Organic Molecular Catalyst for Hydrogen Evolution Reaction
Authors: Yin Xi , Lin Ling , Chung Hoon T , Martinez Ulises , Baker Andrew M. , Maurya Sandip , Zelenay Piotr 
Published: February 2020

Hydrogen evolution reaction (HER) is fundamentally important and critical in water electrolysis and electrochemical hydrogen pumping.1-3 Recently, molecular HER catalysts based on earth-abundant metal complexes have received much research interest for their role as a low-cost alternative to Pt-based catalysts.4-7 However, such molecular catalysts often show poor stability in harsh acidic aqueous environment, which makes them not viable for the most common polymer electrolyte membrane (PEM) electrolyzer. The HER catalysts based on organic molecules and polymers, including N-containing molecules and polymers, have also attracted attention but they suffer from slow kinetics restricting their practicality.8-11

In this presentation, we will demonstrate a new HER catalyst based on a small organic molecule. This catalyst, which is different from materials based on metal complexes, has been evaluated in both aqueous acidic electrolyte and Nafion membrane electrode assembly (MEA), at room temperature and at 80 °C. The interaction of this molecule with acidic electrolytes, including sulfuric acid and Nafion, has also been studied using Fourier-transform Infrared (FTIR) spectroscopy. The results indicate that a unique molecular structure of the catalyst is responsible for significant acceleration in the HER rate, resulting in the onset potential of less than 0.05 V vs. RHE. The durability of this molecule has also been evaluated in H2 pump experiment in MEA revealing respectable stability during a 60 h test at a bias voltage of -0.2 V, with faradaic efficiency close to 100%.
Based on the experimental data and density functional theory (DFT) calculations, the reaction mechanism will be proposed. It will involve an estimate of the Gibbs free energy change along the HER pathway and determination of the rate-limiting reaction steps and corresponding activation energies by transition state searching along the reaction coordinate. Finally, the activity-structure relationship for the studied catalyst will be analyzed and compared with those for similar molecular catalysts.

Acknowledgements

Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged.

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