2022 Annual Meeting

(307i) Kinetic and Thermodynamic Aspects of Voltage As a Driving Force for Ammonia Activation

Authors

Schiffer, Z. - Presenter, Massachusetts Institute of Technology
Manthiram, K., Stanford University
In chemical synthesis, the making and breaking of chemical bonds often requires traversing large energy differences. In fact, in 2012 the basic chemical industry accounted for close to 20% of total delivered energy consumption in the industrial sector, which itself used the most delivered energy of any end-use sector globally (54%). Traditionally, industrial chemical synthesis has relied on pressure and temperature to drive chemical reactions, and the energy to elevate the pressure and temperature generally comes from fossil fuel sources. These chemical reactors also often require centralized locations so that economies of scale make them profitable (e.g., ammonia synthesis). However, with the advent of distributed and accessible renewable electricity, it is attractive to consider driving chemical reactions that are conventionally driven with temperature and pressure with renewable electricity instead. Electrochemical reactors can be driven at mild temperatures and pressures and can take advantage of renewable electricity sources directly, enabling decentralized manufacturing. Thus, electrification of the chemical industry offers an opportunity to further integrate the chemical and energy industries, motivating and leveraging decentralized renewable energy sources and storage for chemical manufacturing.

In this talk, we first look at the broad question of how to compare electrochemical routes with traditional thermochemical routes for chemical transformations, aiming 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?” We present a framework for comparing voltage, temperature, and pressure as thermodynamic driving forces to help quantitatively discriminate between energy sources. Second, we discuss electrochemical utilization of ammonia. Ammonia has one of the largest global production rates by volume, and it is a nexus synthesis molecule: it either directly or indirectly provides nitrogen for a range of molecules including specialty chemicals, polymers, and pharmaceuticals. Ammonia is also attractive as an energy-dense, carbon-free fuel. Because of this, it represents an important target for electrification. We discuss the kinetics of ammonia electro-oxidation, i.e., the breaking of the nitrogen-hydrogen bonds, as well as how an applied potential can help form carbon-nitrogen bonds, an electrochemical analogue to traditional reductive amination. Last, we investigate an energy storage paradigm that leverages ammonium formate, a combination of ammonia and formic acid, to store renewable electricity. We discuss the advantages of this fuel and demonstrate how voltage can aid in the release of energy from this fuel. Overall, we start with the broad question of why and when to use voltage in the chemical transformations, and then we focus on how electrochemistry can aid in ammonia utilization for both synthesis reactions and energy storage purposes.