Noble-Metal-Free Catalysts for Hydrogen Production from Electrolysis of Ammonia

Abstract

Ammonia is one of the largest produced chemicals in the world, reaching 171 million metric tons in 2015 [1]. While ammonia is primarily used as a fertilizer, there is a growing interest in using it as a hydrogen carrier to overcome the significant challenges faced by direct storage and transportation of hydrogen. Ammonia contains 17.8 wt% hydrogen, can be easily liquefied under mild conditions (-33 °C under atmospheric pressure or 0.86 MPa at 20 °C) and the infrastructure for storing and transporting bulk ammonia is well established. On-site catalytic ammonia electrolysis is an ideal approach to generate hydrogen gas on demand, which can be used in fuel cells, engines and turbines. However, ammonia electrooxidation has slow kinetics, and uses Pt-based catalysts due to their low overpotentials. Moreover, Pt is easily poisoned during ammonia electrooxidation and has a high material cost. In addition, the cathodic reaction in the electrolysis of liquid ammonia is proposed to proceed via the reduction of ammonium ion, whose formal potential is 0.60 V more negative compared to that of protons [2]. Therefore, low-cost and stable catalysts are needed to lower the overpotentials for ammonia splitting. In this work, noble-metal-free electrocatalysts were developed for both nitrogen evolution (ammonia oxidation) and hydrogen evolution reactions, NER and HER respectively. For the NER, Fe/N/C catalysts were prepared [3], and the electrocatalytic activity measured in liquid ammonia, NH3(l), with 0.1 M KPF6 or NH4NO3. The onset potential for ammonia oxidation was about 0.6 V vs NHE, which was much lower than the 1.2 V for a Pt disk electrode in [4]. However, the Fe/N/C catalyst degraded slowly when the upper potential limit of the cyclic voltammetry measurements was higher than 1.25 V vs NHE, indicating that the catalyst is oxidized at high potentials. Results from efforts to characterize and passivate the oxidation reactions will be further discussed. For HER, NiMo bimetallic catalysts were deposited onto a rotating glassy carbon disk electrode [5] and the cyclic voltammograms measured. In NH3(l), the onset potential for HER over the NiMo catalyst was about -1.1 V vs NHE, which was very close to that for a Pt disk electrode in NH3(l) with a 1.0 M NH4 + electrolyte [2]. The current density increased significantly as the NH4NO3 concentration increased from 0.1 to 1.0 M. Interestingly the NiMo catalyst showed very large overpotenials for HER in DMF containing 0.1 M NH4PF6 electrolyte. When ammonia was added to the electrolyte, however, the performance was excellent. Onset potentials for HER lower than -0.4 V vs NHE were observed, which is the lowest value reported for HER from non-aqueous ammonia splitting and indicates an alternative mechanism dominates compared to NH3(l). The role of solvent, ammonia and ammonia concentration will also be presented. The implications of the results shown for both NER and HER reactions will be discussed in the context of ammonia electrolysis.