Imagine a sample on an insulating chip. The material sample is contacting two electrodes. The material (a composite, could have liquid component) has a uniform, presumably amorphous, structure between the electrodes - i.e. it is not patterned with different layers. Is there such a material that would generate a voltage between two electrodes? Could some oxidative process occur in atmosphere that would result in charge (electron, hole, or ion) diffusion through the sample and for some reason the creation of potential between the electrodes?