Boosting the Oxygen Evolution Reaction Activity of NiFe2O4 Nanosheets by Phosphate Ion Functionalization.

Here, we demonstrate an effective strategy to constitutionally increase the conductivity and electrocatalytic property of NiFe2O4 by phosphate ion functionalization. The phosphate-ion-modified NiFe2O4 (P-NiFe2O4) nanosheets are readily grown on a carbon cloth by a simple hydrothermal method and followed by a phosphating process. The introduction of phosphate ions on the NiFe2O4 surface is highly beneficial for increasing the charge transport rate and electrocatalytic active sites. As a result, the as-prepared P-NiFe2O4 nanosheets show outstanding electrocatalytic activity toward oxygen evolution reaction (OER), with a low overpotential (231 mV at 10 mA/cm2) and Tafel slope (49 mV/dec). Furthermore, the P-NiFe2O4 electrode has a remarkable stability with no activity fading after 50 h. In addition, the as-fabricated water electrocatalysts exhibit excellent flexibility at the foldable state. These features make the phosphate-ion-functionalized NiFe2O4 electrodes open a new way to develop OER electrocatalysts with high electrochemical property.

Introduction

To address the global warming issue and fossil energy crisis, water electrolysis offers an attractive approach to generate hydrogen fuels as a green and renewable energy.[1−3] As one of the most important reactions, oxygen evolution reaction (OER) has attracted extensive research attention in recent years because of its important role in various hydrogen production and sunlight-powered water splitting processes.[4−6] The major bottleneck for OER is its essentially hysteretic reaction kinetics, and thus, a high-performance catalyst is used to increase the reaction rate.[7,8] Currently, RuO and IrO are the best OER electrocatalysts, but suffer from high cost and resource scarcity.[9,10] Alternatively, in this context, considerable studies have concentrated on nonprecious metal-based electrocatalysts for OER,[11−16] and some low-cost transition metal (Fe, Ni, Co, Mn, etc.) oxides were applied to the OER because of their high electroactivity and stability in alkaline solutions.[17−27] Particularly, because of the electron jump between different valence states of metals in O-sites and additional surface redox-active metal centers, spinel bimetallic oxides AB2O4 (A, B = metal) generally exhibited higher electrochemical activity than the single-metal oxides.[28−40] Of various spinels, ferrite NiFe2O4 is one of the most interesting bimetallic oxides for its attractive advantages, such as high abundance, low cost, environment friendly, and rich valence states.[12,28,30,41] For instance, the atomically thin NiFe2O4 quantum dots obtained by the template-induced in situ growth method showed the high OER performance with a low overpotential (262 mV) and Tafel slope (37 mV/dec).[42] Unfortunately, their constitutionally poor conductivity and large volume expansion during the electrocatalytic reaction process severely limit their electrocatalytic activity and durability. In addition, most of the as-fabricated NiFe2O4-based catalysts are powder, which are liable to aggregation during the reaction processes, resulting in inferior structure stability and low utilization of catalysts. In this regard, there is urgent need to design and prepare new NiFe2O4 nanoarchitectures with abundant active sites and high electrical conductivity for OER.

With all the above considerations, here, we report an efficient phosphate ion modulation strategy to boost the catalytic activity of NiFe2O4 for OER. The phosphate-ion-functionalized NiFe2O4 nanosheets (P-NiFe2O4) were facilely prepared on a flexible carbon cloth (CC) through a hydrothermal approach combined with a phosphating process. Such a flexible conductive structure also provides favorable charge-transfer pathways and mechanical properties, which allow the catalysts to water splitting in a complex environment. The functionalization of NiFe2O4 with phosphate ions could significantly promote the charge-transfer kinetics and make close contact between surface-active sites and reaction medium, enabling that the P-NiFe2O4 obtained excellent electrocatalytic performance for water splitting. Additionally, the P-NiFe2O4 exhibited superior OER catalytic activity with low overpotentials of 231 mV for a current density of 10 mA/cm2 and maintained its catalytic activity for 50 h even at the bending state. The flexible foldable design of the P-NiFe2O4 electrodes has wide applications to OER electrocatalysts in basic media, which fulfills the potential future direction of environment-friendly energy storage and conversion devices.

Results and Discussion

The flexible P-NiFe2O4 electrode was obtained with a two-step process. Typically, free-standing NiFe2O4 nanosheets were grown on CC with a stoichiometric ratio of Ni/Fe = 1:2 through a facile hydrothermal approach and annealed at 500 °C in the air atmosphere for 1 h. The characteristic peaks in the X-ray diffraction (XRD) patterns (Figure a) display the dominant phases of NiFe2O4 (JCPDS card no. 03-0875).[13] The scanning electron microscopy (SEM) images in Figure S1 show that the surface of CC is uniformly covered with a large number of NiFe2O4 nanosheets, with a thickness of about 20 nm. The high-resolution transmission electron microscopy (HRTEM) image (Figure S2a) reveals three distinct lattice fringes of 0.24, 0.25, and 0.48 nm, corresponding to the (222), (311), and (111) planes of NiFe2O4 (JCPDS card no. 03-0875). The selected area electron diffraction (SAED) pattern image (Figure S2b) further confirms the polycrystalline nature of NiFe2O4. It can be seen from the energy-dispersive X-ray spectroscopy (EDXS) analysis that only Fe, Ni, and O are detected from these nanosheets (Figure S2c), indicating that the sample is a highly pure NiFe2O4 phase.

Figure 1

(a) XRD patterns of NiFe2O4 and P-NiFe2O4. (b,c) SEM image of NiFe2O4; (d) TEM, (e) HRTEM, (f) magnified HRTEM image obtained from e1, (g) SAED pattern by the circle in e2, and (h–l) element mapping images of P-NiFe2O4.

To introduce phosphate ions, NiFe2O4 was annealed at 500 °C for 1 h by placing NaH2PO2·H2O in front in a further nitrogen atmosphere. Interestingly, observed from Figure a, the diffraction peak of XRD became weakened, indicating that the crystallinity was significantly reduced. SEM images of P-NiFe2O4 (Figure b,c) reveal that these nanosheets had apparently collapsed upon phosphating process. However, these P-NiFe2O4 nanosheets are still uniformly covered on the surface of CC with a thickness of about 500 nm, which is thicker than that of the original NiFe2O4. In order to test the specific surface area of NiFe2O4 and P-NiFe2O4 nanosheets, we performed the N2 sorption experiments at 77 K by Brunauer–Emmett–Teller (BET; Figure S3). The specific surface area of the P-NiFe2O4 nanosheets is about 8.41 m2/g, lower than that of the NiFe2O4 nanosheets (14.48 m2/g). The transmission electron microscopy (TEM) images of the P-NiFe2O4 (Figure d) demonstrate that each nanosheet consists of numerous different areas with different contrasts. The obvious contrast variation in the HRTEM image of Figure e clearly shows that numerous lattice fringes are present on the nanosheet. Correspondingly, Figure f shows the lattice fringes of 0.25 nm, corresponding to the (311) planes of NiFe2O4. The SAED pattern in Figure g shows that the corresponding area (marked by circle in e2) has an amorphous structure. Furthermore, as shown in Figure h–l, the scanning TEM–EDXS element mappings indicates that the Fe, Ni, P, and O elements are uniformly distributed on the nanosheet. No other impurity was detected for these nanosheets by the EDXS analysis (Figure S4).

In order to study the chemical composition and binding modes of these samples, we further studied Raman and X-ray photoelectron spectroscopy (XPS) of both NiFe2O4 and P-NiFe2O4. As shown in Figure a, the Raman characteristic peaks at 493 and 570 cm–1 belong to T2g mode and the bands at 340 and 703 cm–1 are assigned to Eg and A1g modes, respectively, which reveal the presence of NiFe2O4.[43] No peaks belonging to nickel–iron phosphates or nickel iron hydroxide were observed in Raman spectra, indicating that there was no phase change after phosphate ion modification. The Fe 2p spectra exhibit the two peaks at 711.7 and 724.9 eV corresponding to Fe3+ 2p3/2 and Fe3+ 2p1/2 (Figure b), confirming the main valence state of Fe3+.[44] Furthermore, the Ni 3p core level XPS spectra are displayed in Figure S5. NiFe2O4 exhibits two peaks of Ni located at 855.7 and 873.5 eV, which could be ascribed to Ni 2p3/2 and Ni 2p1/2, respectively, indicating the combined state of Ni2+ in NiFe2O4.[45] As for the P-NiFe2O4 sample, the peaks identified as Fe3+ greatly weaken while the intensity of Fe2+ peaks increase, suggesting that Fe3+ species are reduced to Fe2+ after the surface phosphating process. The lower-valence-state Fe characteristics are favorable for high hardness, fast electron transport, and exposure of more active sites, which are conducive to improvement of catalytic performance. Simultaneously, the P-NiFe2O4 still retains the previous valence state of Ni element (+2) and exhibits two Ni 2p peaks at 856.1 (2p3/2) and 873.9 (2p1/2) eV. Moreover, XPS survey spectra (Figure S5) indicate that NiFe2O4 contains Ni, Fe, O, and C elements without other impurities. In the high-resolution XPS of P 2p spectra, the P-NiFe2O4 exhibits two characteristic peaks of P 2p centered at 133.5 and 134.5 eV, which could be ascribed to (H2PO4–) and (PO3–) (Figure c). The P 2p core level XPS spectrum of the P-NiFe2O4 nanosheets further indicate the existence of (H2PO4–) and (PO3–) species on the P-NiFe2O4 surface.[46]Figure d shows the O 1s XPS spectra of NiFe2O4 and P-NiFe2O4. The NiFe2O4 exhibit one strong peak centered at 530.1 eV corresponding to the oxygen species (−O−) in NiFe2O4 and a shoulder peak located at 532.1 eV corresponding to hydroxyl groups (−OH) on the NiFe2O4 surface. Meanwhile, the P-NiFe2O4 sample peaks can be well convoluted into three parts, centered on 530.5, 531.7, and 533.1 eV, which can be attributed to the oxygen species of (−O−), (H2PO4–), and (PO3–).[47,48] Markedly, the (−OH) species on the NiFe2O4 surface have been replaced by (PO3–) and (H2PO4–) during the surface-functionalized process.

Figure 2

(a) Raman spectra and XPS spectra, (b) Fe2p, (c) P 2p, and (d) O 1s peaks of NiFe2O4 and P-NiFe2O4.

Figure 3

(a) LSV curves, (b) Tafel plots, and (c) Nyquist plots of CC, NiFe2O4, P-NiFe2O4, and IrO2/C. (d) Current density as a function of scan rate for NiFe2O4 and P-NiFe2O4.

The activity of P-NiFe2O4 for electrochemical water oxidation was assessed in KOH (1.0 M) aqueous solution. Figure a shows the plots of current density versus potential for the P-NiFe2O4 electrode, along with NiFe2O4, CC and commercial IrO2 coated on CC (IrO2 loading mass ≈ 0.2 mg/cm2) for comparison. The overpotential of P-NiFe2O4 was 231 mV at 10 mA/cm2, much lower overpotential than IrO2 (344 mV), CC (498 mV), and NiFe2O4 (300 mV). Furthermore, the onset potential of the P-NiFe2O4 electrode (∼1.43 V) is also lower than that of the traditional noble metal base electrocatalyst for OER (IrO2, ∼1.51 V). Figure b collects the corresponding Tafel plots. Likewise, the lowest Tafel slope is observed for P-NiFe2O4 (49 mV/dec), whereas the Tafel slopes for the CC, IrO2, and NiFe2O4 electrodes are 210, 111, and 130 mV/dec, respectively. All these results confirm that the P-NiFe2O4 electrode reveals the highest OER activity. More importantly, our P-NiFe2O4 electrode also presents comparative catalytic performance to other recently reported noble-metal-free OER catalysts (Table S1), such as FeNi/NiFe2O4@NC (316 mV),[49] NiFe2O-NPs (286 mV),[50] NiO/NiFe2O4 (302 mV),[13] Ni3FeN-NPs (241 mV),[51] and CoFe2O4/C (240 mV).[52] The Tafel slope of P-NiFe2O4 nanosheets is only ≈49 mV/dec, which is the smallest compared with other electrodes, and is lower than those of most electrocatalysts shown in Table S1, indicating the favorable electrocatalytic reaction kinetics of the P-NiFe2O4 nanosheets. Turnover frequencies (TOFs) of catalysts were measured for comparison of intrinsic activity. As revealed, the TOF of the P-NiFe2O4 electrode is 0.25 × 10–2 s–1 (overpotential = 300 mV), which is the excellent value reported in previous catalysts.[11,28,53]

Further information on electrochemical performance was obtained by testing electrochemical impedance spectroscopy (Figure c). The charge-transfer resistance (Rct) of the P-NiFe2O4 electrode (0.37 Ω) is smaller than that of NiFe2O4 (17.5 Ω) and IrO2 (2.8 Ω), indicating its fast electron transfer of the electrocatalytic reaction process. The significant reduction of Rct for the P-NiFe2O4 electrode means that it has faster OER kinetics, which is considered to be the main reason for its high OER performance. Electrochemical active surface area (ECSA) is further confirmed through capacitance measurements, and the calculated capacitance of NiFe2O4 and P-NiFe2O4 is 2.5 and 26.5 mF/cm2 (Figures d and S6). The roughness factors of the two samples are approximately 0.5 × 103 and 5.3 × 103, respectively. It is clear that the active surface area of P-NiFe2O4 electrode displays more than 10 times larger of the NiFe2O4 electrode. It is noted that the specific surface area of NiFe2O4 after functionalization was reduced, but its ECSA significantly increased, revealing that the introduction of surface phosphate ions can serve as active sites. As a consequence, the prominent electrocatalytic activity of the P-NiFe2O4 electrode can be ascribed to its faster reaction dynamics, improved electrical conductivity, and more active sites upon phosphate ion functionalization.

Besides astringent requirement of high electrochemical activity, long-term stability is another important consideration in the selection of electrocatalysts. As shown in Figure a, the stability was tested by chronopotentiometry method for 50 h. For the P-NiFe2O4 electrode, in order to achieve the current density of 10 mA/cm2, a overpotential of ∼230 mV needs to be used, which is substantially lower than that of the NiFe2O4 (∼330 mV) and CC (>500 mV) electrodes. In addition, the P-NiFe2O4 electrode shows excellent durability in alkaline solution with an almost constant operating potential for 50 h. In addition, the long-term OER stability of the P-NiFe2O4 electrode was also evaluated by polarization tests at 20, 50, and 100 mA/cm2 for 50 h (Figure S7), and the results reveal that the overpotentials are still unchanged, again instruction the high durableness of this P-NiFe2O4 electrode. The polarization curve of the P-NiFe2O4 electrode after durability test also shows inappreciable differences when compared to the initial data (Figures b and S8). Electrolysis at the fixed current densities was performed sequentially for over 50 h to further highlight the durability of P-NiFe2O4. At the current densities of 10, 20, 50, and 100 mA/cm2 and recovering 10 mA/cm2, the overpotentials remained stable over each 10 h period at around 235, 280, 303, 350, and 234 mV, respectively (Figure c). After 50 h of long-stability test, the linear sweep voltammetry (LSV) curves were collected as shown in Figure S9. The LSV curves of the P-NiFe2O4 electrode can be seen from Figure c, the before and after 50 h of electrolysis nearly overlapping.

Figure 4

(a) Chronopotentiometric (CP) measurements for long-durability tests of the CC, NiFe2O4, and P-NiFe2O4 electrodes. (b) Polarization curves of the P-NiFe2O4 electrode after 50 h at 10 mA/cm2. (c) CP measurements for long-stability tests of the P-NiFe2O4 electrode at various current densities.

We also examined the viability of the as-fabricated P-NiFe2O4 electrode under complex situations. As shown in Figure a,b, the LSV curve of the P-NiFe2O4 electrode was detected under different bending conditions. The good electrocatalytic performance of P-NiFe2O4 was well maintained under different deformation states, implying its good flexibility. Thus, our P-NiFe2O4 holds great promise for the application in state-of-the-art flexible electronics and can adapt to the complex environment of water splitting.

Figure 5

(a) Polarization curves of the P-NiFe2O4 electrode at different distorted states: (b) normal, bending, recovering normal, and bifurcating.

Conclusions

In conclusion, we have demonstrated that the use of phosphate-ion-functionalized NiFe2O4 nanosheets can remarkably boost its OER performance. Because of enhanced conductivity, quick reaction dynamics, and increased active sites after phosphate ion functionalization, the P-NiFe2O4 electrocatalyst yielded high catalytic activity (overpotential: 231 mV (10 mA/cm2); Tafel slope: 49 mV/dec) and excellent long-term durability toward OER (no activity fading at 10 mA/cm2 for 50 h). Moreover, the stability of P-NiFe2O4 was as well remarkable at high current densities of 50 and 100 mA/cm2 for 50 h. Furthermore, the as-fabricated P-NiFe2O4 electrocatalyst exhibited outstanding flexibility at the foldable state. Given the high electrocatalytic property of the f P-NiFe2O4 electrode, the current encouraging discovery might open up a new way for low-cost and environment-friendly electrocatalysts of water splitting.

Experimental Section

Synthesis of P-NiFe2O4 Nanosheets on CC

The NiFe layered double hydroxides were synthesized on CC by a hydrothermal method. First, the CC (3.0 cm × 2.0 cm) were immersed into an ethanol solution (50 mL), containing the (Fe(NO3)3·9H2O) (1.6 g) and Ni(NO3)2·6H2O (0.6 g), for 5 min and further heated on a hotplate at 350 °C for another 3 min. Then, the seeded CCs and the resultant precursor solution (20 mL), containing NiSO4·6H2O (0.1314 g), FeSO4·7H2O (0.2780 g), urea (0.0706 g), and Na3C6H5O7·2H2O (0.0196 g), were moved to a Teflon-lined stainless steel autoclave. Subsequently, the sealed autoclave was heated to 150 °C for 12 h. The samples were washed with deionized water several times and dried. Then, the NiFe2O4 nanosheets were obtained through annealing in air at 500 °C for 1 h. The substrates grown with NiFe2O4 nanosheets were placed in the center of a quartz tube in a chemical vapor deposition furnace equipped with gas flow controllers. The NiFe2O4 nanosheets were placed on the alumina boats containing NaH2PO2·H2O (0.8 g). The P-NiFe2O4 nanosheets were obtained through annealing CC in a nitrogen atmosphere at 500 °C for 1 h.

Material Characterizations and Electrochemical Measurements

The sample compositions were measured with XRD (Rigaku) and Raman spectroscopy (Renishaw). The surface morphologies of the materials were studied by SEM (JSM-6330F) and TEM (FEI Tecnai G2 F30). The valence states of elements of the samples were measured by XPS (VG ESCALAB 250). The specific surface area was tested by BET (ASAP 2020M). For the BET measurement, CC substrates covered with NiFe2O4 and P-NiFe2O4 are directly used for test.

The electrochemical measurements were carried out an electrochemistry workstation (CHI 760D). The electrocatalytic performance of P-NiFe2O4 electrocatalysts was studied in a three-electrode system in KOH (1.0 M) solution. A Ag/AgCl (3 M KCl) electrode was used as the reference electrode and a carbon rod was employed as the counter electrode. The overpotential values used in this study were calculated according to the following equation: overpotential (V) = measured value (vs Ag/AgCl) + 0.197 + 0.0591pH – 1.229 [vs reversible hydrogen electrode (RHE)]. For comparisons, the electrocatalytic activities of NiFe2O4 and CC samples were also measured under the similar conditions. The capacitive current was used to determine the ECSAs of the P-NiFe2O4 with a scan window of 1.03–1.09 V versus RHE. On the basis of the LSV, the TOF [j(mA/cm/(4 × F(96 485C/mol) × m(mg/cm/Mmolecular weight)] of the P-NiFe2O4 catalyst can be calculated.