PT - JOURNAL ARTICLE AU - Le, Jia-Bo AU - Fan, Qi-Yuan AU - Li, Jie-Qiong AU - Cheng, Jun TI - Molecular origin of negative component of Helmholtz capacitance at electrified Pt(111)/water interface AID - 10.1126/sciadv.abb1219 DP - 2020 Oct 01 TA - Science Advances PG - eabb1219 VI - 6 IP - 41 4099 - http://advances.sciencemag.org/content/6/41/eabb1219.short 4100 - http://advances.sciencemag.org/content/6/41/eabb1219.full SO - Sci Adv2020 Oct 01; 6 AB - Electrified solid/liquid interfaces are the key to many physicochemical processes in a myriad of areas including electrochemistry and colloid science. With tremendous efforts devoted to this topic, it is unexpected that molecular-level understanding of electric double layers is still lacking. Particularly, it is perplexing why compact Helmholtz layers often show bell-shaped differential capacitances on metal electrodes, as this would suggest a negative capacitance in some layer of interface water. Here, we report state-of-the-art ab initio molecular dynamics simulations of electrified Pt(111)/water interfaces, aiming at unraveling the structure and capacitive behavior of interface water. Our calculation reproduces the bell-shaped differential Helmholtz capacitance and shows that the interface water follows the Frumkin adsorption isotherm when varying the electrode potential, leading to a peculiar negative capacitive response. Our work provides valuable insight into the structure and capacitance of interface water, which can help understand important processes in electrocatalysis and energy storage in supercapacitors.