Catalysts

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Goal

Discover materials made from Earth-abundant elements to enable: 1) the efficient conversion of water, electrons, and carbon dioxide to carbon-based fuels; and, 2) the efficient conversion of water to oxygen and protons.

Strategy

Thin films of nickel-gallium catalyze the reduction of carbon dioxide to highly reduced products such as methane, ethane, and ethylene.¹

We use a variety of methods such as solution-based growth of nanoparticles, electrodeposition, sputtering, and temperature-programmed reduction to synthesize new heterogeneous – i.e., non-molecular – catalysts.  We are looking for Earth-abundant materials that are stable under the operating conditions for solar fuels devices.  We target specific materials by looking to analogous reactions (such as hydrodesulfurization for hydrogen production) and to theoretical models and predictions.

We apply a variety of techniques to characterize the structure, phase, composition, and surface area of catalysts.  We also apply electrochemical and spectroscopic techniques to evaluate the performance and stability of catalysts.

Highlights

Acid-Stable Catalysis of the Water-Oxidation Reaction by Crystalline Nickel Manganese Antimonate

Fuel-forming electrochemical devices require the oxidation of water to liberate the electrons needed for the cathodic fuel-forming reaction.  Prior to this work, the only well-established active catalysts for water oxidation in acidic electrolytes contained Ru or Ir, two of the scarcest non-radioactive elements on Earth.  In this work, we synthesized a series of solid solutions of NiSb₂O₆ with MnSb₂O₂ and characterized the electrocatalytic activity and stability of the materials for water oxidation in acidic electrolytes.

Chronopotentiometric stability of nickel manganese antimonate electrodes, showing the overpotential required to drive a current density of 10 mA cm-2 in contact with 1.0 M H2SO4 as a function of time. The catalysts were stable for the > 164 h duration of the stability test.²

The Ni_xMn_1-xSb_1.7O_y films exhibited stable water oxidation for > 168 h of continuous operation in 1.0 M sulfuric acid.  The stability of the films depended upon the formation of a crystalline MSb₂O₆ rutile phase.  Among the catalysts explored, Ni_0.5Mn_0.5Sb_1.7O_x exhibited the lowest overpotential, requiring 745 mV of overpotential to drive the water-oxidation reaction at a rate corresponding to a current density of 10 mA cm^-2.

A distinguishing feature of this work was an emphasis on characterizing the catalyst before, during, and after operation, as well as examination of the electrolyte for dissolved metals.  In addition, we also considered the stability of the catalyst without an applied potential, to provide a thorough description of the stability of the catalyst not only under the operating conditions for a solar fuels device, but also under conditions prevalent when a solar fuels device is at rest.

Electrocatalytic Reduction of Water to Hydrogen Gas by Transition-Metal Phosphides

Transmission electron micrographs of (left) Ni₂P nanoparticles and (center) a representative nanoparticle in high resolution showing crystal facets and planes. (center) Polarization data for Ni₂P electrodes in contact with 0.5 M H₂SO₄, showing glassy carbon, Ti foil, and Pt for comparison.³

In collaboration with the Schaak Group at Penn State, we synthesized, characterized, and evaluated the electrocatalytic activity of a series of transition-metal phosphides.  These materials were targeted because a member of this class, specifically nickel phosphide (Ni₂P), is a catalyst of the hydrodesulfurization reaction – a reaction analogous to water reduction.  Ni₂P was the first in the series of transition-metal phosphides to be synthesized and evaluated.  Ni₂P demonstrated water-reduction activity amongst the highest of any non-noble metal electrocatalysts that had been reported previously.

In continued collaboration, our team went on to synthesize and characterize cobalt phosphide, molybdenum phosphide, iron phosphide, tungsten phosphide.  All of these materials demonstrated electrocatalytic activity for water reduction in acid.  Since Ni–Mo can catalyze the reduction of alkaline water, a number of catalysts made from Earth-abundant elements are now available for the reduction of water to hydrogen gas under either acidic or alkaline conditions.

References

1. Torelli, D. A.; Francis, S. A.; Crompton, J. C.; Javier, A.; Thompson, J. R.; Brunschwig, B. S.; Soriaga, M. P.; Lewis, N. S., Nickel–gallium-catalyzed electrochemical reduction of CO₂ to highly reduced products at low overpotentials. ACS Catal. 2016, 6 (3), 2100-2104.
2. Moreno-Hernandez, I. A.; MacFarland, C. A.; Read, C. G.; Brunschwig, B. S.; Papadantonakis, K. M.; Lewis, N., Crystalline Nickel Manganese Antimonate as a Stable Water-Oxidation Catalyst in Aqueous 1.0 M H₂SO₄. Energy Environ. Sci. 2017, 10, 2103-2108.
3. Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E., Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135 (25), 9267-9270.

_{by Kimberly Papadantonakis, June 2016. (last update March 2018)}