Platinum-Group-Metal-Free Electrocatalysts via N + Ions Implantation and Iron Evaporation on Vertically Aligned Carbon Nanotubes

The continuous research on cost-effective nanomaterials for energy conversion in Fuel Cells, Zinc-Air batteries, and Electrolyzers is characterized by platinum-group-metal-free (PGM-free) electrocatalysts. Such materials are based on C, N, O, and non-noble transition metals and are usually synthesized following a chemical route based on organic and inorganic compounds [1–3]. To maximize the catalytic efficiency, the compounds are mechanically mixed and then pyrolyzed one or multiple times at high temperatures (&gt;900°C) and controlled in a non-oxidative atmosphere. The result is the leveling of differences between C- and N-containing precursors. Moreover, the processes also involve one or more acid-washing steps to remove secondary unwanted species, especially for low pH applications, and to create a porous structure [4]. The chemical route is the most widely used for the synthesis of PGM-free, but a poor level of control is exerted on the final result in terms of oxygen and nitrogen moieties. In some cases, it was possible to maximize pyridinic nitrogen functional groups over the others by selecting specific reagents [5,6], de facto narrowing the vastness of choices of precursors. To address the problem of product controllability, alternative approaches for carbonaceous structure modification are investigated. Among them, ion beam techniques for selective ion implantation have been investigated as desirable for large-scale production of fine-tuned optical-related materials and semiconductors such as p- and n-doped carbon nanotubes and graphene [7–10]. Aiming at transposing such results to PGM-free, we investigated the feasibility of replicating the active sites formation within the boundary of a cleaner physical approach instead of the pyrolysis-based route.<br/>The new recipe is thus developed and based on N+ ions implantation via Kaufman apparatus operated at different ion beam energies, followed by iron evaporation from an electron beam evaporator inside an ultra-high-vacuum (UHV) clean chamber. Vertically Aligned Carbon nanotubes (VA-CNT) were chosen as reliable model carbon structures, characterized by an almost total content of sp2 carbon, to avoid spurious contributions from the material and to future transposing the procedure to other common carbonaceous allotropes. The final goal is thus to obtain fine-tuned PGM-free not depending on N-containing compounds.<br/>Upon preparation of five samples at increasing ions beam energies (50 eV, 100 eV, 200 eV, 400 eV, 800 eV), along with control samples, the preliminary investigation based on scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy techniques shown similarities of the final product with the chemical route, illustrating that the formation of active sites is indeed a natural phenomenon only due to the presence of nitrogen inside the carbon matrix. Moreover, within the prepared samples, simply tuning the ion beam energy was also possible to maximize the pyridinic N content minimizing all other moieties. Lastly, electrochemical tests performed on a representative sample showed catalytic activity towards the oxygen reduction reaction (ORR), confirming the feasibility of the process. The preliminary results are thus paving the way to the limitless and nitrogen-compound-free synthesis of critical raw material-free nanoelettrocatalysts.<br/>References<br/>[1] U. Martinez, et al, J. Electrochem. Soc. 166 (2019) F3136.<br/>[2] M.A.C. de Oliveira et al, J. Solid State Electrochem. 25 (2021) 93–104.<br/>[3] W. Moschkowitsch et al, Nanoscale 13 (2021) 4576–4584.<br/>[4] T. Asset et al, Joule 4 (2020) 33–44.<br/>[5] S. Yasuda et al, Chem. Commun. 49 (2013) 9627–9629.<br/>[6] J.H. Dumont et al, ACS Appl. Nano Mater. 2 (2019) 1675–1682.<br/>[7] S. Garaj et al, Appl. Phys. Lett. 97 (2010) 183103.<br/>[8] B. Guo et al, Nano Lett. 10 (2010) 4975–4980.<br/>[9] A. Ishaq et al, New Carbon Mater. 28 (2013) 81–86.<br/>[10] T. Granzier-Nakajima et al, Nanomaterials 9 (2019) 425.