Regulating the active species of Ni(OH)2 using CeO2: 3D CeO2/Ni(OH)2/carbon foam as an efficient electrode for the oxygen evolution reaction

We found that Ni(OH)2 nanosheets of Ni(OH)2/NOSCF decorated with ∼3.3 nm CeO2 NPs displayed enhanced OER performance.


Introduction
Clearly, using electricity to split water into hydrogen and oxygen (2H 2 O / 2H 2 + O 2 ) is one of the most efficient and attractive methods for the production of renewable energy. 1,2 However, large-scale water electrolysis is greatly hindered due to the huge overpotential and signicant efficiency loss for the half-cell of the oxygen evolution reaction (OER). [3][4][5] Although noble-metal based materials (e.g. Ir, Pt) are currently regarded as highefficiency OER catalysts, their low earth abundance and high cost limit their widespread use. [6][7][8] Therefore, in recent years, various efficient and low-cost OER electrocatalysts (such as, Fe, Co, Ni and Mn) with high OER performance (low onset potential, high activity and good stability) in basic electrolytes have been extensively designed and investigated. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] Among them, nickel(II) hydroxide (Ni(OH) 2 )-based materials are attractive electrocatalysts for the OER because of their intrinsic potential for high OER performance and two-dimensional (2D) layered structure. 20 Another key reason for the extensive study of Ni(OH) 2 is that the high oxidation state of Ni III/IV can serve as an active species for OER catalysts. 24 For example, Ye et al. prepared a Ni(OH) 2 -Au hybrid as an OER catalyst, and a significantly enhanced OER performance was exhibited by enhancing the generation of the Ni III/IV active species. However, the poor kinetics and mass-transferability of Ni(OH) 2 as an electrocatalyst for the OER still limit its further development for practical applications.
Cerium(IV) oxide (CeO 2 ) is one of the most important rare earth oxides, is stable in alkaline solution, converts easily between the Ce 3+ and Ce 4+ oxidation states, undergoes reversible oxygen ion exchange (1/2O 2 (gas) + 2e À (solid) 4 O 2À (solid)), and has good ionic conductivity and high oxygenstorage capacity (OSC). [25][26][27] The above unique properties enable CeO 2 to serve as a cocatalyst to enhance the performance of OER catalysts by improving charge transfer and energy conversion efficiency, which can also solve the poor kinetics and mass-transferability problems of Ni(OH) 2 for the OER. However, few studies have focused on the application of CeO 2 nanocrystals in the electrocatalytic eld. Recently, Li et al. developed an efficient OER electrocatalyst by supporting FeOOH/CeO 2 on Ni foam and exhibited enhanced OER performance compared with pure FeOOH. 28 They also demonstrated the unique high OSC properties of CeO 2 , such that CeO 2 can straightway absorb the oxygen produced during the OER and accordingly promote the OER. Therefore, the combination of Ni(OH) 2 and CeO 2 to form a CeO 2 /Ni(OH) 2 hybrid will be an efficient route to improve the electrocatalytic performance of Ni(OH) 2 via improving the energy conversion efficiency, and thereby promoting the generation of active species of Ni III/IV for enhancing the OER performance.
In order to greatly prevent the Ni(OH) 2 nanosheets from aggregation and thus further enhance the OER performance, three dimensional (3D) free-standing carbon foam (CF) is chosen as the substrate for in situ growth of the Ni(OH) 2 nanosheets. The advantages of applying such 3D CF as a substrate can be attributed to the interconnected frameworks with large surface area for effective contact with an aqueous electrolyte and rapid interfacial electron charge transfer. Moreover, the obtained CF is doped by N, O and S elements during carbonization without other extra chemicals being added, where the N, O and S elements come from the melamine resin and sodium bisulte additive of melamine foam (MF) (Fig. S1, ESI †). And N, O and S doped carbon materials are believed to enhance the OER activity. 29 Herein, as we expect, Ni(OH) 2 nanosheets are successfully grown along the frameworks of N, O and S doped CF (NOSCF) and prevent the undesirable aggregation of Ni(OH) 2 nanosheets because of the open cell pores of NOSCF. Then, we prepared uniform CeO 2 NPs of $3.3 nm in size via a one step colloidal synthesis method, and deposited the surface modied-CeO 2 NPs on Ni(OH) 2 nanosheets of the as-designed Ni(OH) 2 /NOSCF to form a self-supported CeO 2 /Ni(OH) 2 /NOSCF electrode, as shown in Fig. 1. As a result of the open cell structure of 3D NOSCF for facile electrolyte transport and strong electronic interactions between CeO 2 NPs and Ni(OH) 2 nanosheets for accelerating the oxidation of Ni II to Ni III/IV , the CeO 2 /Ni(OH) 2 / NOSCF electrocatalyst delivers an excellent water oxidation performance at a lower onset potential, ranking high among the extensive non-noble electrocatalysts studied for the OER. As we know, this is the rst time CeO 2 is combined with a functional Ni(OH) 2 electrocatalyst, which offers an impressive OER performance, and provides insight into the possibility of enhancing OER catalysis by using rare earth CeO 2 -based nanomaterials.

Results and discussion
Design of the CeO 2 /Ni(OH) 2 /NOSCF electrocatalyst N, O and S doped CF was prepared by direct carbonization of melamine foam (MF) in a tube furnace at 700 C for 1 h under protection of a nitrogen atmosphere. Aer carbonization, the volume of CF shrunk to about 70% of MF (inset of Fig. 2a). Then, the NOSCF was further oxidized by dipping into a mixture of acids of HNO 3 (65%) and H 2 SO 4 (98%) with a volume ratio of 1 : 3 for 6 h, which can produce more C]O groups (Fig. S2a, ESI †). 29 It has been revealed that the C atoms in the C]O groups have a stronger electrostatic interaction with OH À ions than other oxygen-containing groups, which can facilitate the formation of crucial M-OOH intermediates (eqn (1)), and thus accelerate the OER reaction. 30,31 The elemental content in the NOSCF was measured by XPS analysis to be about 66.8, 4.3, 24.5 and 0.42 atom% for C, N, O and S, respectively (Fig. S2b, ESI †). Scanning electron microscopy (SEM) images in the inset of Fig. 2a revealed that the asprepared NOSCF possessed an interconnected network architecture, which could make it an ideal substrate for the growth of some electrocatalysts. Then, by using the framework of NOSCF as a nucleation platform, Ni(OH) 2 nanosheets could be uniformly grown in situ along the framework of NOSCF by a simple chemical bath deposition process, 32 which could be observed evidently from the SEM images ( Fig. 2b and c). The selective growth of Ni(OH) 2 nanosheets on the NOSCF could preserve the open-cell structure of the NOSCF (Fig. 2b) and efficiently prevent the aggregation of Ni(OH) 2 nanosheets, indicating that it held a large surface area for electrocatalysis. The vertical Ni(OH) 2 layers could be clearly observed in an enlarged SEM image of the Ni(OH) 2 /carbon foam hybrid (Fig. S3, ESI †). Such nanostructured materials can offer a much rougher surface, which reduces the solid-gas interaction, giving rise to a timely release of adhered gas bubbles and thus enhancing the OER performance.
The grown Ni(OH) 2 nanosheets had a hexagonal phase (a ¼ b ¼ 0.308 nm, c ¼ 0.234, JCPDS: 38-0715), as conrmed by powder X-ray diffraction (PXRD) analysis (Fig. 2d). As shown in the transmission electron microscopy (TEM) image in Fig. 2e, the grown Ni(OH) 2 presented a typical layered structure, and the Fig. 1 Process for the design of a self-supported CeO 2 /Ni(OH) 2 /NOSCF electrode and application for the oxygen evolution reaction.
high resolution (HRTEM) image (inset of Fig. 2e) identied the (101) plane of a hexagonal crystal structure for the Ni(OH) 2 nanosheets with an interplanar spacing of 0.23 nm. The corresponding elemental mapping of the designed Ni(OH) 2 /NOSCF is shown in Fig. 2f; the C, N and S elements were distributed on the whole surface of the frameworks in the NOSCF, and also displayed a very uniform distribution of Ni(OH) 2 . The loading percentage of Ni(OH) 2 in the Ni(OH) 2 /NOSCF composite was estimated to be $63% by thermogravimetric analysis (TGA), as displayed in Fig. S4 (ESI †).
The CeO 2 NPs were synthesized by using cerium(IV) ammonium nitrate ((NH 4 ) 2 Ce(NO 3 ) 6 ) as a precursor in a mixture of solvents of oleylamine and 1-octadecene. As shown in the PXRD pattern in Fig. 3a, the prepared CeO 2 samples presented a cubic phase (space group: Fm 3m, a ¼ b ¼ c ¼ 5.411Å, JCPDS: 34-0394). The TEM image in Fig. 3b showed that the as-synthesized CeO 2 NPs were relatively monodisperse with an average size of $3.3 nm (inset of a histogram of the particle diameters). The good monodispersity of the CeO 2 NPs indicates the retention of the used capping ligand (oleylamine) on the surface of CeO 2 NPs, as demonstrated by Fourier transform infrared (FTIR) spectroscopy (Fig. S5, ESI †). As seen in Fig. 3c, the HRTEM image of the CeO 2 NPs showed clearly crystal lattice fringes with an interplanar spacing of 0.16 nm, which can be ascribed to the (111) crystal plane. The selected area electron diffraction (SAED) pattern shown in Fig. 3d indicated that the synthesized CeO 2 NPs were highly crystallized.
The prepared CeO 2 NPs are hydrophobic due to the long carbon chains of oleylamine (OM) used as surfactants for the reaction, and hence cannot directly disperse in water. In order to generate a hydrophilic surface for combining with the Ni(OH) 2 /NOSCF and testing the OER performance, we employed NaS 2 solution to modify the surface of CeO 2 NPs (Fig. S5, ESI †). 33 As shown in the TEM image in Fig. S6a (ESI †), the CeO 2 NPs still kept their particle morphology with high crystallization aer surface modication, and could be well dispersed in water (digital photo in Fig. S6b, ESI †). The CeO 2 NPs were anchored on the Ni(OH) 2 /NOSCF using a controllable electrophoretic deposition strategy, the details are shown in the Experimental section. 34 All of the diffraction peaks of Ni(OH) 2 (JCPDS: 380715) and CeO 2 (JCPDS: 34-0394) were detected in CeO 2 /Ni(OH) 2 / NOSCF (Fig. 4a). Fig. 4b and c show the representative TEM and HRTEM images of the CeO 2 /Ni(OH) 2 hybrid obtained from CeO 2 /Ni(OH) 2 /NOSCF with a deposition duration of 10 min. It can be observed from Fig. 4b that the Ni(OH) 2 nanosheets are uniformly decorated with CeO 2 NPs. Fig. 4c presents the corresponding HRTEM image with an interplanar spacing of 0.16 nm and 0.23 nm, indexed to the (111) and (101) crystal planes of CeO 2 and Ni(OH) 2 , respectively.
To further investigate the strong electronic interactions between Ni(OH) 2 nanosheets and CeO 2 NPs, UV-vis absorption spectra (Fig. 4d) and X-ray photoelectron spectroscopy (XPS) spectra ( Fig. 4e and f) of Ni(OH) 2 nanosheets, CeO 2 NPs and the CeO 2 /Ni(OH) 2 hybrid were examined. As shown in Fig. 4d, two absorption peaks of the grown Ni(OH) 2 nanosheets located at 385 and 670 nm corresponded to the d-d transitions of Ni II cations. 35 Compared with the pristine Ni(OH) 2 nanosheets, the absorption spectrum of CeO 2 /Ni(OH) 2 was obviously red-shied ($8 nm), indicating the strong electronic interactions between them. 36,37 XPS spectra of Ce 3d and Ni 2p are shown in Fig. 4e and f, respectively. As shown in Fig. 4e, for Ce 3d of CeO 2 , the peaks located at 920-911 eV and 903-893 eV correspond to Ce 3d 3/2 , and the peaks located at 877-866 eV correspond to Ce 3d 5/2 , which demonstrated the coexistence of Ce 3+ and Ce 4+ in the CeO 2 NPs. 38 However, aer CeO 2 NPs were deposited on the Ni(OH) 2 nanosheets, the ratio of Ce 3+ : Ce 4+ in the CeO 2 / Ni(OH) 2 hybrid changed compared with pure CeO 2 NPs, indicating that the valence states of Ce in the CeO 2 /Ni(OH) 2 hybrid rearranged. 28 As shown in Fig. 4f, the XPS spectrum of Ni 2p in Ni(OH) 2 /NOSCF showed two major peaks at 853.2 and 870.8 eV corresponding to Ni 2p 3/2 and Ni 2p 1/2 , respectively, which were characteristic of the Ni 2+ state. 39 And some satellite peaks in the Ni 2p region could also be observed in Fig. 4f. Through careful comparison and analysis, we found that the peaks of Ni 2p 3/2 and Ni 2p 1/2 in the XPS spectrum for CeO 2 /Ni(OH) 2 /NOSCF both shied to lower binding energies of $0.5 eV. Therefore, the ratio change of Ce 3d and peak shis of Ni 2p in the CeO 2 / Ni(OH) 2 hybrid indicate strong electronic interactions between the Ni(OH) 2 nanosheets and CeO 2 NPs.

CeO 2 /Ni(OH) 2 /NOSCF electrocatalyst for OER performance
The water oxidation reaction was applied to study the electronic interactions between Ni(OH) 2 nanosheets and CeO 2 NPs and their effect on the corresponding catalytic activity. The OER performance of the CeO 2 /Ni(OH) 2 /NOSCF electrocatalyst was investigated in 1.0 M KOH (pH ¼ 14). As a comparison, Ni(OH) 2 / NOSCF, CeO 2 /NOSCF, NOSCF and Ir/C were also tested under the same conditions. It was obvious from the cyclic voltammetry (CV) curves (Fig. 5a) that both the electrocatalysts of Ni(OH) 2 / NOSCF and CeO 2 /Ni(OH) 2 /NOSCF clearly showed redox peaks ranging from 1.2 to 1.6 V, which belong to the Ni II /Ni III/IV redox process (Ni(OH) 2 + OH À / NiOOH + H 2 O + e À ). And there was an obvious negative shi of the oxidation potential, changed from 1.46 V to 1.41 V for CeO 2 /Ni(OH) 2 /NOSCF, indicating that CeO 2 /Ni(OH) 2 /NOSCF has higher transfer efficiency from Ni II to Ni III/IV and larger charge capacity than Ni(OH) 2 /NOSCF. 40 This conclusion can be further demonstrated by Nyquist plots. As presented in Fig. S7 (ESI †), compared with Ni(OH) 2 /NOSCF, the CeO 2 /Ni(OH) 2 /NOSCF shows a smaller charge transfer resistance (high frequencies) and reduced mass-transfer resistance (low frequencies). The decrease in mass transfer resistance may contribute to the increased number of Ni III/IV active species.
As shown in the polarization curves in Fig. 5b and statistical data in Fig. 5c, the CeO 2 /Ni(OH) 2 /NOSCF exhibited a lower onset potential of 240 mV than Ir/C and Ni(OH) 2 /NOSCF, surpassing most reported non-noble metal based OER electrocatalysts (Table  S1, ESI †). In particular, the signicant increase of the current density was more obvious when the potential was beyond $1.6 V, which could further demonstrate that the OER activity of Ni(OH) 2 / NOSCF is greatly enhanced when decorated with CeO 2 NPs. In addition, we also optimized the loaded mass ratio of CeO 2 NPs on Ni(OH) 2 /NOSCF and found that when the mass ratio of CeO 2 -: Ni(OH) 2 /NOSCF was 30% (Fig. S8, ESI †), the electrocatalytic activity of CeO 2 /Ni(OH) 2 /NOSCF reached its highest level, and therefore this mass ratio was used in the following experiments.   During the OER process, the highly oxidative Ni III/IV cations are believed to serve as active species, which indicates that the enhanced catalytic activity for CeO 2 /Ni(OH) 2 /NOSCF observed in our study might be a result of increasing Ni II /Ni III/IV transformations. Therefore, we investigated the extent of the Ni II / Ni III/IV transformation by integrated oxidation peak areas (inset of Fig. 5b). 41,42 When CeO 2 NPs were deposited on the Ni(OH) 2 / NOSCF, the Ni II /Ni III/IV extent showed a dramatic increase of about 1.7-fold compared with the Ni(OH) 2 /NOSCF (inset of Fig. 5b), thus CeO 2 NPs potentially facilitated producing more Ni III/IV active species and subsequently led to the improvement of the OER catalytic activity (Fig. 5b).
The enhanced OER activity of CeO 2 /Ni(OH) 2 /NOSCF was more obvious by comparing the Tafel slopes. As shown in Fig. 5c and d, the Tafel slope of CeO 2 /Ni(OH) 2 /NOSCF was 57 mV dec À1 , and it was smaller than those of Ir/C (72 mV dec À1 ), NOSCF (295 mV dec À1 ), CeO 2 /NOSCF (136 mV dec À1 ), and Ni(OH) 2 /NOSCF (65 mV dec À1 ). Through the comparison of the Tafel slopes we could demonstrate that depositing CeO 2 NPs on Ni(OH) 2 /NOSCF could facilitate its OER kinetics, and the OER activity of CeO 2 /Ni(OH) 2 /NOSCF was comparable to many other non-noble metal OER electrocatalysts in alkaline media (Table  S1, ESI †).
We also tested the stability of the designed CeO 2 /Ni(OH) 2 / NOSCF by a chronoamperometry method to evaluate the OER performance. As shown in Fig. 5e, the current density of the OER showed no change during 6 h of continuous operation under various potentials of 0.55, 0.60, 0.65, 0.70, 0.75 and 0.80 V, which suggested that the CeO 2 /Ni(OH) 2 /NOSCF had excellent stability for the OER process. Thus, the CeO 2 /Ni(OH) 2 / NOSCF with its high catalytic activity as well as excellent stability would be a promising candidate for electrochemical water oxidation.

Conclusions
In summary, we have successfully designed a 3D hierarchical Ni(OH) 2 /NOSCF electrode by growing Ni(OH) 2 nanosheets along the carbon frameworks of N, O and S doped CF. The experiments found that Ni(OH) 2 nanosheets of Ni(OH) 2 /NOSCF decorated with $3.3 nm sized CeO 2 NPs displayed enhanced OER performance. Compared with Ni(OH) 2 /NOSCF, the onset potential of CeO 2 NP decorated Ni(OH) 2 /NOSCF decreased from 270 to 240 mV, and the Tafel slope reduced from 65 to 57 mV dec À1 , much better than the benchmark Ir/C. As conrmed by UV-vis and XPS results, as well as electrochemical analysis, the reasons for the enhanced OER performance result from the synergistic effect between CeO 2 NPs and Ni(OH) 2 nanosheets by a 1.7-fold enhancement in the generation of Ni III/IV active species and faster charge transfer. The high OER performance of CeO 2 /Ni(OH) 2 /NOSCF in the present study makes CeO 2 based composites very promising electrocatalysts for water oxidation.