Synergistic charge transfer and proton enrichment in Pt/PDDA-modified CNTs for enhanced hydrogen evolution reaction

Abstract

Herein, Pt nanoparticles/poly dimethyl diallyl ammonium chloride (PDDA) modified carbon nanotubes (CNTs) were synthesized through reduction by ethylene glycol (EG), denoted as EG-Pt/pCNTs. At a Pt loading of 2.31 wt%, EG-Pt/pCNTs were found to exhibit significant electrochemical hydrogen evolution reaction (HER) performance with a Tafel slope of 137 mV dec⁻¹ and 35 mV dec⁻¹ in alkaline and acid solutions, respectively. As control experiments, X-ray photoelectron spectroscopy (XPS) and Raman spectra showed that electrons can be transferred from PDDA to Pt atoms, which was more favorable for adsorbing H* species. Density functional theory (DFT) calculations further confirmed intermolecular charge transfer and charge rearrangement for the Pt atoms. In addition, the experimental results demonstrated that EG-Pt/pCNTs exhibited a strong capacity for proton enrichment. Ultimately, the HER kinetics dramatically improved for EG-Pt/pCNTs. This strategy can be used to transform inert CNTs into highly efficient HER electrocatalysts with low noble metal loading.

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Introduction

To meet the demands of sustainable development and environmental protection, enormous efforts have been made to search for clean energy such as metal–air batteries,¹ fuel cells² and water-splitting systems.³ Hydrogen, as one form of clean energy, holds great promise in replacing conventional fuels owing to its clean and sustainable energy carriers. The electrochemical hydrogen evolving reaction (HER) is believed to be the most effective and sustainable approach for hydrogen generation.^4–8 Therefore, active and durable catalysts are required for efficient electrochemical water splitting. Over the past few decades, Pt-based metals have been considered the most active HER electrocatalysts.^9–14 However, their practical applications remain restricted due to their great expense and limited natural abundance. Thus, there is high demand for developing low cost, highly efficient HER catalysts for improving the HER performance of Pt-based metals.

To date, various transition-metal-based electrocatalysts, including metal oxides/sulfides/phosphides/hydroxides,^15,16 layered double hydroxides,¹⁷ and perovskites,¹⁸ have been developed as efficient HER catalysts. However, their HER performance remains limited owing to their inherent corrosion and easy oxidation in strongly acidic or alkaline solutions. To reduce or replace metal electrocatalysts for environmental protection, carbon-based catalysts have been extensively studied due to their good electron transfer properties and earth abundance.^19–23 As is known to all, pristine carbon materials show rather poor electrochemical catalytic activity towards HER. To improve the electrochemical catalytic activity of pristine carbon materials, it has been proved that the combination of heteroatoms or metal nanoparticles with carbon materials can effectively promote the HER performance owing to the improved active sites and electronic conductivity. Qu et al.²⁴ synthesized N and S dual-doped carbon nanotubes that show excellent HER and OER performance. An overpotential for HER of −0.40 V was achieved at a current density of 5 mA cm⁻² with a Tafel slope of 133 mV dec⁻¹ in 1 mol L⁻¹ KOH solution. Li et al.²⁵ prepared highly dispersive CoP nanocrystal embedded ordered mesoporous carbon, which showed excellent HER performance in 0.5 mol L⁻¹ H₂SO₄ solution. The prepared electrocatalyst showed an overpotential of 112 mV at a current density of 10 mA cm⁻² with a Tafel slope of 57 mV dec⁻¹. Guo et al.²⁶ reported that Co₂P–CoN confined in N-doped carbon nanotubes exhibited an overpotential of 98 mV with a Tafel slope of 57 mV dec⁻¹ in 0.5 mol L⁻¹ H₂SO₄ solution.

Recently, it has been suggested that the electron transfer from a support to Pt atom can significantly enhance the catalytic performance of a catalyst, which can lead to a rearrangement of charge for Pt.^27–31 Ma et al.³¹ synthesized Au–Pt–CdS heteronanostructures in which each component was in direct contact with another two components. In this material system, electrons transferred from Au to CdS and Pt or from CdS to Pt, resulting in the enhancement of the photocatalytic HER performance. Inspired by this, ethylene glycol (EG) and poly dimethyl diallyl ammonium chloride (PDDA) have been introduced for improving the proton concentration and notable interfacial charge redistribution to break the HER catalytic kinetic bottleneck found for CNTs modified by Pt nanoparticles (EG-Pt/pCNTs). PDDA acts as pivotal parts in the following way: the intermolecular charge transfers from PDDA to Pt atoms can induce charge rearrangement for the Pt atoms. As a result, the HER performance of EG-Pt/pCNTs with low metal-loading can be expressively improved.

Results and discussion

Synthesis and characterization

EG-Pt/pCNTs catalysts were synthesized through two steps as: first, PDDA modified CNTs (pCNTs) was prepared via ultrasonication; (2) Pt NPs obtained from the reduction of H₂PtCl₆ by EG were loaded onto pCNTs and named EG-Pt/pCNTs-x, where x presented the volume of the added H₂PtCl₆ solution. The Pt content for the EG-Pt/pCNTs catalysts was determined by inductively coupled plasma-mass spectrometry (ICP-MS) to be 0.42 wt%, 1.38 wt%, 2.31 wt% and 4.65 wt% for EG-Pt/pCNTs-20, EG-Pt/pCNTs-60, EG-Pt/pCNTs-100 and EG-Pt/pCNTs-200, respectively. For comparison, pCNTs modified with Pt NPs synthesized through fast reduction by NaBH₄ denoted as Pt/pCNTs and CNTs loaded with Pt NPs through reduction by EG denoted as EG-Pt/CNTs were also synthesized. In addition, Pt/pCNTs and EG-Pt/pCNTs with different volumes of H₂PtCl₆ solution were also synthesized. The Pt contents in EG-Pt/pCNTs, Pt/pCNTs and Pt/CNTs were shown in Table S1.

As shown in Fig. S1, CNTs exhibited an interconnected porous network architecture and relatively smooth surface. After deposition of the Pt nanoparticles, the surfaces of the Pt/CNTs became rough, and C, O and Pt elements were distributed evenly across the surface, as shown in Fig. S2. The high resolution transmission electron microscope (HRTEM) image showed that Pt nanoparticles (∼4 nm) were deposited on the surface of the CNTs (Fig. S3). For Pt/pCNTs, the HRTEM image exhibited Pt particles (∼4 nm) anchored onto the surface of pCNTs (Fig. S4).³² Compared with CNTs and Pt/CNTs, EG-Pt/pCNTs-100 were found to exhibit the original morphology of the CNTs (Fig. 1a). The corresponding energy dispersive spectrometer (EDS) mapping shows a uniform distribution of C, O, N and Pt elements (Fig. 1e). HRTEM images for EG-Pt/pCNTs-100 were shown in Fig. 1b–d. It can be observed that Pt NPs (approximately 4 nm in size) were successfully anchored onto the CNTs. The HRTEM image of EG-Pt/pCNTs-100 showed interplanar spacings of 0.226 nm and 0.34 nm, which was in line with the (111) facet of Pt and (002) crystalline planes of the CNTs, respectively (Fig. 1c and d).^33,34 From the X-ray diffraction (XRD) patterns obtained for EG-Pt/pCNTs-100 (Fig. S5), a diffraction peak was observed at 2θ = 26.0°, which was in line with the (002) crystalline planes of the CNTs. The diffraction peaks observed at 2θ = 39.6° and 67.7° were consistent with the (111) facet of Pt.³⁵ Fourier translation infrared spectroscopy (FT-IR) measurements were carried out to investigate the functional groups present in the prepared catalysts. As shown in Fig. S6, a peak was observed at 3427 cm⁻¹ due to the stretching vibrations of NH₂ or OH suggested the presence of PDDA.³⁶ The adsorption peaks observed at 2927 cm⁻¹ referred to the symmetrical stretching vibration of methylene (CH₂).³⁷ The peaks observed at 1050 cm⁻¹ and 1390 cm⁻¹ were characteristic of the stretching vibration of the hydroxyl group (–OH), indicating that the presence of hydroxyl group in EG-Pt/pCNTs.³⁸

Fig. 1 SEM image (a), TEM image (b), HRTEM images (c) and (d) and EDX elemental mappings (e) for EG-Pt/pCNTs.

X-ray photoelectron spectroscopy (XPS) measurements were used to investigate the surface chemical states in Pt/CNTs, EG-Pt/CNTs, Pt/pCNTs and EG-Pt/pCNTs. As shown in Fig. S7, Pt, C and O elements were detected from the survey spectra in all four samples, while the N element was detected in Pt/pCNTs and EG-Pt/pCNTs for the introduction of PDDA. As shown in Fig. 2a, the peaks due to Pt 4f_7/2 and Pt 4f_5/2 in the spectra obtained for the Pt/pCNTs and EG-Pt/pCNTs shifted to lower binding energy compared with that for the Pt/CNTs.³⁹ Although a small amount of oxidized Pt species existed, their binding energy shift direction aligned with that of metallic Pt⁰.⁴⁰ The binding energies for the O 1s peaks for the EG-Pt/CNTs, Pt/pCNTs and EG-Pt/pCNTs were found to slightly shift compared with that for the Pt/CNTs as Fig. 2b showed. In the high-resolution N 1s XPS spectra as Fig. 2c showed, the N 1s peak for EG-Pt/pCNTs showed a negative shift compared with that for the Pt/pCNTs, suggesting charge transferred from PDDA to Pt NPs.^41,42 The XPS result suggested electron transferred from PDDA of EG-Pt/pCNTs to Pt NPs. Raman spectroscopy was further used to investigate the interaction between the pCNTs and Pt atom. As shown in Fig. S8, the Raman spectra measured for the EG-Pt/pCNTs showed a slight downshift for the G band, indicating charge transfer from the PDDA to Pt NPs.⁴³ Hence, charge transfer occurred from the PDDA to Pt NPs in EG-Pt/pCNT catalyst.^44,45

Fig. 2 (a) The high-resolution Pt 4f XPS spectra for Pt/CNTs, EG-Pt/CNTs, Pt/pCNTs and EG-Pt/pCNTs; (b) the high-resolution O 1s XPS spectra for Pt/CNTs, EG-Pt/CNTs, Pt/pCNTs and EG-Pt/pCNTs; (c) the high-resolution N 1s XPS spectra for pCNTs, Pt/pCNTs and EG-Pt/pCNTs.

Electrochemical characterization

The electrocatalytic HER activities for the EG-Pt/pCNTs were assessed by applying linear sweep voltammetry (LSV) in 1 mol L⁻¹ KOH solution. As shown in Fig. 3a, GCE and CNTs displayed negligible HER activity. After pCNTs were coated onto the GCE, the HER performance was found to be slightly increased, while the HER performance for the pCNTs was greatly enhanced after the pCNTs were treated with EG (denoted as EG/pCNTs). For comparison, various catalysts, including commercial 20% Pt/C catalyst as a benchmark, were investigated for their HER performance under the identical conditions, as shown in Fig. 3b. For a fair comparison of modification effects under similarly low Pt loadings, EG-Pt/pCNTs-20 was selected as the representative sample, allowing the intrinsic contributions of PDDA and EG to the HER performance to be highlighted independently of Pt content. As presented in Fig. 3b, the commercial Pt/C (20 wt%) reference catalyst showed a nearly zero onset potential and excellent HER activity, while the Pt/CNTs showed low HER performance due to their low Pt content. Among the as-prepared catalysts, EG-Pt/pCNTs-20 required the lowest overpotential of 300 mV to achieve a current density of 10 mA cm⁻² compared with the EG-Pt/CNTs (400 mV) and Pt/pCNTs (460 mV). In addition, the EG-Pt/pCNTs containing different amounts of Pt were also studied for their HER catalytic activity. As shown in Fig. 3c and e, after increasing the volume of H₂PtCl₆ solution from 20 µL to 100 µL, the overpotentials for EG-Pt/pCNTs-20, EG-Pt/pCNTs-60 and EG-Pt/pCNTs-100 were determined to be 300 mV, 178 mV and 175 mV, respectively. While further increasing the volume of H₂PtCl₆ solution to 200 µL led to a shift in the overpotential for EG-Pt/pCNTs-200 to 190 mV, indicating lower HER performance compared with EG-Pt/pCNTs-100.

Fig. 3 (a) LSV curves for HER for GCE, CNTs, pCNTs and EG/pCNTs in 1 mol L⁻¹ KOH solution; (b) LSV curves for HER for EG/pCNTs, Pt/pCNTs, Pt/CNTs, EG-Pt/pCNTs-20, EG-Pt/CNTs and Pt/C in 1 mol L⁻¹ KOH solution; (c) LSV curves for HER for EG-Pt/pCNTs-20, EG-Pt/pCNTs-60, EG-Pt/pCNTs-100, EG-Pt/pCNTs-200 and Pt/C in 1 mol L⁻¹ KOH solution; (d) Tafel plots for Pt/C, EG-Pt/pCNTs-20, EG-Pt/pCNTs-60, EG-Pt/pCNTs-100, EG-Pt/pCNTs-200, EG-Pt/CNTs, Pt/pCNTs and Pt/CNTs. (e) The corresponding histogram for the overpotentials and Tafel slopes.

Fig. 3d and e showed the Tafel plots for the as-prepared catalysts. The Tafel slopes for commercial Pt/C, EG-Pt/pCNTs-20, EG-Pt/pCNTs-60, EG-Pt/pCNTs-100, EG-Pt/pCNTs-200, EG-Pt/CNTs and Pt/pCNTs were determined to be 101 mV dec⁻¹, 166 mV dec⁻¹, 153 mV dec⁻¹, 137 mV dec⁻¹, 146 mV dec⁻¹, 229 mV dec⁻¹ and 340 mV dec⁻¹, respectively. Tafel slope for EG-Pt/pCNTs-100 was found to be 137 mV dec⁻¹, which is only 36 mV dec⁻¹ higher than that of commercial Pt/C.

Furthermore, electrochemical impedance spectroscopy (EIS) was carried out at the open-circuit potential to investigate the kinetics for the catalysts in 1 mol L⁻¹ KOH solution (Fig. S9a). As displayed in Fig. S9a, the charge transfer resistance (R_ct) for the electrocatalyst was found to decrease in the following order: CNTs < pCNTs < EG/pCNTs < Pt/CNTs < Pt/pCNTs < EG-Pt/CNTs < EG-Pt/pCNTs-20. EIS curves for EG-Pt/pCNTs containing different amounts of Pt were also recorded at the open-circuit potential and shown in Fig. S9b. R_ct for the electrocatalysts showed a decrease in the following order: EG-Pt/pCNTs-100 < EG-Pt/pCNTs-60 < EG-Pt/pCNTs-200 < EG-Pt/pCNTs-20. EG-Pt/pCNTs-100 exhibited the lowest R_ct, indicating much faster HER kinetics among the electrocatalysts, resulting from the medium Pt and high conductivity of the CNTs.

Cyclic voltammograms (CV) were recorded to measure the electrochemical double-layer capacitance (C_dl). As shown in Fig. S10, the C_dl values for the electrocatalysts increased in the following order: CNTs (2.7 µF cm⁻²) < pCNTs (3.4 µF cm⁻²) < EG/pCNTs (3.0 µF cm⁻²) < Pt/CNTs (4.4 µF cm⁻²) < Pt/pCNTs (5.3 µF cm⁻²) < EG-Pt/CNTs (6.9 µF cm⁻²) < EG-Pt/pCNTs-20 (9.2 µF cm⁻²). This result revealed that EG-Pt/pCNTs-20 showed the highest exposure for catalytically active sites among the as-prepared electrocatalysts, indicating that PDDA can be used to enhance the exposed active site of Pt/CNTs.

The long-term stability of the EG-Pt/pCNTs-100 electrocatalyst was assessed by conducting CV for 1000 cycles in the potential range of 0.2–0.5 V vs. RHE with a scan rate of 100 mV s⁻¹ in an alkaline environment. As shown in Fig. S11a, the LSV curves after 1000 cycles were observed to remain almost the same as that found before 1000 cycles. Chronoamperometric scanning was further applied to investigate the long-term stability of the EG-Pt/pCNTs-100 electrocatalyst (Fig. S11b), which showed that the current density showed only a slight change after 24 h, indicating the excellent durability of the as-prepared EG-Pt/pCNTs-100 electrocatalyst.

The electrocatalytic HER activities for the as-prepared electrocatalysts were further evaluated in 0.5 mol L⁻¹ H₂SO₄ solution. As shown in Fig. 4a and b, Pt/C catalyst showed an extremely low overpotential and Tafel slope of 35 mV and 47 mV dec⁻¹, respectively. The overpotentials and Tafel slopes for Pt/pCNTs and EG-Pt/CNTs were found to be higher compared with that for the Pt/CNTs. After the introduction of PDDA and EG, the overpotential and Tafel slope for EG-Pt/pCNTs-20 both showed a further decrease compared with the Pt/pCNTs and EG-Pt/CNTs.

Fig. 4 (a) LSV curves for HER for EG/pCNTs-20, Pt/pCNTs-20, Pt/CNTs-20, EG-Pt/pCNTs-20 and Pt/C in 0.5 mol L⁻¹ H₂SO₄ solution; (b) Tafel plots for EG/pCNTs, Pt/pCNTs, Pt/CNTs, EG-Pt/pCNTs-20 and Pt/C in 0.5 mol L⁻¹ H₂SO₄ solution; (c) LSV curves for HER for Pt/CNTs-20, Pt/CNTs-60, Pt/CNTs-100 and Pt/CNTs-200 in 0.5 mol L⁻¹ H₂SO₄ solution; (d) LSV curves for HER for Pt/pCNTs-20, Pt/pCNTs-60, Pt/pCNTs-100 and Pt/pCNTs-200 in 0.5 mol L⁻¹ H₂SO₄ solution; (e) LSV curves for HER for EG-Pt/pCNTs-20, EG-Pt/pCNTs-60, EG-Pt/pCNTs-100, EG-Pt/pCNTs-200 and Pt/C in 0.5 mol L⁻¹ H₂SO₄ solution; (f) the corresponding histogram for the Tafel slopes for Pt/pCNTs-20, Pt/pCNTs-60, Pt/pCNTs-100, Pt/pCNTs-200, EG-Pt/pCNTs-20, EG-Pt/pCNTs-60, EG-Pt/pCNTs-100 and EG-Pt/pCNTs-200.

As controls, Pt/CNTs, Pt/pCNTs and EG-Pt/pCNTs with different Pt loadings were synthesized. Furthermore, their electroactivity towards HER was investigated. As shown in Fig. 4c, with increasing Pt loading, the LSV for Pt/CNTs showed a slight change, indicating that Pt loading had little effect on the HER kinetics for the Pt/CNTs. For the Pt/pCNTs (Fig. 4d and f), the overpotentials for Pt/pCNTs-60, Pt/pCNTs-100 and Pt/pCNTs-200 were determined to be 170 mV, 122 mV and 72 mV, respectively. In addition, the Tafel slopes for Pt/pCNTs-60, Pt/pCNTs-100 and Pt/pCNTs-200 were determined to be 148 mV dec⁻¹, 138 mV dec⁻¹ and 63 mV dec⁻¹, respectively (Fig. S12a). This result suggested that the introduction of PDDA slightly improved the HER kinetics for the Pt/CNTs. As a cationic polymer, PDDA may slightly influence the charge rearrangement of Pt atoms. For EG-Pt/pCNTs (Fig. 4e), with increasing Pt loading, the overpotentials for EG-Pt/pCNTs-20, EG-Pt/pCNTs-60 and EG-Pt/pCNTs-100 and EG-Pt/pCNTs-200 were determined to be 227.6 mV, 48.7 mV, 29 mV and 32.5 mV, respectively. Furthermore, the Tafel slopes for EG-Pt/pCNTs-20, EG-Pt/pCNTs-60, EG-Pt/pCNTs-100 and EG-Pt/pCNTs-200 derived from the corresponding polarization curves were determined to be 87 mV dec⁻¹, 114 mV dec⁻¹, 35 mV dec⁻¹ and 52 mV dec⁻¹, respectively (Fig. S12b). Tafel slopes of EG-Pt/pCNTs showed a dramatic decrease compared to those of Pt/pCNTs. These results indicated that the HER kinetics of EG-Pt/pCNTs synthesized via EG-assisted reduction could significantly enhance, which may be attributed to the increased proton concentration near Pt NPs. Meanwhile, these results showed that the introduction of PDDA could improve HER activity by enriching proton concentration and optimizing interfacial charge transfer of Pt NPs.⁴⁶

Meanwhile, the long-term stability of EG-Pt/pCNTs-100 was also evaluated by CV in an acidic environment. As shown in Fig. S13a, the LSV curves obtained after 1000 cycles showed negligible change compared with that before 1000 cycles. Furthermore, chronoamperometric testing over 24 hours revealed only a minimal decay in current density, (Fig. S13b). Meanwhile, EG-Pt/pCNTs remained in their original morphology indicating the excellent durability of the as-prepared EG-Pt/pCNTs-100 in an acidic and alkaline environment (Fig. S14a and b).

The EG-Pt/PDDA-CNTs catalyst was also characterized by XPS after electrochemical stability testing. After stability testing in acidic medium, the Pt 4f spectrum exhibited binding energies at 68.0 eV and 70.9 eV corresponding to the 4f_7/2 and 4f_5/2 orbitals of metallic Pt⁰, respectively, while peaks at 68.9 eV and 74.0 eV were attributed to Pt²⁺ species, and the signal at 76.7 eV indicated the presence of Pt⁴⁺ species (Fig. S15a).³⁹ In contrast, under alkaline conditions, the Pt 4f spectrum showed significantly enhanced metallic Pt⁰ signals at 66.3 eV and 67.7 eV, with a clear negative binding energy shift compared to the acidic environment (Fig. S16a).³⁹ This shift demonstrated a more pronounced electron transfer effect from PDDA to Pt in alkaline medium, effectively modulating the electronic structure of Pt. The O 1s spectrum in acid showed stable oxygen functionalities (531.3–532.8 eV), whereas alkaline conditions generated a new metal-oxygen interface (529.9 eV) (Fig. S15b and b).³⁹ The N 1s peak shifted from 399.9 eV (acid) to 400.8 eV (alkaline), confirming enhanced PDDA protonation in base (Fig. S15c and S16c).⁴¹ The C 1s spectrum with peaks at 284.7 eV (graphitic carbon), 285.6 eV (C–N), and satellite peaks at 295.1 eV and 296.3 eV further confirmed the integrity of the carbon framework during the reaction (Fig. S15d and S16d).⁴¹ These results demonstrated the catalyst's robust stability and its adaptive surface restructuring in alkaline conditions through enhanced electron transfer and proton enrichment.

In summary, the catalyst not only preserved its structural integrity in acidic alkaline and conditions but also undergone beneficial surface reconstruction. The enhanced synergistic effect of electron transfer and proton enrichment mediated by PDDA provided a fundamental basis for the superior long-term stability of the catalyst in acidic and alkaline environment towards HER.

Mechanism for the improved HER performance

To explore the enhanced catalytic HER kinetics observed for EG-Pt/pCNTs, Pt/CNTs, the roles played by the Pt NPs, PDDA and EG were exploited. EIS experiments were applied to monitor the H* concentrations around Pt for the as-prepared electrocatalysts during HER (Fig. S17 and S18). The impedance data were fitted using the equivalent circuit model in Fig. S19 with the corresponding parameters summarized in Tables S2–S8. In this model, R_s was the uncompensated electrolyte solution resistance, which was similar for all catalysts. The first parallel components (R₁ and C₁) corresponded to the charge-transfer kinetics at the electrode–electrolyte interface, and its low values confirmed good electronic conductivity across the series. The second component (R₂ and a constant phase element C₂), which can be converted into a pseudocapacitance C₂ through calculation, reflected the ability for H* hydrogen adsorption. The time constant τ₂(τ₂ = R₂ × C₂) reflected the relaxation rate of the HER as the overpotential (η) changed.^47–49Fig. 5a and b showed the derived curves τ₂vs. η. The data showed that at low η, τ₂ was small and η dependent. As η increased to a moderate value, τ₂ showed a sharp increase. τ₂ for the as-prepared electrocatalyst increased in the following order: Pt/CNTs < Pt/pCNTs < EG-Pt/CNTs < EG-Pt/pCNTs-20 < EG-Pt/pCNTs-60 < EG-Pt/pCNTs-200 < EG-Pt/pCNTs-100. R₃ and C₃ can be related to the electrode surface porosity response. As a result of low Pt-loading and poor HER activity for the pCNTs and EG/CNTs, Pt cannot play an important role in significantly improving the catalysts. Therefore, EG-Pt/pCNTs prepared through EG reduction exhibited a stronger ability to enrich protons around Pt NPs than Pt/pCNTs prepared via NaBH₄ reduction. This result was consistent with previously reported results.⁴⁶ Thus, the HER activity for the EG-Pt/pCNTs was boosted compared with the Pt/CNTs.

Fig. 5 (a) Plots of τ₂vs. η for Pt/CNTs, Pt/pCNTs, EG-Pt/CNTs and EG-Pt/pCNTs-20 in 0.5 mol L⁻¹ H₂SO₄ solution. (b) Plots of τ₂vs. η for EG-Pt/pCNTs-20, EG-Pt/pCNTs-60, EG-Pt/pCNTs-100 and EG-Pt/pCNTs-200 in 0.5 mol L⁻¹ H₂SO₄ solution.

To gain atomistic insight into the interaction between the Pt NPs, EG molecules and PDDA cationic polymer, we employed density functional theory (DFT) calculations to investigate the properties of bonding polarization direction and charge transfer in the process of the electron coupling interaction. The model system constructed for the EG-Pt/pCNTs is shown in Fig. S18a. The difference charge density Δρ₁ for the EG and Pt(111)-substrate (Fig. S18b) clearly illustrated the electron depletion for EG and the strong interaction between the EG and Pt(111)-substrate via intermolecular charge transfer, corresponding to a Pt–O length of approximately 2.141 Å. In addition, the difference charge density Δρ₂ for EG-Pt(111) and substrate (Fig. S18c) demonstrates that the interaction between the EG-Pt(111) and substrate is a weak van der Waals (vdW) interaction with a Pt–H length of approximately 3.127 Å and that the PDDA cationic polymer may have a significant effect on the charge rearrangement of Pt atoms around the interface between the Pt NPs and PDDA.

Thus, PDDA played an important role in the enhancement of the catalytic activity of Pt/CNTs for HER. A possible mechanism for this enhancement was proposed in Fig. 6. As shown in Fig. 6a, the Tafel slope for EG-Pt/pCNTs in the alkaline condition indicated that the HER mechanism was a Volmer–Heyrovsky mechanism; in that H* combined with another H from H₂O in the electrolyte (H₂O + e⁻ + H* → H₂ + OH⁻). In acidic solution, the Tafel slope for EG-Pt/pCNTs suggested that the mechanism for HER was a Volmer–Tafel mechanism; in that H₂ evolved from the adjacent H* (H* + H* → H₂), as shown in Fig. 6b.^50,51 Under the same condition, the HER activity for the EG-Pt/pCNTs with 2.31 wt% Pt loading was found to be comparable to that for the commercial Pt/C catalyst, which had an almost 8.5-times higher Pt loading. Compared with the other catalysts listed in Tables S9 and S10, EG-Pt/pCNTs still exhibited outstanding HER activity. According to DFT result and the control experiments, the excellent HER activity of the EG-Pt/pCNTs could be due to the following reasons: electrons can transfer from PDDA to Pt, which could induce a charge rearrangement for the Pt atoms that was more beneficial for adsorbing H* species. Moreover, EG and PDDA could effectively increase the exposure of active sites on Pt/CNTs. Meanwhile, CNTs not only acted as a support for PDDA, EG and Pt but also increased the electron conductivity. Therefore, the enhancement of the catalytic activity for HER may originate from intermolecular charge transfer and charge rearrangement for Pt atoms.

Fig. 6 Schematic illustrating the enhanced HER mechanism for EG-Pt/pCNTs in alkaline solution (a) and acidic solution (b).

Conclusions

In conclusion, a new strategy was developed based on PDDA with Pt/CNTs for improving the catalytic kinetics for CNTs loaded with a small amount of Pt. The result showed that HER performance of Pt/CNTs synthesized by reduction of EG was superior to that of Pt/CNTs synthesized by reduction of NaBH₄. This improvement can be attributed to the following factors: PDDA could facilitate transfer electrons to Pt atoms, leading to charge rearrangement for Pt atoms; EG-Pt/pCNTs showed a greater capacity for proton enrichment around Pt NPs; EG-Pt/pCNTs exposed more active sites; the conductivity of EG-Pt/pCNTs significantly increased. Therefore, the electrocatalytic performance of the EG-Pt/pCNTs towards HER can be dramatically improved compared with Pt/CNTs and Pt/pCNTs. EG-Pt/pCNTs with a Pt loading of 2.31 wt% exhibited remarkable electroactive HER performance in both alkaline and acid solutions.

Author contributions

Yuxue Dai: investigation, experiments, data preparation, writing of the manuscript; Dayong Song: methodology and editing the manuscript; Mingju Wang: methodology and editing the manuscript; Lianyu Zhang: data preparation, writing of the manuscript; Yong Zhang: data preparation, writing of the manuscript; Wenwen Liu: reviewing and editing the manuscript; Xuejing Liu: methodology; Chuannan Luo: conceptualization and methodology.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: experimental details including additional synthesis procedures and characterization data; additional electrochemical performance data; and the computational details and data from DFT calculations. See DOI: https://doi.org/10.1039/d5nj01942g.

Acknowledgements

This work was supported by Shandong Agriculture and Engineering University Start-Up Fund for Talented Scholars Shandong (no. 30520037), Shandong Provincial Natural Science Foundation of China (no. ZR2016BM01).

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