Sythesis and characteration of A-Ni–C catalyst

A-Ni–C catalyst was obtained from well-dispersed Ni metal in the graphitized carbon matrix. The synthesized Ni-based metal-organic framework (Ni-MOF) was used as the precursor (Fig. 1). Followed by carbonization at 700 °C in nitrogen (N 2 ) atmosphere, Ni nanoparticles encapsulated in graphene layers (Ni@C) were prepared. Hydrochloric acid (HCl) leaching treatment was used to remove the redundant Ni metal (HCl-Ni@C). After HCl treatment, hollow onion-like carbon nanoshells along with little Ni nanoparticles protected by graphene layers remained. We then applied electrochemical cyclic-potential on the HCl-Ni@C catalysts-decorated electrode, unexpected activation process was observed. Similar activation processes were also reported in the literature13,20. To our surprise, direct constant potential on Ni@C can also activate the electrocatalyst, which was conducted until the electrocatalyst reached the optimized and stable activity. We found Ni single atoms formed during all these activation process, which is verified as discussed later.

Figure 1: Schematic diagram of synthesis and activation process of the Ni–C catalysts. The Ni-MOF used as a precursor consists of an orthorhombic crystal. Atoms are shown as follows: C, black; H, white; O, red; N, blue; Ni, royal blue. Carbonization of the synthesized Ni-MOF was at 700 °C in N 2 atmosphere to obtain Ni@C. HCl leaching treatment was repeated three times to sufficiently dissolve exposed Ni metal. Constant potential and CV treatments were performed to activate the catalysts until they reached the optimal performance and remained stable. During the activation process, Ni single atoms formed in situ anchored on the graphitized carbon. Full size image

The nanostructure of Ni@C was characterized by a transmission electron microscope (TEM). Low-magnification TEM image of Ni@C displays Ni metal inside carbon nanospheres (Supplementary Fig. 1a). The size distribution image (Supplementary Fig. 1b) suggests that the nanospheres have an average size around 10 nm. After HCl leaching, the high-resolution TEM image of HCl-Ni@C shows hollow onion-like nanoshells along with little Ni metal aggregates encapsulated in the graphene layers (Fig. 2a). To further study the structure changes of the catalyst before and after activation, a JEM-ARM200F scanning TEM (STEM) fitted with a double aberration-corrector for both probe-forming and the imaging lenses was used. In the STEM image of HCl-Ni@C, the measured Ni (111) d-spacing (0.204 nm) and Ni (200) d-spacing (0.176 nm) again confirm the existence of well crystalline Ni metal in the graphitic carbon nanoshells (inset of Fig. 2a). The thickness of graphene layer is uneven with exposure of Ni nanoparticles, which would provide possibilities for slow dissolution of Ni metal during the activation process in the acidic solution. The bright-field STEM image of A-Ni–C (Supplementary Fig. 2) illustrates that the measured (002) d-spacing of the graphitic carbon ranges from about 0.33 to 0.36 nm, indicating the carbon structure is somewhat disordered, rather than ideally graphitic, which was also evidenced by the X-ray diffraction (XRD) and Raman analysis. Individual metal atoms in practical catalysts can be discerned in the atomic resolution high-angle annular dark-field (HAADF) images33,34,35. For the sample A-Ni–C, the atomically isolated Ni species (marked by white circles) are dispersed on the partially graphitized carbon matrix (Fig. 2b). Examination of different regions (Supplementary Fig. 2) confirms abundance of Ni single atoms present in A-Ni–C. Considering that the elements of N, F and S have a smaller atomic number than Ni, these impuries can hardly exsit as bright tiny spots in the metal-carbon system. HAADF images of controled samples have also ruled out this possiblity (Supplementary Fig. 3). About 78% of the Ni species counted in the images were present as single atoms away from each other (inset of Fig. 2b). A minority of subnanometer Ni clusters present in the same sample did not have the packed Ni atom structure of Ni nanoparticles.

Figure 2: Micrographs of samples HCl-Ni@C and A-Ni–C. (a) HRTEM image of HCl-Ni@C. Scale bar, 10 nm. Inset of a, BF STEM image of Ni metal encapsulated in graphene layers. Scale bar, 5 nm. The area of graphene with exposure of Ni nanoparticle provides the opportunity that Ni metal would slowly resolve during the activation process. (b) Subångström resolution HAADF STEM image of A-Ni–C. The circles are drawn around isolated Ni atoms. Scale bar, 3 nm. Inset of b, the size distribution figure, which is based on >120 observed Ni species counted from high-magnification images (recorded at 4 × original magnification). Full size image

The multiple peaks on the XRD pattern of the synthesized Ni-MOF (Supplementary Fig. 4a) well match those of the typical crystalline structure of [Ni 2 (L-asp) 2 (bpy)]·CH 3 OH·H 2 O. Ni@C exhibits the cubic phase of Ni metal (JSPDS NO. 04-0850) with a broad shoulder peak in the range 20–30° (2θ), which originates from the partially graphitized carbon specie (Fig. 3a). The diffraction peaks of graphitic carbon become higher and sharper with the increased temperature (Supplementary Fig. 4b). Raman spectrum of Ni@C (Fig. 3b) further confirms that the Ni-MOF is partially graphitized at 700 °C. Among carbon-based catalysts, these graphitic carbon structures formed during the carbonization treatment may significantly enhance the electronic conductivity and corrosion resistance, which would improve the electrocatalytic efficiency29. After sufficiently washed by HCl, little Ni nanoparticles encapsulated in the multi graphene layers remained. The XRD pattern of HCl-Ni@C proves the existence of the Ni metal. However, no Ni–Containing signal was detected in the sample A-Ni–C, primarily attributed to insensitivity of XRD to single atoms or small clusters.

Figure 3: Structure characterization of the Ni–C catalysts. (a) XRD patterns of Ni@C, HCl-Ni@C and A-Ni–C, respectively. The diffraction peaks of Ni in Ni@C and HCl-Ni@C confirm the existence of well crystalline Ni metal. The red line corresponding to A-Ni–C illustrates that no Ni–Containing crystal phase was detected after activation process. (b) Raman spectrum of Ni@C with I D /I G =0.63, indicating the partial graphitization at 700 °C. Full size image

The binding state was investigated by X-ray photoelectron spectroscopy (XPS). As shown in Supplementary Fig. 5, N–C bonds in the samples of Ni@C, HCl-Ni@C and A-Ni–C are in the form of pyridine (398.9 eV), pyrrole (400.4 eV) and graphite (401.3 eV) (refs 28,38). The quantitative analysis of the XPS results reveals that the atomic ratios of N/C in Ni@C, HCl-Ni@C and A-Ni–C are 4.62:100, 3.80:100 and 2.21:100, respectively. The existence of N dopants in the A-Ni–C would inherently increase the ability of interacting with reactants and render high positive charge density on their adjacent carbon atoms, which could also contribute to the high activity of A-Ni–C samples28,29. The Ni 2p XPS spectrum of Ni@C reveals the coexistence of Ni0 (852.8 eV) and Ni2+ (853.7 eV), which can be ascribed to Ni-Ni and Ni-O bonding39,40 (Fig. 4a). After HCl leaching, the Ni 2p XPS spectrum of HCl-Ni@C nearly remains the same as that of Ni@C, suggesting the residue of Ni metal. The existence of Ni-O bonding can be attributed to the slow oxidation of the exposed Ni metal41,42,43. We further collected samples of A-Ni–C for analysis. The XPS analysis of A-Ni–C (Supplementary Fig. 6, Supplementary Table 1) illustrates it contains Ni, C, N, F, S and O elements without any Pt impurity, eliminating the influence of Pt for HER performance. We should point out that F, S and O elements mainly resulted from the Nafion solution. To confirm that trace amount of Ni in the A-Ni–C sample contributes to the HER activity, we determined the Ni contents by indicatively coupled plasma atomic emission spectroscopy. As shown in Supplementary Table 2, the weight ratios of Ni in the Ni@C, HCl-Ni@C and A-Ni–C were 85.0%, 5.3% and 1.5%, respectively. We found that the Ni content decreased during the CV activation process. What is more, no detectable Pt impurities were found in all these samples. In addition, the Ni 2p XPS spectrum of A-Ni–C (Fig. 4b) has a sharp peak ∼860.7 eV, which can be associated with the strong Ni–F binding. Specifically, the peak at 852.8 eV is assigned to Ni0, reflecting Ni–Ni binding of the Ni clusters.

Figure 4: XPS spectra of the Ni–C catalysts. (a) XPS spectra of Ni 2p peaks of Ni@C and HCl-Ni@C. (b) XPS spectrum of Ni 2p peaks of A-Ni–C. The peaks at 852.8 eV and 853.7 eV are assigned to Ni0 and Ni2+, respectively. The sharp peak at 860.7 eV can be correlated with Ni–F binding, and the element F comes from Nafion solution. Full size image

Electrocatalytic HER activity evaluation

The HER electrocatalytic activity was evaluated in 0.5 M H 2 SO 4 using a typical three-electrode set-up. All tests were performed without iR compensation. The lineal sweep voltammetry (LSV) curves showing the normalized current density (j) versus voltage (j versus V) for A-Ni–C and HCl-Ni@C along with commercial Pt/C (5 wt% Pt on Vulcan carbon black), for comparison, are show in Fig. 5a. As expected, Pt/C catalyst exhibits excellent HER performance. HCl-Ni@C requires overpotential (η) of about −440 mV to reach j of 10 mA cm−2. In sharp contrast, A-Ni–C produces j of 10 mA cm−2, 20 mA cm−2 and 100 mA cm−2 at η of −34 mV, −48 mV and −112 mV, respectively. These overpotentials are the lowest among the reported acid-stable and earth-abundant HER electrocatalysts, including recently reported Ni 2 P14, MoP | S16, FeP17, MoS 2 (refs 8,22,24), WS 2 (refs 21,23,25), Mo 2 C (refs 12,13) and chemically doped grapheme-based materials18,19,20 (more details in Supplementary Table 3).

Figure 5: HER electrocatalytic properties of the Ni–C catalysts. (a) LSV curves of HCl-Ni@C, A-Ni–C and Pt/C. (b) Tafel plots obtained from LSV curves in a, indicating significant improvement in activity for HER after activation. (c) EIS spectra of HCl-Ni@C, A-Ni–C and Pt/C at the overpotential of −100 mV. Inset of c represents a close-up view of the spectra at high frequencies. A-Ni–C-modified GC electrode displays much lower impedance than HCl-Ni@C-modified GC electrode, similar to that of Pt/C-modified GC electrode. (d) Chronoamperometric curve of A-Ni–C at η=−45 mV, revealing nearly no apparent deactivation after 25 h. Note: all the electrochemical data of A-Ni–C catalysts were obtained after electrochemical cyclic-potential activation. Full size image

To understand the detailed underlying mechanism of HER activity, Tafel plots based on LSV curves were acquired (Fig. 5b). The linear regions of Tafel plots were fit to the Tafel equation (η=b log j+a, where b is the Tafel slope), yielding Tafel slopes of ∼34, 41, 194 mV per decade for Pt/C, A-Ni–C and HCl-Ni@C, respectively. The value for A-Ni–C does not correspond to one of the standard HER Tafel slopes (29, 38, 116 mV per decade), indicating the HER occurs through a Volmer–Heyrovsky mechanism and electrochemical recombination with an additional proton is the rate-limiting step28. Moreover, the exchange current density (j 0 ) was obtained by extrapolation of Tafel plots (Supplementary Fig. 7). The j 0 of A-Ni–C is about 1.2 mA cm−2 (the same magnitude as the value of 2.5 mA cm−2 for Pt/C), also highlighting the exceptional H 2 evolving efficiency of A-Ni–C catalyst.

The optimal activity of A-Ni–C catalyst would also be attributed to strong chemical and electronic coupling between graphitized carbon and Ni single atoms, permitting highly efficient electrical communication between the catalytic active sites and the underlying electrode substrate. Electrochemical impedance spectroscopy measurements were performed to confirm the hypothesis (Fig. 5c). The charge transfer resistance of A-Ni–C is similar to that of Pt/C, which is much lower than that of HCl-Ni@C. Thus, much faster electron transfer between the catalytic active sites of A-Ni–C and the electrode substrate is one of the key factors contributing to the superior HER kinetics.

Durability of A-Ni–C catalyst

The durability is a key concern of all catalysts. In this study, accelerated degradation measurements were adopted to evaluate the durability. Cyclic voltammetric (CV) sweeps between+905 mV and −95 mV versus the reversible hydrogen electrode potential (versus RHE) at 100 mV s−1 were applied to HCl-Ni@C-decorated glassy carbon (GC) electrode. Post-potential-cycling LSV curves were recorded (Supplementary Fig. 8). HER performance after 8,000 cycles retained almost the same to the test after 4,000 cycles. To further probe the durability of A-Ni–C in the acidic environment, continuous HER at a static overpotential was conducted (Fig. 5d). When operating a constant overpotential of −45 mV, a continuous HER process occurred to generate molecular H 2 (different overpotentials were applied to generate H 2 shown in Supplementary Movie 1). During the first 11 h, the current density of ∼10 mA cm−2 was observed, and then gradually increased to ∼14 mA cm−2, which might be caused by adequate activation of the catalyst. In addition, the current density levelled out at ∼12 mA cm−2. It should be mentioned that the high activity was maintained for >25 h in the chronoamperometric electrolysis (after CV activation, ⩾4,000 cycles). Furthermore, we did the galvanostatic electrolysis test to collect the evolved hydrogen gas to evaluate the Faradaic efficiency of the A-Ni–C catalyst. In Supplementary Fig. 9, the result demonstrates that A-Ni–C gives about 100% Faradaic yield during the electrolysis process. Overall, the high catalytic activity and outstanding durability illustrate the potential of A-Ni–C for cost-effective electrocatalytic hydrogen evolution in water electrolysis system and solar-driven hydrogen system.

Activation processes

Activation processes (constant-potential activation and cyclic-potential activation) were also recorded to investigate changes after activation. Hydrogen bubbles generated at η=−100 mV on Ni@C-decorated GC electrodes with different degrees of activation (Fig. 6a). Constant potential at η of −200 mV (Supplementary Fig. 10) was used to stabilize the catalyst to reach the best performance, simultaneously ruling out the possibility that the high j would result from the reaction between the unsecure Ni metal and the strongly acidic solution. The j gradually improved in the first 2 h and levelled out in the next 2 h. These activation treatments also ensured stable and reproducible results. LSV curves of Ni@C after different periods of constant-potential activation prove the activation process of Ni@C (Fig. 6b). During CV activation process, the catalytic j of HCl-Ni@C gradually increased with the consecutive CV scans and reached the maximum efficiency after about 4,000 cycles (Fig. 6c). Notably, the anodic peaks around 0 V (versus RHE) shifted to lower potentials during the process, indicating a more easily occurring oxidation reaction. Interestingly, the oxidation peak intensities were also enhanced during the CV treatment (Fig. 6d). These signs during CV activation reveal that A-Ni–C could lose electrons more easily in the anodic-going scan. The oxidation peak intensity gradually levelled out when the cathodic catalytic j nearly remained stable (Supplementary Fig. 11). These changes of the oxidation peak corresponding to valance changes of Ni coincide with the trend of the catalytic activity. In addition, we also collected the activated samples after different CV cycles for STEM analysis (Supplementary Figs 12 and 13). The downsizing of the Ni nanoparticles and the formation of single Ni atoms can be evident to be observed during the activation process. Throughout the detailed CV activation process, we summarize that active Ni single atoms would form during the activation process, leading to these changes of the oxidation peak and simultaneously contributing greatly to the improved activity. Furthermore, the HCl-Ni@C samples can also be activated on the fluorine-doped SnO 2 (FTO) glass substrate (Supplementary Figs 14 and 15).