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Corrosion-engineered bimetallic oxide electrode as anode for high-efficiency anion exchange membrane water electrolyzer
Journal Pre-proofs Corrosion-engineered bimetallic oxide electrode as anode for high-efficiency anion exchange membrane water electrolyzer Jooyoung Lee, Hyeonjung Jung, Yoo Sei Park, Seongwon Woo, Nayoung Kwon, Yaolong Xing, Sang Ho Oh, Sung Mook Choi, Jeong Woo Han, Byungkwon Lim PII: DOI: Reference:
S1385-8947(20)33792-X https://doi.org/10.1016/j.cej.2020.127670 CEJ 127670
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Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
21 August 2020 26 October 2020 4 November 2020
Please cite this article as: J. Lee, H. Jung, Y.S. Park, S. Woo, N. Kwon, Y. Xing, S.H. Oh, S.M. Choi, J.W. Han, B. Lim, Corrosion-engineered bimetallic oxide electrode as anode for high-efficiency anion exchange membrane water electrolyzer, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.127670
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Chemical Engineering Journal, 10/2020
Corrosion-engineered bimetallic oxide electrode as anode for high-efficiency anion exchange membrane water electrolyzer
Jooyoung Leea,b,1, Hyeonjung Jungc,1, Yoo Sei Parkb,d, Seongwon Wooa, Nayoung Kwona, Yaolong Xinge, Sang Ho Ohe,*, Sung Mook Choib,*, Jeong Woo Hanc,*, Byungkwon Lima,*
a School
of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea
b
Materials Center for Energy Convergence, Surface Technology Division, Korea Institute of Materials Science (KIMS), Changwon 51508, Republic of Korea c
Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Gyeongbuk 37673, Republic of Korea
d
Department of Materials Science and Engineering, Pusan National University, Busan 46241, Republic of Korea
e
Department of Energy Science, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea
1
These authors contributed equally to this work.
*Corresponding authors. E-mail addresses: [email protected] (S. H. Oh), [email protected] (S. M. Choi), [email protected] (J. W. Han), [email protected] (B. Lim)
Abstract: Developing high-performance and low-cost oxygen-evolving electrodes is one of major challenges in electrochemical water splitting technology. We demonstrated that the aqueous-phase corrosion of a conventional Ni foam in the presence of exotic Fe3+ cations can directly transform it into an electrode with high catalytic activity and stability for oxygen evolution reaction (OER). The surface of the corroded electrode consisted of densely packed, small Ni0.75Fe2.25O4 nanoparticles with sizes less than 5 nm. This electrode required an overpotential of only 192 mV to reach an OER current density of 10 mA/cm2 in 1 M KOH, outperforming the state-of-the-art IrO2 catalyst by 73 mV. Density functional theory calculations revealed that the unique surface structure and iron composition of Ni0.75Fe2.25O4 nanoparticles play a key role in achieving an improved OER activity. When coupled with a Pt/C hydrogen-evolving catalyst, the resulting anion-exchange membrane (AEM) water electrolyzer achieved an overall water-splitting current density as high as 2.0 A/cm2 at a cell voltage of 1.9 V in 1 M KOH, which was 1.7 times that obtained from the IrO2 and Pt/C catalyst pair and also much greater than reported values from other AEM water electrolyzers. By revisiting and exploiting a traditional corrosion process, our work opens a simple, cost-effective, and scalable route to a high-performance OER electrode for efficient AEM water electrolysis.
Keywords: corrosion, nanoparticle bimetallic oxide, oxygen evolution reaction, anion exchange membrane water electrolyzer
1. Introduction Electrochemical water splitting is a promising route to production of hydrogen (H2), which is an eco-friendly, alternative energy source to fossil fuels [1–4]. Polymer-exchange membrane water electrolysis systems are attractive for large-scale production of hydrogen gas at high rate and purity, which include proton-exchange membrane and anion-exchange membrane (AEM) water electrolyzers [5–8]. In particular, an AEM water electrolyzer has attracted much attention because it offers an alkaline environment at a membrane interface, and thus has the advantage that earthabundant transition metal oxides can be used as electrocatalysts for an oxygen evolution reaction (OER) occurring at the anode [9–13]. However, the kinetics of the OER is sluggish, and a substantial overpotential () required for this reaction at practical operating current densities reduces the overall efficiency of water electrolysis [14–19]. Currently, iridium oxide (IrO2) and ruthenium oxide (RuO2) are among the state-of-the-art OER catalysts, but even these precious metal oxides are not effective enough for efficient OER [20–28]. Non-precious metal oxides such as Co3O4, CuxCo3–xO4, and CoFeOx have been explored as OER catalysts for AEM water electrolysis [9,10,13]. Despite recent progress, however, practical application of AEM water electrolyzers in industry is still limited by their low efficiencies, and high-performance, low-cost, and easy-to-fabricate OER electrodes are strongly needed. Corrosion is a chemical process that involves the redox reactions occurring at the interface between a material and its environment, and has long been considered to be undesirable due to its adverse effect leading to damage or failure of a metal or alloy [29–31]. However, recent studies have shown that corrosion can be exploited as a versatile route to produce unconventional nanostructures [32–34]. In this work, we produced a highly active and stable OER electrode simply by corroding a conventional nickel foam (NF) in an aqueous solution containing exotic Fe3+ cations.
The corrosion process led to the evolution of a nanostructure consisting of densely packed, small Ni0.75Fe2.25O4 nanoparticles (NPs) on the NF surface. This corrosion-engineered electrode required a small overpotential of only 192 mV (iR corrected) at an OER current density of 10 mA/cm2 in 1 M KOH, which was 73 mV less than the IrO2 catalyst. The OER electrode was coupled with a cathode loaded with a Pt/C catalyst for hydrogen evolution reaction (HER) to construct an AEM water electrolyzer. Our electrolyzer achieved an overall water-splitting current density as high as 2.0 A/cm2 at a cell voltage of 1.9 V (without iR corrected) in 1 M KOH, which far exceeded the performance of an AEM water electrolyzer based on the IrO2 and Pt/C catalyst pair (1.18 A/cm2 at 1.9 V), as well as those of previously reported AEM water electrolyzers. Our approach does not require synthesis and/or loading of an exotic OER catalyst, and thus provides a convenient and cost-effective route to large-scale production of a high-performance OER electrode for efficient AEM water electrolysis.
2. Experimental section 2.1. Synthesis of Ni0.75Fe2.25O4 NPs arrays on NF To remove a surface oxide layer, a NF with a thickness of 0.8 mm (Welcos Co., Ltd, Korea) was reduced in a tubular furnace at 800 ºC for 2 h with constant H2 flow at a flow rate of 50 sccm. The NF was immersed in 66.7 mM aqueous solution of iron nitrate [Fe(NO3)3·9H2O, Acros] at 95 ºC for 36 h with magnetic stirring, and then cooled down to room temperature. After the synthesis, the Ni0.75Fe2.25O4 nanoparticles array on the NF was washed with deionized water several times and dried in a vacuum oven at 80 ºC for overnight for further characterization and measurements.
2.2. Synthesis of NiFe2O4 NPs
In a typical synthesis, iron chloride (FeCl3·6H2O, Aldrich, 3.2 mmol), nickel chloride (NiCl2, Aldrich, 1.6 mmol), and cationic surfactant (CTAB, Aldrich, 2.7 mmol) were dissolved in deionized water (35 mL), hosted in a 50 mL Teflon-lined stainless autoclave. The pH of the solution was adjusted to 11.0 by adding 2 M NaOH. The autoclave was kept at 130 ºC in a muffle furnace for 15 h and then quenched down to room temperature. The product was washed with ethanol several times and then dried in oven at 80 ºC for overnight. The products were placed in tube furnace and slowly heated to 300 ºC in air [35].
2.3. Characterization The scanning electron microscopy (SEM) images were obtained from a SUPRA 55VP (Carl Zeiss). Transmission electron microscopy (TEM) images were captured using a JEM-2100F microscope operated at 200 kV. Powder X-ray diffraction (XRD) patterns were obtained with a D8-Advances (Bruker AXS) diffractometer, equipped with a rotating anode and a CuKa radiation source (λ = 0.15418 nm). X-ray photoelectron spectroscopy (XPS) spectrums were obtained by using an ECSA2000 (VG Microtech.).
2.4. Electrochemical measurements Polarization curves measurements were performed using a CHI600D electrochemical analyzer (CH instrument) in 1 KOH solution. Pt wire and Hg/HgO (in 1 M NaOH solution) electrodes were used as counter and reference electrodes, respectively. Chronopotentiometric curve measurements were performed with WBCS-3000 (Xeno Co.) in 1 KOH solution. In the preparation of a NiFe2O4 and commercial IrO2 electrodes, a mixture of a catalyst, Ketchen black as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder were prepared with a weight ratio of 70:20:10. These
ingredients were added to N-methyl-2-pyrrolidone (NMP) to prepare the slurry, which was then coated onto a piece of a NF (1 × 1 cm2) and dried in a vacuum oven overnight at 80 ºC for overnight.
2.5. AEM water electrolyzer system fabrication The AEM water electrolyzer systems comprised the anode (Ni0.27Fe2.27O4 NPs on NF), cathode (1 mg/cm2 of commercial Pt/C on NF, 4.9 cm2), anion exchange membrane (AEM, X37-50 Grade T, Dioxide Materials), and gas diffusion layer (NF). IrO2 powder (Sigma Aldrich) was used as an anode catalyst for comparison and loaded on NF with Nafion. The loading amount of IrO2 was 4 mg/cm2. The area of the anode and cathode electrodes of AEM water electrolyzer single-cell are 7.1 cm2 and 4.9 cm2. The single cell was supplied with 1 M KOH as an electrolyte at 24 mL/min and was operated at 42∼45 °C. The AEM water electrolyzer test was performed using a potentiostat (BP2C, ZIVE LAB), and electrochemical performance was analyzed by polarization curves (from 1.4 to 1.9 Vcell) measurements. The long-term testing of AEM water electrolyzer systems was conducted at 500 mA/cm2. Energetic efficiency of the AEM water electrolyzer was calculated by the following equation.
Wh is the electric power to produce hydrogen, H0 is the calorific value of hydrogen (10.8 x106 J/m3, lower heating value) and VH2 is the hydrogen gas volume.
2.6. Computational details
All DFT calculations were carried out by Vienna ab initio Simulation Package (VASP) [36,37]. The projector augmented wave (PAW) method was used for describing electron-ion interactions [38] The exchange-correlation was approximated using the Perdew–Burke–Ernzerhof (PBE) formula with spin polarization [39]. We performed GGA+U method with Ueff (Fe) = 4.5 eV and Ueff (Ni) = 6 eV according to the literature [40]. During the structure optimization, the convergence criterion of total energy was set to 10−4 eV and the atoms were relaxed until the force acting on each atom was less than 0.03 eV/Å. The kinetic energy cutoff and k-points were tested and chosen with a convergence criterion of 10-2 eV. The bottom two layers of four layers were fixed in IrO2 slab models, while the bottom layer of three layers was fixed in NixFe3-xO4 slab models. Each slab model was separated from its neighbors by a sufficiently thick vacuum layer. Ni0.75Fe2.25O4 bulk structure was prepared by substituting Ni atoms at octahedral site with Fe atoms (Fig. S1).
3. Results and discussion A bare NF has a porous, three dimensional structure, and was reduced under a H2 flow to remove a surface oxide layer (Fig. S2). The reduced NF was corroded in an aqueous solution containing Fe(NO3)3 at 95 °C with magnetic stirring. In this case, a corrosion process may involve redox reactions including Ni(s) → Ni2+ + 2e– and Fe3+ + e– Fe2+, which can be driven by the difference in the standard reduction potentials of Ni2+/Ni (0.25 V) and Fe3+/Fe2+ (0.77 V) pairs. In the presence of the added and newly generated cations including Ni2+, Fe2+, and Fe3+, an olation process may lead to the formation of binary NiFe oxide NPs (Fig. 1a) [31,34] Fig. 1b shows photographs of the bare NF and corroded electrode as large as 20 cm × 10 cm. It can be seen that the color of a NF was changed from gray to dark brown after the corrosion. The scanning electron microscopy (SEM) analysis of the corroded electrode revealed the evolution of nanostructures
consisting of densely packed NPs on the electrode surface in order to minimize the total surface energy of small-sized ferrite NPs produced by corrosion (Fig. 1c and d) [4143]. The Brunauer−Emmett−Teller (BET) specific surface area of the corroded electrode was estimated to be 1.91 m2/g (Fig. S3a). The Barrett-Joyner-Halenda (BJH) average pore size of corroded electrode was 8 nm (Fig. S3b). The X-ray diffraction (XRD) pattern of the corroded electrode showed the diffraction peaks at 2θ values of 30.1, 35.6, 43.3, 57.2, and 62.8° (Fig. 1e), which can be indexed to (220), (311), (222), (422), and (440) reflections of an inverse spinel structure, respectively [44,45]. In the X-ray photoelectron spectroscopy (XPS) analysis, the Ni XPS 2p core level spectrum included an intense peak associated with Ni2+ in addition to a weak shoulder peak originating from Ni3+ (Fig. 1f) [46,47]. The Ni3+ sites seem to be generated from the partial oxidation of Ni2+ sites. The Fe XPS 2p core level spectrum showed peaks corresponding to both Fe2+ and Fe3+ (Fig. 1g) [46,47]. When Ni is oxidized, released electrons can be captured by Fe3+ to generate Fe2+. In the O 1s spectrum, peaks centered at 531.2 and 529.9 eV are associated with OH− and O2−, respectively (Fig. 1h). These NPs were detached from the corroded electrode by sonication, and then characterized by transmission electron microscopy (TEM). The TEM image in Fig. 2a clearly shows the formation of NPs with sizes less than 5 nm. The nanoparticles exhibited both multiple twin boundaries and single-crystal structures, as can be seen in high-angle annular dark field scanning TEM (HAADF-STEM) images (Fig. 2b,c and Fig. S4). The lattice spacing of the nanoparticles is found to be 0.208 nm, which is associated with the (400) planes of inverse spinel structure. The Fast Fourier Transformation (FFT) patterns of nanoparticles was in agreement with the result from the XRD pattern analysis (Fig.2b and Fig. S4, insets). The elemental mapping of the NPs showed the uniform distribution of Ni, Fe, and O elements in their structures (Fig. 2d). The energy-
dispersive X-ray spectroscopy (EDS) analysis indicated that the atomic ratio of Ni to Fe in the NPs was approximately 25:75 (Fig. S5). These results indicated the formation of a binary Ni0.75Fe2.25O4 NPs through the corrosion process. We investigated the electrocatalytic properties of the corrosion-derived Ni0.75Fe2.25O4 electrode for the OER. For comparison, we also prepared a NF electrode loaded with NiFe2O4 NPs that had been synthesized by using a previously reported (Fig. S6) [35], hydrothermal method as well as that with a commercial IrO2 catalyst (Fig. S7). Fig. 3a shows OER polarization curves for four different electrodes including a bare NF, which were recorded at room temperature in 1 M KOH solutions at a scan rate of 1 mV/s from positive to negative potential on the reversible hydrogen electrode (RHE) scale. The Ni0.75Fe2.25O4 electrode exhibited a much higher OER activity than other three electrodes, and required an overpotential of only 192 mV to reach a current density of 10 mA/cm2 with iR compensation, which was 185, 82, and 73 mV less than the bare NF, NiFe2O4 NPs, and IrO2 catalyst, respectively. The measured overpotential of the Ni0.75Fe2.25O4 electrode was also lower than those for most of OER anodes that had been explored for AEM water electrolyzers under the same or similar conditions. A Tafel plot reflects the kinetics of an electrochemical reaction on a given catalyst [47–50] A Tafel slope recorded from the Ni0.75Fe2.25O4 electrode was 44 mV/dec, which was much lower than that IrO2 catalyst (67 mV/dec), indicating a higher rate of the OER on the Ni0.75Fe2.25O4 electrode (Fig. 3b). The electrochemical surface areas (ECSA) of the catalyst reflects the number of exposed active sites. The ECSA is proportional to the electrochemical double-layer capacitance (Cdl) from cyclic voltammetry (CV) curves. The Ni0.75Fe2.25O4 electrode had a larger Cdl (28 mF/cm2), which was 16 and 11 mF/cm2 less than the NiFe2O4 electrode and IrO2 electrode, respectively. (Fig. S8). The large Cdl of the Ni0.75Fe2.25O4 electrode indicates the more exposed active site, which is important for the activity of the OER
catalyst. We also evaluated the catalytic stability of the Ni0.75Fe2.25O4 electrode at a constant current density of 50 mA/cm2 for 100 h (Fig. 3c). We did not observe a noticeable increase in the potential required for keeping this current density during the stability test, demonstrating the excellent catalytic stability of the Ni0.75Fe2.25O4 electrode toward the OER. In addition, we investigated the change after OER test of Ni0.75Fe2.25O4 electrode. The SEM, XRD and XPS analysis confirmed that the Ni0.75Fe2.25O4 electrode retained its surface morphology and structure after the stability test (Fig. 3d and Fig. S9). The OER activity is largely affected by the binding energies of OER intermediates, i.e., *O, *OH, and *OOH species adsorbed on the active sites. To explain the higher activity of Ni0.75Fe2.25O4 by obtaining detailed insight into the OER mechanism, we performed DFT calculations for the elementary steps of 4e mechanism (Eqn. (1)-(4)). The asterisk denotes the active site on the catalyst, and the energy of a hydrogen ion and an electron was replaced by that of 1/2 hydrogen gas [51]. The free energy of each step was calculated by Eqn. (5) with assuming the standard state (T = 25 ℃, P = 1 atm, pH = 0). The zero-point energy (ZPE) and vibrational entropy were obtained from the vibrational frequency calculation, while thermodynamic entropy tables were used for water and hydrogen. The step with the highest free energy determines OER operating potential, called a potential determining step (PDS), where over-potential was obtained (Eqn. (6)). Step 1: Step 2:
H2O(l) + ∗ OH ∗
∆G2
Step 3:
O ∗ + H2O(l)
Step 4:
OOH ∗
∆G4
∆G1
OH ∗ + (H + + e ― )
O ∗ + (H + + e ― )
∆G3
(1) (2)
OOH ∗ + (H + + e ― )
(3)
O2(g) + ∗ + (H + + e ― )
(4)
Over-potential:
∆𝐺 = ∆𝐸 + ∆𝑍𝑃𝐸 ― 𝑇∆𝑆 ― 𝑒𝑈 + 𝑘𝑇 ∙ ln [𝐻 + ]
(5)
𝜂 = max (∆G1, ∆G2,∆G3,∆G4)/𝑒 ― 1.23
(6)
We investigated the OER mechanism on Ni0.75Fe2.25O4(311), NiFe2O4(311), and IrO2(110), and their optimized oxygen containing intermediate (OCI) structures and Gibbs free energy profiles were shown in Fig. 4 Several theoretical studies for OER performance on IrO2(110) have already been done [52–55] but we did it again in this work to compare the performance with nickel ferrite under the same calculation conditions. The OER on IrO2 was limited by the oxidation of *O to *OOH, and the over-potential was 607 mV, which is good agreement with previous computational studies (η = 610 mV). It supports that our calculation conditions are reliable and can explain the experimental results in this work. In case of NiFe2O4, the surface Fe with η = 707 mV was the OER active site, and the PDS corresponded to the transformation of *OH to *O. The Ni0.75Fe2.25O4 had two types of Fe sites on the surface, which were distinguished by whether Fe or Ni was located in the lower layer (the former with lower over-potential). Its OER was hindered by the first step and the over-potential was 222 mV. It is consistent with the experimental result that OER activity was Ni0.75Fe2.25O4 > IrO2 > NiFe2O4. In case of spinel-type transition metal oxide, eg occupancy is known as a reliable descriptor for the OER activity. The eg occupancy near 1 balances each step of the 4-e mechanism and results in lower potential barrier [56]. To further elucidate the enhancement of OER performance caused by the Fe substitution into Ni, we calculated eg descriptors from pDOS analysis for Ni0.75Fe2.25O4 and NiFe2O4 and plotted OER overpotential against them (Fig. S10). The OER overpotentials decrease linearly with the eg occupancy increase, because the data is located at the right branch of the volcano plot (eg > 1). Ni has a more filled eg orbital than Fe in both catalysts, and this explains that Fe serves as the main OER active site. It was also confirmed that Fe substituting partial Ni reduces the eg occupancy of Fe, bringing it closer
to the optimal value, and thus results in the improved OER activity. In addition, the unique structure of the corroded electrode consisting of Ni0.75Fe2.25O4 NPs with high density and enrich grain boundary on the NF surface allows intimate electrical contacts between them and exposed more edge sites. These generally achieve low charge transfer resistance and provide many active catalytic sites, thus enabling high OER activity (Fig. S11). Industrial-scale hydrogen production via electrochemical water splitting typically requires high current densities (0.5 A/cm2) [7]. We constructed a single-cell AEM water electrolyzer by coupling the Ni0.75Fe2.25O4 anode with a carbon cloth cathode loaded with a commercial Pt/C catalyst, as well as the same electrolyzer except for the use of a NF loaded with the IrO2 catalyst as an anode (Fig. 5a–c). Polarization curves for the overall water splitting by these two electrolyzers in 1 M KOH solution are shown in Fig. S8. At current densities of 0.5 A/cm2, our Ni0.75Fe2.25O4//(Pt/C)-based AEM water electrolyzer showed better performance than the IrO2//(Pt/C)-based AEM water electrolyzer. The IrO2//(Pt/C)-based electrolyzer required a large overpotential of 612 mV to obtain the overall water-splitting current density of 1.0 A/cm2 (Fig. 5d). In contrast, our electrolyzer achieved the same current density at a cell voltage of 1.75 V ( = 515 mV), which corresponds to electrical-to-fuel efficiency of 83%. At a cell voltage of 1.9 V, our AEM water electrolyzer showed the current density of 2.0 A/cm2 (Fig. 5e), which was 1.7 times that obtained from the IrO2//(Pt/C)-based electrolyzer (1.18 A/cm2). The high frequency resistance (HFR) of Ni0.75Fe0.25O4//(Pt/C)-based AEM water electrolyzer (0.0176 Ω) was lower than IrO2//(Pt/C)-based AEM water electrolyzer (0.0245 Ω) due to the low interface contact resistance between the catalyst layer and conductivity substrate. Despite iR-correction of the polarization curves, the performance of Ni0.75Fe2.25O4//(Pt/C)-based AEM water electrolyzer was showed to be higher than IrO2//(Pt/C)-based AEM water electrolyzer (Fig. S12). Therefore,
performance of our electrolyzer is better than that of IrO2//(Pt/C)-based AEM water electrolyzer in the AEM water electrolyzer performance rather than ohmic resistance. Our electrolyzer also delivered much higher current densities than other single-cell AEM water electrolyzers reported to date under the same or similar operating conditions (Fig. 5f and Table S1). During a 21-h test of our electrolyzer at a constant current density of 0.5 A/cm2, an energetic efficiency was maintained over 75% (Fig. 5g), and the rate of hydrogen production was as high as 14.2 ml/min. Hence, assembly of our electrolyzer may enable to produce hydrogen on a large scale at a much improved efficiency.
4. Conclusion We have demonstrated the fabrication of an OER electrode by harnessing the aqueous-phase corrosion of a conventional NF. This cheap process allowed to directly produce a large-area, highperformance anode for AEM water electrolyzers, without need for additional catalysts. The corroded electrode consisted of a NF with its surface decorated by densely packed, binary Ni0.75Fe2.25O4 NPs, and exhibited a much higher OER activity than the state-of-the-art IrO2 catalyst, which could be attributed to improved OER energetics on the surface of binary Ni0.75Fe2.25O4 NPs as well as high electrical conductivity achieved from intimate electrical contacts between the densely packed Ni0.75Fe2.25O4 NPs and NF surface. This corroded electrode was successfully applied as an anode for a single-cell AEM water electrolyzer, which outperformed that based on the state-of-the-art IrO2 catalyst as well as other previously reported AEM electrolyzers. Our approach is simple, cost-effective, and scalable, and thus offers a promising way for fabricating large-area anodes for efficient OER and water electrolysis. It may also be possible to extend the
corrosion-based approach to produce electrodes with desired surface structures and compositions for catalytic and other applications.
Acknowledgements This work was supported by Basic Science Research Programs (No. 2015R1A2A2A01006325, 2019R1A2C2006997, and 2018R1A2B2002875) through the National Research Foundation (NRF) of Korea funded by the Ministry of Science, ICT & Future Planning.
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Fig. 1. (a) Schematic illustration of corrosion process of a NF. (b) Photographs of bare NF, and corroded NF with a dimension of 20 cm × 10 cm. (c and d) SEM images and (e) XRD pattern taken from the corroded electrode. (f) Ni XPS 2p core level spectrum, (g) Fe XPS 2p core level spectrum and (h) O XPS 1s core level spectrum of a binary Ni0.75Fe2.25O4 NPs.
Fig. 2. (a) TEM, (b, c) HAADF-STEM, and (d) Scanning TEM images of a binary Ni0.75Fe2.25O4 NPs. Insets show the corresponding FFT patterns.
Fig. 3. (a) OER polarization curves for Ni0.75Fe2.25O4 electrode, NiFe2O4 catalyst, IrO2 catalyst, and bare NF electrode recorded in 1 M KOH solution at a scan rate of 1 mV/s from positive to negative potential at room temperature. (b) Tafel plots of the Ni0.75Fe2.25O4 electrode, NiFe2O4 catalyst, and IrO2 catalyst for OER. (c) Chronopotentiometry curve of the Ni0.75Fe2.25O4 electrode at a constant current density of 50 mA/cm2 for 100 h. (d) SEM image taken from the Ni0.75Fe2.25O4 electrode after the stability test.
IrO2
a
c
Ni0.75Fe2.25O4
NiFe2O4
b
Free Energy (eV)
d5
*
G1
*OH
*O
G3
*OOH
G4
O2 (g)
IrO2
4
NiFe2O4
3
Ni0.75Fe2.25O4 PDS: 1.94 eV
2 1
G2
PDS: 1.84 eV
PDS: 1.45 eV
0 Reaction coordinate
Fig. 4. Most stable OCI configurations on the (a) IrO2, (b) NiFe2O4 and (c) Ni0.75Fe2.25O4 surfaces. Red, deep-green, gray, brown, yellow, light-green, and white balls represent oxygen, iridium, nickel, iron, nickel substituted iron, oxygen of OCI, and H, respectively. The top layer is represented by ball-type, while the other layers are represented by line-type. (d) The free energies of the OCIs on the IrO2(110), NiFe2O4(311) and Ni0.75Fe2.25O4(311) surfaces at U = 0 V. The internal subgraph shows the overpotential of each catalyst.
Fig. 5. (a) Schematic illustration of the designed AEM water electrolyzer. Photographs of (b) AEM water electrolyzer and (c) evolved hydrogen. (d) Overpotential at various current density and (e) current density at various cell voltages for Ni0.75Fe2.25O4//(Pt/C) and IrO2//(Pt/C) cells. (f) Comparison of AEM water electrolyzer for the Ni0.75Fe2.25O4//(Pt/C) cell, the reported AEM water electrolyzer cells, and the reported IrO2 based AEM water electrolyzer. (g) Durability cell voltagetime and energetic efficiency plots for the AEM water electrolyzer at constant current density of 500 mA/cm2 for 21 h.
Supplementary Materials for
Corrosion-engineered bimetallic oxide electrode as anode for high-efficiency anion exchange membrane water electrolyzer
Jooyoung Leea,b,1, Hyeonjung Jungc,1, Yoo Sei Parkb,d, Seongwon Wooa, Nayoung Kwona, Yaolong Xinge, Sang Ho Ohe,*, Sung Mook Choib,*, Jeong Woo Hanc,*, Byungkwon Lima,*
a School
of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea. E-mail: [email protected]
b
Materials Center for Energy Convergence, Surface Technology Division, Korea Institute of Materials Science (KIMS), Changwon 51508, Republic of Korea. c
Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Gyeongbuk 37673, Republic of Korea. E-mail:
d
Department of Materials Science and Engineering, Pusan National University, Busan, 46241, Republic of Korea
e
Department of Energy Science, Sungkyunkwan University (SKKU), Suwon, 16419, Republic of Korea.
E-mail: [email protected] (S. H. Oh), [email protected] (S. M. Choi), [email protected] (J. W. Han), [email protected] (B. Lim) *Corresponding authors 1
These authors have contributed equally.
Fig. S1. The most stable configuration of NiFe2O4 and Ni0.75Fe2.25O4 bulk structure. Red, gray, brown, and yellow balls represent oxygen, nickel, iron, and iron substituted nickel, respectively. NiFe2O4 bulk unit cell contains 8 Ni atoms on octahedral site, 8 Fe atoms on tetrahedral site, and 8 Fe atoms on octahedral site. Ni0.75Fe2.25O4 bulk unit cell contains 6 Ni atoms on octahedral site, 8 Fe atoms on tetrahedral site, and 10 Fe atoms on octahedral site. Lattice constant slightly increased (~0.2 Å) due to the difference in atomic radius between Fe and Ni.
Fig. S2. (a, b) SEM images of pristine NF.
Fig. S3. (a) Nitrogen adsorption /desorption isotherms, and (b) pore size distribution of corroded electrode.
Fig. S4. HAADF-STEM image of a binary Ni0.75Fe2.25O4 NP. The image was taken from the sample shown in Fig. 2.
Fig. S5. EDS spectrum of a binary Ni0.75Fe2.25O4 NPs.
Fig. S6. (a) SEM image, (b) XRD pattern, and (c) EDS spectrum of NiFe2O4 NPs.
Fig. S7. SEM image of a commercial IrO2 catalyst.
Fig. S8. Cyclic voltammetry curves of (a) Ni0.75Fe2.25O4 electrode, (b) NiFe2O4 electrode and (c) IrO2 electrode with different scan rate (2−10 mV/s). (d) Electrochemcial double-layer capacitance measurement of the Ni0.75Fe2.25O4 electrode, NiFe2O4 electrode and IrO2 electrode.
Fig. S9. (a) XRD pattern taken from the corroded electrode. (b) Ni XPS 2p core level spectrum, (c) Fe XPS 2p core level spectrum and (d) O XPS 1s core level spectrum of a binary Ni0.75Fe2.25O4 electrode after the stability test.
Fig. S10. The relation between the OER catalytic activity, defined by the calculated overpotentials, and the eg orbital occupancy of the octahedral transition metal. The presence of the Fe component caused the higher calculated value of eg occupancy than the ideal value, 2 [1].
Fig. S11. (a) EIS plots at an applied potential of 1.48 vs. RHE of the Ni0.75Fe2.25O4 electrode and NiFe2O4 catalyst with (b) equivalent circuit diagram.
Fig. S12. Polarization curves ((a) without and (b) with iR corrected) of AEM water electrolyzer for Ni0.75Fe2.25O4//(Pt/C) and IrO2//(Pt/C) recorded in 1 M KOH solution at 50 ℃.
Table S1. A comparison of Ni0.75Fe2.25O4//(Pt/C) AEM water electrolyzer performance with previously reported AEM water electrolyzer.
Cell voltage (V)
j (A/cm2)
1.8
1.315
1.9
2
1.8
0.87
1.8
1.07
2
1.8
0.299
3
1.8
0.399
4
1.9
1.5
5
1.8
1.045
6
1.8
0.4
7
2
0.4
8
Cu0.7Co2.3O4//(Pt/C)
1.8
1
9
Co3O4//(Pt/C)
1.8
0.6
9
pyrochlore//(Pt/C)
1.8
0.5
10
Cu0.81Co2.19O4//(Pt/C)
1.8
0.36
11
Ni//PtNi
1.9
0.25
12
Catalysts Ni0.75Fe2.25O4//(Pt/C)
IrO2//(Pt/C)
ref.
Our work
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8. X. Su, L. Gao, L. Hu, N. A. Qaisrani, X. Yan, W. Zhang, X. Jiang, X. Ruan, G. He, Novel piperidinium functionalized anionic membrane for alkaline polymer electrolysis with excellent electrochemical properties, J. Membr. Sci. 581 (2019) 283–292. 9. X. Wu, K. Scott, CuxCo3−xO4 (0≤ x< 1) nanoparticles for oxygen evolution in high performance alkaline exchange membrane water electrolyzers, J. Mater. Chem. 21 (2011) 12344–12351. 10. J. Parrondo, M. George, C. Capuano, K. E. Ayers, V. Ramani, Pyrochlore electrocatalysts for efficient alkaline water electrolysis, J. Mater. Chem. A 3 (2015) 10819–10828. 11. W.-S. Choi, M. J. Jang, Y. S. Park, K. H. Lee, J. Y. Lee, M.-H. Seo and S. M. Choi, ThreeDimensional Honeycomb-Like Cu0.81Co2.19O4 Nanosheet Arrays Supported by Ni Foam and Their High Efficiency as Oxygen Evolution Electrodes, ACS Appl. Mater. Interfaces 10 (2018) 38663– 38668. 12. M. Faraj, M. Boccia, H. Miller, F. Martini, S. Borsacchi, M. Geppi, A. Pucci, New LDPE based anion-exchange membranes for alkaline solid polymeric electrolyte water electrolysis, Int. J. Hydrog. Energy 37 (2012) 14992–15002. Highlights
A small densely packed Ni0.75Fe2.25O4 nanoparticles were grown on the Ni foam.
The binary Ni0.75Fe2.25O4 nanoparticles exhibited excellent OER activity.
The improved activation sites due to their nanostructures enhanced OER activity.
The electrode applied to high-efficiency AEM water electrolyzer as an anode.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S1385-8947(20)33792-X https://doi.org/10.1016/j.cej.2020.127670 CEJ 127670
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
21 August 2020 26 October 2020 4 November 2020
Please cite this article as: J. Lee, H. Jung, Y.S. Park, S. Woo, N. Kwon, Y. Xing, S.H. Oh, S.M. Choi, J.W. Han, B. Lim, Corrosion-engineered bimetallic oxide electrode as anode for high-efficiency anion exchange membrane water electrolyzer, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.127670
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Chemical Engineering Journal, 10/2020
Corrosion-engineered bimetallic oxide electrode as anode for high-efficiency anion exchange membrane water electrolyzer
Jooyoung Leea,b,1, Hyeonjung Jungc,1, Yoo Sei Parkb,d, Seongwon Wooa, Nayoung Kwona, Yaolong Xinge, Sang Ho Ohe,*, Sung Mook Choib,*, Jeong Woo Hanc,*, Byungkwon Lima,*
a School
of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea
b
Materials Center for Energy Convergence, Surface Technology Division, Korea Institute of Materials Science (KIMS), Changwon 51508, Republic of Korea c
Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Gyeongbuk 37673, Republic of Korea
d
Department of Materials Science and Engineering, Pusan National University, Busan 46241, Republic of Korea
e
Department of Energy Science, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea
1
These authors contributed equally to this work.
*Corresponding authors. E-mail addresses: [email protected] (S. H. Oh), [email protected] (S. M. Choi), [email protected] (J. W. Han), [email protected] (B. Lim)
Abstract: Developing high-performance and low-cost oxygen-evolving electrodes is one of major challenges in electrochemical water splitting technology. We demonstrated that the aqueous-phase corrosion of a conventional Ni foam in the presence of exotic Fe3+ cations can directly transform it into an electrode with high catalytic activity and stability for oxygen evolution reaction (OER). The surface of the corroded electrode consisted of densely packed, small Ni0.75Fe2.25O4 nanoparticles with sizes less than 5 nm. This electrode required an overpotential of only 192 mV to reach an OER current density of 10 mA/cm2 in 1 M KOH, outperforming the state-of-the-art IrO2 catalyst by 73 mV. Density functional theory calculations revealed that the unique surface structure and iron composition of Ni0.75Fe2.25O4 nanoparticles play a key role in achieving an improved OER activity. When coupled with a Pt/C hydrogen-evolving catalyst, the resulting anion-exchange membrane (AEM) water electrolyzer achieved an overall water-splitting current density as high as 2.0 A/cm2 at a cell voltage of 1.9 V in 1 M KOH, which was 1.7 times that obtained from the IrO2 and Pt/C catalyst pair and also much greater than reported values from other AEM water electrolyzers. By revisiting and exploiting a traditional corrosion process, our work opens a simple, cost-effective, and scalable route to a high-performance OER electrode for efficient AEM water electrolysis.
Keywords: corrosion, nanoparticle bimetallic oxide, oxygen evolution reaction, anion exchange membrane water electrolyzer
1. Introduction Electrochemical water splitting is a promising route to production of hydrogen (H2), which is an eco-friendly, alternative energy source to fossil fuels [1–4]. Polymer-exchange membrane water electrolysis systems are attractive for large-scale production of hydrogen gas at high rate and purity, which include proton-exchange membrane and anion-exchange membrane (AEM) water electrolyzers [5–8]. In particular, an AEM water electrolyzer has attracted much attention because it offers an alkaline environment at a membrane interface, and thus has the advantage that earthabundant transition metal oxides can be used as electrocatalysts for an oxygen evolution reaction (OER) occurring at the anode [9–13]. However, the kinetics of the OER is sluggish, and a substantial overpotential () required for this reaction at practical operating current densities reduces the overall efficiency of water electrolysis [14–19]. Currently, iridium oxide (IrO2) and ruthenium oxide (RuO2) are among the state-of-the-art OER catalysts, but even these precious metal oxides are not effective enough for efficient OER [20–28]. Non-precious metal oxides such as Co3O4, CuxCo3–xO4, and CoFeOx have been explored as OER catalysts for AEM water electrolysis [9,10,13]. Despite recent progress, however, practical application of AEM water electrolyzers in industry is still limited by their low efficiencies, and high-performance, low-cost, and easy-to-fabricate OER electrodes are strongly needed. Corrosion is a chemical process that involves the redox reactions occurring at the interface between a material and its environment, and has long been considered to be undesirable due to its adverse effect leading to damage or failure of a metal or alloy [29–31]. However, recent studies have shown that corrosion can be exploited as a versatile route to produce unconventional nanostructures [32–34]. In this work, we produced a highly active and stable OER electrode simply by corroding a conventional nickel foam (NF) in an aqueous solution containing exotic Fe3+ cations.
The corrosion process led to the evolution of a nanostructure consisting of densely packed, small Ni0.75Fe2.25O4 nanoparticles (NPs) on the NF surface. This corrosion-engineered electrode required a small overpotential of only 192 mV (iR corrected) at an OER current density of 10 mA/cm2 in 1 M KOH, which was 73 mV less than the IrO2 catalyst. The OER electrode was coupled with a cathode loaded with a Pt/C catalyst for hydrogen evolution reaction (HER) to construct an AEM water electrolyzer. Our electrolyzer achieved an overall water-splitting current density as high as 2.0 A/cm2 at a cell voltage of 1.9 V (without iR corrected) in 1 M KOH, which far exceeded the performance of an AEM water electrolyzer based on the IrO2 and Pt/C catalyst pair (1.18 A/cm2 at 1.9 V), as well as those of previously reported AEM water electrolyzers. Our approach does not require synthesis and/or loading of an exotic OER catalyst, and thus provides a convenient and cost-effective route to large-scale production of a high-performance OER electrode for efficient AEM water electrolysis.
2. Experimental section 2.1. Synthesis of Ni0.75Fe2.25O4 NPs arrays on NF To remove a surface oxide layer, a NF with a thickness of 0.8 mm (Welcos Co., Ltd, Korea) was reduced in a tubular furnace at 800 ºC for 2 h with constant H2 flow at a flow rate of 50 sccm. The NF was immersed in 66.7 mM aqueous solution of iron nitrate [Fe(NO3)3·9H2O, Acros] at 95 ºC for 36 h with magnetic stirring, and then cooled down to room temperature. After the synthesis, the Ni0.75Fe2.25O4 nanoparticles array on the NF was washed with deionized water several times and dried in a vacuum oven at 80 ºC for overnight for further characterization and measurements.
2.2. Synthesis of NiFe2O4 NPs
In a typical synthesis, iron chloride (FeCl3·6H2O, Aldrich, 3.2 mmol), nickel chloride (NiCl2, Aldrich, 1.6 mmol), and cationic surfactant (CTAB, Aldrich, 2.7 mmol) were dissolved in deionized water (35 mL), hosted in a 50 mL Teflon-lined stainless autoclave. The pH of the solution was adjusted to 11.0 by adding 2 M NaOH. The autoclave was kept at 130 ºC in a muffle furnace for 15 h and then quenched down to room temperature. The product was washed with ethanol several times and then dried in oven at 80 ºC for overnight. The products were placed in tube furnace and slowly heated to 300 ºC in air [35].
2.3. Characterization The scanning electron microscopy (SEM) images were obtained from a SUPRA 55VP (Carl Zeiss). Transmission electron microscopy (TEM) images were captured using a JEM-2100F microscope operated at 200 kV. Powder X-ray diffraction (XRD) patterns were obtained with a D8-Advances (Bruker AXS) diffractometer, equipped with a rotating anode and a CuKa radiation source (λ = 0.15418 nm). X-ray photoelectron spectroscopy (XPS) spectrums were obtained by using an ECSA2000 (VG Microtech.).
2.4. Electrochemical measurements Polarization curves measurements were performed using a CHI600D electrochemical analyzer (CH instrument) in 1 KOH solution. Pt wire and Hg/HgO (in 1 M NaOH solution) electrodes were used as counter and reference electrodes, respectively. Chronopotentiometric curve measurements were performed with WBCS-3000 (Xeno Co.) in 1 KOH solution. In the preparation of a NiFe2O4 and commercial IrO2 electrodes, a mixture of a catalyst, Ketchen black as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder were prepared with a weight ratio of 70:20:10. These
ingredients were added to N-methyl-2-pyrrolidone (NMP) to prepare the slurry, which was then coated onto a piece of a NF (1 × 1 cm2) and dried in a vacuum oven overnight at 80 ºC for overnight.
2.5. AEM water electrolyzer system fabrication The AEM water electrolyzer systems comprised the anode (Ni0.27Fe2.27O4 NPs on NF), cathode (1 mg/cm2 of commercial Pt/C on NF, 4.9 cm2), anion exchange membrane (AEM, X37-50 Grade T, Dioxide Materials), and gas diffusion layer (NF). IrO2 powder (Sigma Aldrich) was used as an anode catalyst for comparison and loaded on NF with Nafion. The loading amount of IrO2 was 4 mg/cm2. The area of the anode and cathode electrodes of AEM water electrolyzer single-cell are 7.1 cm2 and 4.9 cm2. The single cell was supplied with 1 M KOH as an electrolyte at 24 mL/min and was operated at 42∼45 °C. The AEM water electrolyzer test was performed using a potentiostat (BP2C, ZIVE LAB), and electrochemical performance was analyzed by polarization curves (from 1.4 to 1.9 Vcell) measurements. The long-term testing of AEM water electrolyzer systems was conducted at 500 mA/cm2. Energetic efficiency of the AEM water electrolyzer was calculated by the following equation.
Wh is the electric power to produce hydrogen, H0 is the calorific value of hydrogen (10.8 x106 J/m3, lower heating value) and VH2 is the hydrogen gas volume.
2.6. Computational details
All DFT calculations were carried out by Vienna ab initio Simulation Package (VASP) [36,37]. The projector augmented wave (PAW) method was used for describing electron-ion interactions [38] The exchange-correlation was approximated using the Perdew–Burke–Ernzerhof (PBE) formula with spin polarization [39]. We performed GGA+U method with Ueff (Fe) = 4.5 eV and Ueff (Ni) = 6 eV according to the literature [40]. During the structure optimization, the convergence criterion of total energy was set to 10−4 eV and the atoms were relaxed until the force acting on each atom was less than 0.03 eV/Å. The kinetic energy cutoff and k-points were tested and chosen with a convergence criterion of 10-2 eV. The bottom two layers of four layers were fixed in IrO2 slab models, while the bottom layer of three layers was fixed in NixFe3-xO4 slab models. Each slab model was separated from its neighbors by a sufficiently thick vacuum layer. Ni0.75Fe2.25O4 bulk structure was prepared by substituting Ni atoms at octahedral site with Fe atoms (Fig. S1).
3. Results and discussion A bare NF has a porous, three dimensional structure, and was reduced under a H2 flow to remove a surface oxide layer (Fig. S2). The reduced NF was corroded in an aqueous solution containing Fe(NO3)3 at 95 °C with magnetic stirring. In this case, a corrosion process may involve redox reactions including Ni(s) → Ni2+ + 2e– and Fe3+ + e– Fe2+, which can be driven by the difference in the standard reduction potentials of Ni2+/Ni (0.25 V) and Fe3+/Fe2+ (0.77 V) pairs. In the presence of the added and newly generated cations including Ni2+, Fe2+, and Fe3+, an olation process may lead to the formation of binary NiFe oxide NPs (Fig. 1a) [31,34] Fig. 1b shows photographs of the bare NF and corroded electrode as large as 20 cm × 10 cm. It can be seen that the color of a NF was changed from gray to dark brown after the corrosion. The scanning electron microscopy (SEM) analysis of the corroded electrode revealed the evolution of nanostructures
consisting of densely packed NPs on the electrode surface in order to minimize the total surface energy of small-sized ferrite NPs produced by corrosion (Fig. 1c and d) [4143]. The Brunauer−Emmett−Teller (BET) specific surface area of the corroded electrode was estimated to be 1.91 m2/g (Fig. S3a). The Barrett-Joyner-Halenda (BJH) average pore size of corroded electrode was 8 nm (Fig. S3b). The X-ray diffraction (XRD) pattern of the corroded electrode showed the diffraction peaks at 2θ values of 30.1, 35.6, 43.3, 57.2, and 62.8° (Fig. 1e), which can be indexed to (220), (311), (222), (422), and (440) reflections of an inverse spinel structure, respectively [44,45]. In the X-ray photoelectron spectroscopy (XPS) analysis, the Ni XPS 2p core level spectrum included an intense peak associated with Ni2+ in addition to a weak shoulder peak originating from Ni3+ (Fig. 1f) [46,47]. The Ni3+ sites seem to be generated from the partial oxidation of Ni2+ sites. The Fe XPS 2p core level spectrum showed peaks corresponding to both Fe2+ and Fe3+ (Fig. 1g) [46,47]. When Ni is oxidized, released electrons can be captured by Fe3+ to generate Fe2+. In the O 1s spectrum, peaks centered at 531.2 and 529.9 eV are associated with OH− and O2−, respectively (Fig. 1h). These NPs were detached from the corroded electrode by sonication, and then characterized by transmission electron microscopy (TEM). The TEM image in Fig. 2a clearly shows the formation of NPs with sizes less than 5 nm. The nanoparticles exhibited both multiple twin boundaries and single-crystal structures, as can be seen in high-angle annular dark field scanning TEM (HAADF-STEM) images (Fig. 2b,c and Fig. S4). The lattice spacing of the nanoparticles is found to be 0.208 nm, which is associated with the (400) planes of inverse spinel structure. The Fast Fourier Transformation (FFT) patterns of nanoparticles was in agreement with the result from the XRD pattern analysis (Fig.2b and Fig. S4, insets). The elemental mapping of the NPs showed the uniform distribution of Ni, Fe, and O elements in their structures (Fig. 2d). The energy-
dispersive X-ray spectroscopy (EDS) analysis indicated that the atomic ratio of Ni to Fe in the NPs was approximately 25:75 (Fig. S5). These results indicated the formation of a binary Ni0.75Fe2.25O4 NPs through the corrosion process. We investigated the electrocatalytic properties of the corrosion-derived Ni0.75Fe2.25O4 electrode for the OER. For comparison, we also prepared a NF electrode loaded with NiFe2O4 NPs that had been synthesized by using a previously reported (Fig. S6) [35], hydrothermal method as well as that with a commercial IrO2 catalyst (Fig. S7). Fig. 3a shows OER polarization curves for four different electrodes including a bare NF, which were recorded at room temperature in 1 M KOH solutions at a scan rate of 1 mV/s from positive to negative potential on the reversible hydrogen electrode (RHE) scale. The Ni0.75Fe2.25O4 electrode exhibited a much higher OER activity than other three electrodes, and required an overpotential of only 192 mV to reach a current density of 10 mA/cm2 with iR compensation, which was 185, 82, and 73 mV less than the bare NF, NiFe2O4 NPs, and IrO2 catalyst, respectively. The measured overpotential of the Ni0.75Fe2.25O4 electrode was also lower than those for most of OER anodes that had been explored for AEM water electrolyzers under the same or similar conditions. A Tafel plot reflects the kinetics of an electrochemical reaction on a given catalyst [47–50] A Tafel slope recorded from the Ni0.75Fe2.25O4 electrode was 44 mV/dec, which was much lower than that IrO2 catalyst (67 mV/dec), indicating a higher rate of the OER on the Ni0.75Fe2.25O4 electrode (Fig. 3b). The electrochemical surface areas (ECSA) of the catalyst reflects the number of exposed active sites. The ECSA is proportional to the electrochemical double-layer capacitance (Cdl) from cyclic voltammetry (CV) curves. The Ni0.75Fe2.25O4 electrode had a larger Cdl (28 mF/cm2), which was 16 and 11 mF/cm2 less than the NiFe2O4 electrode and IrO2 electrode, respectively. (Fig. S8). The large Cdl of the Ni0.75Fe2.25O4 electrode indicates the more exposed active site, which is important for the activity of the OER
catalyst. We also evaluated the catalytic stability of the Ni0.75Fe2.25O4 electrode at a constant current density of 50 mA/cm2 for 100 h (Fig. 3c). We did not observe a noticeable increase in the potential required for keeping this current density during the stability test, demonstrating the excellent catalytic stability of the Ni0.75Fe2.25O4 electrode toward the OER. In addition, we investigated the change after OER test of Ni0.75Fe2.25O4 electrode. The SEM, XRD and XPS analysis confirmed that the Ni0.75Fe2.25O4 electrode retained its surface morphology and structure after the stability test (Fig. 3d and Fig. S9). The OER activity is largely affected by the binding energies of OER intermediates, i.e., *O, *OH, and *OOH species adsorbed on the active sites. To explain the higher activity of Ni0.75Fe2.25O4 by obtaining detailed insight into the OER mechanism, we performed DFT calculations for the elementary steps of 4e mechanism (Eqn. (1)-(4)). The asterisk denotes the active site on the catalyst, and the energy of a hydrogen ion and an electron was replaced by that of 1/2 hydrogen gas [51]. The free energy of each step was calculated by Eqn. (5) with assuming the standard state (T = 25 ℃, P = 1 atm, pH = 0). The zero-point energy (ZPE) and vibrational entropy were obtained from the vibrational frequency calculation, while thermodynamic entropy tables were used for water and hydrogen. The step with the highest free energy determines OER operating potential, called a potential determining step (PDS), where over-potential was obtained (Eqn. (6)). Step 1: Step 2:
H2O(l) + ∗ OH ∗
∆G2
Step 3:
O ∗ + H2O(l)
Step 4:
OOH ∗
∆G4
∆G1
OH ∗ + (H + + e ― )
O ∗ + (H + + e ― )
∆G3
(1) (2)
OOH ∗ + (H + + e ― )
(3)
O2(g) + ∗ + (H + + e ― )
(4)
Over-potential:
∆𝐺 = ∆𝐸 + ∆𝑍𝑃𝐸 ― 𝑇∆𝑆 ― 𝑒𝑈 + 𝑘𝑇 ∙ ln [𝐻 + ]
(5)
𝜂 = max (∆G1, ∆G2,∆G3,∆G4)/𝑒 ― 1.23
(6)
We investigated the OER mechanism on Ni0.75Fe2.25O4(311), NiFe2O4(311), and IrO2(110), and their optimized oxygen containing intermediate (OCI) structures and Gibbs free energy profiles were shown in Fig. 4 Several theoretical studies for OER performance on IrO2(110) have already been done [52–55] but we did it again in this work to compare the performance with nickel ferrite under the same calculation conditions. The OER on IrO2 was limited by the oxidation of *O to *OOH, and the over-potential was 607 mV, which is good agreement with previous computational studies (η = 610 mV). It supports that our calculation conditions are reliable and can explain the experimental results in this work. In case of NiFe2O4, the surface Fe with η = 707 mV was the OER active site, and the PDS corresponded to the transformation of *OH to *O. The Ni0.75Fe2.25O4 had two types of Fe sites on the surface, which were distinguished by whether Fe or Ni was located in the lower layer (the former with lower over-potential). Its OER was hindered by the first step and the over-potential was 222 mV. It is consistent with the experimental result that OER activity was Ni0.75Fe2.25O4 > IrO2 > NiFe2O4. In case of spinel-type transition metal oxide, eg occupancy is known as a reliable descriptor for the OER activity. The eg occupancy near 1 balances each step of the 4-e mechanism and results in lower potential barrier [56]. To further elucidate the enhancement of OER performance caused by the Fe substitution into Ni, we calculated eg descriptors from pDOS analysis for Ni0.75Fe2.25O4 and NiFe2O4 and plotted OER overpotential against them (Fig. S10). The OER overpotentials decrease linearly with the eg occupancy increase, because the data is located at the right branch of the volcano plot (eg > 1). Ni has a more filled eg orbital than Fe in both catalysts, and this explains that Fe serves as the main OER active site. It was also confirmed that Fe substituting partial Ni reduces the eg occupancy of Fe, bringing it closer
to the optimal value, and thus results in the improved OER activity. In addition, the unique structure of the corroded electrode consisting of Ni0.75Fe2.25O4 NPs with high density and enrich grain boundary on the NF surface allows intimate electrical contacts between them and exposed more edge sites. These generally achieve low charge transfer resistance and provide many active catalytic sites, thus enabling high OER activity (Fig. S11). Industrial-scale hydrogen production via electrochemical water splitting typically requires high current densities (0.5 A/cm2) [7]. We constructed a single-cell AEM water electrolyzer by coupling the Ni0.75Fe2.25O4 anode with a carbon cloth cathode loaded with a commercial Pt/C catalyst, as well as the same electrolyzer except for the use of a NF loaded with the IrO2 catalyst as an anode (Fig. 5a–c). Polarization curves for the overall water splitting by these two electrolyzers in 1 M KOH solution are shown in Fig. S8. At current densities of 0.5 A/cm2, our Ni0.75Fe2.25O4//(Pt/C)-based AEM water electrolyzer showed better performance than the IrO2//(Pt/C)-based AEM water electrolyzer. The IrO2//(Pt/C)-based electrolyzer required a large overpotential of 612 mV to obtain the overall water-splitting current density of 1.0 A/cm2 (Fig. 5d). In contrast, our electrolyzer achieved the same current density at a cell voltage of 1.75 V ( = 515 mV), which corresponds to electrical-to-fuel efficiency of 83%. At a cell voltage of 1.9 V, our AEM water electrolyzer showed the current density of 2.0 A/cm2 (Fig. 5e), which was 1.7 times that obtained from the IrO2//(Pt/C)-based electrolyzer (1.18 A/cm2). The high frequency resistance (HFR) of Ni0.75Fe0.25O4//(Pt/C)-based AEM water electrolyzer (0.0176 Ω) was lower than IrO2//(Pt/C)-based AEM water electrolyzer (0.0245 Ω) due to the low interface contact resistance between the catalyst layer and conductivity substrate. Despite iR-correction of the polarization curves, the performance of Ni0.75Fe2.25O4//(Pt/C)-based AEM water electrolyzer was showed to be higher than IrO2//(Pt/C)-based AEM water electrolyzer (Fig. S12). Therefore,
performance of our electrolyzer is better than that of IrO2//(Pt/C)-based AEM water electrolyzer in the AEM water electrolyzer performance rather than ohmic resistance. Our electrolyzer also delivered much higher current densities than other single-cell AEM water electrolyzers reported to date under the same or similar operating conditions (Fig. 5f and Table S1). During a 21-h test of our electrolyzer at a constant current density of 0.5 A/cm2, an energetic efficiency was maintained over 75% (Fig. 5g), and the rate of hydrogen production was as high as 14.2 ml/min. Hence, assembly of our electrolyzer may enable to produce hydrogen on a large scale at a much improved efficiency.
4. Conclusion We have demonstrated the fabrication of an OER electrode by harnessing the aqueous-phase corrosion of a conventional NF. This cheap process allowed to directly produce a large-area, highperformance anode for AEM water electrolyzers, without need for additional catalysts. The corroded electrode consisted of a NF with its surface decorated by densely packed, binary Ni0.75Fe2.25O4 NPs, and exhibited a much higher OER activity than the state-of-the-art IrO2 catalyst, which could be attributed to improved OER energetics on the surface of binary Ni0.75Fe2.25O4 NPs as well as high electrical conductivity achieved from intimate electrical contacts between the densely packed Ni0.75Fe2.25O4 NPs and NF surface. This corroded electrode was successfully applied as an anode for a single-cell AEM water electrolyzer, which outperformed that based on the state-of-the-art IrO2 catalyst as well as other previously reported AEM electrolyzers. Our approach is simple, cost-effective, and scalable, and thus offers a promising way for fabricating large-area anodes for efficient OER and water electrolysis. It may also be possible to extend the
corrosion-based approach to produce electrodes with desired surface structures and compositions for catalytic and other applications.
Acknowledgements This work was supported by Basic Science Research Programs (No. 2015R1A2A2A01006325, 2019R1A2C2006997, and 2018R1A2B2002875) through the National Research Foundation (NRF) of Korea funded by the Ministry of Science, ICT & Future Planning.
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Fig. 1. (a) Schematic illustration of corrosion process of a NF. (b) Photographs of bare NF, and corroded NF with a dimension of 20 cm × 10 cm. (c and d) SEM images and (e) XRD pattern taken from the corroded electrode. (f) Ni XPS 2p core level spectrum, (g) Fe XPS 2p core level spectrum and (h) O XPS 1s core level spectrum of a binary Ni0.75Fe2.25O4 NPs.
Fig. 2. (a) TEM, (b, c) HAADF-STEM, and (d) Scanning TEM images of a binary Ni0.75Fe2.25O4 NPs. Insets show the corresponding FFT patterns.
Fig. 3. (a) OER polarization curves for Ni0.75Fe2.25O4 electrode, NiFe2O4 catalyst, IrO2 catalyst, and bare NF electrode recorded in 1 M KOH solution at a scan rate of 1 mV/s from positive to negative potential at room temperature. (b) Tafel plots of the Ni0.75Fe2.25O4 electrode, NiFe2O4 catalyst, and IrO2 catalyst for OER. (c) Chronopotentiometry curve of the Ni0.75Fe2.25O4 electrode at a constant current density of 50 mA/cm2 for 100 h. (d) SEM image taken from the Ni0.75Fe2.25O4 electrode after the stability test.
IrO2
a
c
Ni0.75Fe2.25O4
NiFe2O4
b
Free Energy (eV)
d5
*
G1
*OH
*O
G3
*OOH
G4
O2 (g)
IrO2
4
NiFe2O4
3
Ni0.75Fe2.25O4 PDS: 1.94 eV
2 1
G2
PDS: 1.84 eV
PDS: 1.45 eV
0 Reaction coordinate
Fig. 4. Most stable OCI configurations on the (a) IrO2, (b) NiFe2O4 and (c) Ni0.75Fe2.25O4 surfaces. Red, deep-green, gray, brown, yellow, light-green, and white balls represent oxygen, iridium, nickel, iron, nickel substituted iron, oxygen of OCI, and H, respectively. The top layer is represented by ball-type, while the other layers are represented by line-type. (d) The free energies of the OCIs on the IrO2(110), NiFe2O4(311) and Ni0.75Fe2.25O4(311) surfaces at U = 0 V. The internal subgraph shows the overpotential of each catalyst.
Fig. 5. (a) Schematic illustration of the designed AEM water electrolyzer. Photographs of (b) AEM water electrolyzer and (c) evolved hydrogen. (d) Overpotential at various current density and (e) current density at various cell voltages for Ni0.75Fe2.25O4//(Pt/C) and IrO2//(Pt/C) cells. (f) Comparison of AEM water electrolyzer for the Ni0.75Fe2.25O4//(Pt/C) cell, the reported AEM water electrolyzer cells, and the reported IrO2 based AEM water electrolyzer. (g) Durability cell voltagetime and energetic efficiency plots for the AEM water electrolyzer at constant current density of 500 mA/cm2 for 21 h.
Supplementary Materials for
Corrosion-engineered bimetallic oxide electrode as anode for high-efficiency anion exchange membrane water electrolyzer
Jooyoung Leea,b,1, Hyeonjung Jungc,1, Yoo Sei Parkb,d, Seongwon Wooa, Nayoung Kwona, Yaolong Xinge, Sang Ho Ohe,*, Sung Mook Choib,*, Jeong Woo Hanc,*, Byungkwon Lima,*
a School
of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea. E-mail: [email protected]
b
Materials Center for Energy Convergence, Surface Technology Division, Korea Institute of Materials Science (KIMS), Changwon 51508, Republic of Korea. c
Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Gyeongbuk 37673, Republic of Korea. E-mail:
d
Department of Materials Science and Engineering, Pusan National University, Busan, 46241, Republic of Korea
e
Department of Energy Science, Sungkyunkwan University (SKKU), Suwon, 16419, Republic of Korea.
E-mail: [email protected] (S. H. Oh), [email protected] (S. M. Choi), [email protected] (J. W. Han), [email protected] (B. Lim) *Corresponding authors 1
These authors have contributed equally.
Fig. S1. The most stable configuration of NiFe2O4 and Ni0.75Fe2.25O4 bulk structure. Red, gray, brown, and yellow balls represent oxygen, nickel, iron, and iron substituted nickel, respectively. NiFe2O4 bulk unit cell contains 8 Ni atoms on octahedral site, 8 Fe atoms on tetrahedral site, and 8 Fe atoms on octahedral site. Ni0.75Fe2.25O4 bulk unit cell contains 6 Ni atoms on octahedral site, 8 Fe atoms on tetrahedral site, and 10 Fe atoms on octahedral site. Lattice constant slightly increased (~0.2 Å) due to the difference in atomic radius between Fe and Ni.
Fig. S2. (a, b) SEM images of pristine NF.
Fig. S3. (a) Nitrogen adsorption /desorption isotherms, and (b) pore size distribution of corroded electrode.
Fig. S4. HAADF-STEM image of a binary Ni0.75Fe2.25O4 NP. The image was taken from the sample shown in Fig. 2.
Fig. S5. EDS spectrum of a binary Ni0.75Fe2.25O4 NPs.
Fig. S6. (a) SEM image, (b) XRD pattern, and (c) EDS spectrum of NiFe2O4 NPs.
Fig. S7. SEM image of a commercial IrO2 catalyst.
Fig. S8. Cyclic voltammetry curves of (a) Ni0.75Fe2.25O4 electrode, (b) NiFe2O4 electrode and (c) IrO2 electrode with different scan rate (2−10 mV/s). (d) Electrochemcial double-layer capacitance measurement of the Ni0.75Fe2.25O4 electrode, NiFe2O4 electrode and IrO2 electrode.
Fig. S9. (a) XRD pattern taken from the corroded electrode. (b) Ni XPS 2p core level spectrum, (c) Fe XPS 2p core level spectrum and (d) O XPS 1s core level spectrum of a binary Ni0.75Fe2.25O4 electrode after the stability test.
Fig. S10. The relation between the OER catalytic activity, defined by the calculated overpotentials, and the eg orbital occupancy of the octahedral transition metal. The presence of the Fe component caused the higher calculated value of eg occupancy than the ideal value, 2 [1].
Fig. S11. (a) EIS plots at an applied potential of 1.48 vs. RHE of the Ni0.75Fe2.25O4 electrode and NiFe2O4 catalyst with (b) equivalent circuit diagram.
Fig. S12. Polarization curves ((a) without and (b) with iR corrected) of AEM water electrolyzer for Ni0.75Fe2.25O4//(Pt/C) and IrO2//(Pt/C) recorded in 1 M KOH solution at 50 ℃.
Table S1. A comparison of Ni0.75Fe2.25O4//(Pt/C) AEM water electrolyzer performance with previously reported AEM water electrolyzer.
Cell voltage (V)
j (A/cm2)
1.8
1.315
1.9
2
1.8
0.87
1.8
1.07
2
1.8
0.299
3
1.8
0.399
4
1.9
1.5
5
1.8
1.045
6
1.8
0.4
7
2
0.4
8
Cu0.7Co2.3O4//(Pt/C)
1.8
1
9
Co3O4//(Pt/C)
1.8
0.6
9
pyrochlore//(Pt/C)
1.8
0.5
10
Cu0.81Co2.19O4//(Pt/C)
1.8
0.36
11
Ni//PtNi
1.9
0.25
12
Catalysts Ni0.75Fe2.25O4//(Pt/C)
IrO2//(Pt/C)
ref.
Our work
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A small densely packed Ni0.75Fe2.25O4 nanoparticles were grown on the Ni foam.
The binary Ni0.75Fe2.25O4 nanoparticles exhibited excellent OER activity.
The improved activation sites due to their nanostructures enhanced OER activity.
The electrode applied to high-efficiency AEM water electrolyzer as an anode.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: