(Invited) Modified Oxide Electrocatalysts for Renewable Fuels and Energy

Abstract

In this talk I will discuss research to develop and understand modified oxide electrocatalysts for renewable fuels and energy. In the first part, I will focus on catalysts for the oxygen evolution reaction (OER). An important challenge for realizing cheap, stable, and efficient electrocatalysts for producing renewable fuels and energy is to enable transition metal oxides (TMOs) to replace precious metal-based electrocatalysts (e.g. IrOx and RuOx) for OER. Low energy efficiencies of OER resulting from sluggish reaction kinetics and large overpotentials is a major cause of energy losses in photocatalytic systems for solar fuels and in a number of other emergent technologies such as water splitting devices, rechargeable metal-air batteries, and unitized regenerative fuel cells. Insights from two examples of our studies will be discussed, involving a range of spectroscopic techniques for characterization of Ce-modified copper oxide (CuOx) and Ni-modified cobalt (oxy)hydroxides to reveal the importance of chemical state and local structure considerations for the rational design of improved oxide-based OER catalysts. In the case of Ce-modified CuOx, a strong promoting effect of Ce4+ for OER was observed for Ce incorporation (7 at%) into CuOx up to a concentration corresponding to CeO2 phase segregation, as probed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. In the case of Ni-modified CoOxHy, operando Raman spectroscopy and electrochemical techniques were used during OER to identify the composition and local structure of electrodeposited CoOxHy and Ni-modified CoOxHy catalyst films. During OER, Ni-modified CoOxHy films with lower initial crystallinity underwent substantial structural evolution that began with an irreversible transformation of a spinel local structure to an amorphous CoO structure at low anodic potentials. Increasing anodic polarization with elevated oxygen evolution rates caused additional structural conversion of the amorphous CoO structure to a complex phase that can be described as an amalgamation of NiOOH and layered CoO2 motifs (NiOOH-h-CoO2). Formation of this structure, independent of the initial cobalt oxide structure, suggests that this could be the universally active structure for NiCoOxHy catalysts. I will also discuss recent work on phosphorus-doped Hf-Ru oxide (denoted here as HfxRuyPO) electrocatalysts for OER in which we demonstrated a strategy for efficiently promoting the durability of RuO2. This research was motivated by the need to address the rapid corrosion and precipitation of highly efficient precious metal (Ir, Ru, and Rh) nanoparticle catalysts in the harsh environment of OER in proton exchange membrane (PEM) electrolyzers for hydrogen fuel generation. We found that when the HfxRuyPO catalysts possessed a Hf:Ru atomic ratio of 1:2, they exhibited a highly efficient and stable OER activity over 40 hours. By XPS and Raman analyses, the HfPO component was found to play a significant role in improving the durability of RuO2 through the formation of a HfOOH phase, which also possessed a higher OER activity and lower impedance.In the second part of the talk, I will briefly describe our report of an efficient acid-stable N2-plasma treated hafnium oxyhydroxide electrocatalyst for hydrogen evolution and oxidation reactions (HER and HOR). We found that processing Hf oxide with an atmospheric N2 plasma forms an acid-insoluble hafnium oxynitride material, and we propose that under electrochemical environments this material is transformed into an active oxynitride hydroxide that demonstrates unprecedented high catalytic activity and stability for both HER and HOR in strong acidic media for earth-abundant materials. The zero onset potentials and high current densities demonstrate that this material is a promising alternative to Pt group metal (PGM) catalysts, and these results have broad implications for using nitrogen incorporation to activate other non-conductive compounds and films to form new active electrocatalysts. I will also discuss our recent observations on the reaction-driven restructuring of defective PtSe2 into an ultrastable electrocatalyst for the oxygen reduction reaction (ORR), which is of interest for accelerating the commercialization of PEM fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs). We have found that the restructured defective PtSe2 electrocatalyst had 1.3 times the specific activity and 2.6 times the mass activity of a commercial Pt/C catalyst, and maintained this superior ORR activity even after 126,000 cycles in accelerated durability tests.