Boosting hydrogen evolution reaction on few-layer graphdiyne by sp-N and B co-doping

Hydrogen, a clean and efficient energy, has been regarded as a promising alternative to replace fossil fuels.^1–5 Electrochemical water splitting for hydrogen generation, or hydrogen evolution reaction (HER), is the most green and efficient route to obtain hydrogen. Traditionally, Pt-based materials are applied as electrode catalysts for HER, but they always suffered from high cost, rare source, and decreased activity.^6–8 Together with poor durability, the HER process has hampered its practical applications, e.g., electrolyzers.^9,10 It is necessary to design catalysts with tailored activity and durability for HER,^11–13 especially metal-free materials.^11–15 Various strategies and principles have been investigated on interface and surface engineering of metal-free materials to boost the HER activity, including heteroatom doping, adsorption of organic molecules, and structural defect creation.^16–18 Among this, substitution of C atoms with heteroatoms, such as B, N, S, and P, can tune the localized charge distribution and has been regarded as a promising way to modify the adsorption and reduction of hydrogen.^19–21 Notably, when doped with N atoms, the π electrons of carbon materials can be activated by their lone-pair electrons, which can be applied into reduction reactions, but the doping sites are quite difficult to control, and it is strongly related to the catalytic performance. For nitrogen doping, there are different N bonding configurations, including sp-N, pyridinic N, pyrrolic N, amino N, and graphitic N (or quaternary N), which perform different activities for catalysis.²² 

Graphdiyne (GDY) has received much attention since its first synthesis in 2010.²³ It is a new carbon allotrope with a large π conjugated network, in which benzene rings are connected to butadiyne linkages.^24–28 The prominent mechanical, thermal, and electrical properties make it possible to be applied in diverse fields.^29–32 In view of these unique characteristics, fabricating GDY-based catalysts with well-defined structures and distinct properties is highly desirable for electrocatalysis.^33,34 Moreover, GDY, the unique sp hybridized C atoms, give the possibility for chemical modification. Density functional theory (DFT) calculations indicated that the incorporation of B and N into GDY can make the activation more pronounced and, consequently, enhance catalytic activity.^35–37 However, to date, no experimental work has been conducted for this co-doping system in GDY. Previously, we have investigated the site-defined method to introduce sp-N into GDY³⁸ and we found that the sp-N atoms are much more negatively charged than other nitrogen configurations for enhancing the catalytic performance. We can predict that the charge delocalization in GDY induced by sp-N and B atoms is more extensive, facilitating the electron-transfer process for proton reduction and enhancing the HER performance.

In this paper, we carefully designed and fabricated three kinds of B and N co-doped few-layer GDY by controlling the doping orders, including one-stage doping of B and N into few-layer GDY (BNFLGDY), sequential doping of B and N into few-layer GDY (B,NFLGDY, B first), and sequential doping of N and B into few-layer GDY (N,BFLGDY, N first). We found the doping methods strongly related to the configurations of heteroatoms in GDY, including the bonding forms, N/B atomic ratio, and N content, especially the sp-N content, and then present different HER performances. Overall, taking advantage of the synergistic catalytic effects, N,BFLGDY brings the best catalytic activity for HER under alkaline condition, with a low overpotential of 84 mV, a small Tafel slope of 89 mV dec⁻¹, and long-term stability. In addition, the HER catalytic activity of N,BFLGDY can be comparable to Pt/C and superior to the reported metal-free catalysts.

The preparation process of co-doped samples is shown in Fig. 1. The large-area bulk GDY has been prepared via in situ Cu-catalyzed growth according to the reported method.²³ It can be seen from the scanning electron microscopy (SEM) image that the formed GDY film was continuous and uniform with the thickness of ∼1.0 µm (Fig. S1). The transmission electron microscopy (TEM) presents the stacking structure of GDY (Fig. S2). Then, bulk GDY was exfoliated in strong acid solution to few-layer graphdiyne oxide (FLGDYO) nanosheets for the subsequent doping (Fig. S3). Taking the fabrication of N,BFLGDY, for example, FLGDYO was first mixed with melamine (as a nitrogen source) and annealed under an Ar atmosphere. Afterward, the resulting sample was mingled with boron oxide (as a boron source) for further calcination. After washing with hot water, the N,BFLGDY was obtained. Different doping configurations could be obtained by changing the doping order of B and N precursors. All the B and N co-doped samples were fabricated at 900 °C, denoted as the BNFLGDY-900 (one-step doping with melamine and boron oxide); B,NFLGDY-900 (first doped with boron oxide and then melamine); and N,BFLGDY-900 (first doped with melamine and then boron oxide), respectively (Fig. 1).

After exfoliation, FLGDYO presents an ultra-thin two-dimensional structure, exposing abundant edges and sites [Fig. 2(a)]. The average thickness of FLGDYO was ∼1.50 nm, measured by atomic force microscopy (AFM) [Fig. 2(b)]. Figure 2(c) shows the TEM image of N,BFLGDY-900. Compared with FLGDYO, the incorporation of B and N atoms induces structural distortion, making N,BFLGDY-900 pieces wrinkled. A similar phenomenon can be observed in all the doped samples of BFLGDY-900, BNFLGDY-900, and B,NFLGDY-900 (Fig. S4).

Raman features of various samples further supported the structural distortion, in which D bands and G bands are centered at around 1350 and 1590 cm⁻¹ and the vibration of conjugated acetylenic bonds is at ∼2190 cm⁻¹ [Fig. 2(d)]. With the introduction of B and N atoms, the intensity ratio of the D band to G band (I_D/I_G) increases from 0.96 of FLGDYO to 1.03 of N,BFLGDY-900, suggesting successful doping of atoms into the GDY network. The x-ray diffraction (XRD) patterns of GDY samples display a broad background hump, indicating a low crystallinity (Fig. S5).

The typical B K-edge curves in x-ray absorption near-edge structure (XANES) spectroscopy display sharp peaks in the π* region between 190 and 196 eV and a broad span in the σ* region ranging from 196 to 205 eV.³⁹ The narrow and intense peak at ∼192.0 eV is attributed to the π* resonance of h-BN, which is specifically obvious in BNFLGDY-900 [Fig. 2(e)].^40,41 Moreover, the peaks centered at ∼192.2 and ∼193.9 eV correspond to the resonance of BC₂O and BCO₂, respectively, which can be observed from BNFLGDY-900; B,NFLGDY-900; N,BFLGDY-900; and BFLGDY-900 [Fig. 2(e) and Fig. S6].⁴² In addition, peaks at ∼396.8, ∼397.4, ∼398.6, ∼400.3, and ∼406.0 eV in the normalized N K-edge can be attributed to the sp-N, pyri-N, amino-N, grap-N species, and C–N σ* transitions, respectively [Fig. 2(f)].^35,43 The relatively weak pyri-N and strong grap-N in BNFLGDY-900 originated from the formation of h-BN.⁴⁴ 

Together with XPS, the electronic structure was further identified. As shown in Figs. 2(g) and 2(h), the fitted peaks of high-resolution B 1 s spectra centered at 191.4 and 192.5 eV can be attributed to BC₂O and BCO₂, respectively.^45,46 The ratios between these two configurations are different for various samples: B,NFLGDY possesses a higher proportion BCO₂ than N,BFLGDY and BNFLGDY. Notably, in terms of high-resolution B 1s spectra, a specific peak of B–N (190.5 eV) can be detected only from the one-stage doping sample, i.e., BNFLGDY-900, indicating the formation of h-BN [Fig. 2(g)]. The high resolution N 1 s spectra of N,BFLGDY-900 can be deconvoluted into four peaks, assigned as sp-N, pyri-N, amino-N, and grap-N, respectively [Fig. 2(i), Figs. S7 and S8, and Table S2].³⁵ 

The different configurations of heteroatoms in GDY induced different HER performances (Fig. 3). Compared with FLGDY, the doping of heteroatoms into GDY displays enhanced HER activity (Figs. S9 and S10). To clarify, a series of experiments were conducted to evaluate the effects of various atoms and configurations.

First, by comparing N and B doping, we synthesized BFLGDY-900 (only B doping) and NFLGDY-900-NH₃ (only N doping but no sp-N configuration) with boron oxide and NH₃, respectively. The influence of single dopants on HER activity was first evaluated by linear sweep voltammetry (LSV) curves in 1.0M KOH. As can be seen from Figs. 3(a) and 3(d), NFLGDY-900-NH₃ presents a current density of 15.4 mA cm⁻² (at 0.25 V vs RHE), which is superior to BFLGDY-900 of 9.6 mA cm⁻² (at 0.25 V vs RHE), confirming the critical role of N atoms in achieving an excellent HER performance.

Second, by comparing N configurations, samples featuring sp-N (NFLGDY-900-M, dopant of melamine) and excluding sp-N (NFLGDY-900-NH₃, dopant of NH₃) have been accordingly prepared.³⁸ In comparison with NFLGDY-900-NH₃, we noticed that the sp-N doping (NFLGDY-900-M) can bring a higher current density, lower overpotential, and smaller Tafel slope, indicating the decisive influence of the sp-N atom [Figs. 3(b) and 3(c)].

Moreover, when it comes to co-doping, the doping sequence raises obvious importance, which can produce diverse configurations. Figure 4(a) shows the polarization curves of B and N co-doped GDY with various doping approaches. BNFLGDY-900 presents the lowest current density due to the generation of the by-product (h-BN), which is chemically inert and leads to poor electrocatalytic performance [Fig. 2(g)].⁴¹ Although the configuration of B and N is similar in N,BFLGDY-900 and B,NFLGDY-900, the N,BFLGDY-900 sample displays better activity, which may originate from more N content, especially the sp-N content (Tables S1 and S2), making it easier to adsorb hydrogen protons. It is found that N,BFLGDY-900 possessed the highest activity with a current density of 48.6 mA/cm² (at 0.25 V vs RHE). Additionally, the N,BFLGDY-900 exhibits marked kinetics with a Tafel slope of 89 mV dec⁻¹ toward HER compared to BNFLGDY-900 and B,NFLGDY-900 [Fig. 4(b)], suggesting rapid HER kinetics derived from the advantage of co-doping.

Importantly, as reflected from the comparison of overpotentials of various samples, the N,BFLGDY-900 delivers the lowest overpotential of 84 mV (at 10 mA/cm²) in comparison with other doped samples [Fig. 4(c) and Fig. S11]. The above merits of N,BFLGDY-900, including a low Tafel slope and overpotential, are also superior to the previously reported metal-free HER catalysts [Fig. 4(d) and Table S3]. Additionally, the N,BFLGDY-900 shows outstanding long-term stability tests, which maintained 93% of the initial current after 10 000 s scans, while the corresponding value decayed to 79% sharply for Pt/C [Fig. 4(e) and Fig. S12].

Generally, the overall HER process is 2H⁺ + 2e⁻ → H₂. In alkaline solution, the hydrogen intermediates (H^*) are first generated (H₂O + e⁻ + M = M − H* + OH⁻), followed by the formation of the H₂ product (H₂O + e⁻ + M − H* = H₂ + M + OH⁻ or 2M − H* = H₂ + M). The introduction of heteroatoms could modify the electronic structure of catalysts and binding strength of the adsorbed hydrogen. As reported, there exists a synergistic coupling effect between B and N atoms, and N atoms can indirectly activate B atoms to enhance activity.⁴⁷ Together with sp-N, the resulting N,BFLGDY-900 may be more inclined to adsorb hydrogen, leading to the enhanced HER activity ultimately.

In summary, B and N atoms have been co-doped into few-layer GDY for the first time, and we found that the doping configurations are tunable by controlling the doping orders. The introduction of a certain sp-N can decrease the overpotential, and further incorporation of B atoms increases the current density. N,BFLGDY-900 presents the best HER activity and is superior to the currently reported metal-free catalysts. This work may shed light on the delicate design of performance-oriented non-metallic catalysts.

See the supplementary material for the detailed experiments and characterization contents.

This work was supported by the National Key R&D Program of China (Grant No. 2018YFA0703504), the National Natural Science Foundation of China (Grant Nos. 51932001 and 21971244), the Postdoctoral Innovative Talent Support Program (Grant No. BX20190331), and the China Postdoctoral Science Foundation (Grant No. 2020M670457).