Porous Nickel–Iron Oxide as a Highly Efficient Electrocatalyst for Oxygen Evolution Reaction

• DOI: 10.1002/advs.201500199
• Publication Date: 2015-09-11T01:09:56Z
• Authors: Jing Qi; Wei Zhang; Ruijuan Xiang; Kaiqiang Liu; Hong‐Yan Wang; Mingxing Chen; Yongzhen Han; Rui Cao
• Keywords: Oxygen evolution; Nickel oxide

A porous Ni–Fe oxide with improved crystallinity has been prepared as a highly efficient electrocatalytic water oxidation catalyst. It small overpotential, low Tafel slope, and an outstanding stability. The remarkably performance is due to the structure, high extent homogeneous iron incorporation, ameliorative crystallinity, mass transfer resistance. Water splitting for generation of clean sustainable energy resource represents one most promising processes toward environmental remediation.1 Oxygen evolution reaction (OER) bottleneck in it kinetically slow requires overpotential (η) reach substantial current density electrolysis.2 Broad attention therefore attracted seek OER electrocatalysts working at overpotentials reduce input splitting.3 Regarding this, IrO2 RuO2 were reported have good activity.[3] However, use scarce noble metals limited their widespread applications. Toward identifying cheap OER, transition metal oxides,4 hydroxides,5 oxyhydroxides6 based on earth abundant elements extensively examined. Among them, mixed-metal materials containing group (Fe, Co, Ni) generally acknowledged competent candidates aqueous media.[3],[4],[6],7 For fabrication catalysts, widely employed techniques situ electrodeposition method amorphous films surface electrodes. structure electrochemical activity electrodeposited Ni, Ni–Fe, Co–Fe, Ni–Co studied.[4],6, 8 Our recent work first Fe-based film by fast simple cyclic voltammetry (CV) electrocatalyst neutral media turnover frequency (TOF).[5] Besides route, photochemical deposition soft loading mixed Ni–Fe–Co electrode controlled stoichiometry also developed.[4] Alongside methods, synthesized via variety such spin-coating,9 pulsed-laser ablation,10 aerosol spray,11 template synthesis,[5],[7] solid-state reaction,12 well exfoliation bulk materials13 activity. Moreover, layered double hydroxides (LDH) incorporation carbon significant enhancement activity.14 Typically, shown excellent activities basic media.[6],[14] Ni active oxygen center, Fe will improve conductivity material.[4],[6] universally that area contribute improvement, whereas poor arisen from adverse Thermal treatment can ameliorate stability.[7],12 thermal rarely applied largely thus materials.[4] Although considerable efforts devoted fabricating optimized sites or 3D architecture,[5],7,[14],15 documented enhanced stability without damage performance. Thus, desirable develop novel protocols synthesis sites, meanwhile, Herein, we report facile oxides presence organic surfactant Tween 85. By using Tween, which perfect boiling point catalyst this work, be created temperature evaporation surfactant. While traditional method, those carbon-based templates are burnt out under temperatures get structure. Heating elevated not only large cost but limits application instable unfavorable systems. system. creation mild suitable formation phase, preferable reservation area. sample after 200 °C compared room temperature. This outperformed its material counterpart (Tween free sample) OER. In CV studies, efficiently catalyze 0.1 m KOH solution. Small 328 420 mV required densities 10 50 mA cm−2, respectively. potential electrolysis (CPE) η = 387 mV, extremely stable ≈10.2 cm−2 maintained, giving extraordinary TOF value 432 h−1. material, our large-scale production, delivers valuable contribution research. composites obtained through coprecipitation strategy 85, polysorbate above 100 °C. solids collected subjected heat removal construct pores material. X-ray diffraction (XRD) patterns different starting ratios calcination 3 h (all time work), condition highest (see below), Figure 1A. Samples denoted Ni-X-Y, X stands percentage Y degrees Celsius. two samples nickel concentration tended form β-Ni(OH)2 (JCPDS 742075) phase showing selected better (100) (110) reflections 2θ degree 33° 59°, Increased Ni-90-200 Ni-85-200 resulted dominant NiO 780643) weak signals NiFe2O4 742081) observed latter sample. peaks around 62° 75° these assigned NiO, characteristic XRD peak 30° was observed. promote treatment, consistent photoelectron spectroscopy (XPS) studies Smith et al.[4] further increased content existed mixture γ-Fe2O3 871166) even higher content. Ni-50-200 sample, contains actually over 94% suggested XPS, quite phase. With ratio 85:15, tested synergic effects crystal referred 1B. freshly composite showed indexed α-Ni(OH)2 380715). After h, maintained emerging 11° (003) reflection perpendicular layers. phenomenon resembles conversion LDH new direction aging hot solution period time.[6] Due determined element analysis (≈40 wt%), layers probably wrapped molecules leading hydroxide nanosheets Upon released. As consequence, assembled together causing interlayers. result addition S1 (Supporting Information) distinguishable crystalline α-Ni(OH)2, discussions samples. start NiO/NiFe2O4 300 demonstrated 2A, release upon verified infrared spectra studied red line same conditions Tween. before displays distinct C–H stretching vibrations methylene methyl groups (2860, 2940 cm−1), CO/C–O ester/ether (1750, 1130 cm−1). gradually removed trace left treatment. O–H vibration (≈3480 cm−1) scissoring (1700–1500 cm−1 water, 1400–1300 hydroxyl groups) obvious all samples, especially low-temperature gravimetric (TGA) derivative thermogravimetric (DTG) mentioned 2B. starts lose weight sequence absorbed eventually undergoes oxide. transmission electron microscopy (TEM) images (Figure 3A–D), Brunauer–Emmett–Teller (BET) area, pore size distribution measurements (Table S1, Supporting Information, 3E,F). 3A) turned into aggregates comprised domains (darker areas) (lighter illustrated 3B,C. Higher converts more separated nanoparticles 3D. Scanning microscope (SEM) high-resolution TEM (HRTEM) provided Figures S2 S3 Information). Well-dispersed indium tin (ITO) SEM image. HRTEM, lattice fairly NiO. Elemental mapping dispersive (EDX) spectroscopy, suggesting nickel/iron dispersion S4 S5 BET areas distributions examined N2 physical adsorption measurements. 3E shows aforementioned diameter nm. expanded increasing volume during (red line, 3E) significantly than (blue 3E). adsorption–desorption isotherm plot treated type-IV closure P/P0 0.4, mesopores Integrally, capable illustrate procedure 4. doped sheets less Subsequently, evolved Fe-doped complete formed eventually. actual composition information XPS summarized Table foregoing discussed properties. Ni/Fe 2p Ni-X-200 Ni-85-Y presented S6 results Ni-50-Y displayed gray S6A,B indicate very content, much smaller solubility product Fe(OH)3 (Ksp 2.8 × 10−39) Ni(OH)2 5.5 10−16).16 FeIII precipitated solutions easier NiII, when base sufficient synthesis. resolved support 5). Fe-incorporated amount results. 2p3/2 centered 855.5 eV pure 854 eV. doping electronegativity causes valence state NiII.[4],[6] orange binding 857.0 isolated NiFe2O4, whose observed.17 3.1 mol%. fitted 710.2 715.6 developed spectrum ferric environment.18 tested. CVs 6A. Pure self-redox NiII NiIII 1.45 V versus reversible hydrogen (RHE, potentials vs RHE).[5],[6] percent suppress NiII. There hardly notable incorporation. rationalized increase surrounding atoms, become harder oxidized. electron-withdrawing effect nickel, facilitates partial charge possible kinetics.[6] 15%. Further 20% dramatically deteriorates performance, there 30% 6B). notably. 62 1.72 (η 487 mV). decrease activity, reduced change proved analysis, calcination. neither performance.[3],[4],12 believed deteriorate too much. Meanwhile, determinant materials. saturated optimally comparison, 6C best corresponding blank glassy (GC) electrode. dominantly electrocatalyst. addition, benchmarked Ir/C (20 wt% Ir) drop-casted onto GC comparison. Its study oxides, although earlier onset To evaluate electrocatalysts, comparison with/without 6D. blocking molecules. steadily leaving reasonable notice greatly outperform high-temperature up 400 made difference between plain added closer performances. Ni-85-100 former facets 1B converted relatively lower temperature, poorer crystallinity. feature helps reserve resultant detailed comparisons S7 increasingly suppressed along ferrite annealing Solís al.19 extent, interaction calcination, contributes improvement Considering observe 7A, CPE 1.62 conducted ITO electrodes casted line) (purple line), line). CPE, indicating durability. no activation process, oxidation, These features attributed consequence treatment.[7],12 rapid degradation min electrolysis. Before electrolysis, recorded S8 expounded HRTEM S9, Information), almost unchanged morphology Compared dried assisted synthesis, preserves other hand, increases ≈2 initial 1 sluggish period. movie vigorous bubbles (Movie measured calibrated Ocean Optics FOXY probe. During 6 55 C charges passed 141 μmol O2 evolved, gave Faradaic yield >98%. h−1 loaded achieved, among values OER.[4],[5],[6] should noted oxides/hydroxides substantially concentrated alkaline solution11,[14],[15] Au coated substrate electrodes.[6],20 any experimental details. 7B slope 42 decade−1 achieved. Correspondingly, 72 (Ni-85-250 Tween), impedance (EIS) Nyquist plots 8. confirmed 8A EIS potentials, semicircle high-frequency region resistance electrolyte surface, independent applied. following second low-frequency reactions, reference redox efficiency. 8B, ESI S10 reference. lowest work. summary, assistance Likewise, achieved oxidation. From stays constant Synthesis Electrocatalysts: oily mL dissolved NaOH 60 stirring, then Ni(NO3)2 Fe(NO3)3 (the total 2.5 m, 20 mL) dropwise. suspension stirred given 4 h. washed centrifugation (two times), acetone (one time), times) thoroughly. oven required. except Electrochemical Studies: All experiments carried CH Instruments (CHI 660E Analyzer) 15 conventional three-electrode configuration 0.07 cm2 electrode, Ag/AgCl Pt wire auxiliary drop-casting method. 2 mg 30 μL Nafion (5 wt%, DuPont) dispersed water–ethanol 2:1 ultrasonicating suspension. Then 5 Compensation iR drop used CVs. RHE equation: ERHE EAg/AgCl + (0.197 0.0591 pH). Current–potential data acquired implementing two-compartment cells. currents ranging 1.42 every step 600 s being gently stirred. fritted cell mm2) (18.0 aforesaid suspension). range Hz mHz amplitude sinusoidal voltage 1.52 1.67 V. calculated assuming atom involved catalysis (lower limit): i/(4F n). Here, i (A) current, number means four electrons molecule, F Faraday's (96 485.3 mol−1), n moles Analysis produced probe (Model NeoFox). authors grateful Fundamental Research Funds Central Universities, Shaanxi Normal University, "Thousand Talents Program" China, National Natural Science Foundation China Grant No. 21101170. service readers, journal provides supporting supplied authors. Such peer reviewed may re-organized online delivery, copy-edited typeset. Technical issues arising (other missing files) addressed Please note: publisher responsible functionality Any queries content) directed author article.