電解水製氫因為具備永續、高效率以及低汙染的特性被視為替代能源中最具發展潛力的能源之一。電解水的反應透過電壓來驅動陽極的析氧反應(OER)與陰極的析氫反應(HER)，為了降低水分解的工作電壓以及簡化系統，發展具有析氫與析氧的雙功能電極材料是重要的關鍵。一般來說，在析氫反應最有效的觸媒為鉑系金屬，而析氧反應則為銥/釕基的化合物，然而這些金屬的稀少性使他們的價格居高不下，進而限制了實用性，因此人們致力於發展非貴金屬且具備高效分解水的電觸媒材料。其中過渡金屬(鐵鈷鎳)的地球存量高且具備一定的催化能力，且透過混摻不同的元素能進一步提高電解水活性而被廣泛研究。此外，調控電觸媒材料的微結構來增加表面積，讓更多的活性位點進行反應也成為重要研究方向。
本研究利用簡易且低成本的電沉積法，首先在陽極氧化鋁模板中電鍍鈷鐵二元合金奈米線陣列，並改變溶液中的亞鐵離子濃度來調控奈米線中的鐵含量，然後在1 M KOH溶液中以循環伏安法穩定電極表面後，進行線性掃描伏安法(LSV)來評估鈷鐵合金奈米陣列達到10 mA/cm2所需過電位，結果顯示Fe0.22Co0.78之奈米線具備最佳雙功能特性，HER與OER過電位分別為177 mV與306 mV。接著進一步以共鍍方法得到鈷鐵磷合金奈米線，磷的添加能降低兩電極的過電位且不改變鐵含量對過電位影響的趨勢，在相近的鐵分率下，(Fe0.26Co0.74)0.83P0.17在HER與OER的過電位分別為147 mV與284 mV。透過SEM的觀察發現鈷鐵磷合金奈米線在OER的CV掃描下產生了大量片狀氧化物，此片狀氧化物經由TEM、XPS鑑定為氧化鐵與氧化鈷所構成，而片狀的結構能提供更多的反應活性位點應是造成OER過電位下降的主因。最後將此鈷鐵磷合金奈米陣列進行長效性雙功能測試，結果顯示僅需1.66 V即能達到10 mA/cm2，且在12個小時內維持優良的穩定性。

Electrocatalytic hydrogen production is one of the most promising alternative energy sources because of its sustainability, high efficiency and low pollution. The reaction of electrolysis drives the oxygen evolution reaction (OER) at anode and the hydrogen evolution reaction (HER) at cathode by applying a specific overpotential. In order to minimize the overpotential of water decomposition and simplify the system, it is important to develop a bifunctional electrode material that is active for catalyzing both the HER and OER reactions. In tradition, the most effective catalyst for hydrogen evolution is Pt-based metals, and the oxygen evolution reaction are Ir/Ru alloys. However, the scarcity raises the cost and limits the wide employment of these metals accordingly. Therefore, people are committed to the development of precious-metal-free and efficient electrical catalytic materials. Among them, the earth-abundant transition metals (iron/cobalt/nickel) have decent catalytic ability, which have been extensively studied by mixing different elements to further improve the electrocatalytic capability for water splitting. In addition, engineering the microstructure of the electrocatalyst for large surface area and abundant active reaction sites has also become an important research field.
In this study, a low-cost electrodeposition method has been used to electrodeposit cobalt-iron nanowires in an anodized aluminum template (AAO). The iron content in the Co-Fe nanowires is modulated by adjusting the ferrous ion concentration in the electrolyte. After stabilizing electrode in 1 M KOH solution by cyclic voltammetry (CV), a linear scanning voltammetry method (LSV) was performed to evaluate the overpotential required for the iron-cobalt nanowire array at a current density of 10 mA/cm2. The results show that Fe0.22Co0.78 has the best bifunctional performance, with HER and OER overpotentials of 177 mV and 306 mV, respectively. Then, the iron-cobalt-phosphate nanowire is further obtained by the co-plating method. The addition of phosphorus can reduce the overpotentials of both anode and cathode with the optimized iron content in the alloy nanowires. Therefore, under the similar iron fraction, the overpotential of (Fe0.26Co0. 74)0.83P0.17 for HER and OER becomes 147 mV and 284 mV, respectively. It is also found that the iron-cobalt-phosphorus nanowires exhibit a large amount of flaky oxide after the CV potential sweeping during OER reaction according to the SEM images. The flaky oxides were identified by TEM and XPS as iron oxide and cobalt oxide. These oxide sheets can provide more active reaction sites, which is responsible for the decrease of OER overpotential. Finally, the complete cell made of the same electrode material only requires a low voltage of 1.66 V for water splitting at a current density of 10 mA/cm2, and maintains excellent stability within 12 hours.

[1] J. Wang, W. Cui, Q. Liu, Z. Xing, A. M. Asiri, X. Sun, Recent Progress in Cobalt-Based Heterogeneous Catalysts for Electrochemical Water Splitting. Adv. Mater. 28, 215-230 (2016).
[2] C. M. White, R. R. Steeper, A. E. Lutz, The hydrogen-fueled internal combustion engine: a technical review. Int. J. Hydrogen Energy 31, 1292-1305 (2006).
[3] C. Tang, M.-M. Titirici, Q. Zhang, A review of nanocarbons in energy electrocatalysis: Multifunctional substrates and highly active sites. J. Energy Chem. 26, 1077-1093 (2017).
[4] H. Kind, H. Yan, B. Messer, M. Law, P. Yang, Nanowire ultraviolet photodetectors and optical switches. Adv. Mater. 14, 158-160 (2002).
[5] J. Luo, L. Steier, M.-K. Son, M. Schreier, M. T. Mayer, M. Grätzel, Cu2O nanowire photocathodes for efficient and durable solar water splitting. Nano Lett. 16, 1848-1857 (2016).
[6] B. Xiang, P. Wang, X. Zhang, S. A. Dayeh, D. P. Aplin, C. Soci, D. Yu, D. Wang, Rational synthesis of p-type zinc oxide nanowire arrays using simple chemical vapor deposition. Nano Lett. 7, 323-328 (2007).
[7] D. Zhang, S. W. Eaton, Y. Yu, L. Dou, P. Yang, Solution-phase synthesis of cesium lead halide perovskite nanowires. J. Am. Chem. Soc. 137, 9230-9233 (2015).
[8] R. Wagner, W. Ellis, Vapor‐liquid‐solid mechanism of single crystal growth. Appl. Phys. Lett. 4, 89-90 (1964).
[9] M. Zheng, L. Zhang, G. Li, W. Shen, Fabrication and optical properties of large-scale uniform zinc oxide nanowire arrays by one-step electrochemical deposition technique. Chem. Phys. Lett. 363, 123-128 (2002).
[10] L. Liang, Y. Xu, C. Wang, L. Wen, Y. Fang, Y. Mi, M. Zhou, H. Zhao, Y. Lei, Large-scale highly ordered Sb nanorod array anodes with high capacity and rate capability for sodium-ion batteries. Energy Environ. Sci. 8, 2954-2962 (2015).
[11] W. Lee, S.-J. Park, Porous anodic aluminum oxide: anodization and templated synthesis of functional nanostructures. Chem. Rev. 114, 7487-7556 (2014).
[12] C.-G. Kuo, C.-C. Chen, Technique for self-assembly of tin nano-particles on anodic aluminum oxide (AAO) templates. Mater. Trans. 50, 1102-1104 (2009).
[13] A. Li, F. Müller, A. Birner, K. Nielsch, U. Gösele, Hexagonal pore arrays with a 50–420 nm interpore distance formed by self-organization in anodic alumina. J. Appl. Phys. 84, 6023-6026 (1998).
[14] H. Masuda, K. Fukuda, Ordered metal nanohole arrays made by a two-step replication of honeycomb structures of anodic alumina. Science 268, 1466-1468 (1995).
[15] C. Yim, M. Lee, M. Yun, G.-H. Kim, K. T. Kim, S. Jeon, CO 2-selective nanoporous metal-organic framework microcantilevers. Sci. Rep. 5, 10674 (2015).
[16] J. A. Turner, A Realizable Renewable Energy Future. Science 285, 687-689 (1999).
[17] A. Fujishima, K. Honda, Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 238, 37 (1972).
[18] H. Ahmad, S. K. Kamarudin, L. J. Minggu, M. Kassim, Hydrogen from photo-catalytic water splitting process: A review. Renewable Sustainable Energy Rev. 43, 599-610 (2015).
[19] T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting. Chem. Soc. Rev. 43, 7520-7535 (2014).
[20] R. de Levie, The electrolysis of water. J. Electroanal. Chem. 476, 92-93 (1999).
[21] J. Luo, J.-H. Im, M. T. Mayer, M. Schreier, M. K. Nazeeruddin, N.-G. Park, S. D. Tilley, H. J. Fan, M. Grätzel, Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts. Science 345, 1593-1596 (2014).
[22] P.-H. Huang, J.-K. Kuo, Z.-D. Wu, Applying small wind turbines and a photovoltaic system to facilitate electrolysis hydrogen production. Int. J. Hydrogen Energy 41, 8514-8524 (2016).
[23] S. Cobo, J. Heidkamp, P.-A. Jacques, J. Fize, V. Fourmond, L. Guetaz, B. Jousselme, V. Ivanova, H. Dau, S. Palacin, A Janus cobalt-based catalytic material for electro-splitting of water. Nat. Mater. 11, 802 (2012).
[24] H. Wang, H.-W. Lee, Y. Deng, Z. Lu, P.-C. Hsu, Y. Liu, D. Lin, Y. Cui, Bifunctional non-noble metal oxide nanoparticle electrocatalysts through lithium-induced conversion for overall water splitting. Nat. Commun. 6, 7261 (2015).
[25] T. R. Cook, D. K. Dogutan, S. Y. Reece, Y. Surendranath, T. S. Teets, D. G. Nocera, Solar energy supply and storage for the legacy and nonlegacy worlds. Chem. Rev. 110, 6474-6502 (2010).
[26] Y. Yan, B. Y. Xia, B. Zhao, X. Wang, A review on noble-metal-free bifunctional heterogeneous catalysts for overall electrochemical water splitting. J. Mater. Chem. A 4, 17587-17603 (2016).
[27] B. Conway, L. Bai, M. Sattar, Role of the transfer coefficient in electrocatalysis: applications to the H2 and O2 evolution reactions and the characterization of participating adsorbed intermediates. Int. J. Hydrogen Energy 12, 607-621 (1987).
[28] G. F. Chen, T. Y. Ma, Z. Q. Liu, N. Li, Y. Z. Su, K. Davey, S. Z. Qiao, Efficient and stable bifunctional electrocatalysts Ni/NixMy (M= P, S) for overall water splitting. Adv. Funct. Mater. 26, 3314-3323 (2016).
[29] Y. Jiao, Y. Zheng, M. Jaroniec, S. Z. Qiao, Design of electrocatalysts for oxygen-and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. 44, 2060-2086 (2015).
[30] B. Conway, B. Tilak, Interfacial processes involving electrocatalytic evolution and oxidation of H2, and the role of chemisorbed H. Electrochim. Acta 47, 3571-3594 (2002).
[31] A. B. Laursen, A. S. Varela, F. Dionigi, H. Fanchiu, C. Miller, O. L. Trinhammer, J. Rossmeisl, S. Dahl, Electrochemical hydrogen evolution: Sabatier’s principle and the volcano plot. J. Chem. Educ. 89, 1595-1599 (2012).
[32] J. Zhang, T. Wang, D. Pohl, B. Rellinghaus, R. Dong, S. Liu, X. Zhuang, X. Feng, Interface Engineering of MoS2/Ni3S2 Heterostructures for Highly Enhanced Electrochemical Overall‐Water‐Splitting Activity. Angew. Chem. 128, 6814-6819 (2016).
[33] D. V. Esposito, J. G. Chen, Monolayer platinum supported on tungsten carbides as low-cost electrocatalysts: opportunities and limitations. Energy Environ. Sci. 4, 3900-3912 (2011).
[34] B. Zhang, D. Wang, Y. Hou, S. Yang, X. H. Yang, J. H. Zhong, J. Liu, H. F. Wang, P. Hu, H. J. Zhao, Facet-dependent catalytic activity of platinum nanocrystals for triiodide reduction in dye-sensitized solar cells. Sci. Rep. 3, 1836 (2013).
[35] J. Chen, Y. Yang, J. Su, P. Jiang, G. Xia, Q. Chen, Enhanced activity for Hydrogen Evolution Reaction over CoFe Catalysts by Alloying with small amount of Pt. ACS Appl. Mater. Interfaces 9, 3596-3601 (2017).
[36] J. Kibsgaard, C. Tsai, K. Chan, J. D. Benck, J. K. Nørskov, F. Abild-Pedersen, T. F. Jaramillo, Designing an improved transition metal phosphide catalyst for hydrogen evolution using experimental and theoretical trends. Energy Environ. Sci. 8, 3022-3029 (2015).
[37] M. Chhowalla, H. S. Shin, G. Eda, L.-J. Li, K. P. Loh, H. Zhang, The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets. Nat. Chem. 5, 263 (2013).
[38] C. Tang, N. Cheng, Z. Pu, W. Xing, X. Sun, NiSe nanowire film supported on nickel foam: an efficient and stable 3D bifunctional electrode for full water splitting. Angew. Chem. 127, 9483-9487 (2015).
[39] H. Vrubel, X. Hu, Molybdenum boride and carbide catalyze hydrogen evolution in both acidic and basic solutions. Angew. Chem. 124, 12875-12878 (2012).
[40] E. Skúlason, V. Tripkovic, M. E. Björketun, S. d. Gudmundsdóttir, G. Karlberg, J. Rossmeisl, T. Bligaard, H. Jónsson, J. K. Nørskov, Modeling the electrochemical hydrogen oxidation and evolution reactions on the basis of density functional theory calculations. J. PHYS. CHEM. C 114, 18182-18197 (2010).
[41] P. Liu, J. A. Rodriguez, Catalysts for hydrogen evolution from the [NiFe] hydrogenase to the Ni2P (001) surface: the importance of ensemble effect. J. Am. Chem. Soc. 127, 14871-14878 (2005).
[42] P. Xiao, M. A. Sk, L. Thia, X. Ge, R. J. Lim, J.-Y. Wang, K. H. Lim, X. Wang, Molybdenum phosphide as an efficient electrocatalyst for the hydrogen evolution reaction. Energy Environ. Sci. 7, 2624-2629 (2014).
[43] Y. Shi, B. Zhang, Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction. Chem. Soc. Rev. 45, 1529-1541 (2016).
[44] P. Rasiyah, A. Tseung, The role of the lower metal oxide/higher metal oxide couple in oxygen evolution reactions. J. Electrochem. Soc. 131, 803-808 (1984).
[45] B. S. Yeo, A. T. Bell, Enhanced activity of gold-supported cobalt oxide for the electrochemical evolution of oxygen. J. Am. Chem. Soc. 133, 5587-5593 (2011).
[46] M. Zhang, M. De Respinis, H. Frei, Time-resolved observations of water oxidation intermediates on a cobalt oxide nanoparticle catalyst. Nat. Chem. 6, 362 (2014).
[47] J. Rossmeisl, A. Logadottir, J. K. Nørskov, Electrolysis of water on (oxidized) metal surfaces. Chem. Phys. 319, 178-184 (2005).
[48] I. C. Man, H. Y. Su, F. Calle‐Vallejo, H. A. Hansen, J. I. Martínez, N. G. Inoglu, J. Kitchin, T. F. Jaramillo, J. K. Nørskov, J. Rossmeisl, Universality in oxygen evolution electrocatalysis on oxide surfaces. ChemCatChem 3, 1159-1165 (2011).
[49] E. M. Fernández, P. G. Moses, A. Toftelund, H. A. Hansen, J. I. Martínez, F. Abild‐Pedersen, J. Kleis, B. Hinnemann, J. Rossmeisl, T. Bligaard, Scaling relationships for adsorption energies on transition metal oxide, sulfide, and nitride surfaces. Angew. Chem. 120, 4761-4764 (2008).
[50] N. Danilovic, R. Subbaraman, K.-C. Chang, S. H. Chang, Y. J. Kang, J. Snyder, A. P. Paulikas, D. Strmcnik, Y.-T. Kim, D. Myers, Activity–stability trends for the oxygen evolution reaction on monometallic oxides in acidic environments. J. Phys. Chem. Lett. 5, 2474-2478 (2014).
[51] M. E. Lyons, M. P. Brandon, The oxygen evolution reaction on passive oxide covered transition metal electrodes in aqueous alkaline solution. Part 1-Nickel. Int. J. Electrochem. Sci 3, 1386-1424 (2008).
[52] L. Trotochaud, S. L. Young, J. K. Ranney, S. W. Boettcher, Nickel–iron oxyhydroxide oxygen-evolution electrocatalysts: the role of intentional and incidental iron incorporation. J. Am. Chem. Soc. 136, 6744-6753 (2014).
[53] G. Młynarek, M. Paszkiewicz, A. Radniecka, The effect of ferric ions on the behaviour of a nickelous hydroxide electrode. J. Appl. Electrochem. 14, 145-149 (1984).
[54] M. S. Burke, M. G. Kast, L. Trotochaud, A. M. Smith, S. W. Boettcher, Cobalt–iron (oxy) hydroxide oxygen evolution electrocatalysts: the role of structure and composition on activity, stability, and mechanism. J. Am. Chem. Soc. 137, 3638-3648 (2015).
[55] S. Klaus, Y. Cai, M. W. Louie, L. Trotochaud, A. T. Bell, Effects of Fe electrolyte impurities on Ni (OH) 2/NiOOH structure and oxygen evolution activity. J. PHYS. CHEM. C 119, 7243-7254 (2015).
[56] C. Kuhrt, L. Schultz, Formation and magnetic properties of nanocrystalline mechanically alloyed Fe‐Co. J. Appl. Phys. 71, 1896-1900 (1992).
[57] J. C. Slater, Atomic radii in crystals. J. Chem. Phys. 41, 3199-3204 (1964).
[58] A. F. Guillermet, Critical Evaluation of the Thermodynamic Properties of the Iron--Cobalt System. High Temp.--High Press. 19, 477-499 (1987).
[59] Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong, H. Dai, MoS2 nanoparticles grown on graphene: an advanced catalyst for the hydrogen evolution reaction. J. Am. Chem. Soc. 133, 7296-7299 (2011).
[60] S. S. Djokić, Electrodeposition of amorphous alloys based on the iron group of metals. J. Electrochem. Soc. 146, 1824-1828 (1999).
[61] T. M. Harris, Q. D. Dang, The Mechanism of Phosphorus Incorporation during the Electrodeposition of Nickel‐Phosphorus Alloys. J. Electrochem. Soc. 140, 81-83 (1993).
[62] E. Bredael, B. Blanpain, J.-P. Celis, J. Roos, On the Amorphous and Crystalline State of Electrodeposited Nickel‐Phosphorus Coatings. J. Electrochem. Soc. 141, 294-299 (1994).
[63] T. S. Narayanan, S. Selvakumar, A. Stephen, Electroless Ni–Co–P ternary alloy deposits: preparation and characteristics. Surf. Coat. Technol. 172, 298-307 (2003).
[64] D. Lewis, G. Marshall, Investigation into the structure of electrodeposited nickel-phosphorus alloy deposits. Surf. Coat. Technol. 78, 150-156 (1996).
[65] S. C. Petitto, E. M. Marsh, G. A. Carson, M. A. Langell, Cobalt oxide surface chemistry: The interaction of CoO(100), Co3O4(110) and Co3O4(111) with oxygen and water. J. Mol. Catal. A: Chem. 281, 49-58 (2008).
[66] S. Petitto, M. Langell, Surface composition and structure of Co 3 O 4 (110) and the effect of impurity segregation. J. Vac. Sci. Technol., A 22, 1690-1696 (2004).
[67] T. Yamashita, P. Hayes, Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials. Appl. Surf. Sci. 254, 2441-2449 (2008).
[68] J.-C. Dupin, D. Gonbeau, P. Vinatier, A. Levasseur, Systematic XPS studies of metal oxides, hydroxides and peroxides. PCCP 2, 1319-1324 (2000).
[69] W. Li, X. Gao, D. Xiong, F. Xia, J. Liu, W.-G. Song, J. Xu, S. M. Thalluri, M. F. Cerqueira, X. Fu, Vapor–solid synthesis of monolithic single-crystalline CoP nanowire electrodes for efficient and robust water electrolysis. Chem. Sci. 8, 2952-2958 (2017).
[70] M. E. Lyons, R. L. Doyle, M. P. Brandon, Redox switching and oxygen evolution at oxidized metal and metal oxide electrodes: iron in base. PCCP 13, 21530-21551 (2011).
[71] S. Srichandan, Electrical and thermal transport coefficients of CoFe thin films deposited on a microcalorimeter. (2018).
[72] O. Diaz-Morales, I. Ledezma-Yanez, M. T. Koper, F. Calle-Vallejo, Guidelines for the rational design of Ni-based double hydroxide electrocatalysts for the oxygen evolution reaction. ACS Catal. 5, 5380-5387 (2015).
[73] P. E. Blanchard, A. P. Grosvenor, R. G. Cavell, A. Mar, X-ray Photoelectron and Absorption Spectroscopy of Metal-Rich Phosphides M 2P and M 3P (M= Cr− Ni). Chem. Mater. 20, 7081-7088 (2008).
[74] S. Anantharaj, S. R. Ede, K. Sakthikumar, K. Karthick, S. Mishra, S. Kundu, Recent trends and perspectives in electrochemical water splitting with an emphasis on sulfide, selenide, and phosphide catalysts of Fe, Co, and Ni: a review. ACS Catal. 6, 8069-8097 (2016).
[75] Y. Wang, B. Zhang, W. Pan, H. Ma, J. Zhang, 3 D Porous Nickel–Cobalt Nitrides Supported on Nickel Foam as Efficient Electrocatalysts for Overall Water Splitting. ChemSusChem 10, 4170-4177 (2017).
[76] T. Tang, W.-J. Jiang, S. Niu, N. Liu, H. Luo, Y.-Y. Chen, S.-F. Jin, F. Gao, L.-J. Wan, J.-S. Hu, Electronic and morphological dual modulation of cobalt carbonate hydroxides by Mn doping toward highly efficient and stable bifunctional electrocatalysts for overall water splitting. J. Am. Chem. Soc. 139, 8320-8328 (2017).
[77] N. Xu, G. Cao, Z. Chen, Q. Kang, H. Dai, P. Wang, Cobalt nickel boride as an active electrocatalyst for water splitting. J. Mater. Chem. A 5, 12379-12384 (2017).