- Email: [email protected]
Thermal decomposition behavior of nickel-iron hydrotalcite and its electrocatalytic properties of oxygen reduction and oxygen evolution reactions
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e5
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Short Communication
Thermal decomposition behavior of nickel-iron hydrotalcite and its electrocatalytic properties of oxygen reduction and oxygen evolution reactions Zejie Zhang a, Debi Zhou b,*, Xinjun Bao b, Huazhang Yu c, Boyun Huang a a
State Key Laboratory for Powder Metallurgy, Central South University, Changsha, China School of Chemistry and Chemical Engineering, Central South University, Changsha, China c Microvast Power Systems Co., Ltd., Huzhou, China b
article info
abstract
Article history:
The thermal decomposition behavior of NiFe layered double hydroxide (LDH) was investi-
Received 13 August 2018
gated by thermogravimetric analysis-differential scanning calorimetry (TG-DSC). The
Received in revised form
calcined product at 500 C was mainly NiO/FeOx composite oxide, of which FeOx was amor-
18 September 2018
phous oxide; the calcined product at 650 C was mainly NiO/NiFe2O4 composite oxide. The
Accepted 20 September 2018
polarization curves and chronopotentiometry stated that the NiO/FeOx and NiO/NiFe2O4
Available online xxx
showed good electrocatalytic OER and ORR activity; the OER activity of NiO/FeOx was better than that of NiO/NiFe2O4; the ORR activity of NiO/NiFe2O4 was better than that of NiO/FeOx.
Keywords:
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
OER ORR Spinel NiFe LDH Stability
Introduction Since the first industrial revolution, the demand and consumption of energy has become more and more great. The development of new energy sources, represented by hydrogen energy, fuel cells and metal air batteries, is an effective means of alleviating current energy and environmental contradictions. In hydrogen development and metal air batteries, oxygen evolution reaction (OER) is one of the key steps, and the use of oxygen electrodes with high catalytic performance can reduce polarization, thus greatly reducing energy
consumption. Ru and Ir precious metals and the oxygen atoms adsorbed on their surface have moderate bonding strength, which can significantly reduce the over potential for OER and improve the reaction efficiency of oxygen electrodes. Pyrochlore-type composite oxides containing Ir and Ruprecious metals (such as Pb2(Ru2-xPbx)O6.5, Pb2 (Ir2-xPbx)O7-y [1,2]) also have excellent catalytic activity for OER. Taking Pb2(Ru2-xPbx)O6.5 as an example, the reaction mechanism may be that 6-coordination Ru ions convert to 7-coordination intermediates by adsorbing OH. However, Ru, Ir and their oxides are susceptible to corrosion during anodic oxidation, and their service life is short.
* Corresponding author. E-mail address: [email protected] (D. Zhou). https://doi.org/10.1016/j.ijhydene.2018.09.153 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang Z, et al., Thermal decomposition behavior of nickel-iron hydrotalcite and its electrocatalytic properties of oxygen reduction and oxygen evolution reactions, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.09.153
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e5
The main structure of layered double hydroxide (LDH) is composed of two-valence and trivalent metal hydroxide. LDHs can be inserted into layers or ion exchange of various anions (including organic anions, inorganic anions and metal complexes, etc.), thus giving LDHs materials a variety of new physical and chemical properties [3e6]. LDHs material and its calcined products all exist acid and alkali centers, which is a kind of high selectivity, high activity, long life and environment-friendly acid-alkali dual function catalyst or catalyst carrier in industrial multiphase catalysis [7]. Hongjie Dai [8,9] used LDHs materials as OER catalysts in alkaline system, where the LDHs demonstrated good catalytic activity, indicating that the LDHs materials will be of highly potential research and application in the field of electrolytic water and fuel cell. NiFe hydrotalcite is a kind of layered double hydroxide, Ni2þ and Fe3þ respectively occupying octahedral voids which are closely packed by OH in the main body of the laminate. The coprecipitation approach is a conventional method of preparing NiFe hydrotalcite. The preparation of hydrotalcite in this way is characterized by simple operation, large yield, and small grain size (generally <30 nm). However, the hydrotalcite has a large amount of positive charge in the main body of the lamina, and it has a strong adsorption effect on water and anions. When the crystal grains are too small, excessive residual surface energy and electrostatic action are generated, resulting in serious agglomeration of crystal grains, which is disadvantageous to exert a catalytic action in the oxygen electrode. The LDHs material contains a large amount of OH, CO2 3 and other components, and is easily removed under heat to form a composite oxide in which metal ions are highly mixed and dispersed. A transition metal oxide material containing Ni, Co, Fe, Mn is a commonly used catalyst for oxygen reduction or oxygen evolution. If the metal cation of the hydrotalcite is a transition metal cation such as Fe3þ, Co2þ or Ni2þ, the prepared composite oxide material will have a certain electrocatalytic activity. Therefore, in this study, the NiFe LDH (x ¼ 0.295) material with CO2 3 intercalation was used as the precursor, and its thermal decomposition behavior was analyzed by thermogravimetric analysis-differential scanning calorimetry (TGDSC). The calcined product was examined for its electrocatalytic activity and stability. The experiment was evaluated for uncertainty, and the experiment was repeated by others. Many experiments showed that the uncertainty was very small.
polyethylene glycol was added, the mixture hydrothermally reacting at 160 C for 8 h, filtered, washed to neutrality, washed with alcohol, and vacuumed. After drying, a yellow solid was obtained, which is abbreviated as NFL. The prepared NiFe LDH precursor materials were calcined in a muffle furnace at 500 C and 650 C for 1 h to obtain the target products of composite oxide at different temperatures, which were sequentially abbreviated as NFL-500 and NFL-650.
Preparation of air electrode The preparation process of the NFL/CNTs electrode is as follows: the sample and the carbon nanotubes (CNTs) were weighed according to the mass ratio of 1:2 and mixed into a beaker, and an appropriate amount of ethanol was added, ultrasonically dispersed for 30 min, and then 60 wt% PTFE emulsion was added dropwise. The PTFE accounts for 20% of the total weight ratio. The solution was further ultrasonically dispersed for 30 min, placed in an oven and baked into a paste, and rolled to form a film, that is, a catalytic layer. Finally, the catalytic layer and the nickel foam and the diffusion layer were pressed at 3 107 kg cm2 for 10 min and dried at 80 C for 12 h.
Electrochemical characterization The test system is composed of Chi760d electrochemical workstation. The air electrode was used as the working electrode, and the Pt plate was the counter electrode. The HgO/Hg was served as the reference electrode and the electrolyte is 6 mol$L1 KOH solution. In this work, all the potentials were converted to a reversible hydrogen electrode (RHE), E (RHE) ¼ E (Hg/HgO)þ0.098 V þ 0.059 pH. The potential scanning range of the polarization curve is 0.4 Ve0.75 V at a speed of 1 mV$s1.
Results and discussion The LDH material contains a large amount of crystal water, carbonate (or intercalated anions such as nitrate) and hydroxyl groups, and its thermal stability is poor [10e12]. Fig. 1 is the TG-DSC curve of a carbonate intercalated NiFe LDH (x ¼ 0.295) material.
Experimental Synthesis All chemicals were of analytical reagent grade. The preparation process of NiFe LDH precursor is as follows: a mixed metal nitrate solution with a concentration of 0.01 M Ni2þþ0.01 M Fe3þ and a mixed alkali solution with a concentration of 0.02 M NaOHþ0.0059 M Na2CO3 was added dropwise to 200 ml deionized water with stirring for 2 h. The pH value of the reaction solution should be controlled to about 10. After the completion of the dropwise addition, the filter slurry obtained by suction filtration of the liquid was transferred to a hydrothermal reaction kettle, and an appropriate amount of
Fig. 1 e TG-DSC curves of NiFe LDH：a- TG curve, b-DSC curve.
Please cite this article in press as: Zhang Z, et al., Thermal decomposition behavior of nickel-iron hydrotalcite and its electrocatalytic properties of oxygen reduction and oxygen evolution reactions, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.09.153
3
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e5
The standard formula of NiFe LDH material with metal ion ratio of x ¼ 0.295 for carbonate intercalation is Ni0.705Fe0.295(OH)2(CO2 3)0.1475$0.94H2O, and the weight loss rate of removing crystal water is 14.39%, dehydration of carbonate and hydroxyl groups 22.83%. According to the TG curve, it can be seen that the NiFe LDH has an obvious weight loss process from normal temperature to 200 C. The weight loss rate of this process is 13.60%, which is obviously the process of removing crystal water; from 200 C to 450 C, the weight loss rate of this process is 18.49%, which is the process of dehydration of carbonate and hydroxyl. Since the TG curve process is a dynamic thermal scanning process, there is a certain error between the actual weight loss rate and the theoretical weight loss rate. It can be seen from the DSC curve that the first endothermic peak appears at 188 C, which corresponds to the endothermic removal process of interlayer crystallization water; the second endothermic peak appears at 303 C, corresponding to the endothermic removal of carbonate and the hydroxy endothermic dehydration process of the layer; there is a small exothermic peak and a small endothermic peak between 550 C and 610 C. Since no mass change occurs at this time, it should be related to the crystal transformation of the calcined product and the formation of a solid solution [13]. The NiFe LDH precursor was treated at 500 C and 650 C, the XRD spectrum of the obtained product and NiFe LDH is shown in Fig. 2. The 2q angle characteristic diffraction peaks corresponding to NiFe LDH presented at 11.3 , 22.8 , 34.1 , 38.4 , 45.8 , 59.8 , 61.1 , 64.2 . Symmetrical characteristic diffraction peaks of (003), (006), (110), (113) crystal planes and asymmetric characteristic diffraction peaks of (012), (015), (018), (016) crystal planes are included in the spectrum. The NFL-500 has only the characteristic diffraction peak of NiO(JCPDF 47-1049); the thermogravimetric analysis shows that the calcination is performed before 500 C; the mass loss is mainly due to water, hydroxy and carbonate loss, indicating that the iron (III) is mainly amorphous in the form of iron oxide, so the characteristic diffraction peak cannot be obtained, and the NFL-500 product is a NiO/FeOx composite oxide. At the same time, the diffraction spectrum in the
sweeping range is slightly disordered, and the characteristic diffraction peak of NiO is obviously broadened, indicating that the crystallinity of NiO is not high, and the grain size is small. When the calcination temperature is raised to 650 C, there is a distinct NiFe2O4(JCPDF 97-0040) spinel phase in the NFL-650, that is, NiO undergoes crystal transformation with amorphous FeOx to form a NiFe2O4 spinel phase. Since NiO and NiFe2O4 belong to the same cubic system, the unit cell parameter a ¼ 8.348 A of NiFe2O4 is calculated to be twice the unit cell parameter a ¼ 4.174 A of NiO (Table 1), so the diffraction angles 2q of NiO phase of (111), (200), (220) and (311) are very close to the diffraction angles of NiFe2O4 phase of (222), (400), (440), and (533), and the diffraction peaks of the two phases overlap. Therefore, the calcined product at 650 C is a solid solution of the spinel-type NiO/NiFe2O4 composite oxide. The calcined products of NiFe LDH at 500 C and 650 C were dispersed on CNTs to prepare NFL-500/CNTs and NFL650/CNTs electrodes. The electrocatalytic activity and stability of the electrode were investigated. Fig. 3 shows the polarization of NFL/CNTs, NFL-500/CNTs and NFL-650/CNTs electrodes in OER and ORR processes. In the OER process, the initial oxygen evolution potential of each electrode is sequentially in the order of NFL-650/CNTs > NFL500/CNTs > NFL/CNTs, and under the same current density, the potential of each electrode is still NFL-650/CNTs > NFL500/CNTs > NFL/CNTs, as shown in Table 2. The potentials of the electrodes are respectively 1.576 V(NFL-650/CNTs), 1.551 V(NFL-500/CNTs), and 1.482 V(NFL/CNTs) at the current density of 100 mA$cm2. In the ORR process, the NFL/CNTs electrode mainly exhibits CNTs activity, so its ORR activity is the worst, and the NFL-650/CNTs electrode has the highest ORR activity, as shown in Table 3. The electrode potentials were 0.753 V (NFL-650/CNTs), 0.719 V (NFL-500/CNTs), and 0.572 V (NFL/CNTs) at a current density of 50 mA$cm2. NiFe LDH forms an iron oxide after heat treatment [14], and NiFe2O4 spinel which is formed at a higher temperature is an efficient oxygen reduction catalyst, and therefore the catalytic oxygen reduction activity is increasing with temperature. On the other hand, the structure of NiFe LDH is destroyed after heat treatment, and the oxygen evolution activity of the formed nickel oxide is lower than that of NiFe LDH material, but higher than that of NiFe2O4 spinel [9,15]. So the NiFe LDH material decreases with the calcination temperature, and its catalytic oxygen evolution activity decreases. In general, the bifunctional catalyst for OER and ORR was prepared by heat treatment of NiFe LDH, which opened up the idea of preparation of bifunctional electrocatalytic materials. The chronopotentiometry curves NFL-500/CNTs and NFL650/CNTs electrodes for OER and ORR were measured at a current density of 100 mA$cm2, as shown in Fig. 4. In the OER
Table 1 e Characteristic parameters of NiO and NiFe2O4. Phase
Fig. 2 e XRD patterns of NFL-500, NFL-650 and NiFe LDH.
NiO NiFe2O4
Diffraction angle
Interplanar spacing
Cell parameters
2q（degree）
A) d111(
a( A) b ( A) c ( A)
37.25 18.40
2.410 4.820
4.174 4.174 4.174 8.348 8.378 8.348
Please cite this article in press as: Zhang Z, et al., Thermal decomposition behavior of nickel-iron hydrotalcite and its electrocatalytic properties of oxygen reduction and oxygen evolution reactions, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.09.153
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e5
Fig. 3 e Polarization curves of NiFe LDH/CNTs, NFL-500/ CNTs and NFL-650/CNTs electrodes.
Table 2 e Potential of NiFe LDH/CNTs, NFL-500/CNTs and NFL-650/CNTs electrodes at different current density for OER. j (mA$cm2)
20 50 100 200
Potential (V vs. RHE) NiFe LDH/ CNTs
NFL-500/ CNTs
NFL-650/ CNTs
1.404 1.444 1.482 1.542
1.421 1.499 1.551 1.620
1.498 1.536 1.576 1.646
process, the potentials of the NFL-650/CNTs and NFL-500/ CNTs electrodes were 1.576 V and 1.550 V, respectively. After 2.5 h, the potentials were 1.577 V and 1.551 V, and the over potentials of the two electrodes only increased with ~1 mV, the variation is very small, indicating that the two electrodes have high stability for OER. In the ORR process, the potentials of the NFL-650/CNTs and NFL-500/CNTs electrodes were 0.681 V and 0.592 V, respectively. After 2.5 h, the potential were 0.679 V, 0.589 V; the over potential of the NFL-650/CNTs electrode only increased by ~2 mV; the over potential of the NFL-500/CNTs electrode only increased by ~3 mV, and the potential change was also very small, indicating that the two electrodes also have high stability in the ORR. Comparing with the NiFe LDH precursor, the NiO/FeOx (NFL-500) composite oxide and the spinel NiO/NiFe2O4(NFL650) composite oxide not only manifest good OER activity, but
Table 3 e Potential of NiFe LDH/CNTs, NFL-500/CNTs and NFL-650/CNTs electrodes at different current density for ORR. j (mA$cm2)
20 50 100
Potential（V vs. RHE） NiFe LDH/ CNTs
NFL-500/ CNTs
NFL-650/ CNTs
0.708 0.572
0.780 0.719 0.592
0.797 0.753 0.681
Fig. 4 e Chronopotentiometry curves of NFL-500/CNTs and NFL-650/CNTs electrodes.
also good ORR activity. The OER activity of NiO/FeOx (NFL-500) composite oxide is higher. Meanwhile, the ORR activity of NiO/ NiFe2O4(NFL-650) is the best. Moreover, the two materials also have excellent OER and ORR stability and are good dualfunctional catalytic materials.
Conclusion In summary, the thermal decomposition behavior of NiFe LDH materials was analyzed, and the electrocatalytic properties for ORR and OER of the calcined products were studied. The TGDSC analysis showed that the thermal decomposition behavior of NiFe LDH material was mainly divided into three stages. The calcined product at 500 C was mainly NiO/FeOx composite oxide, of which FeOx was amorphous oxide; the calcined product at 650 C was mainly NiO/NiFe2O4 composite oxide. The electrochemical analysis exhibited that the NiO/ FeOx (NFL-500) composite oxide and the NiO/NiFe2O4(NFL-650) composite oxide had excellent and stable OER and ORR activity.
references
[1] Prakash J, Tryk DA, Yeager EB. Kinetic investigations of oxygen reduction and evolution reactions on lead ruthenate catalysts. J Electrochem Soc 1999;146(11):4145e51. [2] Ten Kortenaar MV, Vente JF, Ijdo DJW, Muller S, Kotz R. Oxygen evolution and reduction on iridium oxide compounds. J Power Sources 1995;56(1):51e60. [3] Yun SK, Pinnavaia TJ. Layered double hydroxides intercalated by polyoxometalate anions with Keggin (aH2W12O406-), Dawson (a-P2W18O626-), and Finke (Co4(H2O)2 (PW9O34210-) structures. Inorg Chem 1996;35(23):6853e60. [4] Dixit M, Mishra M, Joshi PA, Shah DO. Physico-chemical and catalytic properties of Mg-Al hydrotalcite and Mg-Al mixed oxide supported copper catalysts. J Ind Eng Chem 2013;19(2):458e68. [5] Jin L, Liu Q, Sun Z, Ni X, Wei M. Preparation of 5-fluorouracil/ b-cyclodextrin complex intercalated in layered double hydroxide and the controlled drug release properties. Ind Eng Chem Res 2010;49(22):11176e81.
Please cite this article in press as: Zhang Z, et al., Thermal decomposition behavior of nickel-iron hydrotalcite and its electrocatalytic properties of oxygen reduction and oxygen evolution reactions, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.09.153
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e5
[6] Yu XY, Luo T, Jia Y, Xu RX, Gao C, Zhang YX, et al. Threedimensional hierarchical flower-like Mg-Al-layered double hydroxides: highly efficient adsorbents for as (V) and Cr(VI) removal. Nanoscale 2012;4(11):3466e74. [7] Choudary BM, Madhi S, Chowdari NS, Kantam ML, Sreedhar B. Layered double hydroxide supported nanopalladium catalyst for Heck-, Suzuki-, Sonogashira-, and Stille-type coupling reactions of chloroarenes. J Am Chem Soc 2002;124(47):14127e36. [8] Gong M, Li Y, Wang H, Liang Y, Wu JZ, Zhou J, et al. An advanced Ni-Fe layered double hydroxide electrocatalyst for water oxidation. J Am Chem Soc 2013;135(23):8452e5. [9] Li Y, Gong M, Liang Y, Feng J, Kim JE, Wang H, et al. Advanced zinc-air batteries based on high-performance hybrid electrocatalysts. Nat Commun 2013;4:1805. [10] MacKenzie KJD, Meinhold RH, Sherriff BL, Xu Z. 27Al and 25Mg solid-state magic-angle spinning nuclear magnetic resonance study of hydrotalcite and its thermal
[11]
[12]
[13]
[14] [15]
5
decomposition sequence. J Mater Chem 1993;3(12): 1263e9. Hudson MJ, Carlino S, Apperley DC. Thermal conversion of a layered (Mg/Al) double hydroxide to the oxide. J Mater Chem 1995;5(2):323e9. Cardoso LP, Valim JB. Competition between three organic anions during regeneration process of calcined LDH. J Phys Chem Solid 2004;65(2):481e5. Unnikrishnan R, Narayanan S. Metal containing layered double hydroxides as efficient catalyst precursors for the selective conversion of acetone. J Mol Catal Chem 1999;144(1):173e9. Adzic R. Recent advances in the kinetics of oxygen reduction. New York: Wiley-VCH; 1998. Subbaraman R, Tripkovic D, Chang KC, Strmcnik D, Paulikas AP, Hirunsit P, et al. Trends in activity for the water electrolyser reactions on 3d M (Ni, Co, Fe, Mn) hydr (oxy) oxide catalysts. Nat Mater 2012;11(6):550e7.
Please cite this article in press as: Zhang Z, et al., Thermal decomposition behavior of nickel-iron hydrotalcite and its electrocatalytic properties of oxygen reduction and oxygen evolution reactions, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.09.153
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Short Communication
Thermal decomposition behavior of nickel-iron hydrotalcite and its electrocatalytic properties of oxygen reduction and oxygen evolution reactions Zejie Zhang a, Debi Zhou b,*, Xinjun Bao b, Huazhang Yu c, Boyun Huang a a
State Key Laboratory for Powder Metallurgy, Central South University, Changsha, China School of Chemistry and Chemical Engineering, Central South University, Changsha, China c Microvast Power Systems Co., Ltd., Huzhou, China b
article info
abstract
Article history:
The thermal decomposition behavior of NiFe layered double hydroxide (LDH) was investi-
Received 13 August 2018
gated by thermogravimetric analysis-differential scanning calorimetry (TG-DSC). The
Received in revised form
calcined product at 500 C was mainly NiO/FeOx composite oxide, of which FeOx was amor-
18 September 2018
phous oxide; the calcined product at 650 C was mainly NiO/NiFe2O4 composite oxide. The
Accepted 20 September 2018
polarization curves and chronopotentiometry stated that the NiO/FeOx and NiO/NiFe2O4
Available online xxx
showed good electrocatalytic OER and ORR activity; the OER activity of NiO/FeOx was better than that of NiO/NiFe2O4; the ORR activity of NiO/NiFe2O4 was better than that of NiO/FeOx.
Keywords:
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
OER ORR Spinel NiFe LDH Stability
Introduction Since the first industrial revolution, the demand and consumption of energy has become more and more great. The development of new energy sources, represented by hydrogen energy, fuel cells and metal air batteries, is an effective means of alleviating current energy and environmental contradictions. In hydrogen development and metal air batteries, oxygen evolution reaction (OER) is one of the key steps, and the use of oxygen electrodes with high catalytic performance can reduce polarization, thus greatly reducing energy
consumption. Ru and Ir precious metals and the oxygen atoms adsorbed on their surface have moderate bonding strength, which can significantly reduce the over potential for OER and improve the reaction efficiency of oxygen electrodes. Pyrochlore-type composite oxides containing Ir and Ruprecious metals (such as Pb2(Ru2-xPbx)O6.5, Pb2 (Ir2-xPbx)O7-y [1,2]) also have excellent catalytic activity for OER. Taking Pb2(Ru2-xPbx)O6.5 as an example, the reaction mechanism may be that 6-coordination Ru ions convert to 7-coordination intermediates by adsorbing OH. However, Ru, Ir and their oxides are susceptible to corrosion during anodic oxidation, and their service life is short.
* Corresponding author. E-mail address: [email protected] (D. Zhou). https://doi.org/10.1016/j.ijhydene.2018.09.153 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang Z, et al., Thermal decomposition behavior of nickel-iron hydrotalcite and its electrocatalytic properties of oxygen reduction and oxygen evolution reactions, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.09.153
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e5
The main structure of layered double hydroxide (LDH) is composed of two-valence and trivalent metal hydroxide. LDHs can be inserted into layers or ion exchange of various anions (including organic anions, inorganic anions and metal complexes, etc.), thus giving LDHs materials a variety of new physical and chemical properties [3e6]. LDHs material and its calcined products all exist acid and alkali centers, which is a kind of high selectivity, high activity, long life and environment-friendly acid-alkali dual function catalyst or catalyst carrier in industrial multiphase catalysis [7]. Hongjie Dai [8,9] used LDHs materials as OER catalysts in alkaline system, where the LDHs demonstrated good catalytic activity, indicating that the LDHs materials will be of highly potential research and application in the field of electrolytic water and fuel cell. NiFe hydrotalcite is a kind of layered double hydroxide, Ni2þ and Fe3þ respectively occupying octahedral voids which are closely packed by OH in the main body of the laminate. The coprecipitation approach is a conventional method of preparing NiFe hydrotalcite. The preparation of hydrotalcite in this way is characterized by simple operation, large yield, and small grain size (generally <30 nm). However, the hydrotalcite has a large amount of positive charge in the main body of the lamina, and it has a strong adsorption effect on water and anions. When the crystal grains are too small, excessive residual surface energy and electrostatic action are generated, resulting in serious agglomeration of crystal grains, which is disadvantageous to exert a catalytic action in the oxygen electrode. The LDHs material contains a large amount of OH, CO2 3 and other components, and is easily removed under heat to form a composite oxide in which metal ions are highly mixed and dispersed. A transition metal oxide material containing Ni, Co, Fe, Mn is a commonly used catalyst for oxygen reduction or oxygen evolution. If the metal cation of the hydrotalcite is a transition metal cation such as Fe3þ, Co2þ or Ni2þ, the prepared composite oxide material will have a certain electrocatalytic activity. Therefore, in this study, the NiFe LDH (x ¼ 0.295) material with CO2 3 intercalation was used as the precursor, and its thermal decomposition behavior was analyzed by thermogravimetric analysis-differential scanning calorimetry (TGDSC). The calcined product was examined for its electrocatalytic activity and stability. The experiment was evaluated for uncertainty, and the experiment was repeated by others. Many experiments showed that the uncertainty was very small.
polyethylene glycol was added, the mixture hydrothermally reacting at 160 C for 8 h, filtered, washed to neutrality, washed with alcohol, and vacuumed. After drying, a yellow solid was obtained, which is abbreviated as NFL. The prepared NiFe LDH precursor materials were calcined in a muffle furnace at 500 C and 650 C for 1 h to obtain the target products of composite oxide at different temperatures, which were sequentially abbreviated as NFL-500 and NFL-650.
Preparation of air electrode The preparation process of the NFL/CNTs electrode is as follows: the sample and the carbon nanotubes (CNTs) were weighed according to the mass ratio of 1:2 and mixed into a beaker, and an appropriate amount of ethanol was added, ultrasonically dispersed for 30 min, and then 60 wt% PTFE emulsion was added dropwise. The PTFE accounts for 20% of the total weight ratio. The solution was further ultrasonically dispersed for 30 min, placed in an oven and baked into a paste, and rolled to form a film, that is, a catalytic layer. Finally, the catalytic layer and the nickel foam and the diffusion layer were pressed at 3 107 kg cm2 for 10 min and dried at 80 C for 12 h.
Electrochemical characterization The test system is composed of Chi760d electrochemical workstation. The air electrode was used as the working electrode, and the Pt plate was the counter electrode. The HgO/Hg was served as the reference electrode and the electrolyte is 6 mol$L1 KOH solution. In this work, all the potentials were converted to a reversible hydrogen electrode (RHE), E (RHE) ¼ E (Hg/HgO)þ0.098 V þ 0.059 pH. The potential scanning range of the polarization curve is 0.4 Ve0.75 V at a speed of 1 mV$s1.
Results and discussion The LDH material contains a large amount of crystal water, carbonate (or intercalated anions such as nitrate) and hydroxyl groups, and its thermal stability is poor [10e12]. Fig. 1 is the TG-DSC curve of a carbonate intercalated NiFe LDH (x ¼ 0.295) material.
Experimental Synthesis All chemicals were of analytical reagent grade. The preparation process of NiFe LDH precursor is as follows: a mixed metal nitrate solution with a concentration of 0.01 M Ni2þþ0.01 M Fe3þ and a mixed alkali solution with a concentration of 0.02 M NaOHþ0.0059 M Na2CO3 was added dropwise to 200 ml deionized water with stirring for 2 h. The pH value of the reaction solution should be controlled to about 10. After the completion of the dropwise addition, the filter slurry obtained by suction filtration of the liquid was transferred to a hydrothermal reaction kettle, and an appropriate amount of
Fig. 1 e TG-DSC curves of NiFe LDH：a- TG curve, b-DSC curve.
Please cite this article in press as: Zhang Z, et al., Thermal decomposition behavior of nickel-iron hydrotalcite and its electrocatalytic properties of oxygen reduction and oxygen evolution reactions, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.09.153
3
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e5
The standard formula of NiFe LDH material with metal ion ratio of x ¼ 0.295 for carbonate intercalation is Ni0.705Fe0.295(OH)2(CO2 3)0.1475$0.94H2O, and the weight loss rate of removing crystal water is 14.39%, dehydration of carbonate and hydroxyl groups 22.83%. According to the TG curve, it can be seen that the NiFe LDH has an obvious weight loss process from normal temperature to 200 C. The weight loss rate of this process is 13.60%, which is obviously the process of removing crystal water; from 200 C to 450 C, the weight loss rate of this process is 18.49%, which is the process of dehydration of carbonate and hydroxyl. Since the TG curve process is a dynamic thermal scanning process, there is a certain error between the actual weight loss rate and the theoretical weight loss rate. It can be seen from the DSC curve that the first endothermic peak appears at 188 C, which corresponds to the endothermic removal process of interlayer crystallization water; the second endothermic peak appears at 303 C, corresponding to the endothermic removal of carbonate and the hydroxy endothermic dehydration process of the layer; there is a small exothermic peak and a small endothermic peak between 550 C and 610 C. Since no mass change occurs at this time, it should be related to the crystal transformation of the calcined product and the formation of a solid solution [13]. The NiFe LDH precursor was treated at 500 C and 650 C, the XRD spectrum of the obtained product and NiFe LDH is shown in Fig. 2. The 2q angle characteristic diffraction peaks corresponding to NiFe LDH presented at 11.3 , 22.8 , 34.1 , 38.4 , 45.8 , 59.8 , 61.1 , 64.2 . Symmetrical characteristic diffraction peaks of (003), (006), (110), (113) crystal planes and asymmetric characteristic diffraction peaks of (012), (015), (018), (016) crystal planes are included in the spectrum. The NFL-500 has only the characteristic diffraction peak of NiO(JCPDF 47-1049); the thermogravimetric analysis shows that the calcination is performed before 500 C; the mass loss is mainly due to water, hydroxy and carbonate loss, indicating that the iron (III) is mainly amorphous in the form of iron oxide, so the characteristic diffraction peak cannot be obtained, and the NFL-500 product is a NiO/FeOx composite oxide. At the same time, the diffraction spectrum in the
sweeping range is slightly disordered, and the characteristic diffraction peak of NiO is obviously broadened, indicating that the crystallinity of NiO is not high, and the grain size is small. When the calcination temperature is raised to 650 C, there is a distinct NiFe2O4(JCPDF 97-0040) spinel phase in the NFL-650, that is, NiO undergoes crystal transformation with amorphous FeOx to form a NiFe2O4 spinel phase. Since NiO and NiFe2O4 belong to the same cubic system, the unit cell parameter a ¼ 8.348 A of NiFe2O4 is calculated to be twice the unit cell parameter a ¼ 4.174 A of NiO (Table 1), so the diffraction angles 2q of NiO phase of (111), (200), (220) and (311) are very close to the diffraction angles of NiFe2O4 phase of (222), (400), (440), and (533), and the diffraction peaks of the two phases overlap. Therefore, the calcined product at 650 C is a solid solution of the spinel-type NiO/NiFe2O4 composite oxide. The calcined products of NiFe LDH at 500 C and 650 C were dispersed on CNTs to prepare NFL-500/CNTs and NFL650/CNTs electrodes. The electrocatalytic activity and stability of the electrode were investigated. Fig. 3 shows the polarization of NFL/CNTs, NFL-500/CNTs and NFL-650/CNTs electrodes in OER and ORR processes. In the OER process, the initial oxygen evolution potential of each electrode is sequentially in the order of NFL-650/CNTs > NFL500/CNTs > NFL/CNTs, and under the same current density, the potential of each electrode is still NFL-650/CNTs > NFL500/CNTs > NFL/CNTs, as shown in Table 2. The potentials of the electrodes are respectively 1.576 V(NFL-650/CNTs), 1.551 V(NFL-500/CNTs), and 1.482 V(NFL/CNTs) at the current density of 100 mA$cm2. In the ORR process, the NFL/CNTs electrode mainly exhibits CNTs activity, so its ORR activity is the worst, and the NFL-650/CNTs electrode has the highest ORR activity, as shown in Table 3. The electrode potentials were 0.753 V (NFL-650/CNTs), 0.719 V (NFL-500/CNTs), and 0.572 V (NFL/CNTs) at a current density of 50 mA$cm2. NiFe LDH forms an iron oxide after heat treatment [14], and NiFe2O4 spinel which is formed at a higher temperature is an efficient oxygen reduction catalyst, and therefore the catalytic oxygen reduction activity is increasing with temperature. On the other hand, the structure of NiFe LDH is destroyed after heat treatment, and the oxygen evolution activity of the formed nickel oxide is lower than that of NiFe LDH material, but higher than that of NiFe2O4 spinel [9,15]. So the NiFe LDH material decreases with the calcination temperature, and its catalytic oxygen evolution activity decreases. In general, the bifunctional catalyst for OER and ORR was prepared by heat treatment of NiFe LDH, which opened up the idea of preparation of bifunctional electrocatalytic materials. The chronopotentiometry curves NFL-500/CNTs and NFL650/CNTs electrodes for OER and ORR were measured at a current density of 100 mA$cm2, as shown in Fig. 4. In the OER
Table 1 e Characteristic parameters of NiO and NiFe2O4. Phase
Fig. 2 e XRD patterns of NFL-500, NFL-650 and NiFe LDH.
NiO NiFe2O4
Diffraction angle
Interplanar spacing
Cell parameters
2q（degree）
A) d111(
a( A) b ( A) c ( A)
37.25 18.40
2.410 4.820
4.174 4.174 4.174 8.348 8.378 8.348
Please cite this article in press as: Zhang Z, et al., Thermal decomposition behavior of nickel-iron hydrotalcite and its electrocatalytic properties of oxygen reduction and oxygen evolution reactions, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.09.153
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e5
Fig. 3 e Polarization curves of NiFe LDH/CNTs, NFL-500/ CNTs and NFL-650/CNTs electrodes.
Table 2 e Potential of NiFe LDH/CNTs, NFL-500/CNTs and NFL-650/CNTs electrodes at different current density for OER. j (mA$cm2)
20 50 100 200
Potential (V vs. RHE) NiFe LDH/ CNTs
NFL-500/ CNTs
NFL-650/ CNTs
1.404 1.444 1.482 1.542
1.421 1.499 1.551 1.620
1.498 1.536 1.576 1.646
process, the potentials of the NFL-650/CNTs and NFL-500/ CNTs electrodes were 1.576 V and 1.550 V, respectively. After 2.5 h, the potentials were 1.577 V and 1.551 V, and the over potentials of the two electrodes only increased with ~1 mV, the variation is very small, indicating that the two electrodes have high stability for OER. In the ORR process, the potentials of the NFL-650/CNTs and NFL-500/CNTs electrodes were 0.681 V and 0.592 V, respectively. After 2.5 h, the potential were 0.679 V, 0.589 V; the over potential of the NFL-650/CNTs electrode only increased by ~2 mV; the over potential of the NFL-500/CNTs electrode only increased by ~3 mV, and the potential change was also very small, indicating that the two electrodes also have high stability in the ORR. Comparing with the NiFe LDH precursor, the NiO/FeOx (NFL-500) composite oxide and the spinel NiO/NiFe2O4(NFL650) composite oxide not only manifest good OER activity, but
Table 3 e Potential of NiFe LDH/CNTs, NFL-500/CNTs and NFL-650/CNTs electrodes at different current density for ORR. j (mA$cm2)
20 50 100
Potential（V vs. RHE） NiFe LDH/ CNTs
NFL-500/ CNTs
NFL-650/ CNTs
0.708 0.572
0.780 0.719 0.592
0.797 0.753 0.681
Fig. 4 e Chronopotentiometry curves of NFL-500/CNTs and NFL-650/CNTs electrodes.
also good ORR activity. The OER activity of NiO/FeOx (NFL-500) composite oxide is higher. Meanwhile, the ORR activity of NiO/ NiFe2O4(NFL-650) is the best. Moreover, the two materials also have excellent OER and ORR stability and are good dualfunctional catalytic materials.
Conclusion In summary, the thermal decomposition behavior of NiFe LDH materials was analyzed, and the electrocatalytic properties for ORR and OER of the calcined products were studied. The TGDSC analysis showed that the thermal decomposition behavior of NiFe LDH material was mainly divided into three stages. The calcined product at 500 C was mainly NiO/FeOx composite oxide, of which FeOx was amorphous oxide; the calcined product at 650 C was mainly NiO/NiFe2O4 composite oxide. The electrochemical analysis exhibited that the NiO/ FeOx (NFL-500) composite oxide and the NiO/NiFe2O4(NFL-650) composite oxide had excellent and stable OER and ORR activity.
references
[1] Prakash J, Tryk DA, Yeager EB. Kinetic investigations of oxygen reduction and evolution reactions on lead ruthenate catalysts. J Electrochem Soc 1999;146(11):4145e51. [2] Ten Kortenaar MV, Vente JF, Ijdo DJW, Muller S, Kotz R. Oxygen evolution and reduction on iridium oxide compounds. J Power Sources 1995;56(1):51e60. [3] Yun SK, Pinnavaia TJ. Layered double hydroxides intercalated by polyoxometalate anions with Keggin (aH2W12O406-), Dawson (a-P2W18O626-), and Finke (Co4(H2O)2 (PW9O34210-) structures. Inorg Chem 1996;35(23):6853e60. [4] Dixit M, Mishra M, Joshi PA, Shah DO. Physico-chemical and catalytic properties of Mg-Al hydrotalcite and Mg-Al mixed oxide supported copper catalysts. J Ind Eng Chem 2013;19(2):458e68. [5] Jin L, Liu Q, Sun Z, Ni X, Wei M. Preparation of 5-fluorouracil/ b-cyclodextrin complex intercalated in layered double hydroxide and the controlled drug release properties. Ind Eng Chem Res 2010;49(22):11176e81.
Please cite this article in press as: Zhang Z, et al., Thermal decomposition behavior of nickel-iron hydrotalcite and its electrocatalytic properties of oxygen reduction and oxygen evolution reactions, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.09.153
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e5
[6] Yu XY, Luo T, Jia Y, Xu RX, Gao C, Zhang YX, et al. Threedimensional hierarchical flower-like Mg-Al-layered double hydroxides: highly efficient adsorbents for as (V) and Cr(VI) removal. Nanoscale 2012;4(11):3466e74. [7] Choudary BM, Madhi S, Chowdari NS, Kantam ML, Sreedhar B. Layered double hydroxide supported nanopalladium catalyst for Heck-, Suzuki-, Sonogashira-, and Stille-type coupling reactions of chloroarenes. J Am Chem Soc 2002;124(47):14127e36. [8] Gong M, Li Y, Wang H, Liang Y, Wu JZ, Zhou J, et al. An advanced Ni-Fe layered double hydroxide electrocatalyst for water oxidation. J Am Chem Soc 2013;135(23):8452e5. [9] Li Y, Gong M, Liang Y, Feng J, Kim JE, Wang H, et al. Advanced zinc-air batteries based on high-performance hybrid electrocatalysts. Nat Commun 2013;4:1805. [10] MacKenzie KJD, Meinhold RH, Sherriff BL, Xu Z. 27Al and 25Mg solid-state magic-angle spinning nuclear magnetic resonance study of hydrotalcite and its thermal
[11]
[12]
[13]
[14] [15]
5
decomposition sequence. J Mater Chem 1993;3(12): 1263e9. Hudson MJ, Carlino S, Apperley DC. Thermal conversion of a layered (Mg/Al) double hydroxide to the oxide. J Mater Chem 1995;5(2):323e9. Cardoso LP, Valim JB. Competition between three organic anions during regeneration process of calcined LDH. J Phys Chem Solid 2004;65(2):481e5. Unnikrishnan R, Narayanan S. Metal containing layered double hydroxides as efficient catalyst precursors for the selective conversion of acetone. J Mol Catal Chem 1999;144(1):173e9. Adzic R. Recent advances in the kinetics of oxygen reduction. New York: Wiley-VCH; 1998. Subbaraman R, Tripkovic D, Chang KC, Strmcnik D, Paulikas AP, Hirunsit P, et al. Trends in activity for the water electrolyser reactions on 3d M (Ni, Co, Fe, Mn) hydr (oxy) oxide catalysts. Nat Mater 2012;11(6):550e7.
Please cite this article in press as: Zhang Z, et al., Thermal decomposition behavior of nickel-iron hydrotalcite and its electrocatalytic properties of oxygen reduction and oxygen evolution reactions, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.09.153