A pH Indiscriminate, Iridium-Supported ZIF Derived Bifunctional Oxygen Electrocatalyst
Authors: Simon Benedict A. , Mehmood Asad , Brandon Nigel P , Kucernak Anthony R. J. 
Published: December 2020

Large scale energy storage solutions are an imperative feature of our future, greener energy grid. Recently, many megawatts-worth of solar photovoltaic arrays and wind turbines have been added to the energy network. While this sets a largely positive trajectory, the nature of their unpredictable, uncontrollable and intermittent output poses a threat to grid stability. For their continued addition to the energy network, storage solutions are needed to control their energy output. To date, very low volumes of commercial electrochemical storage system exist, relative to pumped hydro storage systems. A wider breadth of distributable electrochemical devices is needed to harness the crucial, seismic change the energy network has ahead. Among the potentially significant emerging technologies include the metal-air and hybrid redox flow batteries. [1]

Expanding the range of catalysts used in electrochemical devices is of vital importance for the exploration of new types of energy storage system. Reducing the use of expensive platinum group metal catalysts is a common goal among researchers too. This work develops and optimizes the synthesis of an iridium (Ir)-supported ZIF 67-derived nitrogen doped carbon (Ir/NC) bifunctional oxygen catalyst using a previously developed synthesis protocol [2] and attempts to fully characterize the best performing material using a series of physical characterization techniques including BET, EDX, ICP, SEM, TEM, XAS, XPS and XRD.

On inspection of the surface of the material via TEM, a better description of the catalyst active site can be deciphered. Ir particles are shown to be co-located with the cobalt metal agglomerates and clearly demonstrated to positively affect the catalyst by providing the majority of the oxygen evolution reaction (OER) performance. XPS and XRD analyses shed additional light on the surface chemistry of the catalyst and the former serves as a crucial indicator of both the nitrogen content and the form of Ir at the catalyst surface. ICP measurements, often used to calculate catalyst ‘mass activity’, are systematically carried out too, and we rigorously demonstrate the importance of thorough sample preparation and the dangers associated metal-content misrepresentation.

Several aspects of the physical characterization findings carried out in this work are rationalized and confirmed during electrochemical tests using a rotating ring disk electrode. As expected, inclusion of Ir in the catalyst augments the OER performance [3] and has very little effect on the oxygen reduction reaction performance – assertions are therefore made about the active site for the oxygen reactions. Investigation of catalyst performance in different pH media shows extremely high OER performance in acid, however, durability tests show that the catalyst performs best overall in basic media and may find utility in the previously mentioned devices.

In attempting to make a clear comparison between this catalyst and previously reported materials, a pervasive issue associated with defining performance of these catalysts is found. The ΔE metric, commonly used to compare bifunctional oxygen catalysts, is calculated using arbitrary potentials (E10 and E1/2, which, when used to compare single function catalysts is perfectly valid). We suggest an alternative ‘Figure of Merit’ which provides a potentially alternative performance metric and better way of comparing bifunctional catalysts.

In summary, we synthesize and characterize a new material and carry out fundamental electrochemical tests that reveal a material with compelling performance and promising qualities for next generation energy storage/conversion devices.

[1]: Electrochemical Energy Storage for Renewable Sources and Grid Balancing, Moseley, P.T. and Garche, J., Elsevier (2015).

[2]: A. Mehmood, J. Pampel, G. Ali, H. Y. Ha, F. Ruiz‐Zepeda and T.‐P. Fellinger, Facile Metal Coordination of Active Site Imprinted Nitrogen Doped Carbons for the Conservative Preparation of Non‐Noble Metal Oxygen Reduction Electrocatalysts. Adv. Energy Mater., 8, 1701771 (2008).

[3]: T. Reier, M. Oezaslan and P. Strasser, Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt Catalysts: A Comparative Study of Nanoparticles and Bulk Materials. ACS Catal., 2, 8, 1765-1772 (2012).

Figure 1