F P d o A a b c d a A R R A A K U C C O W 1 w m u t a ( b t s c k I d h t o ( h 0 Applied Surface Science 434 (2018) 1153–1160 Contents lists available at ScienceDirect Applied Surface Science jou rn al h om ep age: www.elsev ier .com/ locate /apsusc ull Length Article ulse electrodeposition of CoFe thin films covered with layered ouble hydroxides as a fast route to prepare enhanced catalysts for xygen evolution reaction lan M.P. Sakitaa,b, Rodrigo Della Nocec, Elisa Vallésb,d, Assis V. Benedetti a,∗ Instituto de Química, UNESP-Universidade Estadual Paulista, 14800-900 Araraquara, Brazil Ge-CPN (Thin Films and Nanostructures Electrodeposition Group), Dpt. Ciència de Materials i Química Física, Martí i Franquès 1, 08028 Barcelona, Spain Centro de Química Estrutural-CQE, Departament of Chemical Engineering, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal Institute of Nanocience and Nanotechnology (IN2UB), Universitat de Barcelona, Spain r t i c l e i n f o rticle history: eceived 14 August 2017 eceived in revised form 16 October 2017 ccepted 6 November 2017 vailable online 8 November 2017 a b s t r a c t A novel, ultra-fast, and one-step method for obtaining an effective catalyst for oxygen evolution reaction is proposed. The procedure consists in direct electrodeposition, in a free-nitrate bath, of CoFe alloy films covered with layered double hydroxides (LDH), by potentiostatic mode, in continuous or pulsed regime. The catalyst is directly formed on glassy carbon substrates. The best-prepared catalyst material reveals a mixed morphology with granular and dendritic CoFe alloy covered with a sponge of CoFe-LDH containing eywords: ltra-fast electrodeposition oFe alloy oFe-layered double hydroxide xygen evolution reaction a Cl interlayer. An overpotential of �10 mA = 286 mV, with a Tafel slope of 48 mV dec−1, is obtained for the OER which displays the enhanced properties of the catalyst. These improved results demonstrate the competitiveness and efficacy of our proposal for the production of OER catalysts. © 2017 Elsevier B.V. All rights reserved. ater splitting . Introduction Water electrolysis reaction (H2O → H2 + 1/2O2), also called ater splitting (WS), has attracted a great interest in last years ainly because it can be used for large-scale energy storage by sing renewable energy sources [1,2]. However, one of the reac- ions involved in WS, the oxygen evolution reaction (OER), shows low kinetics attributed to a four-electron multi-step reaction 4OH− → 2H2O + O2 + 4e−, in alkaline medium) [3]. The OER can e enhanced by using catalysts that not only accelerate the reac- ion, but also decrease its overpotential. Oxides of noble metals uch as IrO2 and RuO2 have shown good performance as electro- atalysts for water oxidation, inducing low overpotential and fast inetics, which are attributed to the different oxidation states of ridium and Ruthenium oxides and to the exchange between the ifferent oxidation states during the reaction [4]. Nevertheless the igh cost of the precursor metallic salts to prepare these catalysts, heir low abundance, and the difficult extraction and manufacture f these metals are some drawbacks that limit their use for OER. ∗ Corresponding author. E-mail addresses: benedeti@iq.unesp.br, avbenedetti@gmail.com A.V. Benedetti). ttps://doi.org/10.1016/j.apsusc.2017.11.042 169-4332/© 2017 Elsevier B.V. All rights reserved. Therefore, much effort has been done to lower the produc- tion colour costs of OER catalysts, especially for obtaining (oxy)hydroxides and layered double hydroxides (LDHs) of earth- abundance and cost-effective transition metals [5]. The catalytic properties of these earth-abundant metal oxides/hydroxides are related to their ability to exchange oxidation states, e.g. Ir and Ru oxides, which make them potential materials for WS applications [6,7]. Among the preparation methods of oxides/hydroxides of tran- sition metals with enhanced electrochemical properties for OER, the solvothermal [2,8] and hydrothermal [9,10] synthesis have shown to be reasonable methods for the preparation of catalysts with low overpotential for the WS reaction. Their high cost of pro- duction due to the time-consuming of synthesis and the need of using high temperature and pressure are obstacles that hinder their widespread utilization, however. Additionally, their large- scale production implies a solvent separation step, which requires the utilization of centrifuges and filtering systems that makes the process costly. Electrodeposition may be used for the preparation of (oxy)hydroxides of transition metals directly over the substrate due to its benefits such as low cost and no need of binders [11,12]. Typically, researchers have tested this possibility by elec- trodepositing the metal (oxy)hydroxides using nitrate-containing https://doi.org/10.1016/j.apsusc.2017.11.042 http://www.sciencedirect.com/science/journal/01694332 http://www.elsevier.com/locate/apsusc http://crossmark.crossref.org/dialog/?doi=10.1016/j.apsusc.2017.11.042&domain=pdf mailto:benedeti@iq.unesp.br mailto:avbenedetti@gmail.com https://doi.org/10.1016/j.apsusc.2017.11.042 1 rface S m ( o s f t e M a p 2 2 c P o g a d 2 ( s w 5 b i p d t t s t d 2 m P t a s t v 2 d E a g ( L l s a 154 A.M.P. Sakita et al. / Applied Su edium because the cathodically induced nitrate reduction NO3 − + 7H2O + 8e− → NH4 + + 10OH−) produces significant amount f OH− ions that react with metal ions to form hydroxides on the ubstrate surface [12,13]. In this work, we propose a novel, facile, ast, nitrate-free, and one-step synthesis to directly obtain CoFe hin films covered by layered double hydroxides (CoFe//LDH) with nhanced catalytic electrochemical properties for water splitting. oreover, catalysts obtained by means of pulse electrodeposition re optimized and compared with those obtained by continuous otential application. . Experimental section .1. Deposition solution The deposition solution was prepared by dissolving the metal hloride salts (FeCl2·4H2O Sigma-Aldrich 99.0% and CoCl2·6H2O anreac Analytical Grade) in Milli-Q quality water to reach 0.1 M f each metallic salt, and 0.5 M KCl as supporting electrolyte. Oxy- en was removed from the solution using argon during 20 min to void the Fe2+ ion oxidation. The solution pH was about 2.5–3, and id not change by the addition of dilute acids or bases. .2. Electrodeposition process The deposition of CoFe//LDH was performed on glassy carbon GC) electrodes with geometric area of 0.071 cm2, using potentio- tatic method. GC electrode was polished to mirror-like finishing ith alumina slurry of 0.3 �m. The potentials were applied for s for all the deposited materials. Other tests were performed y applying potentials from 1 to 60 s but the optimum condition n terms of catalytic activity was obtained for the deposits pre- ared at 5 s, both in continuous and in pulse regimes. The pulsed eposition was performed with equal ton and toff, with a total ime of ton = 5 s. The scheme 1 (Supplementary material) shows he described process for pulsed deposition. For the electrodepo- ition process, Ag|AgCl|KCl3M reference electrode was used while he counter electrode was a Ti/Rh spiral. All the potentials of the eposition process are referred to this reference electrode. .3. Electrochemical studies The electrochemical studies were performed using a icrocomputer-controlled potentiostat/galvanostat AUTOLAB GSTAT30 and GPES software. For the water splitting experiments, he reference electrode was a reversible hydrogen electrode (RHE) nd a Ti/Rh wire was employed as counter electrode. The water plitting experiments were carried out using a 1 M NaOH solu- ion and the polarization curves were obtained at 5 mV s−1. The oltammetric curves showed in this work have no IR correction. .4. Surface characterization XPS (X-ray photoelectron spectroscopy) experiments were con- ucted in a PHI 5500 Multitechnique System (from Physical lectronics) with a monochromatic X-ray source. X ray diffraction (XRD) experiments were registered in a PAN- lytical X’Pert PRO MPD powder diffractometer Bragg-Brentano eometry and �/� goniometer. Nickel filtered Cu K� radiation � = 1.5418 Å) and a work power of 45 kV–40 mA were used. Raman spectroscopy was performed using a Jobin Yvon-Horiba abRam HR800 equipment and the 532 nm green excitation wave- ength from a Nd:YAG laser. FE-SEM (Field emission scanning electron microscopy) analy- is was performed using a JEOL J-7100 operating with electron cceleration of 2 keV. cience 434 (2018) 1153–1160 3. Results and discussion A voltammetric study of the CoFe deposition process on glassy carbon substrates was performed to select the best conditions for the preparation of CoFe//LDH catalysts. Potentials sufficiently neg- ative to induce H2 evolution, with consequent increase of the local pH, were selected to prepare the oxidized catalysts. The aim was to obtain deposits composed of CoFe alloy directly covered with LDH. Fig. SM1 shows the cyclic voltammograms of the solution contain- ing Fe(II) and Co(II), at different scan rates, from which the onset of the deposition process was detected at about −1.0 V at 10 mV s−1. After that, a diffusion-controlled peak for the reduction of Co(II) and Fe(II) was observed at –1.12 V. The scan to more negative potentials shows a fast increase of the cathodic current at −1.25 V, related to the hydrogen evolution reaction (HER). From this study, we tested deposition potentials in the −1.0 to −1.8 V range, selecting values ca. −1.4 V as the more adequate to avoid pure CoFe alloy deposi- tion and excessive hydroxides formation. Short deposition times were used in order to obtain thin particulate deposits with a high area/volume ratio and adherent to the glassy carbon substrate. Figs. 1 and SM2 show the SEM images of deposits obtained in continuous at −1.4 V and pulse deposition with different ton. For all samples, at least three different structures can be observed: a granular underlayer mainly composed of nanograins, most likely, of CoFe alloy, a second layer related to metallic oxides, and a pine-like CoFe alloy top layer. The pulsed deposits obtained with 1 s ≤ ton ≤ 0.5 s (Figs. 1B and C and SM2B and C) present less metal underlayer, more homogeneous and spongious layer of oxides, and low proportion of dendritic structures, due to the recovery of the metallic ion concentration near the electrode surface caused by the toff step. However, when ton decreases to <0.5 s, alloy dendrites increase again, because the toff time is not enough to recover the surface concentration. The three types of structures observed in the deposits (granular, oxide interlayer and dendritic) can be justified by the shape of the chronoamperometric curves recorded during the material deposition. For the continuous deposition (Fig. 2), at the initial times, an increase of current density, typical of a nucle- ation process, is observed, which corresponds to the formation of CoFe nuclei. Subsequently, the contribution of a new current (observed in Fig. 2 at potentials of −1.4 and −1.8 V) referred to HER over the initial deposit appears [14,15]. This simultaneous reaction drastically consumes the H+ near the electrode surface, increasing the local pH and favoring the instantaneous hydrox- ides formation [16,17]. Moreover, the more superficial pine-like structure, e.g. dendritic growth, might be a consequence of the simultaneous alloy formation and hydrogen evolution during the electrodeposition process [18]. In the pulsed deposition (Fig. 2B) the current corresponding to HER decreases as the ton also diminishes, although the formation of CoFe alloy catalyzes the HER and always induces the formation of the oxide layer. However, when the pulses are very short, the deposits are mainly composed of the alloy. Therefore, the more adequate deposits as OER catalysts will be, probably, those obtained by means of pulsed deposition with ton values in the range 0.5 to 1 s, which present the higher proportion of the oxide layer. The morphology of the deposits is similar to that obtained by Burke et al. [19] for CoFe//LDH, where LDH represents a Layered Double Hydroxide structure, but in our work the simultaneous H+ electro- reduction and local pH increase directly induce the formation of oxidized species. The XRD patterns of representative deposits obtained at −1.4 V show very small peaks over the signal of the GC substrate (Fig. 3A). In order to clearly detect the structure of the deposits, several replicate deposits were obtained on GC and accumulated on a sil- icon holder; Fig. 3B shows now very visible peaks corresponding to crystalline phases in the deposits: the peaks (marked in red in A.M.P. Sakita et al. / Applied Surface Science 434 (2018) 1153–1160 1155 F ant po F ) 0.05 F � a s [ a g c ig. 1. FE-SEM images of deposit obtained on GC at −1.4 V/Ag|AgCl|KCl3M A) const ) pulse deposition with 5 pulses of ton = toff of B) 1 s, C) 0.5 s, D) 0.25 s, E) 0.1 s and F ig. 3) at 44.95, 65.5 and 82.9◦2� are assigned to a cubic phase of -CoFe alloy (PDF #49-1568). The peaks marked in blue, located t 11.6, 22.9, 34.2, 59.3 and 60.6◦2�, can be assigned to CoFe-LDH, imilar structure that those of NiFe//LDH, shown by Hunter et al. 20], and CoFe//LDH (PDF #50-235 card) in which the intercalate nion is CO3 2−. Therefore, the deposits formed present, over the ranular CoFe alloy, oxidized species of Fe and/or Co, formed as a onsequence of the local pH variation during electrodeposition and tential during 5 s: insert (A) top-layer; (B) intermediate layer; (C) underlayer. B to s, being in all cases the total time in on of 5 s. hydrogen evolution, in a layered structure with, probably, interca- lated with Cl- anion, in excess in the solution. XRD pattern of the deposit obtained for 60 s on GC (Fig. SM3) corroborates the infor- mation extracted from Fig. 3. This structure is confirmed by XPS (Fig. 4). The hydroxides structure is constituted by a layered dou- ble hydroxide of Co2+ and Fe3+ (LDH), with chloride inserted in the interlayer. 1156 A.M.P. Sakita et al. / Applied Surface Science 434 (2018) 1153–1160 Fig. 2. i-t curves obtained on glassy carbon at different protocols, A) continous deposition during 5 s at different potentials; B) during 10 s by pulse deposition with 1 s pulse. Electrodeposition bah 100 mM CoCl2 + 100 mM FeCl2 + 500 mM KCl aqueous solution. F n sub d d i T ( a d T s I h C c i p y C t o 7 ig. 3. XRD patterns of deposits obtained at −1.4 V, a) directly on the glassy carbo ashed line: �-CoFe. The XPS survey spectrum (Fig. 4a) reveals that the CoFe//LDH eposited at −1.4 V is composed by Cl, O, Fe, and Co, while carbon s observed due to the glassy carbon substrate and atmospheric CO2. he global composition of the electrodeposits was obtained by EDS more penetrative technique), which reveals a Co:Fe atomic ratio round 1:1, and a chloride content decreasing as the pulse time ecreases (from 4% to 0%, See supplementary material Table S1). his global Co:Fe ratio in the deposit agrees with the bath compo- ition and, according to Burke et al. [19], is a good value for the OER. n addition, the proposed method permits obtaining deposits with igh contents of iron, which is very interesting in economic aspects. hlorinated species such as iron hydroxychloride (Fe2(OH)3Cl) and obalt hydroxychloride (Co2(OH)3Cl) may depict similar behav- or for chloride incorporation, but the XRD analysis indicates the ossible formation of CoFe//LDH with intercalated Cl−. The hydrox- chlorides and LDH with intercalated chloride depict a content of l around 20 at.% [20,21], suggesting that in the deposit, a mix- ure of oxidized species is grown. The high resolution XPS spectra f Co 2p (Fig. 4b) shows two characteristic peaks at 782.1 and 98.2 eV related to Co 2p1/2 and Co 2p3/2, respectively. The dif- strate, and b) in powder form on silicon holder substrate. Dotted line: CoFe//LDH, ference between the binding energy of Co 2p1/2 and Co 2p3/2 is around 16 eV, which is characteristic of Co(OH)2 species, reveal- ing that such species have predominantly the Co2+ oxidation state [22]. The small peak at 777.3 eV is related to Co0 species, which is in agreement with the metallic underlayer mentioned above [23]. The deconvoluted Fe 2p spectra (Fig. 4c) displays two peaks at 712.6 and 725.6 eV related to Fe 2p3/2 and Fe 2p1/2, respectively, and a satel- lite peak of Fe 2p3/2 at 717.8 eV, which are commonly attributed to Fe3+ species [24,25]. This is supported by the FeOOH structure found by Raman (Fig. 5). The deconvoluted spectra of O 1s (Fig. 4d) displays peaks at 528.2, 530.1 and 532.3 eV, which are attributed to O2−, OH− and O− ions [26], respectively. The XPS results reveal that a mixture of Fe and Co LDH composes the deposits obtained and a metallic underlayer mainly composed by cobalt are formed at −1.4 V, during the fast deposition process, as expected. Representative Raman spectra of CoFe//LDH obtained at −1.4 V (Fig. 5A) show bands in the range from 250 to 700 cm−1 that are typ- ical of Co(OH)2 (black dashed lines) and of �-FeOOH (lepidocrocite – red dashed lines) [22,27]. A small band at 682 cm−1 (blue dashed line) can be assigned to active Ag mode attributed to a cubic inverse- A.M.P. Sakita et al. / Applied Surface Science 434 (2018) 1153–1160 1157 c) Fe 2p and d) O 1s of the deposit obtained at −1.4 V/Ag|AgCl|KCl3M during 5 s. s v S t t o s o b d X t R � o 5 f S t t f o N O u ( w ( ( Table 1 Kinetic and catalytic parameters of the CoFe electrodeposits obtained at different pulsed time. Deposition pulse time/s Tafel Slope/m V dec−1 Ovepotential/mV at 10 mA cm−2 Onset over- potential/mV Bare GC 94 447 343 314 62 272 1 50 292 261 0.5 48 286 258 0.25 45 291 264 Fig. 4. a) XPS survey spectrum and high resolution spectra for: b) Co 2p, pinel of mixed CoFe oxides [28] and also to Fe O H bending ibration [29]. Other bands at high wavenumbers are shown in Fig. M4 and can be assigned to the carbon substrate (2920 cm−1) and o the presence of O H bonds (3520 cm−1) [29,30], which indicate he presence of hydroxides. The band at 523 cm−1 is a contribution f the two kinds of hydroxides, which can be associated with a (Ag) ymmetric stretching mode of Co(OH)2 and a characteristic band f lepidocrocite. However, the presence of lepidocrocite is detected y the band at 257 cm−1 that identifies the �-FeOOH phase which iffers from other phases such as �-FeOOH and �-FeOOH [27]. The RD pattern reveals a small peak at 11.9◦2� that could be attributed o -FeOOH (akaganeite) or to CoFe-LDH which contrasts with the aman results that evidences the �-FeOOH phase. The presence of -FeOOH is an indicative of the formation of LDH and ledipocrocite n the electrode surface. The attribution of the shoulder band at 95 cm−1 is unknown by the authors and will be investigated in urther studies. Other bands at high wavenumbers are shown in Fig. M4 and can be assigned to the carbon substrate (2920 cm−1) and o the presence of O H bonds (3520 cm−1) [29,30], which indicate he presence of hydroxides (Fig. 5B). The efficacy of the obtained CoFe//LDH deposits as catalysts or OER has been tested. For this purpose, polarization curves f deposits formed at different potentials were recorded in 1 M aOH solution, at 5 mV s−1, until the oxygen evolution (Fig. 6). nset potential, Tafel slopes, and overpotential at 10 mA cm−2, as sual [31,32], were determined for the different obtained deposits Table 1). In all cases, the behavior of the deposits clearly improved ith respect to bare glassy carbon substrates. The overpotential also the onset potential and Tafel slopes) for the OER improved decrease) when pulse deposition was used, being the best result 0.1 44 299 269 0.05 52 303 269 obtained for pulses of 0.5 s. Therefore, the CoFe//LDH samples pre- pared at –1.4 V with pulsed time of 0.5 s have better catalytic properties for OER, which is justified by the high proportion of hydroxides layer in the deposits and its spongious structure, with a very high active surface. Nevertheless, when duration of the pulses was very low, less proportion of the oxidized layer was formed, with less catalytic effect for OER. The composition of the films obtained by EDS (Table SM1) corroborates the lower content of both oxides and chloride as the pulse time decreases. In this sense, CoFe alloy shows lower catalytic properties for OER than the LDH formed during the electrodeposition. This is also confirmed when the cat- alytic response to OER of deposits obtained at different potentials, potentiostatic continuous protocol, is compared (Fig. 7). The best effectiveness for the OER is observed for the deposits obtained at −1.4 V, formed by CoFe//LDH. When deposits are formed at less negative deposition potential (−1.0 V), hydrogen evolution is low significant, and the low variation of local pH is not able to induce 1158 A.M.P. Sakita et al. / Applied Surface Science 434 (2018) 1153–1160 F A s d o f a s F Fig. 7. A) Polarization curves for deposits obtained at different potentials in contin- uous mode and recorded in 1 M NaOH solution at 5 mV s−1. ig. 5. Raman spectra of the deposit obtained at −1.4 V/Ag|AgCl|KCl3M during 5 s. t A) low and B) high wavelength. ignificant hydroxides formation. On the other hand, when the eposition potential is excessively negative (−1.8 V), only a layer f low conductive oxidized species is formed. The best conditions or the preparation of the OER catalysts are, then, those than lead to granular conductive underlayer of CoFe alloy covered with a LDH tructure containing hydroxide species with intercalated chloride. or the global process occurring on a hydrous oxide electrode pre- 1.2 1.3 1.4 1.5 1.6 0.00 0.02 0.04 0.06 Con tinou s 1 s pulse 0.5 s pu lse 0.25 s pulse 0.1 s pu lse 0.05 s pulse Fig. 6. a) Polarization curves and b) Tafel plot of the deposit pared at −1.4 V, the Tafel slope can be attributed to the second step, which is the rate-determining step [33]. In order to assure the durability of the catalyst, stability tests were performed, by applying to the deposits a current density of 10 mA cm−2 during 5 h. When the deposits obtained in continuous or pulsed conditions were compared (Fig. 8), an overpotential of 313 mV with a slight increase of 66 �V min−1 were observed for the deposits obtained in continuous by the deposition protocol, while the deposits prepared by pulse deposition mode with pulses of 0.5 s, show a lower overpotential (298 mV) and excellent stabil- ity (increase of only 1.2 nV min−1). This corroborates the excellent performance of the CoFe//LDH deposits obtained at −1.4 V by the pulse deposition procedure as catalysts for OER. In Table 2 the per- formance of the best deposits obtained in this work with respect to OER is compared with the performance of other catalysts based on transition metals proposed in literature. Our proposal for OER catalysts preparation improves clearly the previous results because not only allows to prepare catalysts with lower overpotential, Tafel slope and onset potential for the reaction, but also implies an eas- ier preparation method, (one-step electrodeposition process in a simple bath) and, moreover, much shorter preparation time(few seconds only). -3.0 -2.5 -2.0 -1.5 1.45 1.50 1.55 1.60 Continou s 1 s pulse 0.5 s pulse 0.25 s pulse 0.1 s pulse 0.05 s pul se E / V v s R H E s obtained at different pulsed times with Eon = −1.4V. A.M.P. Sakita et al. / Applied Surface Science 434 (2018) 1153–1160 1159 Table 2 Comparison of catalytic parameters and experimental conditions of catalysts preparation for OER. Material Overpotential (10 mA cm−2)/mV Tafel slope/mV dec−1 Electrolyte Substrate Preparation Time Reference CoFe//LDH – continuous 314 60 1 M KOH GC 5 s This Work CoFe//LDH – pulsed 0.1 286 48 1 M KOH CG 10 s (ton = 5s) This Work CoFe35 LDH 351 49 0.1 M KOH GC 2.5 h [34] CoxFe3-xO4 420 53 1 M NaOH Au and Cu 45 to 150 s [11] PI/CNT-Co(OH)2 317 49 KOH PI/CNT 10 min [35] CoFe-LDH 300 83 1 M KOH NF 48 hours [36] Co0.54Fe0.46OOH 390 47 0.1 M KOH GC 21–46 h [32] CoFeOx 270 36 1 M KOH NF 2000 s [37] Fe-Co composite 283 34 Co nanoparticles 390 – Co3O4/N-rmGO 310 67 F o 4 t c a o i s w d o a l p o a A 8 t a a A t 0 [ [ [ [ [ [ [ [ ig. 8. Stability test obtained by applying 10 mA cm−2 during 5 h for the deposit btained at −1.4 V/Ag|AgCl|KCl3M at constant and pulsed deposition at 0.5 s. . Conclusion In summary, a novel, facile, fast, nitrate-free, and one-step elec- rosyntehsis has been proposed to directly prepare CoFe thin films overed by layered double hydroxides with enhanced catalytic ctivity for OER. The Co and Fe oxidized species at the surface f the composite material and the presence of the LDH structure ntercalating chloride directly formed over a glassy carbon sub- trate result in an efficient and cost-effective electrocatalyst for ater splitting. The CoFe//LDH prepared at −1.4 V by continuous eposition and pulsed deposition (0.5 s) shows an overpotential f 314 mV and 286 mV (at 10 mA cm−2) with a Tafel slope of 62 nd 48 mV dec−1, respectively, indicating to be a promising cata- yst for OER. The adjustment of the electrosynthesis potential and ulses is fundamental in order to avoid the formation of pure alloy r excessive hydroxides. Furthermore, a new approach to prepare binder-free electrocatalyst has been developed. cknowledgments The authors thank the Brazilian funding CAPES (proc. no. 8881.132671/2016-01), CNPq (proc. no. 141257/2014-8), Por- uguese FCT (project PEst-OE/QUI/UI0100/2013), EU ERDF (FEDER) nd the Spanish Government grants (TEC2014-51940-C2-R). The uthors thank the CCiT-UB for the use of their equipment. ppendix A. Supplementary data Supplementary material related to this article can be found, in he online version, at doi:https://doi.org/10.1016/j.apsusc.2017.11. 42 [ 1 M KOH CFP 30 min [38] 0.1 M KOH GC 1 h [39] 0.1 M KOH CFP 13 h [40] References [1] L. Gong, D. Ren, Y. Deng, B.S. Yeo, Efficient and stable evolution of oxygen using pulse-electrodeposited Ir/Ni oxide catalyst in Fe-spiked KOH electrolyte, ACS Appl. Mater. Interfaces 8 (2016) 15985–15990, http://dx.doi. org/10.1021/acsami.6b01888. [2] M. Gong, Y. Li, H. Wang, Y. Liang, J.Z. Wu, J. 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enhanced cata... 1 Introduction 2 Experimental section 2.1 Deposition solution 2.2 Electrodeposition process 2.3 Electrochemical studies 2.4 Surface characterization 3 Results and discussion 4 Conclusion Acknowledgments Appendix A Supplementary data References