Characterization. The XRD patterns of [email protected], [email protected], and [email protected] are shown in Fig. 1a. [email protected] and [email protected] showed well-defined XRD patterns which can be indexed to the fcc structure of Ni (PDF# 98-005-3809) 11,15. However, no peaks corresponding to Fe were observed in the XRD pattern of [email protected], indicating the substitution of Fe with Ni having the fcc structure. The existence of Fe in [email protected] was confirmed by XPS as shown in Fig. 1b. The XRD pattern of [email protected] exhibited peaks both for both cubic NiP2 (PDF#98-002-2221) and hexagonal Ni5P4 (PDF#98-010-8462). Additionally, the peaks at 46.20°, 48.23°, and 58.96°, ascribed to orthorhombic FeP, were also observed in the XRD pattern of [email protected] 49. These results indicated that the [email protected] catalyst was successfully phosphodized and consisted of NiP2, Ni5P4, and FeP.
The surface elemental composition and valence state of the [email protected] and [email protected] catalysts were analyzed by XPS. Detailed scans of the Ni2p, Fe2p, and P2p regions are shown in Fig. 1b − 1f. For [email protected], the Ni2p high-resolution spectra displayed peaks at binding energies of 856.4 and 874.3 eV, which were assigned to the Ni2p3/2 and Ni2p1/2 states, respectively, indicating the existence of Ni2+ and Ni3+ ions and thus partial oxidation of Ni at the surface 50. The satellite peaks corresponding to Ni 2p3/2 and 2p1/2 spin-orbit couplings were observed at 861.7 and 880.15 eV, respectively 51. Additionally, the Fe2p peaks of [email protected] (Fig. 1c) located at 712.1 and 725.4 eV were ascribed to Fe 2p3/2 and 2p1/2, respectively, indicating the successful incorporation of Fe species into the Ni structure, considering the nonexistence of a crystalline structure including Fe (Fig. 1a). In the case of [email protected], peaks were observed at 852.78 and 870.16 eV in the Ni2p (Fig. 1d) region, where these values are close to the binding energy of Niδ+ in Ni5P4 and NiP2, respectively; the result, therefore, indicated increased metallicity of [email protected] as compared to [email protected] Additionally, the peaks at 855.82 and 874.62 eV are due to Ni-POx 33,37, while the other peak at 861.13 eV is a satellite peak37. The deconvoluted Fe2p spectrum (Fig. 1e) revealed Fe2p3/2 peaks at 706.43 and 711.80 eV, derived from Fe-P and Fe-O (corresponding to Fe-P-Ox)35,37. Moreover, the peaks at 719.28 and 728.40 eV were assigned to the Fe2p1/2 state of Fe-P and Fe-O, respectively 37. The P2p spectrum displayed two peaks at binding energies of 128.9 and 129.5 eV (Fig. 1f), corresponding to the 2p3/2 and 2p1/2 states, respectively, suggesting the existence of a strong bond between Pδ− and the metal35,37. The peak at 133.4 eV corresponds to the PO43− or P2O5 species originating from the oxidation of phosphorus upon exposure to air35. Elemental analysis based on XPS results shows the ratio of Ni:Fe:P (15:7:78). The results suggest that the bonds between Ni and Fe were changed to metal-P bonds by phosphorization, forming heterogeneous metal-P mixtures (i.e., NixPy and FeP). It is obviously reported that phosphide center of the metal phosphide becomes partially negative, making surface of metal positive51. Positive metal surface can more easily attract hydroxide ion and make UOR earlier, reducing its overpotential and increasing catalytic performance.
The SEM micrographs of the [email protected], [email protected], and [email protected] samples are shown in Fig. 2. The pristine CF comprised carbon fibers with a diameter of ~ 16 µm, forming an open network 3D structure. As seen in Fig. 2b and 2c, the individual fibers were completely coated with a thin layer of NixPy and/or FeP as evidenced by EDX (Suppl. Fig. S1 and S2) and XPS analysis (Fig. 1). The high-resolution image (inset of Fig. 2a − d) revealed a rough surface with granular metal deposits that coalesced to form a continuous thin film. After phosphidation at 550°C, 3D flower-like P-NiFe architecture on CF was formed as shown in the inset of Fig. 2d. The structure of P-NiFe coated on CF was further characterized by HRTEM, as shown in Fig. 2(e − h). The P-NiFe structure comprised Ni5P4, NiP2, and FeP phases, which were identified by their lattice parameters as shown in Fig. 2(f − h), in line with the XRD analysis. This result suggests intimate contact and strong interactions between the Ni5P4, NiP2, and FeP species in the hybrid structure. Additionally, uniform distribution of Ni, Fe, and P in the P-NiF nanoparticles was observed by TEM elemental EDX mapping (Suppl. Fig S3).
Electrochemical Properties. The electrochemical performances of [email protected], [email protected], and [email protected] catalyst electrodes in the UOR was analyzed by LSV using 1 M KOH and 0.33 M urea at a scan rate of 5 mV s− 1, as depicted in Fig. 3. For comparison, bare carbon cloth (CC) and Pt on carbon cloth ([email protected]) were also analyzed. The oxidation peak at 1.51 V in the LSV plot in 1 M KOH (Fig. 3a) was ascribed to the formation of active NiOOH sites for water oxidation 49. As shown in Fig. 3a, the electrode potential of [email protected] for the UOR decreased considerably to 1.44 V to attain a current density of 200 mA cm− 2 compared to 1.64 V for the OER (i.e., water oxidation), which indicates the oxidation current increased considerably in the presence of urea, thus indicating that H2 production by urea electrolysis was more energy efficient than water electrolysis. Figure 3a also shows the activities of the different catalysts for the UOR. Clearly, [email protected] required the lowest potential to attain a given current density for H2 production, indicating its superior UOR activity. Importantly, the electrochemical activity of [email protected] for the UOR exceeds that reported in the literature (Table S1). Additionally, the Tafel slope of [email protected] was 107.2 mV dec− 1 which is much lower than those of [email protected] and [email protected], as shown in Fig. 3b, further indicating faster kinetics of the UOR on P- [email protected] [52, 53].
EIS measurements were conducted in a 1 M KOH solution to investigate the charge transfer rate (Rct) and double layer capacitance (Cdl), as shown in Fig. 3c. The EIS spectra were fitted with an equivalent circuit, as shown in the inset of Suppl. Fig. S4. The [email protected] electrode exhibited a significantly reduced Rct compared to [email protected] and [email protected], indicating considerably enhanced charge transfer kinetics of the UOR on the [email protected] catalyst 52. The smaller Rct value of [email protected] was possibly due to the improved conductivity of metallic bonds such as Ni5P4, NiP2, and FeP, as the main factor [54, 55]. Furthermore, [email protected] featured the highest Cdl value, which was determined from the constant phase element value of the equivalent circuit (Suppl. Fig. S4), suggesting that [email protected] the highest electrochemically active surface area of the [email protected] 53. This might be due to the higher valence state of Ni at the surface of [email protected], as evidenced by the XPS analysis. Intimate contacts among the different crystal phases (Ni5P4, NiP2, and FeP) might also affect the electronic structure, making it more favorable for the UOR as previously reported based on empirical and computational approaches 54,55. Additionally, the electrochemical stability of [email protected] was higher than that of [email protected] and [email protected] (Suppl. Fig. S4), plausibly owing to the formation of the metallic phosphide-rich surface of [email protected], which could resist structural collapse during the Ni2+ to Ni3+ transition 56.
The HER performance of [email protected] was also analyzed by LSV in KOH with and without urea to examine its bifunctional catalytic activity for the UOR and HER. For comparative purposes, the HER activity of bare CF, [email protected], [email protected], and [email protected] was also studied for their HER performances, where the activity was indicated by the potential of the catalysts to achieve a given current density 2,4,57. As evident from Fig. 4a, bare CF was inactive for the HER, with a negligible current density. The potential required for the [email protected] electrode to attain 10 mA cm− 2 was lowered from 0.166 to 0.142 V by Fe doping, and further considerably lowered to 65 mV by phospidation; however, it is still high than that (25 mV) of commercial [email protected] Additionally, [email protected] catalyst electrode had the highest stability among the evaluated catalysts (Suppl. Fig S5). Interestingly, upon adding 0.33 M urea, a negative shift of only 10 mV was observed at a current density of 100 mA cm− 2 (Fig. 4b), indicating that urea had little impact on the electrocatalytic activity for the HER. Figure 4c illustrates the Tafel plots of [email protected], [email protected], [email protected], and [email protected] for the HER; demonstrating that [email protected] had a considerably lower Tafel slope of 41.4 mV dec− 1 than [email protected] and [email protected], which is comparable with that (34.2 mV dec− 1) of [email protected]
Two electrode urea electrolysis cells were constructed using the bifunctional catalyst electrodes as both the anode and cathode. As seen in Fig. 5, the urea electrolyzer with [email protected] electrodes featured a current density of 10 mA cm− 2 at a cell voltage of 1.42 V, which is lower than that (1.61 V) of the cell with [email protected] Furthermore, a current of 100 mA cm− 2 in the urea electrolyzer with [email protected] electrodes was achieved at a low cell voltage of 1.77 V, which exceeds those reported for Ni3N/NF 58, Ni3N NA/CC 59, NF/CoPx 60, Ni2P/CC 52, MoS2/Ni3S2 61, Fe11.1%-Ni3S2/NF 26, and Mo-NiP2 62, as summarized in Suppl. Table S2. The [email protected] urea electrolyzer also exhibited good long-term electrochemical stability, as the current density remained stable for 8 h of operation at an applied voltage of 1.70 V after the initial drop due to concentration polarization  (Suppl. Fig. S6).