In recent years, asymmetric membranes based on mixed ionic-electronic conductors (MIEC) have gained importance in practical gas separations. MIEC ceramic materials show high-energy efficiency and high-temperature resistance, which allows direct integration in industrial processes. Thin layers are supported on porous substrates that provide mechanical strength. In the asymmetric membrane manufacture, the control of support porosity and microstructure is crucial. Colloidal processing is an interesting method that allows controlling the final microstructure in both surfaces and bulk, with high reproducibility. Here, the development of asymmetric membranes with a top functional layer made of Ce_0.8Gd_0.2O_1.9/Ni₂FeO₄ composite is presented and aims to maximize oxygen permeation and membrane robustness. The porous substrate is prepared by slip casting while the functional layers by screen-printing. The effect of pore former volume and particle morphology were studied. The combination of spherical and flake-like PMMA particles enabled to generate open porosity suitable for fast gas transport.