물분해 반응의 과전압을 낮추기 위한 상호보완적 NiMoFe과 NiMoFeP 전기화학 촉매의 합성 및 태양광 물분해 시스템으로의 응용 연구

Abstract

Solar water splitting is an eco-friendly method of hydrogen production by solar energy. The semiconductor that absorbs light energy larger than its bandgap splits water into hydrogen and oxygen gas by photo-excited electron and hole. Solar water splitting systems can be classified into three types: particulate photocatalyst, photoelectrochemical (PEC) cell, and photovoltaic (PV) electrolyser. Among them, only PEC cell and PV electrolyser can have high solar-to-hydrogen (STH) efficiencies and can separate pure hydrogen gas easily. Currently, hydrogen production by PEC cell and PV electrolyser is not economically viable, and therefore it is imperative to increase STH efficiency and to develop stable and low-cost solar water splitting systems for commercialization. Both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in water splitting have multi electron transfer steps, and thus require overpotentials to overcome the kinetic activation energy of each reaction. Efficient electrocatalysts can minimize the overpotentials of both reactions. Actual overall water splitting (OWS) is achieved by operating HER and OER in a single electrolyte at the same pH. Complementary electrocatalysts that have high catalytic activities in both the HER and OER under the same pH condition can reduce fabrication costs and simplify the water splitting system when they are applied simultaneously as the HER and OER catalysts. In this thesis, NiMoFe (NMF)-based complementary electrocatalysts are synthesized. The synthesized electrocatalysts are applied to both the cathode and anode of the electrolytic cell to perform efficient OWS. In addition, efficient low-cost electrocatalysts are also a key factor in increasing STH efficiencies of PEC cell and PV electrolyser for commercialization. Hence, we apply NMF-based electrocatalysts to various solar water splitting systems. PV electrolyser with STH efficiency of 12.3% is developed by combining our electrocatalysts with tandem perovskite solar cells (PSCs). Also, PEC cell composed of NiMo/TiO2/CdS/CIGS photocathode is fabricated, representing half-cell STH efficiency of 2.8%. In Chapter 1, we emphasize the importance of hydrogen energy and explain the theoretical background of solar water splitting. We take a closer look at the features of PEC cell and PV electrolyser to be addressed in this thesis. In addition, the principles and research status of HER and OER electrocatalysts are identified. In Chapter 2, OWS system using NiMoFe (NMF) as HER catalyst and NiMoFeP (NMFP) as OER catalyst is suggested. Complementary electrocatalysts based on NMF alloy, which is a ternary transition metal compound, are synthesized for highly active and stable OWS. Ni catalysts have been actively studied due to their intrinsic HER and OER activities, and additional Mo and Fe incorporation into Ni catalyst can further improve the HER and OER activity of NMF, respectively. To further enhance the OER activity, NMF is phosphorized to produce quaternary NMFP. We employ X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) to investigate the change in the chemical states and local structural motifs of NMFP during OER. OWS performance with NMF and NMFP surpasses the performance with Pt and Ru, which are commercialized noble metal electrocatalysts. Furthermore, our complementary electrocatalysts are combined with tandem PSCs for PV-electrolysis application, producing a remarkably high STH efficiency of 12.3 %. In Chapter 3, NiMo is used as HER co-catalyst in CIGS (Cu(In,Ga)(S,Se)2) photocathode to produce high photocurrent. Developing a cost-effective co-catalyst and ensuring the durability under various pH conditions are critical issues in the PEC application of CIGS photocathode. We catalyze the CIGS photocathode with NiMo as a non-noble metal co-catalyst to enhance the PEC efficiency, and employ thin TiO2 layer by ALD to passivate the surface of CIGS photocathode and to improve the stability under various pH conditions ranging from 0.4 to 14. Ni-Mo/CdS/CIGS photocathode yields photocurrent highly comparable to Pt/CdS/CIGS. Furthermore, the TiO2 passivation effectively prevents the photocorrosion of CdS and the dissolution of the Mo back contact, which are the main causes of the instability of CIGS photocathode. In summary, we synthesize NMF-based complementary electrocatalysts that promote both HER and OER, and further improve the OER activity of NMF through post phosphorization. The electrocatalysts are applied to the PV electrolyser with PSCs and to the PEC cell with CIGS photocathode, each representing the best STH efficiency among reported studies. Synthesis of electrocatalysts and their applications in this thesis present important methodologies for satisfying the low cost, high efficiency, and stability of future solar water splitting system.