Influence of Redox-Inactive Cations on the Structure and Electrochemical Reactivity of Synthetic Birnessite, a Heterogeneous Analog for the Oxygen-Evolving Complex

Abstract

Electrochemical protocols were developed for the facile potentiostatic deposition of birnessite films, supported on Au substrates, to serve as a structural motif for oxygen evolution reaction electrocatalysts. The elimination of prolonged cation-exchange submersion dramatically reduced the synthesis time scale from days to minutes. The electrodeposited films were characterized using a combination of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, scanning tunneling microscopy, and X-ray photoelectron spectroscopy. The prepared birnessite films were crystalline, monophasic oxide materials that contained Mn3+, Mn4+, traces of Mn2+, and the intercalant of choice. Redox-inactive Na+, Ca2+, Sr2+, Y3+, and Zn2+ cations showed minimal influence on the voltammetric behavior of birnessite in the presence of Mn2+(aq). Slightly more significant effects emerged during potential cycling and chronopotentiometry of birnessite films in 0.1 M NaOH. The potential needed to sustain a current density of 10 mA cm–2 in 0.1 M NaOH increased according to the sequence Na+ < Ca2+ < Sr2+ < Y3+ < Zn2+. The sequence, with slight inversions in the order, was reminiscent of the trend in the heterometal-dependent modulation of the half-wave potential of the redox couple Mn3+Mn24+/Mn34+ in nonaqueous solutions of heterometallic manganese–dioxido cluster systems. Unlike the case of the homogeneous cluster catalysts, the electrochemical reactivity of intercalated birnessite films did not vary linearly with the pKa of the redox-inactive cations.