10.1002_smll.201906766.pdf

1906766 (1 of 7) 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimwww.small-CommuniCationUltrathin Amorphous Nickel Doped Cobalt Phosphates with Highly Ordered Mesoporous Structures as Efficient Electrocatalyst for Oxygen Evolution ReactionLan Yang, Hao Ren, Qinghua Liang, Khang Ngoc Dinh, Raksha Dangol, and Qingyu Yan*L. Yang, Dr. H. Ren, Dr. Q. Liang, K. N. Dinh, R. Dangol, Prof. Q. Y. YanSchool of Materials Science and EngineeringNanyang Technological UniversitySingapore 639798, SingaporeE-mail: alexyanntu.edu.sgThe ORCID identification number(s) for the author(s) of this article can be found under https:/doi.org/10.1002/smll.201906766.DOI: 10.1002/smll.201906766Owing to the depletion of fossil fuel and increasing concern on global warming, there is an urgent desire for sustainable and clean energy resources. Hydrogen is deemed as one of the most promising candidate, which could be produced from water electrolysis.1 The main challenge is that the anodic reaction of water splittingoxygen evolution reaction (OER) exhibits slug-gish kinetics and high energy barrier (overpotential) because it involves four electronelectron process and OO bond forma-tion. Highly active electrocatalysts are needed to fasten reaction kinetics and lower that overpotential. However, the benchmark OER catalysts so far are still based on the expensive and low-abundance noble metals such as IrO2 and RuO2.2The past decade has witnessed extraordinary progress on overcoming the bottleneck of OER electrocatalysis by transition metal phosphides,3 dichalcogenides,4 nitrides,5 and thiophos-phate materials.6 Nevertheless, the real active sites of the above materials remain controversial because their phase, structure, Herein, the facile preparation of ultrathin (3.8 nm in thickness) 2D cobalt phosphate (CoPi) nanoflakes through an oil-phase method is reported. The obtained nanoflakes are composed of highly ordered mesoporous (3.74 nm in diameter) structure and exhibit an amorphous nature. Attractively, when doped with nickel, such 2D mesoporous Ni-doped CoPi nanoflakes display decent electrocatalytic performances in terms of intrinsic activity, and low kinetic barrier toward the oxygen evolution reaction (OER). Particularly, the optimized 10 at% Ni-doped CoPi nanoflakes (denoted as Ni10-CoPi) deliver a low overpotential at 10 mA cm2 (320 mV), small Tafel slope (44.5 mV dec1), and high stability for OER in 1.0 m KOH solution, which is comparable to the state-of-the-art RuO2 tested in the same condition (overpotential: 327 mV at 10 mA cm2, Tafel slope: 73.7 mV dec1). The robust frame-work coupled with good OER performance enables the 2D mesoporous Ni10-CoPi nanoflakes to be a promising material for energy conversion applications.and composition usually undergo obvious evolution under strong oxidation condition.3c,7 It has been reported that the phosphate group plays an important role in facilitating adsorption and stabilizing catalytic centers,8 however transition metal phosphates are rarely explored as electrocatalysts for OER. Among the few reports on phosphate materials as OER catalyst, their performances are still far from satisfactory.8c,9 As known, the elec-trocatalytic performance depends on many factors, such as the number of accessible active sites, mass transport efficiency, and the ability to resist corrosion in elec-trolytes, which are related to the internal compositional properties as well as the external structure and morphology.3c,10Hence, there are multiple effective strategies to optimize the OER perfor-mance of transition metal phosphates. First, cation doping can improve the intrinsic activity. Mixed metal cations show more favorable adsorption/desorption of oxygen-containing intermediates than that of the single-metal counterparts.8b,c The transition metal Fe, Co, Ni and their mixed compounds are excellent catalysts in alkaline OER reactions, and different combinations of these three elements can significantly change the properties of the catalysts. It was reported that moderate amount of Fe and Ni doping in cobalt-based catalysts yielded higher OER activity and lower overpo-tential, which are associated with reduction of the adsorption energy on the catalyst surface, and Fe, Ni substitutions were revealed to facilitate surface reconstruction into active Co oxy-hydroxides under OER conditions.11 Due to the similar atomic structure of Co and Ni subgroup elements, Ni doping in CoPi is beneficial to keep the original morphology and structure. Moreover, reducing the thickness is another effective way since it can accelerate the mass transport.12 Mass transport is an important parameter to describe the transfer rate of electrocat-alysts to the electrolyte. Faster mass transport leads to higher electrocatalysts utilization, which means better performance of the electrocatalysts for the same mass loading.13 Furthermore, a highly ordered porous structure such as mesopores can offer higher surface area, which means more active sites are exposed to electrolyte.14 The increased effectively active surface area facilitates the adsorption/desorption process and yields better electrocatalytic activity. Besides, the design of amorphous struc-ture has merits such as short-range structure ordering and high Small 2020, 19067661906766 (2 of 7) 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimwww.small-density of active sites density.15 Compared to crystallized mate-rials, the disordered structures of amorphous electrocatalysts offer more active sites for highly efficient water oxidation.In consideration of the above merits, herein, a facile oil-phase method is employed to realize the compositional, mor-phological, and lattice structural engineering of transition metal phosphates. Particularly, the amorphous 10 at% Ni-doped CoPi nanoflakes (denoted as Ni10-CoPi) composed of highly ordered mesoporous structure were prepared as an efficient and elec-trostable OER electrocatalyst. The obtained mesoporous Ni10-CoPi nanoflakes show an ultrathin thickness of 3.8 nm and an average pore diameter of 3.74 nm. During OER process, the as-prepared Ni10-CoPi electrodes show a decent activity with low overpotential of 320 mV at 10 mA cm2 and small Tafel slope of 44.5 mV dec1.We prepared the mesoporous CoPi nanoflakes with dif-ferent Ni doping content of 0%, 5%, 10%, and 15% based on a modified oil-phase method (see the Experimental Section for details).16 The Co/Ni atomic ratio of the as-obtained CoPi, Ni5-CoPi, Ni10-CoPi, and Ni15-CoPi was confirmed by individu-ally coupled plasma-optical emission spectrometry (ICP-OES), which is close to that was used in the corresponding synthesis (Table S1, Supporting Information).X-ray diffraction (XRD) patterns of the synthesized CoPi, Ni5-CoPi, Ni10-CoPi, and Ni15-CoPi (Figure 1a) show no sharp diffraction peak, indicating the amorphous nature of the tested samples. To investigate the chemical bonds and oxidation states of the obtained materials, X-ray photoelectron spectroscopy (XPS) was performed on CoPi and Ni10-CoPi (Figure 1bd; Figure S1, Supporting Information). In the high-resolution Co 2p XPS spectrum (Figure 1b), the spinorbit doublets of Co 2p3/2 and Co 2p1/2 are located at binding energy of 797.4 and 781.64 eV, respectively. The spin energy interval is around 16.76 eV, suggesting the existence of Co2+.17 In the corresponding Ni 2p profile (Figure 1c), the main two strong peaks observed at the binding energy of 856.23 and 873.88 eV can be attributed to 2p3/2 and 2p1/2 from the Ni2+ species, respectively.18 The P 2p (Figure 1d) and O 1s (Figure S1b, Supporting Information) XPS spectra show the feature peaks at 133.7 and 531.2 eV, respectively, which matches the characteristic peaks of (PO4)3.19 The P 2p and C 1s (Figure S1c, Supporting Infor-mation) spectra revealed the existence of PC bonding with the corresponding fea-ture peak showing at 132.7 and 285.2 eV, respectively.20To characterize the morphology of the obtained materials, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were carried out. The low-magnification TEM images clearly reveal the nanoflakes morphology of Ni10-CoPi with a lateral size of 800 nm (Figure 2a). The low and high-magnifica-tion SEM images in Figure S2 (Supporting Information) also confirmed such 2D layered nanoflakes structure. Furthermore, the high-resolution TEM (HRTEM) images in Figure 2b,c show that there are many channels on the nanoflakes. The distance between the centers of two neighboring channels (defines as Dcenter) is measured to be 3.74 nm (inset of Figure 2c), indicating the mesoporous structure of the obtained material. The HRTEM images of CoPi, Ni5-CoPi, and Ni15-CoPi show the same mesoporous morphology and very similar pore diameters of 3.42, 3.4, and 3.75 nm, respectively (Figure S3, Supporting Informa-tion). The selected area electron diffraction (SAED) pattern fur-ther confirms the amorphous nature of the material (inset of Figure 2b), which agrees well with the XRD result above. The low-angle XRD in Figure 2d shows three resolved diffraction peaks. According to Braggs law (2d sin = n), the d values for the three peaks are calculated to be 1.87, 1.26, and 0.94 nm. The three d values are are 1/2, 1/3, 1/4 of the Dcenter, thus indi-cating that the three peaks can be accordingly indexed as (200), (300), and (400) reflections and proves that the mesopores are highly ordered distributing along the 100 direction. The low-angle XRD patterns of Co-Pi, Ni5-CoPi, and Ni15-CoPi nano-flakes (Figure S4, Supporting Information) also show the same correlation between the d values and Dcenter, indicating that Ni-doping does not affect the structure and morphology of the syn-thesized materials. It is worth noting that the thickness of the flake measured by atomic force microscopy (AFM) (3.8 nm) is very close to d and Dcenter (Figure S5, Supporting Informa-tion), indicating that the nanoflake may contain only one layer of the mesoporous channels. The ultrathin nature of the mate-rials is conducive to mass transport and charge transfer, which Small 2020, 1906766Figure 1. a) XRD patterns of CoPi (in black), Ni5-CoPi (in red), Ni10-CoPi (in blue), and Ni15-CoPi (in green). bd) XPS spectrums of CoPi and Ni10-CoPi in Co 2p region, Ni 2p region, and P2p region.1906766 (3 of 7) 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimwww.small-leads to high electrocatalytic performance toward water oxida-tion. The BrunauerEmmettTeller (BET) surface area obtained from N2 adsorptiondesorption isotherms is 35 m2 g1, and the pore size distribution calculated by the BarrettJoynerHalenda (BJH) method shows a narrow peak with a mean value of 4.5 nm (Figure S6, Supporting Information). The mesoporous structures provide a large surface area and expose more active sites. Besides, mesoporous structure has the advantages of reg-ular channel structure and narrow pore size distribution, which is conducive to the adsorption and desorption process during OER. Moreover, the energy-dispersive X-ray spectroscopy (EDX) elemental mapping confirmed the presence of P, Co, and Ni and their uniform distribution overlapping the Ni10-CoPi nanoflakes, which is consistent with the XPS results (Figure S7, Supporting information).To study the electrocatalytic performance of Ni10-CoPi, we tested the Ni10-CoPi nanoflakes for OER in the alkaline solu-tion of 1.0 m KOH. A three-electrode system was used with a glassy-carbon electrode coated with catalyst as the working electrode, carbon rod, and Hg/HgO as the contact and refer-ence electrode, respectively. The OER activities of CoPi, Ni5-CoPi, Ni15-CoPi, and commercial RuO2 were also evaluated for comparison.In the iR-compensated polarization curves obtained by steady-state linear sweep voltammetry (LSV), it can be seen that the Ni10-CoPi electrode exhibits an onset overpotential of 272 mV (Figure 3a), which is obviously lower than those of CoPi (302 mV), Ni5-CoPi (286 mV), and Ni15-CoPi (279 mV). The polarization curves without iR-correction (Figure S8, Sup-porting Information) show an onset overpotential of 273 mV for Ni10-CoPi, which is lower than those of CoPi (303 mV), Ni10-CoPi (287 mV), and Ni15-CoPi (280 mV). Moreover, Tafel slopes obtained from Tafel equation indicate that Ni10-CoPi electrode has faster OER kinetics with a smaller Tafel slope of 44.5 mV dec1. The Tafel slope of CoPi, Ni5-CoPi, and Ni15-CoPi is 68.3, 55.1, and 51.2 mV dec1, respectively (Figure 3b). Tafel slope of CoPi is more than 60 mV dec1, indicating that the discharge of OH1 is the rate-determining step, while the Tafel slope of Ni-doped CoPi is in the range of 4060 mV dec1, implying that the adsorption of OH1 on the catalysts is the rate-determining step. This revealed that Ni doping moves the rate-limiting step to the end of the mul-tielectron transfer reaction and speed up the OER kinetic process, which is very important for improving the catalytic activity. Besides, the performance of Ni10-CoPi also surpasses the tested OER electrocatalyst RuO2, which requires 327 mV Small 2020, 1906766Figure 2. a) TEM image, b,c) HRTEM images, and d) low-angle XRD pattern of Ni10-CoPi. The inset in (b) is the SAED pattern of Ni10-CoPi. The inset in (c) is the enlarged TEM image, showing the distance between neighboring channels is 3.74 nm.1906766 (4 of 7) 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimwww.small-to drive a current density of 10 mA cm2 and the corre-sponding Tafel slope is 73.7 mV dec1 (Figure S9, Supporting Information). The respective 10, i.e., required overpotential to realize the current density of 10 mA cm2, for the optimized Ni10-CoPi nanoflakes is 320 mV, which is smaller than that of CoPi (354 mV), Ni5-CoPi (335 mV), and Ni15-CoPi (325 mV) (Figure 3c). To investigate the effect of mesoporous structure on OER, we also synthesized Ni10-CoPi nanoflakes without mesoporous structures by improving the reaction temperature (see the Experimental Section) and tested the OER performance under same condition (Figure S10, Supporting Information). The TEM images (Figure S10ac, Supporting Information) show that there is no as obvious channels as those on Ni10-CoPi with mesoporous structure, indicating that too high reaction temperature is not conducive to the formation of this morphology. In the OER polarization curve (Figure S10d, Sup-porting Information), Ni10-CoPi without mesoporous structure exhibits the performance nowhere near as good as Ni10-CoPi with mesoporous structure, showing an overpotential of 350 mV at 10 mA cm2. This contrast further proved that mesoporous structure plays an important role in OER pro-cess by providing large surface area and making more active sites exposed. More importantly, the chronoamperometry test performed on carbon paper shows that a stable current den-sity of 20 mA cm2 was maintained for 20 h with ignorable change less than 5%, demonstrating the excellent durability of the electrode (Figure 3d). The XRD characterization shows that the Ni10-CoP