Joseph H. Montoya

A highly active and stable IrOx/SrIrO3 catalyst for the oxygen evolution reaction

Oxygen electrochemistry plays a key role in renewable energy technologies such as fuel cells and electrolyzers, but the slow kinetics of the oxygen evolution reaction (OER) limit the performance and commercialization of such devices. Here we report an iridium oxide/strontium iridium oxide (IrOx/SrIrO3) catalyst formed during electrochemical testing by strontium leaching from surface layers of thin films of SrIrO3. This catalyst has demonstrated specific activity at 10 milliamps per square centimeter of oxide catalyst (OER current normalized to catalyst surface area), with only 270 to 290 millivolts of overpotential for 30 hours of continuous testing in acidic electrolyte. Density functional theory calculations suggest the formation of highly active surface layers during strontium leaching with IrO3 or anatase IrO2 motifs. The IrOx/SrIrO3 catalyst outperforms known IrOx and ruthenium oxide (RuOx) systems, the only other OER catalysts that have reasonable activity in acidic electrolyte.

Materials for solar fuels and chemicals

The conversion of sunlight into fuels and chemicals is an attractive prospect for the storage of renewable energy, and photoelectrocatalytic technologies represent a pathway by which solar fuels might be realized. However, there are numerous scientific challenges in developing these technologies. These include finding suitable materials for the absorption of incident photons, developing more efficient catalysts for both water splitting and the production of fuels, and understanding how interfaces between catalysts, photoabsorbers and electrolytes can be designed to minimize losses and resist degradation. In this Review, we highlight recent milestones in these areas and some key scientific challenges remaining between the current state of the art and a technology that can effectively convert sunlight into fuels and chemicals.

Insights into CC Coupling in CO2 Electroreduction on Copper Electrodes

We present a first‐principles theoretical study of carbon–carbon coupling in CO2 electroreduction on the copper 2 1 1 surface. Using DFT, we have determined kinetic barriers to the formation of a CC bond between adsorbates derived from CO. The results of our nudged elastic band calculations demonstrate that kinetic barriers to CC coupling decrease significantly with the degree of hydrogenation of reacting adsorbates. We also show that this trend is not affected by the electrical fields present at the solid‐electrolyte interface during electrocatalysis. Our results explain how copper can catalyze the production of higher hydrocarbons and oxygenates in the electrochemical environment, despite producing only single carbon atom products in gas‐phase catalysis, and how CC bonds can be formed at room temperature in the electrochemical environment, whereas substantially higher temperatures are needed in the Fischer–Tropsch catalysis. The unique feature of the electrochemical environment is that the chemical potential of hydrogen (electrons and protons) can be varied through the applied potential. This allows a variation of the degree of hydrogenation of the reactants and thus the activation barrier for CC coupling.

Theoretical Insights into a CO Dimerization Mechanism in CO2 Electroreduction

In this work, we present DFT simulations that demonstrate the ability of Cu to catalyze CO dimerization in CO2 and CO electroreduction. We describe a previously unreported CO dimer configuration that is uniquely stabilized by a charged water layer on both Cu(111) and Cu(100). Without this charged water layer at the metal surface, the formation of the CO dimer is prohibitively endergonic. Our calculations also demonstrate that dimerization should have a lower activation barrier on Cu(100) than Cu(111), which, along with a more exergonic adsorption energy and a corresponding higher coverage of *CO, is consistent with experimental observations that Cu(100) has a high activity for C-C coupling at low overpotentials. We also demonstrate that this effect is present with cations other than H(+), a finding that is consistent with the experimentally observed pH independence of C2 formation on Cu.

The Challenge of Electrochemical Ammonia Synthesis: A New Perspective on the Role of Nitrogen Scaling Relations

The electrochemical production of NH3 under ambient conditions represents an attractive prospect for sustainable agriculture, but electrocatalysts that selectively reduce N2 to NH3 remain elusive. In this work, we present insights from DFT calculations that describe limitations on the low-temperature electrocatalytic production of NH3 from N2 . In particular, we highlight the linear scaling relations of the adsorption energies of intermediates that can be used to model the overpotential requirements in this process. By using a two-variable description of the theoretical overpotential, we identify fundamental limitations on N2 reduction analogous to those present in processes such as oxygen evolution. Using these trends, we propose new strategies for catalyst design that may help guide the search for an electrocatalyst that can achieve selective N2 reduction.

Understanding trends in C-H bond activation in heterogeneous catalysis.

While the search for catalysts capable of directly converting methane to higher value commodity chemicals and liquid fuels has been active for over a century, a viable industrial process for selective methane activation has yet to be developed. Electronic structure calculations are playing an increasingly relevant role in this search, but large-scale materials screening efforts are hindered by computationally expensive transition state barrier calculations. The purpose of the present letter is twofold. First, we show that, for the wide range of catalysts that proceed via a radical intermediate, a unifying framework for predicting C-H activation barriers using a single universal descriptor can be established. Second, we combine this scaling approach with a thermodynamic analysis of active site formation to provide a map of methane activation rates. Our model successfully rationalizes the available empirical data and lays the foundation for future catalyst design strategies that transcend different catalyst classes.

Improving Oxygen Electrochemistry through Nanoscopic Confinement

Oxygen evolution and reduction offer a promising method of grid‐level energy storage that could facilitate widespread adaptation of solar and wind power. However, the efficiency of these technologies is fundamentally limited by high overpotentials, which stem from correlations between adsorption energies of different reaction intermediates. We propose a scheme to circumvent these scaling relationships by defining a three‐dimensional nanoscopic catalyst structure that capitalizes on different interactions between the intermediates and the catalyst owing to confinement. These nanoscopic channels reduce the theoretical overpotential for oxygen evolution on RuO2 by over 200 mV, corresponding to a 10 % increase in theoretical catalyst efficiency compared with a two‐dimensional RuO2 surface. This approach may hold promise for other oxygen‐evolution catalysts or, more broadly, to other reactions limited by (intermediate) adsorption‐energy scaling relationships.

Acetaldehyde as an Intermediate in the Electroreduction of Carbon Monoxide to Ethanol on Oxide-Derived Copper.

Abstract Oxide‐derived copper (OD‐Cu) electrodes exhibit unprecedented CO reduction performance towards liquid fuels, producing ethanol and acetate with >50 % Faradaic efficiency at −0.3 V (vs. RHE). By using static headspace‐gas chromatography for liquid phase analysis, we identify acetaldehyde as a minor product and key intermediate in the electroreduction of CO to ethanol on OD‐Cu electrodes. Acetaldehyde is produced with a Faradaic efficiency of ≈5 % at −0.33 V (vs. RHE). We show that acetaldehyde forms at low steady‐state concentrations, and that free acetaldehyde is difficult to detect in alkaline solutions using NMR spectroscopy, requiring alternative methods for detection and quantification. Our results represent an important step towards understanding the CO reduction mechanism on OD‐Cu electrodes.

Electroreduction of Methanediol on Copper

We have used density functional theory calculations to study intermediates in the electroreduction of methanediol on copper. We find that methanediol, which is the hydrated form of formaldehyde, may be reduced to methanol with a limiting potential close to the experimental onset for reduction of aqueous formaldehyde.Graphical Abstract

Direct Water Decomposition on Transition Metal Surfaces: Structural Dependence and Catalytic Screening

Density functional theory calculations are used to investigate thermal water decomposition over the close-packed (111), stepped (211), and open (100) facets of transition metal surfaces. A descriptor-based approach is used to determine that the (211) facet leads to the highest possible rates. A range of 96 binary alloys were screened for their potential activity and a rate control analysis was performed to assess how the overall rate could be improved.Graphical Abstract