Hydrogen and Electric Power Generation from Liquid Microjets: Design Principles for Optimizing Conversion Eﬃciency Nadine Schwierz,* ,†,§ Royce K. Lam, †,‡,§ Zach Gamlieli, † Jeremiah J. Tills, † Alvin Leung, † Phillip L. Geissler, †,‡,¶ and Richard J. Saykally* ,†,‡ † Department of Chemistry, University of California, Berkeley, California 94720, United States ‡ Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ¶ Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ABSTRACT: Liquid water microjets have been successfully employed for both electrical power generation and gaseous hydrogen production, but the demonstrated eﬃciencies have been low. Here, we employ a combination of a modiﬁed Poisson−Boltzmann description, continuum hydrodynamic equations, and microjet electrokinetic experiments to gain detailed insight into the origin of the streaming currents produced in pure water. We identify the contributions to the streaming current from speciﬁc ion adsorption at the solid/liquid interface and from long-ranged electrostatic interactions, ﬁnding that the portion originating from the latter dominate at charged surfaces. The detailed understanding aﬀorded by theory and the close agreement with experimental results elucidates design principles for optimizing hydrogen production and power generation. Changing the sign of the surface charge density through targeted use of surface coatings via silanization switches the primary charge carrier between hydronium and hydroxide and therefore switches the corresponding production of molecular hydrogen to oxygen at the target electrode. Moreover, hydrophobic surface coatings reduce dissipation due to ﬂuid/solid friction, thereby increasing the conversion eﬃciency. ■ INTRODUCTION Micro- and nanoﬂuidic devices have shown promise for energy conversion and have received signiﬁcant attention over the past few years. For instance, electrokinetic currents can be produced by forcing water through porous materials or through well- deﬁned micron- or nanometer-sized channels. 1−18 Liquid microjet electrokinetics allow for the conversion of hydrostatic pressure directly into electrical energy and molecular hydrogen and also increase the conversion eﬃciency (>10%) by eliminating back conduction due to electroosmotic ﬂow. 1,2 Xie et al. have recently reported up to 48% eﬃciency in the conversion of kinetic energy to potential energy for a droplet train driven into a strong electric ﬁeld. 19 It has long been recognized that streaming currents arise from the asymmetric distribution of anions and cations in an interfacial electric double layer. At charged interfaces, this double layer is formed from electrostatic interactions of the ions with the charged surface. Speciﬁc ion adsorption or repulsion at a solid/electrolyte interface also contributes to the asymmetric charge distribution, leading to anomalous ion- speciﬁc electrokinetic eﬀects in uncharged channels. 20 How- ever, the question of whether the streaming current in pure water results from the selective adsorption of hydroxide ions to a solid interface, 1 or whether the electrostatic interactions of hydronium and hydroxide with surface charges might play an important role, has not yet been addressed. We employ a combination of modiﬁed Poisson−Boltzmann (PB) theory, including nonelectrostatic ion−surface interac- tions, continuum hydrodynamics, and microjet electrokinetic measurements to identify the origin of the streaming current. This approach identiﬁes the contributions to the streaming current from long-ranged electrostatic interactions and speciﬁc adsorption of hydronium and hydroxide at the solid/liquid interface. At charged surfaces, the contribution from long- ranged electrostatic interactions results in streaming currents that are signiﬁcantly larger than those induced by speciﬁc ion adsorption to uncharged interfaces. The detailed understanding aﬀorded by this theory and the close match between the theoretical and the experimental results allows us to adjust surface properties and apparatus design to optimize power generation and conversion eﬃciency. In particular, targeted functionalization of the microjet surface via silanization is used to vary the surface charge density and the surface hydro- phobicity. Changing the sign of the surface charge reverses the sign of the streaming current and switches between hydronium and hydroxide as the primary charge carrier and therefore between the production of molecular hydrogen or oxygen. Additionally, increasing the surface hydrophobicity reduces energy loss resulting from ﬂuid/surface frictional forces, thereby increasing the conversion eﬃciency. ■ METHODS A. Streaming Current for Liquid Microjets. Electro- kinetic streaming currents originate from the overlap between Received: April 13, 2016 Revised: June 8, 2016 Published: June 9, 2016 Article pubs.acs.org/JPCC © 2016 American Chemical Society 14513 DOI: 10.1021/acs.jpcc.6b03788 J. Phys. Chem. C 2016, 120, 14513−14521 Downloaded by UNIV OF CALIFORNIA BERKELEY at 09:56:52:245 on June 11, 2019 from https://pubs.acs.org/doi/10.1021/acs.jpcc.6b03788.