Effects of surface modification of carbon nanofibers on their electrocatalytic activity for hydrogen evolution reaction of water electrolysis

Abstract

The effects of surface modification of CNF on their electrocatalytic activity for hydrogen evolution reaction (HER) were investigated. Firstly, Oxygen-containing functional groups were introduced onto the CNF surface (labeled as CNF-OX) by a simple sonochemical oxidation in mixed acids (concentrated sulfuric acid and nitric acid) and then the nitrogen-containing functional groups were introduced onto the CNF-OX surface (labeled as CNF-ON) by sonochemical treatment in ammonia, and the B-doped CNF was synthesized by mixing CNF-OX and boric acid in a ratio following by pyrolysis at 800℃ under N2 atmosphere (labeled as B-CNF-OX). The XPS results showed that the ultrasonic treatments could introduce oxygen- and nitrogen-containing functional groups onto the CNF surface successfully, the oxygen atoms content of CNF-OX was increased from 1.96% to 6.01% compared to the untreated CNF (CNF-UN), and nitrogen atoms content of CNF-ON was increased from 0% to 1.02% compared to CNF-OX. The pyrolysis of boric acid could introduce B element onto the CNF-OX surface where the B atoms content of B-CNF-OX was 1.00%. The linear sweep voltammetry (LSV) test results showed that the HER electrocatalytic activity of the modified CNFs were higher than CNF-UN, and among them CNF-ON had the highest electrocatalytic activities for HER than the others, the onset overpotential of CNF-ON was 344 mV, which shifted 240 mV positively compared to CNF-UN, and the electrocatalytic activity of B-CNF-OX was enhanced by B-doping treatment compared to CNF-OX. The Tafel test indicated CNF-ON had smallest Tafel slop of 154 mV/dec and largest exchange current density of 6.68 μA/cm2. The electrochemical impedance spectroscopy (EIS) was carried out to investigate the complex interfacial properties of the CNFs modified electrode, and the results showed that CNF-ON had the smallest charge transfer resistance of 1578 Ω among all the catalysts, which means the nitrogen-doped CNF had higher activity than the oxygen- and boron-doped CNF. All above indicated that the doping of heteroatom (O, B, N) onto the CNFs surface had effects on their electrocatalytic activity for HER, especially the introduction of N atom onto the CNF surface had excellent improvement.