Spectroscopic Elucidation of Phase Evolution in NiO/YSZ Under Oxidative and Reductive Gas Environments
Authors: Kumar Neelesh , Mosevitzky Lis Bar , Nikolla Eranda , Wachs Israel E 
Published: August 2024

High temperature co-electrolysis of CO2 and H2O serves a viable, energy efficient, and scalable route to produce syngas (CO+ H2), which can be used to generate a variety of utility hydrocarbons. Solid oxide electrolysis cell (SOEC) has proven to be a promising technology for CO2 utilization and a carbon neutral route to curb CO2 emissions. The design of cathode catalyst is critical as it must facilitate the activation of CO2 and H2O and also exhibit high electronic and oxygen ion conductivity. The state-of-the-art cathode material used in SOECs is Ni/YSZ (8 mol% Yttria Stabilized Zirconia) ceramic composite (known as cermet), whose performance is dependent on various operating factors including temperature, composition of feed gas stream, presence of contaminants like SOx/NOx etc. There is a very limited number of studies on the evolution of the cathode materials under process level conditions. To begin with, effect of different oxidative and reductive gas environments and temperature over Ni/YSZ is studied using in situ Raman spectroscopy.
Typical flue gas composition is chosen as the basis for the concentration of different components like 15% CO2, 10% H2O, 5% O2 with argon. Under oxidative conditions (O2, CO2, H2O) with temperature variation range of RT-800 0C, NiO/YSZ vibrational modes revealed that it is composed of NiO and YSZ phases. It is seen that there is thermal broadening in the bands of NiO and YSZ, thermal effects reversible upon cooling down and red shift in the NiO bands exemplified the lattice expansion. Exposing the NiO/YSZ to 5% H2 at high temperature resulted in the formation of metallic Ni, which is corroborated by the absence of the Ni-O vibrational modes in Raman spectrum at 400 0C. YSZ phase is quite stable in H2 at elevated temperature.
After reduction of NiO/YSZ to get Ni/YSZ, exposure to oxidative environments (i.e., O2, CO2, H2O) aided in the formation of defect laden NiOx phase on YSZ. Specifically, NiOx formation starts at 350 0C under oxygen and at 450 0C under CO2 and H2O. Under 15% CO2, there is an overlap between the bands associated to the carbonate like species and the NiO phase but there is no formation of coke at these conditions as D and G bands were not observed (Fig. 1). It could be due to the oxidation of Ni that obliterated coke formation. The blue shift at 1046 cm-1 (2P mode of NiO) signifies the shortening of bond length in Ni-O with gradual conversion of Ni to NiOx. Compared to the NiOx phase formed under 5% O2 and 15% CO2 which has a sharp feature in Raman spectra, there is a broad band of NiOx phase formed under 10 % H2O in the region 480-580 cm-1
. This observation demonstrates the variation in the microcrystalline size of the defect laden NiOx phase formed under exposure to different oxidizers.
Another set of experiments were done with the dry hydrogen (5 % H2) and humidified hydrogen (5% H2 + 10 % H2O) to study the role of presence of water on the reduction of NiO/YSZ with temperature variation between 120 – 800 0C. It was found that reduction of NiO to Ni is not affected by the presence of water. Under both conditions this transformation occurs between 400-600 0C. The results from the current study provide the key information on the effects of temperature and gas compositions over Ni/YSZ which serves as a guide for the process level design and operation of SOECs.

References

Juliana Carneiro et al, Industrial & Engineering Chemistry Research 2020 59(36), 15884-15893.

John Kirtley et al, The Journal of Physical Chemistry C2013, 117 (49), 25908-25916.

A. Naumenko et al, Phys Chem Solid State 2008, 9, 121–125.

Figure 1