Sample preparation
We prepared thin film samples of MnO2 and TiO2-modified MnO2 using reactive sputter deposition.32
For the unmodified samples, 40 nm thick films (nominally) of MnO2 were deposited onto the
substrates using a power of 140 W in a 5 mTorr Ar/O2 mixture with a ratio of 5/1. For the TiO2
modified samples, a 35 nm MnO2 film was grown first, onto which a 5 nm thick mixed oxide layer was
added. The mixed layer was prepared by co-sputtering manganese and titanium and the rates were
calibrated with an in-chamber Quartz Crystal Microbalance. The power used for manganese was kept
at 140 W while titanium was sputtered at 150 W. According to the calibration performed with an inchamber
QCM, the TiO2 rate was 20 % of the total deposition rate. All depositions were carried out
with a substrate temperature of 200 ⁰C. Prior to deposition, the EQCM crystals were cleaned in an
ultrasonic bath first in acetone, then isopropanol and finally Millipore water (18.2 MΩ). The gold
polycrystalline disks were polished with a 0.25 μm diamond paste, sonicated in isopropanol and
water, then plasma cleaned and annealed to 700 ⁰C in two consecutive cycles. After electrochemical
tests, the remaining MnO2 film was removed by scanning the working electrode towards 0 VRHE.
The Electrochemical Quartz Crystal Microbalance crystals were purchased from Stanford Research
Systems and the gold polycrystalline disks from Pine Instruments. The electrochemically active area
on the EQCM samples is nominally 1.37 cm2 but only 0.38 cm2 in the center is sensitive to the
frequency measurement. Therefore, we applied a mask of TiO2 on the non-sensitive area and
deposited the MnO2 catalyst in the sensitive area only. Frequency measurements have been
converted to mass changes by using the Sauerbrey equation.32,78
Electrochemical tests
The samples were tested in 0.05 M H2SO4, Merck Suprapur. In the BioLogic software package, EC-Lab,
a test protocol was designed including the following procedure: the electrode is inserted under
potential control at 1.4 VRHE, cyclic voltammetry (CV) is performed to evaluate activity,
electrochemical impedance spectroscopy (EIS) is used to evaluate the Ohmic losses, the Ohmic drop is
compensated at 85 %, the electrode is held at 1.4 V vs RHE to stabilize, chronoamperometry is
performed (CA) at 1.8 V vs. RHE and finally CA is performed at 1.9 V vs. RHE. The electrochemical tests