Sign Inversion Of Stress Charge Coefficient Of Nanoporous Nickel And Its Actuation Properties

Metallic actuators (metallic muscles) have recently received great attention due to low operating voltage, moderate strength and strain compared to piezoelectric ceramics and conducting polymers, but are limited to precious metals like gold, platinum and their alloys. It is a great challenge to develop earth-abundant metallic actuators with superior performance. Here, we report a nanoporous nickel actuator with state-of-the-art actuation properties through engineering design of nanostructured surface with oxide layer. The actuator exhibits giant reversible stain of up to 2%and ultrahigh work density of 11.76 MJ/m3 (the highest among the known actuator materials, an enhanced factor of-300 compared with skeletal muscles), as well as low materials cost (less than 1/2000 of gold). Moreover, this nickel actuator can perform over 10,000 cycles with strain preservation of over 70%.The surface stress-charge coefficient, ζ reflects the response of surface stress to the change of superficial electric charge. The results of actuation of np-Ni reveal a negative sign of C for the clean surface of nanoporous nickel, and an unusual positive sign of ζ for the oxide-covered surface. This provides another evidence for the sign-inverted behavior of nanoporous metals with distinct surface state. Interestingly, the oxidized nickel surface cannot recover the conventional negative value of ζ after the reduction process, which is related to the nickel oxide adhere to the nickel surface, β-NiO, which is difficult to be reduced even at quite negative potentials.