The Potential of Transition Metal Doped Nickel Hydroxide for Electrochemical Ammonia Oxidation and Energy Applications

Abstract

Current society is challenged with climate change and aims to adapt and improve to mitigate climate change. This is an extremely challenging transition that requires a paradigm shift from our current fossil fuel driven world to a primarily renewable electricity based one. Increasing the electricity generation alone would not be sufficient, as balance between supply and demand is required. Solar and wind power are intrinsically intermittent with strong diurnal and seasonal variations. 

Therefore, the need arises for electricity storage to compensate for intermittence. Within this thesis we focus on the combined system that can handle both situations, the Ni-Fe alkaline battery and electrolyser. Previously the Ni-Fe battery suffered from limited round-trip efficiency due to hydrogen and oxygen generation. However, when utilized within an integrated electrolysis application, the gas production is intentionally applied for long-term energy storage.
So, within chapter 4, we aimed at improving the Ni electrode by reducing the amount of required nickel, while also improving both electrolyser and battery properties. Depending on the amount of copper substitution, a significant improved electrochemical activity can be obtained. In addition, the Cu doped nickel hydroxide showed stability for over 1000 cycles with the amount of dopant reduced to Ni_0.95Cu_0.05(OH)₂. Overall, this would thus result in requiring 40% less nickel for the same observed capacity.
 
In addition to decarbonizing the electricity production, it is mandatory to replace current industrial processes that require fossil fuel and feedstock with renewable energy and feedstock based alternative processes. Within this Thesis we focused on the oxidation of ammonia to nitrite and nitrate. These products are mainly applied within the synthetic fertilizer production. Ammonia oxidation to nitrites and nitrates is currently performed in the Ostwald process with Pt/Rh gauze. As the ammonia electrooxidation is feasible in ammonia fuel cells, electrochemical oxidation could have potential for replacement of this thermal Ostwald process.
Therefore, in chapter 2, we focus on applying the doped nickel hydroxide materials to increase the performance for ammonia oxidation to nitrate and nitrite. Co, Mn and Cu as dopants showed promising results. Furthermore, it became clear that the reaction was dependent on e Ni(II)/Ni(III) equilibrium as the reaction also occurs through indirect oxidation via the charged NiOOH phase. Under continuous operation at 25 mA/cm² a high faradaic efficiency is obtained with a 97% NO₂^-:NO₃^- selectivity. 

The above sparks interest in further investigation of NiCu_0.2 as promising catalyst in chapter 3, while taking into account the significant of the setup layout. The setup contains a Nafion 117 membrane to keep the counter electrode and nitrite separate, as it would otherwise result in reduction of the nitrite to nitrogen gas or back to ammonia. The Ni_0.8Cu_0.2(OH)₂ can perform ammonia oxidation, with limited oxygen evolution, from 2.5 up to 400 mA/cm². At high initial ammonia concentration of 1 M, more than 75% was able to be converted in 3 hours at this 400 mA/cm² with a faradaic efficiency of 96%. Thus, this work reveals a potential approach for replacing the Ostwald process with electrochemistry within the near future.