Green hydrogen production by water electrolysis is a viable sustainable technology for achieving carbon neutrality. Proton exchange membrane (PEM) water splitting is preferable to alkaline water electrolysis due to the advantages of high compatibility to intermittent renewable electricity, large current density, high hydrogen purity, and great conversion efficiency. Carbon-supported platinum (Pt) catalysts are the most active electrocatalysts for hydrogen evolution reaction (HER), but high catalyst loading of 0.5~1.0 mgPt cm-2 is commonly used in the PEM water electrolyzer (PEMWE), which hinders its large-scale application due to the high cost and scarcity of Pt. Therefore, it is crucial to develop efficient, stable, and structurally clear Pt-based catalysts.
Recently, the research group of professor Zheng Hu at the Key Laboratory of Mesoscopic Chemistry of MOE has achieved precise regulation of Pt active sites on hierarchical nitrogen-doped carbon nanocages (hNCNC) by a thermal-driven Pt migration, from edge-hosted Pt-N2Cl2 single sites in initial Pt1/hNCNC-70°C catalyst to Pt clusters/nanoparticles and back to in-plane Pt-NxC4-x single sites. Thereinto, Pt-N2Cl2 presents the optimal HER activity (6 mV@10 mA cm-2) while Pt-NxC4-x shows poor HER activity (321 mV@10 mA cm-2) due to their different Pt coordination. Operando characterizations demonstrate the low-coordinated Pt-N2 intermediates derived from the Pt-N2Cl2 under working condition are the real active sites for HER, which enable the multi-H adsorption mechanism with an ideal H* adsorption energy of nearly 0 eV thereby the high activity, as revealed by theoretical calculations. In contrast, the high-coordinated Pt-NxC4-xsites only allow the single-H adsorption with a positive adsorption energy thereby the low HER activity. Accordingly, with an ultralow Pt loading of only 25 μgPt cm-2, the proton exchange membrane water electrolyzer assembled using Pt1/hNCNC-70°C as cathodic catalyst achieves an industrial-level current density of 1.0 A cm-2 at a low cell voltage of 1.66 V and high durability, showing great potential applications.This study provides an insight into the correlation of HER performance with Pt dispersion and coordination, the formation of unsaturated Pt-N2 active-site and the related HER mechanism, which much favors the illumination of the elusive literature results as well as the rational design of advanced Pt SSCs for ultralow Pt usage in the industrial-level PEMWE.
Figure 1. Morphology and structure of typical Pt/hNCNC-T (T=70, 300, 600 and 900°C). (a-d) HAADF-STEM images of Pt/hNCNC-70°C (a), -300°C (b), -600°C (c), -900°C (d). Yellow and red circles in (b) highlight the Pt sites with adjoining bi-atoms and multi-atoms, respectively. (e) XPS spectra of Pt 4f. (f) Normalized XANES spectra at the Pt L3-edge, with Pt foil and PtO2 for reference. Inset is the local enlargement. (g) k3-weighted R-space Fourier transformed spectra from EXAFS. (h) Wavelet transform Pt L3-edge EXAFS. (i) Schematic diagram of Pt dispersion evolution upon heating.
Figure 2. Electrocatalytic HER performances of typical Pt/hNCNC-T catalysts. (a) Polarization curves. (b) Tafel slopes (Dotted lines are derived experimental data from figure 2a; solid lines are fitted data). (c) h10 and mass activities at 50 mV. (d) Polarization curves of Pt1/hNCNC-70°C before and after 5000 and 10000 CV cycles. Note: The data for commercial 20 wt% Pt/C are also shown for comparison.
Figure 3.Operando characterizations and DFT calculations on HER mechanism of Pt1/hNCNC-70°C. (a-d) OperandoRaman spectra (a), Operando XAFS (b, c) and free energy diagram (d) for HER of Pt/hNCNC-70°C. Inset in (b) is the local enlargement. (e) Free energy diagrams for HER of Pt1/hNCNC-900°C. (f) DOS of Pt1/hNCNC-70°C and -900 °C.
Figure 4. PEMWE performances. (a) Schematic diagram of device. (b) Polarization curves. (c) Cell voltages at different current densities. (d) Pt mass activity. (e) Chronopotentiometry curve at 1.0 A cm-2. Note: The data for commercial 20 wt% Pt/C are also shown for comparison.
The related paper entitled “Understanding Pt active sites on nitrogen-doped carbon nanocages for industrial hydrogen evolution with ultralow Pt usage” has been published on Journal of the American Chemical Society on November 25, 2024 (Paper link: https://doi.org/10.1021/jacs.4c11445, DOI: 10.1021/jacs.4c11445). Ph.D. student Jingyi Tian is the first author. Prof. Xizhang Wang, Prof. Qiang Wu and Prof. Zheng Hu from our department are co-corresponding authors. This work was jointly supported by the National Key Research and Development Program of China (No. 2021YFA1500900), the National Natural Science Foundation of China (Nos. 52071174), and the Natural Science Foundation of Jiangsu Province, Major Project (BK20212005).