Topographic control of electrical conductivity for enhanced electrokinetic energy conversion

Experimental and theoretical analyses of electrokinetic flow demonstrate simple but effective strategies by which electrical conduction can be controlled through the surface topography to improve the conversion of mechanical energy into electrical energy. The present study is conducted in slit microchannels with glass and silicon substrates over a broad range of electrolyte concentrations (10⁻⁵ to 10⁻¹ M). At low ionic concentrations that are desirable for enhancing conversion efficiency, electrical conductivity arising from an excess concentration of mobile charges at the liquid–solid interface becomes the primary factor limiting the maximum achievable streaming potential. Similarly to observations in nanoscale systems, an “excess” conductivity that is unexpectedly high for silica surfaces emerges at very low concentrations (< 0.1 mM) in the studied slit channels with microscale height. This excess surface conductivity is influenced by the micro/nanoscale surface topography and can be thus reduced by employing engineered nanostructured surfaces and/or segmented channel sections. A compact analytical model considering surface charge and topography is proposed to quantitatively account for experimentally determined streaming voltages. The results of this work can guide the rational design of more efficient systems for electrokinetic energy conversion devices.