An Approach to Understanding the Electrocatalytic Activity ...

Article pubs.acs.org/JPCC

An Approach to Understanding the Electrocatalytic Activity Enhancement by Superexchange Interaction toward OER in Alkaline Media of Ni−Fe LDH Miguel A. Oliver-Tolentino,† Juvencio Vázquez-Samperio,‡ Arturo Manzo-Robledo,§ Rosa de Guadalupe González-Huerta,§ Jorge L. Flores-Moreno,∥ Daniel Ramírez-Rosales,⊥ and Ariel Guzmán-Vargas*,‡ †

Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada, Instituto Politécnico Nacional, Calzada Legaria 694, Col. Irrigación, México D.F. 11500, Mexico ‡ ESIQIE-Departamento de Ingeniería Química, Laboratorio de Investigación en Materiales Porosos, Catálisis Ambiental y Química Fina, Instituto Politécnico Nacional, UPALM Edif. 7 P.B. Zacatenco, GAM, México, DF 07738, Mexico § ESIQIE-Departamento de Ingeniería Química, Laboratorio de Electroquímica y Corrosión, Instituto Politécnico Nacional, Edif. Z-5 3er piso, UPALM, Zacatenco, GAM, México, DF 07738, Mexico ∥ ́ Area de Química de Materiales, Universidad Autónoma Metropolitana-Azcapotzalco, Av. San Pablo 180, Col. Reynosa Tamaulipas, 02200 México, DF, Mexico ⊥ ESFM-Departamento de Física, Instituto Politécnico Nacional, UPALM Edif. 9 Zacatenco, GAM, México, DF 07738, Mexico S Supporting Information *

ABSTRACT: In the present work, the hydrotalcite-like materials known as layered double hydroxides (LDHs) were synthesized. The Ni−Al and Ni−Fe materials with different Ni/Fe ratio were obtained by coprecipitation method at variable pH. The LDH structure was verified by X-ray diffraction, Fourier transform infrared, and Raman spectroscopy. No secondary extra phases were observed for any material. The electronic properties were evaluated by UV−vis spectroscopy, while the magnetic ones were followed by electron paramagnetic resonance (EPR). The results suggested that sample H/ Ni−Fe2 (Ni/Fe = 2) has a ferrimagnetic behavior as a result of the combined action of NiII−OH−NiII, FeIII−OH−NiII, and FeIII−OH− FeIII pairs across the layers and ferromagnetic interactions operating between layers. Furthermore, the material H/Ni−Fe1 (Ni/ Fe = 1.5) showed a combination of paramagnetic and ferromagnetic interactions which favors a superexchange interaction among metal centers through the OH bridges across the cationic sheets; the superexchange interaction enhances the electrocatalytic activity on the oxygen evolution reaction (OER) in alkaline media. On the other hand, XPS experiments showed that the H/Ni− Fe1 did not exhibit structural changes after electrochemical processes. The activity toward the OER was in the order H/Ni−Fe1 > H/Ni−Fe2 > H/Ni−Al, as was confirmed using in situ linear sweep voltammetry (LSV) coupled with mass spectrometry (differential electrochemical mass spectrometry).

1. INTRODUCTION The efficient production of hydrogen by using renewable energy resources is a key component in the development of future energy-storage technologies. One method of producing hydrogen is from water electrolysis induced by renewable sources such as sunlight or wind. The efficiency of this process is limited by the oxygen evolution reaction (OER), which occurs simultaneously with hydrogen evolution.1,2 The hydrogen evolution reaction (HER) is a relatively simple reaction that readily occurs at low overpotential on many metals. Whereas the oxygen evolution mechanism involves several steps having large reaction barriers, and then leading to large overpotentials to drive the reaction at practical rates. This large overpotential significantly decrease the efficiency, as the extra © 2014 American Chemical Society

energy is dissipated as low-quality heat, limiting the possibility of hydrogen-large scale production from water splitting.3,4 On the other hand, the most active OER catalysts are IrO or RuO2 working in acid or alkaline conditions; however, these materials suffer for their scarcity and high cost of precious metals.5 In this context, extensive efforts have been taken to develop highly active, durable, and low-cost alternatives. In particular, the activities of transition-metal-based catalyst for OER were proposed to relate to the 3d electron number; the surface of this metal exhibited eg orbitals which could bond with surfaceReceived: July 11, 2014 Revised: September 1, 2014 Published: September 5, 2014 22432

dx.doi.org/10.1021/jp506946b | J. Phys. Chem. C 2014, 118, 22432−22438

The Journal of Physical Chemistry C

Article

and 0.017 mol of Fe(NO3)3·9H2O (Aldrich) in 100 mL of deionized water. While maintaining the first solution under vigorous stirring, the second solution was slowly added by means of a peristaltic pump. After complete addition, the resultant slurry was stirred for 2 h at room temperature; the pH of the suspension was 9.5. Finally, the suspension was stirred for 2 days at 50 °C, and then the solid obtained was separated by centrifugation, rinsed thoroughly with warm distilled water, and dried overnight at 80 °C. The solids obtained were labeled H/Ni−Fe1 and H/Ni−Fe2 with Ni/Fe ratios of 1.5 and 2, respectively, and H/Ni−Al, Ni/Al = 4. The modified carbon paste electrodes were prepared by mixing graphite powder (Alfa Aesar, 99.9995%), silicon oil (Aldrich), and the corresponding LDH at 20 wt %. The mixture was mechanically homogenized and inserted in a 2 mm diameter cylinder (0.0314 cm2). The surface contact on the electrode was made with a platinum wire. The X-ray diffraction (XRD) structural characterization of LDHs was performed in a Philips X’PERTPRO instrument using Cu Kα1 radiation (λ = 1.542 Å, 45 kV, and 40 mA); whereas the spectroscopic analyses were recorded in a Fourier transform infrared (FT-IR) spectrophotometer PerkinElmer RX-1 with an attenuated total reflectance accessory (ATR), and a Raman spectrophotometer Labram HR800 with a laser operating at 784.29 nm. XPS studies were performed by using a Thermo Scientific KAlpha X-ray photoelectron spectrometer with a monochromatized Al Kα X-ray source (1,487 eV). Narrow scans were collected at 60 eV analyzer pass energy and 400 μm spot size. The electronic properties were performed with a UV−visNIR spectrophotometer PerkinElmer Lambda 950. Electron paramagnetic resonance (EPR) measurements were carried out at room temperature and 77 K using a JEOL JES-RES3X continuous wave EPR spectrometer. Typical EPR spectral parameters were as follows: X-band frequency = 9.1642 GHz, modulation amplitude = 3.2 G, modulation frequency = 100 kHz. Electrochemical analyses were carried out at room temperature in a potentiostat−galvanostat VERSASTAT3-400 (Princeton Applied Research). A three-electrode standard electrochemical cell was used for the cyclic voltammetry (CV) measurements at 5 mV s−1 with a carbon rod and a saturated calomel electrode (SCE), respectively. For these experiments, the working electrode was made from the synthesized materials, immersed in a carbon paste electrode (CPE) matrix. For in situ experiments during polarization in the OER region, DEMS coupled with a homemade electrochemical cell made of Teka-Peek was used. Briefly, the electrochemical cell was connected to the chamber containing the quadrupole mass spectrometer (MS, Prisma QMS300, Pfeiffer) through a precision valve, which allows the isolation of the ion source from the electrochemical cell forming a small prechamber. The DEMS system allows using three electrochemical cells connected to the vacuum system. A duo pumps evacuates the latter, whereas the vacuum in the chamber containing the MS is obtained from a turbo molecular pump (the working pressure was ca. 7 × 10−6 mbar for all set of experiments described therein). The amount of species reaching the MS, throughout a porous membrane (60 μm thick, 0.2 μm pore diameter, and 50% porosity) can be controlled with the dosing valve located between the electrochemical cell and the prechamber. Mass spectrometric profiles (ionic current (Ii) versus potential (E)) and faradic

anion adsorbents. Among the materials that have been studied are perovskites,6 nickel oxides-based materials.7,8 Specially, the Fe plays a critical, but not yet understood, role in enhancing the activity of the Ni-based oxygen evolution reaction (OER) electrocatalysts.9,10 Recently layer double hydroxide (LDH) materials have showed promising results.11−15 LDHs belong to the anionic clays family or the hydrotalcitelike compounds. LDHs result from the stacking of positively charged brucite-like octahedral layers. The positive charge results from the replacement of part of the divalent metal (MII) cations by trivalent (MIII) ones. The spaces between layers host solvated anions (An−) that compensate the positive charge of layers. Thus, these materials exhibit specific properties as anion exchangers,16 and hydrotalcite-like compounds can be represented by the general formula: [MII1 − xMIII x(OH)2 ]x + [An −x / n]·mH 2O

In the LDH family, the metallic cations can be from the transition group and thus undergo redox reactions in the range of applied potential. This inclusion of transition metals in the layers has been proposed to enhance the charge transport of the material. Such charge transport can be thought as due to a mixed mechanism involving an “electron hopping” along the layers; which is ascribable to an inner redox reaction between oxidized and reduced forms of the MII/MIII couple, and a migration of anions inside the interlayers to compensate the positive extra charge.17,18 In particular, the Ni-based LDH has been studied as electrochemical sensor,19 electrochemical pseudocapacitor,20 and electrocatalyst.21,22 Nevertheless, few works, involving Ni−Fe LDH, devoted to water electrolysis has been published. Recently Gong et al. reported the performance of LDH Ni−Fe (Ni/Fe = 5), and the main results pointed out high electrocatalytic activity and stability for oxygen evolution reaction in alkaline media than commercial precious metalbased catalysts.11 Lu et al. showed that 3D architecture films of vertically aligned Ni−Fe LDH nanoplates exhibited excellent performance in the OER: small onset overpotential, low Tafel slope, large anodic current density and prominent electrochemical durability.12 Tang et al. synthesized carbon quantum dot/Ni−Fe LDH nanoplates, this complex system exhibited high catalytic activity toward water electrolysis and good stability for oxygen evolution.13 Long et al. reported a low overpotential (as low as 0.195 V) of catalytic activity toward OER using hybrid catalyst of NiFe-LDH and graphene.14 However, until now it has not been reported some explanation that allow us to understand the activity toward the OER from the LDH properties. In the study herein, the synthesis of Ni−Al and Ni−Fe LDH with different Ni/Fe ratios is reported. The materials were evaluated as electrocatalysts in OER, and their activity was explained from the Ni−Fe material magnetic behavior. The OER takes place with the highest faradic performances as also demonstrated by the in situ differential electrochemical mass spectrometry (DEMS) technique.

2. EXPERIMENTAL SECTION The Ni−Al and Ni−Fe LDHs were prepared by coprecipitation as described elsewhere. As an example, H/Ni−Fe2 was prepared by dissolving 0.117 mol of NaOH (Aldrich) and 0.034 mol of Na2CO3 (Aldrich) in 100 mL of deionized water; the pH of this solution was 13.4. A second solution was prepared by dissolving 0.034 mol of Ni(NO3)2·6H2O (Aldrich) 22433



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the FT-IR spectra for LDH materials. The intense and broad lines are observed between 3416 and 3388 cm−1 and ascribed to the O−H stretching vibration mode of water molecules which are intercalated within the interlaminar space. Besides interlaminar water molecules, hydrogen-bonded hydroxyl groups along the cationic sheets might also be responsible for broadening the band. The medium-intensity band around 1630 cm−1 is related to the bending mode of those hydrogens bonded to water molecules. In fact, this band intensity is proportional to the amount of intercalated water in the studied sample. The stretching mode of CO32− anions is responsible for the sharp and strong band around 1360 cm−1. The stretching and bending vibrational modes were attributed to hydroxometallic octahedral complexes which constitute the [MII1−xMIIIx(OH)2]x+ cationic brucite-like sheets are responsible for absorption at lower wavenumbers ( H/Ni−Fe2 > H/Ni−Al. On the other hand, the current−potential characteristic obtained using CV for H/Ni−Al displays well-defined anodic and cathodic peaks associated with redox process for the couple NiIII/NiII with a peak-to-peak potential ΔEp = 200 mV; see inset in Figure 3. This electrochemical behavior is well-known and is due to insertion/desertion of OH− ions from the LDHinterlayer space during nickel-sites oxidation/reduction by electron hopping mechanism along the brucite structure inducing electroneutrality. Then the redox process can be represented by reaction R1:18 H/Ni II−Al + OH−sol ⇄ H/Ni III(OH−)ie −Al + e−

(R1)

The CVs for LDH materials are exhibited in Figure S2. From these profiles, an amplification range between 1.2 and 1.4 V/ RHE showed the faradic peak at 1.3 V/RHE associated with reduction of NiIII to NiII during cathodic scan (see inset), suggesting that in the H/Ni−Fe solids also nickel oxidation takes place during anodic scan. It is worth mentioning that nonfaradic processes were observed at CPE free of LDH (figure not shown). In order to confirm the generation of molecular oxygen during anodic polarization in the LDH-based materials, the in situ DEMS technique was employed. For such an approach, an alternative electrochemical cell connected to the MS was used. The scan rate was fixed at 1 mV s−1. Figure 4A−D shows the current−potential characteristics corresponding to the OER onto the LDH, in argon-purged 1 M KOH solution. Notice that, for the sample H/Ni−Fe1, the magnitude of the faradic current (Figure 4A) is higher when it is compared with sample H/Ni−Fe2 (Figure 4D). This faradic current represents, in fact, the production of oxygen at the interface of the electrode, as it can be observed from the mass-to-charge ratio (m/z = 32) obtained by DEMS in Figure 4B and E for H/Ni−Fe1 and H/ Ni−Fe2, respectively. However, such a mass signal is higher for 22435



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H/Ni II−MIII + OH−sol ⇄ H/Ni III(OH−)ie −MIII + e− (R2)

H/Ni III(OH−)ie −MIII + OH−sol ⇄ H/Ni III(OH−)ie −MIII(OH)ad + e−

(R3)

H/Ni III(OH−)ie −MIII(OH)ad + OH−sol ⇄ H/Ni III(OH−)ie −MIII + 1/2O2 + H 2O + e−

(R4)

where sol is solution, ie is interlayer space, ad is adsorbed, and M is FeIII or AlIII. On the other hand, the specific electrocatalytic performance of the system can be given in terms of the turnover number (TON), defined here as the number of oxygen molecules generated at 1.62 V per second and per catalytic site. The TON number was calculated (see the Supporting Information) assuming that all available nickel sites are involved in the electrochemical reaction.8 The TON values obtained were H/ Ni−Al ∼ 0.05, H/Ni−Fe2 ∼ 9.93, and H/Ni−Fe3 ∼ 38.08 s−1, whereas the currents at a potential of 1.65 V/RHE were 46, 120, and 922 μA for H/Ni−Al, H/Ni−Fe2, and H/Ni−Fe1, respectively. These values put in evidence that, as discussed above, H/Ni−Fe1 exhibited the highest electrocatalytic activity toward OER, attributed to increase the amount of Fe which induced partial-charge transfer mechanism, which activates Ni centers.10 This could be explained due to electron delocalization on the layer surface, which permits the improvement of electron hopping through the brucite layer. Such electron delocalization is due to the arrangement of the Fe atoms in the lattice, which spins from a specific magnetic structure that favors the superexchange interaction. A greater superexchange interaction implies a greater electron delocalization. The effect of anionic interference in the O2 production for H/Ni−Fe1 material, using Cl− and NO3− in a ratio 2:1 with respect to OH−, is observed in Figure 5B. The results showed that the Tafel slope decrease in the presence of Cl− and NO3− (see inset). Similar behavior was observed for H/Ni−Fe2. This fact can be explained in terms of (i) insertion in the interlayer space or (ii) adsorption on the layer of Cl− or NO3− during the anodic polarization. However, due to the strong interaction of OH− with the brucite layers and the prioritization of anionic exchange, only the insertion of OH− could be favored by the application of an anodic potential. This was verified by cyclic voltammetry profiles for H/Ni−Al in the same solution as that in Figure 5 (figure not shown). From this, the i−E

Figure 4. Current−potential characteristics for (A) H/Ni−Fe1 and (D) H/Ni−Fe2, in 1 M KOH and scan rate of 1 mV s−1. Panels (B− C), (E), and (F) are the mass signal profiles obtained using DEMS during anodic polarization.

H/Ni−Fe1 as expected for the magnitude of faradic current from Figure 4A. In addition, for both samples analyzed in Figure 4, no evidence of secondary redox reaction (i.e., corrosion) was observed as the mass signal corresponding to carbon dioxide (m/z = 44) was not perturbed; see Figure 4C− F. In this context, for the case of sample H/Ni−Al and CPE free of LDH, the magnitude of the faradic current is less intense and then the production of oxygen decreases (Figure S3). The DEMS analysis then shows nicely that the electrocatalytic activity toward OER is in the order H/Ni−Fe1 ≫ H/N−Fe2 > H/Ni−Al > CPE. Tafel plots obtained from polarization curves were fashioned (Figure 5A). The resulting Tafel slopes were ca. 34, 36, and 37 mV dec −1 for H/Ni−Al, H/Ni−Fe2, and H/Ni−Fe1, respectively. These values are smaller than those reported for the system Ir/C which exhibited a Tafel slope of ca. 40 mV dec−1,11 indicating that, within experimental error, the OER mechanism is similar for all set of these materials. Such a reaction mechanism might be related with (i) surface oxidation by one electron electrochemical step R2; (ii) adsorption step R3; and (iii) a one electron-electrochemical rate-determining step for oxygen production R4.7 A general process suggested is described by the next reactions.

Figure 5. (A) Tafel plots of (a) H/Ni−Al, (b) H/Ni−Fe1, and (c) H/Ni−Fe2 obtained from i−E characteristic of Figure 3. (B) Linear sweep voltammetry of H/Ni−Fe1 in (a) 1 M KOH, (b) 1 M KOH + 0.5 M KNO3, and (c) 1 M KOH+ 0.5 M KCl. Inset: Tafel plots. 22436



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Figure 6. XPS spectra of (A) Ni 2p and (B) Fe 2p for H/Ni−Fe1 (a) before the electrochemical process and after 50 cycles in different solutions of (b) 1 M KOH, (c) 1 M KOH + 0.5 M KNO3, and (d) 1 M KOH+ 0.5 M KCl.

Production of molecular oxygen (m/z = 32) and the absence of secondary interfacial-electrochemical reactions during anodic polarization (i.e., corrosion) were confirmed using LSV coupled with mass spectrometry (DEMS). Whereas Tafel slopes were around 40 mV dec−1, indicating that every materials rules out by the same mechanism reaction, where one electron electrochemical is the rate-determining step for O2 production. On the other hand, experiments carried out at different anionbased solutions did not show important catalytic effect on OER. The good stability of LDH materials after the electrochemical process was observed by XPS spectra. This study identifies the importance of magnetic behavior in the electrochemical process.

characteristics did not exhibit significant differences in shape of the signal peak potential related with oxidation and reduction of nickel centers. However, some changes in the reaction mechanism were observed, verifying that OER occurs on the brucite layers, according to reactions R2−R4. In addition, the current obtained for H/Ni−Fe1 at 1.63 V/ NHE was more intense in solution containing Cl− + OH− (curve c, Figure 6.). Whereas lower current intensity was attained using solutions based on NO3− + OH− ions (curve b) and with OH− free of NO3− and Cl− (curve a). This behavior might be associated with the adsorption of Cl− on the layersurface producing the oxidation of Cl− to Cl2. Conversely, for a solution based on NO3− ions, more studies are necessary in order to better understand the implied phenomena causing lower faradic efficiency. The TON values calculated were 25 s−1 in Cl− and 17 s−1 in NO3−, indicating that current due to OER decreases. Furthermore, the stability of H/Ni−Fe1 structure after the induced electrochemical process was evaluated by XPS (Figure 6). The spectra of the raw sample (i.e., before inducedelectrochemical reaction) (curve a) show the typical binding energy corresponding to Ni 2p3/2 and 2p1/2 located at ca. 855.5 and 873.2 eV, respectively (Figure 6A). Whereas the binding energies for Fe 2p3/2 and 2p1/2 are positioned at 712 and 725.3 eV (Figure 6B), confirming the oxidation state (2+) for Ni and (3+) for Fe. In addition, using high resolution XPS, nickel species were identified and qualitatively analyzed using the decomposition model propose by Grosvenor et al.,32 including nickel hydroxide and oxyhydroxide spectra (Figure S4). The Ni 2p doublet spectrum of each sample was reconstructed using a Gaussian−Lorentzian mix function and Shirley background subtraction. To decompose the experimental spectrum, it was necessary to use seven double contributions: Ni(OH)2 located at 454.9 and 872.5 ± 0.2 eV, for Ni 2p3/2 and Ni 2p1/2 peaks, respectively. While Ni2+multiplet and satellite contributions were located taking into account the results reported in ref 32. The high resolution XPS spectra after 50 cycles using cyclic voltammetry did not show important structural modifications.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1. FTIR and Raman spectra of H/Ni−Al, H/Ni−Fe1, and H/Ni−Fe2. Figure S2. Cyclic voltammetry of H/Ni−Al, H/Ni−Fe1, andH/Ni−Fe2 in 1 M KOH; inset, cathodic sweep. Figure S3. DEMS profiles of H/Ni−Al and CPE, in 1 M KOH. Figure S4. XPS spectra of Ni 2p deconvolution, and calculation of turnover number (TON). To Luis Lartundo from CNMN-IPN for XPS analysis. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*Tel: +52 5557296000. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Projects SIP-IPN 20140793, CONACYT 101319, and SECITI DF (ICVIDF/ 193/2012). A.M.-R. thanks IPN-CONACYT 160333-DEMS.

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4. CONCLUSION LDH materials with different Ni/Fe ratios using a coprecipitation method at variable pH were synthesized. The iron content in the structure of these materials plays an important role on their magnetic properties. It may be postulated that the superexchange interaction is one of the main causes that promotes the enhancement of the H/Ni−Fe1 catalytic activity toward OER; the catalytic activity associated with TON value of this sample was higher than H/Ni−Fe2.

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