CoHPi Nanoflakes for Enhanced Oxygen Evolution Reaction - ACS

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CoHPi Nanoflakes for Enhanced Oxygen Evolution Reaction Jingjing Wang, and Hua Chun Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17257 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 25, 2018

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ACS Applied Materials & Interfaces

CoHPi Nanoflakes for Enhanced Oxygen Evolution Reaction Jingjing Wang and Hua Chun Zeng* Department of Chemical and Biomolecular Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Keywords: transition metal hydrogen phosphate, template-directing, ultrathin flake, OER electrocatalyst, mass and charge transfer

ABSTRACT: Electrochemical splitting of water to produce hydrogen and oxygen is an important process for many energy storage and conversion devices. Developing efficient, robust, low-cost, and earth-abundant electrocatalysts for the oxygen evolution reaction (OER) is therefore of great importance. Herein, we report a novel method to prepare two-dimensional cobalt hydrogen phosphate (CoHPi) through chemical conversion of α-Co(OH)2 precursor at room-temperature. The CoHPi nanoflakes with the thickness of 3 nm contain HPO42− anions which have been demonstrated to serve as a proton acceptor in proton coupled electron transfer process (PCET) of OER. Due to their ultrathin structure and the PCET merit of anions, the CoHPi nanoflakes show enhanced OER activity as well as excellent stability in prolonged OER operation. Through further mechanism study, the observed performances can be ascribed to enriched active sites, surface superhydrophilicity and rapid electron/proton and mass transfers.

1. INTRODUCTION The oxygen evolution reaction (OER) is one of the key processes for many energy storage and conversion devices, such as hydrogen production from water splitting, regenerative fuel cells, and rechargeable metal-air batteries. However, the intrinsic thermodynamic “up-hill” of OER requires extra potential to drive the process. To increase the reaction rate and decrease the energy consumption, therefore, efficient electrocatalysts with low over-potentials are generally considered to be the key to boost this process.1-3 To date, the noble metal oxides, such as IrO2 and RuO2 are demonstrated as the state-of-the-art electrocatalysts for OER in both acidic and alkaline mediums, but their high cost and scarcity limit their industrial applications. To circumvent this cost hindrance, over the last decades, materials based on first-row transition metals, including their layered double hydroxides (LDHs),4 perovskite,4-5 oxides,6-8 phosphides9 and chalcogenides10, have been widely investigated, and many of them have exhibited considerable electrocatalytic activity toward OER. It has been well known now that the resultant OER performance can be significantly influenced by intrinsic and extrinsic factors of electrocatalysts such as their chemical composition, crystal structure, size-control and particle distribution, hierarchical porosity, dopant modification and

surface engineering, electrical conductivity, as well as electrode architecture and so on.11-14 Among the above studied materials, in particular, transition metal phosphates (MPi) can be singled out as a class of important OER catalysts. Compared to other transition metal-based catalysts, in particular, MPi have demonstrated remarkable electrocatalytic performances in both alkaline medium and neutral medium.15-16 In recent years, modification on MPi systems has also received considerable research interest; such works include doping17 and integrating with carbon materials (e.g., graphene oxide GO)18 and nitrogen-doped carbon,19 which lead to even better OER performances. Nevertheless, it is noted that the above reported works were mostly focused on PO43− based transition metal phosphates. To further exploit this well studied system, on the one hand, it has been reported that the hydrogen phosphate ion (HPO42−) is serving as a proton acceptor in proton coupled electron transfer (PCET) process which accounts for the robust OER performance of MPi.20 In order to take full advantage of PCET, therefore, we believe that synthesizing and investigating HPO42− based MPi for the same OER must become important in the near future in addition to the PO43− based MPi. On the other hand, another way to improve OER performance is the morphological control of electrocatalysts. Because of the presence of large

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open surfaces, for example, two-dimensional materials (e.g., ultrathin nanoflakes) are generally conceived to be more favorable in terms of facilitating charge and mass transfers in electrolyte when compared with their bulk or particulate counterparts. In this regard, it seems that the development of hydrogen-phosphate based MPi (MHPi) with a twodimensional (2D) morphology (i.e., developing 2D-MHPi electrocatalysts) should therefore be highly desirable. As part of our recent research endeavors, the current work will address the above two important issues (compositional (i.e., from MPi to MHPi) and dimensional (i.e., from 3D to ultrathin 2D)) related to the development of MPi-based electrocatalysts. In selecting suitable synthetic strategies to form 2D-MHPi, we recognized that common transition metal hydroxides, including their LDHs, are a class of very versatile solid precursors that can be further transform into various targeted materials.10, 21-23 Using this synthetic strategy, indeed, two-dimensional CoHPi nanoflakes (shorthanded as 2D-CoHPi hereafter, where anions are primarily HPO42−) samples could be prepared from alpha cobalt hydroxide (α-Co(OH)2) at room temperature. More specifically, hexagonal platelets of αCo(OH)2 served as both a Co source and a sacrificial template for the preparation of CoHPi nanoflakes. The obtained 2D-CoHPi were then used as catalysts for OER application, which exhibited excellent electrocatalytic activity. Finally, XPS technique was also employed in order to address mechanistic aspects of the enhanced OER performance. 2. EXPERIMENTAL SECTION 2.1. Chemicals. The following chemicals were used as received without any further purification: CoCl2∙6H2O (99%, Sigma-Aldrich), NaCl (+99%, Sigma-Aldrich), ethanol (99.99%, Fisher), hexamethylenetetramine (HMT, +99%, Alfa-Aesar), Na2HPO4∙12H2O (+99%, Sigma-Aldrich), NaOH (99.0%, VWR), KOH (85%, Merck), carbon black (Nacalai Tesque Inc), ethanol (analytical reagent grade, VWR), ruthenium (IV) oxide (RuO2, 99.9%, Aldrich), ultrapure water (Merck), perfluorosulfonic acid-PTFE copolymer (Nafion solution, 5% w/w in water, Alfa-Aesar), and phosphate buffer solution (PBS, pH = 7, Merck). 2.2. Synthesis of α-Co(OH)2 precursor. The α-Co(OH)2 nanoplatelets, giving the composition as Co(OH)1.70(CO3)0.02Cl0.26∙0.56H2O, was synthesized according to a previously reported method.24 Typically, 0.952 g of CoCl2∙6H2O, 0.234 g of NaCl, and 1.050 g of HMT were dissolved in 200 mL of a cosolvent made from deionized water and ethanol (9:1, v/v). The reaction solution was then heated at 90oC under magnetic stirring. After being heated for about 1 h, a suspension containing green particles resulted. The solid product was filtered and washed with deionized water and anhydrous ethanol for several times, and finally air-dried at 60oC.

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2.3. Synthesis of CoHPi nanoflakes (namely, 2D-CoHPi). Briefly, 50 mg of the above prepared α-Co(OH)2 precursor was dispersed into 50 mL of an aqueous solution of 1.0 M Na2HPO4 (with an initial pH value of 8.6). The suspension was then stirred under room temperature for 20 h for transformative reaction. The final product was isolated using the same procedure described for the pristine material (Section 2.2). In order to probe the reaction mechanism, we also prepared samples using the same method, but with different reaction times (1 h, 2 h, and 3 h). The obtained samples are denoted as CoHPi-1 h, CoHPi-2 h and CoHPi-3 h, respectively, according to their reaction times. 2.4. Synthesis of Pi intercalated Co(OH)2 (i.e., Co-Pi-OH). In this synthesis, 50 mg of the above prepared α-Co(OH)2 precursor was dispersed into 50 mL of an aqueous solution of 1.0 M Na2HPO4 with a pH value of 12, adjusted using a NaOH solution at high concentration (10 M). The suspension was stirred under room temperature for 20 h. After exchange of gallery anions with phosphate ions (mainly PO43− under this condition),25 the exchanged products were isolated using the same procedure described for the pristine material. 2.5. Synthesis of porous Co2(P2O7) nanoflakes. By heat treatments at 500oC under air or argon atmospheres, the CoHPi nanoflakes can be converted to Co2(P2O7) nanoflakes with irregular pores. Typically, the dried sample of 2DCoHPi was placed in the middle of a tube furnace, then it is heated from room temperature to 500 °C with a ramping rate of 5 °C∙min−1 and a flow rate of 50 mL∙min−1 of Ar or Air for 2 h. After heating, the sample was cooled naturally. 2.6. Electrochemical measurements. 3.0 mg of each of the above catalyst powders and 2.0 mg carbon black were dispersed in a mixed water and ethanol (3:1, v/v) solution (0.9 mL), and then 30 μL of Nafion solution (5 wt%) was added. The suspension was treated in an ultrasonic bath for 30 min to prepare a homogeneous ink. The working electrode was prepared by dripping 4.2 μL of the catalyst ink onto a polished glassy carbon electrode with a diameter of 3 mm (catalyst loading 0.2 mg/cm 2), and subsequent drying at room temperature for 12 h. The electrochemical measurements were conducted on a CH Instruments model 760E electrochemical workstation using 1.0 M KOH (pH = 14) and phosphate buffer solution (PBS, pH = 7) as electrolytes, platinum gauze as a counter electrode, and Ag/AgCl electrode with 3.0 M KCl as a reference electrode. During the measurements, the working electrode was constantly rotated at 2000 rpm to remove generated O2. All linear scan voltammograms (LSV) polarization curves were corrected with 95% iRcompensation. The measured potentials vs Ag/AgCl was converted to a reversible hydrogen electrode (RHE) according to the Nernst equation (ERHE = EAg/AgCl + 0.059 pH + 0.21).


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To have a better quantification of performance of the above electrocatalysts, the turnover frequency (TOF) value is calculated from the equation: TOF = Ј∙A/4F∙m, where J is the measured current density, A is the surface area of the working electrode, F is the Faraday constant (96485 C/mol), m is the number of moles of active materials loaded on the electrodes. The relative electrochemical active surface area of the catalysts was further estimated by determining their electrochemical double-layer capacitance (Cdl), which is linearly proportional to the effective surface area. Cdl (mF∙cm−2) was determined from the CV curves measured in a 0.10 V potential range without faradaic process at different scan rates according to the following equation: Jc = Cdl∙ ν, where Jc and ν are the capacitive current density (mA∙cm−2) and scan rate (V∙s−1) respectively. By plotting the average capacitive current density at the open circuit potential against the scan rate, the measured slope of the plot represents the Cdl. 2.7. Materials characterization. The microscopic features of the samples were characterized by scanning electron microscopy (SEM, JEOL-6700F) equipped with an energydispersive X-ray (EDX) analyzer (Oxford INCA), transmission electron microscopy (TEM, JEOL JEM- 2010, 200 kV), and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F, 200 kV). The elemental mapping was done by energy-dispersive X-ray spectroscopy (EDX, Oxford Instruments, model 7426). The wide-angle X-ray diffraction patterns were taken on Bruker D8 Advance system (Cu Kα radiation). Inductively coupled plasma (ICP) analysis (Dual-view Optima 5300 DV ICP-OES) was used to measure the elemental compositions of the above studied samples. X-ray photoelectron spectroscopy (XPS, AXIS-HSi, Kratos Analytical) analysis was conducted using a monochromatised Al Kα exciting radiation (hν = 1286.71 eV) with a constant analyzer-pass-energy of 40.0 eV. All binding energies (BE) were referenced to the C 1s peak arising from CC bonds; their BE was set at 284.5 eV. Chemical bonding information of the above samples was also gathered with Fourier transformed infrared spectroscopy (FTIR, Bio-Rad FTS-3500ARX). The contact angles of water droplets on the samples were measured using a contact angle meter (Kyowa Interfacial Science Japan, CA-D). N2 adsorption−desorption experiments were performed by a Quantachrome Instruments NOVA 4200e surface area and pore size analyzer at 77.3 K after overnight degassing in ﬂowing N2 at 150°C. 3. RESULTS AND DISCUSSION

Figure 1. (a) TEM image, (b) SEM image, and (c, e) HRTEM images, (d) SAED, and (f) elemental mapping images of asprepared CoHPi nanoflakes. Morphological characterization of the as-prepared 2DCoHPi is reported in Figure 1. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images (Figure 1a, b) depict that the CoHPi nanoflakes have a flowerlike morphology and their size is about 1−1.5 μm in diameter. The thickness of individual nanoflake is only about 3 nm (Figure 1e) which indicates such a morphology will be efficient in materials utilization. In addition, the high resolution TEM image in Figure 1c together with the selected area electronic diffraction in Figure 1d reveals that the 2D-CoHPi sample is essentially amorphous. Elemental mapping images displayed in Figure 1f elucidate that elements of 2D-CoHPi (i.e., Co, O, and P) are uniformly distributed, indicating the formation of a homogenous solid despite its amorphous nature. Furthermore, Figure S1a verifies the atomic ratio of Co and P of 2D-CoHPi is around 1, which corresponds well with the XRD pattern given in Figure S1b, affirming that the CoHPi nanoflakes are basically a mixture of CoHPO4 with different number of crystal waters. By selecting different reaction times, we found that the morphological evolution of CoHPi nanoflakes from αCo(OH)2 precursor can be broadly divided into four stages, as illustrated in Figure 2. Initially, the starting -Co(OH)2 has a hexagonal platelet form in high crystallinity with a width in the ranges of 2−3 µm and a thickness around 15 nm (Figure 2a, b and Figure S2). It serves as the Co source as well as the template for the formation of CoHPi



nanoflakes. When it is then added to the 1.0 M Na 2HPO4 solution (Section 2.3), the pristine -Co(OH)2 undergoes the transformative reaction at room temperature. After 2 h of reaction, for instance, it becomes porous and its surface coarsening can be identified clearly (Figure 2c, d), which can be related to an initial dissolution of -Co(OH)2. From Figure 2e and f, we can see that, with the continuous dissolution of the metal hydroxide, CoHPi nanoflakes begin

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to form at the porous surface of -Co(OH)2 at 3 h, while the hexagonal platelets are getting more dissolved. Finally, as shown in Figure 2g and h, the hexagonal -Co(OH)2 platelets are completely replaced by the ultrathin 2D-CoHPi, giving rise to a flowerlike architecture (Section 2.3).

Figure 2. Schematic illustration of the formation of CoHPi nanoflakes from the hexagonal -Co(OH)2 platelets. TEM images of samples related to this structural evolution: (a, b) hexagonal -Co(OH)2 platelet precursor, (c, d) samples obtained after 2 h of reaction, (e, f) after 3 h of reaction, and (g, h) final 2D-CoHPi sample obtained after 20 h of reaction.


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bound OH groups, and the peak at 1615 cm −1 is due to the bending mode of water molecules.26 For the spectrum of Co(OH)2, the weak bands at 845 and 962 cm−1 are characteristic of carbonate ions and other absorptions below 1000 cm−1 are associated with Co−O stretching and Co−OH bending vibrations. For the spectra of 2 h and 20 h, the absorptions at 1045 cm−1 and 1047 cm−1 are corresponding to stretching vibration of P−OH, and the peaks at 582 cm−1 and 598 cm−1 belong to P−OH wagging vibrational modes, which elucidate the formation of CoHPi phase.27-28 Note the small peak appears at 787 cm−1 (24 h) can also be assigned to the bending mode outside the plane of the P−OH group.29 Comparing the intensities of the two peaks at 1045 cm−1 and 1047 cm−1, we can infer that the conversion from the -Co(OH)2 precursor to the 2D-CoHPi product is just started at 2 h (Figure 3a), and is much more completed at 20 h of the reaction. In addition, from the N2 physisorption isotherms presented in Figure 4, we can see a remarkable increase of the specific surface area (Brunauer−Emmett−Teller (BET) method) for the CoHPi sample, which is as high as 106.4 m2/g, while the specific surface area of -Co(OH)2 precursor is about 31.1 m2/g.

Figure 3. (a) XRD patterns and (b) FTIR spectra of Co(OH)2 precursor and 2D-CoHPi samples after reacting for 2 h and 20 h (see Section 2.3). In order to confirm the above transformation mechanism, we pursued a combined investigation with XRD and FTIR techniques. Indeed, the XRD patterns and FTIR spectra displayed in Figure 3 are correlated well with the observed evolution. For example, the -Co(OH)2 precursor has a typical hydrotalcite-like phase with two prominent low-angle reflections at 11.0o and 22.1o which belong to 003 and 006 reflections respectively. Considering the thickness of the platelet (~15 nm), it can be estimated that the Co(OH)2 platelet is comprised of 18−20 single brucite-like Co(OH)2 layers. After 2 h of reaction, the diffraction angles of this hydrotalcite-like solid precursor remain unchanged but the crystallinity decreases slightly. After 20 h, interestingly, the product becomes amorphous, accompanied with a significant morphological change (Figure 2g, h). Consistent with this phase evolution, the production of CoHPi phase is also evidenced by our FTIR results. Shown in Figure 3b, the large bands centered at 3500 cm−1 of these FTIR spectra can be assigned to the O−H stretching modes of interlayer water molecules and of H-

Figure 4. (a) N2 adsorption-desorption isotherms and (b) volumetric pore size distribution of -Co(OH)2 precursor and CoHPi samples.



Figure 5. TEM images of samples prepared at higher pH values: (a) and (b) pH = 10, (c) and (d) pH = 11, and (e) and (f) pH = 12, adjusted using a 10 M NaOH solution. From the above comparative studies, we found that the pH value and the concentration of Na2HPO4 solution are the two most critical factors for the preparation of ultrathin CoHPi nanoflakes. Specifically, the pH value of 1.0 M Na2HPO4 solution is about 8.6, but by adding NaOH solution with high concentration (10 M; Section 2.4), we can adjust its pH values to 10, 11 or 12. As reported in Figure 5, the dissolution of -Co(OH)2 precursor will be restrained at higher pH values, which inhibits the transformation of Co(OH)2 and thus the formation of CoHPi nanoflakes. When the pH value of 12 is adopted, the original platelet-like morphology of -Co(OH)2 can be well reserved, but the product is determined to be mainly phosphate anion intercalated Co-LDH (Co-Pi-OH; Section 2.4) by XRD, FTIR (Figure S4) and XPS measurements (Figure S5). In addition, as shown in Figure S6, the products turn to be rhombus nanosheets when a lower concentration of Na2HPO4 reaction solution is used, because the concentration of Na2HPO4 can also influence the pH value, which in turn gives a profound impact on the reaction speed of such a conversion.

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Figure 6. (a) iR-corrected polarization curves (b) Tafel plots of -Co(OH)2 platelet precursor, commercial RuO2, CoPi-OH and 2D-CoHPi obtained at different times in 1.0 M KOH. Table 1. Summary of the overpotentials (η) at 10 mA/cm2 and Tafel slopes for different electrocatalysts prepared in this work. electrolyte

 [V]

Tafel slope [mV dec-1]

-Co(OH)2

1.0 M KOH

0.346

110

CoHPi-1 h

1.0 M KOM

0.335

68

CoHPi-2 h

1.0 M KOH

0.325

41

CoHPi-3 h

1.0 M KOH

0.314

39

2D-CoHPi

1.0 M KOH

0.314

31

Catalysts

RuO2

1.0 M KOH

0.356

91

Co-Pi-OH

1.0 M KOH

0.352

106

CoHPi-500Air*

1.0 M KOH

0.324

39

CoHPi-500Ar*

1.0 M KOH

0.340

42

-Co(OH)2

pH = 7 PBS


307

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2D-CoHPi

pH = 7 PBS

RuO2

pH = 7 PBS

0.682

229 362

* Their LSVs and the relevant Tafel plots are shown in Figure S8.

In order to shed light on the effects of compositional and structural modifications to the CoHPi system, the above prepared samples were further tested for their electrochemical performance toward OER. Firstly, a comparison among these samples and a commercial RuO 2 was made in a standard three electrode system using Ag/AgCl as a reference electrode and Pt plate as a counter electrode in 1.0 M KOH electrolyte. During the scanning, the working electrode was continually rotated at 2000 rpm to remove generated oxygen gas and limit the diffusion effect. Linear scan voltammograms (LSV) were obtained with a scan rate of 10 mV/s using GCE modified with the corresponding catalysts, as demonstrated in Figure 6a. As a benchmark, the overpotentials (η) at the current density of 10 mA/cm2 and Tafel slopes (Figure 6b) in each case were extracted and compared in Table 1. As we can see, OER activities of these samples measured from 1 to 3 h have an increasing trend and the final CoHPi nanoflakes exhibit the highest OER activity among all the samples. More specifically, the overpotential required at a current density of 10 mA/cm2 is as low as 314 mV, whereas that of Co(OH)2 precursor and RuO2 are much bigger, 346 mV and 356 mV, respectively. In addition, the corresponding Tafel slope of 2D-CoHPi, 31 mV/decade, is much lower than the value of 110 mV/decade for -Co(OH)2 and 91 mV/decade for RuO2, even outperforming most of the transition metal based OER catalysts reported so far.30-32 We attribute the superior Tafel slope of 2D-CoHPi sample to the structural advantage of our ultrathin 2D morphology and the chemical modification with HPO42− (i.e., in replacing PO43−); both factors indeed efficiently facilitate mass transfer as well as electron and proton transfer in CoHPi nanoflakes. However, the sample of Co-Pi-OH shows a similar activity with Co(OH)2 precursor with an overpotential of 352 mV and a Tafel slope of 106 mV/decade which is much higher when compared with 2D-CoHPi. This can be ascribed to lacking the proton in the intercalated anion (PO43−, instead of HPO42−), and its similar crystal structure (Figure S3) and thick plate-like morphology (Figure 5e, f) with the original -Co(OH)2 precursor. In addition, the electrochemical performance of CoHPi nanoflakes after heat treatments (Section 2.5) under different temperatures (400oC, 500oC and 600oC) and atmospheres (air and argon) was also explored and demonstrated in Figure S8. From the results, we can see that the products obtained at 500oC show the best electrocatalytic activities among the three treatment temperatures regardless under air or argon atmosphere. Moreover, the electrocatalytic activity in air environment at 500oC, with an overpotential of 324 mV and a Tafel slope of 38.6 mV/decade, is superior to that in argon environment, with an overpotential of 340 mV and a Tafel slope of 42.2 mV/decade. This can be accounted for more irregular pores

present on the nanoflakes after heat treatment in air (versus in argon; Figure S6). Nevertheless, when comparing with the as-prepared 2D-CoHPi, the heated samples result in lower electrocatalytic activities in spite of the additional pores. From the XRD patterns (Figure S7), we find the CoHPi nanoflakes were turned into Co2(P2O7) after these heat treatments. Therefore, the lower electrocatalytic activities can be ascribed to losing the compositional merit of HPO42−, which further verifies our conclusion.

Figure 7. (a) TOF values of the catalysts as a function of overpotential for -Co(OH)2 platelet precursor, commercial RuO2 and final CoHPi nanoflakes (b) Chronopotentiometry curves at a constant current density of 10 mA/cm2 for CoHPi nanoflakes. To have a more quantitative and more objective evaluation of electrocatalytic activity, the turnover frequency (TOF) is also calculated by taking all the transition metal ions (i.e., cobalt ions in the current study) in the catalysts into consideration (the ICP results are given in Table S1). As displayed in Figure 7a, once again, the TOF of 2D-CoHPi keeps the highest and presents a much faster increase with overpotential than other samples, indicating the high activity of cobalt ions. In addition to the electrocatalytic activity, the stability of the as-prepared 2DCoHPi catalyst was investigated using



chronopotentiometric (CP) measurement at a constant current density of 10 mA/cm2. As displayed in Figure 7b, the high activity of 2D-CoHPi can be maintained well over a prolonged CP operation of 24000 s, though it is slightly deteriorated, but still comparable with other best electrocatalysts for OER reported in literature.12,30 Moreover, the electrochemical performances were also investigated using a phosphate buffer solution (PBS, pH = 7, Merck). From the LSV curves in Figure 8a, it can be seen that the 2D-CoHPi sample and its -Co(OH)2 platelet precursor both show lower onset potentials than commercial RuO 2, but the 2D-CoHPi sample exhibits the smallest Tafel slope of 229 mV/decade, which is 78 mV/decade smaller than Co(OH)2 platelet precursor and 133 mV/decade smaller than commercial RuO2 (Figure 8b), respectively. Note that the peaks around 1.5 V for 2D-CoHPi and -Co(OH)2 can be assigned to the precatalytic oxidation of Co species, the Co2+/Co3+ and the Co3+/Co4+ transitions, which are commonly observed prior to catalytic water oxidation. This quantitative comparison can further conform the faster mass transfer as well as electron and proton transfer in our CoHPi nanoflakes.

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Figure 8. (a) Polarization curves, (b) Tafel plots of Co(OH)2 platelet precursor, commercial RuO2 and final CoHPi nanoflakes in pH = 7 phosphate-buffered solution. In order to probe the mechanistic origin of this promoting effect of 2D-CoHPi sample on OER performance, we also carried out a detailed surface analysis for this sample using X-ray photoelectron spectroscopy (XPS). Firstly, the electronic states of each ion of the -Co(OH)2 precursor and prepared 2D-CoHPi sample were investigated. For Co 2p, the spectra of -Co(OH)2 precursor (Figure 9a) and 2D-CoHPi sample (Figure 9c), both display well separated 2p3/2 and 2p1/2 doublets owing to spin-orbit splitting together with shakeup satellite peaks. Particularly, in Figure 9a, the 2p3/2 branch could be further deconvoluted into two peaks with a dominating peak at 780.8 eV and a small peak at 782.5 eV, corresponding to Co−O and Co−Cl, respectively, which also indicates the Co oxidation states mainly to be Co2+.33-34 On contrast, the 2p3/2 branch in Figure 9c deconvoluted into a weaker peak at 780.9 eV and a stronger peak at 783.2 eV. Herein, the peak at 780.9 eV is similar with the peak at 780.8 eV in Figure 9a, which also corresponds to Co−O. However, the stronger peak at 783.2 eV is different from the small peak at 782.5 eV of Figure 9a. Actually, it is a characteristic of 2D-CoHPi, corresponding to the formation of Co−O−P.19, 35 In the high-resolution O 1s region, for -Co(OH)2 precursor in Figure 9b, the peak at 530.9 eV can be assigned to O ions in crystal lattice,30 which belongs to the O−Co, whereas the higher binding energy peaks at 531.8 eV and 532.8 eV are associated with surface hydroxyl and adsorbed water molecular.34, 36 For 2D-CoHPi sample displayed in Figure 9d, the peak at 530.8 eV can be assigned to O ions in crystal lattice as well, which corresponds to P−O−Co and O=P.37 In addition, the spectrum also possesses two strong peaks at 531.9 eV and 532.8 eV, associated with surface hydroxyl and P−O−H respectively, indicating a large amount of hydroxyls at surface. Furthermore, the component at 533.6 eV of O 1s spectrum is attributed to physisorbed and chemisorbed water at or near the surface, manifesting abundant water molecules at the surface.38 To further confirm the above findings, we also measured the wettability of CoHPi nanoflakes (Section 2.7). Probed with a Theta/Attension optical tensiometer, the 2D-CoHPi sample indeed has an average contact angle of nearly 0o (Figure 9f), revealing the presence of its superhydrophilic property. In Figure 9e, the broad P 2p peak, which should theoretically be an unresolved doublet with 2p1/2 and 2p3/2 components, was deconvoluted into two corresponding peaks at 134.9 eV and 133.4 eV respectively. In particular, the main component at 133.4 eV is attributed to the HPO 42− group.39 Therefore, the XPS results not only verify the composition of 2D-CoHPi, but also shows the existence of plentiful (P)O−H groups and water molecules on the surface which contribute to the observed superhydrophilicity.40


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Figure 9. (a) Co 2p and (b) O 1s XPS spectra of -Co(OH)2 platelet-like precursor, (c) Co 2p, (d) O 1s, (e) P 2p XPS spectra of the as-prepared 2D-CoHPi sample and (f) Photograph of the 2D-CoHPi substrate after loading a water droplet. To better understand the surface chemistry related to OER process, XPS measurements of CoHPi nanoflakes after OER test were also carried out. As displayed in Figure 10a, the 2p3/2 peak of Co species shifts to 279.3 eV indicating that the majority of Co species on the surface turn to be Co3+ after OER.41-43 Besides, the O 1s spectrum of the used sample in Figure 10b shows that the peak of the (P)O−H species at 532.9 eV becomes dominantly strong, while the peak of the P−O−Co species (at 530.8 eV) almost diminishes, which indicates the generation of a significant amount of surface phases of hydrated oxides or oxyhydroxides. Finally, the P 2p peaks of the 2D-CoHPi sample after OER test (Figure 10c) are totally vanished, indicating the generation of an oxide or

hydroxide-like surface overlayer. Generally speaking, these findings also correspond well with other reports for the first-row transition-metal-based electrocatalysts.30, 44-47 Based on the above results and in comparison with those reported in literature (Table S2), the improved catalytic performances for the CoHPi nanoflakes are believed to be derived from the following two factors. First, the CoHPi nanoflakes are much hydroxylated as a result of better deprotonation ability of HPO42−, and the surface hydroxyls serve as the active sites, leading to more surface hydrated oxides and oxyhydroxides phases. Second, the enhanced adsorption of H2O molecules onto the surface of



2D-CoHPi sample can further facilitate the OER process. Clearly, the ultrathin 2D structure and HPO 42− ions investigated in the current work are beneficial for faster proton and electron transportation as well as mass transfer, as investigated and demonstrated in this work.

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4. CONCLUSION In summary, in this work we have devised a synthetic method to prepare a highly efficient oxygen evolution catalyst, ultrathin 2D-CoHPi, through a chemical conversion of α-Co(OH)2 precursor at room-temperature. Based on an ex-situ investigation on intermediates and products at different reaction stages, we found that the conversion of αCo(OH)2 to 2D-CoHPi actually involves pre-dissolution and reprecipitation (i.e., regrowth), in which the hexagonal platelets of α-Co(OH)2 serve as both a Co metal source and a sacrificial template for the formation of CoHPi nanoflakes. Importantly, due to their ultrathin structure and the PCET merit of HPO42− anions, the CoHPi nanoflakes show enhanced OER performance. Through the further mechanistic study, the observed low overpotential, extraordinarily small Tafel slope as well as high endurability of 2D-CoHPi catalyst can be ascribed to several structural and chemical parameters such as large surface area, increased active sites, surface superhydrophilicity as well as fast electron/proton and mass transfers. This work demonstrates that there is still room of engineering product morphology and enabling anions with better PCET property for well-established electrocatalytic materials such as the cobalt-based phosphates and their related derivatives.

ASSOCIATED CONTENT Supporting Information Additional SEM images, TEM images, EDX, XRD patterns, XPS spectra, heating treatments, electrocatalytic measurements and ICP-MS data of the samples.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS J.J.W. would like to thank National University of Singapore for providing her postgraduate scholarship. The authors gratefully acknowledge the financial support provided by the Ministry of Education, Singapore, and National University of Singapore. This project is also partially funded by the National Research Foundation (NRF), Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) program.

REFERENCES Figure 10. (a) Co 2p, (b) O 1s, (c) P 2p XPS spectra of 2DCoHPi sample after OER test in 1.0 M KOH.

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