  La$_2$NiO$_4$ is a very promising cathode material for intermediate and low
temperature solid oxide cell applications, due to its good electronic and ionic
conductivity, together with its high oxygen exchange activity with a low
activation energy. Oxygen incorporation and transport in La$_2$NiO$_4$ (L2NO4)
thin films is limited by surface reactions. Hence, tailoring the morphology is
expected to lead to an overall improvement of the electrode performance. We
report on the growth of nano-architectured La$_2$NiO$_4$ thin film electrodes
by Pulsed Injection Metal Organic Vapour Deposition (PI-MOCVD), achieving
vertically gapped columns with multi-fold active surface area, leading to much
faster oxygen exchange. This nano-columnar structure is rooted in a dense
bottom layer serving as good electronic and ionic conduction pathway. The
microstructure is tuned by modification of the growth temperature and
characterised by SEM, TEM and XRD. We studied the effect of surface activity by
electrical conductivity relaxation measurements in fully dense and
nano-columnar La$_2$NiO$_4$ thin films of various thicknesses grown on several
different single crystal substrates. Our results demonstrate that the increased
surface area, in combination with the opening of different surface
terminations, leads to a significant enhancment of the total exchange activity
in our films with optimized nano-architectured microstructure.
