PMC

(a–e) TMO particles gradually change from single crystalline to ultra-small interconnected crystalline NPs. Long-term battery cycling may result in the break-up of the particle. (f) The galvanostatic cycling profile of CoO/CNF galvanostatic cycling.

(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.

(a) The HER activity of 2-cycle NiFeO_x/CFP is significantly improved from its pristine counterpart and close to the Pt/C benchmark. (b) Two-cycle NiFeO_x/CFP as HER and OER bifunctional catalyst in 1 M KOH for overall water splitting. Ir/C and Pt/C as OER and HER benchmarks are tested side by side. With the mass loading increased from 1.6 to 3 mg cm⁻² (the dash line), the water-splitting activity of 2-cycle NiFeO_x/CFP outperforms the benchmark combination. (c) Long-term stability of 2-cycle NiFeO_x/CFP bifunctional catalyst. The voltage to achieve 10 mA cm⁻² electrolysis current shows an activation process, followed by a stable 1.55 V for 100-h continuous operation. As a sharp contrast, Ir and Pt combination shows an efficient starting voltage but followed by a fast decay. By increasing the mass loading to 3 mg cm⁻², 2-cycle NiFeO_x/CFP further lowers the voltage to 1.51 V to achieve 10 mA cm⁻² current for over 200 h without decay.

(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.