隨著電動車，金屬空氣電池等的蓬勃發展，科技對水電解反應之效率的要求也越來越高。而目前在水電解反應的研究又以析氧反應最為甚，因其反應機制的緩慢和觸媒的選擇性等問題，造成實際應用上的瓶頸。
有鑒於此，本研究開發了一個新型電催化觸媒，以鎳鐵層狀複金屬氫氧化物（NiFe Layered Double Hydroxide,NiFe LDH）為基底,修飾以無析氧活性的鈰氧化物，藉此提升整體活性。此設計意在能夠有效地提升觸媒每單個活性位點的催化能力，進而促進觸媒的整體析氧催化活性。鎳鐵均是地球富含的元素，且透過簡便水熱法合成的NiFe LDH/CNT已被證實是析氧反應中新的高效能基準觸媒。而氧化鈰在析氧反應中雖無催化活性，但其他文獻指出部分還原態的氧化鈰與主體觸媒間可有協同作用，且可加快反應中氧氣的擴散速度。本實驗把兩者結合起來探討如何利用部分還原的CeO2來影響NiFe LDH的活性。
研究結果顯示，水熱法合成的6.28%CeOx /NiFe LDH觸媒，CeOx與NiFe LDH間有較強的電子相互作用力（XPS結果）且在析氧反應中有卓越的比活性，在電位1.55V時，純NiFe LDH的電流密度僅有0.2A/cm2Ni2+/3+，而6.28%CeOx /NiFe LDH觸媒在相同電位下的電流密度則可達到1A/cm2Ni2+/3+，較前者提升5倍。6.28%CeOx /NiFe LDH觸媒的質量活性上雖略遜色於純NiFe LDH，但也說明經二次水熱法修飾CeOx後NiFe LDH結構會遭到一定破壞，活性位的數量損失較大。穩定性測試中，6.28%CeOx /NiFe LDH的表現依舊優於NiFe LDH，在1M KOH電解質電流密度為10mA/cm2的嚴苛環境下1天後仍有約94%的保留率，而參考觸媒NiFe LDH則僅有約92%。
此外，為改善二次水熱法損失NiFe LDH活性位數量之問題，本研究還進行了合成方法的優化，將CeOx的生長由水熱法改為微波法。結果顯示，經改善後的6.4%CeOx /NiFe LDH在質量活性上確有提高（1.54VRHE時40.3 mA/mgNiFeLDH ），經XAS結果顯示CeOx的修飾可縮短金屬與氧的鍵長，降低Ni-O, Ni-M配位數，導致結構鬆散，氧缺陷數增加。合成之觸媒成功提高了NiFe LDH的催化活性，穩定性亦良好。

Nowadays, Electrical vehicle and metal-air battery has blossomed and attracted various of attention among people and these technologies usually require high efficiency on water splitting reaction. Water splitting(H2O→H2+1/2 O2), ideally driven by solar power, is one of the most efficient and sustainable ways to produce molecular hydrogen(H2) and oxygen (O2) gases in high purity. If available, water splitting would have the potential to solve the global urgent energy needs of future societies without any environmental cost. Water oxidation also known as oxygen evolution reaction(OER), one half-reaction of water splitting, has long been the bottleneck due to its four proton-coupled electron-transfer process that requires such a high redox potential.
Herein, we report a newly designed OER nanocatalyst, NiFe LDH(Layered Double Hydroxide) based electrochemical catalyst is decorated by CeOx (CeOx/NiFe LDH), to improve its site activity and further the whole catalyst activity. Ni and Fe are earth abundant elements and their product NiFe LDH has been proved as a new benchmark in OER. Meanwhile, having little activity in OER, CeO2, still could be a nice synergistic catalyst and accelerate the dispersion rate of oxygen in OER revealed from some literatures. In this work, we try to combine these two composites and investigate how partial reduced CeO2 affect the activity of NiFe LDH.
The 6.28%CeOx /NiFe LDH derived from the hydrothermal method, has an excellent specific activity(1 A/cm2Ni2+/3+) under 0.1M KOH, almost 5 folds higher than pure NiFe LDH. The XPS results show that there is a strong electronic interactions between NiFe LDH and CeOx and it leads to an enhanced OER performance. The stability of this catalyst could also be good for its 94% retention under 1M KOH for 24h. Nevertheless, due to the structural damage in NiFe LDH from the second hydrothermal treatment, mass activity of this catalyst is less satisfying.
To further optimize the mass activity, we developed a catalyst, synthesized by the microwave method(6.4%CeOx /NiFe LDH). In this case, both better mass activity(40.3 mA/mgNiFeLDH at 1.54VRHE ) and specific activity(0.33A/cm2) could be achieved. The XAS result reveals that the decoration by CeOx could decrease the coordination number between Ni-O and Ni-M, creating a more loose structure and larger amount of oxygen vacancies. Finally, 6.4%CeOx /NiFe LDH with enhanced activity and long-time stability was successfully prepared.

1. Marom, R.A., S. Francis，Leifer, Nicole，Jacob, David，Aurbach, Doron, A review of advanced and practical lithium battery materials. Journal of Materials Chemistry, 2011. 21(27).
2. Ni, M.L., Michael K. H.，Leung, Dennis Y. C.，Sumathy, K., A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production. Renewable and Sustainable Energy Reviews, 2007. 11(3): p. 401-425.
3. Diaz-Morales, O.C.-V., Federico，de Munck, Casper，Koper, Marc T. M., Electrochemical water splitting by gold: evidence for an oxide decomposition mechanism. Chemical Science, 2013. 4(6).
4. Lee, Y.S., J.May, K. J.Perry, E. E.Shao-Horn, Y., Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J Phys Chem Lett, 2012. 3(3): p. 399-404.
5. Carmo, M.F., D. L.Merge, J.Stolten, D., A comprehensive review on PEM water electrolysis. International Journal of Hydrogen Energy, 2013. 38(12): p. 4901-4934.
6. Suen, N.T.H., S. F. Quan, Q. Zhang, N. Xu, Y. J. Chen, H. M., Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives. Chem Soc Rev, 2017. 46(2): p. 337-365.
7. http://www.infomine.com/investment/. 2019.
8. Suntivich, J.M., Kevin J. Gasteiger, Hubert A. Goodenough, John B. Shao-Horn, Yang, A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. 2011. 334(6061): p. 1383-1385.
9. Frydendal, R.P., Elisa A. Knudsen, Brian P. Wickman, Björn Malacrida, Paolo Stephens, Ifan E. L. Chorkendorff, Ib, Benchmarking the Stability of Oxygen Evolution Reaction Catalysts: The Importance of Monitoring Mass Losses. ChemElectroChem, 2014. 1(12): p. 2075-2081.
10. Cherevko Serhiy, G.S., Kasian Olga, Kulyk Nadiia, Grote Jan-Philipp, Savan Alan, Shrestha Buddha Ratna, Merzlikin Sergiy, Breitbach Benjamin, Ludwig Alfred, Mayrhofer Karl J. J., Oxygen and hydrogen evolution reactions on Ru, RuO 2 , Ir, and IrO 2 thin film electrodes in acidic and alkaline electrolytes: A comparative study on activity and stability. Catalysis Today, 2016. 262: p. 170-180.
11. Y. Matsumoto, S.Y., T. Nishida, and E. Sato Oxygen Evolution on La1–xSrxFe1–yCoyO3 Series Oxides. Journal of the Electrochemical Society, 1980.
. 127: p. 2360-2364.
12. Burns, R.G., Mineralogical applications of crystal field theory
Cambridge University Press, 1993.
13. Li, M.X., Y. Liu, X. Bo, X. Zhang, Y. Han, C. Guo, L., Facile synthesis of electrospun MFe2O4 (M = Co, Ni, Cu, Mn) spinel nanofibers with excellent electrocatalytic properties for oxygen evolution and hydrogen peroxide reduction. Nanoscale, 2015. 7(19): p. 8920-30.
14. Tung, C.W.H., Y. Y.Shen, Y. P. Zheng, Y. Chan, T. S. Sheu, H. S. Cheng, Y. C. Chen, H. M., Reversible adapting layer produces robust single-crystal electrocatalyst for oxygen evolution. Nat Commun, 2015. 6: p. 8106.
15. Wang, H.-Y.H., Ying-Ya Chen, Rong Chan, Ting-Shan Chen, Hao Ming Liu, Bin, Ni3+-Induced Formation of Active NiOOH on the Spinel Ni-Co Oxide Surface for Efficient Oxygen Evolution Reaction. Advanced Energy Materials, 2015. 5(10).
16. Yan, K.W., Guosheng Jin, Wei, Recent Advances in the Synthesis of Layered, Double-Hydroxide-Based Materials and Their Applications in Hydrogen and Oxygen Evolution. 2016. 4(3): p. 354-368.
17. Fan, G.L., F.Evans, D. G.Duan, X., Catalytic applications of layered double hydroxides: recent advances and perspectives. Chem Soc Rev, 2014. 43(20): p. 7040-66.
18. Xu, Z.P.S., G. Lu, C. Q. Lu, G. Q., Dispersion and size control of layered double hydroxide nanoparticles in aqueous solutions. J Phys Chem B, 2006. 110(34): p. 16923-9.
19. Song, F. and X. Hu, Exfoliation of layered double hydroxides for enhanced oxygen evolution catalysis. Nat Commun, 2014. 5: p. 4477.
20. Gong, M.L., Y.Wang, H.Liang, Y.Wu, J. Z.Zhou, J.Wang, J.Regier, T.Wei, F.Dai, H., An advanced Ni-Fe layered double hydroxide electrocatalyst for water oxidation. J Am Chem Soc, 2013. 135(23): p. 8452-5.
21. Xu, H.C., J. Shan, C. Wang, B. Xi, P. Liu, W. Tang, Y., MOF-Derived Hollow CoS Decorated with CeOx Nanoparticles for Boosting Oxygen Evolution Reaction Electrocatalysis. Angew Chem Int Ed Engl, 2018. 57(28): p. 8654-8658.
22. Xia, D.-c., et al., Graphene/Ni–Fe layered double-hydroxide composite as highly active electrocatalyst for water oxidation. Materials Research Bulletin, 2016. 74: p. 441-446.
23. Li, X., et al., In-situ intercalation of NiFe LDH materials: An efficient approach to improve electrocatalytic activity and stability for water splitting. Journal of Power Sources, 2017. 347: p. 193-200.
24. Xu, H.W., B. Shan, C. Xi, P. Liu, W. Tang, Y., Ce-Doped NiFe-Layered Double Hydroxide Ultrathin Nanosheets/Nanocarbon Hierarchical Nanocomposite as an Efficient Oxygen Evolution Catalyst. ACS Appl Mater Interfaces, 2018. 10(7): p. 6336-6345.
25. Xu, L.J., Q. Xiao, Z. Li, X. Huo, J. Wang, S. Dai, L., Plasma-Engraved Co3 O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen Evolution Reaction. Angew Chem Int Ed Engl, 2016. 55(17): p. 5277-81.
26. Chen, S.M., Min Liu, Xi Hong, Shiyu Lu, Zhouguang Sang, Shangbin Liu, Kaiyu Liu, Hongtao, A high-rate cathode material hybridized by in-site grown Ni–Fe layered double hydroxides and carbon black nanoparticles. Journal of Materials Chemistry A, 2016. 4(13): p. 4877-4881.
27. 林享德, 過渡金屬摻雜二氧化鈦修飾鎳鐵層狀氫氧化物觸媒於鹼性氧氣析出反應之研究. 國立台灣科技大學化學工程系碩士學位論文, 2018.
28. Dionigi, F.R., T.，Pawolek, Z.，Gliech, M.，Strasser, P., Design Criteria, Operating Conditions, and Nickel-Iron Hydroxide Catalyst Materials for Selective Seawater Electrolysis. ChemSusChem, 2016. 9(9): p. 962-72.
29. Louie, M.W. and A.T. Bell, An investigation of thin-film Ni-Fe oxide catalysts for the electrochemical evolution of oxygen. J Am Chem Soc, 2013. 135(33): p. 12329-37.
30. Zhu, X.T., Cheng，Wang, Hao-Fan，Zhang, Qiang，Yang, Chaohe，Wei, Fei, Dual-sized NiFe layered double hydroxides in situ grown on oxygen-decorated self-dispersal nanocarbon as enhanced water oxidation catalysts. Journal of Materials Chemistry A, 2015. 3(48): p. 24540-24546.
31. Koper, M.T.M., J.Electroanal.Chem, 2011. 660: p. 254.
32. Ji, Z.W., X.，Zhang, H.，Lin, S.，Meng, H.，Sun, B.，George, S.，Xia, T.，Nel, A. E.，Zink, J. I., Designed synthesis of CeO2 nanorods and nanowires for studying toxicological effects of high aspect ratio nanomaterials. ACS Nano, 2012. 6(6): p. 5366-80.
33. Hunter, B.M.H., W. Winkler, J. R. Gray, H. B. Müller, A. M., Effect of interlayer anions on [NiFe]-LDH nanosheet water oxidation activity. Energy & Environmental Science, 2016. 9(5): p. 1734-1743.
34. Long, X.L., J. Xiao, S. Yan, K. Wang, Z. Chen, H. Yang, S., A strongly coupled graphene and FeNi double hydroxide hybrid as an excellent electrocatalyst for the oxygen evolution reaction. Angew Chem Int Ed Engl, 2014. 53(29): p. 7584-8.
35. Bao, J.Z., X.Fan, B.Zhang, J.Zhou, M.Yang, W.Hu, X.Wang, H.Pan, B.Xie, Y., Ultrathin Spinel-Structured Nanosheets Rich in Oxygen Deficiencies for Enhanced Electrocatalytic Water Oxidation. Angew Chem Int Ed Engl, 2015. 54(25): p. 7399-404.