Source: https://pubs.rsc.org/en/content/articlehtml/2019/qm/c9qm00052f
Timestamp: 2019-04-18 15:29:35+00:00

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Except for the noble metal catalysts, 3d transition metal based electrocatalysts, especially hydroxides, have receive focused interest due to their fascinating electric structure and catalysis activity.2,11–18 It has been demonstrated that compositing transition metal hydroxides with other materials could lead to enhanced catalytic performances, due to the tuned electric structure, more catalysis sites, or improved charge transfer.19–23 Very recently, noble metals of Au and Pt have been utilized to boost the electrocatalytic activities of non-noble-metal based catalysts and achieved substantial improvements in catalytic performance, through fabricating noble metal-catalyst interfacial interactions.4,24–30 For instance, Ravichandran et al. reported the boosting effect of Pt on the HER performance.4 Fester and co-workers have successfully demonstrated the strong promotion of Au on the OER activity of cobalt oxide.25 Han et al. developed carbon cloth supported Pt nanoparticles–CoS2 as a bifunctional electrocatalyst for overall water splitting with a low cell potential of 1.55 V to generate a current density of 10 mA cm−2, showing the attractive boosting effect of Pt nanoparticles for the application of water electrolysis.29 However, to date, the study of the utilization of Pd to enhance the water electrolysis activity of current non-noble-metal catalysts, especially for hydroxides, is still at a nascent stage.
Enlightened by the aforementioned ideas, we develop a two-step synthesis strategy to fabricate Pd decorated NiFe LDH on nickel foam (NF) to achieve improved bifunctionality for water splitting, which includes the initial hydrothermal synthesis of NiFe LDH flowers on NF and the following electrodeposition of ultrafine Pd nanoparticles onto the NiFe LDH nanosheets. The electrocatalytic measurements reveal that the as-obtained Pd-NiFe LDH/NF electrode exhibits enhanced catalytic activity towards both the OER and HER in 1 M KOH due to the introduced interfacial interactions between Pd and NiFe LDH. Moreover, Pd-NiFe LDH/NF serves as an excellent bifunctional electrocatalyst and drives high-efficiency overall water electrolysis in a two-electrode system. This study demonstrates an effective technology to substantially boost the water splitting activities of transition metal materials by simultaneously improving the intrinsic activity, site density, and electrical conductivity with the help of trace noble metals, which could be utilized for exploring other advanced electrocatalysts.
The schematic synthesis procedure of Pd-NiFe LDH/NF is shown in Fig. 1, which includes the initial hydrothermal growth of NiFe LDH on the NF substrate and the subsequent electrodeposition of ultrafine Pd nanoparticles on NiFe LDH/NF. The morphology of the obtained Pd-NiFe LDH/NF is recorded with SEM. In the panoramic view of Fig. 2a, the flowerlike building blocks constructed with nanosheet subunits are observed for Pd-NiFe LDH. In the magnified view (Fig. 2b), these nanosheets possess an ultrathin thickness of several nanometres and a diameter ranging from 500 to 2000 nm. Such ultrathin features of the nanosheets make them the ideal platform for catalysis applications, supplying a short charge diffusion path for fast charge transfer and high surface area for the contact of the active sites with the electrolyte. The TEM of Fig. 2c further reveals the sheet-assembly architecture with the nanosheets radiating from the central area with specific spacing between adjacent nanosheets. The sheet-assembled flower-like structure is beneficial for electrolyte infiltration and contact. Note that, from the SEM images and low-magnification TEM image, the Pd particles cannot be observed clearly due to their ultra-small particle sizes. The HRTEM (Fig. 2d) shows that ultrafine dark nanoparticles are uniformly decorated on the NiFe LDH nanosheets. No obvious aggregated Pd particles are observed, assuring the exposed active sites and stable catalysis output. The dark ultrafine Pd nanoparticles possess a particle size of less than 5 nm. The ultralow particle size and good dispersion could be attributed to the benefits of the electrodeposition method and the wonderful nanosheet substrates. In the inset of Fig. 2d, the lattice fringe of the (012) planes for NiFe LDH with an interplanar distance of 0.26 nm is detected, however, the Pd particles show an amorphous phase without distinct lattice fringes. Interestingly, the defects and lattice distortion of NiFe LDH are observed along the borders of the Pd particles, implying that Pd particles could induce additional catalytic sites. The EDX mapping of Fig. 2e further confirms the good dispersion of Pd particles on the nanosheet substrate. One can see from the EDX mapping that the distributions of Pd, Ni, and Fe elements are well overlapped, indicating the uniformly and fully distributed ultrafine Pd nanoparticles on the NiFe LDH nanosheets. In the powder XRD pattern of Pd-NiFe LDH (Fig. 3), all diffraction peaks are in good agreement with NiFe LDH.14,23 The strong and sharp diffraction peaks of (003), (006), (012), and (015) confirm the high phase purity of the obtained NiFe LDH. No XRD peaks corresponding to Pd are detected, due to the amorphous phase of Pd.
Fig. 1 The schematic synthesis procedure of Pd-NiFe LDH/NF.
Fig. 2 Morphology and structure characterization of Pd-NiFe LDH/NF. (a and b) SEM images, (c) TEM image, (d) HRTEM image, and (e) EDX elemental mapping of Pd, Ni, and Fe elements. The inset in (d) shows the Pd particles (labeled with red circle) and the lattice fringe corresponding to the (012) crystal of NiFe LDH. Scale bars: (a) 10 μm; (b) 100 nm; (c) 2 μm; (d) 10 nm.
Fig. 3 XRD pattern of Pd-NiFe LDH.
XPS measurements of Pd-NiFe LDH were performed to determine the possible electric interaction between the decorated Pd and NiFe LDH, which is of special importance for the catalytic activity. As shown in the high-resolution Ni 2p XPS region of Fig. 4a, two XPS regions of Ni 2p3/2 and Ni 2p3/2 with their corresponding satellite peaks are observed from both samples of NiFe LDH/NF and Pd-NiFe LDH/NF.14,20,23,30 Notably, the Ni 2p XPS region of Pd-NiFe LDH/NF shows a slight positive shift compared to that of NiFe LDH/NF, with a positive shift of 0.6 eV at the Ni 2p3/2 peak, suggesting the increased oxidation degree of the surface Ni species after Pd loading. Fig. 4b shows the Fe 2p XPS spectra, in which two XPS regions related to Fe 2p3/2 and Fe 2p1/2 are detected.14,30 Similar to Ni 2p, a slight positive shift is also observed in the Fe 2p spectra. The visible valence state changes of Ni and Fe in Pd-NiFe LDH suggest the strong electronic interaction between the loaded Pd and NiFe LDH nanosheets. The observation of the Pd 3d XPS spectra (Fig. 4c) with two regions of Pd 3d5/2 and Pd 3d3/2 confirms the successful electrodeposition of Pd on NiFe LDH. Fig. 4d displays the O 1s XPS regions in NiFe LDH and Pd-NiFe LDH, which are in agreement with the characteristic M–OH bonds in metal LDH.20,30 Interestingly, the O 1s peak of Pd-NiFe LDH shifts to the negative binding energy region compared to that of NiFe LDH, further confirming the electric interaction between the ultrafine Pd and NiFe LDH.
Fig. 4 High-resolution XPS spectra of (a) Ni 2p, (b) Fe 2p, (c) Pd 3d, and (d) O 1s regions of Pd-NiFe LDH/NF with fitting curves. The corresponding XPS spectra of NiFe LDH/NF are supplied for comparison.
The as-obtained Pd-NiFe LDH/NF is firstly subjected to the OER test in 1 M KOH electrolyte using a three-electrode system, in which Pd-NiFe LDH/NF directly serves as a working electrode. Fig. 5a shows the LSV curves of various Pd-NiFe LDH/NF samples obtained with different electrodeposition times, which indicates that the Pd-NiFe LDH/NF catalyst with 300 s electrodeposition of Pd exhibits the best OER activity among all the samples. This optimized sample is used for all other electrochemical tests. To reveal the effect of deposited Pd on the OER activity, the LSV curves of Pd-NiFe LDH/NF and NiFe LDH/NF are depicted in Fig. 5b. Clearly, Pd-NiFe LDH/NF exhibits remarkably improved OER activity compared to NiFe LDH/NF, affording an OER current density of 10 mA cm−2 at an ultralow overpotential of 156 mV, which is 117 mV lower than that of NiFe LDH/NF. With potential increasing, Pd-NiFe LDH/NF delivers high current densities of 50 and 100 mA cm−2 at overpotentials as low as 219 and 291 mV, respectively. Notably, both Pd-NiFe LDH/NF and NiFe LDH/NF show superior OER activities to the benchmark RuO2 catalyst. As shown in Table 1, Pd-NiFe LDH/NF demonstrates lower overpotential at an OER current of 10 mA cm−2 than the listed electrocatalysts, including the metal hydroxide based catalysts of NiFe LDH/NF (240 mV),7 NiFe LDH-NS@DG10 (210 mV),14 Cu@NiFe LDH (199 mV),18 NiFe-LDH/NiCo2O4/NF (290 mV at 50 mA cm−2),20 Cu@CoFe LDH (240 mV),22 NiFe LDH@NiCoP/NF (220 mV),23 NiFe LDH (263 mV),28 and FeOOH nanosheets (265 mV),34 the precious metal-boosted transition metal catalysts of Au@Co(OH)2 (220 mV),26 Co(OH)2–Au–Ni(OH)2 (340 mV),27 SAu/NiFe LDH (237 mV),28 Pt–CoS2/CC (300 mV),29 and Pt/NiFe-LDH (230 mV),30 and other transition metal catalysts of Ni/Ni8P3 (270 mV at 30 mA cm−2),31 V-Ni2P (257 mV),32 MoS2/Ni3S2 (218 mV),36 and NiCoP/N-rGO (310 mV at 40 mA cm−2),37 showing impressive OER activity.
Fig. 5 Electrochemical OER measurements of Pd-NiFe LDH/NF in 1 M KOH. (a) Polarization curves of various Pd-NiFe LDH/NF electrodes obtained with different electrodeposition times. (b) Polarization curves and corresponding (c) Tafel plots of Pd-NiFe LDH/NF, NiFe LDH/NF and RuO2. (d) Nyquist plots of Pd-NiFe LDH/NF and NiFe LDH/NF.
The corresponding Tafel plots are obtained to understand the boosting effect of Pd on the OER kinetics. In Fig. 5c, a smaller Tafel value of 58 mV dec−1 is achieved for Pd-NiFe LDH/NF than that (64 mV dec−1) of NiFe LDH/NF, indicating that the deposited Pd could facilitate the OER kinetics of NiFe LDH. There are five steps for a generally accepted OER process on transition metal based catalysts, including: (1) M + OH− → MOH + e−1; (2) MOH + OH− → MO + H2O + e−1; (3) MO + OH− → MOOH + e−1; (4) MOOH + OH− → MO2 + H2O + e−1; (5) MO2 → M + O2.29,31 It is believed that the slightly higher oxidation state of Mn+ sites possesses more 3d electron orbits and is more favorable for electron acceptance, which are of great importance to improve the adsorption of OH− and lead to a lower energy barrier for the OER intermediate transformation.29,31,32 In this work, the deposited Pd induces the higher oxidation state of Ni and Fe species, which should be responsible for the improved OER activity and Tafel value of Pd-NiFe LDH/NF compared with NiFe LDH/NF. The EIS measurements of Pd-NiFe LDH/NF and NiFe LDH/NF electrodes are performed at the overpotential of 100 mV to further explore the OER kinetics. As shown in Fig. 5d, the Nyquist plots of both electrodes show the straight slope, indicating the low ion diffusion resistance.33 Pd-NiFe LDH/NF delivers a smaller interfacial charge resistance than NiFe LDH/NF, showing that Pd could improve the OER kinetics.
The electrochemical double-layer capacitances (Cdl) of Pd-NiFe LDH/NF (Fig. 6a) and NiFe LDH/NF (Fig. 6b) are measured to estimate their electrochemical active surface area (ECSA). As depicted in Fig. 6c, Pd deposition could induce an increase of Cdl, resulting in a higher Cdl of 1.3 mF cm−2 for Pd-NiFe LDH/NF than that of 1.0 mF cm−2 for NiFe LDH/NF, which is of great importance for enhanced OER performances. In addition to ECSA, the intrinsic OER activity is also an important factor for the catalytic performance of the catalysts, which could be evaluated by the TOF values.34,35 In Fig. 6d, Pd-NiFe LDH/NF shows much higher TOF values than NiFe LDH/NF at different overpotentials, indicating that Pd could promote the intrinsic catalytic activity of NiFe LDH. Moreover, the mass activities of Pd-NiFe LDH/NF and NiFe LDH/NF are calculated based on the mass of metals in the catalysts and shown in Fig. 6e. Pd-NiFe LDH/NF shows substantially improved mass activity compared with NiFe LDH/NF, for instance, with a higher mass activity of 217.8 A g−1 than 40.8 A g−1 for NiFe LDH/NF at an overpotential of 300 mV. This result indicates that ultralow dose of Pd could dramatically improve the OER activity, suggesting the possible catalytic synergism between Pd and NiFe LDH. The OER durability test of Pd-NiFe LDH/NF is further conducted at a constant overpotential of 190 mV. As shown in Fig. 6f, an almost constant current density of 20 mA cm−2 can be maintained for a long time of 60 h, showing excellent stability. After the durability test, the LSV curve (Fig. 6f) of Pd-NiFe LDH/NF is collected and displayed as the inset in Fig. 6f, which shows a slight positive shift compared to the LSV curve obtained from the freshly prepared catalyst, with a 15 mV increase in overpotential at 100 mA cm−2, further confirming the salient OER stability.
Fig. 6 CV curves of (a) Pd-NiFe LDH/NF and (b) NiFe LDH/NF at scan rates from 20 to 120 mV s−1. (c) The capacitive currents as a function of scan rate for Pd-NiFe LDH/NF and NiFe LDH/NF. (d) TOF values and (e) mass activity (based on the metal mass) of Pd-NiFe LDH/NF and NiFe LDH/NF at different overpotentials. (f) The chronoamperometric measurement at an overpotential of 190 mV. The inset shows the LSV curves of Pd-NiFe LDH/NF electrodes before and after a durability test.
In addition to the high OER activity, the Pd-NiFe LDH/NF electrode is evaluated for HER properties in 1 M KOH. In the HER LSV curves of Fig. 7a, Pd-NiFe LDH/NF shows much improved HER activity compared to NiFe LDH/NF, achieving a low overpotential of 130 mV at 10 mA cm−2, which is 93 mV lower than that of NiFe LDH/NF. As is shown in Table 1, this value of 130 mV is comparable to the best reported catalysts, such as NiFe LDH-NS@DG10 (115 mV at 20 mA cm−2),10 Cu@NiFe LDH (116 mV),18 LDH@NiCoP/NF (120 mV),23 Pt/NiFe-LDH (101 mV),30 Ni/Ni8P3 (130 mV),31 V-Ni2P (108 mV),32 FeOOH nanosheets (108 mV),34 MoS2/Ni3S2 (110 mV),36 and NiCoP/N-rGO (115 mV),37 showing competitive HER activity. The improved HER kinetics of Pd-NiFe LDH/NF compared to NiFe LDH/NF is observed from the Tafel profiles (Fig. 7b). A remarkably depressed Tafel value of 46 mV dec−1 is achieved for Pd-NiFe LDH/NF, which is much smaller than that (80 mV dec−1) for NiFe LDH/NF, showing that Pd could facilitate the HER kinetics. The TOF values and mass activities of Pd-NiFe LDH/NF and NiFe LDH/NF for the HER at different overpotentials are compared in Fig. 7c and d. Compared with NiFe LDH/NF, Pd-NiFe LDH/NF exhibits much improved TOF values and mass activities. For instance, Pd-NiFe LDH/NF shows TOF of 17.4 s−1 and mass activity of 59.5 A g−1 at an overpotential of 200 mV, which are higher than the TOF of 4.1 s−1 and mass activity of 14.0 A g−1 for NiFe LDH/NF, respectively. Such results indicate that the deposited Pd substantially enhances the intrinsic HER activity of NiFe LDH. The chronoamperometric measurement on the Pd-NiFe LDH/NF electrode is performed at a fixed overpotential of 217 mV to evaluate the HER stability. As shown in Fig. 7e, no obvious cathode current fade is observed from the Pd-NiFe LDH/NF electrode over a severe time of over 60 h, showing the excellent catalysis stability.
Fig. 7 Electrochemical HER measurements of Pd-NiFe LDH/NF in 1 M KOH. (a) Polarization curves and corresponding (b) Tafel plots of Pd-NiFe LDH/NF, NiFe LDH/NF and Pt/C. (c) TOF values and (d) mass activity (based on the metal mass) of Pd-NiFe LDH/NF and NiFe LDH/NF at different overpotentials. (e) The chronoamperometric measurement at overpotential of 217 mV.
In alkaline solution, a two-electron transfer HER process is widely accepted, which is composed of three possible steps: (1) the initial Volmer reaction M + H2O + e−1 → M–H + OH−1; the following Heyrovsky step M–H + H2O + e−1 → M + H2 + OH−1 or the Tafel recombination path 2M–H → 2M + H2. The Tafel value of 46 mV dec−1 suggests that the HER on the Pd-NiFe LDH/NF catalyst is through the Volmer–Heyrovsky mechanism.29,36 The significantly important step during a Volmer–Heyrovsky path is the initial Volmer step. Based on the XPS results, the deposited Pd not only increases the oxidation state of Ni and Fe species, but also decreases the electron density of O species. Interestingly, the slightly increased oxidation state of Ni and Fe can offer empty d orbitals for enhanced H adsorption on the catalyst surface, thus benefiting the kinetics of the Volmer step.36 Simultaneously, the increased electron density of O may serve as a Brønsted base to trap H and enhance the H2O dissolution, similar to the role of P in ref. 32, thus further boosting the HER electrocatalysis. Based on the aforementioned analysis, the introduction of ultrafine Pd could modulate the chemical states of Ni, Fe, and O species, thus inducing improved intrinsic HER activity and reaction kinetics of NiFe LDH.
Inspired by the impressive HER, and especially the OER activities, the bifunctionality of the Pd-NiFe LDH/NF is further investigated for overall water splitting in a two-electrode system. As is shown in Fig. 8a, the ultralow cell potential of 1.514 V is achieved to afford current densities of 10 mA cm−2 in 1 M KOH, respectively. Such excellent bifunctionality performance is among the recently reported best bifunctional catalysts in Table 1, e.g., NiFe LDH-NS@DG10 (1.5 V at 20 mA cm−2),14 Cu@NiFe LDH (1.54 V),18 LDH@NiCoP/NF (1.57 V),23 Pt–CoS2/CC (1.55 V),29 Pt/NiFe-LDH (1.561 V),30 V-Ni2P (1.563 V),32 and NiCoP/N-rGO (1.57 V at 20 mA cm−2).37 The chronoamperometric test (inset of Fig. 8b) shows that the bifunctional Pd-NiFe LDH/NF electrode assembled electrolyzer maintains a smooth line with a constant catalytic current density of 20 mA cm−2 for 30 h, when the cell potential is fixed at 1.56 V. Moreover, the LSV curve (Fig. 8b) collected from the durability tested cell shows no detectable changes compared with the initial LSV curve. Both chronoamperometric and LSV measurements indicate the satisfactory stability of bifunctional Pd-NiFe LDH/NF electrodes. Note that the Ni, Fe, and Pd element contents of the Pd-NiFe LDH/NF electrodes are measured with ICP-MS before and after the long-term OER and HER tests. The results show that no detectable changes are observed for the contents of Ni, Fe, and Pd elements, indicting the good stability of the Pd-NiFe LDH/NF electrode during the harsh durability test.
Fig. 8 (a) LSV polarization curves of electrochemical overall water splitting using Pd-NiFe LDH/NF as a bifunctional catalyst in 1 M KOH. The inset shows the O2/H2 bubbles generated from dual-functional Pd-NiFe LDH/NF electrodes. (b) LSV curves obtained from bifunctional Pd-NiFe LDH/NF electrodes before and after the durability test. The inset shows the chronoamperometric measurement at static potential of 1.56 V.
Ni foam (1 × 1 cm2) is totally immersed into 3 M HCl solution and subjected to ultrasonic treatment for 30 min, and is then rinsed with lots of water and ethanol in turn. The synthesis of NiFe LDH flowers on NF is via a hydrothermal approach. 30 mL of solution containing 0.446 g of Ni(NO3)2·6H2O, 0.183 g of Fe(NO3)3·9H2O, 0.14 g of NH4F, and 0.66 g of urea is poured into a 50 mL autoclave (Anhui Kemi Machinery Technology Co., Ltd), and then the clean NF is immersed. The sealed autoclave is heated at 120 °C for 6 h. After being naturally cooled, NiFe LDH/NF is washed with water and ethanol, and then dried in a vacuum oven at 60 °C overnight. The mass loading of NiFe LDH on NF is about 0.9 mg cm−2. The anchoring of ultrafine Pd nanoparticles on NiFe LDH/NF is via a simple electrodeposition approach. In a conventional three-electrode system, the as-obtained NiFe LDH/NF serves as a working electrode, a parallel positioned graphite rod and a saturated calomel electrode (SCE) are utilized as the auxiliary electrode and the reference electrode, respectively. The electrodeposition procedure is performed in the electrolyte of 0.05 M NaCl and 0.3 mM PdCl2 with an applied potential of −1.0 V vs. SCE for the optimized time of 300 s.
The microstructure and morphology of Pd-NiFe LDH/NF are characterized with a transmission electron microscope (TEM, FEI Tecnai G2 F30) equipped with an energy dispersive X-ray spectroscopy (EDX), scanning electron microscope (SEM, JEOL JSM-7500F), and powder X-ray diffraction (XRD, Philips X’-pert X-ray diffractometer). The X-ray photoelectron spectroscopy (XPS) data of Pd-NiFe LDH/NF and NiFe LDH are collected from an RBD upgrade PHI-5000c ESCA system (PerkinElmer). The Ni, Fe, and Pd element contents are determined with inductively coupled plasma-mass spectrometry (ICP-MS) on an Agilent 7500a.
The electrochemical OER and HER tests of Pd-NiFe LDH/NF are conducted on a CHI760D (CH Instruments, Shanghai, China) in a three-electrode cell containing 1 M KOH electrolyte. Pd-NiFe LDH/NF is directly employed as a working electrode. The saturated calomel electrode (SCE) and a graphite rod serve as a reference electrode and counter electrode, respectively. The linear sweep voltammetry (LSV) measurement is performed at a scan rate of 5 mV s−1. The potentials are calibrated with a reversible hydrogen electrode (RHE). The commercial Pt/C and RuO2 are utilized as benchmark electrocatalysts for the HER and OER tests, respectively. The cyclic voltammetry (CV) of Pd-NiFe LDH/NF is collected to determine the electrochemical double-layer capacitance (Cdl) in the potential range where no Faradaic processes occur at various scan rates. Electrochemical impedance spectroscopy (EIS) is recorded at onset overpotential (overpotential for generating a current density of 1 mA cm−2) in the frequency range from 0.1 Hz to 100 kHz. Turnover frequency (TOF) is adopted to assess the intrinsic catalytic activity per active site for Pd-NiFe LDH/NF and NiFe LDH/NF, and is determined based on the following equation: TOF = I/(4 × F × n) for the OER. I is current in the LSV curve, 4 corresponds to the four-electron mechanism for the OER process, F is the Faraday constant, and n is the moles of all metal atoms assuming that all of them are active. Note that a fraction of metal sites is inactive during the OER; therefore, the calculated TOF values are lower than the actual performance. For the HER with a two-electron mechanism, the TOF equation should be TOF = I/(2 × F × n).
In summary, a two-step synthesis method, including the initial hydrothermal synthesis of NiFe LDH flowers on NF and the following electrodeposition of Pd on NiFe LDH, is utilized for the development of ultrafine Pd nanoparticle decorated NiFe LDH as an advanced bifunctional electrocatalyst for water splitting. The introduction of Pd induces defects and lattice distortion as additional active sites, as well as facilitates charge transfer. Moreover, the electric interaction between Pd and NiFe LDH generates an increase of oxidation state of Ni and Fe species, simultaneously inducing a decrease of the electron density of O species. Based on the aforementioned advantages, the as-obtained Pd-NiFe LDH catalyst exhibits higher intrinsic activity, improved active sites, and facilitated reaction kinetics compared to NiFe LDH. The as-obtained Pd-NiFe LDH/NF electrode shows impressive catalytic activity and durability stability for water electrolysis. To acquire a current density of 10 mA cm−2, Pd-NiFe LDH only needs low overpotentials of 156 and 130 mV for the OER and HER, respectively. Importantly, an ultralow cell potential of 1.514 V is obtained at a current density of 10 mA cm−2 for overall water splitting. This work contributes to the exploration of trace noble metal-boosted functional materials in energy storage and conversion.
We are thankful for the financial support from the Natural Science Foundation (2016GGX104019) of Shandong Province.
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