高学庆，杨树姣，张伟，曹睿

陕西师范大学化学化工学院，应用表面与胶体化学教育部重点实验室，西安 710119

1 Introduction

The conversion and storage of renewable energy can be realized by electrocatalytic water splitting into hydrogen and oxygen1–3. Owing to the multi-step proton-coupled electron transfer (PCET) processes4,5and sluggish kinetics of O－O bonding step6,7, OER has become the bottleneck limiting the overall efficiency of water splitting8–13. OER occurs in PSII of green plant in nature, where the CaMn4O5cluster is the active center14,15. Over billions of years, this system has evolved to be the most efficient water oxidation system16.

It is very important to learn from natural oxygen evolution catalysts for the development of artificial oxygen evolution catalysts17–26. The CaMn4O5cluster consists of Mn－O and Ca－O in a chair-shape cube, in which five oxygen atoms act as bridges connecting the five metal atoms27. This geometrically asymmetric structure facilitates structural rearrangement in OER processes and reduces the activation energy required for intermediate formation28,29. It should be noted that the CaMn4O5cluster is not isolated30. Two water molecules are connected to the Ca atom and the Mn atom outside the deformed cube,respectively31. In addition to water molecules, there are many amino acid residues around the CaMn4O5cluster16,32–34. Water molecules and these amino acid residues are connected to the CaMn4O5cluster by hydrogen bonds, which can not only stabilize the structure of the CaMn4O5cluster, but also play an important role in OER process35. Bonded water molecules can provide initial reactants, facilitating rapid reactions36. The surrounding amino acid residues facilitate the transfer of protons and electrons in the OER process and speed up the reaction rate35. Besides the four water molecules those are closely linked to the CaMn4O5cluster, it has been found that there are more than 1300 water molecules in a single PSII monomer37,38. Some of these water molecules, together with numerous amino acid residues, form an extensive hydrogen bond network and can serve as channels for protons39–42. Recently, many works have been done to simulate the structure of the CaMn4O5cluster and study its OER process19–21,23,24,30,43,44, but few simulate the structure and function of the hydrogen bond network around the CaMn4O5cluster.

In this work, an inorganic-organic hybrid material was synthesized to model the hydrogen bond network in PSII. We prepared amines and water interlayer manganese phosphate nanosheets, in which manganese phosphate with asymmetric structure mimics the effect of CaMn4O5cluster. Amines between layers simulate the amino acid residues, and inserted water molecules simulate water molecules around the CaMn4O5cluster. The hydrogen bond network formed by interlayer amines and water in the whole manganese phosphate nanosheets simulates the hydrogen bond network formed by rich amino acid residues and water molecules in PSII. Compared with manganese phosphate nanosheets with broken hydrogenbonding network (amine-free or water-free), manganese phosphate nanosheets with continuous and complete hydrogenbonding network exhibit the best OER performance.

2 Experimental

2.1 General materials

All reagents, including MnCl2·4H2O (99.0%, Fuchen, China),H3PO4(≥ 85.0%, Sinopharm Chemicals, China),ethylenediamine (EDA, 99.0%, Sinopharm Chemicals, China),NaH2PO4·2H2O (99.99%, Alfa, America), Na2HPO4(99.99%,Alfa, America), Na3PO4(≥ 98.0%, Sinopharm Chemicals,China), KOH (≥ 85.0%, Sinopharm Chemicals, China), and Nafion (5% (w, mass fraction), DuPont, America) were purchased from commercial suppliers and used without further purification. Milli-Q water of 18 MΩ·cm was used in all experiments.

2.2 Syntheses of materials

In a typical procedure, 4 mmol·L−1MnCl2∙4H2O and 71 mmol·L−1H3PO4were dissolved in 50 mL of water by ultrasonic treatment for 10 min. Then, 2.1 g of EDA was added slowly until a large amount of white precipitate was formed. The obtained manganese phosphate (EDAI)(H2O)MnPi sample was collected by centrifugation/washing with water several times and dried at room temperature.

For control studies, manganese phosphate (EDAI)MnPi and(H2O)MnPi samples were also synthesized. For the preparation of (EDAI)MnPi, the above prepared (EDAI)(H2O)MnPi sample was heat treated in muffle furnace for 30 min. The temperature was adjusted until water molecules between layers were completely removed.

For (H2O)MnPi, the preparation method is as follows: 50 mL mixed aqueous solution containing 4 mmol·L−1MnCl2·4H2O and 71 mmol·L−1H3PO4was treated by ultrasound for 10 min.Subsequently, 1.0 mol·L−1KOH aqueous solution was added slowly until a large amount of white precipitate was formed.After centrifugation and washing, manganese phosphate nanosheets (H2O)MnPi was obtained.

2.3 Physical characterizations

The powder X-ray diffraction (XRD) patterns were obtained with a Rigaku D/Max2550VB+/PC X-ray diffractometer using CuKαradiation (λ= 0.15406 nm) at 40 kV and 100 mA. The scanning electron microscope (SEM) images were observed on a Hitachi SU8020 cold-emission field emission SEM with an accelerating voltage of 5 kV. The transmission electron microscopy (TEM) images were taken using a FEI Tecnai G2 F20 field TEM operated at 200 kV. The samples were dispersed and loaded on carbon-coated copper grids for TEM analysis.Energy-dispersive X-ray analysis (EDX) and the elemental mappings were conducted on an AMETEK Materials Analysis EDX equipped on the SEM. Thermogravimetric analyses were carried out by heating the dry powder sample at a rate of 5 °C·min−1with nitrogen flow at 100 mL·min−1over 25 °C to 900 °C in a TA Instruments SDT Q600.

2.4 Electrochemical studies

All electrochemical experiments were performed using a CH Instruments Electrochemical Analyzer (CHI 660E) at room temperature. A standard three-electrode system, which consists of a graphite rod as the counter electrode, a saturated Ag/AgCl as the reference electrode, and the glassy carbon electrode as the working electrode, was used for all electrochemical experiments.Potentials in this study were reported against the reversible hydrogen electrode (RHE) based on the equation:ERHE=EAg/AgCl+(0.197 + 0.0591 × pH) V. Linear sweep voltammograms (LSV),at a scan rate of 50 mV·s−1, were recorded in a 15 mL of N2-saturated 0.05 mol·L−1pH = 7 phosphate buffered saline (PBS)solution. The LSV measurements were iR compensated at 100%.The proton conductivity of sample was studied by electrochemical impedance spectroscopy (EIS). The EIS was recorded under 1.9 V (vsRHE) from 0.1 Hz to 1 MHz at the amplitude of the sinusoidal voltage of 5 mV. The Nyquist plots were thus obtained based on the EIS data. Controlled potential electrolysis (CPE) test was recorded under the same experimental setup without the iR drop compensation. The working electrode was prepared through a drop-casting method and the typical procedure was as follows: a catalyst ink was prepared by dispersing 2 mg of the sample powder in 1 mL of water-ethanol solution at volume ratio of 2 : 1 containing 20 μL of Nafion solution. The mixture was treated by sonication until a homogeneous suspension was obtained. The working electrode was prepared by loading 5 μL of the catalyst ink evenly on the effective working area of the glassy carbon electrode and dried at room temperature. The mass loadings of the catalysts on the working electrodes were all 0.14 mg·cm−2.

3 Results and discussion

(EDAI)(H2O)MnPi was prepared at room temperature by coprecipitation of MnCl2·4H2O, H3PO4and EDA. In the preparation process, EDA regulates the pH of the whole system and acts as an organic base. In addition, EDA stabilizes the entire structure of (EDAI)(H2O)MnPi.

The morphology of the (EDAI)(H2O)MnPi sample was characterized by SEM and TEM. Aggregates of nanosheets are shown in Fig. 1a. The enlarged SEM (Fig. 1b) and TEM (Fig.1c) images show that the nanosheet has the length of about 1.5 μm and width of 700 nm. The X-ray diffraction (XRD) pattern of the sample is shown in Fig. 1d. The diffraction peaks of 2θat 8.1, 16.8 and 24.3 degree can be indexed to (200), (400) and(600) crystal planes of (C2H10N2)[Mn2(HPO4)3](H2O) with a monoclinic structure (CCDC 127292)45. AFM measurement shows that the thickness of (EDAI)(H2O)MnPi nanosheet is about 16 nm (Fig. 1e). This value corresponds to the thickness of about 7 unit cells of the monoclinal((C2H10N2)[Mn2(HPO4)3](H2O) (a= 2.1961 nm). The SEMEDX (Fig. 1f) of the (EDAI)(H2O)MnPi sample indicates that Mn, P, O, C and N elements are uniformly distributed in the whole sample.

Fig. 1 (a, b) SEM and (c) TEM images of (EDAI)(H2O)MnPi.(d) XRD pattern and (e) AFM image of (EDAI)(H2O)MnPi.(f) The SEM image and corresponding EDX elemental mapping images of (EDAI)(H2O)MnPi.

The crystal structure of (EDAI)(H2O)MnPi sample, namely(C2H10N2)[Mn2(HPO4)3](H2O), belongs to the monoclinic crystal system,P21/nspace group (a= 2.1961,b= 0.9345,c=0.6639 nm;β= 91.06°;V= 1.3623 nm3;Z= 4) (Fig. 2a). It contains three different kinds of manganese atoms (Fig. 2b).Mn(1) is five-coordinated, and forms an asymmetric triangular double cone with the surrounding oxygen. This asymmetric structure can reduce the energy of the structure rearrangement to form key intermediates, which is conducive to catalyzing oxygen evolution43. Mn(2) and Mn(3) are six-coordinated and form octahedron with the surrounding oxygen. The manganese atoms are bridged between phosphate oxygen and water oxygen.

The inorganic skeleton of (EDAI)(H2O)MnPi sample consists of [Mn2(HPO4)3]2−anions, forming manganese phosphate layers(Fig. 2a). Ethylenediamine cations [C2H10N2]2+(EDAI) are located between manganese phosphate layers to compensate the charge. In addition, water molecules are also located between manganese phosphate layers (Fig. 2c, left). The interlayer EDAI and water molecules are linked by hydrogen bonds to the oxygen atoms of the phosphate ions in the manganese phosphate layer(Fig. 2c, right). A large number of hydrogen bonds are also formed between [Mn2(HPO4)3]2−units within the manganese phosphate layer. Therefore, (EDAI)(H2O)MnPi has a rich,extensive and continuous hydrogen-bond network, which would have similar function to the hydrogen-bond network in PSII. The network of hydrogen bonds can accelerate the transfer rate of protons, thus expediting electrocatalytic water oxidation40,41.

Fig. 2 (a) Polyhedral view of (EDAI)(H2O)MnPi, showing the layered structure. (b) The coordination geometry of the three different Mn sites with Mn－O bond lengths (unit in Å, 1 Å = 0.1 nm)and (c) hydrogen bond networks in (EDAI)(H2O)MnPi.

Fig. 3 (a) The XRD patterns of (EDAI)(H2O)MnPi samples after heat treatment at different temperatures. (b) The XRD patterns of(EDAI)MnPi and (EDAI)(H2O)MnPi samples. (c) SEM image and(d) possible structure of (EDAI)MnPi sample.

The above prepared (EDAI)(H2O)MnPi sample was heattreated in muffle furnace for 30 min at different temperatures.The XRD patterns of these obtained samples are shown in Fig.3a. At 230 °C, the (200) diffraction peak splits, and a weak new peak with higher 2θvalue appears. Due to the loss of some lattice water, the interlayer space was reduced for partial layers of the sample46. At 235 °C, the original (200) diffraction peak is very weak, indicating that the lattice water molecules disappear completely. With the rise of temperature, the sample gradually becomes amorphous. On the basis of the above temperaturecontrol experiments, it can be concluded that the sample obtained at 235 °C does not contain water. This sample is denoted as (EDAI)MnPi (Fig. 3b). The morphology of(EDAI)MnPi was characterized by SEM. As shown in Fig. 3c and Fig. S1 (in Supporting Information), the morphology is almost unchanged and remains nanosheet structure as the(EDAI)(H2O)MnPi sample does. The possible structure diagram of (EDAI)MnPi is shown in Fig. 3d.

Fig. 4 (a) The XRD pattern and SEM image of (H2O)MnPi.(c) Polyhedral view and (d) coordination geometry of the three different Mn sites with Mn－O bond lengths (unit in Å) in Mn3(PO4)2·7H2O structure.

Fig. 5 The thermal gravimetric analysis (TGA) and derivative thermogravimetric (DTG) analysis studies of (EDAI)(H2O)MnPi,(EDAI)MnPi, and (H2O)MnPi samples.

The (H2O)MnPi sample without amine was obtained by substituting ethylenediamine with KOH during the synthesis.The XRD pattern of (H2O)MnPi is shown in Fig. 4a. (H2O)MnPi is a mixture of Mn3(PO4)2·7H2O and Mn3(PO4)2·3H2O. Due to the instability of Mn3(PO4)2·7H2O, it is easy to lose part of the crystal water and becomes Mn3(PO4)2·3H2O. The (H2O)MnPi sample has similar nanosheet morphology as the abovementioned two MnPi samples (Fig. 4b). Nam research group has analyzed the structure of Mn3(PO4)2·3H2O in detail, which contains six different kinds of manganese, two with distorted octahedral coordination and four with distorted trigonal bipyramidal geometry28. The structure of Mn3(PO4)2·7H2O is shown in Fig. 4c and 4d. Mn3(PO4)2·7H2O contains three different kinds of Mn sites, two octahedral coordination and one distorted five-coordination, which is similar to three Mn sites in(EDAI)(H2O)MnPi.

Thermal gravimetric analysis (TGA) and derivative thermogravimetric (DTG) analysis further show the difference between (EDAI)(H2O)MnPi, (EDAI)MnPi, and (H2O)MnPi. As shown in Fig. 5, (EDAI)(H2O)MnPi lost interlayer water mainly at about 150 °C, and lost water completely at 235 °C. The weightlessness at 300 °C is caused by the departure of interlayer ethylenediamine ions. For (EDAI)MnPi sample, the loss at 300 °C comes from the departure of ethylenediamine ions.(H2O)MnPi, without ethylenediamine ions, has no characteristic weightlessness of ethylenediamine ions at 300 °C.

The electrocatalytic water oxidation performance of these manganese phosphate nanosheets was characterized by linear sweep voltammetry (LSV) in a 0.05 mol·L−1pH = 7 PBS solution. As shown in Fig. 6a, compared with (EDAI)MnPi (610 mV) and (H2O)MnPi (580 mV), (EDAI)(H2O)MnPi shows a lower overpotential of 520 mV to drive a current density of 1 mA·cm−2. The catalytic oxygen evolution performance of(EDAI)(H2O)MnPi is the highest among the reported manganese phosphate OER catalysts in neutral solutions, as shown in Table S1 (in Supporting Information).

Fig. 6 (a) LSV polarization curves of (EDAI)(H2O)MnPi, (EDAI)MnPi, and (H2O)MnPi samples. (b) The anodic charging current at 0.84 V plotted against the scan rates, the slopes of which are capacitances of samples. (c) The normalized OER activity comparison of samples. The original activity is normalized by the ECSA of the materials determined in Fig. 6b. (d) CPE of the (EDAI)(H2O)MnPi at 1.86 V (vs RHE) without iR compensation.

The electrochemical surface area (ECSA) analyses of(EDAI)(H2O)MnPi, (EDAI)MnPi, and (H2O)MnPi samples are shown in Fig. 6b and Fig. S2 (in Supporting Information).Considering the different ECSA of different materials, the water oxidation activities of three materials were normalized by ECSA(Fig. 6c). It indicates that the (EDAI)(H2O)MnPi sample has the highest intrinsic OER activity among these manganese phosphate nanosheet samples. (EDAI)(H2O)MnPi contains both EDAI and H2O. EDAI may provide the receptor for proton transfer in OER39–41, and H2O may offer the initial reactant.More importantly, EDAI and H2O form a rich, extensive and continuous hydrogen-bond network. The network of hydrogen bonds has high proton conductivity46and increases the transfer rate of protons. EIS tests show that the proton conductivity of the(EDAI)(H2O)MnPi is higher than that of (EDAI)MnPi and(H2O)MnPi toward OER (Fig. S3, in Supporting Information),indicating that the rich hydrogen-bond network structure in(EDAI)(H2O)MnPi facilitates proton transfer in the electrocatalysis.

In order to study the stability of (EDAI)(H2O)MnPi in OER,constant voltage electrolysis (CPE) was performed without iR compensation. The CPE experiment showed that(EDAI)(H2O)MnPi remained active at least 10 h (Fig. 6d). The SEM image and XRD pattern of the (EDAI)(H2O)MnPi sample after OER electrolysis (Figs. S4 and S5, in Supporting Information) show negligible morphology and crystal structure change. It demonstrates that the (EDAI)(H2O)MnPi sample has good OER stability under neutral conditions.

4 Conclusions

We prepared manganese phosphate nanosheets with interlayer ethylenediamine ions and water molecules by a simple coprecipitation method at room temperature. Ethylenediamine ions and water molecules form a rich, extensive and continuous hydrogen bond network, which increases the proton transfer rate and plays a very important role in the OER process. Compared with manganese phosphate nanosheets (EDAI)MnPi and(H2O)MnPi with damaged hydrogen bond network, manganese phosphate nanosheets (EDAI)(H2O)MnPi exhibit enhanced OER activity in neutral conditions. This work may provide guidance for the design of water oxidation catalysts with rich hydrogen bond network.

Supporting Information: available free of chargeviathe internet at http://www.whxb.pku.edu.cn.