N-deposited carbon supported FeNi alloys as electrocatalysts for oxygen evolution reaction

Water electrolysis is a renewable energy production method that decomposes water molecules into oxygen (O2) and hydrogen (H2) gases by employing electricity. Released H2 is utilized as an energy carrier whereas O2 is used in the energy conversion systems such as fuel cells. Water electrolysis systems are composed of an anode electrode where an oxygen evolution reaction (OER) occurs and a cathode electrode that is responsible for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). OER is often regarded as the main bottleneck of water electrolysis because of its sluggish kinetics which limits cell efficiency. The thermodynamic potential of the water electrolysis reaction is 1.23 V but there is a need for higher potential due to the various activation barriers. Thus, there has been extensive research for novel electrocatalysts. Noble metal-based oxides have been offered as promising candidates towards OER thanks to their superior activity. However, because of their poor stability and high cost, first-row transition metals such as nickel (Ni), iron (Fe), cobalt (Co), and manganese (Mn) have drawn a lot of attention for the OER in alkaline media. Ni showed superior properties among the others thanks to its corrosion resistance and ductility towards OER. Further research studies have revealed that the combination of Ni with Fe has increased the OER activity. Carbon-based electrocatalysts for OER have been found promising thanks to their good conductivity and high stability both in acidic and alkaline electrolytes. Later, it has been found that carbon materials with metal or nitrogen (N) doping have a higher surface area and the number of active sites as a result of the increasing dispersion. Consequently, uphill energy states of the catalytic intermediates have been decreased by the structural defects as a result of N doping. Accordingly, polyacrylonitrile (PAN) has been offered as advantageous carbon support due to N atoms in the structure. PAN-derived carbon-supported electrocatalysts have been synthesized via the electrospinning method in the literature up to date, which is an expensive method, and it is hard to optimize. Recently, a new synthesis approach which is "in-situ synthesis" has been proposed by our research group as an easier and inexpensive method. In the first part of this thesis, single metal-based (Ni) PAN-derived carbon-supported electrocatalysts were synthesized with the in-situ method which is a cheap and easy process. Then, the structures of these materials were characterized and investigated for OER activity with the electrochemical characterization methods. In addition, the chelating effect was examined using different chelating agents such as 2-2'-bipyridine and ethylenediamine, to observe the formation of the smaller nanoparticles. In the second part of the thesis, bimetallic systems (i.e. alloys) were synthesized in different ratios of Ni and Fe with the in-situ method. Structural and electrochemical characterizations were conducted to examine the relationship between Fe and Ni and their effect on the OER activity. In addition, the chelating effect was examined with the best resulting chelating agent. Consequently, 40% Fe1:Ni1@NC-bipy was exhibited the smallest overpotential, smallest Tafel slope, highest surface area, good crystallinity, and homogeneous dispersion of the nanoparticles.