Operation of Calcium-Birnessite Water-Oxidation Anodes: Interactions of the Catalyst with Phosphate Buffer Anions

Abstract

Investigating the interfaces between electrolytes and electrocatalysts during electrochemical water oxidation is of tremendous importance for an understanding of the factors influencing catalytic activity and stability. Here, the interaction of a wellestablished, nanocrystalline and mesoporous Ca-birnessite catalyst material (initial composition K0.2Ca0.21MnO2.21·1.4 H2O, initial Mn-Oxidation state ~+3.8) with an aqueous potassium phosphate buffer electrolyte at pH 7 was studied by using various electron microscopy and spectroscopy techniques. In comparison to electrolyte solutions not containing phosphate, Ca-birnessite electrodes show especially high and stable oxygen evolution activity in phosphate buffer. During electrolysis, partial ion substitutions of Ca2+ by K + and OH- / O 2- by HnPO4 (3-n)- were observed, leading to the formation of a stable, partially disordered Ca-K-Mn-HnPO4-H2O layer on the outer and the pore surfaces of the electrocatalyst. In this surface layer, Mn(III) ions are stabilized, which are often assumed to be of key importance for oxygen evolution catalysis. Furthermore, evidence for the formation of [Ca/PO4/H2O]- complexes located between the [MnO6] layers of the birnessite was found using Ca 2p and Ca L-edge the soft X-ray synchrotron-based spectroscopy. A possible way to interpret the obviously very favorable, “special relationship” between (hydrogen)phosphates and Ca-birnessites in electrocatalytic water oxidation would be that HnPO4 (3-n)- anions are incorporated into the catalyst material where they act as stabilizing units for Mn3+ centers and also as “internal bases” for the protons released during the reaction.

Atom Surface Dynamics of Manganese Oxide under Oxygen Evolution Reaction-Like Conditions Studied by In Situ Environmental Transmission Electron Microscopy

“…MLLS fitting is quite sensitive to energy calibration, and thus, a slight difference in the spectrometer calibration can cause significant deviations. However, the accordance with the XANES analysis indicates that our spectrometer energy calibration matches that of the used reference spectra. This was additionally confirmed by O K edge spectroscopy in HV, whereas in H 2 O/O 2 vapor, it is dominated by the gas spectra (see Figure S13).…”

“…For the printed electrode, the valence reaches 3.9 ± 0.2 in H 2 O compared to 3.6 ± 0.3 of the pressed electrodes. An increase of the Mn oxidation state from 3.2 to 3.5 after electrochemical operation is also established in post-operando XANES analysis . This value is slightly reduced compared to this work.…”

“…However, it cannot be clarified whether the dynamics is an intrinsic property of hydrated birnessite layers or caused by OER. In previous research, , an interaction of Ca 2+ with phosphorus ions from a phosphate buffer electrolyte and the formation of a phosphorous-rich amorphous surface layer were observed only after OER. The in situ study of the dynamic surface layer presented here can well explain the formation of such an amorphous surface layer.…”

“…In previous studies, , the formation of an amorphous surface layer in correlation to the ion exchange of Ca 2+ with H n PO 4 (3– n )– has been observed during OER, hinting to a pronounced atom dynamics at the electrolyte birnessite interface. In situ transmission electron microscopy (TEM) experiments enable the study of the evolution of the catalyst surface under reactive conditions in real space.…”

“…This might be a key characteristic for OER catalysis by manganese oxides. 19 In previous studies, 26,27 the formation of an amorphous surface layer in correlation to the ion exchange of Ca 2+ with H n PO 4…”