Thermodynamic Discrimination between Energy Sources for Chemical Reactions

doi:10.1016/j.joule.2020.12.014

Joule

Volume 5, Issue 1, 20 January 2021, Pages 135-148

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Highlights

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Reaction-independent, non-dimensional expression for chemical equilibrium is developed
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Temperature, pressure, and voltage are compared as driving forces for chemical synthesis
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Visualization reveals divide between electro- and thermochemical reactions

Context & Scale

The equilibrium of a chemical reaction can be shifted using various driving forces, including temperature (thermal energy), pressure (mechanical energy), and voltage (electrical energy). These driving forces are often compared with technoeconomic analyses that discriminate between mature chemical synthesis routes where associated costs can be tabulated. Here, we demonstrate how thermodynamic analyses provide a framework to compare and discriminate between energy sources for chemical reactions. This methodology is useful for comparisons of less developed synthetic routes where accurate costs cannot be ascribed. Specifically, our analysis provides a universal, non-dimensional framework through which the effect of temperature, pressure, and voltage on reactions can be understood. This is articulated in both the context of chemical equilibria and energy exchanges, providing understanding across industrially practiced and next-generation chemical synthesis routes.

Summary

Chemical transformations traverse large energy differences, yet the comparison of energy sources to drive a reaction is often done on a case-by-case basis; there is no fundamentally driven, universal framework with which to analyze and compare driving forces for chemical reactions. In this work, we present a reaction-independent expression for the equilibrium constant as a function of temperature, pressure, and voltage. With a specific set of axes, all reactions are represented by a single $(x, y)$ point, and a quantitative divide between electrochemically and thermochemically driven reactions is visually evident. Additionally, we show that our expression has a strong physical basis in work and energy fluxes to the system, although specific data about operating conditions are necessary to provide a quantitative energy analysis. Overall, this universal equation and facile visualization of chemical reactions provides a consistent thermodynamic framework for comparing electrochemical versus thermochemical energy sources without knowledge of detailed process parameters.

Graphical Abstract

Keywords

electrochemistry

chemical thermodynamics

electrochemical synthesis

energy sources

equilibrium constant

chemical equilibrium

chemical synthesis

thermochemical reactions

electrochemical reactions

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