(20c) Discriminating between Energy Sources for Chemical Reactions

The chemical and energy industries have historically strongly influenced each other, and recent developments in the energy industry due to penetration of renewable electricity and development of renewable fuels will continue to bring the industries closer together. However, the choice of energy source used to drive a chemical reaction is often decided on a case-by-case basis with qualitative justifications provided after the fact; there is no universal framework in which to analyze and compare chemical reactions. We therefore aim to answer the question "If I can apply mechanical energy (pressure), thermal energy (temperature), or electrical energy (voltage) to a chemical reaction, which should I use?" In particular, we are working to understand both when and why one should drive a chemical reaction by applying a voltage, raising the temperature, or increasing the pressure.

In this work, we present a universal expression for the equilibrium constant of a chemical reaction as a function of thermodynamic driving forces and demonstrate how all reaction equilibrium contours can be plotted with non-dimensional groups that scale with temperature, pressure, and voltage. With a specific set of axes, all reactions can be represented by a single (x,y) point and a clear divide between electrochemically and thermochemically driven reactions is visually evident. This visualization shows that reactions with large enthalpies generally cannot be driven solely by temperature and pressure, whereas such reactions can be driven electrochemically according to their thermodynamic equilibrium. Converting from temperature, pressure, and voltage to heat and work fluxes qualitatively supports this conclusion, although more specific data about reaction operation such as equipment efficiencies is necessary to provide a quantitative energy analysis. This universal equation and facile visualization of chemical reactions enables quick and informed justification for electrochemical versus thermochemical energy sources.