Modulating between 2e− and 4e− pathways in the oxygen reduction reaction with laser-synthesized iron oxide-grafted nitrogen-doped carbon

Abstract

In this study, we demonstrate the tuning of the oxygen reduction reaction (ORR) using iron/iron oxide nanoparticle grafted laser-patterned nitrogen-doped carbon (LP-NC) electrodes. Depending on the preparation route, i.e. addition of a molecular Fe(NO₃)₂ precursor before (route 1) or after pre-carbonization (route 2) of the citric acid/urea precursors, either the 4e⁻ or the 2e⁻ pathway in the ORR is facilitated leading to either H₂O or H₂O₂ as a reaction product, respectively. The kinetic reaction conditions afford mixed valence metal oxide nanoparticles embedded in LP-NC in the form of either Fe₂O₃/Fe or Fe₂O₃/FeO/Fe, respectively, facilitated by an in situ carbothermal reduction during the laser-induced carbonization. In HR(S)TEM analysis we found evidence for the occurrence of Fe₂O₃ in the η- or α-phase, depending on the preparation route. Reciprocally, the graphitization is also affected by the preparation route leading to either homogeneous graphitization or a locally graphitized shell structures around the nanoparticles. In the 4e⁻ mediated ORR facilitated by η-Fe₂O₃/Fe@LP-NC onset potentials as low as 0.70 V (vs. RHE) with a H₂O₂ production efficiency of 4% and 10% in alkaline and neutral electrolyte, respectively, were determined. On the other hand, α-Fe₂O₃/FeO/Fe@LP-NC presents an onset potential for the 2e⁻ mediated ORR as low as 0.77 V with a H₂O₂ production efficiency of nearly 80%. The changes in selectivity and physicochemical properties of the electrocatalysts by applying simple modifications in the synthetic route point to laser-patterning as a very promising route to scale up designer electrodes for electrochemical conversion.