Development of transition metal oxide and nitride electrocatalysts for energy conversion technologies

Julian Lorenz, Institute of Engineering Thermodynamics, German Aerospace Center (DLR), Oldenburg, Germany

A sustainable energy system based on chemical energy carriers like hydrogen (H2) and ammonia (NH3) requires efficient and abundant electrocatalysts for their synthesis and conversion in different energy technologies.

Anion exchange membrane water electrolysis (AEMWE) is a cost-effective approach for H2 synthesis if made of PGM-free (platinum group metals) electrocatalysts like transition metal oxides catalyzing the kinetically-hampered oxygen evolution reaction (OER). On the other hand, synthesis of indispensable green NH3 may be achieved by the electrochemical ammonia synthesis (EAS) from water and nitrogen. Transition metal nitrides may offer an energetic advantage as electrocatalyst, because the nitrogen reduction reaction (NRR) is expected to be catalyzed via the Mars-van Krevelen mechanism. However, NRR research field suffers from false-positive results due to low productions rates, low Faradaic efficiencies and complex contamination issues.

Structured as well as doped Co3O4 spinel and NiO rock salt materials were investigated by different electrochemical techniques for the OER. Mesostructured Co3O4 was studied by the surface interrogation mode of scanning electrochemical microscopy (SI-SECM).[1] SI-SECM is a transient in situ technique that allows quantification of reactive species formed during the OER. Facet-controlled and doped (Co, Mn, Fe) Ni (hydr)oxides were evaluated for the OER by rotating disk electrode (RDE) technique.[2] Structural peculiarities were studied by synchrotron-based X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) or (in situ) Raman spectroscopy. The Fe- and Co-doped NiO(111) nanosheets outperformed pure NiO, whereas the hydroxide analogue Fe-doped Ni(OH)2 material showed the best OER performance. However, in situ doping by Fe3+ addition to the electrolyte further improves the OER activity compared to compounded catalysts materials highlighting the effect of surface-confined active sites. Moreover, incorporation of these materials into catalyst layers and their qualification for real AEMWE operation requires application-oriented electrochemical techniques. Adoption of the cell design and experimental parameters enabled reliable testing of porous gas diffusion electrodes (GDE) in a commercial half-cell setup under industrial-relevant conditions of up to 60 °C and 1 A cm-2 in 1 M KOH electrolyte.

For the NRR, Zr-based nitrides were developed[3,4], their electrochemical activity tested in a GDE setup and ion chromatography[5] is used for the trace analysis of ammonium. Only consideration of all three interconnected steps can give an indication of genuine NRR activity, where structural characterization after NRR turnover experiments elucidates the stability of the material. The ZrN-based GDEs showed rather effects of contaminations and catalyst deactivation, while ZrN and VN films deposited by chemical vapor deposition (CVD) hints towards some activity which has to be verified by isotope-labelled experiments.

References
[1] J. Lorenz, M. Yu, H. Tüysüz, C. Harms, A. Dyck, G. Wittstock, J. Phys. Chem. C 2020, 124, 7737-7748. [2] K.K. Rücker, D.H. Taffa, O. Bisen, M. Risch, D. Hayes, E. Brim, R.R. Richards, C. Harms, M. Wark, J. Lorenz, J Phys. Chem. C. 2025, 129, 20, 9341-9355. [3] S. C. H. Bragulla, A.R. von Seggern, J. Lorenz, C. Harms, M. Wark, K.A. Friedrich, ChemCatChem, 2024, e202400613. [4] J.-P. Glauber, J. Lorenz, J. Liu, B. Müller, S. Bragulla, A. Kostka, D. Rogalla, M. Wark, M. Nolan, C. Harm, A. Devi, Dalton Trans. 2024, 53, 15451-15464. [5] S. C. H. Bragulla, J. Lorenz, C. Harms, M. Wark, K.A. Friedrich, ChemSusChem, 2023, 16, e20220221.