*3.4. Synthesis of Cu/Cu3P Nanoarrays*

To prepare Cu/Cu3P nanoarrays, NaH2PO<sup>2</sup> was placed at the center of the tube furnace and the Cu/CuO nanoarray was placed at the downstream side of the furnace at carefully adjusted locations to set the temperature, and the distance between them was measured to be approximately 24 cm. After flushing with Ar, the center of the furnace was elevated to 270 ◦C with a heating rate of 2 ◦C/min and held at this temperature for 60 min. For comparison, samples at different phosphating temperatures were synthesized by the same process.

#### *3.5. Material Characterization*

XRD patterns were collected using an M21X diffractometer (MAC Science Co. Ltd., Japan) with high-intensity Cu Kα radiation (λ = 1.541Å). The morphology of the products was characterized via scanning electron microscopy (SEM, ZEISS SUPRA55). The HRTEM images were collected on a FEI Tecnai F20 electron microscope operated at 200 kV. The elemental compositions and valence states of the samples were determined by XPS. XPS measurements were performed using a Thermo Fisher Scientific, Escalab-250Xi spectrometer with an Al K*α* X-ray resource. The C 1s contamination peak was used for charge correction (284.8 eV).

#### *3.6. Electrochemical Measurements*

The electrochemical performances of Cu/Cu3P nanoarrays were evaluated with the CHI 660D electrochemical workstation. All the electrochemical measurements were conducted in a typical three-electrode setup with an electrolyte solution of 0.5 M H2SO<sup>4</sup> using Cu/Cu3P nanoarrays as the working electrode, a graphite plate as the counter electrode and Ag/AgCl as the reference electrode. In all measurements, the Ag/AgCl reference electrode was calibrated with respect to a reversible hydrogen electrode (RHE). Linear sweep voltammetry (LSV) measurements were conducted in 0.5 M H2SO<sup>4</sup> with a scan rate of 2 mV s−<sup>1</sup> . All the potentials reported in our work were versus the RHE according to E vs. RHE = E vs. Ag/AgCl + E<sup>θ</sup> vs. Ag/AgCl + 0.059 pH. Impedance measurements were carried out with a frequency range from 0.1 Hz to 10 kHz at the open-circuit potentials.

#### **4. Conclusions**

In summary, we have successfully prepared Cu/Cu3P nanoarrays via a facile two-step synthetic strategy, including the hydrothermal synthesis of a Cu/CuO nanoarray precursor and a low-temperature phosphorization process in an Ar atmosphere. The as-prepared Cu/Cu3P nanoarrays, as electrocatalysts, display excellent catalytic performance and durability for electrochemical hydrogen generation. Specifically, the Cu/Cu3P nanoarray-270 exhibits a low onset overpotential (96 mV) and a small Tafel slope (131 mV dec−<sup>1</sup> ). The excellent electrocatalytic efficiency of the Cu/Cu3P nanoarray catalyst for the HER

can be attributed to its unique architecture. The nanoarray structure provides more active sites and contributes to the diffusion of the products. The Cu/Cu3P nanoarrays show good electrical conductivity, which is favorable to a faster transfer rate of electrons. This strategy provides an efficient technique that can be extended to other metal phosphides and metal-based nanostructures, thus creating a new opportunity in hydrogen production.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/catal12070762/s1, Figure S1: Characterization of the morphology and structure of samples: (a) SEM image of Cu nanosheet, (b) XRD patterns of Cu nanosheets, and (c) XRD patterns of Cu/CuO nanoarrays; Figure S2: The high-magnification SEM images of samples: (a,b) SEM image of Cu/CuO nanoarrays; Figure S3: The high-magnification SEM images of (a) CuO nanoplate, (b) Cu3P nanoplate; Figure S4: XPS survey spectrum of Cu /Cu3P nanoarrays; Figure S5: The SEM images and XRD patterns of samples at different phosphating time: (a,b) 1 h; (c,d) 1.5 h; Figure S6: The pure Cu nanosheet was directly phosphated and calcined: (a) SEM image; (b) XRD pattern; Table S1: Comparison of HER catalytic performance of Cu/Cu3P nanoarray-270 and other non-noble-metal electrocatalysts in acidic media.

**Author Contributions:** The manuscript was written with contributions from all authors. Conceptualization, R.D. and J.L.; methodology and investigation, R.D. and X.X.; data curation, M.X.; formal analysis, R.D. and X.X.; writing—original draft preparation, R.D.; writing—review and editing, R.D. and J.L.; visualization, M.X.; project administration, R.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (Program No. 21905232) and Natural Science Basic Research Plan in Shaanxi Province of China (Program No. 2022JM-236).

**Data Availability Statement:** The data are available upon request from the corresponding author.

**Acknowledgments:** The authors thank G. Wang and H.Y. Gao (University of Science and Technology Beijing, Beijing 100083, China) and X.W. Zhang and M.Y. Han (Beijing Normal University, 100088, China) for their help.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

