Jesus Barrio Hermida

Abstract:

Metal single atoms in nitrogen doped carbon materials (M-NC) have attracted plenty of attention during the last decades in the field of electrocatalysis for oxygen reduction and carbon dioxide conversion amongst others. In the cathode of proton exchange membrane fuel cells Fe-NC are the most promising solution to scarce and expensive Platinum-group-metal catalysts,[1] and in the cathode of CO₂ conversion electrolysers, Fe and Ni-NC are predicted to be as active as Au or Ag.[2] However, their controlled synthesis and stability for practical applications remains challenging. Approaches to enhancing their catalytic performance include increasing the loading of Fe single atoms, for example by decoupling high temperature pyrolysis and Fe coordination atoms, or enhancing the intrinsic activity of the FeN_x sites, for example by engineering of coordination environment or creation of dual atom catalysts.[3–5] However, the metal utilization within these materials remains very low owing to the lack of scaffolds that combine adequate micro- and mesoporosity.
In this work we employ inexpensive 2,4,6-Triaminopyrimidine (TAP) with MgCl₂.6H₂O as porogen to prepare a highly porous N-doped carbon material.[6] The hydrogen bonding between nitrogen moieties of TAP and the water molecules of the Mg salt allows an optimal interaction during pyrolysis that leads to remarkable porosity in the nitrogen-doped material (~3300 m² g^-1) and very available N sites for Fe or Ni coordination. The subsequent low temperature metal coordination (Figure 1) results in a highly active O₂ reduction to electrocatalyst with a mass activity 4.0 A g^-1 at 0.8 VRHE in acid electrolyte, and one the highest turnover frequency for CO₂ reduction reported to date for M-NC materials.[7] Additionally in-situ nitrite stripping reveals a high active site density of >2×10¹⁹ sites g^-1; and a electrochemical active site utilisation of 52% and 76% for Fe and Ni-NC, respectively, up to our knowledge the highest reported to date. Aberration corrected high-angle annular dark field scanning transmission electron microscopy, time-of-flight secondary ion mass spectrometry, X-ray absorption extended fine structure and density functional theory calculations were employed to assess the electrochemical stability and the intrinsic activity of the active sites.