TMO NPs on carbon nanofibres for characterizations

We first grow CoO NPs on carbon nanofibres (CNFs) to study the morphology evolutions and the corresponding improvements in OER activities under different galvanostatic cycle numbers (see Methods). The pristine CoO NPs are ∼20 nm in diameter and uniformly distributed on CNFs (Supplementary Fig. 1, we denote this sample as pristine CoO/CNF). Transmission electron microscopy (TEM) and the corresponding fast Fourier transform (FFT) images suggest the monocrystalline nature of pristine CoO NPs (Fig. 2a). The spacing of (111) atomic planes is measured to be 0.24 nm, consistent with previous studies28. The CoO/CNF was then assembled in a lithium-ion battery pouch cell for galvanostatic lithiation (charge) and delithiation (discharge) processes (Fig. 1f, see Methods)27. Small charge/discharge current (compared with regular battery cycling) was selected for thorough reaction (see Methods), which also helps to maximally maintain the integration of the particles for long-term stability. The morphology of CoO begins to change after one cycle of the charge/discharge process (we denote the cycled samples as 1-cycle, 2-cycle and 5-cycle CoO/CNF). While the whole lattices are still visible, the fringes become curvy and loose compared with pristine CoO (Fig. 2b). Defects are created during the cycling process, as suggested by the blurred areas present in the zoomed-in TEM image. The average (111) spacing of 1-cycle CoO is ∼0.26 nm, slightly expanded from the pristine 0.24 nm. This lattice expansion and distortion in the first cycle lower the energy barrier for a small lattice domain to change orientation, preparing for the large particle to be further transformed into smaller particles in the following cycles. The TEM images of 2-cycle CoO/CNF show that the monocrystalline CoO particle is converted into interconnected crystalline NPs, with ultra-small sizes ∼2 nm (Fig. 2c). The FFT image with significantly more diffraction spot patterns than pristine CoO also suggest that many lattice orientations are present in this single CoO particle. The ultra-small NPs create boundaries, defects and dislocations, which are considered to be active sites of electrocatalysis29. Two neighbouring NPs merge together at the boundary without any visible gaps present, suggesting that they are strongly interconnected with each other that ensures good electrical and mechanical contact for efficient and stable catalysis. Similar structures are also observed in NiO, FeO and NiFeO x NPs (Supplementary Fig. 2). As indicated by the TEM images of 5-cycle CoO/CNF (Fig. 2d and Supplementary Fig. 3), further cycles do not significantly reduce the sizes of the interconnected NPs or even convert them into amorphous, suggesting that ultra-small NPs have reached the minimum domain sizes under the specific cycling condition. In areas away from the integrated particle, we observe that several ultra-small CoO crystals are detached, which indicates that more cycling number harms the integration of the whole particle and may also loosen the contacts between the interconnected NPs. X-ray diffraction spectroscopy of pristine CoO has three distinguished peaks, which however disappear in all of the battery-cycled samples (Supplementary Fig. 4), indicating that the sizes of the interconnected NPs are under the X-ray coherence length27. Raman spectra of pristine and 2-cycle CoO/CNF confirm that the phase of CoO is not changed after the battery cycling process (Supplementary Fig. 5)28.

Figure 2: TEM images and OER activities of battery-cycled CoO/CNF. (a) TEM image of pristine CoO/CNF. The lattice structure and the FFT pattern indicate the single-crystalline nature of the pristine particle. (b) With a blurred lattice orientation still visible, TEM image of 1-cycle CoO/CNF exhibits defects, lattice distortions and expanded (111) spacing. (c) TEM image of 2-cycle CoO/CNF shows the ultra-small, interconnected NPs. The sizes are ∼2–5 nm. (d) TEM image of 5-cycle CoO/CNF shows similar domain size to the 2-cycle one. The yellow dash line in the upper image represents the boundary of the whole particle. The zoom-in image indicates the detachment of the ultra-small NPs from the mother particle. Scale bars in a–d, upper, 5 nm; lower, 2 nm. (e) OER catalytic activities of CoO/CNF on CFP in 0.1 M KOH under different galvanostatic cycles. The polarization scan rate is 5 mV s−1. Two-cycle CoO/CNF gives the best performance. (f) The Tafel plots of OER polarization curves. (g) Electrochemical double layer capacitance of CoO/CNF under different cycles. The error bars include three identical samples tested for each cycle number. Full size image

To examine the electrochemical OER catalytic activities, pristine CoO/CNF were drop casted onto commercial carbon fibre paper (CFP) substrates, followed by 1, 2 and 5 galvanostatic cycles, respectively (Supplementary Fig. 6, see Methods). The as-prepared catalysts were tested in 0.1 M KOH solution. All of the potentials are referred to reversible hydrogen electrode (RHE) and have been iR corrected unless noted (see Methods). Pristine CoO/CNF shows a sluggish OER process with an onset potential around 1.59 V and a Tafel slope of 69.8 mV per decade (Fig. 2e). The activity of 1-cycle CoO/CNF is significantly improved, achieving a reduced onset potential to ∼1.55 V while exhibiting a slightly increased Tafel slope of 83.7 mV per decade. The increased surface area, atomic defects and distortions created during the first cycle in Fig. 2b are considered to contribute to the improved catalytic activity. The OER performance is continuously improved after two galvanostatic cycles, as additional surface areas and active sites are introduced by those ultra-small interconnected NPs (Fig. 2c,e). While the Tafel slope (73.6 mV per decade) of 2-cycle CoO/CNF is not changed much, the onset potential is further lowered to ∼1.51 V, significantly improving the OER activity which reaches 10 mA cm−2 anodic current at ∼1.57 V. Five-cycle CoO/CNF shows a degraded OER performance compared with the 2-cycle sample, consistent with the analysis of the TEM image (Fig. 2d) that some of the ultra-small NPs are detached from and lose electrical contact with the mother particle. The electrochemical double layer capacities of the catalysts, which represent the active surface areas, are obtained by applying cyclic voltammograms at a series of scanning rates (Fig. 2g and Supplementary Figs 7 and 8). The trend of the capacity versus the cycle number agrees well with that of the OER activity, where 2-cycle CoO/CNF exhibits the largest capacity. Therefore, we conclude that two galvanostatic cycles is an optimized condition for improving the catalytic performance of as-synthesized TMO NPs. While the conversion from monocrystalline particle to polycrystalline NPs helps to significantly increase the active sites and surface areas, whether those ultra-small crystalline NPs become amorphous under the OER conditions is worth to be further examined. The TEM image of 2-cycle CoO/CNF after OER catalysis is shown in Supplementary Fig. 9, in which the structures and sizes of interconnected crystalline NPs are well maintained and no sign of amorphization process is observed. No Li signal is observed in 2-cycle CoO/CNF by electron energy loss spectroscopy as shown in Supplementary Fig. 10, indicating that the concentration of residual Li is lower than the electron energy loss spectroscopy detection limit. In addition, the molar ratio of Li to Co in 2-cycle CoO/CNF is determined to be 1:23.4 by inductive coupled plasma mass spectroscopy, suggesting the negligible amount of residual Li after the cycling process. To shed light on how Li doping influences the catalytic activities, we doped CoO/CNF with Li by charging the electrode to 1 V versus Li+/Li (right above the conversion reaction plateau, Li to Co ratio was determined to be 1:7 by inductive coupled plasma mass spectroscopy). The OER performance shows a slightly decay compared with pristine CoO/CNF in Supplementary Fig. 11, indicating that Li doping does not contribute to the improvement in OER performance. Combined with the analysis of the great contributions from the increased surface areas as well as capacitances, we therefore conclude that the very small amount of residual Li does not play a role in improving the OER catalysis. We also rule out the possibility of background contributions by performing battery cycling on bare CNF in Supplementary Fig. 12.

TMO NPs synthesized on CFP for high-performance catalysis

To avoid the long-term stability and large current bubble-releasing issues of TMO NPs on CNF (due to the use of binder and the hydrophobic nature of carbon respectively), we directly synthesize TMO catalysts on CFP substrates including CoO/CFP, NiO/CFP, Fe 3 O 4 /CFP and the mixed oxide of NiFeO x /CFP (Supplementary Figs 13 to 16, see Methods). The mass loadings of the catalysts are ∼1.6 mg cm−2 and the Ir and Pt benchmarks are 0.5 mg cm−2 (Supplementary Fig. 17, see Methods). Galvanostatic cycling shows its general efficacy in improving all of the TMOs from their pristine counterparts, with significantly reduced onset potentials as well as overpotentials to achieve 20 mA cm−2 OER current (Fig. 3a–d and Supplementary Figs 18–20). It is interesting to notice that 2-cycle NiO/CFP shows a significantly increased NiO to NiOOH oxidation peak, again confirming the impressively increased surface areas and active sites, which suggests the potential applications of the galvanostatic cycling method in supercapacitors30. The best OER performance comes from 2-cycle NiFeO x /CFP (Fig. 3d and Supplementary Fig. 18). In 0.1 M KOH, 20 wt% Ir/C reaches 10 and 20 mA cm−2 at ∼1.53 and 1.58 V, respectively. As a comparison, the OER activity of 2-cycle NiFeO x /CFP outperforms this noble metal, with only 1.48 (η OER 10 mA =250 mV) and 1.50 V (η OER 20 mA =270 mV) to achieve the corresponding currents (Supplementary Figs. 18e,f). This highly efficient catalyst exhibits even better OER performance as pH increases to 14 (1 M KOH; Fig. 3e). To avoid the overlap of the NiO to NiOOH oxidation peak with the OER onset currents, we scanned the voltage from the positive to the negative direction (the inset of Fig. 3e) and determine the onset potential of 2-cycle NiFeO x /CFP in 1 M KOH to be ∼1.43 V (η OER onset =200 mV), nearly 40 mV lower than Ir/C. The OER current of 2-cycle NiFeO x /CFP then ramps up quickly to 200 mA cm−2 at only 1.51 V. This high OER activity benefits from the small Tafel slope of 31.5 mV per decade which does not show the curve bending as observed in pristine NiFeO x /CFP and Ir/C, suggesting the improved kinetic and bubble-releasing processes by galvanostatic cycling (Fig. 3f). To avoid the oxidation peak and therefore obtain a larger range of current for NiFeO x /CFP Tafel slope analysis, we reversely swept the I–V curve as shown in Supplementary Fig. 21 and calculated the Tafel slope to be 34.2 mV per decade, very close to the forward sweeping result. It is worth noting that the voltage sweeping rate in all of the tests is 5 mV s−1, which is slow enough to reach the steady state for accurate analysis of Tafel slopes (Supplementary Fig. 22). The reverse scanning method also helps to reveal an interesting conclusion that in more concentrated KOH solution the oxidation process can go deeper on the surface of the NiFeO x catalyst (Supplementary Fig. 23). Very small oxidation peaks of CoO and Fe 3 O 4 were also observed in Supplementary Fig. 24. Stability of the battery-cycled TMO is of our concern that whether these ultra-small interconnected NPs can tolerate the violent condition of gas evolution. An impressive OER stability of 2-cycle NiFeO x /CFP is shown in Fig. 3g, with 10 mA cm−2 anodic current at ∼1.46 V (η OER 10 mA =230 mV) for over 100 h without degradation. The high activity and long-term stability confirm the strong interactions between those ultra-small, interconnected NPs, outperform the OER catalysts reported so far, and consequently makes this material attractive for practical use in the future.

Figure 3: OER activities and stability of pristine and 2-cycle TMO/CFP catalysts. (a–d) The general efficacy of galvanostatic cycling in improving the OER activities of Co, Ni, Fe and NiFe oxides in 0.1 M KOH. Two-cycle NiFeO x /CFP exhibits better performance than the Ir/C benchmark. (e,f) The OER polarization curves and the corresponding Tafel plots of pristine and 2-cycle NiFeO x /CFP in 1 M KOH. The polarization scanned from positive potential to negative in the inset indicates the onset potential of 2-cycle NiFeO x /CFP at ∼1.43 V. The Tafel slope of 2-cycle NiFeO x /CFP is 31.5 mV per decade, better than the Ir/C benchmark. (g) Two-cycle NiFeO x /CFP exhibits an excellent OER stability, achieving 10 mA cm−2 anodic current at only 1.46 V versus RHE for over 100 h without degradation. This is better than the Ir/C benchmark. Full size image

Efficient HER catalysts in alkaline solutions such as transition metals and their alloys have been well investigated15,16,17, but the HER activities of TMOs are rarely developed22, which limits the study of high-performance bifunctional OER and HER catalysts for overall water splitting. The HER activity of 2-cycle NiFeO x /CFP as an efficient OER catalyst is also tested in 1 M KOH, which shows a small onset potential of −40 mV, significantly improved from its pristine counterpart with a large onset of −310 mV (Fig. 4a). The Tafel slope increases from 84.6 to 150 mV per decade after the battery cycling process, which may be related to a change of the reaction pathway or a mass transport limit (Supplementary Fig. 25)31,32. A small overpotential of −88 mV is required for 2-cycle NiFeO x /CFP to reach −10 mA cm−2 cathodic current, which is not far from the Pt benchmark of −23 mV. Together with the other half reaction of OER, the galvanostatic cycling method creates an attractive bifunctional NiFeO x /CFP water-splitting catalyst to compete with the combination of Pt and Ir benchmarks. The overall water-splitting polarization of 2-cycle NiFeO x /CFP bifunctional catalyst exhibits a slightly larger onset voltage than the benchmark combination, but quickly catches up with them due to the facile kinetic and bubble-releasing processes (Fig. 4b). In addition, the sizes of O 2 and H 2 bubbles observed on 2-cycle NiFeO x /CFP electrodes under 200 mA cm−2 are distinctively smaller than those on the benchmark electrodes, indicating the great capability for large current operations (Supplementary Movie 1). The long-term stability testing further illustrates the advantages of 2-cycle NiFeO x /CFP over those noble metals (Fig. 4c). With a slightly higher starting voltage to achieve 10 mA cm−2 of constant water-splitting current, 2-cycle NiFeO x /CFP exhibits a gradually increased catalytic activity and surpasses the benchmark combination after 1-h operation. Gas chromatography measurements of 2-cycle NiFeO x /CFP water electrolysis confirm a high faradic efficiency of O 2 and H 2 , calibrated by the benchmark electrodes (Supplementary Fig. 26). During the long-term stability testing, it is possible for the oxidation process (MO to MOOH) to get deeper at a very slow rate, gradually reaching to a limit. This may help to create additional active sites and refresh the boundaries of the interconnected particles, which slightly increases the activity. The gas evolution may also help to remove surface residues from the battery cycling, which contribute to the activation process observed22. The voltage stabilizes at ∼1.55 V (η overall 10 mA =320 mV) for 100-h continuous operation, in a sharp contrast to the benchmark combination. In addition, the water-splitting performance of our catalyst can be further improved simply by increasing the mass loading to 3 mg cm−2 (Fig. 4b,c, see Methods). The high-mass catalyst further brings the voltage down to 1.51 V (η overall 10 mA =280 mV) to achieve 10 mA cm−2 current, with remarkable stability of over 200 h with no sign of decay. Overall water splitting in neutral electrolyte is also tested in Supplementary Fig. 27, which however shows much lower activity compared with that in the alkaline solution.