Defect-Rich NiCeOx Electrocatalyst with Ultrahigh Stability and Low

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Defect-Rich NiCeO Electrocatalyst with Ultrahigh Stability and Low Overpotential for Water Oxidation Jun Yu, Qi Cao, Yanbo Li, Xia Long, Shihe Yang, J. Kenji Clark, Mamiko Nakabayashi, Naoya Shibata, and Jean-Jacques Delaunay ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00191 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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ACS Catalysis

Defect-Rich NiCeOx Electrocatalyst with Ultrahigh Stability and Low Overpotential for Water Oxidation

Jun Yu,† ‡ Qi Cao,† Yanbo Li,§ Xia Long,‡ Shihe Yang,‡ J. Kenji Clark,† Mamiko Nakabayashi,¶ Naoya Shibata,¶ Jean-Jacques Delaunay*, †

†

School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656,

Japan ‡

Guangdong Key Lab of Nano-Micro Material Research, School of Chemical Biology and

Biotechnology, Peking University Shenzhen Graduate School, Shenzhen, China §

Institute of Fundamental and Frontier Science, University of Electronic Science and Technology

of China, Chengdu 610054, China ¶

Institute of Engineering Innovation, The University of Tokyo, 2-11-16, Yayoi, Bunkyo-ku,

Tokyo 113-8656, Japan

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ABSTRACT For the utilization of renewable energy resources to become widespread, efficient energy storage devices must be developed. Electrocatalysts for the oxygen evolution reaction (OER) are needed for a wide variety of such devices, including fuel-cells, metal-air batteries and photoelectrochemical cells. Here we demonstrate a defect-rich NiCeOx layer, directly synthesized on a Ni substrate through a simple two-step dip-coating/annealing process, as a highly active and stable OER catalyst made from earth-abundant materials. With a low overpotential of 295 mV at 10 mA/cm2 and a stability of over 200 hours, NiCeOx boasts one of the best performances reported in the literatures. This good performance is a result of the large number of oxygen vacancy defects introduced into NiCeOx by the diffusion of Ni from the Ni substrate into a deposited CeOx film during the annealing step. The oxygen vacancy defects not only supply an abundance of active sites but also decrease the mass-transfer resistance, resulting in the large electrochemically active surface area and high OER performance. This work demonstrates the potential of developing defect-rich Ce-containing materials as robust OER catalysts.

KEYWORDS: oxygen evolution reaction; NiCeOx layer; Ni diffusion; defects; long-term stability


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ACS Catalysis

For society to adapt to the growing demand for clean energy, renewable energy sources must be utilized to a greater degree. Unfortunately, the vast majority of renewable energy sources, such as solar energy and wind energy, can only produce energy intermittently.1,2 For these renewable energy sources to replace conventional fossil fuel energy sources, energy storage technologies must be improved.3,4 A wide variety of energy conversion devices, including hydrogen fuel-cells, metal-air batteries and photoelectrochemical cells, have been explored to convert electrical or solar energy to chemical energy in order to store the energy produced by renewable energy sources. In all of these devices, the oxygen evolution reaction (OER) is a critical reaction.5–10 Because the OER is a four-electron reaction, it has a large activation barrier, and a large overpotential is required to drive the electrochemical reaction.11,12 Finding a suitable OER catalyst with a high efficiency, low overpotential, low-cost and long-term stability is key to enabling widespread adoption of renewable energy sources. The best OER catalysts reported so far have generally been iridium or ruthenium-based oxides. The scarcity of both iridium and ruthenium, however, significantly limits the widespread use of these catalysts.13,14 First-row transition metals such as Ni- and Co-based oxides, have recently been studied as low-cost alternatives to iridium and ruthenium-based oxide catalysts.14–18 Ni- and Cobased mixed oxides such as NiFeOx, NiCoOx and NiCeOx have been studied as the benchmarking OER electrocatalysts.15 For the current density of 10 mA/cm2, NiFeOx was shown to have the lowest overpotential of 350 mV, and NiCeOx with the highest overpotential of 430 mV.15 Both of these are higher than the 320 mV overpotential of IrOx.15 In order to improve the activity of NiCeOx, the Au mid-layer was added. The overpotential of this NiCeOx-Au material was decreased to 280 mV. During a 2-hour stability test, however, the overpotential of NiCeOx-Au increased with the testing time and ended up with 310 mV.19 It is therefore needed to design and synthesize a noble metal free Ni- or Co- based catalyst with a low overpotential and long-time stability. Defects in catalysts can enhance the OER performance of the catalysts by altering the local electron density distribution in the vicinity of the defects and providing catalytically active sites.20– 22

Ceria is an example of a non-stoichiometric catalyst material that exhibits a higher catalytic

activity when defects are introduced into it. The enhancement of the catalytic activity of ceria by introducing defects has been demonstrated for the CO oxidation reaction and the selective catalytic reduction of NOx.23–26 These reactions, however, usually occur at high temperature, and whether


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or not defects in ceria and ceria based catalysts can enhance catalytic activity in low temperature reactions, such as water electrolysis, has yet to be discerned. Nickel foam (NF) was reported to be a good substrate for OER because of its high electronic conductivity and large surface area.17,27,28 The NF substrate can be directly used as the source of nickel for the synthesis of nickel-based catalysts. By doing this, the advantages of NF can be retained, and the NF forms a strong connection with the surface catalyst which is good for the long-term stability of the catalyst. In this work, we synthesize a NiCeOx layer rich with oxygen vacancy defects and show that it behaves as an excellent catalyst for the OER. The NiCeOx layer is directly synthesized on commercially available nickel foam (NF) using a simple, environmentally friendly and reproducible synthesis method that consists of first dip-coating the NF in a cerium pre-cursor and then annealing the sample at 400 °C in air. During the annealing step, Ni atoms diffuse from the NF substrate into the deposited layer, forming a NiCeOx layer that has a large number of oxygen vacancy defects. The oxygen vacancy defects are shown to enhance the catalytic activity of the NiCeOx catalyst by offering abundant active OER sites and decreasing the mass-transfer resistance. Furthermore, the NiCeOx catalyst is found to extremely stable, making it viable as an electrocatalyst in industrial scale water splitting.


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ACS Catalysis

Figure 1. Morphology and chemical composition characterizations of the as-prepared NiCeOx catalysts. (a), (b) SEM images of the NiCeOx sample with different magnifications; (c) the crosssectional STEM image of the top NiCeOx layer (the upper part) and the NiOx/NF substrate (the lower part) of the NiCeOx sample. The dark gap between the NiCeOx layer and NiOx/NF substrate was created during the cutting of the cross-section with a FIB and is not present in the original material. Elemental mapping of the STEM image shown in (c) are given for Ce (d), Ni (e) and O (f); (g) elemental line profiles for Ce, Ni and O across a scan line indicated by the dashed white line shown in (c). The inset table shows the elemental composition at the five locations numbered in (c). The resolution of the line EDS is finer than 1 nm. The NiCeOx sample was synthesized by depositing a cerium salt solution on NF (Figure S1a) and then annealing the sample in air to form a NiCeOx layer on top of the NF, as can be seen in


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Figure 1a. From the cross-sectional SEM image of the sample (Figure S2), the thickness of the NiCeOx layer was found to vary from several hundred nanometers thick to several micrometers thick. Cracks, that are believed to have formed as a result of a combination of thermal stress in the layer and the release of gas produced by the decomposition of nitrate ions during the annealing process, were also observed. The cracking of the layer resulted in gaps between portions of the layer where the underlying substrate was exposed. Upon SEM characterization of these areas of the substrate (Figure 1b), the substrate surface morphology was found to be extremely similar to that of the NiO sample (Figure S1b). A cross-sectional STEM image of the NiCeOx sample, prepared by FIB-TEM, is shown in Figure 1c. The upper and lower bright regions in Figure 1c represent the NiCeOx layer and NF/NiO substrate respectively, with the dark region between these two being a gap that was created during the FIB cutting process. STEM EDS was then used to determine elemental composition of the cross-sectional sample. Figure 1d, 1e and 1f illustrate the Ce, Ni and O distribution in the crosssectional sample. In the top layer, all the three elements (Ce, Ni and O) were detected. Understandably, within the gap only trace amounts of O – from carbonaceous compounds created during the FIB cutting process – are detected. Below the gap, a thin layer (about 100 nm thick) composed of only Ni and O can be observed. The rest of the sample was found to contain only Ni. The above results show that the sample is composed of a NiCeOx top layer, a thin NiOx middle layer, and a Ni base layer (the NF substrate). In Figure 1g, the concentration of each element as a function of the distance along the dashed line of Figure 1c, starting from the top of the NiCeOx layer, is illustrated. Figure 1g shows that although the Ni, Ce and O concentrations remain relatively constant throughout most of the NiCeOx film, at the interface between the NiCeOx layer and the NiOx layer (just before the gap in the graph), the Ni concentration increases while the Ce concentration decreases. This is further supported by the concentration values reported for areas 1, 2 and 3 in the inset table of Figure 1g. The elemental concentrations of Ni, Ce and O in areas 1 (NiCeOx layer surface), 2 (NiCeOx layer), 3 (the interface of NiCeOx and NiOx layer), 4 (NiOx layer) and 5 (Ni substrate) of Figure 1c are shown in the inset of Figure 1g. The distribution of Ni atoms in the NiCeOx layer, and the increase in the Ni concentration close to the interface suggests that Ni atoms from the substrate diffused into the top layer and mixed with the deposited Ce during the annealing process, resulting in the formation of the NiCeOx layer.


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ACS Catalysis

Figure 2. (a) X-ray diffraction patterns and (b) Raman spectra of the Ni, NiO and NiCeOx samples, and (c) XPS spectra of the NiO/CeO2 and NiCeOx samples. The peak labeled a in the XPS spectra is ascribed to the Ni 2p peak. XRD measurements were performed on the Ni, NiO, NiO/CeO2 and NiCeOx samples to examine the crystal structure of the four samples of Ni, NiO, NiO/CeO2 and NiCeOx, and are shown in Figure 2a. For the Ni sample, only peaks associated with Ni (JCPDS 65-2865) were found. The annealed NiO sample had an additional four peaks, the (111), (200), (220) and (311) peaks, which were attributed to NiO (JCPDS 65-2901). The observed peaks for NiO/CeO2 were the same with that for NiO, and no peaks for ceria were found. For the NiCeOx sample, a single weak peak corresponding to the (200) plane of NiO was found; it is believed that this peak is from the polycrystalline NiOx intermediate layer. No peaks for ceria were found, suggesting that Ni and Ce mixed uniformly in the top layer and formed an amorphous NiCeOx. The HRTEM images and the selected area electron diffraction (SAED) patterns of the NiCeOx sample (Figure S3) further confirmed this result. Figure 2b shows the Raman spectra of the Ni, NiO, NiO/CeO2 and NiCeOx samples. The peaks at 546 cm-1, 1093 cm-1 and 1511 cm-1 in the NiO sample spectrum were attributed to the one-phonon TO/LO mode of NiO, the two-phonon 2LO mode of NiO and the two-magnon mode of NiO, respectively.29 For the NiCeOx sample, three peaks of 224 cm-1, 582 cm-1 and around 1160 cm-1 are observed. Crystalline CeO2 is known to have a strong F2g Raman peak at 464 cm-1 related to its crystalline structure.30 In addition, the Ce-O bond has a 2LO (second-order longitudinal) mode at 1160 cm-1.30,31 The presence of ions with the oxidation states lower than Ce4+ in the CeO2 has been shown to induce a Raman band, known as the D band, from 500 to 700 cm-1.30,32,33 This


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band is associated with the presence of oxygen vacancy defects created in the non-stoichimometric CeO2-y by the 3+ coordinated ions. In addition to the introduction of the D band, the F2g band will be weakened and becomes asymmetric and broad.34 In the Raman spectrum of the NiCeOx sample, there is no F2g band, which suggests that there is no crystalline CeO2 with a fluorite structure in the layer.31 The strong intensity of the peak at 582 cm-1 (D band) indicates the formation of a large number of oxygen vacancy defects within the NiCeOx layer. The oxygen vacancy defects should be related to the formation of Ce3+ because of the incorporation of Ni into CeO2, as suggested by the previous published literatures30–32. Furthermore, the amorphous structure of NiCeOx contributes to the broadness of the peak.31,34 The peak at 224 cm-1 is related to Ce-OH vibrations which result from surface defects. Different types of hydroxyl groups originate from the dissociation of surface adsorbed water and doubly bridging hydroxyl groups on reduced ceria are detected in the Raman spectra.35,36 The Raman peak at 1160 cm-1 is observed; however, it has a very low intensity compared to the D band. This indicates that the Ce-O bond has been weakened, and suggests that electron transfer from Ni to the Ni-Ce-O bond has occurred.30,31 The NiO/CeO2 sample on the other hand, only peaks associated with NiO were observed because NiO substrate disturbed the diffusion of Ni. This clearly demonstrates that the oxygen vacancy defects in NiCeOx layer are induced by the incorporation of Ni into CeO2. Two parameters, namely the dip-coating cycles Y and the annealing temperature T, were investigated to optimize the synthesis process of the NiCeOx catalyst layers. The Raman spectra of these catalyst layers are shown in Figure S4. The dip-coating cycles of 15 and the annealing temperature T of 400 °C were found to be the best condition for fabricating NiCeOx catalyst layer with a large concentration of oxygen vacancy defects. XPS was carried out on the NiO, NiO/CeO2 and NiCeOx samples to further analyze the electron interactions between nickel and cerium oxide, and the results are shown in Figure S5 and Figure 2c. The observed XPS peaks in the NiO sample could be attributed to the Ni 2p3/2 peak (854.1 eV) and the Ni 2p1/2 peak (872.5 eV) of NiO.37 Additional intense satellite peaks (861.3 eV and 879.9 eV) characteristic of NiO were also observed, as shown in Figure S5. This confirms that the NiO sample is indeed composed of NiO. In the NiO/CeO2 sample, a weak Ni 2p signal, associated with NiO, was observed. It is believed that this is due to cracks in the CeO2 film exposing the underlying NiO. For both the NiO/CeO2 and NiCeOx samples, Ce 3d peaks were observed. The Ce 3d band is composed of ten individual peaks, that are labeled on Figure 2c as v,


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ACS Catalysis

v'', v''', u, u'', u''', v0, v', u0 and u'. The v, v'', v''', u, u'' and u''' peaks represent the 3d104f0 state of Ce4+, and the v0, v', u0 and u' peaks represent the 3d104f1 state of Ce3+.19,26,37 The band energies of v0, v', u0 and u' for the NiO/CeO2 sample are 880.6 eV, 883.9 eV, 898.8 eV and 905.5 eV, and the band energies are 878.8 eV, 882.8 eV, 898.3 eV and 906.3 eV for the NiCeOx sample. Based on the relative areas of the Ce3+ and Ce4+ peaks, it was possible to determine that the Ce3+ concentration is 24% for NiO/CeO2 sample and 37% for the NiCeOx sample. The increase in the concentration of Ce3+ is a result of the reduction of Ce4+ by the removal of lattice O2- anions via oxygen vacancy generation.26 This, along with the change in the band energies for the NiCeOx sample, indicates that there are strong electronic interactions between the nickel and cerium oxide in the NiCeOx sample.28 This suggests that electron interactions between the nickel and cerium oxide in NiCeOx results in Ce4+ ions being reduced to Ce3+ and the formation of oxygen vacancies.


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Figure 3. Electrochemical performance of the catalytsts. (a) Polarization curves of Ni, NiO, NiO/CeO2 and NiCeOx samples for the OER with a scan rate of 10 mV s-1. (b) Current density at OCP vs CV scan rate for the Ni, NiO, NiO/CeO2 and NiCeOx samples. The slope of current density at OCP vs scan rate represents the double-layer capacitance. (c) Nyquist plots of Ni, NiO, NiO/CeO2 and NiCeOx samples obtained at 1.50 V vs RHE. The inset shows the electrical equivalent circuit. (d) Potential trace of the NiCeOx sample obtained by fixing the current density for electrolysis at 10 mA/cm2. The electrolyte was 1 M KOH (pH ≈ 14).

Table 1. Electrochemistry Parameters for Each Catalyst Investigated in 1 M KOH Catalyst

CDL/cm2

ECSA/cm2

RCT/Ω

Tafel/mV dec-1

Ni

1.19 mF

29.75

174

54

NiO

1.31 mF

32.75

370

42

NiO/CeO2

10.8 mF

270

36

66

NiCeOx

40.4 mF

1010

12

66

To test the electrochemical activity of the samples prepared, the polarization curves of the samples (Figure 3a) were obtained using Linear Sweep Voltammetry (LSV). The Ni sample showed the lowest current density for the applied potentials, and had the largest overpotential of 400 mV for the current density of 10 mA/cm2. The NiO sample exhibited a slightly better performance, with an overpotential of 380 mV for 10 mA/cm2. The overpotential of 10 mA/cm2 for NiO/CeO2 was 360 mV. The best performance was shown by the NiCeOx sample which had a low overpotential of only 295 mV to obtain the current density of 10 mA/cm2. The electrochemically active surface area (ECSA) of each sample was estimated by the double-layer capacitance.15,38 In order to know the double-layer capacitance, we first obtained CV curves of the capacitance current in the non-Faradaic voltage region (a 0.1 V potential window centered on the OCP) for several different scan rates (Figure S6). The rate of change in the current at OCP with respect to the scan rate corresponds to the double-layer capacitance.39 For this reason, the current at OCP was plotted against the scan rate for the four samples, and a line of best fit was fitted for each sample’s data set, as shown in Figure 3b. The double layer capacitance was 1.19 mF, 1.31


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ACS Catalysis

mF, 10.8 mF and 40.4 mF for the Ni, NiO, NiO/CeO2 and NiCeOx samples, respectively. The ECSA can be calculated according to the formula ECSA = CDL/Cs, where a specific capacitance of Cs = 0.040 mF cm-2 was used in this work.15 The calculated ECSA values for the four samples as well as other relevant electrochemistry parameters are summarized in Table 1. The ECSA of NiO was found to be only slightly larger than that of Ni, while the ECSA of NiO/CeO2 was much larger than both. The ECSA of NiCeOx was nearly 31 times that of NiO and 4 times that of NiO/CeO2. This is consistent with the observed current densities (JNiCeOx >> JNiO/CeO2 > JNiO > JNi), as a larger ECSA means a sample has more active sites and therefore can catalyze more reactions at one time and sustain a large current. The charge transfer resistances (RCT) of the samples were obtained from their Nyquist plots (Figure 3c). As shown in Table 1, the RCT of NiO/CeO2 (36 Ω) was much smaller than Ni (174 Ω) and NiO (370 Ω), and NiCeOx had the smallest RCT of only 12 Ω at an applied bias of 1.5 V vs RHE. The high oxygen storage capacity and good ionic/electronic conductivity of CeO2 surface layer of NiO/CeO2 result in the sharp decrease of RCT compared with pure NiO.28,40,41 The smallest RCT of NiCeOx indicates the oxygen vacancy defects in CeOx can further promote the mobility of lattice oxygen and enhance the ionic conductivity.24 Thus, NiCeOx has the lowest mass-transfer resistance, suggesting favorable OER kinetics. As shown in Figure S7, the Tafel slopes of the NiCeOx and NiO/CeO2 (66 mV dec-1) were slightly larger than those of the Ni (54 mV dec-1) and NiO (42 mV dec-1). The fast OER kinetics of a catalyst is reflected by its low Tafel slope. This result suggests that the OER kinetics became slower compared to pure Ni and NiO as a result of adding CeO2. The high OER activity of NiCeOx was mainly due to its abundant active sites and low charge transfer resistance. Shannon W. Boettcher and co-authors have studied the role of Fe on the OER activity of Ni(OH)2 catalyst.42 They found the OER activity of Ni(OH)2 catalyst was greatly improved when Ni(OH)2 doped by Fe. They reported that even a very small concentration of Fe ions in the KOH electrolyte would incorporate into the Ni(OH)2 catalyst during the electrochemical measurements. We believe that this should be the reason for the good performance of the Ni and NiO sample for the oxygen evolution reaction in our experiments. To verify the excellent OER activity of the NiCeOx sample is not from the Fe impurities in KOH electrolyte. We invested the OER activity of the Ni-Pre sample (Ni precursor instead of Ce precursor), as shown in Figure S8. Also, the electrochemical performance of the NiO and NiO/CeO2 samples during the long-term water electrolysis were studied, as shown in Figure S9.


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The strong electronic interactions between nickel and cerium oxide induced the oxygen vacancy defects in NiCeOx, as shown by the XPS and Raman results. The large number of oxygen vacancy defects supply an abundance of electrochemically active OER sites and decrease the masstransfer resistance, resulting in the large electrochemically active surface area and high OER performance of NiCeOx. When the lower performance of the NiO/CeO2 sample – which did not show any evidence of defects within the CeO2 in its Raman spectrum – is considered, the importance of oxygen vacancy defects for Ce-containing in achieving a high OER performance is abundantly clear. Controlled-current water electrolysis (Figure 3d) was done to test the long-term performance and stability of the NiCeOx sample. With a fixed current density of 10 mA/cm2, the potential remained stable at ca. 1.525 V vs RHE for more than 200 h. The corresponding faradaic efficiency was nearly 100%, as shown in Figure S10. SEM images of the NiCeOx sample after the 200-hour stability test are shown in Figure S11. The morphology remained nearly the same as the fresh NiCeOx sample. The Raman spectrum and XPS spectrum of the NiCeOx sample after the 200-hour water electrolysis test are shown in Figure S12. The amount of oxygen vacancy defects is retained, however the type of oxygen vacancy defects seems to be different and this change may have been induced by a decrease in the surface Ce concentration. This phenomenon is still under investigation.

Figure 4. Stability characterizations of the catalysts. (a) Polarization curves of the NiCeOx catalyst after the stability test of 0 h, 100 h (NiCeOx-100) and 200 h (NiCeOx-200) for OER with a scan rate of 10 mV s-1. (b) Current density at OCP vs CV scan rate for the NiCeOx-100 catalyst. The slope of current density at OCP vs scan rate represents the double-layer capacitance. (c) The ions concentration of Ni, Ce and Fe (data acquired by inductive coupled plasma emission spectroscopy)


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ACS Catalysis

in the electrolyte during the 200-hour water electrolysis process. N.D. stands for a value below the measurement limit of 0.005 mg/L. The electrolyte was 1 M KOH (pH ≈ 14). To further investigate the OER stability of the NiCeOx catalyst, the polarization curve (Figure 4a) and the ECSA (Figure S13 and Figure 4b) of the NiCeOx catalyst after 100-hour water electrolysis (NiCeOx-100) were obtained. Through 100-hour water electrolysis, more Ni2+ was oxidized to Ni3+, as confirmed by the peak at ca. 1.37 V vs RHE in Figure 4a, thus improving OER performance of the NiCeOx-100. The OER performance of the NiCeOx-200 is very similar to that of NiCeOx-100, thus indicating that the NiCeOx catalyst tends to be stable after the available Ni2+ was oxidized to Ni3+. The electrochemically active surface area (ECSA) of the NiCeOx-100 catalyst was estimated by the double-layer capacitance. The double layer capacitance was 44.4 mF (the same method for the NiCeOx catalyst) for the NiCeOx-100 catalyst, which was a little larger than that of the NiCeOx catalyst (40.4 mF). This indicates that the ECSA of the NiCeOx catalyst increased a little during the long-term water electrolysis, which is consistent with the LSV result. Finally, ICP of the electrolyte after the 200-hour water electrolysis was performed to check the presence of dissolved ions (Ni, Ce) and contaminant ions (Fe). As shown in Figure 4c, no Ni, Ce and Fe ions were detected in the electrolyte after the 200-hour water electrolysis test. This indicates that the NiCeOx catalyst is very stable and not dissolved during the long-term stability test. The OER performance of NiCeOx compared to that of many other OER metal oxide catalysts reported recently (Table S1), including precious metals, Ni-based and Co-based catalysts. In addition to the NiCeOx having an initial activity and the activity after a 200-hour continuous test comparable to that of other OER metal oxide catalysts, the NiCeOx sample outperforms other metal oxide catalysts in terms of stability. NiCeOx-Au (Ni:Ce 95: 5) was reported to have a low overpotential of 280 mV for a current density of 10 mA/cm2, but the overpotential increased with the testing time and end up with 310 mV during a 2-hour stability test.19 The long-time stability (over 200 h) of our NiCeOx sample may be attributed to the high Ce concentration (Ni:Ce 52:48) and the synthesis method (NF substrate as the source of nickel). It is noted that the overpotential for 10 mA/cm2 was above 470 mV for the reported NiCeOx-Au (Ni:Ce 50:50). The low overpotential (295 mV) of our NiCeOx sample is attributed to the large number of active sites and favorable OER kinetics induced by the abundant oxygen vacancy defects. The low overpotential and long-term stability of NiCeOx achieved in this work make it an ideal OER catalyst.


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Figure 5. Suitability to industrial application of the NiCeOx catalyst. (a) Polarization curves of the NiCeOx sample for the OER with a scan rate of 10 mV s-1 in 6 M KOH at 25 °C and 60 °C. 0 h and 15 h represent the polarization curves taken before and after the 15-hour water electrolysis, respectively. Inset shows the overpotentials for the current density of 200 mA/cm2. (b) Potential trace of the NiCeOx sample obtained by fixing the current density for electrolysis at 50 mA/cm2 at 60 °C. The electrolyte was 6 M KOH. In the following, the suitability to industrial application of the reported NiCeOx is investigated. For this purpose, the highly concentrated electrolyte of 6 M KOH was used in the OER measurements. As shown in Figure 5a, the OER performance of the NiCeOx sample improved drastically when the operating temperature was increased from 25 °C to 60 °C, as indicated by the overpotential decrease from 450 mV to 300 mV for the current density of 200 mA/cm2. After a 15-hour water electrolysis test performed at 60 °C with the fixed current density of 50 mA/cm2 (shown in Figure 5b), the OER performance of the NiCeOx sample slightly increases exhibiting a slight decrease to 270 mV of the overpotential for the current density of 200 mA/cm2. During the 15-hour water electrolysis test, the formation of Ni3+ oxidized from Ni2+ is found to increase as revealed by the larger peak area at ca. 1.37 V for the blue polarization curve (15 h) compared with the red polarization curve (0 h). In conclusion, the high current density obtained

at low

overpotential and high stability of the NiCeOx sample operating at 60 °C in 6 M KOH make it suitable for industrial application.


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In summary, a defect-rich NiCeOx layer was directly synthesized on nickel foam using a simple two-step dip-coating/annealing method. Through thermal diffusion, Ni atoms from the nickel foam substrate transfer to the top layer and mix with deposited Ce uniformly to form a NiCeOx layer. The electronic interactions between the Ni atoms and cerium oxide result in an oxygen vacancy defect-rich NiCeOx layer that has a very high electrochemically active surface area and a low mass-transfer resistance, resulting the high OER performance. The NiCeOx catalyst, with the low overpotential of 295 mV for 10 mA/cm2 and the stability of over 200 hours, is one of the best OER catalysts ever reported. Moreover, when the NiCeOx catalyst was tested under industrial conditions (6 M KOH, 60 °C), the overpotential for the current density of 200 mA/cm2 was only 270 mV, and the catalyst layer was stable for more than 15 hours. The excellent performance and low-cost, environmentally friendly and reproducible synthesis method of NiCeOx catalyst make it suitable for industry application. This work might open a new avenue for developing Ce-containing OER catalysts with high efficiency and stable performance by introducing defects. Nickel foam can be used directly as the source of nickel for the synthesis of Ni-based catalysts.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental section and additional characterizations and analysis of data for the catalysts

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Jean-Jacques Delaunay: 0000-0003-2175-0620 Xia Long: 0000-0002-9705-1589


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The XRD and electron microscopy characterizations were conducted at the Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Part of this work was supported by the JSPS Core-to-Core program (Advanced Research Networks type A), Japan (JSPS)-Korea (NRF) Bilateral program and Grantsin-Aids for Specially Promoted Research and KAKENHI Grant number (17H03229). J.Y. thanks the support from China Scholarship Council (201506210091). Part of this work was supported by the Shenzhen Peacock Plan (KQTD2016053015544057) and the National Science Foundation of China (contract number 21703003).

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Defect-Rich NiCeOx Electrocatalyst with Ultrahigh Stability and Low

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