Role of Ion transport in silicon-based Hydrovoltaic devices Tarique Anwar and Giulia Tagliabue EPFL, Switzerland Evaporation driven fluid flow in porous or nanostructured materials has recently opened a new paradigm for converting thermal energy from the ambient into electrical energy, via an electrokinetic pathway [1] . Although many recent studies have shown that ion transport is the governing phenomenon in these so- called Hydrovoltaic devices, there is a lack of fundamental understanding as to how the solid-liquid interfacial parameters, confinement size, and liquid properties can modulate the overall performance. In this work, we leverage nanofabrication techniques to realize ordered arrays of Silicon nanopillars and we carefully study their hydrovoltaic response. In particular, we change in a controlled manner both the geometrical parameters and the liquid properties (ion type and concentration), to correlate their effect to the open circuit voltage and power output. Importantly, by combining experiments with numerical modelling we can provide deeper insight into the relevant governing parameters that modulate the electrokinetic response of hydrovoltaic devices. Overall, our controlled nanostructuring approach, which lies in between single nanochannel studies and macro-scale porous system characterization, offers critical insight into how to enhance the energy conversion performance of evaporation- driven hydrovoltaic devices. The three main experimental techniques we employ are a direct measurement of open circuit voltage (OCV), I-V characteristics, and electrochemical impedance spectroscopy (EIS). These measurements were carried out for the series of devices with different geometrical parameters and a wide range of electrolyte concentrations for different cationic and anionic components of the simple salts. The study of concentration dependence on OCV was carried out by sweeping concertation up to 4M starting from 0.1 μM. We observe non-monotonic behavior, which can be attributed to the increase in charge screening effect at high concentrations due to ion adsorption at the stern plane of the electrical double layer and ion-ion correlation [2] . Furthermore, the effect of geometrical parameters on OCV was due to the change in the streaming of ions and the associated ionic resistance. We quantify the effective surface charge by determining correlations from our simulation results. For determining the output power, we measured the I-V characteristics for the series of geometrical parameters in various electrolyte concentrations. From moderate-to-high concentration, an increase in power can be attributed to the increase in the advection of ions. At lower concentrations, a slight increase in power output was a consequence of electrical double-layer overlap. The nature of the dependence of output power on the geometrical parameters was similar to the OCV. The EIS measurements were carried out to investigate the effect of ion diffusion and the associated resistive and capacitive elements in the equivalent electrical circuit model of our system [3] . Overall, this study highlights the importance of electrokinetic parameters and nanofabrication process parameters on the performance of hydrovoltaic devices made of uniform array nanostructured silicon. References 1. Chem.Int.Ed.2020,59,10619–10625 2. Rev. Lett. 119 , 026002 3. Langmuir 2014, 30, 36, 10950–10961
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