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Page 4 of 11 Chen et al. Energy Mater 2022;2:200033 https://dx.doi.org/10.20517/energymater.2022.36

working electrode was as follows. Firstly, a certain amount of PtCu and twice as much carbon (XC-72) were
mixed and dispersed in a 0.05 wt.% Nafion solution of isopropanol/water (7:3) by 1 h of ultrasonication to
obtain a catalyst ink of 2.5 mg /mL. Then, 6 μL of catalyst ink were drop-casted onto the freshly polished
cat
GCE, followed by drying at ambient temperature. Finally, a working electrode was obtained and 15 μg of
catalysts were coated onto the GCE (~150 μg /cm 2 GCE ), followed by measurements of the ORR polarization
cat
and cyclic voltammetry (CV) curves. For reference, 20 wt.% commercial Pt/C was made into an ink
following a similar route as above and 6 μg of Pt were coated onto GCE. All measurements were carried out
in a 0.1 M HClO aqueous solution at 30 ℃ in a water bath. ORR polarization curves were measured in an
4
O -saturated solution at a rotation rate of 1600 rpm with a sweep rate of 5 mV/s between 0.1 and 1.06 V vs.
2
RHE. CV curves were measured in an Ar-saturated solution with a sweep rate of 50 mV/s between 0.05 and
1.15 V vs. RHE.

The electrochemically active surface area (ECSA, cm ) was obtained according to:
2

ECSA =∫ JdE/v sweep /ρ (1)

Where ∫ JdE is the integral of the absorption/desorption peak (A·V), v sweep is the sweep rate of the cyclic
2
voltammetry (V/s) and ρ is the charge density of the adsorbed/desorbed atom or molecule (C/cm ). In this
work, the hydrogen desorption region of the CV curve was chosen to calculate the ECSA and the value of ρ H
2
was 210 μC/cm .
2
The mass activities (MA, mA/mg ) and specific activities (SA, mA/cm ) are obtained according to equations
M
(2) and (3), respectively:
MA = j /m (2)
M
k
SA = j /ECSA (3)
k

where j is the kinetic current (mA), which can be obtained according to the Koutecky–Levich equation:
k
1/j = 1/j +1/j (4)
d
k
where j is the actual current (mA) obtained from a point on the polarization curve and j is the limiting
d
diffusion current (mA) obtained from the maximum current of the polarization curves. In this work, the j at
0.90 V was chosen to calculate the MA and SA.

CO-stripping voltammetry was carried out to determine the ECSA of the PtCu aerogels more delicately.
2
The ECSA was calculated using the CO stripping peak with a ρ of 420 μC/cm according to equation (1).
CO
Pure PtCu was made into ink of 0.5 mg /mL following a similar route as above. To cover all the surface, 40
cat
μL of catalyst ink was drop-casted onto the freshly polished GCE. The route of CO-stripping started with 20
min of Ar purging to remove O from the electrolyte. CO adsorption was then operated by holding the
2
electrode at 0.1 V vs. RHE for 20 min with CO purging, followed by another 20 min of Ar purging to
remove CO from the electrolyte. The CO-stripping peaks were measured via CV in an Ar-saturated solution
with a sweep rate of 20 mV/s between 0.0 and 1.2 V vs. RHE.