Conducting Poly-Adenine Electrocatalyst for Hydrogen Evolution Reaction

Abstract

1. Introduction Hydrogen is a promising energy carrier, but is currently produced by steam reforming of fossil fuels to discharge CO2. Water splitting by electricity from renewable resources is an ideal solution. However, expensive and rare metals such as Pt and Ru are needed as electrocatalysts for their high activity and durability.We have recently found that metal-free organic conductive polymer made by polymerization of guanine (PG) exhibits a high catalytic activity for hydrogen evolution reaction (HER) close to that of Pt.1,2,3 Hydrogen bonding sites in PG are supposed to stabilize the reaction intermediates. It is also interesting that a nucleic acid as one of the key components of DNA has such properties for HER catalysis.In this work, we have explored other nucleic acids, namely, adenine, cytosine and thymine for their capabilities of polymerization by oxidative chemical vapor deposition (oCVD) and HER activities. 2. Experimental oCVD was carried out in a 3-zone heating quartz tubular furnace, in which 0.5 g of monomer (set to 300~360°C depending on melting points), oxidant (mixture of 0.5 mL sulfuric acid and 0.5 g sodium sulfate, 360°C) and carbon felt (CF) substrate (1.5 cm × 2.5 cm, Jing Long Te Tan, 360°C) were put under N2 stream (3 L/min). The deposition was maintained for 30 minutes after the set temperatures were reached and then allowed to cool down. FT-IR spectra were measured to check formation of conductive polymers. Linear sweep voltammetry (LSV) was performed at 10 mV/s on the polymer coated CF electrodes in a 0.1 M sulfuric acid under N2 to evaluate overvoltage for hydrogen evolution reaction (HER) at 10 mA/cm2. 3. Results and Discussion A black layer was formed by oCVD of adenine. Its FT-IR spectrum exhibited broad peaks around 3000 cm-1, indicative of hydrogen bonds, as well as several strong peaks below 1000 cm-1, which shows presence of free charge carriers to evoke infrared-activated vibration (IRAV). Since similar features were observed in the FT-IR of PG,2 formation of conductive poly-adenine (PA) is reasonably expected. While oCVD of cytosine also yielded similar black film, suggesting formation of PC, that of thymine did not afford any product on the substrate, despite several trials to change temperatures and other deposition conditions. The difference can be reasoned as adenine and cytosine bear cross-linkable amino groups, whereas thymine does not have one.Preliminary electrochemical study revealed an overvoltage of CA. 700mV, far larger than that of PG (290 mV).2 While PG has a hydrogen donating -NH group and a hydrogen accepting ketone group in α-position, PA does have an -NH group but not a ketone. Such difference in the structure of hydrogen bonding site is a possible reason for the large difference of the catalytic activity. Further studies about the HER electrocatalysis are under investigation.