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 Mn^(3+), Mn^(4+), traces of Mn^(2+), and the intercalant of choice. Redox-inactive Na^+, Ca^(2+), Sr^(2+), Y^(3+), and Zn^(2+) cations showed minimal influence on the voltammetric behavior of birnessite in the presence of Mn^(2+)(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^+ < Ca^(2+) < Sr^(2+) < Y^(3+) < Zn^(2+). 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 Mn^(3+)Mn_2^(4+)/Mn_3^(4+) 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 pK_a of the redox-inactive cations.