Abstract text (incl. figure legends and references)

The understanding of molecular mechanisms of water oxidation reactions is very important for development of renewable energy storage solutions. Inspired by a naturally occuring water splitting catalyst, the Mn4CaO5 particles [1], many first-row transition metal compounds are being synthesized and studied as catalysts in oxygen-evolution reactions (OER) at various conditions [2]. Copper(II) complexes are considered to be very promising, with the [Cu(TMC) (H2O)](NO3)2 (TMC = 1,4,8,11‐ tetramethyl‐1,4,8,11‐tetraazacyclotetradecane) compound (further, the Cu(II)-complex) reported among the most efficient copper-based catalyst for electrocatalytic OER in neutral aqueous solutions [3, 4, 5]. However, the control over the stability and catalytic activity of the compound is handicapped by a complicated reaction path, and general lack of understanding of its molecular mechanism [5].

In our study, we followed the OER in the presence of the Cu(II)-complex by employing the most advanced analytical and computational techniques, including in-situ vis-spectroelectrochemistry, in-situ electrochemical liquid-phase transmission electron microscopy (EC-LPTEM) and extended X-ray absorption fine structure (EXAFS) studies.

Our in-situ EC-LPTEM and post-reaction analytical and electron diffraction studies revealed that already in the first anodic scan, small CuO nanoparticles formed at the surface of working electrode at a significantly lower potential than required for OER (Figure). This indicates that these can be the true catalysts rather than the Cu(II)-complex. Further, during the OER, the small CuO particles dissolved to form larger ones. Moreover, during cyclic voltammetry (CV) runs, we observed the formation of a very thin, nearly continuous Cu-oxide film at the working electrode of the reactor cell of the TEM holder. These data accord very well with the electrochemical impedance spectroscopy (EIS) curves acquired in a separate experiment.

We show that even at the very beginning of the OER, the compound can undergo transformation. We also show that the actual OER mechanism in the presence of the Cu(II)-complex is more complicated than a mere molecular mechanism. All these evidences that the metal catalyst cannot be presumed to stay intact during the OER, unless the reaction path is known and fully controlled. In-depth studies of the particular reaction path using the advanced morphological and analytical methods are therefore crucially important for the catalyst design.

Figure.

(a) Degradation of Cu-complexes during anodic scan of cyclic voltammetry (Ag/AgCl reference electrode). TEM micrographs presents the snap-shots of the early stages of the nucleation of the copper oxide particles (bright yellow) at the surface of the working electrode (blue). The phase state change of the material at the electrolyte is verified by electron diffraction pattern (eDP) analyses: (b) the eDP of the Pt-working electrode. (c) Pt-electrode & CuO-phase particles. (d, e) The EDS SI elemental content analyses by of the electrode adjacent region (frame in d). The EDS SI confirms the formation of CuO phase in the electrolyte. (f) Schematic of the CuO unit cell.

[1] Y. Umena, K. Kawakami, J. R. Shen, N.Kamiya, Nature, 473 (2011).

[2] M. M. Najafpour et al., Chem. Rev. 116 (2016).

[3] X. Liu et al., Electrochem. Commun. 46 (2014).

[4] M. M. Najafpour, S. Mehrabani, Y. Mousazade, M. Holynska, Dalton Trans. 47 (2018).

[5] F. Yu et al., Chem. Commun. 52 (2016).