(415c) Achievable Alcohol Concentrations and Membrane Requirements for Artificial Photosynthetic System

Artificial photosynthesis of liquid fuels is a potential source for clean energy. Alcohols are particularly attractive products because of their high energy density and market value per amount of energy input. The major challenges in photo/electrochemical synthesis of alcohols from sunlight, water and CO₂ are low product selectivity, high membrane fuel-crossover losses, and high cost of product separation from the electrolyte. The maximum achievable concentrations of various alcohols in the catholyte are of the order of 1 mol % at partial current densities of 10 mA cm^-2. However such concentrations can only be achieved about 100 h of batch operation and result in in 100% fuel-crossover if a Nafion 117 membrane is used to separate the catholyte and anolyte compartments. Reducing the alcohol permeability of the ion-exchange membrane by an order of magnitude only increases the liquid fuel concentration to 10 mol%, but also raises the batch operation time needed to achieve this concentration to 1000 h. To overcome these limitations, we propose an artificial photosynthesis scheme for direct synthesis and separation to almost pure ethanol with minimum product crossover using saturated salt electrolytes. The ethanol produced in the saturated salt electrolytes can be readily phase separated into a microemulsion, which can be collected as pure products in a liquid-liquid extractor. A novel design of an integrated artificial photosynthetic system is proposed that continuously produces >90 wt% pure ethanol using a polycrystalline copper cathode at a current density of 0.85 mA cm^-2. The annual production rate of > 90 wt% ethanol using such a photosynthesis system operating at 10 mA cm^-2 (12% solar-to-fuel (STF) efficiency) can be 15.27 million gallons per year per square kilometer, which corresponds to 7% of the industrial ethanol production capacity of California.