Title: Development of Transition Metal Carbide and Nitride Electrocatalysts for Chemical Energy Storage and CO2 Conversion
Description: The rapid influx of solar energy and the desire to utilize carbon dioxide (CO2) will require large-scale energy storage and CO2 conversion technologies. Electrocatalytic devices can substantially impact both challenges, but improvements to electrocatalyst cost, activity, and selectivity are needed. Transition metal carbides provide a unique framework to reduce the loading of expensive catalyst metals while tuning the electrocatalytic activity and selectivity. Transition metal nitrides have many similar properties as carbides, and their synthesis inherently avoids the unwanted carbonaceous overlayer associated with carbide synthesis. Here it is shown that carbides and nitrides enable lower platinum-group metal (PGM) loadings and improve the activity and selectivity of electrocatalysts for reactions of water electrolysis and electrochemical CO2 reduction. Atom-thick layers of Pt were deposited onto niobium carbide (NbC) thin films to assess hydrogen evolution reaction (HER) activity. The Pt/NbC thin film, with one monolayer of Pt on NbC, performed similarly to bulk Pt.

This correlated well with density functional theory (DFT) calculations of the hydrogen binding energy on the Pt/NbC surface. Potential applications of transition metal nitrides as electrocatalyst support materials were explored by synthesizing thin film nitrides of niobium and tungsten. The stability of each nitride was evaluated across broad potential-pH regimes to create a pseudo-Pourbaix diagram for each one. The films were each modified with atom-thick layers of Pt and were evaluated for HER performance in acid and alkaline electrolyte. Thin layers of Pt on WN and NbN showed Pt-like HER performance in acid and are promising candidates for high-surface area catalysts. To address the issue of high iridium (Ir) loading for the oxygen evolution reaction (OER) at the water electrolyzer anode, core-shell Ir-metal nitride particles were synthesized that contained 50% of the Ir mass loading of benchmark IrO¬2 particles. Iridium-iron nitride (Ir/Fe4N) showed increased activity on a mass-Ir basis and on a per-site basis, compared to IrO2.

The core-shell morphology and stability under reaction conditions were confirmed with electron microscopy and in-situ X-ray absorption spectroscopy. Electrochemical reduction of CO2 to a mixture of CO and H¬2 (synthesis gas) was achieved on the palladium hydride (PdH) electrocatalyst. The product mixture can then be used as feedstock for the Fischer–Tropsch process and methanol synthesis. The syngas production performance was optimized by evaluating shape controlled PdH particles, bimetallic PdH, and PdH supported on transition metal carbides. At each step, the phase transition from Pd to PdH was monitored under reaction conditions with synchrotron-based X-ray absorption spectroscopy and X-ray diffraction. We also performed an overall carbon balance for catalytic transformation of CO2 to methanol via four reaction schemes, including one relying on electrocatalytic syngas production. The analysis revealed that hybrid electrocatalytic/thermocatalytic processes are most promising for resulting in overall CO2 reduction, but current densities of recently reported electrocatalysts need to increase to make the process economically feasible.