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Chen et al. Energy Mater 2022;2:200033 https://dx.doi.org/10.20517/energymater.2022.36 Page 5 of 11

RESULTS AND DISCUSSION
Structure and composition of PtCu aerogels
The morphology of the products was investigated by SEM and TEM. As shown in Figure 2A and B, the
PtCu aerogels are sponge-like and exhibit an open hierarchical porous structure. The TEM image
[Figure 2C] reveals that the PtCu aerogels were constructed of nanospheres of ~2 nm in diameter,
displaying a homogeneously porous structure. The TEM image at higher magnification [Figure 2D] shows
that the average spacing of the lattice fringe was measured to be 0.218 nm, which can be assigned to the
(111) facet of the face-centered cubic (FCC)-structured PtCu nanoparticles. Moreover, it displayed a high
crystallinity of PtCu.

The elemental distribution of the PtCu aerogels was investigated using high-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy
(EDX). As shown in Figure 2E, both the Pt and Cu elements were evenly distributed in the PtCu aerogel
framework, indicating that the structural unit nanospheres consisted of a bimetallic PtCu alloy. The XRD
pattern [Figure 2F] shows that the as-prepared PtCu was quite similar to the standard FCC-structured PtCu
crystal [obtained from the Joint Committee on Powder Diffraction Standards (JCPDS) File No. 48-1549].
The crystalline properties revealed by XRD agree with the TEM and HAADF-STEM-EDX analysis.

As a result, we detected the formation of PtCu aerogels following the steps below (as shown in Figure 3). At
2+
2-
the primary stage of the reaction, both PtCI and Cu were rapidly reduced by the strong reductant NaBH
4
4
together and numerous crystal nuclei of the PtCu alloy formed. The nucleus then grew into nanospheres
and BH4 was decomposed into H . The violent reaction destabilized the hydrogel and accelerated the
-
2
attachment of the nanospheres. Finally, the nanospheres were fused into a network and the aerogels formed
in a very short time.
With the addition of citrate, the PtCu similarly grew into a sponge-like hierarchical porous structure
[Figure 4A and B] and the average spacing of the lattice fringe was measured to be 0.218 nm [Figure 4D].
However, the nanospheres grew into a larger size of ~20 nm in diameter [Figure 4C] with lower
crystallinity, indicating that the fusion was hindered by the coordination of citrate. This phenomenon
further authenticated that the gelation of PtCu followed the rapid nucleation mechanism.

2
The topological surface area of the PtCu aerogels was measured to be 33.0 m /g by the Brunauer-Emmett-
Teller (BET) method. The nitrogen adsorption-desorption isotherms of the PtCu aerogels are shown in
Figure 5.

Measured by ICP-AES, the exact composition of the PtCu aerogels was Pt Cu , in good accordance with
55
45
their feed ratios.
Electrocatalytic properties of PtCu aerogels
The electrochemical catalysis of the products was investigated using the rotating disk electrode technique
via a classic three-electrode system in 0.1 M HClO . Commercial 20 wt.% Pt/C catalysts were also evaluated
4
as references. Figure 6A shows the CV curves of all these catalysts measured in an Ar-saturated 0.1 M
HClO solution at 30 ℃ with a sweep rate of 50 mV/s. According to equation (1), the ECSA values for the
4
PtCu aerogels/C and Pt/C were determined to be 43.6 and 54.3 m /g, respectively. CO stripping
2
voltammetry [Figure 6D] was used to investigate the ECSA of the catalyst more delicately and was recorded
in an Ar-saturated 0.1 M HClO solution at 30 ℃ with a sweep rate of 20 mV/s. According to equation (1),
4
the ECSA value for PtCu was determined to be 26.8 m /g. The single peak in CO stripping curves indicated
2
that the PtCu exposed mainly one kind of facet, in good accordance with the TEM measurement.