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Chen et al. Energy Mater 2022;2:200033 https://dx.doi.org/10.20517/energymater.2022.36 Page 3 of 11

Figure 1. Digital photographs of PtCu and PtCu(CS) aerogel formation at different stages (A) before and (B) 5 min, (C) 10 min, (D) 20
min, (E) 30 min, (F) 40 min, (G) 50 min, (H) 1 h and (I) 5 h after the addition of NaBH . 4

aerogel was almost totally formed. The resulting mixture was settled still for another 4 h and finally sank to
the bottom of the tube [Figure 1H]. The products were collected by centrifugation. The products were then
washed with ultrapure water twice, ethanol/methanol (1:3) four times and ultrapure water twice. The
washed products were frozen and freeze-dried in a vacuum for 12 h using a LGJ-10 freeze dryer. Finally, the
PtCu aerogels were obtained.

To investigate the forming process of the PtCu aerogels, a control group was conducted. The synthesis of
the control group followed a similar route but with citrate as a stabilizer in the primary step. As a result, the
formation of the aerogel was slower than that without citrate [Figure 1E]. The resulting products were
designated as citrate-stable platinum-copper [PtCu(CS)] aerogels.

Characterization of structure and composition
X-ray diffraction (XRD) measurements were carried out using a Rigaku Miniflex600 powder diffractometer
equipped with Cu-Kα radiation, with the tube operated at a 40 kV accelerating voltage and a 15 mA current.
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained
using Zeiss Merlin Compact and JEOL JEM-2100 microscopes, respectively. The samples were ground into
powders for the XRD and SEM analysis. The samples were dispersed in ethanol, dropped on the ultrathin
copper film and dried for the TEM analysis.

The topological surface area of the PtCu aerogels was investigated using nitrogen adsorption and desorption
isotherms collected on a Micromeritics ASAP 2460 system. The chemical composition of the PtCu aerogels
was confirmed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) using an IRIS
Intrepid II XSP spectrometer. The samples for ICP-AES were prepared by dissolving them with aqua regia,
followed by dilution.

Electrochemical measurements
Electrochemical measurements were conducted in a typical three-electrode cell using a CHI630E
electrochemical workstation. Platinum foil was used as the counter electrode and a reversible hydrogen
electrode (RHE) acted as the reference. The working electrode was a glassy carbon electrode (GCE) of 5 mm
in diameter (Tianjin Aidahengsheng) coated with a uniform thin-film catalyst. The preparation of the