DNA-inspired polymers as metal-free electrocatalysts for hydrogen evolution reaction in acidic media

Abstract

Metal-free hydrogen-bonding conductive polymer electrocatalysts (HCPCs) are emerging alternatives to platinum-group metals for hydrogen evolution reaction (HER), especially in acidic media. Herein, we employ oxidative chemical vapor deposition (oCVD) to polymerize nucleobases consisting of deoxyribonucleic acids (DNAs), namely, adenine, guanine, cytosine and thymine to prepare putative HCPCs. The former three polymerized into PA (poly-adenine), PG (poly-guanine) and PC (poly-cytosine), whereas thymine failed to polymerize. Interestingly, only PG exhibited a high catalytic activity ($k^{\circ }$= 6.12 × 10⁻⁸ cm s^-1) for HER, achieving an overpotential of 411 mV for 10 mA cm⁻² when it was deposited onto a carbon felt, while PA and PC showed no catalytic activity. Cathodic charge transfer coefficients (α_c) clearly smaller than 0.5 were determined from the large Tafel slopes and thus imposed high overpotentials. DFT calculations and consideration about hydrogen-bonded network structure of the polymers suggested a Volmer-Tafel pathway as the predominant mechanism of the HER by HCPCs. Since PG is predicted to have the highest number of hydrogen bonding sites based on structural comparison, it offers closely positioned catalytic active sites, thereby promoting the rate-determining step known as the Tafel step.