Catalytic Activity Boosting of Nickel Sulfide toward Oxygen Evolution

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Letter

Catalytic Activity Boosting of Nickel Sulphide towards Oxygen Evolution Reaction via Confined Overdoping Engineering Chao Han, Weijie Li, Chaozhu Shu, Hai-peng Guo, Huakun Liu, Shi Xue Dou, and Jia-Zhao Wang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00932 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on July 30, 2019

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ACS Applied Energy Materials

Catalytic Activity Boosting of Nickel Sulphide towards Oxygen Evolution Reaction via Confined Overdoping Engineering Chao Han,ǁ§ Weijie Li,ǁ§ Chaozhu Shu,ǁ‡ Haipeng Guo,ǁ Huakun Liu,ǁ Shixue Dou,ǁ Jiazhao Wangǁ* ǁ Institute for Superconducting and Electronic Materials, AIIM Building, Innovation Campus, University of Wollongong, Squires Way, North Wollongong, NSW 2500, Australia. ‡

College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, 1 Dongsan Lu, Erxianqiao, Chengdu 610059, Sichuan, P. R. China.

KEYWORDS: nickel sulphides, confined overdoping, OER, porous carbon substrate, nanodomains, Zinc-Air battery

ACS Paragon Plus Environment

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ABSTRACT: Electro-catalysis for the oxygen evolution reaction (OER) plays an irreplaceable role in numerous, green and efficient energy conversion or storage techniques such as water electrolysis, fuel cells, and metal–air batteries. High-performance catalysts are always needed despite the sluggish kinetics of four electron-transfer OER process. In this paper, for the first time, employing a simple, new strategy of “confined Fe overdoping”, the OER activity of Ni3S2 in alkaline solution is significantly boosted, showing overpotential of 350 mV at 10 mA.cm-2, which is even lower than the benchmark IrO2. The designed catalyst (Meso C-NiFeS) is composed by mesoporous highly graphited N doped carbon and nanodomains/defects/strains rich NiFeS nanoparticles. The mesoporous carbon support facilitates mass/electron transfer, while confined Fe overdoping leads to smaller and defects/strain rich nanodomains. DFT calculations proved that Fe doping could induce compressing strains, which is benefit for OER process; modify electronic states of Ni3S2 and act as active sites at the same time. This overdoping strategy can trigger synergic effect combining size decreasing, electronic structure modifying and defects/strain engineering. Moreover, this simple strategy is easy to be implanted to other catalysts.

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With the gradual depletion of fossil fuels and growing crisis on environmental pollution, there is more and more urgent need for exploiting the green and renewable energy conversion and storage systems, such as alkaline water electrolysis, fuel cells, and metal–air batteries. However, one important factor that keeps these systems from wide applications to date is the sluggish kinetics (high overpotential) of the four electron-transfer oxygen evolution reaction (OER).1-8 For example, rechargeable Zinc–Air battery (ZAB) is considered as one of the most promising advanced energy storage technologies due to its advantages of high energy density of 1086 Wh.kg-1 (four times that of the state-of-the-art lithium-ion battery), low cost, and environmental friendliness.1-8 One of the most significant barriers for wide application of rechargeable ZABs is the high charging potential, which is originated from the sluggish oxygen evolution reaction (OER) on air cathode in nature.9 Although noble metals such as IrO2-based materials are well known as the most efficient OER catalysts, their scarcity, high cost, and poor stability limit their widespread utilization.10 Transition metal sulphides (TMSs), particularly cobalt and nickel sulphides, have been extensively studied due to their earth abundance and theoretically high catalytic activity towards OER,2, 10-16 however, the performances of TMSs are still unsatisfactory compared with benchmark IrO2. Since the oxygen evolution reaction involves a complex electrochemical process at the threephase interface of a gas (O2), a liquid (OH-), and a solid (catalyst), building a high specific area mass/electron transfer substrate for these processes has become a consensus for catalysts design.17 Furthermore, decreasing the size of the catalysts, increasing the intrinsic catalytic activity; while introducing enough amounts of strains or defects have all been recognized as effective measures to further increasing performance of catalysts.18-20 Until now, however, there is no any report to accompany these three benefits simultaneously.


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In this paper, simply by employing the strategy of “confined overdoping”, the three benifits are synergically realized and a high active catalyst towards the OER in alkaline solution is obtained. The designed catalyst is composed by embedding Fe overdoped Ni3S2 nanoparticles in a highly graphitized, N doped mesoporous carbon substrate. The porous carbon substrate facilitates the mass transfer of O2 and OH-, improves the electrical accessibility of active sites; and isolates catalytic nanoparticles from aggregation. And different from the traditional low amount doping (or common doping), the high-amount doping (overdoping) of Fe into Ni3S2 lattice induces the composition segregation. Finally, the composition segregation in a confined carbon shell not only facilitates formation of smaller nanodomains, modifies the electronic structure of different nanodomains, but also introduces strians, lattice distortions and grain boundaries. Proved by rotating disk electrode (RDE) tests and Zinc-Air battery tests, the overdoped catalyst exhibited boosted catalytic activity towards the OER in alkaline solution than the pristine Ni3S2. As presented in Figure 1, environmentally benign and low-cost chitosan was hydrolysed into a clear gel with the help of acetic acid. After adding of metal salt, the chitosan gel was mixed with mercaptosuccinic acid (MSA) decorated silica nanoparticles, and the metal ions were attracted onto silica nanoparticles. Then, the gel was dried and carbonized twice to enhance the electrical conductivity.21 Finally, a sulphide nanoparticle-decorated mesoporous carbon structure was obtained after removing the silica templates using a strong alkaline solution. The X-ray diffraction patterns of the three samples are respectively shown in Figure S1(a). It is clearly seen that the widened diffraction peaks of Meso C could be ascribed to the graphite-2H (JCPDS 411487), indicating the highly graphitic nature of the Meso C sample. The Meso C-NiS could be indexed to Ni3S2 (JCPDS 44-1418); while the pattern of Meso C-NiFeS shifted slightly towards left comparing with Meso C-NiS sample due to the increased lattice parameters. The scanning


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electron microscope (SEM) images of the three samples look almost the same [Figure S1(b-d)], showing uniform meso-pores with diameter around 80 nm, which coincides with the size of the silica templates [Figure S2]. Compositions of the three samples were also analysed using energydispersive X-ray spectroscopy (EDS). The result was listed in Table S1. Besides C, N, Ni, Fe, O, and S, Si is also identified. The concentration of N is between 1-2 at.% ; atom ratio of Ni and S in Meso C-NiS sample is around 3:2. Negligible S (0.202 at.%) is found in the Meso C sample comparing with the other two.

Figure 1. Schematic illustration of the synthesis procedure. Consistent with the XRD and SEM results, the low and high magnification dark field transmission electron microscope images of the Meso C sample showed highly graphitized, mesoporous structure [Figure 2(a-b)]. The fast Fourier transform (FFT) pattern in Figure 2(b) presents an obvious diffraction ring with a diameter of 6 nm-1, corresponding to a lattice fringe spacing of 0.33 nm [Figure 2(c)], which is close to standard layer spacing of graphite (0.335 nm).


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Figure 2(d) shows a typical area with three adjacent pores and a graphitic wall around 6.48 nm in thickness. Compared with the Meso C sample, besides the mesopores, obvious black dots are observed in the Meso C-NiS sample [Figure 2(e)]. High-magnification TEM image [Figure 2(f)] identified these black dots to be nanoparticles around 5-10 nm with a 2 nm layer of graphitized carbon coating. This layer of carbon is formed due to the carbonazation of fluidity chitosan gel. The carbon layer effectively hinders the growth and agglomeration of sulphide nanoparticles via a self-limiting effect (or confinement), which is pretty important for stability of the nanoparticles and catalytic activity boosting. At the same time, the graphite layer also offers excellent electrical contact between the nanoparticles and the carbon substrate. The inversed fast Fourier transform (IFFT) image of the area enclosed by the white square in Figure 2(g) displays clear and intact lattice fringes with interplanar spacing of 0.204 nm and 0.235 nm, corresponding to (202) and (021) planes of Ni3S2 phase (JCPDS 44-1418), respectively. The calculated angle between the two planes (72°) coincides with real measured value (73°); confirming the validity of the indexing. Two clear peaks located at 857.29 eV and 875.07 eV in the zero-energy loss peak calibrated electron energy loss spectrum (EELS) of Ni element in the Meso C-NiS nanoparticles coincide with the reported data on Ni3S2 [Figure 2(h)].22-24 Elemental mapping of the Meso C-NiS sample was shown in Figure S3. It can be seen that the mesoporous carbon substrate also contains nitrogen, while the nanoparticles are sulphur- and nickel-rich. Table S2 displays the Ni and S atomic ratio extracted from the mapping of a Meso C-NiS nanoparticle is Ni (57.80 at.%) and S (42.20 at.%), close to 3:2, coincides with Table S1 and the TEM index results in Figure 2(g); indicating the formation of well-crystallized Ni3S2 nanoparticles in the Meso C-NiS sample.


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Basic structure of the Meso C-NiFeS sample is also mesoporous carbon embedded with wellcrystallized nanoparticles ranging 5-10 nm [Figure 2(i-j)]. However, unlike the uniform and nea-

Figure 2. (a) Dark field low-magnification TEM image of the Meso C sample; (b) Highmagnification TEM image of the Meso C sample; (c) Fast Fourier transform (FFT) pattern of (b), where a diffraction ring is detected, implying a highly graphitic sample with a lattice spacing of 0.33 nm; (d) High-magnification TEM image showing walls of the Meso C sample; (e) Lowmagnification bright field TEM image of the Meso C-NiS sample; (f) High-magnification TEM


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image of a nanoparticle in the Meso C-NiS sample; the dashed red line indicates the real nanoparticle, while the outer shell is a layer of graphitic carbon; (g) Inverse FFT image of the area enclosed by the white square in (f), the zone axis is indexed to [-2,-1,2]; (h) Electron energy loss spectrum (EELS) of Nickel element in the Meso C-NiS nanoparticle; (i) Low-magnification bright field TEM image of the Meso C-NiFeS sample; (j) High-magnification TEM image of nanoparticles in the Meso C-NiFeS sample; the dashed red line indicates the real nanoparticles, while the outer shell is a layer of graphitic carbon; the white dashed line indicates different regions in the nanoparticle; (k) Inverse FFT image of the region enclosed by the white square in (j); (l) Electron energy loss spectra (EELS) of Nickel and Iron elements, corresponding to different parts of the nanoparticles shown in (j), the black arrows mark the shifting of Fe L3 peak; (m) Element mapping of a nanoparticle in the Meso C-NiFeS sample. -lly perfect single crystalline Ni3S2 nanoparticles of the Meso C-NiS sample, the nanoparticles in Meso C-NiFeS exhibited obvious boundaries and lattice distortion, [Figure 2(j-k)]. Although the EELS of different areas in Figure 2(l) both showed Fe and Ni peaks, different intensities and a slightly deviation of the Fe peaks in regions A, B, and C indicated different Fe content and environments. The Fe and Ni EELS spectra matched well with reported FeS phase and Ni3S2, respectively.22-26 Distinct Fe-rich and Ni rich-areas could be detected from the element mapping of the NiFeS nanoparticle [Figure 2(m)]. The deviation in the composition could also be verified by the line scanning and EDS results of different regions shown in Figure S4, from which the darker and whiter parts are Fe-rich and Fe-poor phase, respectively. Based on EELS and Figure S4, composition of region A, B, C shown in Figure 2(j) is simply defined as Ni2Fe0.12S, Ni2.25Fe0.75S2, and Ni2FeS3.4, respectively. This composition segragation is caused by overdoping of Fe and complex Ni-Fe-S phase diagram [Figure S5]. Moreover, as illustrated in Figure S6,


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this composition segragation phenomenon occurs in most nanoparticles. The composition deviation leads to new phases, clear boundaries and lattice distortions in Figure 2(j-k). Geometric phase analysis (GPA) is also conducted using Digital Microscopy software pack. The strain distribution is determined using selected area as a reference (or zero-strain area). As shown in Figure 3, comparing with nearly-zero strain Ni3S2 nanoparticle, strains (or lattice distortion), either tension (Green to White, lattice expansion) or compressing (Green to Black, lattice shrink), are introduced after the overdoping of Fe. These high energy strains could increase active surface area and affect catalytic activity according to references and following theory calculations.20, 27

Figure 3. Geometric phase analyze of the (a) Ni3S2 nanoparticle and (b) NiFeS nanoparticle. The red square areas are selected as reference (or non-strained area). Corresponding color maps are also shown.


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To further analyze the compositions of the samples, X-ray photoelectron spectroscopy (XPS) was also conducted. As presented in Figure S7(a), no other peaks apart from Fe, Ni, C, N, O, and S were found in the three samples. The compositions of each sample were calculated from the whole survey and summarized in Table S3. Unlike the results shown in Table S1, no Si is found, and the element ratios are different because XPS is a general surface analysis tool, while the penetration depth is rather limited comparing with EDS. The C 1s peaks of the three samples shown in Figure S7(b) could be de-convoluted to four peaks: graphitic C (-C-C-) at 284.6 eV, Nsp2 C at 285.4 eV, N-sp3 C at 286.7 eV and oxidized C (-C-O-) at 288.8 eV.28-29 The high content of graphitic C is in agreement with the XRD and Figure 2(b-c). The ratios of different types of carbon are shown in Table S4. Both the EDS [Table S1] and XPS result [Figure S7(a) and Table S3] showed that the mesoporous carbon substrate is Nitrogen doped, with the atomic ratio ranging from 1.1 to 3.6 %. The N 1s core spectra [Figure S7(c)] shows that the element N exists in the forms of pyridinic N (398 eV), pyrrolic N (400 eV), graphitic N (401 eV), and oxidized N (403 eV).28-31 The graphitic N could improve the electrical conductivity of the carbon substrate, while pyridinic N and pyrrolic N could serve as metal-coordination sites. It is widely accepted that graphitic-N could increase the limiting current density, while pyridinic-N could improve the onset potential for the OER.32 It is easy to conclude from Table S5 that from Meso C to Meso CNiFeS, the content of pyridinic N and pyrrolic N decreased from 36.72 at.% to 27.53 at.% and from 42.49 at.% to 26.53 at.%, respectively; while the graphitic N increased from 14.00 at.% to 36.65 at.%. The two major peaks at 164.1 and 165.2 eV in Meso C are assigned to S 2p3/2 and S 2p1/2 spectra for the -C-S-C- covalent bonds of thiophene-type Sulphur, while the peaks at 166.9 eV and 168.3 eV are consistent with –C-S(O)2-C-sulfone bridges.33-34 Besides oxidized S, in the


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Meso C-NiS and Meso C-NiFeS samples, S mainly exists in the form of S2-, as indicated by two peaks at 162.5 eV and 164.1 eV. The intensity of the S spectrum in the Meso C sample is lower than for the other two samples, indicating lower S content in the Meso C sample, which agrees with the results shown in Table S1 and Table S3. No Fe peaks are found in the Meso C and Meso C-NiS samples. For the Meso C-NiFeS sample, the main peaks at around 711 eV and 724 eV are assigned to 2p1/2 and 2p3/2 peak of Fe2+ in FeS phase, while the two peaks at 716 eV and 730 eV are the satellite peaks.25-26 The XPS peaks of Meso C-NiS at 854.3 eV and 871.4 eV were assigned to the Ni 2p3/2 and 2p1/2 peaks of Ni3S2.22-24 The other two peaks at 859.7 eV and 877 eV are the corresponding satellite peaks. The Ni 2p3/2 peak of the Meso C-NiFeS shifts 0.6 eV to lower binding energy compared with the Meso C-NiS sample because of the Fe doping, indicating the existence of electronic interactions between Ni and Fe cations. The XPS spectra of Fe and Ni also coincide with the EELS data in Figure 2(h) and (l).


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Figure 4. (a) OER performance of the samples in 1 M KOH at a rotation rate of 1600 rpm; (b) Tafel curves for the OER; (c) Chronoamperometric stability of Meso C-NiFeS for the OER at 1600 rpm in 1 M KOH; (d) Measured electrochemical double-layer capacitance (Cdl) from current-voltage curves at different scanning rates. The oxygen evolution reaction (OER) performance was evaluated by linear sweep voltammetry (LSV) in Ar-saturated 1 M KOH at a scan rate of 10 mV/s. As shown in Figure 4(a), the Meso C-NiFeS shows the best OER performance with the lowest overpotential of 350 mV to achieve the current density of 10 mA∙cm−2, which is 54 and 87 mV lower than those of Meso C-NiS and Meso C, respectively. All three samples are more effective than commercial IrO2.35-37 The Meso C-NiFeS sample also exhibits the Tafel slope of 93 mV.dec−1, which is higher than benchmark IrO2 but significantly lower than Meso C and Meso C-NiS sample


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[Figure 4(b)]. The high OER catalytic activity of Meso C-NiFeS sample is also proved by the electrochemical impedance spectra (EIS) tested at overpotential of 350 mV. The Nyquist plots and the equivalent circuit are also presented in Figure S8. The Rs, Rct, CPE and Wo represent the contact resistance of electrode, charge transfer (reaction) resistance at the interface, constant phase element and the Warburg diffusion element, respectively. Figure S8(a) demonstrated the EIS of Meso C, Meso C-NiS and Meso C-NiFeS sample in the first cycle of OER test. The fitted Rct of Meso C, Meso C-NiS and Meso C-NiFeS reach 1725 Ω, 573 Ω and 101 Ω, respectively, corresponding to the OER catalytic activity sequence of Meso C < Meso C-NiS < Meso C-NiFeS. A comparison table is displayed in Table S6, the OER catalytic activity is higher than the benchmark IrO2 and some transition metal based compounds.38-40 The mass activity and turnover frequency (TOF) of the catalysts were calculated to evaluate their catalytic performance, on the assumption that all metal sites and N sites in the carbon substrate are both effective for the OER reactions. Detailed calculations are presented in Supporting Information. Thermogravimetric analysis (TGA) [Figure S9] was performed in air to calculate the weight ratios of the metal sulphides and mesoporous carbon. The Meso C sample contains 92.24 wt.% C and 7.76 wt.% residual silica template sealed in the mesoporous carbon substrate. The weight increase of Meso C-NiS and Meso C-NiFeS from 100 °C to 400 °C is ascribed to oxidation of the sulphides into sulphates, while the following weight decrease is due to decomposition of sulphates to oxides.41 The calculated weight ratios of sulphides in Meso CNiS and Meso C-NiFeS were found to be 41.63 % and 40.44 %, respectively. The calculated OER mass activity and TOF for Meso C-NiFeS are 786.9 A∙g−1 and 0.055 s−1 (at 1.6 V vs. RHE), respectively [Figure S10(a-b)]; 2.73 times and 1.65 times higher than the Meso C-NiS sample. The catalysts also demonstrate excellent OER stability in alkaline solution up to 20000 seconds


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[Figure 4(c)], proved by almost constant potentials at a set current density of 5 mA∙cm−2. The morphology of the Meso C-NiFeS sample after the chronoamperometric test is shown in Figure S11. The basic structure that the nanoparticles embedded in a mesoporous carbon substrate shows almost no change after the electrochemical stability study [Figure S11(a-c)]. Moreover, the overdoping induced small nanodomains still existed, as presented by the EELS and element mapping results in Figure S11(d-e). All the results demonstrated the robustness of the designed catalysts. The EIS of Meso C-NiFeS before (Initial) during (10000 s) and after (20000 s) the chronoamperometric test are displayed in Figure S8(b). The Rct of Meso C-NiFeS sample just increased slightly from 101 Ω to 112 Ω after 20000 seconds of stability test, demonstrating a good stability. To reveal the reason for the boosted OER catalytic activity of Meso C-NiFeS than Meso NiS, several characterizations including Raman spectroscopy, specific area analysis, pore size analysis, and electrochemical active surface area (ECSA) measurements were performed. The Raman spectra (Figure S12) and BET curves (Figure S13) illustrate that the Meso C, Meso CNiS, and Meso C-NiFeS have almost the same degree of graphitization [same ratio of the D to G band intensities (ID/IG)], surface area (around 83-85 m2/g), and pore-size distribution (mainly around 80 nm). As described by the XPS results on N element, however, the higher content of graphitic N in Meso C-NiFeS leads to higher electrical conductivity. This is related to the higher catalytic activity of Fe towards the formation of graphitic N.42-43 Furthermore, the ECSA is evaluated by double layer capacitance (Cdl) method [Figure 4(d)]. The Cdl of Meso C-NiFeS (13.79 mF∙cm-2) is higher than those of Meso C (3.46 mF∙cm-2) and Meso C-NiS (10.52 mF∙cm2),

indicating a larger effective electrochemical surface area. This corresponds with the small

nanodomains and rich defects in NiFeS [Figure 2(j-k)], caused by confined overdoping with Fe.


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In addition, compared with the pure phase of Ni3S2, Fe doping could also modify the electronic status and enhance the catalytic activity, as proved by following theoretical calculations. To further uncover the effect of Fe doping on OER in alkaline solution, the first principle density functional theory (DFT) calculations have been performed. In the first step, to simplify the situation, the pristine Ni3S2 and Ni2.25Fe0.75S2 [Region B in Figure 2(j)] are selected as target materials. 20 different atomic configurations for Ni2.25Fe0.75S2 supercell with 24 Cation sites are enumerated and ranked with formation energy. Finally, their most stable structures are depicted in Figure 5(a). Comparing the optimized cell dimensions of Ni3S2 and Ni2.25Fe0.75S2 (Table S7), it is easy to deduce that intrinsic compressing strains are aroused by the Fe doping. Based on their structures, the corresponding projected density of states (PDOS) are also calculated and illustrated in Figure 5(b). With the introduction of Fe, the DOS of pristine Ni3S2 is largely modified. To be more specific, Fe atoms create new states between the band gap of pristine Ni3S2, leading to improved electrical conductivity.


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Figure5. (a) Crystal structure of the pristine Ni3S2 lattice and most energy favorable Ni2.25Fe0.75S2; (b) Projected density of states (PDOS) of Ni3S2 and Ni2.25Fe0.75S2; (c) Four different adsorption sites of OH on the most active (110) face, adsorption energy of each site is also calculated; (d) Calculated reaction path of pristine Ni3S2 and Ni2.25Fe0.75S2. Hydroxyl group adsorption at {100} and {110} surfaces are evaluated as they showed lower (less negative) adsorption energy. And for each of the surface, two types of symmetrically inequivalent atomic orientations are evaluated, as visualized in Figure S14. Finally the {110}


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surface II is turned out to the most active as it shows the most negative binding energy with OH group. Further calculations [Figure 5(c)] proved that Fe sites would attract strongly of OH group compared with Ni sites. Thereby Fe sites are proved to be more active during OER process. Moreover, the optimized size parameters of basic calculation cell [box shown in Figure 5(c)] before and after adsorption of OH group are listed in Table S8. The adsorption of OH leads to lattice shrunk and compression strain. The intrinsic compression strain in the Ni2.25Fe0.75S2 nanoparticles would promote adsorption of OH group; contribute significantly to higher OER performance. 27 On base of the previous calculations, the reaction paths are constructed to demonstrate the energy evolution during OER [Figure 5(d)]. Energy difference between the four steps under open circuit potential (U=0 V, magenta line), equilibrium OER potential (red line) and minimum potential where all steps are downhill (minimum OER potential, blue line), were calculated. The calculated OER overpotential for pristine Ni3S2 and Ni2.25Fe0.75S2 are 340 mV and 180 mV respectively, indicating higher OER activity after Fe doping. In all, the highly graphitized mesoporous carbon substrate and thin carbon layer coating provide enough surface area and electrical conductivity for the catalysts; improve the electron transfer and mass transfer of O2 and OH-; offer excellent stability for the catalytic nanoparticles in alkaline solution; isolate the nanoparticles and prevent aggregation. Besides, the “confined Fe overdoping” leads to composition deviation, which could introduce small nanodomains, modified electronic states, as well as compressing strains, enhancing intrinsic catalytic activity towards the OER reaction.


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Figure 6. (a) Discharge-charge polarization curves for Zn-Air batteries at different current densities; (b) Electrochemical impedance spectra for the assembled Zn-air batteries, with the upper right inset showing the spectrum of the Meso C-NiFeS sample and the lower right inset the equivalent circuit; (c) Cycling performance of ZABs using Meso C-NiS and Meso C-NiFeS as air cathode. To further prove the effectiveness of the designed catalysts towards OER in alkaline solution. The performances of the alkaline electrolyte based ZABs using Meso C-NiFeS and Meso C-NiS catalysts are shown in Figure 6. The open circuit voltage of Meso C-NiS and Meso C-NiFeS reached 1.19 V and 1.21 V in air, respectively. Meso C-NiFeS showed the lowest charging voltage, indicating the least polarization at different charge/discharge current densities and best catalytic activity towards the oxygen evolution reaction (OER) [Figure 6(a)]. Electrochemical impedance spectroscopy (EIS) tests were conducted to investigate the electrochemical kinetics. The Nyquist plots and the equivalent circuit are presented in Figure 6(b). As presented in Table S9, Rct of the Meso C-NiFeS (12.3 Ω) was lower than the Meso C-NiS (41.6 Ω) and Meso C (420 Ω), demonstrating the high catalytic activity of Meso C-NiFeS. The cycling performances of ZABs using Meso C-NiS and Meso C-NiFeS as air cathode are plotted in Figure 6(c). The Meso C-NiFeS sample showed higher discharge voltage and lower charge voltage than the Meso C-NiS sample throughout all the cycles. After 300 cycles (around 100 h), the discharge and


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charge voltage of the Meso C-NiS sample increased from1.09 V to 1.12 V and from 2.00 V to 2.10 V, respectively, while those of Meso C-NiFeS increased from 1.13 V to 1.14 V and 1.98 V to 2.06 V, respectively. The voltaic efficiency of Meso C-NiFeS was 57.1% in the first cycle and 55.3% at the 300th cycle, showing a superb stability. The performance of the ZABs using the Meso C-NiFeS stands out the Meso C-NiS sample. In summary, a simple confined overdoping strategy is proposed to fully release the potential of Ni3S2 towards oxygen evolution reaction (OER) in the alkaline solution. The designed catalyst is obtained by encapsulating Fe overdoped Ni3S2 nanoparticles in an N doped mesoporous carbon substrate. Porous carbon facilitates the mass/electron transfer, and improves the electrical accessibility of active sites. The confined Fe overdoping makes composition deviation, leading to small nanodomians with modified electronic structure while introducing defects and intrinsic strains in lattice, causing the enhancement of intrinsic activity. The rotating disk electrode measurements showed that the Fe overdoped composite possessed higher catalytic activity towards the OER both than its undoped counterpart and benchmark IrO2. Moreover, at a current density of 2 mA∙cm−2, the Zn−Air battery based on this deliberately designed catalyst showed lowest charging voltage; a small overpotential of 0.84 V and excellent stability over 300 cycles (100 hours) without obvious increase in the round-trip overpotential, outperforming its counterpart based on the undoped sample. All these results indicate the intriguing potential of confined overdoping for developing catalysts with high activity and stability. Experimental Methods All the chemicals are purchased from Sigma-Aldrich Pty. Ltd. and used without further purification.


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Synthesis of micro silica spheres as template: 225 ml ethanol, 75 ml H2O, and 10 ml NH3∙H2O (28 wt.%, ≥ 99.99%, trace metals basis) were mixed and stirred at room temperature for 5 min, and then 9 ml tetraethoxysilane (TEOS) (≥ 99%) was added dropwise into the mixed solution, which was followed by continuous stirring for 24 hours. The silica template was centrifuged at 15000 rpm and re-dispersed in 200 ml water via ultrasonication. Then, the resultant turbid suspension was further mixed with 0.9 g mercaptosuccinic acid (97%) by stirring to make precursor A. Synthesis of Meso C-NiFeS: The typical synthesis procedure for Meso C-NiFeS started from mixing 1.2 g chitosan (medium molecular weight), 10 ml acetic acid (≥ 99%), and 190 ml water to obtain a homogeneous clear solution (precursor B). Then, 50 ml precursor B, 50 ml precursor A, 0.3 mmol NiCl2 (98%), and 0.1 mmol FeCl2 (97%) were mixed by stirring. After evaporating the water from the mixture, the gel was carbonized at 750 °C for 4 hours in argon with a heating ramp of 5 °C/min. The collected black powder was ground and annealed for a second time at 850 °C for 4 hours to improve the crystallinity of the carbon. The sample was then washed with hot KOH solution (4 M) for 24 h to remove the silica templates before being washed with HCl solution (1 M) to remove alkaline and metal nanoparticles. Finally, the sample was washed with distilled water, followed by drying in a vacuum oven. For the Meso C-NiS sample, all the procedures were the same, except that only 0.3 mmol NiCl2 was added into precursors A and B. As a comparison, a Meso C sample was also synthesised without the addition of a metal salt. Physical characterization: The materials phase and crystallinity were determined by X-ray diffraction (XRD), which was carried out at room temperature with an X-ray diffractometer (GBC-MMA) using Cu-Kα1 radiation (λ = 0.153 nm). The microstructures and compositions of the samples were characterized by scanning electron microscopy (SEM; JEOL JSM-7500FA


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operated at 10–20 kV) and aberration-corrected transmission electron microscopy (TEM; JEOL JEM-ARM200F operated at 200 kV), along with energy dispersive X-ray (EDS) spectroscopy. TEM samples were prepared by dispersing the samples in ethanol. One drop of the suspension was then dropped onto a holey carbon grid, and the ethanol was allowed to evaporate. X-ray photoelectron spectroscopy (XPS) was conducted using a SPECS PHOIBOS 100 Analyser installed in a high-vacuum chamber with the base pressure below 10–8 mbar. All the spectra were calibrated with C 1s at 284.6 eV. Analysis of the XPS data was carried out using the commercial Casa XPS 2.3.15 software package. For specific surface area and pore size analysis, the samples were first degassed at 180 °C for 5 hours and then put into liquid nitrogen for Brunauer-EmmettTeller (BET) testing. The adsorption branch in the relative pressure range of P/P0 < 0.3 and the Brunauer−Emmet−Teller model were applied to the isotherms to determine the apparent surface area. Thermogravimetric analysis (TGA) was conducted on a TGA2 machine (from Mettler Toledo Inc.) using an alumina crucible from room temperature to 800 °C with the heating rate of 10 °C/min in air. Catalytic performance test: All electrochemical measurements were carried out on a conventional three-electrode system with a rotating disk electrode (Biologic SP-300). For the OER tests, an Hg/HgO/0.1M NaOH electrode was used as the reference electrode, and platinum was used as the counter electrode. 1 mg of the samples and 50 μl 5% Nafion solution were put into 450 μl isopropanol and dispersed by ultrasonication for at least 60 min to form a homogeneous ink. After that, 5 μl of this ink was carefully dropped onto the glassy carbon electrode (GCE) with a surface area of 0.196 cm2 and dried in air atmosphere to form a uniform catalyst mass loading of about 0.05 mg∙cm−2. As a comparison, IrO2 electrode was also prepared using the same mass loading of 0.05 mg.cm-2 of IrO2. The potentials were converted to reversible


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hydrogen electrode (RHE) by using ERHE = EAg/AgCl + 0.059 × pH + 0.164 V. The polarization curves of the OER were recorded with a scan rate of 10 mV.s−1 and a rotation rate of 1600 rpm in Ar saturated 1 M KOH (purged with pure Ar for at least 20 min). The electrochemical impedance spectra (EIS) was performed with the three-electrode system in 1 M KOH solution, with an overpotential of 350 mV and the frequency ranging from 100 KHz to 0.1 Hz. An iR compensation of 95 % is applied for the entire catalytic performance test. Zinc-Air battery performance test: To make the air cathode for the ZAB, a gas diffusion layer (GDL) was first prepared in the following steps. Carbon black and polytetrafluoroethylene (PTFE, 1 μm) were mixed in ethanol with a weight ratio of 3:7 to make a paste, and the paste was then coated onto cleaned nickel foam using a doctor blade; after being dried, the coated nickel foam was annealed in air at 350 °C for 30 min. After being cold pressed, the GDL was cut into 4 cm × 4 cm pieces. 8 mg catalyst was well dispersed in 5 ml ethanol and then uniformly dispersed onto the GDL to make the air cathode. Zinc foil (0.25 mm thick) was polished and used directly as the anode. The electrolyte was a water solution containing 6 M KOH and 0.2 M Zn(CH3COO)2. The battery discharge and charge voltages were measured by the galvanodynamic method at different current densities. Discharge-charge cycling was conducted using a constant current density of 2 mA∙cm−2 with 10 min discharge and 10 min charge time. The back side of the GDL was purged with pure oxygen. Electrochemical impedance spectroscopy (EIS) was performed at open circuit voltage from 1 MHz to 0.1 Hz. Theoretical calculations: First principle density functional theory (DFT) calculations have been performed for atomic configuration optimization and energetic calculations. In all the slab models, the vacuum space has been set to be more than 15Å. For all calculations, the PerdewBurke-Ernzerhof (PBE) functional and projector augmented-wave (PAW)44-46 method is adopted


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using the Vienna ab initio simulation package (VASP). The dispersive van der Waals interactions between the surface and molecules were included using the DFT-D2 method of Grimme.47-48 In each calculation, an energy cutoff of 500 eV was adopted while higher cutoff will have and energy difference of less than 0.01 eV. When performing the structure optimizations, the system is regarded as converged when the force per atom is less than 0.01 eV/Å.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Additional information on Calculations, SEM, TGA, XPS, Raman, BET, Composition Analyses, and Catalytic Activity Characterizations are in the Supporting Information. AUTHOR INFORMATION Corresponding Author * Email: [email protected]

Author Contributions All authors have given approval to the final version of the manuscript. § C. H. and W. J. L. contributed equally.

Notes The authors declare no competing financial interest.


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Funding Sources This work is supported by Australian Research Council (ARC) through a Discovery project (DP180101453) to cover the experimental expenditure. Dr. W. Li wants to thank the support from Discovery Early Career Researcher Award via DE180101478.

ACKNOWLEDGMENT We also would like to thank Dr. Dongqi Shi for the XPS measurement and Dr. Tania Silver for polishing the manuscript. The authors also want to thank the Institute for Superconducting and Electronic Materials (ISEM) and the Electron Microscopy Centre (EMC) at the University of Wollongong (UoW) for their support.

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Catalytic Activity Boosting of Nickel Sulfide toward Oxygen Evolution

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