Synthesis and characterization of the Co-NG catalyst

To prepare the Co-NG catalyst, a precursor solution was first prepared by sonicating GO and cobalt salts (CoCl 2 ·6H 2 O; weight ratio GO/Co=135:1) in water. The well-mixed precursor solution, as depicted in Fig. 1a, was then freeze-dried to minimize re-stacking of the GO sheets. The Co-NG catalyst was finally obtained by heating the dried sample under a NH 3 atmosphere to dope the GO with nitrogen. Control samples of nitrogen-doped graphene (NG) and Co-containing graphene (Co-G, with no N doping) were also prepared. A detailed preparation procedure is described in the Methods section. The morphology of the Co-NG was examined by scanning electron microscopy (SEM); Fig. 1b reveals that the Co-NG has similar morphologic features to graphene with sheet-like structures. Transmission electron microscopy (TEM; Fig. 1c) shows Co-NG nanosheets with ripples observed on the surface. No cobalt-derived particles were found by SEM or TEM on the Co-NG nanosheets, underscoring the smallness in size of the Co. The Co-NG could be formed into a paper by filtration of Co-containing GO suspension and subsequent NH 3 treatment (Fig. 1d).

Figure 1: Preparation and morphology characterizations. (a) Schematic illustration of the synthetic procedure of the Co-NG catalyst. (b) SEM image of the Co-NG nanosheets. Scale bar, 2 μm. (c) TEM image of the Co-NG nanosheets atop a lacey carbon TEM grid. Scale bar, 50 nm. (d) SEM image showing the cross-section view of the Co-NG paper with thickness of 15 μm, prepared by filtration of Co-containing GO suspension followed by NH 3 annealing. Scale bar, 20 μm. The inset shows the optical image of a 2 × 1 cm2 Co-NG paper. Full size image

To probe the compositions of Co-NG, X-ray photoelectron spectroscopy (XPS; Fig. 2a) showed the presence of C, N and O peaks in the samples of Co-NG and NG, whereas the N peak was absent in Co-G. No significant signals were found at the Co region in the Co-NG. To determine the Co content, inductively coupled plasma optical emission spectrometry (ICP-OES) was performed after digesting the powdered sample in HNO 3 . By combined use of XPS and ICP-OES, the Co-NG was determined to be 0.57 at% Co, 8.5 at% N, 2.9 at% O and 88.2 at% C, as summarized in Fig. 2b. The Co content in NG with no intentional addition of Co is negligible (<0.005 at% by ICP-OES). The XPS detailed scan in the Co region (Fig. 2c) of Co-NG shows two peaks at a binding energy of 781.1 and 796.2 eV, corresponding to the 2p 3/2 and 2p 1/2 levels, respectively. The peak positions and the separation of 15.1 eV between these two peaks indicates the presence of Co(III)24. The N 1s (Fig. 2d) can be deconvoluted into different types of nitrogen25,26, namely pyridinic and N-Co (398.4 eV), pyrrolic (399.8 eV), graphitic (401.2 eV) and N-oxide (402.8). The small difference in the binding energies between pyridinic N and N-Co prevents further deconvolution27. From the peak intensity, the N was dominated by the pyridinic/N-Co species. The C 1s and O 1s XPS were shown in Supplementary Fig. 1. The presence of Co and N was further confirmed by the energy-dispersive X-ray spectroscopy (EDS) spectrum (Supplementary Fig. 2) taken in the area shown in Fig. 2e of the scanning transmission electron microscopy (STEM) image. The EDS line scan in Fig. 2e reveals the close-proximity distributions of the Co and N elements.

Figure 2: Compositional characterizations on the Co-NG. (a) XPS survey spectra of the Co-NG, NG and Co-G. (b) Chart showing the percentages of cobalt, nitrogen, oxygen and carbon in the Co-NG measured by XPS and ICP-OES. (c,d) High-resolution XPS Co 2p and N 1s spectra, respectively. (e) STEM image of the Co-NG nanosheet. Scale bar, 20 nm. Inset is the EDS elemental line scan from A to B showing the presence of C, N and Co elements. Full size image

Atomic structure analysis by HAADF and EXAFS

To investigate the atomic structure of the Co-NG nanosheet, we used high-angle annular dark field (HAADF) imaging in an aberration-corrected STEM. The bright-field STEM image (Fig. 3a) shows the defective structures of the GO-derived graphitic carbon. The corresponding HAADF image (Fig. 3b) clearly shows that several bright dots, corresponding to heavy atoms (Co in this case), are well dispersed in the carbon matrix. The size of these dots is in the range of 2–3 Å, indicating that each bright dot corresponds to one individual Co atom. The enlarged view of the selected region (Fig. 3c) reveals that each Co atom is centred by the light elements (C, N and/or O). Additional STEM images are provided in Supplementary Fig. 3. To probe the possible bonding between the cobalt and the light elements in the Co-NG, we performed extended X-ray absorption fine structure (EXAFS) analysis at the Co K-edge, using both a wavelet transform (WT) and Fourier transform. WT-EXAFS analysis is a powerful method for separating backscattering atoms that provides not only a radial distance resolution, but also resolution in the k-space28. The discrimination of atoms can be identified even when these atoms overlap substantially in R-space. The k2-weighted χ(k) signals (Fig. 3d) and the corresponding Fourier transforms (Fig. 3e) of the Co-NG and Co-G samples show quite similar profiles, suggesting no substantial differences in the coordination environments of the Co atoms. The existence of only one single strong shell, which is usually characteristic of amorphous or poorly crystalline materials, at ∼1.5 Å in R-space (Fig. 3e) is indicative of a large structural disorder around Co sites, consistent with the abundant misplacement and voids observed in the aberration-corrected STEM images. Figure 3f shows the WT contour plots of the two signals based on Morlet wavelets (κ=3, σ=1) with optimum resolution at the first shell29. The intensity maximum A is well-resolved for the Co-NG (3.4 Å−1) and Co-G (3.2 Å−1). Since the locations of the WT maxima are highly predictable, they allow qualitative interpretation of the scattering path origins. The WT maximum is known to be affected by the path length R, Debye–Waller factors σ2, energy shift ΔE and atomic number Z, and this corresponds to the same location of the maximum in the q-space magnitude30. For an isolated Co–C path (R=2 Å), the WT maximum at 3.2 Å−1 in the q-space magnitude showed little dependence on R, σ2 and ΔE, but it is largely affected by different Z (3.5 Å−1 for Co-N path, 4.3 Å−1 for Co-O path, and 6.8 Å−1 for Co-Co path; Supplementary Fig. 4). As a result, by comparison, the WT maximum A at 3.2 Å−1 for the Co-G can be associated with the Co-C path, and 3.4 Å−1 for the Co-N path within the Co-NG. A small difference of ∼0.1 Å−1 between the maxima A for the Co-NG (3.4 Å−1) and the calculated Co-N path (3.5 Å−1) might arise from the much shorter length of the actual Co-N path than 2 Å. The maximum feature B at 9.0 Å−1 might result from the effect of side lobes and the multiple scattering paths between the light atoms, instead of from the Co–Co path, which exhibits a maximum at 6.8 Å−1. The validity of the above WT-EXAFS interpretation was confirmed by a least-squares curve fitting analysis carried out for the first coordination shell of Co (Supplementary Figs 5 and 6 and Supplementary Note 1).

Figure 3: Structural characterizations on the Co-NG. (a) Bright-field aberration-corrected STEM image of the Co-NG showing the defective and disordered graphitic carbon structures. Scale bar, 1 nm. (b) HAADF-STEM image of the Co-NG, showing many Co atoms well-dispersed in the carbon matrix. Scale bar, 1 nm. (c) The enlarged view of the selected area in b. Scale bar, 0.5 nm. (d,e) The k2-weighted EXAFS in k-space and their Fourier transforms in R space for the Co-NG and Co-G, respectively. (f) Wavelet transforms for the Co-NG and Co-G. The location of the maximum A shifts from 3.2 Å−1 for Co-G to 3.4 Å−1 for Co-NG, indicating the presence of Co-N bonding in Co-NG. The vertical dashed lines are provided to guide the eye. Full size image

Taken together, the data indicate that in the Co-NG the Co is atomically dispersed in the nitrogen-doped graphene matrix and it is in the ionic state with nitrogen atoms in the cobalt’s first coordination sphere. Hence, nitrogen doping of the graphene provides sites for Co incorporation.

HER activity evaluation

The HER catalytic activity of the Co-NG was evaluated using a standard three-electrode electrochemical cell. The catalyst mass loading on a glassy carbon electrode was 285 μg cm−2. Figure 4a shows the linear-sweep voltammograms (LSVs) at a scan rate of 2 mV s−1 in 0.5 M H 2 SO 4 after iR-compensation for the Co-NG electrode along with the two control samples of NG and Co-G. The commercial Pt/C (20 wt% platinum on Vulcan carbon black, Alfa Aesar) with the same mass loading was also included as a reference point. As expected, the Pt/C exhibits superior HER catalytic activity with a near zero onset η. The Co-NG catalyst shows excellent HER activity, as evidenced by the very small onset η of ∼30 mV (inset in Fig. 4a), beyond which the current density increases sharply. The onset η is defined here as the potential at a current density of −0.3 mA cm−2, which is chosen to match the onset η determined by the Tafel plot (shown later). The η needed to deliver 1 and 10 mA cm−2 were determined to be ∼70 and ∼147 mV, respectively. The Faradaic efficiency of the Co-NG catalyst was determined to be ∼100% by gas chromatography (Fig. 4b, Supplementary Fig. 7 and Supplementary Note 2), confirming the cathode current is due to the generation of H 2 . It should be noted that these η values are much smaller than those of Co-based molecular complexes31,32,33, and further suggesting that the Co-NG system is one of the best solid-state earth-abundant catalysts, including MoS 2 (refs 15, 34), WS 2 (ref. 35), CoP36 and MoP37. Also, this ‘pseudo-metal-free’ catalyst (which contains only 0.57 at% metal) shows much higher activity than all the recently reported metal-free catalysts (Supplementary Table 1 and Supplementary Note 3). As control samples, the NG and Co-G show poor activity towards HER with onset η larger than 200 mV, indicating that the active sites in Co-NG are associated with the Co and N. Tafel analysis (Fig. 4c) gives Tafel slope values of 31, 82, 117 and 144 mV decade−1 for Pt/C, Co-NG, NG and Co-G, respectively. Notably, the Tafel plot for the Co-NG catalyst becomes linear at low η of ∼30 mV.

Figure 4: HER activity characterizations. (a) LSV of NG, Co-G, Co-NG and Pt/C in 0.5 M H 2 SO 4 at scan rate of 2 mV s−1. The inset shows the enlarged view of the LSV for the Co-NG near the onset region. (b) Plot showing the molar number of H 2 produced as a function of time. The straight line represents the theoretically calculated amounts of H 2 assuming 100% Faradaic efficiency, and the scattered dots represent the produced H 2 measured by gas chromatography. The overlapping of these two sets of data indicates that nearly all the current is due to H 2 evolution. The error bars arise from instrument uncertainty. (c) Tafel plots of the polarization curves in a. (d) TOF values of the Co-NG catalyst (black line) along with TOF values for other recently reported catalysts. Full size image