Electrochemical Oxidation of Hydrothermal Liquefaction-Derived Aqueous Phase for Simultaneous H2 Production
Authors: Bhatwadekar Swanand Sadashiv , Riedel Nickolas W , Lin Fan , Boruah Bhanupriya , Strange Lyndi E. , Lopez-Ruiz Juan A. 
Published: February 2024

Hydrothermal liquefaction (HTL) is a thermochemical process in which high water containing feedstocks (95%-75%) such as food waste or sewage sludge is converted into fuels. There are four different phases generated during HTL, namely biocrude (oil phase), biochar (solid phase), aqueous phase and gaseous phase. The biocrude phase needs to be processed with hydrogen (H2) via catalytic hydrotreating to produce gasoline, diesel, jet oil. The aqueous phase (AP) contains up to 5 wt% organic compounds mostly in the form of carboxylic acids and amines and inorganics (e.g., Na, K) and cannot be treated with traditional wastewater treatment technologies (e.g., anaerobic digestion). Hence, it is necessary to treat this wastewater so it can be released safely.
Electrocatalysis represents a novel approach to treat this AP waste stream while simultaneously generating the H2 needed for hydrotreating. Instead of using traditional water electrolysis (H2O → 2 H+ + 2 e- + ½ O2, E° = − 1.18 V, ∆H° = 275 kJ/mol H2), we explore electrocatalytic oxidation (ECO) of organic molecules (e.g., CH3COOH + 2 H2O → 2 CO2 + 8 e- + 8 H+, E° = − 0.02 V, ∆H° = 53 kJ/mol H2) to simultaneously remove organics from the HTL-derived AP while simultaneously producing H2. The HTL-derived AP can have different concentration and composition of both organics as well as inorganics, which depend on the source feedstock. The ECO of different HTL-derived wastewaters was evaluated in this study. It was studied, how the concentration and source of AP as well as the electrochemical reaction conditions affect the organic removal as well as the H2 production in both batch cell and continuous flow-cell configuration. Our results show that the presence of organics decreases the current density as well as causes anode deactivation after 40 h of operation in the continuous flow cell. However, the cathode performance and stability remained unaffected for more than 1,000 h. Different strategies were evaluated to enhance the life of the electrode and running the system under alkaline conditions improved the anode stability 5 folds

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