Carbon-Promoted Pt-Single Atoms Anchored on RuO2 Nanorods to Boost Electrochemical Hydrogen Evolution

While efficient for electrochemical hydrogen evolution reaction (HER), Pt is limited by its cost and rarity. Traditional Pt catalysts and Pt single-atom (aPt) catalysts (Pt-SACs) face challenges in maintaining kinetically favorable HER pathways (Volmer–Tafel) at ultralow Pt loadings. Herein, carbon-promoted aPts were deposited on RuO2 without the addition of reductants. aPts confined on carbon-supported RuO2 nanorods (aPt/RuO2NR/Carbon) promoted “inter-aPts” Tafel. aPt/RuO2NR/Carbon is the Pt-SAC that retained underpotentially deposited H; additionally, its HER onset overpotential was “negative”. The aPt/RuO2NR/Carbon exhibited 260-fold higher Pt mass activity (imPt)/turnover frequency (TOF) (522.7 A mg–1/528.4 s–1) than that of commercial Pt/C (1.9 A mg–1/1.9 s–1). In an ultralow Pt loading (0.19 μg cm–2), the HER rate-determining step maintained Volmer–Tafel and the Pt utilization efficiency was 100.3%.


Figures. S1-S30:
Tafel plots of Ptm/C, HR-TEM images of Ptm/C, CVs of the Ptm/C/GCE, EDS mapping results, high-resolution XPS spectra of C+RuO2(Pt 2+ ), C+RuO2, C and Pt/C, k 3 -weighted FT-EXAFS spectra, RuO2 and Pt content in aPt/RuO2NR/Carbon, EIS results, imPt/TOF for aPt/RuO2NR/C, imPt for the modern Pt-SACs, anodic LSVs for Hupd on aPt/RuO2NR/C, CO stripping curves on aPt/RuO2/C, tracking RuO2 content by CVs of aPt/RuO2NR/C, stability tests of aPt/RuO2NR/Carbon in HER through the CV scanning and the XRD analysis, and price activities of aPt/RuO2NR/C.Tables S1 and S2: The best-fit FT-EXAFS parameters of aPt/RuO2/C catalysts and HER performances of Pt-SACs and Pt-catalysts.

Data S1
Influences of Pt loading for HER mechanism on Ptm/C The Volmer-Tafel mechanism (V-T) is the dominant HER rate-determining step (RDSH) for Ptcatalysts in acidic aqueous solutions.During V-T, two active H atoms (H*) on the Pt surface form an H2 molecule via chemical bonding without electron transfer.This results in the reduction of the HER by electron transfer kinetics (Scheme S1).The Tafel slope on the Tafel plot, typically set at 30 mV dec -1 at 25 °C, represents a standard value for V-T when evaluating the kinetics of HER.
The HER polarization curves and corresponding Tafel plots of the micrometer-sized XC-72 graphite carbon-supported nanometer-sized Pt (~5 nm) catalysts (denoted as Ptm/C, where m represents the Pt wt.%, with values of 10, 20, and 40) are shown in Figure S1.The 10 of the HER on Ptm/C increases with decreasing Pt loading on the electrode.Based on the Tafel slope assessment, the RDSH of Ptm/C shifts from the V-T to the Volmer-Heyrovsky mechanism (V-H) as the Pt loading on the electrode decreases (Figures.S1 and S2).This transition is achieved by adjusting the catalyst ink dilution.V-H is H* that continually reacts with H + and electrons to form H2 molecules.The distance between adjacent H*-adsorbed Pt active sites (Dee) influences the bonding possibility of the two H* atoms (Scheme S1 and Figure S2).The HR-TEM images of Ptm/C are shown in Figure S2.The average size of the Pt nanoparticles is ~5 nm.Assuming that the density of the Pt nanoparticles on each C particle is homogeneous, the final Pt loading on the electrode is varied by tuning the Ptm/C ink concentration.The aggregation of Ptm/C particles in high Pt loading (2~0.5 g cm -2 ) shortens the Dee; however, the actual Dee is observed at low Pt loading (<0.2 g cm -2 ) (Scheme S1 and Figure S2).Dee is constrained by the spacing of Pt anchored to C in Ptm/C.Ptm/C is limited to further reducing the Pt loading in HER applications.

Electrochemical determination of Dee
According to our previous report, a combination of Pt-electrochemical surface area (ECSA) and the anodic charge (QPt) from the anodic stripping of Pt during anodic CV scanning on Pt-based catalysts covering GCE was used to calculate the mean diameter of Pt particles (DPt) and the number of Pt particles on the substrate (NPt).S1-S6 The method was also extended to calculate the Pt diameter in atomic size (less than 1 nm) in this study, as the retention of Hupd on a Pt/RuO2NR/Carbon can be used to measure the electrochemical surface area (ECSA).The Pt content can be directly evaluated from QPt, assuming four electrons are transferred per Pt atom.
The Pt content evaluated directly from QPt is consistent with the results obtained by ICP-MS and TGA.ECSA was determined by measuring the areas (charges) under the electro-oxidation of adsorbed Hupd (Figure S3).A conversion factor of 0.21 mC cm -2 was used to determine the ECSA.
Assuming that the Pt particles are uniformly distributed and spherical in shape on the substrate, ECSA and QPt can be calculated using equations (S1) and (S2), respectively.
where n = 4 represents the number of electrons transferred for the anodic stripping of Pt, F is the Faraday constant, ρPt is the density of Pt (21.09 g cm -3 ), and MPt is the atomic mass of Pt.The QPt/ECSA ratio is used for evaluating the DPt of Pt as shown in equation (S3).NPt can then be obtained from DPt.
The inter-particle distance (Dee) represents the distance between two Pt particles (edge-to-edge distance between each Pt particle) on a substrate.We assumed that all Pt particles are spherical for the calculation of Dee.The Pt particles were monodispersed and homogenously distributed on the carbon support (Scheme S1).Nitrogen adsorption and desorption studies were performed at 77 K on a Micromeritics TriStar 3000 adsorption apparatus.ABET was measured using the BET method.

Data S3
The stability of aPt/RuO2NR/Carbon in HER process To assess its stability in the HER process, aPt/RuO2NR/Carbon was subjected to continuous potential scanning, cycled 5000 times from 0.6 V to -0.1 V versus RHE, at a scan rate of 0.05 V/s.
The polarization curves for the first and 5000th cycles show almost identical behaviors without any obvious change, as depicted in Figure S26.While Pt ions could transfer to the electrolyte due to the anodic potential being too positive (> 0.8 V versus RHE), they appear to remain stable during the cathodic operation in the HER process.ICP-MS was used to track the metal ions in the electrolyte solution.The content of Pt 2+ and Ru 3+ is less than 0.05 M.This supports that the HER process did not cause the possible dissolution of Pt and RuO2 during HER.The XRD of pristine aPt/RuO2NR/C is added in Figure S27.The diffraction peaks of RuO2NR were observed, but the diffraction signals of aPt were not detected, which supports the notion that the active sites of aPt are at the atomic scale.After the HER stability test, the diffraction peaks of RuO2NR did not show significant changes, and there was still no appearance of aPt diffraction signals.This supports that the HER process did not cause the agglomeration of aPt to form Pt clusters.
For practical application, the stability of aPt/RuO2NR/Carbon for HER was further evaluated at a large current density, ~120 mA cm -2 (the overpotential of 100 mV). Figure S28a shows that the chronoamperometric curve of aPt/RuO2NR/Carbon only retained 50% after 1h.To track the stability of aPt/RuO2NR/Carbon, CVs of aPt/RuO2NR/Carbon after electrolysis static overpotentials of 100 mV for HER for 12h in 0.5 M H2SO4aq were provided in Figure S28b.Despite the current density dropping to 50% of its original value during the first hour of continuous HER operation, it remained stable for 12 hours, with no significant increase in Pt 2+ and Ru 3+ in the electrolyte solution.However, CV tracking results showed that the onset potential for HER shifted in the cathodic direction from 0.05 V to 0.0 V, which is close to the onset potential of bulk Pt, after the high current density HER test.Since there was no significant decrease in Pt content in the catalysts, the reduction in HER activity is likely due to the strong polarization of the electrode surface under high current density, leading to the migration and aggregation of Pt single atoms on the catalyst surface, forming larger Pt clusters.This issue needs to be addressed for the future application of aPt/RuO2NR/Carbon in practical high-current density operations.

Scheme S1
The electrochemical process evaluated Dee vs. Pt loading; Dee influences the bond formation between two H*, and the HER mechanism.

Figure S1
Figure S1 (a)(c)(e) Polarization curves of Ptm/C with various Pt loadings for HER; (b)(d)(f) The related Tafel plots.

Figure S2 (Figure S3 Figure S4 Figure S6 Figure S7
Figure S2 (a) Tafel slopes and b) 10s of Ptm/C various with Pt loading on the electrode; c) HR-TEM images of Ptm/C with various Pt loadings in low magnification (above) and high magnification (below). .

Figure S10 (Figure S11
Figure S10 (a) The HR-TEM images of aPt/RuO2NR/GE (ultra-small dark dots are aPts marked by red cycles); (b) The corresponding HAADF-STEM images (ultra-small bright dots are aPts).

Figure S25 TrackingFigure S26
Figure S25 Tracking RuO2 content by CVs of aPt/RuO2NR/C recorded in the non-Faradaic potential range (electrochemical double layer).

Figure S27 Figure S28 .
Figure S27 Stability tests of aPt/RuO2NR/Carbon in HER through the XRD analysis: XRD patterns (Cu Kα) of (black line) C and aPt/RuO2NR/C (blue line) before and (red line) after 5000 potential cycles are displayed.

Figure S29 Figure S30 .
Figure S29 Price activities of aPt/RuO2NR/C and Pt/C at a η of 60 mV.(price activities estimation based on the prices of Ru ($14,146/Kg) and Pt ($28,932/Kg) as found on a website dated 3/21/2024.)

Table S1 .
The best-fit FT-EXAFS parameters of aPt/RuO2/C catalysts

Table S2 .
HER performances of Pt-SACs and Pt-catalysts