High-Efficiency and Durable Water Oxidation under Mild pH Conditions

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High-Efficiency and Durable Water Oxidation under Mild pH Conditions: An Iron Phosphate−Borate Nanosheet Array as a NonNoble-Metal Catalyst Electrode Weiyi Wang,†,‡ Danni Liu,‡ Shuai Hao,‡ Fengli Qu,§ Yongjun Ma,⊥ Gu Du,∥ Abdullah M. Asiri,¶ Yadong Yao,*,† and Xuping Sun*,‡ †

College of Materials Science and Engineering and ‡College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, Shandong, China ⊥ Analytical and Test Center, Southwest University of Science and Technology, Mianyang 621010, China ∥ Chengdu Institute of Geology and Mineral Resources, Chengdu 610064, China ¶ Chemistry Department, King Abdulaziz University, Jeddah 21589, Saudi Arabia §

S Supporting Information *

lytes.20−30 Such phosphate or borate catalysts are robust, inexpensive, and self-healing, and they are usually grown on conductive substrates as catalyst films by electrodeposition from metal-ion-containing potassium phosphate (KPi) or potassium borate (KBi) solutions. Compared to cobalt and nickel, iron is more earth-abundant and cheaper with less toxicity and rich redox properties for dioxygen activation in both biological and biomimetic environments.31,32 Unfortunately, the electrodeposition preparation of such an iron-based catalyst film from FeIII is difficult because FeIII ions precipitate out from water under neutral or near-neutral conditions. Although iron-based films can be deposited from FeII solutions with the presence of protonaccepting buffer anions33 or in acetate buffer,34 particular care must be taken to avoid FeII oxidation during deposition, and such catalyst films need large overpotentials to drive water oxidation (η7 mA cm−2 = 630 mV33 and η1 mA cm−2 = 480 mV34) in 0.1 M phosphate-buffered saline. Herein, we report our recent effort toward this direction in developing an iron phosphate−borate nanosheet array on carbon cloth (Fe−Pi−Bi/CC) via direct electrochemcial topotactic conversion from an iron phosphide nanoarray (FeP/CC) in 0.1 M KBi, with the use of FeP as both iron and phosphorus resources. As a 3D electrode, such an Fe−Pi−Bi/CC can drive a geometrical catalytic current density of 10 mA cm−2 at overpotentials of 434 and 383 mV in 0.1 and 0.5 M KBi, respectively. Remarkably, it also shows superior long-term electrochemical durability with 97.8% Faradaic efficiency (FE) for an oxygen evolution reaction (OER). A Fe−Pi−Bi nanoarray was developed via oxidative polarization of a FeP nanoarray on CC (see Figure S1 for preparation details).35 The X-ray photoelectron spectroscopy (XPS) spectra for FeP are presented in Figure S2, and the peaks at 706.7 and 129.5 eV are close to the binding energies (BEs) for iron and phosphorus in FeP, respectively.36 Figure 1 shows the XPS spectra in the Fe 2p, B 1s, P 2p, and O 1s regions for Fe−Pi−Bi. As shown in Figure 1a, the BEs of Fe 2p1/2 and Fe 2p3/2 appear at 724.6 and 711.2 eV, respectively, suggesting the formation of iron

ABSTRACT: It is highly desired but still remains a key challenge to develop iron-based large-surface-area arrays as heterogeneous water oxidation catalysts that perform efficiently and durably under mild pH conditions for solar-to-hydrogen conversion. In this work, we report the in situ derivation of an iron phosphate−borate nanosheet array on carbon cloth (Fe−Pi−Bi/CC) from an iron phosphide nanosheet array via oxidative polarization in a potassium borate (KBi) solution. As a 3D catalyst electrode for water oxidation at mild pH, such a Fe−Pi− Bi/CC shows high activity and strong long-term electrochemical durability, and it only demands an overpotential of 434 mV to drive a geometrical catalytic current density of 10 mA cm−2 with maintenance of its activity for at least 20 h in 0.1 M KBi. This study offers an attractive earthabundant catalyst material in water-splitting devices toward the large-scale production of hydrogen fuels under benign conditions for application.

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ntensive recent attention has focused on electrocatalytic and photocatalytic water splitting toward the storage of intermittent renewable energy as hydrogen fuels.1−4 To achieve more energy-efficient hydrogen production, active catalysts must be implemented to overcome the large overpotentials.5−9 Proton-exchange membrane electrolyzers respond well to fluctuations in power inputs but suffer from the use of noblemetal catalysts with low abundance and high cost.10−13 This issue can be avoided by alkaline electrolysis.14−16 Such harsh chemical environments for both techniques, however, limit the types of photoelectrodes and cell components.17 The water oxidation reaction at the anode suffers from sluggish kinetics with a large overpotential.18−20 So, efficient earth-abundant water oxidation catalysts (WOCs) at mild pH need to be developed. Cobalt and nickel have emerged as interesting iron group elements because of their catalytic power toward environmentally friendly water oxidation, and recent years have witnessed the rapid development of cobalt/nickel-based heterogeneous WOCs in neutral or near-neutral electro© XXXX American Chemical Society

Received: December 31, 2016

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DOI: 10.1021/acs.inorgchem.6b03171 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. XPS spectra for Fe−Pi−Bi in the (a) Fe 2p, (b) B 1s, (c) P 2p, and (d) O 1s regions.

with higher oxidation states (Fe3+).33,34 In the B 1s region (Figure 1b), the BE of 191.2 eV corresponds to the core levels of central boron atoms in borate species.37 The peak at 132.9 eV in the P 2p region (Figure 1c) reveals that P exists in the form of phosphate.38 The BE at 530.1 eV in the O 1s region (Figure 1d) is assigned to the central oxygen atoms in phosphate39 or borate.40 All of these observations demonstrate the successful conversion of FeP to Fe−Pi−Bi after oxidative polarization. Figure 2a presents the X-ray diffraction (XRD) patterns for CC, FeP/CC, and Fe−Pi−Bi/CC. CC shows two strong peaks at 26° and 43°, while additional peaks characteristic of the FeP phase are observed for FeP/CC (JCPDS 78-1443).41 After oxidative polarization, only diffraction peaks of CC are observed, suggesting the formation of amorphous species. Scanning electron microscopy (SEM) analysis shows the full coverage of bare CC by a FeP nanosheet array (Figure 2b). Note that amorphous Fe−Pi−Bi still retains its nanoarray feature but with a rough surface (Figure 2c). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analyses (Figure 2d−g) also confirm the conversion of crystalline FeP to amorphous Fe−Pi−Bi. Energy-dispersive X-ray (EDX) elemental mapping images (Figure 2h) reveal the uniform distriubution of iron, boron, phosphorus, and oxygen elements in Fe−Pi−Bi. The formation mechanism for a Fe−Pi−Bi nanoarray could be proposed as follows. Oxidative polarization leads to the oxidation of iron and phosphorus at the FeP surface to form iron cations and phosphate anions, respectively, and the electrostatic attractive forces between iron cations and anions of phosphate and borate subsequently drive them to precipitate as Fe−Pi−Bi on a FeP nanosheet.42 Such a process proceeds repeatedly until a FeP nanoarray is transformed completely to a Fe−Pi−Bi nanoarray. We investigated the OER activity of Fe−Pi−Bi/CC in 0.1 M KBi with a scan rate of 2 mV s−1 using a three-electrode configuration (loading: ∼2.21 mg cm−2). For comparison, similar tests for bare CC, RuO2 on CC (RuO2/CC), and FeP/ CC with the same loading were also performed. Figure 3a shows their linear-sweep voltammetry (LSV) curves on the reversible hydrogen electrode (RHE) scale. Note that all currents presented are corrected against the ohmic potential drop,43 and the current densities are based on the projected geometric

Figure 2. (a) XRD patterns for CC, FeP/CC, and Fe−Pi−Bi/CC. SEM images of (b) FeP/CC and (c) Fe−Pi−Bi/CC. TEM and HRTEM images of FeP (d and e) and Fe−Pi−Bi (f and g). (h) EDX elemental mapping images of iron, boron, phosphorus, and oxygen for Fe−Pi−Bi.

area of an electrode. It can be observed that RuO2/CC shows excellent OER activity with a low overpotential, whereas bare CC has poor OER activity. FeP/CC is also capable of catalyzing OER with an overpotential of 520 mV to drive a geometrical catalytic current density of 10 mA cm−2. In sharp contrast, Fe−Pi−Bi/CC has OER activity much superior to that of FeP/CC and only requires a much smaller overpotential of 434 mV to afford 10 mA cm−2. The overpotential of Fe−Pi−Bi/CC compares favorably with the behaviors of reported non-noble-metal OER catalysts in neutral or near-neutral media, including iron-based film/ indium−tin oxide (ITO; η7 mA cm−2 = 630 mV33 and η1 mA cm−2 = 480 mV34), FeOOH/ITO (η10 mA cm−2 = 560 mV),44 FeOOH/ fluorine-doped tin oxide (FTO; η10 mA cm−2 = 550 mV),45 Fe−Ci/ FTO (η10 mA cm−2 = 560 mV),46 NiOx−Fe−Bi (η5 mA cm−2 = 522 mV),47 Co−Mo (η1 mA cm−2 = 500 mV),48 LiMnP2O7 (η0.5 mA cm−2 = 680 mV),49 Cu-TPA (η5 mA cm−2 = 710 mV),50 etc. A more detailed comparison is listed in Table S1. The OER kinetics is also estimated by the corresponding Tafel plots for these electrodes (Figure 3b). The Tafel slope for Fe−Pi−Bi/CC is 94 mV dec−1, smaller than that of FeP/CC (114 mV dec−1), implying a more rapid OER rate for Fe−Pi−Bi/CC. The activity of a catalyst can also be expressed in terms of turnover frequency (TOF), which is measured using the reported method.51 For TOF calculations, the number of active sites associated with the redox iron species needs to be known. As observed, the oxidation peak current for iron shows a linear dependence on the scan rate (Figure S3), and the slope can be derived from the linear relationship (see the Supporting Information for calculation details). The TOF for Fe−Pi−Bi/ CC was calculated as 0.22 mol of O2 s−1 at η = 530 mV, larger than that of reported non-noble-metal OER catalysts like Co−Pi B

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Information for details). The CDL values for Fe−Pi−Bi/CC and FeP/CC are calculated to be 0.757 and 0.560 mF cm−2 (Figure S7), respectively, implying a higher surface roughness for Fe− Pi−Bi/CC and thus the exposure of more active sites.54 The evolved oxygen on Fe−Pi−Bi/CC was measured quantitatively using a calibrated pressure sensor to monitor the pressure change in an anode compartment of the H-type electrolytic cell.55 The FE of this electrode for OER was determined as 97.8% by comparing the amount of experimentally quantified oxygen with the theoretically calculated oxygen (Figure S8). In summary, a Fe−Pi−Bi nanoarray has been successfully in situ derived from a FeP nanoarray in 0.1 M KBi via oxidative polarization. As a catalyst electrode for OER, this nanoarray exhibits superior activity and long-term electrochemical durability, with a lower overpotential of 434 mV to drive 10 mA cm−2. This study not only provides an attractive low-cost catalyst material for high-performance electrochemical water oxidation but also offers us a universal preparative methodology toward an iron-based 3D catalyst electrode via topotactic electrochemical transformation for catalysis and sensing applications under benign conditions.56−60

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b03171. Experimental section, FE determination, time-dependent current density curves, XPS spectra, cyclic voltammograms, SEM image, capacitive current versus scan rate curves, LSV curves, and Table S1 (PDF)

Figure 3. (a) LSV curves for CC, RuO2/CC, FeP/CC, and Fe−Pi−Bi/ CC for OER. (b) Tafel plots for RuO2/CC, FeP/CC, and Fe−Pi−Bi/ CC. (c) Multicurrent process of Fe−Pi−Bi/CC. The current density started at 4 mA cm−2 and ended at 40 mA cm−2, with an increment of 4 mA cm−2 per 500 s without iR correction. (d) LSV curves recorded for Fe−Pi−Bi/CC before and after 500 CV cycles. (e) Time-dependent current density curve at a fixed overpotential of 510 mV in 0.1 M KBi. (f) LSV curves of Fe−Pi−Bi/CC in 0.1, 0.2, and 0.5 M KBi for OER.

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(0.002 mol of O2 s−1; η = 410 mV),23 Ni−Bi (0.01 mol of O2 s−1; η = 600 mV),30 iron-based film/ITO (0.21 mol of O2 s−1; η = 530 mV),34 and FeOOH/ITO (0.17 mol of O2 s−1; η = 600 mV).44 Figure 3c displays a multistep chronopotentiometric curve for Fe−Pi−Bi/CC in 0.1 M KBi. The potential immediately levels off at 1.64 V vs RHE at the starting current value and remains unchanged for the rest of the 500 s, and the other steps also show similar results, implying excellent mass transportation and mechanical robustness of the Fe−Pi−Bi/CC electrode.52 The stability of Fe−Pi−Bi/CC was probed using continuous cyclic voltammetry (CV) scanning. After 500 CV cycles, the Fe−Pi− Bi/CC electrode exhibits a negligible loss in the catalytic current density (Figure 3d). The electrolysis result (Figure 3e) demonstrates that this catalyst can maintain its catalytic activity for at least 20 h, and SEM image (Figure S4) indicates that such an electrode retains its morphological integrity after a long-term stability test. The effect of different concentrations of KBi on the OER process was also investigated. Fe−Pi−Bi/CC only needs an overpotential of 383 mV to drive 10 mA cm−2 in 0.5 M KBi (Figure 3f) with strong durability (Figure S5). We further measured the OER activity for Fe−Pi−Bi/CC under different pH values (Figure S6). As observed, increased pH leads to decreased overpotential. To obtain the electrochemically active surface area for Fe−Pi− Bi/CC, we determined the double-layer capacitances (CDL) at the solid/liquid interface for both Fe−Pi−Bi/CC and FeP/CC by CV.53 The cyclic voltammograms were collected in the region of 1.04−1.14 V vs RHE, where the current response should only be due to the charging of the double layer (see the Supporting

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.Y.). *E-mail: [email protected] (X.S.). ORCID

Xuping Sun: 0000-0001-5034-1135 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21575137 and 21375076) and the Key Technologies Research and Development Program of Sichuan Province (Grant 2017GZ0419).

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