As the world continues to search for new ways to support the growth of a sustainable energy future, electrochemical conversions arise as a potential method to both store intermittent renewable electricity and produce biofuels and biochemicals. In the hydrothermal liquefaction (HTL) of biomass, a biocrude and an aqueous phase containing carboxylic acids, alcohols, etc. are produced. Under conventional bio-crude upgrading into biofuels, an external source of hydrogen (H₂) is required, which is traditionally sourced from fossil fuels. Instead, we propose to electrocatalytically oxidize (ECO) the aqueous phase to clean the wastewater while simultaneously producing H₂ via the hydrogen evolution reaction (HER).
In this work, we compare the performance of heterogeneous electrocatalysts for the ECO and HER of real HTL-derived wastewater as a function of current density and potentials at room temperature and atmospheric pressure. We synthesized HER catalysts using both platinum-group metals (PGM) as well as base-group metals (BGM) with equimolar loadings. While it is well known that PGM nanoparticles catalyze HER, our results show that BGM catalysts can outperform the PGM catalysts in terms of lowest energy requirement needed to generate H₂(Figure 1). For example, the BGMs required up to five times lower energy than PGMs regardless of the current density explored. We also evaluated the performance of our proprietary RuO₂-based ECO catalysts which show selective oxidation of organic molecules (e.g., CH₃COOH + 2H₂O ->2CO₂ + 8e^- + 8H⁺, E°=−0.02V, 53 kJ/mol) instead of traditional water electrolysis (H₂O→2H⁺+ 2e^-+ ½O₂, △H°=275 kJ/mol H₂, E°=−1.18 V), which has a five times lower energy demand.

<b>Figure </b><b>1</b><b>:</b> Specific energy requirements for hydrogen (H<sub>2</sub>) production of BGM and PGM electrocatalysts with equimolar metal loadings obtained at steady state current/current density values. These results were obtained in a continuous flow cell at room temperature and atmospheric pressure using real HTL-derived aqueous wastewater.

Figure 1: Specific energy requirements for hydrogen (H₂) production of BGM and PGM electrocatalysts with equimolar metal loadings obtained at steady state current/current density values. These results were obtained in a continuous flow cell at room temperature and atmospheric pressure using real HTL-derived aqueous wastewater.

Dr. Fan Lin

Chemical Engineer, Pacific Northwest National Laboratory

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