrating that the Mn₃O₄/CNTs reduce oxygen in a one‐step four electrons mechanism and with a similar kinetics as Ptbased
electrocatalysts. Moreover, the current density keeps at 80% even after 12 h at 0.58 V
displaying therefore a higher stability than Pt-based catalysts. These findings are attributed to the
few nm-thick conformal and catalytic active coating obtained by atomic layer deposition, which also
protects the underneath CNT substrate from corrosion.