Communication pubs.acs.org/JACS
An Alkaline-Stable, Metal Hydroxide Mimicking Metal−Organic Framework for Efficient Electrocatalytic Oxygen Evolution Xue-Feng Lu,‡ Pei-Qin Liao,‡ Jia-Wei Wang, Jun-Xi Wu, Xun-Wei Chen, Chun-Ting He, Jie-Peng Zhang,* Gao-Ren Li,* and Xiao-Ming Chen MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China S Supporting Information *
Metal−organic frameworks (MOFs) are very attractive as catalysts for their highly ordered structures, large porosities, and diversified pore surfaces.17−20 Especially, their surfaces are mostly contributed from the highly ordered internal pores as observed in the crystal structures, which are not only beneficial for mechanism study but also can be easily functionalized by open metal sites (OMSs) with Lewis acidity and/or redox property.21−24 Some MOFs have been used as precursors to fabricate metal oxide/porous carbon nanocomposites as OER catalysts.25−28 However, it seems that MOFs themselves are unsuitable as OER catalysts,29−32 considering their poor stabilities (in water, especially basic/acidic conditions) and low electrical conductivities. Here, we report a strategy to integrate the advantages of metal oxides/hydroxides and MOFs. An open framework consisting of cobalt hydroxide chains is synthesized by metathesis of its cobalt chloride analogue, giving not only drastically improved OER activities but also a direct proof of the advantage of hydroxide ligand via the intraframework coupling pathway. We selected a metal azolate framework [Co2(μ-Cl)2(bbta)] (MAF-X27-Cl in the guest-free form, H2bbta =1H,5H-benzo(1,2-d:4,5-d′)bistriazole) as an OER candidate, for the high stabilities of this kind of MOFs and its high-concentration (5.88 mmol g−1, 6.69 mmol cm−3), oxidizable OMSs (from Co(II)Null to Co(II)-OH2 or Co(III)-OH) (Figure 1).33 Chemical stability tests showed that MAF-X27-Cl could retain its original crystallinity in acidic (0.001 M HCl) or strong alkaline (1.0 M KOH) solution for at least 1 week (Figure S1). Such a high chemical stability has been rarely reported for MOFs.34−36 Linear sweep voltammetry (LSV) was performed at pH = 14 for MAF-X27-Cl (microcrystalline powder coated on glassycarbon electrode (GCE) with Nafion binder). By repeating the LSV, the OER performance increased gradually and finally (after 24 h) reached an overpotential of 387 mV at 10 mA cm−2, being 121 mV lower than for the initial sample (Figure S2). X-ray photoelectron spectroscopy (XPS) analyses of MAF-X27-Cl after LSV tests showed that the Cl content disappeared, while the O content increased (Figure S3). Also, the color of the catalyst changed from pink to light pink, indicating that the Cl− ligand of MAF-X27-Cl might be replaced by OH−, giving a new catalyst [Co2(μ-OH)2(bbta)] (MAF-X27-OH in the guest-free form). Further experiments showed that the modification could be furnished by immersing MAF-X27-Cl in a solution of 1.0 M
ABSTRACT: Postsynthetic ion exchange of [Co2(μCl)2(btta)] (MAF-X27-Cl, H2bbta =1H,5H-benzo(1,2d:4,5-d′)bistriazole) possessing open metal sites on its pore surface yields a material [Co2(μ-OH)2(bbta)] (MAFX27-OH) functionalized by both open metal sites and hydroxide ligands, giving drastically improved electrocatalytic activities for the oxygen evolution reaction (an overpotential of 292 mV at 10.0 mA cm−2 in 1.0 M KOH solution). Isotope tracing experiments further confirm that the hydroxide ligands are involved in the OER process to provide a low-energy intraframework coupling pathway.
T
he electrochemical oxygen evolution reaction (OER) is the core process for a number of renewable energy systems such as metal−air batteries and water splitting.1−3 As a fourelectron process, OER always suffers from slow kinetics and needs high-performance catalysts to reduce the electrochemical overpotential.4 Precious metal oxides, IrO2 and RuO2, are so far the most efficient OER catalysts, but their high cost and scarcity, as well as low durability are impractical for large-scale applications.5,6 Other OER catalysts (mostly metal oxides/ hydroxides)7 generally show unsatisfactory catalytic activities with overpotentials of ca. 350−500 mV at 10 mA cm−2 (expected for a 10% efficient solar-to-fuels conversion device)5,6,8−10 at pH = 14. To improve the electrocatalytic performance, general strategies consider the catalyst particle size/morphology (surface structure and area), catalyst/electrode contact (electrical conductivity), composite effect, etc.1,11,12 Obviously, the chemical structure of the electrocatalyst especially on the solid/liquid interface is the most fundamental issue.13 However, the surface structures of conventional catalysts are usually different from those deduced from their crystal structures and difficult to determine directly.14 For example, while the metal atom/ion plays a critical role for OER because it coordinates and discharges the H2O/OH− species, the particle surfaces exhibit predominantly as metal oxides,7 or more accurately hydroxides in the aqueous, strongly oxidizing environment.15 It has been proposed by computational simulation that the surface hydroxide ligands can have a lower energy barrier than an external (solution) H2O/OH− to couple with the discharged species,16 but the advantage of such an intraframework coupling pathway has not been experimentally verified or rationally utilized because of the uncertain and uncontrollable catalyst surface structures. © XXXX American Chemical Society
Received: March 25, 2016
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DOI: 10.1021/jacs.6b03125 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society
Figure 1. (a) Three-dimensional coordination network and pore surface structures of MAF-X27-OH. (b−e) Local coordination environments of (b) water-appended MAF-X27-Cl, (c) guest-free MAF-X27-Cl, (d) water-appended MAF-X27-OH, and (e) guest-free MAF-X27-OH. (f) Solid−liquid coupling pathway for MAF-X27-Cl. (g) Intraframework coupling pathway for MAF-X27-OH.
cm3 g−1 (crystallographic values 0.51 and 0.57 cm3 g−1) for MAFX27-Cl and MAF-X27-OH, respectively, demonstrating good sample crystallinity and purity (Figure S8 and Table S2). Notably, the surface areas of MAF-X27-Cl and MAF-X27-OH are much higher than for inorganic OER catalysts (
