Low-Temperature Wastewater Electrolysis for H2 and Clean Water Generation

Abstract

Water electrolysis represents a sustainable approach to produce green hydrogen (H2); however, the high energy cost (H2O → 2H+ + 2e- + ½O2, ∆H° = 275 kJ/mol H2, E° = −1.18 V) associated with water electrolysis and electricity cost (>$50/MWh) results in a minimum H2 selling price ≈$5/kWh. Hence, we need to find alternative process to generate H2 to meet the current U.S. Department of Energy's (DOE's) Energy Earthshots of $1/kg H2 in one decade.Wastewater electrolysis represents an alternative approach to generate H2 with renewable sources by electrochemically oxidizing organic molecules to generate H2 (e.g., CH3COOH + 2H2O → 2CO2 + 8e- + 8H+, ∆H° = 53 kJ/mol, E° = −0.02 V,) which has up to 5 times lower energy requirement than water electrolysis. As opposed to water electrolysis that requires clean water streams, wastewater electrolysis uses readily available waste streams that need cleanup anyway to generate H2; hence, integrating a second application in addition to H2 generation.1-3 In this work, we compare the performance of heterogeneous electrocatalysts for the electrocatalytic oxidation (ECO) of organic molecules present in wastewater as well as electrocatalysts for the H2 evolution reaction (HER) at room temperature. ECO and HER electrocatalysts were developed and tested using real wastewaters derived from the hydrothermal liquefaction (HTL) of food waste. We synthesized the electrocatalysts using both platinum-group metals (PGM) as well as base-group metals (BGM) with equimolar loadings and tested them as a function of potential. Our results show that while PGM-based electrocatalysts were required to perform ECO of HTL-derived wastewater4-6, BGM-based electrocatalysts were able to perform HER similarly to PGM electrocatalysts in HTL-derived wastewater. We performed a preliminary techno-economic analysis to outline a path towards $1/kg H2 via wastewater electrolysis and identified the next research and development areas. References Andrews, E.; Lopez-Ruiz, J. A.; Egbert, J. D.; Koh, K.; Sanyal, U.; Song, M.; Li, D.; Karkamkar, A. J.; Derewinski, M. A.; Holladay, J.; Gutiérrez, O. Y.; Holladay, J. D., Performance of Base and Noble Metals for Electrocatalytic Hydrogenation of Bio-Oil-Derived Oxygenated Compounds. ACS Sustainable Chem. Eng. 2020, 8 (11), 4407-4418.Lopez-Ruiz, J. A.; Qiu, Y.; Andrews, E.; Gutiérrez, O. Y.; Holladay, J. D., Electrocatalytic valorization into H2 and hydrocarbons of an aqueous stream derived from hydrothermal liquefaction. J. Appl. Electrochem. 2021, 51 (1), 107-118.Lopez-Ruiz, J. A.; Andrews, E.; Akhade, S. A.; Lee, M.-S.; Koh, K.; Sanyal, U.; Yuk, S. F.; Karkamkar, A. J.; Derewinski, M. A.; Holladay, J.; Glezakou, V.-A.; Rousseau, R.; Gutiérrez, O. Y.; Holladay, J. D., Understanding the Role of Metal and Molecular Structure on the Electrocatalytic Hydrogenation of Oxygenated Organic Compounds. ACS Catalysis 2019, 9 (11), 9964-9972.Qiu, Y.; Lopez-Ruiz, J. A.; Zhu, G.; Engelhard, M. H.; Gutiérrez, O. Y.; Holladay, J. D., Electrocatalytic decarboxylation of carboxylic acids over RuO2 and Pt nanoparticles. Appl. Catal. B-Environ. 2022, 305, 121060.Lopez-Ruiz, J. A.; Qiu, Y.; Andrews, E.; Gutiérrez, O. Y.; Holladay, J. D., Electrocatalytic valorization into H2 and hydrocarbons of an aqueous stream derived from hydrothermal liquefaction. J. Appl. Electrochem. 2021, 51 (1), 107-118.Qiu, Y.; Lopez-Ruiz, J. A.; Sanyal, U.; Andrews, E.; Gutiérrez, O. Y.; Holladay, J. D., Anodic electrocatalytic conversion of carboxylic acids on thin films of RuO2, IrO2, and Pt. Appl. Catal. B-Environ. 2020, 277, 119277. Figure 1