Electrocatalysis


Introduction

Electrocatalysts are critical to increase reaction rates and control selectivity in many electrochemical fuel production and consumption reactions. In the Jaramillo Group, we develop new electrocatalyst materials for processes including hydrogen evolution and oxidation, oxygen evolution and reduction, and carbon dioxide reduction. These reactions are necessary for electricity generation in hydrogen fuel cells, H2 production through water electrolysis, and CO2 conversion to create useful fuels and chemicals. Our approach is to understand the catalyst properties that control activity, selectivity, and stability by combining catalyst synthesis and electrochemical performance testing with physical and chemical characterization as well as computational and theoretical modeling performed by collaborators. Using these insights, we design new electrocatalyst materials with improved performance.

Hydrogen Evolution Reaction


The hydrogen evolution reaction (HER, 2 H+ + 2 e → H2) is the cathodic reaction in electrochemical water splitting. The HER is a classic example of a two-electron transfer reaction with one catalytic intermediate, and offers the potential to produce H2, a critical chemical reagent and fuel. Driving the HER with renewable sources of energy can lead to a sustainable source of hydrogen fuel that can stored, transported and used in a zero-emission fuel cell of combustion engine. Achieving high energetic efficiency for water splitting requires the use of a catalyst to minimize the overpotential necessary to drive the HER. Platinum is the best known catalyst for HER and requires very small overpotentials even at high reaction rates in acidic solutions. However, the scarcity and high cost of Pt limits its widespread technological use. We study the fundamental material properties that determine catalytic activity for the HER. By designing new catalyst materials for the HER, we have expanded our understanding of the surface structures and properties that govern HER activity and stability. By applying this knowledge towards next generation catalyst design, we have developed several earth-abundant HER catalysts, including sulfide- and phosphide-based materials, with activities approaching that observed for platinum.

Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is a pertinent process in many electrochemical energy conversion and storage technologies, including fuel cells and rechargeable metal-air batteries. In acidic conditions, only platinum-based catalysts have shown sufficient activity and durability for practical purposes, although comes at a high cost and relies on limited global platinum supplies. Research in our laboratory focuses on designing improved activity ORR catalysts, with significantly reduced or eliminated platinum contents. Some strategies that have been employed to achieve this include the use of solid solution or intermetallic platinum alloys, shape controlled nanostructures to achieve preferential facet exposure, or the preparation of core-shell nanoparticles that can tune the interactions (i.e., binding energies) with ORR intermediate species. Our aim is to develop an improved understanding of the ORR process occurring on various surfaces, providing fundamental insight that can rationally guide our design of new catalyst materials.


In alkaline conditions, the ORR is a much more facile process and provides opportunity for non-platinum catalyst alternatives, including metal oxides, nanostructured carbons and silver-based catalysts. The ORR process on these surfaces is poorly understood in comparison to platinum, and the activity still lags behind that of platinum on a turn over frequency basis. We aim to develop new material designs and develop a thorough understanding of the surface structures and properties that influence intermediate adsorption energies and ORR activity on various alkaline-based catalysts.

The ORR can also proceed by a 2 electron reduction mechanism, forming hydrogen peroxide species. While this is undesirable from an energy efficiency standpoint for fuel cells (only 2 electrons transferred per oxygen molecule to form hydrogen peroxide as opposed to 4 to form water), this electrochemical reaction is advantageous because it could replace the energy intensive anthraquinone process conventionally used to synthesize hydrogen peroxide. This electrochemical synthesis could, for example, be coupled to renewable sources of energy (i.e., wind or solar) to provide on site generation of hydrogen peroxide. As a powerful oxidizer, distributed production of hydrogen peroxide could play an important role for drinking water treatment or sterilization applications.

CO2 Reduction Reaction


Electrochemical reduction of CO2 has a potential of becoming a major contributor to sustainable production of fuels and chemicals through the use of renewable CO2 free energy sources. However, the development of an effective catalyst is vital, as there are currently no industrial scale operations that utilize this technology due to the low energetic efficiency. In our group, we focus on gaining fundamental understanding of the surface chemistry by tuning some of the key catalyst characteristics, such as the composition, the surface structure, and the morphology, as well as other factors, such as electrolyte composition and reaction conditions. By implementing the insights gained from our work, we aim to design effective catalysts that would allow for the industrialization of the technology.