# The Evolution of Solid Oxide Fuel Cell Materials
## 3 Materials Systems
### Advanced Concepts  
In order to attain improved performance and downscaling so as to be compatible with the design specifications that will be required, solid oxide cells (SOC) need to benefit from beyond-state-of-the-art concepts for nanoengineering materials [111]. In essence, practical applications will need to approach the scaling rate of Moore's Law as it has been observed in microelectronics for many decades. In reality, this is hindered by the poor thermochemical stability, limited electrode catalytic activity and the difficulties in the efficient introduction of nanoscale materials, all of which undermine the use of SOCs as a portable energy source. The improvement of the SOC air electrode can be achieved using scarce materials or by the introduction of advanced ceramic nanostructures formed using mixed ionic-electronic conducting oxides (MIECs) such as doped cobaltites, ferrites and manganites [112]. It is critical in MIEC materials to balance the free energy of the oxygen redox cycle and the enthalpy of oxide reduction as this will ensure significant electrocatalytic activity over the free surface combined with enhanced oxygen diffusion [113]. In thin-film MIECs, the dense microstructure leads to significant in-plane percolation, and the potential of nanostructuring through cutting-edge thin-film techniques is a way to unravel nanoscale effects (i.e., fast oxygen reduction kinetics diffusion) [23]. MIEC thin films are beneficial for SOC functional layers as they do not have areas with high oxygen chemical potential gradients or high electronic current density that are deleterious for device performance; however, there are still issues that need to be considered such as dopant segregation toward the surface that can lead to limited thermal stability [114]. Controlling and limiting cation self-diffusion is important to limit the formation of insulating secondary phases and also to retain surface activity in the long term [115; 116]. The community has addressed the improvement of MIEC thin films' performance through chemical doping, inclusion of interfaces, surface decoration and strain engineering [117; 118; 119; 120; 121]. The difficulty that emerges is to mitigate cation segregation and to improve the oxygen reduction reaction (ORR) kinetics [122]. To improve the oxygen diffusion at intermediate temperatures, MIEC can be paired with oxygen conductors to form nanocomposites, with applications as SOC electrodes and dual-phase membranes for oxygen separation. Nanocomposites are structurally stable and have the benefit of ionic and electronic species transport [123; 124]. Vertically aligned nanostructures (VANs) are considered as an alternative to the nanocomposite electrode functional  
Figure 3: Static atomistic simulation results of the activation energies of migration calculated in the STO-LCO interface [109]. layers [125]. VANs have been studied for nearly a decade and, in particular, nanocolumn in matrix mesostructures exhibited important functionalities such as controlled resistive switching, enhanced ionic diffusion and photocatalytic properties [126; 127; 128; 129]. The interplay of interfacial effects (elastic strain ionic and electronic redistribution) inspired the investigation of self-assembled MIEC-fluorite thin-film vertical nanocomposites for applications in SOFCs [37; 130; 131]. The benefit of these studies is the realization of the VAN approach; however, their inclusion of inert model substrates and unrealistic electrolyte layers leads to incoherent interfaces with low interface density, and these in turn restrain the potential of VANs and inhibit their use in realistic electrochemical devices. The recent study by [PERSON] et al. [25] described the construction of a self-assembled thin-film VAN nanocomposite using an electronic conductor (La\({}_{0.8}\)Sr\({}_{0.2}\)MnO\({}_{3}\) (LSM)) and an ionic conductor (Ce\({}_{0.8}\)Sm\({}_{0.2}\)O\({}_{2}\) (SDC)). Importantly, VAN has long-range order and phase alternation with nanosized column width (\(\approx\)10 nm) [25].