In view of the transfer from fossil fuels to renewable energies, electrochemical reactions offer the chance of a fast and environmentally friendly production of chemicals or the transfer of chemical energy into electric energy (e.g. via fuel cells). While Me-N-C catalysts are already well-established as promising alternative to Pt/C in fuel cell applications [1], some of them might also be of interest for the electrochemical production of H₂O₂ (hydrogenperoxide) as an intermediate product.[2, 3] So far the largest efforts in electrochemical production of hydrogen peroxide are made by using supported metal (e.g. Pd, Au-Pd, Pd-Hg) nanoparticles [4, 5] and nitrogen coordinated metal complexes [6, 7] but recently also non-precious MNC catalysts have attracted attention due to their high ORR activity.[7-9]

In a previous publication we have shown that the presence of protonated nitrogen groups can enhance the ORR activity of Fe-N-C catalysts.[10] In addition theoretical calculations reveal that the largest enhancement in terms of required overpotential would be expected for the combination of MnN₄ sites with carboxylic group.[11] In this work, we try to apply this promotor concept to our MNC catalysts (M= Co, Fe, Mn) by introducing targeted carboxylic groups via plasma treatments in different gas atmospheres (O₂, CO₂). Fe-N-C was selected as most promising candidate for FC application, Co-N-C as possible candidate for hydrogen peroxide formation and Mn-N-C due to previous findings on the improvement of the overpotential by carboxylic groups.[11] Rotating ring disk (RRDE) studies were performed to investigate the positive effect of plasma treatments on the activity and selectivity of the MNC catalysts. In addition, the initial and the modified MNCs were characterized using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy to identify the plasma induced structural changes and their effect on the activity and selectivity data.

References:

1. Kramm, U.I., et al., Correlations between Mass Activity and Physicochemical Properties of Fe/N/C Catalysts for the ORR in PEM Fuel Cell via 57Fe Mössbauer Spectroscopy and Other Techniques. Journal of the American Chemical Society, 2014. 136(3): p. 978-985.

2. Bonakdarpour, A., et al., Impact of Loading in RRDE Experiments on Fe–N–C Catalysts: Two- or Four-Electron Oxygen Reduction? Electrochemical and Solid-State Letters, 2008. 11(6): p. B105-B108.

3. Hasché, F., et al., Electrocatalytic hydrogen peroxide formation on mesoporous non-metal nitrogen-doped carbon catalyst. Journal of Energy Chemistry, 2016. 25(2): p. 251-257.

4. Edwards, J.K., et al., Comparison of supports for the direct synthesis of hydrogen peroxide from H2 and O2 using Au–Pd catalysts. Catalysis Today, 2007. 122(3–4): p. 397-402.

5. Verdaguer-Casadevall, A., et al., Trends in the Electrochemical Synthesis of H2O2: Enhancing Activity and Selectivity by Electrocatalytic Site Engineering. Nano Letters, 2014. 14(3): p. 1603-1608.

6. Mase, K., K. Ohkubo, and S. Fukuzumi, Efficient Two-Electron Reduction of Dioxygen to Hydrogen Peroxide with One-Electron Reductants with a Small Overpotential Catalyzed by a Cobalt Chlorin Complex. Journal of the American Chemical Society, 2013. 135(7): p. 2800-2808.

7. Yamanaka, I., et al., Catalytic Synthesis of Neutral Hydrogen Peroxide at a CoN2Cx Cathode of a Polymer Electrolyte Membrane Fuel Cell (PEMFC). ChemSusChem, 2010. 3(1): p. 59-62.

8. Yamanaka, I., et al., Electrocatalysis of heat-treated cobalt-porphyrin/carbon for hydrogen peroxide formation. Electrochimica Acta, 2013. 108: p. 321-329.

9. Park, J., et al., Highly Selective Two-Electron Oxygen Reduction Catalyzed by Mesoporous Nitrogen-Doped Carbon. ACS Catalysis, 2014. 4(10): p. 3749-3754.

10. Herranz, J., et al., Unveiling N-Protonation and Anion-Binding Effects on Fe/N/C Catalysts for O2 Reduction in Proton-Exchange-Membrane Fuel Cells. The Journal of Physical Chemistry C, 2011. 115(32): p. 16087-16097.

11. Busch, M., et al., Beyond the top of the volcano? – A unified approach to electrocatalytic oxygen reduction and oxygen evolution. Nano Energy, 2016. 29: p. 126-135.