Electrodeposited MnO2/Huynh et al.5
The amount of catalyst loaded onto 1 cm2 of substrate was estimated from the reported deposition charge.
The standard amount encompassed 6 mC of charge. From 6 the overall deposition reaction is described with
two electrons per MnO2 unit. Using this assumption a TOFmin of 0.0833 s-1 per mA/cm2 can be estimated. The
activity results used for comparison is taken from Figure 1 in 5 where the catalyst was measured at steady
state (60-90 seconds at each potential) in diluted phosphoric acid mixed with KNO3 and KOH (pH 2.5).
δ-MnO2/Takashima7
The electrodes in this work were prepared by spray deposition using a 0.5 mM colloidal solution of δ-MnO2. In
total, 6 ml of this solution was sprayed onto a 3x3 cm FTO substrate resulting in a loading of 3.33 x 10-7
mol/cm2. Assuming that this quantity is representative for Mn atoms the TOFmin is 0.0077 s-1 per mA/cm2.
Activity data is taken from figure 6a in 7 where the catalyst is tested with cyclic voltammetry in aqueous
Na2SO4 mixed with H2SO4 and NaOH resulting in a pH of 4. The original data is shown on an SHE scale. In order
to compare on the RHE scale, we have converted it by adding 59 mV per pH value.
These estimations are summarized in Figure 1. Morita et al. prepared their catalyst using thermal
decomposition3, a method typically resulting in surfaces with a very high roughness.8 Consequently, the
actual TOF for the catalyst reported by Morita et al. would likely be orders of magnitude lower than the
TOFmax; on this basis our thin film catalyst is the most active.
DFT calculations on bulk and planar MnO2
The mixing of Ti or Ir into the bulk of rutile MnO2 was investigated by evaluating 3 concentrations, 25, 50 and
75 %. The calculation parameters used are as described in the main text. The stabilities of the mixed oxides are
evaluated similarly to the termination energy:
Δ������������������ ������������ = ���������������� − ���������������� − (���� ∙ ������ ����,������ − ���� ∙ ������ ����,����������)
Here ������������������ ������������ is the energy of formation for the mixed oxide, ���������������� the total energy of the mixed structure,
�������������� the energy of the original MnO2 structure, ������ ����,������ the energy of a unit cell of the guest material,
������ ����,������2 the energy of a unit cell of the MnO2 and ���� the amount of substituted atoms, which is substituted.
Surface incorporation was also investigated for 25 and 50 % surface sites substituted. It is seen in Table S1 that
none of the concentrations results in negative energy of formation for bulk mixtures or surface incorporation.
This is also consistent with results from the materials project database where the only stable mixed oxide with
Mn and Ti is a perovskite type, TiMnO3, while for Mn and Ir no stable mixed oxide are found.9,10
Calculated values
The values calculated with DFT can be seen below in Table S1.
Termination
material
Step
termination
eV/MO2
Kink
termination
eV/MO2
Bulk mixing
Mn0.75X0.25O2
eV/MO2
Surface
Mn0.75X0.25O2
eV/MO2
Lattice
parameter
a Å
Lattice
parameter
c Å
MnO2 - - - - 4,499 2,902
TiO2 -0.23 -0.35 0.01 0.1 4,708 2,958