(353b) A Highly Efficient Fe-Rich Confined Catalyst within Layered Zirconium Phosphate for the Oxygen Evolution Reaction in Alkaline Media

Water electrolyzers offer a renewable pathway towards the synthesis of hydrogen; however, their overall efficiency is limited by the sluggish kinetics of the oxygen evolution reaction (OER).¹ Alkaline water electrolyzers are an attractive technology due to their compatibility with earth-abundant catalysts at both the anode and cathode.^2,3 However, high surface area, active, and stable catalyst are limited and the use of layered materials have shown promising performance towards the OER. (ref) Previously, our group has investigated α- and θ-zirconium phosphate as a 3D layered support structure for transition metal oxides for the OER in alkaline media.⁴ Focusing on single metal Ni, Co, and Fe, oxides intercalated into the ZrPO_x sheets or adsorbed onto the surface of the ZrPO_x particles, we demonstrated that surface bound species exhibited similar overpotentials relative to their metal-intercalated counterparts. These electrochemical studies indicated that no intrinsic performance enhancements were achieved by intercalated catalytic species in nanoscopic channels within zirconium phosphate.

In this study, we improved the performance of mono-metal adsorbed systems through the ion-exchange of Ni and Fe cations to produce an adsorbed Fe-rich catalyst. The activity and stability of the Fe-rich catalyst was further enhanced via intercalation into the ZrPO_x layers. Improved stability by 2-3 orders of magnitude and a reduced overpotential at 10 mA cm^-2 by ~200 mV is obtained at an intercalated composition of Ni_0.1Fe_0.9 versus the adsorbed counterpart. We have demonstrated that the Ni-Fe within the interlayer is an available active site by blocking the surface Ni-Fe sites through a grafting study. After blocking the surface Ni-Fe sites with octadecyl isocyanate, the Ni^II/Ni^III redox feature remains apparent suggesting electrochemical accessibility throughout the layers. To probe the origin of the Ni-Fe activity enhancement via intercalation we characterized the crystal structure, conductivity, electrochemical surface area, oxidation state, morphology, and performance as a function of Ni-Fe composition. Furthermore, OER activities of adsorbed and intercalated Fe and co-intercalated Ni-Fe are assessed by density functional theory calculations to provide atomic-level insights and provide evidence of the stability of zirconium phosphate under the OER in alkaline conditions. In short, a compositional-structural-activity dependence is presented for Ni-Fe zirconium phosphate. The data suggests that the improved activity and stability of an intercalated Ni_0.1Fe_0.9 system stems from the confined environment provided by the layered structure of zirconium phosphate.

References:

(1) Ursua, A.; Gandia, L. M.; Sanchis, P. Hydrogen Production From Water Electrolysis: Current Status and Future Trends. Proc. IEEE 2012, 100 (2), 410–426.

(2) Xu, D.; Stevens, M. B.; Cosby, M. R.; Oener, S. Z.; Smith, A. M.; Enman, L. J.; Ayers, K. E.; Capuano, C. B.; Renner, J. N.; Danilovic, N.; et al. Earth-Abundant Oxygen Electrocatalysts for Alkaline Anion-Exchange-Membrane Water Electrolysis: Effects of Catalyst Conductivity and Comparison with Performance in Three-Electrode Cells. ACS Catal. 2018, 7–15.

(3) Wei Seh, Z.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355 (6321), eaad4998.

(4) Sanchez, J.; Ramos-Garcés, M. V.; Narkeviciute, I.; Colón, J. L.; Jaramillo, T. F. Transition Metal-Modified Zirconium Phosphate Electrocatalysts for the Oxygen Evolution Reaction. Catalysts 2017, 7 (12), 132.

(5) Strickler, A. L.; Escudero-Escribano, M.; Jaramillo, T. F. Core–Shell Au@Metal-Oxide Nanoparticle Electrocatalysts for Enhanced Oxygen Evolution. Nano Lett. 2017, 17 (10), 6040–6046.