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Monolithic nanoporous NiFe alloy by dealloying laser processed NiFeAl as electrocatalyst toward oxygen evolution reaction
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Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction Xiaodan Cui, Boliang Zhang, Congyuan Zeng, Hao Wen, Shengmin Guo* Department of Mechanical & Industrial Engineering, Louisiana State University, Baton Rouge, LA 70803, USA
article info
abstract
Article history:
In this paper, monolithic nanoporous NieFe electrocatalyst is developed by dealloying a
Received 9 May 2018
NieFeeAl alloy, fabricated by a laser-based manufacturing method, as the electrocatalyst
Received in revised form
toward oxygen evolution reaction (OER). The nanoporous NieFe alloy displays an improved
14 June 2018
electrochemical performance by exhibiting an OER current density of 100 mA/cm2 at
Accepted 19 June 2018
442 mV overpotential in the 1 M KOH aqueous solution, which is better than that of laser
Available online xxx
processed bulk NieFe alloy (464 mV). The structures, crystallinities, and chemical compositions of the nanoporous NieFe electrocatalyst are characterized by SEM, XRD and
Keywords:
EDXS. With an activity comparable to the electrocatalysts reported to date, the monolithic
Oxygen evolution reaction
nanoporous NieFe electrodes are also highly flexible in mechanical configurations, due to
Nanoporous
the combined advantages of both laser and dealloying processes, which provide new op-
NieFe alloy
tions for OER electrode design.
Laser processing
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Dealloying
Background Since clean energy and environmental protection attract global concerns, the demand for renewable and clean energy sources has been increasing rapidly [1e9]. Water electrolysis is a promising method for hydrogen generation [10e14]. Oxygen evolution reaction (OER) is the redox reaction occurring at the anode and electrolyte interface during water electrolysis, which is also the sluggish portion of water splitting. The reaction rates strongly depend on the applied overpotential. With a suitable heterogeneous electrocatalysis, due to the formation of intermediate species, an energy shortcut at a much lower overpotential is allowed. However, due to the general needs of expensive noble metal-based
electrocatalysts, such as Ir, Ru, and Pt, the commercialization of water splitting is hindered [15e17]. Recently, more research efforts have been devoted to explore earth-abundant and ecofriendly electrocatalysts to replace noble metals. The reported materials for OER [18e21] include transition metals and alloys [22e25], metal oxides [26e28], metal chalcogenides [29], metal pnictides [30,31], organometallic [32] and non-metallic materials [33e35]. The most decisive factor for the catalytic activity of an electrocatalyst is the intrinsic binding strength toward the reaction species, though further modifications could improve the performance to some degree. A moderate strength is required for an electrocatalyst as stronger bindings would hinder product repelling while weaker bindings would result in difficult intermediates stabilization [36]. It is found that d-
* Corresponding author. E-mail address: [email protected] (S. Guo). https://doi.org/10.1016/j.ijhydene.2018.06.109 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109
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block metals usually have a moderate absorption to main group elements; since there are unpaired electrons on their outermost shell s orbitals and secondary outermost shell d orbitals [37,38]. In addition, a proper geometric configuration of the electrocatalyst would strongly increase the catalytic activity. If the lattice spacing is too large, diatomic reactant absorption and diatomic product formation would be inhibited; and if atoms are too closely packed, strong reactant repulsion would be resulted [39,40]. Recently, NieFe alloy stands out as a most appealing category for OER electrocatalysts, due to its moderate absorption affinity to reaction intermediates, good match with the reaction coordination, and low material cost [41e45]. Along with an ideal composition, an optimal electrocatalyst also requires a high specific surface area, which not only influences the localized electronic structure of surface atoms, but also alters the availability of active sites [46e49]. Nanostructuring is the most straight forward way to modify the structure of a catalyst, owing to the minimization of particle sizes and the increased exposed specific surface area. Therefore a nanostructured Nie Fe alloy possesses a great potential for low-cost and highperformance OER electrocatalyst. Compared to traditional powder-based electrocatalysts, monolithic electrocatalyst displays great advantages in simplifying the electrode assembling process and avoiding electrical connection issues [50]. For example, for hydrogen generation, Xie et al. reported a self-standing monolithic cobalt oxide array for NaBH4 hydrolysis that effectively inhibited catalyst aggregation and avoided the tedious and timeconsuming process for catalyst separation [51]. As a rapidly
developing manufacturing method, laser-based additive manufacturing is an excellent technique to prepare complex templates for monolithic electrocatalyst. In addition, laserbased additive manufacturing can achieve the desired alloy compositions without tedious chemical synthesis and can achieve a strong metallic bond to the substrate avoiding the usage of binders. For instance, Laser Powder Stream, or LENS®-Laser Engineered Net Shaping™ process is advantaged in free forming/coating capability to make large and complex parts. Due to the small laser spot size and thus a rapid cooling rate, laser processed parts have unique microstructures and material properties. For example, Cebollero et al. developed thin ceramic membranes for electrolyte-supported solid oxide fuel cells through laser processing which effectively improved the electrolyte-electrode contact and thus reduced cathodic polarizations [52]. Dealloying is an efficient strategy to develop nanostructures, which can develop microscale channels to facilitate mass transfer and nanoscale features to provide more active sites at the same time [53e55]. In this regard, Xu et al. fabricated nanoporous PtCo and PtNi alloy ribbons through a mild dealloying process. Those materials displayed excellent activity and stability toward methanol electroxidation [56]. Herein, monolithic nanoporous NieFe alloy electrodes are developed by dealloying laser-processed Nie FeeAl alloy samples as the electrocatalyst toward OER. Its crystal structures, morphologies and chemical compositions are characterized by XRD, SEM and EDXS. Its electrochemical performance toward OER is evaluated by linear sweep voltammetry, Tafel slope, and electrochemical impedance spectrometry.
Fig. 1 e Illustration of NieFeeAl dealloying process. Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109
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Experimental To fabricate the NieFeeAl alloy samples, nickel powders (3e7 mm, Alfa Aesar, >99.9%, metals basis), iron powders (Sigma Aldrich, >99%, metals basis) and aluminum powders (325 mesh, Alfa Aesar, 99.5%, metals basis), with Ni: Fe: Al atomic ratio ¼ 6:4:10, were mixed in a polystyrene jar through ball milling. Next the well mixed powders were compressed in a 12.7 mm dry pressing die. The as-prepared disks were then put into the chamber of a custom laser-based additive manufacturing system equipped with an ytterbium fiber laser (IPG model: VLR-200-AC-Y11) and a ProSeries II scan head. The laser beam scanned the surfaces of the disk shaped samples at a power of 77 W with a spot size of 50 mm at a scan speed of 200 mm/s and a hatch spacing of 0.254 mm within a chamber protected by argon gas. The resulted alloy sample was labeled as NiFeAl. The bulk NieFe alloy sample was fabricated the same way without adding aluminum, and labeled as NiFe_B. The Ni:Fe ratio is 6:4 due to its optimal performance based on our previous study [57]. To fabricate the monolithic nanoporous NieFe alloy samples, the laser processed NiFeAl alloy samples were dealloyed in 10 wt% KOH aqueous solution for 24 h. The dealloyed samples were labeled as NiFe_P. The NieFeeAl ternary phase diagram is calculated by ThermoCalc™ software, with the assistance of TCNI8 database. The crystal structures of the samples were characterized by an X-ray diffraction (XRD) using a Panalytical Empyrean multipurpose diffractometer equipped with PreFIX modules with Cu Ka radiation. The surface morphologies of the samples were studied by an FEI Quanta 3D FEG scanning electron microscope (SEM) equipped with energydispersive X-ray spectroscopy (EDXS) at an acceleration voltage of 20 kV. The electrochemical measurements were carried out in a three-electrode system using a CHI 650C electrochemical workstation. The tested samples served as the working electrodes in a 1 M KOH aqueous electrolyte, with Pt wire and saturated calomel electrode (SCE) serving as counter electrode and reference electrode, respectively. Linear sweep voltammetry (LSV) was carried out in the potential range of 0e1 V vs. SCE at a scan rate of 2 mV/s. Electrochemical impedance spectra (EIS) measurements were carried out at an overpotential of 438 mV with an applied voltage aptitude of 5 mV in the frequency range from 0.1 Hz to 105 Hz. Cyclic voltammetry (CV) measurements were carried out for 250 cycles in the potential range of 0e1 V vs. SCE at a scan rate of 10 mV/s.
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The following reaction happens in the dealloying process: 2AlðsÞ þ 2OH ðaqÞ þ 2H2 OðlÞ/2AlO 2 ðaqÞ þ 3H2 ðgÞ. The surface morphology of the NiFe_B, NiFeAl and NiFe_P are characterized by SEM images in Fig. 2. As displayed in Fig. 2(a),(b), the surfaces of NiFe_B are composed of parallel scan tracks bonded to each other coherently. The majority of the pores are distributed along the molten tracks. As shown in
Results and discussion The dealloying mechanism is illustrated in Fig. 1. At the beginning, Al atoms are rapidly dissolved into the alkaline solution from the alloy matrix, leaving Ni and Fe atoms with a high rate of vacancy. Then, the continuous dissolving of Al causes Ni and Fe enrichment at the solid and liquid interface. Next, Ni and Fe atoms aggregate into more stable crystal nucleus to minimize the surface energy and then grow into grains of NieFe intermetallic compounds. By extending this process, the grains tend to grow into ordered nanostructures.
Fig. 2 e SEM images of NiFe_B (aeb), NiFeAl (ced), and NiFe_P (eej).
Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109
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Fig. 2(c),(d) the surface of NiFeAl is much rougher than that of NiFe_B, with disconnected scan tracks, high porosity, intensified balling and large line cracks. This could be due to the high instability of melting pools formed during the manufacturing process. Because the melting point of Al (660 C) is critically lower than that of Ni (1455 C) and Fe (1538 C), during the solidification process, Ni and Fe solidify preferentially, leading to the unsolidified liquid phase enriched with Al, resulting in interrupted melting pools with balls and caves. On the other hand, due to the much larger thermal expansion coefficient of Al (a ¼ 23.1 106/K, at 20 C) than that of Ni (a ¼ 13 106/K) and Fe (a ¼ 11.8 106/K), the thermal coefficient mismatch leads to a large residue stress and surface tension during the rapid cooling of laser processing, resulting in cracks and pores. These structural imperfectness can serve as reaction centers for initializing dealloying process and promoting the formation of nanostructures and micro-channels. The SEM images of NiFe_P are displayed in Fig. 2 e)e(j). As observed in Fig. 2(e), the surface of NiFe_P is very porous and rough. After increasing the image magnification to Fig. 2(f), a lot of micro-scale channels are observed, which can facilitate OH ion diffusion and O2 gas transportation. On the relatively smooth areas (Fig. 2(g),(h)), the surface is converted to an array structure constructed by nanorods with ~100 nm diameter interconnected by ultrathin nanowires. On the areas along molten tracks and inside the surface pores (Fig. 2(i),(j)), well-defined coral-like nanoparticles are clearly observed. The nanocorals have a unit size ~5 mm composed of thin blades of ~500 nm width. An open source image processing program, ImageJ, is applied to estimate the porosity of the NiFe_P sample. The overall porosity of NiFe_P is estimated to be around 45%. Therefore, monolithic nanoporous NieFe alloy with an ultra-high specific surface area is developed through dealloying the laser-processed Nie FeeAl alloy samples. Fig. 3(a) displays the calculated ternary NieFeeAl phase diagram at 900 C. It can be seen upon solidification, the Nie FeeAl alloy with a Ni: Fe: Al atomic ratio ¼ 6:4:10 marked with
a red star falls into the region of b2 phase, with secondary a and g phases and an Al rich liquidus phase. The XRD patterns of NiFe_B, NiFeAl and NiFe_P are displayed in Fig. 3(b). It can be seen that the bulk NiFe_B sample is mainly composed of a FCC structure composed of a g-Ni (JCPDF card no. 01-070-0989) phase with 2q ¼ 44.605 , 51.979 and 76.591 corresponding to (111), (200) and (220) planes with the lattice parameter of a ¼ 3.516 A; a cubic FeNi3 phase (JCPDF card no. 03-065-3244) with 2q ¼ 44.121 , 51.404 and 75.661 corresponding to (111), (200) and (220) planes with a ¼ 3.552 A; a cubic taenite FeNi phase (JCPDF card no. 00-003-0016) with 2q ¼ 43.173 , 50.674 and 74.679 corresponding to (111), (200) and (220) planes with a ¼ 3.601 A and trivial amount of a-Fe phase (JCPDF card no. 00-001-1252) with 2q ¼ 44.142 and 65.186 corresponding to (111) and (220) planes with a ¼ 2.860 A. It is observed from the NiFeAl spectrum, it is composed of a BCC matrix with 2q ¼ 31.101 , 44.496 and 65.059 corresponding to (110), (200) and (202) planes with a ¼ 4.060 A, an FCC structure matrix 2q ¼ 44.765 , 52.216 and 77.063 corresponding to (111), (200), (220) planes with a ¼ 3.503 A and a high-intensity cubic Al phase (JCPDF card no. 04-013-0326) with 2q ¼ 38.510 , 44.765 , 65.165 and 78.317 corresponding to (111), (200), (220) and (311) planes with a ¼ 4.0458 A. This is because during the rapid thermal cycle of laser processing, the diffusion is highly insufficient and non-equilibrium Al segregations occur. Therefore, the Ni and Fe atoms tend to grow into NieFe intermetallic compounds resulting in a NieFe based BCC and FCC matrixes with minor amount of Al atoms altering lattice distances. For the spectrum of NiFe_P sample, it is clearly found that the Al phase disappeared from the BCC and FCC structures, indicating a successful dealloying process that has removed majority of Al atoms from the alloy matrix. As a result, the nanoporous NiFe_P is mainly composed of NieFe based alloys, which are active components toward OER in the alkaline environment. EDXS elemental mapping analysis and energy-dispersive spectrum were executed to characterize elemental composition and distribution in the nanoporous NiFe_P sample. It is
Fig. 3 e (a) Ternary NieFeeAl phase diagram at 900 C; (b) XRD spectra of NiFe_B, NiFeAl and NiFe_P. Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109
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observed the atomic ratios of Ni, Fe, Al, O are 45%, 17%, 8%, 30%, respectively. The loss of Fe during the dealloying process is due to the dissolution of Fe into the alkaline solution. As shown in Fig. 4(a),(b), Ni and Fe atoms are distributed uniformly all over the surface. As shown in Fig. 4(c), the vast majority of surface Al atoms have been dissolved and the signal of Al is mainly from the underneath substrate in the areas exposed through pores of the printed layer. In addition, due to the in-situ metal oxidation and hydroxylation in the dealloying process, oxygen signal is also observed uniformly distributed all over the sample as shown in Fig. 4(d), indicating the formation of M O (M ¼ Al, Ni, Fe) and M OH bonds. To evaluate the electrocatalytic performance, LSV was carried out on the NiFeAl, the NiFe_B and NiFe_P electrodes in
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a 1 M KOH aqueous electrolyte (Fig. 5). It is firstly observed that in the alkaline solution, a layer of metal hydroxides rapidly forms on the surface of the NiFe_B electrode at an OER overpotential of ~295 mV due to the oxidation of Ni(OH)2 into NiOOH. No peaks were observed on the spectrum of NiFe_P or NiFeAl due to a full hydroxylation during the dealloying process for NiFe_P sample and thermodynamic hydroxylation favorability of Fe and Al over Ni for NiFeAl sample [58]. It is also observed, the NiFe_B electrode exhibits an apparently lower activity toward OER compared to the nanoporous NiFe_P electrode. The NiFe_B electrode requires an overpotential of 464 mV to reach a 100 mA/cm2 current density. For nanoporous NiFe_P electrode, it only requires an overpotential of 442 mV to reach an OER current density of 100 mA/cm2,
Fig. 4 e (a) (b) (c) and (d) EDXS elemental mappings of Ni, Fe, Al and O elements in nanoporous NiFe_P respectively, (e) Energy-dispersive spectrum of NiFe_P. Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109
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Fig. 5 e The OER polarization curves of NiFeAl, the NiFe_B and NiFe_P electrodes in a 1 M KOH aqueous electrolyte in the potential range of 0e1 V vs. SCE at a scan rate of 2 mV/s.
which is comparable to reported OER electrocatalyst to date [16,58e61]. At an overpotential of 500 mV, the LSV current density of NiFe_P is 142 mA/cm2, while it is only 122 mA/cm2 and 61 mA/cm2 for NiFe_B and NiFeAl electrodes. With increased surface area and amount of surface active sites, electron transfer process at the electrode and electrolyte interface is effectively accelerated. In addition, the interconnected microchannel structure increases the amount of pathways for ion and gas diffusion. The overpotential h vs. log (current density) is plotted in Fig. 6. To understand the electrochemical mechanism of the OER catalytic process, the linear regions of polarization curves were fitted to Tafel plots, using the equation h ¼ a þ b log (j), where b is the Tafel slope to determine the kinetic constant of
Fig. 6 e Tafel Slope of NiFeAl, NiFe_B and NiFe_P electrodes in a 1 M KOH aqueous electrolyte in the potential range of 0e1 V vs. SCE at a scan rate of 2 mV/s.
electrode reactions. A small slope of Tafel plot indicates rapid electron transfer at the electrode and electrolyte interface at the applied potentials. On a Tafel plot, once the onset potential is reached, a linear slope is well defined, where electrochemical reaction is the rate determining step [62]. As shown in Fig. 6, the slope values of the Tafel regions at low overpotential range for NiFeAl, NiFe_B and NiFe_P electrodes are 158, 46 and 36 mV/decade, respectively. The lower Tafel slope of NiFe_P is contributed by ultra-small nanoparticles, which increase active sites on the electrode surface and effectively accelerate electron transfer process at the electrode and electrolyte interface. By increasing the overpotential, the rate determining step evolves from electrochemical reaction to mass diffusion [62e65], with the slope gradually increasing to ~ 600 mV/dec. It can be seen, NiFe_P reaches the mass diffusion control region at a lower overpotential than that of NiFe_B. Because the higher surface roughness and wider distribution of microchannels of NiFe_P enhance the oxygen gas repelling and mass diffusion, leading to a full surface coverage earlier. As shown in Fig. 7, The Nyquist plots of EIS were taken at an overpotential of 438 mV in the frequency range of 105 Hze0.1 Hz on the NiFeAl, NiFe_B and NiFe_P electrodes. The equivalent circuit is calculated using the CHIe650C electrochemical workstation software package with the fitted results listed in Table 1. The equivalent circuit is composed of Rs e solution resistance, Rint e electrode/electrolyte interface resistance, Rct e charge transfer resistance, CPE1 e constant phase element for interfacial capacitance, and CPE2 e constant phase element for double layer capacitance [66]. The intercept of the plot with the real axis (Rs) represents the overall ohmic resistance, including the bulk resistance of the material, series resistance, and contact resistance [67]. The semicircle at high frequency range is from metal hydroxide intermediates formation and absorption process, whose diameter (Rint) indicates the rate of the electrode/electrolyte interfacial process [68]. It is observed, the Rint of NiFe_P (0.010 U cm2) spectrum is smaller than that of Ni4Fe6_B (0.023 U cm2), indicating a faster OH absorption and intermediates formation at the electrode/electrolyte interface. It is also noticed, the CPE1 of NiFe_P (3.061 mF sn1 cm2) is much higher than that of NiFe_B (2.041 mF sn1 cm2), indicating more surface atoms on NiFe_P electrode are exposed for reaction species absorption and intermediates formation, attributed to the enlarged specific surface area from the nanoporous structure. Since the n1 values are close to the value of unity, the CPE1 is close to pure capacitance. The semicircle on low frequency range describes the electron charge transfer process of OH/O2 redox reaction at the electrode/electrolyte interface [68], with a diameter of Rct representing the charge transfer resistance between reactant to product [69]. A smaller Rct indicates a faster electron transfer. It can be seen, the Rct of NiFe_P (0.490 U cm2) is smaller than that of NiFe_B electrode (0.573 U cm2). The CPE2 illustrates the double layer capacitance, which doesn't contribute to electrochemical reaction kinetics [70,71]. The n2 value of NiFe_P (0.608) is much lower than that of NiFe_B (0.802), indicating a rougher and more porous surface. It is found, due to formation of nanostructures, the overall electrocatalytic activity is apparently improved by the increased specific surface area
Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109
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Fig. 7 e The EIS spectra of the NiFeAl, NiFe_B and NiFe_P electrodes at an overpotential of 438 mV with an applied voltage aptitude of 5 mV in the frequency range from 0.1 Hz to 105 Hz. Symbols: experimental data; solid line: fitting data; inset: equivalent circuit and enlarged image of high frequency range.
Table 1 e EIS parameters.
NiFeAl NiFe_B NiFe_P
Rs (U cm2)
Rint (U cm2)
CPE1 (mF sn1 cm2)
n1
Rct (U cm2)
n2
CPE2 (F sn1 cm2)
1.117 1.323 1.243
0.037 0.023 0.010
0.612 2.041 3.061
0.999 0.999 0.999
1.690 0.573 0.490
0.680 0.752 0.608
0.085 0.160 0.198
Fig. 8 e (a) 1st cycle and 250th cycle of cyclic voltammetry of NiFe_P electrodes in a 1 M KOH aqueous electrolyte in the potential range of 0e1 V vs. SCE at a scan rate of 10 mV/s (b) time-dependent potential response during 15-h chronoamperometric water-splitting reaction, at a constant DC potential of 288 mV overpotential.
and active sites. The nanostructure enhances the reactant absorption and intermediates formation, and accelerates the electrode/electrolyte interface electron transfer; the interconnected micro channels increase the pathways for mass
transfer and facilitate the ion diffusion and gas transportation. To study the redox behavior and stability of the nanoporous NiFe_P electrocatalyst, 250 CV cycles were carried out
Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109
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Fig. 9 e XPS core spectra of (a) Ni 2p, (b) Fe 2p, (c) Al 2p, and (d) O 1s of as-prepared fresh NiFe_P and NiFe_P after 15-h chronoamperometric water-splitting reaction.
in the potential range of 0e1 V vs. SCE at a scan rate of 10 mV/ s. The spectra of the 1st cycle and the 250th cycle are displayed in Fig. 8(a). A cathodic peak at 60 mV overpotential is observable, which can be assigned to the reduction of Ni(III) to
Ni(II) [72]. In addition, after 250 CV cycles, no significant current density decay is observed, indicating the good stability of NiFe_P electrocatalyst. To further evaluate the stability of the nanoporous NiFe_P electrocatalyst, chronoamperometry was
Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109
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applied at a 288 mV overpotential for 15 h. As shown in Fig. 8(b), it has an initial current density of 10 mA/cm2. Although the current density experienced a slight decay in the first 3 h, the current density maintains fairly constant, ~10 mA/cm2, to 15 h. To study the chemical compositions of as-prepared fresh NiFe_P and NiFe_P after 15-h chronoamperometric watersplitting reaction, XPS spectra are displayed in Fig. 9. As displayed in Fig. 9(a), the Ni 2p spectrum of fresh NiFe_P sample has two main pairs of de-convoluted peaks, which can be attributed to Ni0 and Ni2þ/Ni3þ, respectively [73,74]. It can be seen, after 15h chronoamperometric water-splitting reaction, the relative intensity of Ni2þ/Ni3þ peaks to Ni0 peaks are getting higher, indicating more Ni atoms have been oxidized/hydro-oxidized during the water-splitting process. As displayed in Fig. 9(b), the Fe 2p spectrum has two main pair of de-convoluted peaks attributed to Fe0 and Fe2þ/Fe3þ, respectively [75]. The chronoamperometric process slightly increased the ratio of Fe2þ/Fe3þ to Fe0. As displayed in Fig. 9(c), the Al atoms are mostly oxidized/hydrooxidized to Al3þ with little metallic Al remaining in the alloy after 15 h. It can be seen, the O 1s core spectra are composed of M O (M ¼ Ni, Fe, Al), M OH and M-OOH bonds [76,77]; the increased ratio of M-OOH bonds and decreased ratio of M O bonds confirm that metals are further oxidized/hydro-oxidized after the 15-h chronoamperometric water-splitting reaction.
Conclusions In conclusion, monolithic nanoporous NieFe alloy electrocatalyst is developed by dealloying laser processed NieFeeAl alloy for OER. The nanoscale pores derived from dealloying process can provide high specific surface area and more active sites for catalytic reactions. The microscale channels can provide more pathways for gas diffusion and ion transportation. In addition, as a monolithic electrode, the electron conduction can be effectively facilitated and the use of substrate and binders can be eliminated. Compared to bulk NieFe electrocatalyst, the nanoporous NieFe electrode, even at a less favorable Ni:Fe composition after the dealloying process, exhibits an apparently improved electrocatalytic activity and only requires 442 mV overpotential to achieve an OER current density of 100 mA/cm2 in 1 M KOH aqueous solution, which is comparable to the performance of reported electrocatalysts to date. Overall, the laser-based manufacturing method possesses high flexibility in developing complex shaped preforms for monolithic nanoporous electrodes, which provides a new fabrication method for next generation electrode design and electrocatalyst engineering. It's well known that nanoporous structures are prone to structure degradation. Further process optimization can be performed to improve the composition and stability of monolithic nanoporous electrocatalysts.
Acknowledgements This publication is based upon work supported by NSF-Consortium for innovation in manufacturing and materials (CIMM) program (grant number # OIA-1541079).
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Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction Xiaodan Cui, Boliang Zhang, Congyuan Zeng, Hao Wen, Shengmin Guo* Department of Mechanical & Industrial Engineering, Louisiana State University, Baton Rouge, LA 70803, USA
article info
abstract
Article history:
In this paper, monolithic nanoporous NieFe electrocatalyst is developed by dealloying a
Received 9 May 2018
NieFeeAl alloy, fabricated by a laser-based manufacturing method, as the electrocatalyst
Received in revised form
toward oxygen evolution reaction (OER). The nanoporous NieFe alloy displays an improved
14 June 2018
electrochemical performance by exhibiting an OER current density of 100 mA/cm2 at
Accepted 19 June 2018
442 mV overpotential in the 1 M KOH aqueous solution, which is better than that of laser
Available online xxx
processed bulk NieFe alloy (464 mV). The structures, crystallinities, and chemical compositions of the nanoporous NieFe electrocatalyst are characterized by SEM, XRD and
Keywords:
EDXS. With an activity comparable to the electrocatalysts reported to date, the monolithic
Oxygen evolution reaction
nanoporous NieFe electrodes are also highly flexible in mechanical configurations, due to
Nanoporous
the combined advantages of both laser and dealloying processes, which provide new op-
NieFe alloy
tions for OER electrode design.
Laser processing
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Dealloying
Background Since clean energy and environmental protection attract global concerns, the demand for renewable and clean energy sources has been increasing rapidly [1e9]. Water electrolysis is a promising method for hydrogen generation [10e14]. Oxygen evolution reaction (OER) is the redox reaction occurring at the anode and electrolyte interface during water electrolysis, which is also the sluggish portion of water splitting. The reaction rates strongly depend on the applied overpotential. With a suitable heterogeneous electrocatalysis, due to the formation of intermediate species, an energy shortcut at a much lower overpotential is allowed. However, due to the general needs of expensive noble metal-based
electrocatalysts, such as Ir, Ru, and Pt, the commercialization of water splitting is hindered [15e17]. Recently, more research efforts have been devoted to explore earth-abundant and ecofriendly electrocatalysts to replace noble metals. The reported materials for OER [18e21] include transition metals and alloys [22e25], metal oxides [26e28], metal chalcogenides [29], metal pnictides [30,31], organometallic [32] and non-metallic materials [33e35]. The most decisive factor for the catalytic activity of an electrocatalyst is the intrinsic binding strength toward the reaction species, though further modifications could improve the performance to some degree. A moderate strength is required for an electrocatalyst as stronger bindings would hinder product repelling while weaker bindings would result in difficult intermediates stabilization [36]. It is found that d-
* Corresponding author. E-mail address: [email protected] (S. Guo). https://doi.org/10.1016/j.ijhydene.2018.06.109 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109
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block metals usually have a moderate absorption to main group elements; since there are unpaired electrons on their outermost shell s orbitals and secondary outermost shell d orbitals [37,38]. In addition, a proper geometric configuration of the electrocatalyst would strongly increase the catalytic activity. If the lattice spacing is too large, diatomic reactant absorption and diatomic product formation would be inhibited; and if atoms are too closely packed, strong reactant repulsion would be resulted [39,40]. Recently, NieFe alloy stands out as a most appealing category for OER electrocatalysts, due to its moderate absorption affinity to reaction intermediates, good match with the reaction coordination, and low material cost [41e45]. Along with an ideal composition, an optimal electrocatalyst also requires a high specific surface area, which not only influences the localized electronic structure of surface atoms, but also alters the availability of active sites [46e49]. Nanostructuring is the most straight forward way to modify the structure of a catalyst, owing to the minimization of particle sizes and the increased exposed specific surface area. Therefore a nanostructured Nie Fe alloy possesses a great potential for low-cost and highperformance OER electrocatalyst. Compared to traditional powder-based electrocatalysts, monolithic electrocatalyst displays great advantages in simplifying the electrode assembling process and avoiding electrical connection issues [50]. For example, for hydrogen generation, Xie et al. reported a self-standing monolithic cobalt oxide array for NaBH4 hydrolysis that effectively inhibited catalyst aggregation and avoided the tedious and timeconsuming process for catalyst separation [51]. As a rapidly
developing manufacturing method, laser-based additive manufacturing is an excellent technique to prepare complex templates for monolithic electrocatalyst. In addition, laserbased additive manufacturing can achieve the desired alloy compositions without tedious chemical synthesis and can achieve a strong metallic bond to the substrate avoiding the usage of binders. For instance, Laser Powder Stream, or LENS®-Laser Engineered Net Shaping™ process is advantaged in free forming/coating capability to make large and complex parts. Due to the small laser spot size and thus a rapid cooling rate, laser processed parts have unique microstructures and material properties. For example, Cebollero et al. developed thin ceramic membranes for electrolyte-supported solid oxide fuel cells through laser processing which effectively improved the electrolyte-electrode contact and thus reduced cathodic polarizations [52]. Dealloying is an efficient strategy to develop nanostructures, which can develop microscale channels to facilitate mass transfer and nanoscale features to provide more active sites at the same time [53e55]. In this regard, Xu et al. fabricated nanoporous PtCo and PtNi alloy ribbons through a mild dealloying process. Those materials displayed excellent activity and stability toward methanol electroxidation [56]. Herein, monolithic nanoporous NieFe alloy electrodes are developed by dealloying laser-processed Nie FeeAl alloy samples as the electrocatalyst toward OER. Its crystal structures, morphologies and chemical compositions are characterized by XRD, SEM and EDXS. Its electrochemical performance toward OER is evaluated by linear sweep voltammetry, Tafel slope, and electrochemical impedance spectrometry.
Fig. 1 e Illustration of NieFeeAl dealloying process. Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109
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Experimental To fabricate the NieFeeAl alloy samples, nickel powders (3e7 mm, Alfa Aesar, >99.9%, metals basis), iron powders (Sigma Aldrich, >99%, metals basis) and aluminum powders (325 mesh, Alfa Aesar, 99.5%, metals basis), with Ni: Fe: Al atomic ratio ¼ 6:4:10, were mixed in a polystyrene jar through ball milling. Next the well mixed powders were compressed in a 12.7 mm dry pressing die. The as-prepared disks were then put into the chamber of a custom laser-based additive manufacturing system equipped with an ytterbium fiber laser (IPG model: VLR-200-AC-Y11) and a ProSeries II scan head. The laser beam scanned the surfaces of the disk shaped samples at a power of 77 W with a spot size of 50 mm at a scan speed of 200 mm/s and a hatch spacing of 0.254 mm within a chamber protected by argon gas. The resulted alloy sample was labeled as NiFeAl. The bulk NieFe alloy sample was fabricated the same way without adding aluminum, and labeled as NiFe_B. The Ni:Fe ratio is 6:4 due to its optimal performance based on our previous study [57]. To fabricate the monolithic nanoporous NieFe alloy samples, the laser processed NiFeAl alloy samples were dealloyed in 10 wt% KOH aqueous solution for 24 h. The dealloyed samples were labeled as NiFe_P. The NieFeeAl ternary phase diagram is calculated by ThermoCalc™ software, with the assistance of TCNI8 database. The crystal structures of the samples were characterized by an X-ray diffraction (XRD) using a Panalytical Empyrean multipurpose diffractometer equipped with PreFIX modules with Cu Ka radiation. The surface morphologies of the samples were studied by an FEI Quanta 3D FEG scanning electron microscope (SEM) equipped with energydispersive X-ray spectroscopy (EDXS) at an acceleration voltage of 20 kV. The electrochemical measurements were carried out in a three-electrode system using a CHI 650C electrochemical workstation. The tested samples served as the working electrodes in a 1 M KOH aqueous electrolyte, with Pt wire and saturated calomel electrode (SCE) serving as counter electrode and reference electrode, respectively. Linear sweep voltammetry (LSV) was carried out in the potential range of 0e1 V vs. SCE at a scan rate of 2 mV/s. Electrochemical impedance spectra (EIS) measurements were carried out at an overpotential of 438 mV with an applied voltage aptitude of 5 mV in the frequency range from 0.1 Hz to 105 Hz. Cyclic voltammetry (CV) measurements were carried out for 250 cycles in the potential range of 0e1 V vs. SCE at a scan rate of 10 mV/s.
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The following reaction happens in the dealloying process: 2AlðsÞ þ 2OH ðaqÞ þ 2H2 OðlÞ/2AlO 2 ðaqÞ þ 3H2 ðgÞ. The surface morphology of the NiFe_B, NiFeAl and NiFe_P are characterized by SEM images in Fig. 2. As displayed in Fig. 2(a),(b), the surfaces of NiFe_B are composed of parallel scan tracks bonded to each other coherently. The majority of the pores are distributed along the molten tracks. As shown in
Results and discussion The dealloying mechanism is illustrated in Fig. 1. At the beginning, Al atoms are rapidly dissolved into the alkaline solution from the alloy matrix, leaving Ni and Fe atoms with a high rate of vacancy. Then, the continuous dissolving of Al causes Ni and Fe enrichment at the solid and liquid interface. Next, Ni and Fe atoms aggregate into more stable crystal nucleus to minimize the surface energy and then grow into grains of NieFe intermetallic compounds. By extending this process, the grains tend to grow into ordered nanostructures.
Fig. 2 e SEM images of NiFe_B (aeb), NiFeAl (ced), and NiFe_P (eej).
Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109
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Fig. 2(c),(d) the surface of NiFeAl is much rougher than that of NiFe_B, with disconnected scan tracks, high porosity, intensified balling and large line cracks. This could be due to the high instability of melting pools formed during the manufacturing process. Because the melting point of Al (660 C) is critically lower than that of Ni (1455 C) and Fe (1538 C), during the solidification process, Ni and Fe solidify preferentially, leading to the unsolidified liquid phase enriched with Al, resulting in interrupted melting pools with balls and caves. On the other hand, due to the much larger thermal expansion coefficient of Al (a ¼ 23.1 106/K, at 20 C) than that of Ni (a ¼ 13 106/K) and Fe (a ¼ 11.8 106/K), the thermal coefficient mismatch leads to a large residue stress and surface tension during the rapid cooling of laser processing, resulting in cracks and pores. These structural imperfectness can serve as reaction centers for initializing dealloying process and promoting the formation of nanostructures and micro-channels. The SEM images of NiFe_P are displayed in Fig. 2 e)e(j). As observed in Fig. 2(e), the surface of NiFe_P is very porous and rough. After increasing the image magnification to Fig. 2(f), a lot of micro-scale channels are observed, which can facilitate OH ion diffusion and O2 gas transportation. On the relatively smooth areas (Fig. 2(g),(h)), the surface is converted to an array structure constructed by nanorods with ~100 nm diameter interconnected by ultrathin nanowires. On the areas along molten tracks and inside the surface pores (Fig. 2(i),(j)), well-defined coral-like nanoparticles are clearly observed. The nanocorals have a unit size ~5 mm composed of thin blades of ~500 nm width. An open source image processing program, ImageJ, is applied to estimate the porosity of the NiFe_P sample. The overall porosity of NiFe_P is estimated to be around 45%. Therefore, monolithic nanoporous NieFe alloy with an ultra-high specific surface area is developed through dealloying the laser-processed Nie FeeAl alloy samples. Fig. 3(a) displays the calculated ternary NieFeeAl phase diagram at 900 C. It can be seen upon solidification, the Nie FeeAl alloy with a Ni: Fe: Al atomic ratio ¼ 6:4:10 marked with
a red star falls into the region of b2 phase, with secondary a and g phases and an Al rich liquidus phase. The XRD patterns of NiFe_B, NiFeAl and NiFe_P are displayed in Fig. 3(b). It can be seen that the bulk NiFe_B sample is mainly composed of a FCC structure composed of a g-Ni (JCPDF card no. 01-070-0989) phase with 2q ¼ 44.605 , 51.979 and 76.591 corresponding to (111), (200) and (220) planes with the lattice parameter of a ¼ 3.516 A; a cubic FeNi3 phase (JCPDF card no. 03-065-3244) with 2q ¼ 44.121 , 51.404 and 75.661 corresponding to (111), (200) and (220) planes with a ¼ 3.552 A; a cubic taenite FeNi phase (JCPDF card no. 00-003-0016) with 2q ¼ 43.173 , 50.674 and 74.679 corresponding to (111), (200) and (220) planes with a ¼ 3.601 A and trivial amount of a-Fe phase (JCPDF card no. 00-001-1252) with 2q ¼ 44.142 and 65.186 corresponding to (111) and (220) planes with a ¼ 2.860 A. It is observed from the NiFeAl spectrum, it is composed of a BCC matrix with 2q ¼ 31.101 , 44.496 and 65.059 corresponding to (110), (200) and (202) planes with a ¼ 4.060 A, an FCC structure matrix 2q ¼ 44.765 , 52.216 and 77.063 corresponding to (111), (200), (220) planes with a ¼ 3.503 A and a high-intensity cubic Al phase (JCPDF card no. 04-013-0326) with 2q ¼ 38.510 , 44.765 , 65.165 and 78.317 corresponding to (111), (200), (220) and (311) planes with a ¼ 4.0458 A. This is because during the rapid thermal cycle of laser processing, the diffusion is highly insufficient and non-equilibrium Al segregations occur. Therefore, the Ni and Fe atoms tend to grow into NieFe intermetallic compounds resulting in a NieFe based BCC and FCC matrixes with minor amount of Al atoms altering lattice distances. For the spectrum of NiFe_P sample, it is clearly found that the Al phase disappeared from the BCC and FCC structures, indicating a successful dealloying process that has removed majority of Al atoms from the alloy matrix. As a result, the nanoporous NiFe_P is mainly composed of NieFe based alloys, which are active components toward OER in the alkaline environment. EDXS elemental mapping analysis and energy-dispersive spectrum were executed to characterize elemental composition and distribution in the nanoporous NiFe_P sample. It is
Fig. 3 e (a) Ternary NieFeeAl phase diagram at 900 C; (b) XRD spectra of NiFe_B, NiFeAl and NiFe_P. Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109
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observed the atomic ratios of Ni, Fe, Al, O are 45%, 17%, 8%, 30%, respectively. The loss of Fe during the dealloying process is due to the dissolution of Fe into the alkaline solution. As shown in Fig. 4(a),(b), Ni and Fe atoms are distributed uniformly all over the surface. As shown in Fig. 4(c), the vast majority of surface Al atoms have been dissolved and the signal of Al is mainly from the underneath substrate in the areas exposed through pores of the printed layer. In addition, due to the in-situ metal oxidation and hydroxylation in the dealloying process, oxygen signal is also observed uniformly distributed all over the sample as shown in Fig. 4(d), indicating the formation of M O (M ¼ Al, Ni, Fe) and M OH bonds. To evaluate the electrocatalytic performance, LSV was carried out on the NiFeAl, the NiFe_B and NiFe_P electrodes in
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a 1 M KOH aqueous electrolyte (Fig. 5). It is firstly observed that in the alkaline solution, a layer of metal hydroxides rapidly forms on the surface of the NiFe_B electrode at an OER overpotential of ~295 mV due to the oxidation of Ni(OH)2 into NiOOH. No peaks were observed on the spectrum of NiFe_P or NiFeAl due to a full hydroxylation during the dealloying process for NiFe_P sample and thermodynamic hydroxylation favorability of Fe and Al over Ni for NiFeAl sample [58]. It is also observed, the NiFe_B electrode exhibits an apparently lower activity toward OER compared to the nanoporous NiFe_P electrode. The NiFe_B electrode requires an overpotential of 464 mV to reach a 100 mA/cm2 current density. For nanoporous NiFe_P electrode, it only requires an overpotential of 442 mV to reach an OER current density of 100 mA/cm2,
Fig. 4 e (a) (b) (c) and (d) EDXS elemental mappings of Ni, Fe, Al and O elements in nanoporous NiFe_P respectively, (e) Energy-dispersive spectrum of NiFe_P. Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109
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Fig. 5 e The OER polarization curves of NiFeAl, the NiFe_B and NiFe_P electrodes in a 1 M KOH aqueous electrolyte in the potential range of 0e1 V vs. SCE at a scan rate of 2 mV/s.
which is comparable to reported OER electrocatalyst to date [16,58e61]. At an overpotential of 500 mV, the LSV current density of NiFe_P is 142 mA/cm2, while it is only 122 mA/cm2 and 61 mA/cm2 for NiFe_B and NiFeAl electrodes. With increased surface area and amount of surface active sites, electron transfer process at the electrode and electrolyte interface is effectively accelerated. In addition, the interconnected microchannel structure increases the amount of pathways for ion and gas diffusion. The overpotential h vs. log (current density) is plotted in Fig. 6. To understand the electrochemical mechanism of the OER catalytic process, the linear regions of polarization curves were fitted to Tafel plots, using the equation h ¼ a þ b log (j), where b is the Tafel slope to determine the kinetic constant of
Fig. 6 e Tafel Slope of NiFeAl, NiFe_B and NiFe_P electrodes in a 1 M KOH aqueous electrolyte in the potential range of 0e1 V vs. SCE at a scan rate of 2 mV/s.
electrode reactions. A small slope of Tafel plot indicates rapid electron transfer at the electrode and electrolyte interface at the applied potentials. On a Tafel plot, once the onset potential is reached, a linear slope is well defined, where electrochemical reaction is the rate determining step [62]. As shown in Fig. 6, the slope values of the Tafel regions at low overpotential range for NiFeAl, NiFe_B and NiFe_P electrodes are 158, 46 and 36 mV/decade, respectively. The lower Tafel slope of NiFe_P is contributed by ultra-small nanoparticles, which increase active sites on the electrode surface and effectively accelerate electron transfer process at the electrode and electrolyte interface. By increasing the overpotential, the rate determining step evolves from electrochemical reaction to mass diffusion [62e65], with the slope gradually increasing to ~ 600 mV/dec. It can be seen, NiFe_P reaches the mass diffusion control region at a lower overpotential than that of NiFe_B. Because the higher surface roughness and wider distribution of microchannels of NiFe_P enhance the oxygen gas repelling and mass diffusion, leading to a full surface coverage earlier. As shown in Fig. 7, The Nyquist plots of EIS were taken at an overpotential of 438 mV in the frequency range of 105 Hze0.1 Hz on the NiFeAl, NiFe_B and NiFe_P electrodes. The equivalent circuit is calculated using the CHIe650C electrochemical workstation software package with the fitted results listed in Table 1. The equivalent circuit is composed of Rs e solution resistance, Rint e electrode/electrolyte interface resistance, Rct e charge transfer resistance, CPE1 e constant phase element for interfacial capacitance, and CPE2 e constant phase element for double layer capacitance [66]. The intercept of the plot with the real axis (Rs) represents the overall ohmic resistance, including the bulk resistance of the material, series resistance, and contact resistance [67]. The semicircle at high frequency range is from metal hydroxide intermediates formation and absorption process, whose diameter (Rint) indicates the rate of the electrode/electrolyte interfacial process [68]. It is observed, the Rint of NiFe_P (0.010 U cm2) spectrum is smaller than that of Ni4Fe6_B (0.023 U cm2), indicating a faster OH absorption and intermediates formation at the electrode/electrolyte interface. It is also noticed, the CPE1 of NiFe_P (3.061 mF sn1 cm2) is much higher than that of NiFe_B (2.041 mF sn1 cm2), indicating more surface atoms on NiFe_P electrode are exposed for reaction species absorption and intermediates formation, attributed to the enlarged specific surface area from the nanoporous structure. Since the n1 values are close to the value of unity, the CPE1 is close to pure capacitance. The semicircle on low frequency range describes the electron charge transfer process of OH/O2 redox reaction at the electrode/electrolyte interface [68], with a diameter of Rct representing the charge transfer resistance between reactant to product [69]. A smaller Rct indicates a faster electron transfer. It can be seen, the Rct of NiFe_P (0.490 U cm2) is smaller than that of NiFe_B electrode (0.573 U cm2). The CPE2 illustrates the double layer capacitance, which doesn't contribute to electrochemical reaction kinetics [70,71]. The n2 value of NiFe_P (0.608) is much lower than that of NiFe_B (0.802), indicating a rougher and more porous surface. It is found, due to formation of nanostructures, the overall electrocatalytic activity is apparently improved by the increased specific surface area
Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109
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Fig. 7 e The EIS spectra of the NiFeAl, NiFe_B and NiFe_P electrodes at an overpotential of 438 mV with an applied voltage aptitude of 5 mV in the frequency range from 0.1 Hz to 105 Hz. Symbols: experimental data; solid line: fitting data; inset: equivalent circuit and enlarged image of high frequency range.
Table 1 e EIS parameters.
NiFeAl NiFe_B NiFe_P
Rs (U cm2)
Rint (U cm2)
CPE1 (mF sn1 cm2)
n1
Rct (U cm2)
n2
CPE2 (F sn1 cm2)
1.117 1.323 1.243
0.037 0.023 0.010
0.612 2.041 3.061
0.999 0.999 0.999
1.690 0.573 0.490
0.680 0.752 0.608
0.085 0.160 0.198
Fig. 8 e (a) 1st cycle and 250th cycle of cyclic voltammetry of NiFe_P electrodes in a 1 M KOH aqueous electrolyte in the potential range of 0e1 V vs. SCE at a scan rate of 10 mV/s (b) time-dependent potential response during 15-h chronoamperometric water-splitting reaction, at a constant DC potential of 288 mV overpotential.
and active sites. The nanostructure enhances the reactant absorption and intermediates formation, and accelerates the electrode/electrolyte interface electron transfer; the interconnected micro channels increase the pathways for mass
transfer and facilitate the ion diffusion and gas transportation. To study the redox behavior and stability of the nanoporous NiFe_P electrocatalyst, 250 CV cycles were carried out
Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109
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Fig. 9 e XPS core spectra of (a) Ni 2p, (b) Fe 2p, (c) Al 2p, and (d) O 1s of as-prepared fresh NiFe_P and NiFe_P after 15-h chronoamperometric water-splitting reaction.
in the potential range of 0e1 V vs. SCE at a scan rate of 10 mV/ s. The spectra of the 1st cycle and the 250th cycle are displayed in Fig. 8(a). A cathodic peak at 60 mV overpotential is observable, which can be assigned to the reduction of Ni(III) to
Ni(II) [72]. In addition, after 250 CV cycles, no significant current density decay is observed, indicating the good stability of NiFe_P electrocatalyst. To further evaluate the stability of the nanoporous NiFe_P electrocatalyst, chronoamperometry was
Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109
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applied at a 288 mV overpotential for 15 h. As shown in Fig. 8(b), it has an initial current density of 10 mA/cm2. Although the current density experienced a slight decay in the first 3 h, the current density maintains fairly constant, ~10 mA/cm2, to 15 h. To study the chemical compositions of as-prepared fresh NiFe_P and NiFe_P after 15-h chronoamperometric watersplitting reaction, XPS spectra are displayed in Fig. 9. As displayed in Fig. 9(a), the Ni 2p spectrum of fresh NiFe_P sample has two main pairs of de-convoluted peaks, which can be attributed to Ni0 and Ni2þ/Ni3þ, respectively [73,74]. It can be seen, after 15h chronoamperometric water-splitting reaction, the relative intensity of Ni2þ/Ni3þ peaks to Ni0 peaks are getting higher, indicating more Ni atoms have been oxidized/hydro-oxidized during the water-splitting process. As displayed in Fig. 9(b), the Fe 2p spectrum has two main pair of de-convoluted peaks attributed to Fe0 and Fe2þ/Fe3þ, respectively [75]. The chronoamperometric process slightly increased the ratio of Fe2þ/Fe3þ to Fe0. As displayed in Fig. 9(c), the Al atoms are mostly oxidized/hydrooxidized to Al3þ with little metallic Al remaining in the alloy after 15 h. It can be seen, the O 1s core spectra are composed of M O (M ¼ Ni, Fe, Al), M OH and M-OOH bonds [76,77]; the increased ratio of M-OOH bonds and decreased ratio of M O bonds confirm that metals are further oxidized/hydro-oxidized after the 15-h chronoamperometric water-splitting reaction.
Conclusions In conclusion, monolithic nanoporous NieFe alloy electrocatalyst is developed by dealloying laser processed NieFeeAl alloy for OER. The nanoscale pores derived from dealloying process can provide high specific surface area and more active sites for catalytic reactions. The microscale channels can provide more pathways for gas diffusion and ion transportation. In addition, as a monolithic electrode, the electron conduction can be effectively facilitated and the use of substrate and binders can be eliminated. Compared to bulk NieFe electrocatalyst, the nanoporous NieFe electrode, even at a less favorable Ni:Fe composition after the dealloying process, exhibits an apparently improved electrocatalytic activity and only requires 442 mV overpotential to achieve an OER current density of 100 mA/cm2 in 1 M KOH aqueous solution, which is comparable to the performance of reported electrocatalysts to date. Overall, the laser-based manufacturing method possesses high flexibility in developing complex shaped preforms for monolithic nanoporous electrodes, which provides a new fabrication method for next generation electrode design and electrocatalyst engineering. It's well known that nanoporous structures are prone to structure degradation. Further process optimization can be performed to improve the composition and stability of monolithic nanoporous electrocatalysts.
Acknowledgements This publication is based upon work supported by NSF-Consortium for innovation in manufacturing and materials (CIMM) program (grant number # OIA-1541079).
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Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109
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Please cite this article in press as: Cui X, et al., Monolithic nanoporous NieFe alloy by dealloying laser processed NieFeeAl as electrocatalyst toward oxygen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.109