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Boosting Overall Water Splitting via FeOOH Nanoflakes Decorated PrBa0.5Sr0.5Co2O5+# Nanorods Zonghuai Zhang, Beibei He, Liangjian Chen, Huanwen Wang, Rui Wang, Ling Zhao, and Yansheng Gong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12372 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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ACS Applied Materials & Interfaces

Boosting Overall Water Splitting via FeOOH Nanoflakes Decorated PrBa0.5Sr0.5Co2O5+δ Nanorods Zonghuai Zhang, Beibei He*, Liangjian Chen, Huanwen Wang, Rui Wang, Ling Zhao, Yansheng Gong* Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, China.

Abstract The development of efficient, robust and low-cost catalyst for water electrolysis is critically important for renewable energy conversion. Herein, we fulfil a significant improvement on electrocatalytic activity for both oxygen-evolution reaction (OER) and hydrogen-evolution

reaction

(HER)

by

constructing

a

novel

hierarchical

PrBa0.5Sr0.5Co2O5+δ (PBSC)@FeOOH catalyst. The optimized PBSC@FeOOH-20 catalyst consisted of layered perovskite PBSC nanorods and 20 nm thick amorphous FeOOH nanoflakes exhibits an excellent electrocatalytic activity for OER and HER in 0.1 M KOH media, delivering a current density of 10 mA cm-2 at overpotentials of 390 mV for OER and 280 mV for HER, respectively. The substantially enhanced performance is probably attributed to the hierarchical nanostructure, the good charger transfer capability and the strong electronic interactions of FeOOH and PBSC. Importantly, an alkaline electrolyzer integrated PBSC@FeOOH-20 catalyst as both anode and cathode shows a highly active overall water splitting with a low voltage of 1.638 V at 10 mA cm-2 and high stability during continuous operation. This study provides new insights into exploring efficient 1 ACS Paragon Plus Environment

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bifunctional catalysts for overall water splitting, and it suggests that the rational design of oxyhydroxide/perovskite heterostructure shows great potential as a promising type of electrocatalysts.

Keywords: double perovskite, oxygen-evolution reaction, hydrogen-evolution reaction, overall water splitting, electronic interactions

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1. Introduction

Water splitting into high purity hydrogen and oxygen is regarded as one of the most efficient approach for the pursuit of sustainable and economical energy.1-2 Due to sluggish kinetics of the two half splitting reactions: O2 and H2 evolution reactions (OER and HER, respectively), the exploitation of highly active and durable catalysts is of great importance.3 The state-of-the-art catalysts are based on noble metal materials, e.g., Ir/Ru oxides for OER and Pt based catalysts for HER, respectively.3-4 Given their scarcity and high cost, the utilization of noble metal based materials should be drastically reduced or even completely replaced to realize commercial application. Such alternatives include transition metal oxides/hydroxides5-7 for OER in alkaline electrolyte as well as transition metal carbides,8 selenides,9 nitrides,10 chalcogenides,11 phosphides12 and borides13 for HER in acidic solution. For the sake of cost-effective and efficient consideration, the development of bifunctional catalysts simultaneously catalyzing OER and HER in the same media is highly imperative and remains challenging. To address this, enormous efforts have been devoted to developing alternatives, and the transition metal oxides/phosphides/chalcogenides have achieved significant progress for OER and HER.14-16 As a potential type of candidate, the perovskites family are gaining much attention due to their compositional flexibility, cost-effective, and tunable electrocatalytic activity.1718

Recently, the perovskite oxides have been reported as a novel category of promising

bifunctional OER/HER catalysts in alkaline environment.19-27 For example, Shao-Horn et 3 ACS Paragon Plus Environment


al.28-29 reported that Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) perovskite provided a higher intrinsic OER activity in alkaline solution comparable to commercial IrO2 catalyst. They further revealed that excellent OER activity can be attributed to an approximately half-filled eg orbital.28 However, it is reported that the stability of BSCF catalyst is inferior because of the readily surface amorphization and A-site cations leaching during OER practical operation.30 Later, layered double perovskites are developed for OER owing to their faster surface oxygen exchange and diffusion rates relative to simple perovskites. Layered perovskites LnBaCo2O5+δ (Ln = La, Sm, Nd, Pr, Ho and Gd)31, especially PrBaCo2O5+δ (PBC) with an eg of ~1.2

displays a substantially enhanced intrinsic OER activity and

durability than BSCF catalyst. Further promoting OER performance could be achieved by chemical substitution32-33 and nanostructure engineering.34-36 For instance, tailored PrBa0.5Sr0.5Co1.5Fe0.5O5+δ nanofiber34-35 and La0.7Sr0.3Co0.25Mn0.75O336 with controllable hollow tubular structure have been found to provide excellent intrinsic activity and high mass activity. These findings indicate that the electrocatalytic activity highly relies on morphology and electronic structure of catalysts. On the other hand, the activity of perovskites as HER catalysts are rarely studied to date. Recent studies of SrNb0.1Co0.7Fe0.2O337 and Pr0.5(Ba0.5Sr0.5)0.5Co0.8Fe0.2O319 has revealed their effectiveness for catalyzing HER. Apart from chemical substitution and nanostructure strategy, surface modification through incorporating other efficient electrocatalysts is also envisioned to generate synergistically active sites for promoting electrochemical performance.38 As known, 4 ACS Paragon Plus Environment

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FeOOH is reported to show potential promise towards water splitting

on account of its

low cost and open structure.39-41 Unfortunately, the pristine FeOOH usually exhibits inferior activity due to its extremely low electrical conductivity and strong bonding between surface active sites and OH- group, which hinders its fully activation.42-43 In recent studies, hybridizing nano-sized FeOOH on conductive materials (e.g. Ni foam, CNT, metal/metal oxide) as hybrid catalysts have been proved to exhibit excellent OER activities.44-47 The matrices not only enhance the electrical conductivity of the composite catalysts, but also provide high electrochemical surface area (ECSA). Herein, a decorated PrBa0.5Sr0.5Co2O5+δ (PBSC) hollow nanorods with ultrathin amorphous FeOOH nanoflakes (denoted as PBSC@FeOOH) is proposed and synthesisd as an effective catalysts for overall water electrolysis. The PBSC nanorods with an average diameter of 100 nm are obtained by electro-spinning method with large electrochemical active surface area, while the FeOOH layer is fabricated via atomic layer deposition deposition (ALD) ZnO and subsequent sacrificial template accelerated hydrolysis process. The amorphous FeOOH layer with controlled thickness could guarantee uniform growth on topmost of the PBSC surface, and amorphous component is found to be more electrochemical active relative to its crystalline structure experimentally.38, 48-49 Benefiting from the hierarchical nanostructure as well as the synergetic catalytic effect of FeOOH and PBSC, the optimized PBSC@FeOOH-20 shows competitive activity for both OER and HER in alkaline conditions. As a result, by employing PBSC@FeOOH-20 as both cathode and anode in alkaline media, a water splitting current density of 10 mA cm-2 is achieved at 5 ACS Paragon Plus Environment


1.638 V with good long-term durability.

2. Experimental Section

Synthesis of PrBa0.5Sr0.5Co2O5+δ nanorods The PrBa0.5Sr0.5Co2O5+δ nanorods were synthesized by electrostatic spinning method. Stoichiometric amount of Pr(NO3)3·6H2O (4 mmol ， Aladdin， 99.9 %) ， Ba(NO3)2 (2 mmol，Aladdin，99 %)，Sr(NO3)2 (2 mmol，Aladdin，99.9 %) and Co(NO3)2·6H2O (8 mmol，Aladin，99.9 %) were dissolved in 20 mL of N,N-dimethylformamide (DMF) followed by strongly stirring. Then, 2.5 g of polyvinylpyrrolodone (PVP) powders were introduced to the homogeneous solution and the obtained solution was stirred over night to ensure the well dissolution of PVP. The resultant solution was then transferred into a plastic injector equipped with a 20-gauge metal nozzle. The relative humidity was controlled to be 30-35 % during the electro-spinning process. The applied voltage between the collector and needle tip was 17 kV with the distance of 22 cm. The feeding rate of such solution was fixed at 6 μL min-1. A piece of Al foil was wrapped on the rotating drum to collect the electrospinning fiber. The as-spun nanofibers were dried under vacuum at 60 ºC for 12 h, and then sintered in air at 800 ℃ for 5 h. The primary product was obtained and represented as PBSC. Synthesis of PBSC@FeOOH PBSC@FeOOH with core-shell structure were synthesized via a combination of 6 ACS Paragon Plus Environment

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atomic-layer-deposition (ALD) and sacrificial template-accelerated hydrolysis. ZnO was evenly loaded onto the surface of PBSC nanofibers by ALD at 170℃ using diethyl zinc and deionized water as the Zn and O precursors, respectively. Three different thicknesses of ZnO was deposited by controlling the cycle number of 50, 100 and 150 cycles. Then the obtained PBSC@ZnO precursor was immersed into the 6 M Fe(NO3)3 solution and kept overnight to ensure ZnO convert into FeOOH completely. After that, three different samples with the FeOOH thickness of 10 nm, 20 nm and 30 nm were obtained by centrifugal

and

drying,

denoted

as

PBSC@FeOOH-10,

PBSC@FeOOH-20,

PBSC@FeOOH-30, respectively. The FeOOH as a reference catalyst was prepared by a similar ALD and sacrificial template-accelerated hydrolysis process on carbon cloth. Materials characterization Phase structures of as-prepared catalysts were determined by X-ray diffraction (XRD) on Bruker (D8advanced, Cu Kα radiation). Scanning electron microscopy (SEM, SU-8010) and transmission electron microscopy (TEM, Tecnai G2 F20 U-TWIN) images equipped with Energy Dispersive Spectrum (EDS) analyses were carried out to survey the microand nanostructures. The electronic configuration are analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). Specific surface area was identified by Brunauer Emmet Teller (BET, ASAP-2460) system with N2 as an adsorptive medium. The FT-IR spectra were measured on FT-IR spectrometer (Nicolet 460, Thermo Scientific America) using the KBr pellet technique. 7 ACS Paragon Plus Environment


Electrochemical characterization For the electrochemical tests, a refined slurry coating technology was used to prepare the catalyst electrodes. 40 mg of catalyst, 10 mg of ketjen black carbon and 250 uL Nafion solution (5wt. %, Aldrich) were dispersed in 5 mL ethanol, followed by ultrasonication for 2 h to form the catalyst ink. The OER and HER tests were carried out in a three-electrode cell using a rotating ring-disk electrode (RDE) device (Pine Research Instrumentation). Catalyst-coated glassy ―2 carbon (GC) electrode (3 mm in diameter) with catalyst loading of ~0.2264 mg cmdisk in

RDE was used as the working electrode. Additionally, commercial IrO2 and 20 wt. % Pt/C (Sigma-Aldrich 99.9%) were studied as reference catalysts. Calomel electrode (1 M, 0.24 V versus NHE) was applied as the reference electrode. A Pt plate for OER and a graphite rod for HER tests were served as the counter electrodes, respectively. The measured potentials were calibrated in reference to the standard reversible hydrogen electrode (RHE) and corrected for iR losses, where R value was the ohmic resistance of the electrolyte. The LSV curves were measured from 0 to 1 V versus RHE for OER and from 0.1 to -0.6 V versus RHE for HER with a potential scan rate of 10 mV s-1. Cycle voltammetry (CV) was conducted to assess the electrochemical double-layer capacitance (Cdl). The potential was scanned as a function of scan rates from 1.10 to 1.15 V versus RHE with not faradic current observed. The chronopotentiometry (CP) tests were performed to evaluate the long-term stability. Electrochemical impedance spectroscopy (EIS) measurements were conducted from 106 Hz to 10-2 Hz at 1.65 V versus RHE for OER and -0.3 V versus RHE for HER. 8 ACS Paragon Plus Environment

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Catalytic performances of the PBSC and PBSC@FeOOH catalysts were evaluated on a Gamry electrochemical workstation (Reference 3000) in a two-electrode device for full water-splitting. The electrodes were prepared via drop-casting of ink onto Ni Foam (NF) with a size of 2 cm × 2 cm, the catalyst loading was ~1.6 mg cm-2. The volume of generated oxygen and hydrogen was measured with a constant current density for 100 min by a water drainage method.

3. Results and discussion

Figure 1a illustrates the three-step fabrication process for PBSC hollow nanorods@FeOOH nanoflakes. In the first step, the perovskite PBSC is engineered through facile electrospinning technique with tailored nanostructure. PBSC is a layered double perovskite with Pr and Ba/Sr cations ordered in alternating layers along the c axis, and abundant active oxygen vacancies exists in PrOx layer.34 The X-ray diffraction spectra shown in Figure 1b reveals that all peaks are well indexed to double perovskite PBSC, which is consistent with the literature.34 After the second and third synthetic process, Fe3+ ions hydrolyze into FeOOH colloid and H+. This hydrolysis reaction takes place preferentially on the ALD-ZnO layer because of the consumption of H+ by ZnO, resulting in the formation of FeOOH nanoflakes on PBSC nanorods surface. As seen in Figure 1b, the diffraction peaks in XRD pattern of PBSC@FeOOH-20 are the same as those of pristine PBSC, indicating an amorphous form of the topmost FeOOH layer. In relative to bare 9 ACS Paragon Plus Environment


PBSC, Raman spectroscopy of PBSC@FeOOH-20 exhibits the characteristic peaks of FeOOH in the low wave number region (Figure 1c),50 clearly demonstrating the formation of FeOOH on the PBSC surface. The as-spun precursory PBSC, which is synthesized by electrospinning method, displays a nanofiber structure with the average diameter of 150~200 nm (Figure 2a). After calcination, the PBSC nanofibers shrink and break into nanorods with a decreasing diameter of ~100 nm (Figure 2b). This hollow structure with discontinuous mesopores pores inside not only exposures more active sites, but also facilitates the facile mass transfer of reactant products. After the deposition of FeOOH with the thickness of 20 nm, the entire PBSC surface becomes rough and is uniformly occupied by porous FeOOH particles (Figure 2c). For comparison, FeOOH layer with thickness of 10 nm and 30 nm (the thickness is controlled by ALD cycling) are also prepared as shown in Figure S1. To gain more information about the microstructure, the specific surface area of PBSC and PBSC@FeOOH-20 are probed by the Brunauer-Emmett-Teller (BET) method. As verified in Figure 2d, the PBSC@FeOOH-20 possesses a BET surface area of 15.43 m2 g-1, slightly higher than that of bare PBSC (13.77 m2 g-1). The BET results indicate that a larger surface area is obtained after FeOOH decoration. The specific surface areas of PBSC@FeOOH10, PBSC@FeOOH-30, and FeOOH samples are also obtained from N2 adsorptiondesorption experiments and the results are presented in Figure S2. The TEM images of PBSC and PBSC@FeOOH-20 are displayed in Figure 3a and Figure 3c to further reveal the morphology and structure. The double perovskite structure 10 ACS Paragon Plus Environment

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of PBSC is confirmed in Figure 3b (d(001) spacing of 0.762 nm). The fast Fourier transform pattern along [110] zone axis inside Figure 3b shows the presence of superlattice reflections along the c axis, which provides the compelling evidence of the double interplanar spacing along c-direction. After the precipitated FeOOH decoration, the initially smooth surface of PBSC is uniformly covered by a loose flaky FeOOH with ~20 nm thickness (Figure 3c). The cross section morphology (Figure 3d) highlights the strong connection between the perovskite core and decoration shell, which is conducive to the electronic interaction and the biphasic synergy catalysis. Additionally, the lattice fringes on the core match the (001) lattice plane spacing (0.760 nm) of PBSC (inset of Figure 3d), in agreement with the XRD result. The scanning TEM corresponding elemental mapping images in Figure 3e clearly reflect the uniform distribution of O, Pr, Ba, Sr and Co elements throughout the whole nanorods, whereas the Fe element mainly concentrates in the surface shell layer. The semi-quantiattive anlysis of catalyst elements is listed in Figure 3f. The atomic ratio of Pr: Ba: Sr: Co is in agreement with the expected stoichiometry, and the Fe content is around 6.76 wt. %. Figure 4a shows the OER activity for the as-prepared catalysts as well as commercial IrO2. Compared with bare PBSC nanorods, the onset potential of PBSC@FeOOH-20 and PBSC@FeOOH-10 shift significantly to negative potential region with increasing current densities at the same overpotential. PBSC@FeOOH-20 catalyst exhibits the highest current densities among these catalysts, suggesting a best OER activity. For example, the current density for PBSC@FeOOH-20 catalyst at 1.6 V is 5.37 mA cm-2, which is 2.92, 5.57 and 11 ACS Paragon Plus Environment


1.91 times than pristine PBSC, FeOOH and IrO2 catalysts, respectively. Moreover, the PBSC&FeOOH-20 requires an overpotential of 390 mV at 10 mA cm-2, which is 67, 80 and 112 mV less than that of PBSC, IrO2 and FeOOH, respectively. The optimized PBSC@FeOOH-20 catalyst displays higher intrinsic OER activity (normalized the activity to the surface area of the electrocatalysts) relative to PBSC and FeOOH individuals (Figure S3), denoting the possible synergistic enhancement of PBSC and FeOOH. Tafel plots (Figure 4b) further reveal the superiority of PBSC@FeOOH-20 (53 mV dec-1) over PBSC (114 mV dec-1) and IrO2 (78 mV dec-1) with regard to OER dynamics. Such superior activity of PBSC@FeOOH-20 is comparable to the representative perovskite-based OER catalysts reported (Table S1). In addition to electrocatalytic activity, long-term durability is also a significant factor to assess practical feasibility of catalysts. Figure 4c shows that PBSC@FeOOH-20 displayed 10.3 % increased OER overpotential over the 12 h CP test at a current density of 10 mA cm-2, which is more stable than the reported IrO2 catalyst.51 The CVs of PBSC@FeOOH-20 for its 1st and 500th cycles inset in Figure 4c reveal little current loss over continuous cycling scans, confirming the good stability of PBSC@FeOOH-20 as well. After the long-term durability test, the TEM investigation (Figure S4) reveals the well-preserved morphology of PBSC@FeOOH-20 after OER, and both the XRD pattern and XPS spectra (Figure S5) scarcely change, consistent with the robust stability of the PBSC@FeOOH-20 electrode under alkaline OER conditions. As shown in Figure 4d, the variation of HER activities for the as-synthesized catalysts is similar with that of the OER activities. PBSC@FeOOH-20 demonstrates the improved 12 ACS Paragon Plus Environment

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HER activity than pristine PBSC. The overpotential needed by PBSC@FeOOH-20 catalyst at -10 mA cm-2 is 0.280 V, which is higher than that of 20 % Pt/C (0.08 V) while much lower than those of individual PBSC (0.393 V) and FeOOH (0.406 V). In addition, a smaller Tafel slope (70 mV dec-1) of PBSC@FeOOH-20 is obtained relative to that of other catalysts (Figure 4e). Of note, the activity of PBSC@FeOOH-20 is comparable to recently discovered perovskite-based HER catalysts (Table S2). We also carried out the CP test to investigated the HER catalytic durability of PBSC@FeOOH-20, as shown in Figure 4f. One can see that there is no appreciable deactivation observed during continuously 24 h operation. Together with the 1000 cycling test (inset of Figure 4f), it demonstrates the excellent durability of PBSC@FeOOH-20 for HER. Extensive morphological and microstructural analysis (Figure S6 and Figure S7a) confirm that structural changes do not take place in the PBSC@FeOOH-20 electrode during the HER catalysis. While XPS analysis in Figure S7b and Figure S7c indicate a partial reduction from Fe3+ to Fe2+ under the HER conditions, which indicates that the original FeOOH might be partially reduced at least in the near surface region. To further understand the origin of superior bi-functional activity of PBSC@FeOOH20, the electrochemically active surface area (ECSA) and electrochemical impedance spectroscopy (EIS) analysis are carried out. A CV method (Figure S8) is carried out to determine the double layer capacity (Cdl), which is in direct proportion to the ECSA.52 As shown in Figure 5a, the Cdl of PBSC@FeOOH-20 is 10.05 mF cm-2, which is approximately 2.15 times higher than that for PBSC, implying a significantly increased 13 ACS Paragon Plus Environment


active sites with 20 nm FeOOH layer decoration. In addition, PBSC@FeOOH-20 catalyst possesses rapid charge transfer rate for efficient electrocatalysis. As shown in Figure 5b, the charge-transfer resistance (Rct) is determined from the semicircle diameter of Nyquist plot, which shows that the Rct values of the catalysts for OER and HER firstly increase and then decrease with FeOOH thickness. It indicates the smaller charge transfer resistance and correspondingly faster charge transfer kinetics of PBSC@FeOOH-20 for OER/HER electrocatalysis. Furthermore, the mechanism and the intermediates for OER and HER are different. Therefore, the positive synergy and the negative effect by poor electronic conduction of PBSC@FeOOH-30 are different for OER and HER, which result in the different impedance order for OER and HER. Understanding the factors which contribute to the high electrochemical performance will guide the rational design and construction of efficient electrocatalysts. As shown in Figure 4, PBSC@FeOOH-20 delivers higher OER and HER activity than PBSC and FeOOH individuals, considering an intimate interface between PBSC and FeOOH to boost the water electrocatalysis activity. To further understand the reason responsible for the enhanced performance, the surface states of PBSC@FeOOH, PBSC and FeOOH are intensively investigated by XPS analysis. Figure 6a shows the XPS survey of PBSC and PBSC@FeOOH-20 samples. O, Pr, Ba, Sr and Co elements could be detected on the two samples, whereas Fe exists only on PBSC@FeOOH-20 sample. The core-level spectra of Co 2p & Ba 3d for these two catalysts are presented in Figure 6b. The overlapping between Co 2p and Ba 3d spectra makes it different to identify the surface Co valence by peak fitting, 14 ACS Paragon Plus Environment

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while the Co oxidation state could be inferred from the shift of peak position. Compared to pristine PBSC, the positions of two main peaks of PBSC@FeOOH-20 shift to higher binding energies (DE=0.60 eV) obviously, suggesting the higher oxidative state of Co species in PBSC surface induced by FeOOH decoration. Oxidized Co species is generally believed to is facile for OH- adsorption and provide abundant active site,53-54 which is consistent with the ECSA results (Figure 5a), and benefit for OER. Figure 6c presents the XPS spectra of Fe 2p for FeOOH and PBSC@FeOOH-20. The Fe 2p spectra includes two typical spin-orbit doublets characteristics of Fe 2p3/2 at 711.1 eV and Fe 2p1/2 at 724.9 eV, which are well indexed to the reported Fe3+ in oxyhydroxides.44 Compared to FeOOH, the Fe 2p peaks of PBSC@FeOOH-20 shift to lower binding energy (DE=0.30 eV), implying the presence of reduced Fe species. Figure 6d shows the O 1s spectra of PBSC@FeOOH-20, PBSC and FeOOH. The peaks at 529.7 eV and 531.3 eV correspond to Fe-O and OH- of FeOOH, respectively.44 For perovskite PBSC, the peak at around 528.7 eV and 531.1 eV are ascribed to lattice oxygen and OH- groups on the surface, respectively.55 The O 1s peak of lattice oxygen for PBSC@FeOOH-20 shifts negatively of ~0.27 eV than that of PBSC. Compared with those of FeOOH, both the peaks of OH- and Fe-O for PBSC@FeOOH-20 shift to the higher binding energies. Based on above results, it could be concluded that an electronic interaction occurs between perovskite PBSC core and oxyhydroxide FeOOH shell. In addition, larger peak shifts of Co 2p@Ba 3d and Fe 2p for PBSC@FeOOH-20 indicate strong electronic interactions between PBSC and FeOOH compared to PBSC@FeOOH-30 sample (Figure S9), which results in the synergy and 15 ACS Paragon Plus Environment


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facilitated charge transfer process for electrocatalytic reactions, as demonstrated elsewhere for hybrid.44-45, 56 Generally, the recognized OER mechanism in alkaline condition could be described as:57

OH   *  HO *+e

(1)

HO * OH   O *  H 2O +e 

(2)

O * OH   HOO * +e 

(3)

HOO * OH   *  O2 +e 

(4)

The asterisk denotes the surface electrochemical active sites. The OER activity mainly depends on the binding energies between electro-catalysts surface and different reaction intermediates (OH*, O* and OOH*).58 The bind of OHad is relatively strong for pristine FeOOH,43 when 20 nm thick FeOOH layer deposits on the PBSC surface, a suitable interaction of PBSC and FeOOH with OH- species could be induced, and the optimal electrocatalytic activity for OER is obtained. On the other hand, the first step of HER process in alkaline media is generally the reduction of water to H and OH- ( H 2O +e   OH   H ), then OH- species and H atoms adsorb onto the catalysts surface (namely a Volmer process). It is followed by either a Heyrovsky process which is determined by the rate of desorption of OHads ( H 2O +H ads +e   H 2  OH  ), or a Tafel process which is determined by the rate of desorption of Hads recombination ( H ads +H ads  H 2 ). When Volmer, Heyrovsky or Tafel process is the rate-determining step, the corresponding Tafel slope is 120, 40 and 30 mV 16 ACS Paragon Plus Environment

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dec-1, respectively.59 From Figure 4e, it is seen that the Tafel slope of PBSC@FeOOH based catalysts are 108~70 mV dec-1, indicating that the hybrid catalysis might be controlled by the Volmer process. It has proved that the adsorption of OH- is more important during Volmer process under alkaline condition.19,

22

The HER activity of

FeOOH is limited due to the strongly adsorbed hydroxyl ions. When the thin FeOOH layer decorated to PBSC surface, the strong electronic interaction modulates the local electronic structure of Fe cations in the FeOOH and reduces the adsorption energy between the transition metal sites and generated hydroxyl ions from water reduction,19,

60

and thus

benefits HER kinetics. The results are consistent with the EIS results (Figure 5b), that PBSC@FeOOH-20 has the smallest charge transfer resistance towards OER/HER processes. Therefore, the good charger transfer capability of PBSC@FeOOH-20 is an indication of optimized structure as well as the synergistic effect between perovskite oxide (PBSC) and hydroxide (FeOOH). Figure 7a confirms the superb bi-functional performance for OER and HER of PBSC@FeOOH-20 in alkaline environment (0.1 M and 1 M KOH). To test the practical feasibility of bifunctional PBSC@FeOOH-20 catalyst, a two electrode electrolyzer loading PBSC@FeOOH-20 on Ni foam (NF) as both cathode and anode is assembled and measured as shown in Figure 7b. For comparison, two-electrode electrolyzers comprised of NF//NF, PBSC//PBSC, and commercial IrO2(+)//Pt/C(-) are also configured. Notably, the PBSC@FeOOH-20 electrodes afford a current density of 10 mA cm-2 at a relatively low potential of 1.638 V, which compares favorably to those recently reported non-precious 17 ACS Paragon Plus Environment


metal based bifunctional catalysts (Table S3). Although the IrO2//Pt/C catalyzes with a lower voltage of around 1.55 V at 10 mA cm−2, PBSC@FeOOH-20//PBSC@FeOOH-20 is comparable to IrO2//Pt/C when the applied voltage is higher than 1.8 V in catalyzing water splitting. In addition, the electrolyzer based on PBSC@FeOOH-20 electrodes maintains superior durability as manifested by CP test with negligible potential change at increasing current densities of 10, 20 and 30 mA cm-2 for continuously 36 h operation (Figure 7c), whereas the reference IrO2//Pt/C couple exhibits poor stability of PBSC@FeOOH-20 for water splitting (Figure S10). During the durability test of PBSC@FeOOH-20 catalyst, H2 and O2 bubbles are continuously released from the electrode surfaces (inset of Figure 7c). The evolution of generated gases is further measured in Figure 7d. From the result, the amounts of generated gases are quite close to the theoretical calculation quantities, and the faradaic efficiency is approximated to 98 %. Therefore, PBSC@FeOOH-20 is as promising bi-functional catalyst with highly activity and good durability for practical application of alkaline water electrolysis application.

4. Conclusion

In summary, a hierarchical PBSC@FeOOH with core-shell structure is proposed and fabricated as a novel electrocatalyst for water splitting in alkaline solution. PBSC hollow nanorods is prepared by facial electrospinning method and ultrathin amorphous FeOOH nanoflakes is fabricated with the assistance of ALD strategy. The as-synthesized 18 ACS Paragon Plus Environment

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PBSC@FeOOH-20 catalyst with optimized FeOOH thickness (20 nm) exhibits excellent OER and HER activity and stability, and the impressive performance probably originate from the hierarchical nanostructure as well as the synergistic effect of PBSC and FeOOH. The synergy can be attributed to strong electronic interaction between PBSC and FeOOH, which favorably tune the interaction between intermediate products and catalysts. Of note, an alkaline electrolyzer with the optimized PBSC@FeOOH-20 as anode and cathode presents efficient performance for the full water splitting, with 10 mA cm-2 current density reached by supplying only 1.638 V and high electrochemical durability. The present work highlights new designing for biphasic synergic catalyst for overall water splitting.

Supporting Information

Additional SEM, TEM, XRD, XPS analysis and more electrochemical tests. Tables comparing OER, HER and water-splitting catalysts in this work and other perovskite-based catalysts in recent literatures.

Author information

Corresponding Authors * E-mail: [email protected] (B.B. He) * E-mail: [email protected] (Y. S. Gong)

ORCID 19 ACS Paragon Plus Environment


Beibei He: 0000-0001-0802-1519 Huanwen Wang: 0000-0001-9880-7723 Rui Wang: 0000-0001-5403-1628 Ling Zhao: 0000-0002-9500-3110 Yansheng Gong: 0000-0001-8197-9481

Notes The authors declare no competing financial interest.

Acknowledgements

The work was supported by the National Natural Science Foundation of China (Grant No. 21401171 and Grant No. 51402266).

References

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Figure Captions

Figure 1. (a) Fabrication schematics of integrated PBSC@FeOOH-20 catalysts with coreshell structure; (b) XRD patterns and (c) Raman spectrum of PBSC and PBSC@FeOOH20 samples. Figure 2. High magnification SEM images of (a) the as-spun PBSC nanowires, (b) bare PBSC hollow nanorods and (c) PBSC@FeOOH-20; (d) N2 adsorption-desorption isotherms of PBSC and PBSC@FeOOH-20 samples, (e)The corresponding Barrett-JoynerHalenda (BJH) Pore size distribution. 25 ACS Paragon Plus Environment


Figure 3. (a) Low magnification TEM images of PBSC nanorods, (b) high-magnification TEM images of PBSC and fast Fourier transform (FFT) pattern inset, revealing the double perovskite structure; (c) Low magnification TEM images of PBSC@FeOOH-20 with coreshell structure, (d) high-magnification TEM images of amorphous FeOOH decorated PBSC catalyst; (e) STEM-EDS element mapping of PBSC@FeOOH-20 catalyst, (f) EDS spectrum measured at position P1 in (e). Figure 4. (a) Linear sweeping voltammograms (LSVs) and (b) Tafel plots of PBSC, PBSC@FeOOH-10, PBSC@FeOOH-20, PBSC@FeOOH-30, FeOOH and commercial IrO2 catalysts for OER in 0.1 M KOH solution at 1600 rpm at a scan rate of 10 mV s-1, (c) Chronopotentiometry curves of PBSC@FeOOH-20 catalyst at a constant anodic current density of 10 mA cm-2, inset are polarization curves initially and 500th cycles; (d) LSVs and (e) Tafel plots of PBSC, PBSC@FeOOH-10, PBSC@FeOOH-20, PBSC@FeOOH30, FeOOH and commercial 20% Pt/C catalysts for HER in 0.1 M KOH solution at 1600 rpm at a scan rate of 10 mV s-1, (f) Chronopotentiometry curves of PBSC@FeOOH-20 catalyst at a constant cathodic current density of -10 mA cm-2, inset are polarization curves initially and 1000th cycles. Figure 5. (a) Linear fitting of the capacitive currents versus CV scan rates for PBSC and PBSC@FeOOH-20, (b) Nyquist plots of PBSC and PBSC@FeOOH-20 catalysts recorded at OER potential of 1.65 V and HER potential of -0.3 V (vs. RHE). Figure 6. (a) Full XPS spectrum of PBSC and PBSC@FeOOH-20 samples, (b) High resolution XPS spectra of Ba 3d & Co 2p of PBSC and PBSC@FeOOH-20, (c) Fe 2p of FeOOH and PBSC@FeOOH-20, (d) O 1s of PBSC, FeOOH, and PBSC@FeOOH-20 catalysts. Figure 7. (a) OER and HER activities of PBSC@FeOOH-20 in 0.1 M and 1 M KOH solutions, (b) Polarization curves of Ni foam//Ni foam (NF//NF), PBSC//PBSC, PBSC@FeOOH-20//PBSC@FeOOH-20 and IrO2(+)//Pt/C(-) for overall water splitting in 1 M KOH, (c) Chronopotentiometry curves of water electrolysis using PBSC@FeOOH-20 26 ACS Paragon Plus Environment

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as both anode and cathode at current densities of 10, 20 and 30 mA cm-2, (d) Evolution of H2 and O2 gas from PBSC@FeOOH-20 electrodes at a current density of 10 mA cm-2. The theoretical gas volume of H2 and O2 is calculated based on the ideal gas law and cumulative charge volume passed during electrolysis.



Figure 1


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Figure 2



Figure 3


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Figure 4



Figure 5


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Figure 6



Figure 7


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Graphical abstract

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Boosting Overall Water Splitting via FeOOH ... - ACS Publications

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