We demonstrate that nanofabrication of 3D dendritic CoNi alloy foams with an open porous structure can be achieved by electrodeposition onto a single-crystalline Cu(111) substrate at ambient conditions. The very low wettability of this substrate caused by its low surface energy allows tailoring the CoNi deposit morphology. This is concluded from a comparison of polycrystalline Cu substrates with single-crystalline ones of different orientations. The advantages of the present CoNi alloy foams are low internal stresses and good mechanical stability on the substrate. In a second step, by comparing the catalytic properties of the achieved foam with those of CoNi layers obtained on polycrystalline Cu substrates, it is shown that the morphology of the CoNi layers has a decisive influence on the kinetics of the surface redox reaction. The higher reaction rate makes the open foam suitable as catalyst for oxygen evolution in electrolysers. The reversibility of the redox process provides great potential for the achieved porous layers to be used as positive material in alkaline batteries.