- Email: [email protected]
Electroless Deposition of Nickel-Cobalt Nanoparticles for Hydrogen Evolution Reaction
Available online at www.sciencedirect.com
ScienceDirect www.materialstoday.com/proceedings Materials Today: Proceedings 22 (2020) 268–274
2018 2nd International Conference on Nanomaterials and Biomaterials, ICNB 2018, 10–12 December 2018, Barcelona, Spain
Electroless Deposition of Nickel-Cobalt Nanoparticles for Hydrogen Evolution Reaction Luigi A. Dahonog and Mary Donnabelle L. Balela* Sustainable Electronic Materials Group, Department of Mining, Metallurgical and Materials Engineering, University of the Philippines Diliman, Quezon City, Philippines 1101
Abstract Spherical nickel-cobalt (Ni-Co) nanoparticles with an average particle size of 53.16 nm were formed by a low temperature electroless deposition (70 °C) in ethylene glycol. A minute amount of chloroplatinic acid (H2PtCl6) was added to facilitate heterogeneous nucleation on the surface of Pt nanoparticles. XRD analysis revealed the formation of bimetallic Ni-Co nanoparticles instead of an alloy. EDX analysis showed that the nanoparticles are 49.47 wt% Ni and 50.53 wt% Co, which are close to the initial amounts. The bimetallic Ni-Co nanoparticles were tested for hydrogen evolution reaction (HER) activity. At a current density of 1 and 10 mA/cm2, the Ni-Co nanoparticles exhibited an overpotential of 77.28 and 430.65 mV vs. RHE, respectively, in 1 M KOH. © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the 2018 2nd International Conference on Nanomaterials and Biomaterials. Keywords: nickel; cobalt; nanoparticles; hydrogen evolution reaction
* Corresponding author. Tel.: +63-02-9818500 local 3171 ; fax: +63-02-981-8500 loc. 3171. E-mail address: [email protected] 1876-6102 © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the 2018 2nd International Conference on Nanomaterials and Biomaterials.
L.A. Dahonog and M.D.L. Balela / Materials Today: Proceedings 22 (2020) 268–274
269
Nomenclature Ni HER Co LSV
nickel hydrogen evolution reaction cobalt linear sweep voltammetry
1. Introduction Different technologies are currently being developed to alleviate the issues regarding energy sources. Considered to be an efficient energy source, hydrogen has been produced by coal gasification [1], biomass pyrolysis [2] and electrolysis [3-6]. Among these, electrolysis or electrochemical water splitting is recognized to be an effective green method to separate hydrogen and oxygen from water. Two important half reactions are involved in water electrolysis. Oxygen evolution reaction (OER) occurs at the anode while hydrogen evolution reaction (HER) at the cathode [3]. Current researches focus on the development of catalysts which can reduce the overpotential and improve the efficiency of HER at the cathode. Owing to their high efficiencies, platinum (Pt) – based compounds are commonly used as catalysts for HER [7-9]. However, their high costs and low abundance limits their use in largescale productions and commercialization. Among non-noble metals, nickel (Ni) is considered to have an adsorption energy for hydrogen close to Pt [10, 11]. It was also identified that Ni alloys showed better electrocatalytic activity towards HER in alkaline media compared to Ni metal alone [10].Synergistic effects of different phases improved the catalytic activity of Ni in alkaline media. In fact, Ni alloyed with Cu [12, 13], Mo [14], Fe [15] and Co [16] have already been studied by other researchers for HER. In addition, smaller particle size would result to an increase in the number of active catalytic sites. Thus, controlling the size of the catalyst is an effective strategy to enhance the HER activity. Metallic nanomaterials have been synthesized using different approaches. Others have performed ball milling [17], chemical vapor deposition (CVD) [18], polyol technique [19] and simple chemical reduction method or electroless deposition [20-25]. Among these, chemical reduction from metallic salts using a strong reducing agent is the most effective way of preparing metallic nanoparticles. In this work, Ni-Co nanoparticles were synthesized using electroless deposition at 70 °C in ethylene glycol with hydrazine as the reducing agent. Very small amount of chloroplatinic acid was added to introduce minute Pt nanoparticles in solution that could serve as nucleation sites for the Ni-Co nanoparticles. In addition, it might be interesting to see any contributing effect of Pt on the HER activity of the bimetallic Ni-Co. 2. Methodology 2.1. Materials Solutions were prepared using nickel chloride (NiCl2, Fluka Chemika) and cobalt chloride hexahydrate(CoCl2 · 6H2O, Vetec) as sources of Ni and Co, respectively. Hydrazine monohydrate (N2H4, 98%, Sigma Aldrich), sodium hydroxide (NaOH, Macron Fine Chemicals) and chloroplatinic acid (H2PtCl6, Sigma Aldrich) were used as reducing agent, OH- source and nucleating agent, respectively. Ethylene glycol was used as the solvent. 2.2. Synthesis First, 27 mL ethylene glycol containing 0.10 M metallic salts (0.05 M NiCl2 + 0.05 M CoCl2), 0.30 M NaOH and 0.30 mM H2PtCl6 was prepared. A reducing agent solution was prepared by adding 1.0 mL N2H4 and 0.10 M NaOH in 27 mL ethylene glycol. Both solutions were heated at 70 °C before mixing. The reaction was then kept at 70 °C for 1 h. After the reaction, the samples were magnetically collected and washed several times with ethanol. Pure Ni and Co nanoparticles were also prepared separately using the method described above.
270
L.A. Dahonog and M.D.L. Balela / Materials Today: Proceedings 22 (2020) 268–274
2.3. Characterization The morphology of the prepared samples was observed using scanning electron microscope (SEM, Hitachi SU8230). Elemental analysis was performed using energy dispersive X-ray spectrometer (EDX, Phenom XL). The phase composition and crystalline structure were determined using X-Ray diffraction (XRD). Linear sweep voltammetry (LSV) was performed to evaluate the electrochemical behavior of the samples in 1.00 M potassium hydroxide (KOH) at 1.00 mV/s in the potential range of 0 to -0.6 V (vs RHE, reversible hydrogen electrode). A three-electrode set-up was used in the experiment. A double junction silver/silver chloride (Ag/AgCl, Metrohm) immersed in 3.00 M potassium chloride (KCl) and platinum (Pt) wire were employed as the reference and counter electrode, respectively. The working electrode was prepared by dispersing the as-prepared powders (10 mg) in few drops of ethanol to form a slurry. The slurry was then pasted on a 1.0 cm2 carbon fiber paper resulting to a catalyst loading of 0.5 mg/cm2. Measured potentials (vs. Ag/AgCl) were converted RHE using the Nernst equation given below: ERHE = E (vs. Ag/AgCl) + E°Ag/AgCl + 0.059pH
(1)
where ERHE is the converted potential, E(vs. Ag/AgCl) is the measured potential, and EoAg/AgCl is 0.1967 V at 25 oC. 3. Results and discussion 3.1. Characterization of Ni-Co nanoparticles Figure 1 shows the SEM images of the Ni, Co and Ni-Co nanoparticles produced after electroless deposition in ethylene glycol. All samples appear agglomerated, which is due to their inherent magnetic properties. It is known that both Ni and Co are ferromagnetic at room temperature. As seen in Fig. 1a-b, the pure Ni and Co nanoparticles have an average diameter of 20.57 and 24.10 nm, respectively. On the other, Fig. 1c shows that the Ni-Co nanoparticles have relatively larger particle size of about 53.16 nm compared to pure Ni and Co. It is possible that growth was favored during the formation of the bimetallic nanoparticles as indicated by their large particle size. The average particle size was determined by image analysis using Image J from several SEM images. Then again, the calculated average sizes for all samples are all less than 100 nm, indicating suggesting high surface area. This suggests that chloroplatinic acid effectively provided small nucleation sites for the growth of the Ni, Co and Ni-Co nanoparticles [21-23]. Pt ions are more easily reduced in solution compared to Ni and Co ions. Subsequently, Pt nanoparticles were first formed in the solution and act as heterogenous seeds for the growth of Ni and Co. The EDX spectrum of the Ni-Co nanoparticles is presented in Fig. 1c. EDX analysis showed the presence of Ni, Co, oxygen (O) and silicon (Si) in the samples. Si is attributed to the substrate used during the analysis, while O is due to the presence of surface oxides and adsorbed water on the nanoparticles. Without considering the weight concentrations of O and Si, the bimetallic sample is composed of 49.47 wt% Ni and 50.53 wt% Co, which is near to the initial mole ratio of Co and Ni before electroless deposition. Pt was not detected EDX, which can be due to its very low concentration. The XRD patterns of the nanoparticles formed after electroless deposition in ethylene glycol are shown in Fig. 2. Characteristic peaks of Ni at 2θ = 44.5 and 51.60° were identified from the diffraction pattern in Fig. 2a. These peaks correspond to the (111) and (200) planes of face centered cubic (fcc) Ni, respectively. Unlike the Ni nanoparticles, no distinct peaks were observed for Co nanoparticles as shown in Fig. 2b. This suggests that the Co nanoparticles formed were possibly amorphous. On the other hand, the XRD pattern of the Ni-Co nanoparticles showed overlapping peaks attributed to both fcc Ni and Co. In fact, the peak at 2θ = 43.34° showed splitting, possibly due to the overlapping of the (111) fcc Ni and (111) fcc Co. Additionally, a very broad peak at 2θ = 50.67° could be due to both (200) fcc Ni and (200) fcc Co. The presence of separate Ni and Co peaks indicates that alloying of Ni and Co was not successful possibly due to the low reaction temperature and the lack of post treatment. Interestingly, the XRD pattern shows that the fcc structure is the preferred structure for the bimetallic nanoparticles.
L.A. Dahonog and M.D.L. Balela / Materials Today: Proceedings 22 (2020) 268–274
271
It is possible that Ni promotes the formation of the fcc Co even though the thermodynamically stable phase at room temperature is hexagonal close-packed.
Fig. 1. Corresponding SEM images of (a) Ni, (b) Co and (c) Ni-Co nanoparticles, and (d) EDX spectra of Co-Ni nanoparticles formed by electroless deposition in ethylene glycol.
Fig. 2. XRD pattern of the (a) Ni, (b) Co and (c) Ni-Co nanoparticles formed by electroless deposition in ethylene glycol.
3.2. Electrochemical characterization of Ni-Co nanoparticles Figure 3 shows the electrochemical activity of the as-prepared nanoparticles formed by electroless deposition in ethylene glycol. Bare carbon fiber paper was also tested for HER. From the graph, it was observed that the carbon fiber paper showed little catalytic activity in the potential window experimented (0 to -0.6 V vs. RHE), having a
272
L.A. Dahonog and M.D.L. Balela / Materials Today: Proceedings 22 (2020) 268–274
maximum current density of 8 mA/cm2. This is low compared to the current density obtained when the metal nanoparticles were deposited on the carbon fiber paper. At a current density of 1 mA/cm2, the Ni, Co and Ni-Co nanoparticle catalysts exhibited overpotentials of 76.52, 130.28 and 77.28 mV, respectively. The corresponding Tafel slope is 204.92, 184.47 and 166.89 mV/dec. Although Ni showed the lowest overpotential which can be attributed to its particle size, Ni-Co showed the lowest tafel slope. Co, on the other, has small particle size but it should be noted that the catalytic properties of Co are worse than of Ni. This shows that Ni-Co is the most active among the three leading to an enhanced HER rate at moderately increased overpotential. Lupi showed that Co concentrations ranging between 41 and 64 wt% are best to obtain high current density [16]. In this work, 50.53% Co was present in the sample. This shows that the synergism among the catalytic properties of Ni and the high hydrogen adsorption property of Co is best for HER. On the other hand, considering the solar-to-hydrogen efficiency, the Ni, Co and Ni-Co electrode achieved an overpotential of 333.82, 333.55 and 430.65 mV vs. RHE at 10 mA/cm2. 10 mA/cm2 reflects the 10% efficient solar-to-fuel conversion device. The stability of the Ni-Co electrodes was tested through multiple cyclic voltammetries. The polarization curves of the sample before and after 500 cycles were compared. After 500 cycles, the overpotential of the Ni-Co electrode was found to be 67.20 mV vs. RHE at 1 mA/cm2. The Ni-Co electrode showed a decrease in onset overpotential after 500 cycles. The improved performance after 500 cycles suggests that some of the bimetallic electrocatalysts are being activated during use. It is possible that oxide layers were already present on the surface of the electrode during the initial polarization curve. These oxide layers were possibly amorphous, which explains why they were not detected by XRD. It can be observed that cathodic currents are also present before HER, which could be due to the reduction of these amorphous surface oxides In general, the LSV curve of the Ni-Co electrode was shifted positively after 500 cycles. Higher current density was also observed for the Ni-Co electrode after long-term cycling. Consequently, the Tafel slope of the Ni-Co electrode after multiple cycles was slightly improved. In theory, Tafel slopes for HER catalyst have values ranging between 30 to 120 mV/dec [16]. A Tafel slope of 160.08 mV/dec may suggests that the reduction of the surface oxides affected the reaction. One problem during hydrogen evolution is the formation of tiny gas bubbles on the surface of the catalyst. This possibly limits the mass transport and the active surface area to participate in the reaction.
Fig. 3. Corresponding (a) LSV curves and (b) tafel slopes of the as-prepared catalysts in 1 M KOH ; (c) LSV curves and (d) tafel slope of the Co-Ni electrode before and after 500 cycles.
L.A. Dahonog and M.D.L. Balela / Materials Today: Proceedings 22 (2020) 268–274
273
4. Conclusion In summary, bimetallic Ni-Co nanoparticles were formed by electroless deposition in ethylene glycol in the presence of chloroplatinic acid as nucleating agent. The Ni-Co nanoparticles are spherical with an average diameter of 56.13 nm. Since Ni and Co nanoparticles are ferromagnetic, the nanoparticles were agglomerated. XRD revealed that the bimetallic nanoparticles are composed of separate Ni and Co nanoparticles. When used as catalyst for hydrogen evolution reaction, individual Ni and Co as well as bimetallic Ni-Co nanoparticles exhibited an overpotentials of 76.52, 130.28 and 77.28 mV vs. RHE, respectively. Among the three, the bimetallic catalyst showed the lowest tafel slope of 166.89 mV/dec. This was shifted positively after 500 cycles indicating an improved performance of the catalyst. The synergistic effect of using bimetallic catalyst showed stronger activity compared to single phase catalysts. Acknowledgments This study is supported by the Commission on Higher Education under CHED-Newton Agham Institutional Links for the project “Affordable Electrolyzer Technology Based on Transition Metal Catalyst for Energy Storage” Project start date: 25 October 2017. The authors also thank the College of Engineering, University of the Philippines Dilliman for the support. References [1] Gnanapragasam, Nirmal V., Bale V. Reddy, and Marc A. Rosen. "Hydrogen production from coal gasification for effective downstream CO2 capture." International Journal of Hydrogen Energy 35(10) (2010): 4933-4943 [2] Jiang, Hongtao, Yeru Wu, Hao Fan, and Jianbing Ji. "Hydrogen Production from Biomass Pyrolysis in Molten Alkali." AASRI Procedia 3 (2012): 217-223 [3] Santos, Diogo M., César A. Sequeira, and José L. Figueiredo. "Hydrogen production by alkaline water electrolysis." Química Nova 36(8) (2013):1176-1193 [4] Ahn, Sang H., Byung-Seok Lee, Insoo Choi, Sung J. Yoo, Hyoung-Juhn Kim, EunAe Cho, Dirk Henkensmeier, Suk W. Nam, Soo-Kil Kim, and Jong H. Jang. "Development of a membrane electrode assembly for alkaline water electrolysis by direct electrodeposition of nickel on carbon papers." Applied Catalysis B: Environmental 154-155 (2014):197-205 [5] Ahn, Sang H., Sung J. Yoo, Hyoung-Juhn Kim, Dirk Henkensmeier, Suk W. Nam, Soo-Kil Kim, and Jong H. Jang. "Anion exchange membrane water electrolyzer with an ultra-low loading of Pt-decorated Ni electrocatalyst." Applied Catalysis B: Environmental 180 (2016): 674-679 [6] Dahonog, Luigi A., and Mary D. Balela. "Hydrothermal Synthesis of NiCo2O4 Nanowires on Carbon Fiber Paper for Hydrogen Evolution Catalyst." Key Engineering Materials 775 (2018): 139-143 [7] An, Li, Liang Huang, Panpan Zhou, Jie Yin, Hongyan Liu, and Pinxian Xi. "A Self-Standing High-Performance Hydrogen Evolution Electrode with Nanostructured NiCo2O4/CuS Heterostructures." Advanced Functional Materials 25(43) (2015): 6814-6822 [8] Gao, M.Y., C. Yang, Q.B. Zhang, Y.W. Yu, Y.X. Hua, Y. Li, and P. Dong. "Electrochemical fabrication of porous Ni-Cu alloy nanosheets with high catalytic activity for hydrogen evolution." Electrochimica Acta 215 (2016): 609-616 [9] Li, Kui, Yang Li, Yuemin Wang, Junjie Ge, Changpeng Liu, and Wei Xing. "Enhanced electrocatalytic performance for the hydrogen evolution reaction through surface enrichment of platinum nanoclusters alloying with ruthenium in situ embedded in carbon." Energy & Environmental Science 11(5) (2018): 1232-1239 [10] Tao, Shasha, Florent Yang, Jona Schuch, Wolfram Jaegermann, and Bernhard Kaiser. "Electrodeposition of Nickel Nanoparticles for the Alkaline Hydrogen Evolution Reaction: Correlating Electrocatalytic Behavior and Chemical Composition." ChemSusChem 11(5) (2018): 948-958 [11] Sun, Qiangqiang, Yujuan Dong, Zenglin Wang, Shiwei Yin, and Chuan Zhao. "Synergistic Nanotubular Copper-Doped Nickel Catalysts for Hydrogen Evolution Reactions." Small 14(14) (2018): 1704137, doi:10.1002/smll.201704137 [12] Yin, Zuwei, and Fuyi Chen. "A facile electrochemical fabrication of hierarchically structured nickel–copper composite electrodes on nickel foam for hydrogen evolution reaction." Journal of Power Sources 265 (2014): 273-281 [13] Shen, Yi, Yongfang Zhou, Duo Wang, Xi Wu, Jia Li, and Jingyu Xi. "Nickel-Copper Alloy Encapsulated in Graphitic Carbon Shells as Electrocatalysts for Hydrogen Evolution Reaction." Advanced Energy Materials 8(2) (2017): 1701759, doi:10.1002/aenm.201701759 [14] Fang, Ming, Wei Gao, Guofa Dong, Zhaoming Xia, SenPo Yip, Yuanbin Qin, Yongquan Qu, and Johnny C. Ho. "Hierarchical NiMo-based 3D electrocatalysts for highly-efficient hydrogen evolution in alkaline conditions." Nano Energy 27 (2016): 247-254 [15] Du, Yunmei, Zijian Li, Yanru Liu, Yu Yang, and Lei Wang. "Nickel-iron phosphides nanorods derived from bimetallic-organic frameworks for hydrogen evolution reaction." Applied Surface Science 457 (2018): 1081-1086 [16] Lupi, C., A. Dell'Era, and M. Pasquali. "Nickel–cobalt electrodeposited alloys for hydrogen evolution in alkaline media." International Journal of Hydrogen Energy 34(5) (2009): 2101-2106
274
L.A. Dahonog and M.D.L. Balela / Materials Today: Proceedings 22 (2020) 268–274
[17] Ban, Irena, Janja Stergar, Miha Drofenik, Gregor Ferk, and Darko Makovec. "Synthesis of copper–nickel nanoparticles prepared by mechanical milling for use in magnetic hyperthermia." Journal of Magnetism and Magnetic Materials 323(17) (2011): 2254-2258 [18] Moravec, Pavel, Jiří Smolík, Helmi Keskinen, Jyrki M. Mäkelä, Snejana Bakardjieva, and Valeri V. Levdansky. "NiOx Nanoparticle Synthesis by Chemical Vapor Deposition from Nickel Acetylacetonate." Materials Sciences and Applications 02(04) (2011): 258-264 [19] Carroll, Kyler J., J. U. Reveles, Michael D. Shultz, Shiv N. Khanna, and Everett E. Carpenter. "Preparation of Elemental Cu and Ni Nanoparticles by the Polyol Method: An Experimental and Theoretical Approach." The Journal of Physical Chemistry C 115(6) (2011): 2656-2664 [20] Kawamori, Makoto, Shunsuke Yagi, and Eiichiro Matsubara. "Nickel Alloying Effect on Formation of Cobalt Nanoparticles and Nanowires via Electroless Deposition under a Magnetic Field." Journal of The Electrochemical Society 159(2) (2011): E37-E44, doi:10.1149/2.062202jes. [21] Balela, Mary D., Lalaine M. Dulin, Erica A. Garcia, and M. J. Tica. "Synthesis and Characterization of Cobalt-Nickel Nanowires Formed under External Magnetic Field." Applied Mechanics and Materials 799-800 (2015): 120-124, doi:10.4028/www.scientific.net/amm.799800.120. [22] Balela, Mary D., Shunsuke Yagi, and Eiichiro Matsubara. "Room-Temperature Synthesis of Cobalt Nanoparticles by Electroless Deposition in Aqueous Solution." Electrochemical and Solid-State Letters 13 (2) (2010): D4, doi:10.1149/1.3265525. [23] Balela, Mary D., Shunsuke Yagi, and Eiichiro Matsubara. "Electroless Deposition of Cobalt Nanowires in an Aqueous Solution under External Magnetic Field." Electrochemical and Solid-State Letters 14(6) (2011): D68, doi:10.1149/1.3568829. [24] Balela, Mary D., and Kathy L. Amores. "Electroless deposition of copper nanoparticle for antimicrobial coating." Materials Chemistry and Physics 225 (2019): 393-398 [25] De Guzman, Nathaniel, and Mary D. Balela. "Growth of Ultralong Ag Nanowires by Electroless Deposition in Hot Ethylene Glycol for Flexible Transparent Conducting Electrodes." Journal of Nanomaterials 2017 (2017): 1-14, doi:10.1155/2017/7896094.
ScienceDirect www.materialstoday.com/proceedings Materials Today: Proceedings 22 (2020) 268–274
2018 2nd International Conference on Nanomaterials and Biomaterials, ICNB 2018, 10–12 December 2018, Barcelona, Spain
Electroless Deposition of Nickel-Cobalt Nanoparticles for Hydrogen Evolution Reaction Luigi A. Dahonog and Mary Donnabelle L. Balela* Sustainable Electronic Materials Group, Department of Mining, Metallurgical and Materials Engineering, University of the Philippines Diliman, Quezon City, Philippines 1101
Abstract Spherical nickel-cobalt (Ni-Co) nanoparticles with an average particle size of 53.16 nm were formed by a low temperature electroless deposition (70 °C) in ethylene glycol. A minute amount of chloroplatinic acid (H2PtCl6) was added to facilitate heterogeneous nucleation on the surface of Pt nanoparticles. XRD analysis revealed the formation of bimetallic Ni-Co nanoparticles instead of an alloy. EDX analysis showed that the nanoparticles are 49.47 wt% Ni and 50.53 wt% Co, which are close to the initial amounts. The bimetallic Ni-Co nanoparticles were tested for hydrogen evolution reaction (HER) activity. At a current density of 1 and 10 mA/cm2, the Ni-Co nanoparticles exhibited an overpotential of 77.28 and 430.65 mV vs. RHE, respectively, in 1 M KOH. © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the 2018 2nd International Conference on Nanomaterials and Biomaterials. Keywords: nickel; cobalt; nanoparticles; hydrogen evolution reaction
* Corresponding author. Tel.: +63-02-9818500 local 3171 ; fax: +63-02-981-8500 loc. 3171. E-mail address: [email protected] 1876-6102 © 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the 2018 2nd International Conference on Nanomaterials and Biomaterials.
L.A. Dahonog and M.D.L. Balela / Materials Today: Proceedings 22 (2020) 268–274
269
Nomenclature Ni HER Co LSV
nickel hydrogen evolution reaction cobalt linear sweep voltammetry
1. Introduction Different technologies are currently being developed to alleviate the issues regarding energy sources. Considered to be an efficient energy source, hydrogen has been produced by coal gasification [1], biomass pyrolysis [2] and electrolysis [3-6]. Among these, electrolysis or electrochemical water splitting is recognized to be an effective green method to separate hydrogen and oxygen from water. Two important half reactions are involved in water electrolysis. Oxygen evolution reaction (OER) occurs at the anode while hydrogen evolution reaction (HER) at the cathode [3]. Current researches focus on the development of catalysts which can reduce the overpotential and improve the efficiency of HER at the cathode. Owing to their high efficiencies, platinum (Pt) – based compounds are commonly used as catalysts for HER [7-9]. However, their high costs and low abundance limits their use in largescale productions and commercialization. Among non-noble metals, nickel (Ni) is considered to have an adsorption energy for hydrogen close to Pt [10, 11]. It was also identified that Ni alloys showed better electrocatalytic activity towards HER in alkaline media compared to Ni metal alone [10].Synergistic effects of different phases improved the catalytic activity of Ni in alkaline media. In fact, Ni alloyed with Cu [12, 13], Mo [14], Fe [15] and Co [16] have already been studied by other researchers for HER. In addition, smaller particle size would result to an increase in the number of active catalytic sites. Thus, controlling the size of the catalyst is an effective strategy to enhance the HER activity. Metallic nanomaterials have been synthesized using different approaches. Others have performed ball milling [17], chemical vapor deposition (CVD) [18], polyol technique [19] and simple chemical reduction method or electroless deposition [20-25]. Among these, chemical reduction from metallic salts using a strong reducing agent is the most effective way of preparing metallic nanoparticles. In this work, Ni-Co nanoparticles were synthesized using electroless deposition at 70 °C in ethylene glycol with hydrazine as the reducing agent. Very small amount of chloroplatinic acid was added to introduce minute Pt nanoparticles in solution that could serve as nucleation sites for the Ni-Co nanoparticles. In addition, it might be interesting to see any contributing effect of Pt on the HER activity of the bimetallic Ni-Co. 2. Methodology 2.1. Materials Solutions were prepared using nickel chloride (NiCl2, Fluka Chemika) and cobalt chloride hexahydrate(CoCl2 · 6H2O, Vetec) as sources of Ni and Co, respectively. Hydrazine monohydrate (N2H4, 98%, Sigma Aldrich), sodium hydroxide (NaOH, Macron Fine Chemicals) and chloroplatinic acid (H2PtCl6, Sigma Aldrich) were used as reducing agent, OH- source and nucleating agent, respectively. Ethylene glycol was used as the solvent. 2.2. Synthesis First, 27 mL ethylene glycol containing 0.10 M metallic salts (0.05 M NiCl2 + 0.05 M CoCl2), 0.30 M NaOH and 0.30 mM H2PtCl6 was prepared. A reducing agent solution was prepared by adding 1.0 mL N2H4 and 0.10 M NaOH in 27 mL ethylene glycol. Both solutions were heated at 70 °C before mixing. The reaction was then kept at 70 °C for 1 h. After the reaction, the samples were magnetically collected and washed several times with ethanol. Pure Ni and Co nanoparticles were also prepared separately using the method described above.
270
L.A. Dahonog and M.D.L. Balela / Materials Today: Proceedings 22 (2020) 268–274
2.3. Characterization The morphology of the prepared samples was observed using scanning electron microscope (SEM, Hitachi SU8230). Elemental analysis was performed using energy dispersive X-ray spectrometer (EDX, Phenom XL). The phase composition and crystalline structure were determined using X-Ray diffraction (XRD). Linear sweep voltammetry (LSV) was performed to evaluate the electrochemical behavior of the samples in 1.00 M potassium hydroxide (KOH) at 1.00 mV/s in the potential range of 0 to -0.6 V (vs RHE, reversible hydrogen electrode). A three-electrode set-up was used in the experiment. A double junction silver/silver chloride (Ag/AgCl, Metrohm) immersed in 3.00 M potassium chloride (KCl) and platinum (Pt) wire were employed as the reference and counter electrode, respectively. The working electrode was prepared by dispersing the as-prepared powders (10 mg) in few drops of ethanol to form a slurry. The slurry was then pasted on a 1.0 cm2 carbon fiber paper resulting to a catalyst loading of 0.5 mg/cm2. Measured potentials (vs. Ag/AgCl) were converted RHE using the Nernst equation given below: ERHE = E (vs. Ag/AgCl) + E°Ag/AgCl + 0.059pH
(1)
where ERHE is the converted potential, E(vs. Ag/AgCl) is the measured potential, and EoAg/AgCl is 0.1967 V at 25 oC. 3. Results and discussion 3.1. Characterization of Ni-Co nanoparticles Figure 1 shows the SEM images of the Ni, Co and Ni-Co nanoparticles produced after electroless deposition in ethylene glycol. All samples appear agglomerated, which is due to their inherent magnetic properties. It is known that both Ni and Co are ferromagnetic at room temperature. As seen in Fig. 1a-b, the pure Ni and Co nanoparticles have an average diameter of 20.57 and 24.10 nm, respectively. On the other, Fig. 1c shows that the Ni-Co nanoparticles have relatively larger particle size of about 53.16 nm compared to pure Ni and Co. It is possible that growth was favored during the formation of the bimetallic nanoparticles as indicated by their large particle size. The average particle size was determined by image analysis using Image J from several SEM images. Then again, the calculated average sizes for all samples are all less than 100 nm, indicating suggesting high surface area. This suggests that chloroplatinic acid effectively provided small nucleation sites for the growth of the Ni, Co and Ni-Co nanoparticles [21-23]. Pt ions are more easily reduced in solution compared to Ni and Co ions. Subsequently, Pt nanoparticles were first formed in the solution and act as heterogenous seeds for the growth of Ni and Co. The EDX spectrum of the Ni-Co nanoparticles is presented in Fig. 1c. EDX analysis showed the presence of Ni, Co, oxygen (O) and silicon (Si) in the samples. Si is attributed to the substrate used during the analysis, while O is due to the presence of surface oxides and adsorbed water on the nanoparticles. Without considering the weight concentrations of O and Si, the bimetallic sample is composed of 49.47 wt% Ni and 50.53 wt% Co, which is near to the initial mole ratio of Co and Ni before electroless deposition. Pt was not detected EDX, which can be due to its very low concentration. The XRD patterns of the nanoparticles formed after electroless deposition in ethylene glycol are shown in Fig. 2. Characteristic peaks of Ni at 2θ = 44.5 and 51.60° were identified from the diffraction pattern in Fig. 2a. These peaks correspond to the (111) and (200) planes of face centered cubic (fcc) Ni, respectively. Unlike the Ni nanoparticles, no distinct peaks were observed for Co nanoparticles as shown in Fig. 2b. This suggests that the Co nanoparticles formed were possibly amorphous. On the other hand, the XRD pattern of the Ni-Co nanoparticles showed overlapping peaks attributed to both fcc Ni and Co. In fact, the peak at 2θ = 43.34° showed splitting, possibly due to the overlapping of the (111) fcc Ni and (111) fcc Co. Additionally, a very broad peak at 2θ = 50.67° could be due to both (200) fcc Ni and (200) fcc Co. The presence of separate Ni and Co peaks indicates that alloying of Ni and Co was not successful possibly due to the low reaction temperature and the lack of post treatment. Interestingly, the XRD pattern shows that the fcc structure is the preferred structure for the bimetallic nanoparticles.
L.A. Dahonog and M.D.L. Balela / Materials Today: Proceedings 22 (2020) 268–274
271
It is possible that Ni promotes the formation of the fcc Co even though the thermodynamically stable phase at room temperature is hexagonal close-packed.
Fig. 1. Corresponding SEM images of (a) Ni, (b) Co and (c) Ni-Co nanoparticles, and (d) EDX spectra of Co-Ni nanoparticles formed by electroless deposition in ethylene glycol.
Fig. 2. XRD pattern of the (a) Ni, (b) Co and (c) Ni-Co nanoparticles formed by electroless deposition in ethylene glycol.
3.2. Electrochemical characterization of Ni-Co nanoparticles Figure 3 shows the electrochemical activity of the as-prepared nanoparticles formed by electroless deposition in ethylene glycol. Bare carbon fiber paper was also tested for HER. From the graph, it was observed that the carbon fiber paper showed little catalytic activity in the potential window experimented (0 to -0.6 V vs. RHE), having a
272
L.A. Dahonog and M.D.L. Balela / Materials Today: Proceedings 22 (2020) 268–274
maximum current density of 8 mA/cm2. This is low compared to the current density obtained when the metal nanoparticles were deposited on the carbon fiber paper. At a current density of 1 mA/cm2, the Ni, Co and Ni-Co nanoparticle catalysts exhibited overpotentials of 76.52, 130.28 and 77.28 mV, respectively. The corresponding Tafel slope is 204.92, 184.47 and 166.89 mV/dec. Although Ni showed the lowest overpotential which can be attributed to its particle size, Ni-Co showed the lowest tafel slope. Co, on the other, has small particle size but it should be noted that the catalytic properties of Co are worse than of Ni. This shows that Ni-Co is the most active among the three leading to an enhanced HER rate at moderately increased overpotential. Lupi showed that Co concentrations ranging between 41 and 64 wt% are best to obtain high current density [16]. In this work, 50.53% Co was present in the sample. This shows that the synergism among the catalytic properties of Ni and the high hydrogen adsorption property of Co is best for HER. On the other hand, considering the solar-to-hydrogen efficiency, the Ni, Co and Ni-Co electrode achieved an overpotential of 333.82, 333.55 and 430.65 mV vs. RHE at 10 mA/cm2. 10 mA/cm2 reflects the 10% efficient solar-to-fuel conversion device. The stability of the Ni-Co electrodes was tested through multiple cyclic voltammetries. The polarization curves of the sample before and after 500 cycles were compared. After 500 cycles, the overpotential of the Ni-Co electrode was found to be 67.20 mV vs. RHE at 1 mA/cm2. The Ni-Co electrode showed a decrease in onset overpotential after 500 cycles. The improved performance after 500 cycles suggests that some of the bimetallic electrocatalysts are being activated during use. It is possible that oxide layers were already present on the surface of the electrode during the initial polarization curve. These oxide layers were possibly amorphous, which explains why they were not detected by XRD. It can be observed that cathodic currents are also present before HER, which could be due to the reduction of these amorphous surface oxides In general, the LSV curve of the Ni-Co electrode was shifted positively after 500 cycles. Higher current density was also observed for the Ni-Co electrode after long-term cycling. Consequently, the Tafel slope of the Ni-Co electrode after multiple cycles was slightly improved. In theory, Tafel slopes for HER catalyst have values ranging between 30 to 120 mV/dec [16]. A Tafel slope of 160.08 mV/dec may suggests that the reduction of the surface oxides affected the reaction. One problem during hydrogen evolution is the formation of tiny gas bubbles on the surface of the catalyst. This possibly limits the mass transport and the active surface area to participate in the reaction.
Fig. 3. Corresponding (a) LSV curves and (b) tafel slopes of the as-prepared catalysts in 1 M KOH ; (c) LSV curves and (d) tafel slope of the Co-Ni electrode before and after 500 cycles.
L.A. Dahonog and M.D.L. Balela / Materials Today: Proceedings 22 (2020) 268–274
273
4. Conclusion In summary, bimetallic Ni-Co nanoparticles were formed by electroless deposition in ethylene glycol in the presence of chloroplatinic acid as nucleating agent. The Ni-Co nanoparticles are spherical with an average diameter of 56.13 nm. Since Ni and Co nanoparticles are ferromagnetic, the nanoparticles were agglomerated. XRD revealed that the bimetallic nanoparticles are composed of separate Ni and Co nanoparticles. When used as catalyst for hydrogen evolution reaction, individual Ni and Co as well as bimetallic Ni-Co nanoparticles exhibited an overpotentials of 76.52, 130.28 and 77.28 mV vs. RHE, respectively. Among the three, the bimetallic catalyst showed the lowest tafel slope of 166.89 mV/dec. This was shifted positively after 500 cycles indicating an improved performance of the catalyst. The synergistic effect of using bimetallic catalyst showed stronger activity compared to single phase catalysts. Acknowledgments This study is supported by the Commission on Higher Education under CHED-Newton Agham Institutional Links for the project “Affordable Electrolyzer Technology Based on Transition Metal Catalyst for Energy Storage” Project start date: 25 October 2017. The authors also thank the College of Engineering, University of the Philippines Dilliman for the support. References [1] Gnanapragasam, Nirmal V., Bale V. Reddy, and Marc A. Rosen. "Hydrogen production from coal gasification for effective downstream CO2 capture." International Journal of Hydrogen Energy 35(10) (2010): 4933-4943 [2] Jiang, Hongtao, Yeru Wu, Hao Fan, and Jianbing Ji. "Hydrogen Production from Biomass Pyrolysis in Molten Alkali." AASRI Procedia 3 (2012): 217-223 [3] Santos, Diogo M., César A. Sequeira, and José L. Figueiredo. "Hydrogen production by alkaline water electrolysis." Química Nova 36(8) (2013):1176-1193 [4] Ahn, Sang H., Byung-Seok Lee, Insoo Choi, Sung J. Yoo, Hyoung-Juhn Kim, EunAe Cho, Dirk Henkensmeier, Suk W. Nam, Soo-Kil Kim, and Jong H. Jang. "Development of a membrane electrode assembly for alkaline water electrolysis by direct electrodeposition of nickel on carbon papers." Applied Catalysis B: Environmental 154-155 (2014):197-205 [5] Ahn, Sang H., Sung J. Yoo, Hyoung-Juhn Kim, Dirk Henkensmeier, Suk W. Nam, Soo-Kil Kim, and Jong H. Jang. "Anion exchange membrane water electrolyzer with an ultra-low loading of Pt-decorated Ni electrocatalyst." Applied Catalysis B: Environmental 180 (2016): 674-679 [6] Dahonog, Luigi A., and Mary D. Balela. "Hydrothermal Synthesis of NiCo2O4 Nanowires on Carbon Fiber Paper for Hydrogen Evolution Catalyst." Key Engineering Materials 775 (2018): 139-143 [7] An, Li, Liang Huang, Panpan Zhou, Jie Yin, Hongyan Liu, and Pinxian Xi. "A Self-Standing High-Performance Hydrogen Evolution Electrode with Nanostructured NiCo2O4/CuS Heterostructures." Advanced Functional Materials 25(43) (2015): 6814-6822 [8] Gao, M.Y., C. Yang, Q.B. Zhang, Y.W. Yu, Y.X. Hua, Y. Li, and P. Dong. "Electrochemical fabrication of porous Ni-Cu alloy nanosheets with high catalytic activity for hydrogen evolution." Electrochimica Acta 215 (2016): 609-616 [9] Li, Kui, Yang Li, Yuemin Wang, Junjie Ge, Changpeng Liu, and Wei Xing. "Enhanced electrocatalytic performance for the hydrogen evolution reaction through surface enrichment of platinum nanoclusters alloying with ruthenium in situ embedded in carbon." Energy & Environmental Science 11(5) (2018): 1232-1239 [10] Tao, Shasha, Florent Yang, Jona Schuch, Wolfram Jaegermann, and Bernhard Kaiser. "Electrodeposition of Nickel Nanoparticles for the Alkaline Hydrogen Evolution Reaction: Correlating Electrocatalytic Behavior and Chemical Composition." ChemSusChem 11(5) (2018): 948-958 [11] Sun, Qiangqiang, Yujuan Dong, Zenglin Wang, Shiwei Yin, and Chuan Zhao. "Synergistic Nanotubular Copper-Doped Nickel Catalysts for Hydrogen Evolution Reactions." Small 14(14) (2018): 1704137, doi:10.1002/smll.201704137 [12] Yin, Zuwei, and Fuyi Chen. "A facile electrochemical fabrication of hierarchically structured nickel–copper composite electrodes on nickel foam for hydrogen evolution reaction." Journal of Power Sources 265 (2014): 273-281 [13] Shen, Yi, Yongfang Zhou, Duo Wang, Xi Wu, Jia Li, and Jingyu Xi. "Nickel-Copper Alloy Encapsulated in Graphitic Carbon Shells as Electrocatalysts for Hydrogen Evolution Reaction." Advanced Energy Materials 8(2) (2017): 1701759, doi:10.1002/aenm.201701759 [14] Fang, Ming, Wei Gao, Guofa Dong, Zhaoming Xia, SenPo Yip, Yuanbin Qin, Yongquan Qu, and Johnny C. Ho. "Hierarchical NiMo-based 3D electrocatalysts for highly-efficient hydrogen evolution in alkaline conditions." Nano Energy 27 (2016): 247-254 [15] Du, Yunmei, Zijian Li, Yanru Liu, Yu Yang, and Lei Wang. "Nickel-iron phosphides nanorods derived from bimetallic-organic frameworks for hydrogen evolution reaction." Applied Surface Science 457 (2018): 1081-1086 [16] Lupi, C., A. Dell'Era, and M. Pasquali. "Nickel–cobalt electrodeposited alloys for hydrogen evolution in alkaline media." International Journal of Hydrogen Energy 34(5) (2009): 2101-2106
274
L.A. Dahonog and M.D.L. Balela / Materials Today: Proceedings 22 (2020) 268–274
[17] Ban, Irena, Janja Stergar, Miha Drofenik, Gregor Ferk, and Darko Makovec. "Synthesis of copper–nickel nanoparticles prepared by mechanical milling for use in magnetic hyperthermia." Journal of Magnetism and Magnetic Materials 323(17) (2011): 2254-2258 [18] Moravec, Pavel, Jiří Smolík, Helmi Keskinen, Jyrki M. Mäkelä, Snejana Bakardjieva, and Valeri V. Levdansky. "NiOx Nanoparticle Synthesis by Chemical Vapor Deposition from Nickel Acetylacetonate." Materials Sciences and Applications 02(04) (2011): 258-264 [19] Carroll, Kyler J., J. U. Reveles, Michael D. Shultz, Shiv N. Khanna, and Everett E. Carpenter. "Preparation of Elemental Cu and Ni Nanoparticles by the Polyol Method: An Experimental and Theoretical Approach." The Journal of Physical Chemistry C 115(6) (2011): 2656-2664 [20] Kawamori, Makoto, Shunsuke Yagi, and Eiichiro Matsubara. "Nickel Alloying Effect on Formation of Cobalt Nanoparticles and Nanowires via Electroless Deposition under a Magnetic Field." Journal of The Electrochemical Society 159(2) (2011): E37-E44, doi:10.1149/2.062202jes. [21] Balela, Mary D., Lalaine M. Dulin, Erica A. Garcia, and M. J. Tica. "Synthesis and Characterization of Cobalt-Nickel Nanowires Formed under External Magnetic Field." Applied Mechanics and Materials 799-800 (2015): 120-124, doi:10.4028/www.scientific.net/amm.799800.120. [22] Balela, Mary D., Shunsuke Yagi, and Eiichiro Matsubara. "Room-Temperature Synthesis of Cobalt Nanoparticles by Electroless Deposition in Aqueous Solution." Electrochemical and Solid-State Letters 13 (2) (2010): D4, doi:10.1149/1.3265525. [23] Balela, Mary D., Shunsuke Yagi, and Eiichiro Matsubara. "Electroless Deposition of Cobalt Nanowires in an Aqueous Solution under External Magnetic Field." Electrochemical and Solid-State Letters 14(6) (2011): D68, doi:10.1149/1.3568829. [24] Balela, Mary D., and Kathy L. Amores. "Electroless deposition of copper nanoparticle for antimicrobial coating." Materials Chemistry and Physics 225 (2019): 393-398 [25] De Guzman, Nathaniel, and Mary D. Balela. "Growth of Ultralong Ag Nanowires by Electroless Deposition in Hot Ethylene Glycol for Flexible Transparent Conducting Electrodes." Journal of Nanomaterials 2017 (2017): 1-14, doi:10.1155/2017/7896094.