Selectivity of Control of Oxygen Reduction Reaction over Mesoporous Transition Metal Oxides Catalysts for Electrified Purification Technologies
Authors: Wu Zhixing , Vagin Mikhail Yu. , Boyd Robert , Ding Penghui , Pshyk Oleksandr , Greczynski Grzegorz , Odén Magnus , Björk Emma M 
Published: February 2024

Introduction: The use of abundant feedstocks, such as water and air, in manufacturing of value-added products is important in achieving the UN’s Sustainable Development Goals.1 In this context, electrified direct synthesis of chemicals from oxygen in the air is of special interest since it avoids greenhouse gas emissions if the electricity used to drive the synthesis comes from sustainable sources such as solar- and wind-power. The controlling of oxygen reduction path over catalysts incorporation with oxygen evolution reaction as auxiliary reaction enables the electrosynthesis of value-added chemicals, oxygen and hydroxyl radical. The development of cheap materials e.g., mesoporous transition metal oxides, is urged to replace the expensive and scarce of platinum group metal (PGM)-based catalysts, with the ability to provide competitive reaction rate in alkaline media.2

Experimental: A template-free hydrothermal method is developed for the synthesis of mesoporous NiO and NiCo2O4. The materials’ physicochemical properties are characterized by X-ray diffraction, physisorption, electron microscopy, selected electron area diffraction, and X-ray photoelectron spectroscopy. The electrochemical properties and electrocatalytic oxygen activity are evaluated via cyclic voltammetry and electrochemical impedance spectra in alkaline electrolyte using a standard three electrodes electrochemical working station equipped with (ring) rotating disk electrode (RDE/RRDE) techniques.
The catalysts coated on carbon fiber paper (CFP) are used as electrode assembled with anion exchange membrane in two-electrodes-electrochemical cell for device test. NiO- and NiCo2O4-modified CFP are used as cathode and anode, respectively, for constructing electrochemical generator of hydroxyl radical. 50 mL min-1 O2 flow was maintained in catholyte to enable oxygen saturation. 20 mg L-1 rhodamine B (RhB)was fed into cathode. Off-line UV-vis measurements were used for RhB quantification. In Electrochemical oxygen purifier, NiCo2O4-modified CFP are used as electrodes. 0.1 - 3 M KOH aqueous solutions were used as anolyte and air-saturated catholyte. The volume of produced oxygen on the anode after 2 h was collected and measured by a water displacement method.

Results and Discussion: A facile template-free hydrothermal synthesis is developed for bifunctional oxygen electrocatalysts (OER and ORR) of NiO and NiCo2O4. Physicochemical characterization shows that both NiO and NiCo2O4 are mesoporous with the pore size within a range of 2 - 8 nm and specific surface areas of 60 and 120 m2 g-1, respectively. In addition, NiO and NiCo2O4 possess cubic crystal structures with the corresponding crystal sizes of ~6.0 and 9.3 nm.
Electrochemical analysis shows that mesoporous NiCo2O4 has about two times larger capacitance and eight times smaller charge transfer resistance compared to NiO. NiCo2O4 shows higher electrocatalytic activity in OER and selectivity to water as the terminal product of ORR. On the contrary, ORR over NiO yielded hydroxyl radicals, as a product of a Fenton-like reaction of H2O2. The product selectivity in ORR is used to construct two electrolyzers of electrified purification of oxygen and generation of hydroxyl radicals. The optimization of configuration of hydroxyl radical generator enables the degradation of RhB up to 90% after 1 hour, and the electrochemical oxygen purifier shows high efficiency of about 100%.

Significance: It is demonstrated that the synthesized mesoporous transition metal oxides via a simple template-free hydrothermal route, and the materials can be used as alternatives to PGM-based electrocatalysts for driving electrochemical cell towards to various products. The work also clearly shows how the composition and structure of catalysts affect the oxygen reactions, and the selectivity of ORR used for producing value-added chemicals coupling with OER. It is shown that the established electrochemical oxygen purifier and hydroxyl radical generator have a high conversion efficiency.

Schematic 1. (a) Scheme of operation of the hydroxyl radical generator, (b) the degradation efficiency and rate constants as function of current densities; (c) Scheme of the oxygen purifier, and (d) the generated oxygen volume as a function of working power of the electrochemical cell with different anolyte concentrations.

Acknowledgements: This work was supported by the competence center FunMat-II funded by the Swedish Agency for Innovation Systems (Vinnova, grant no 2016-05156), the Swedish Energy Agency (project no 42022-1), and Swedish Research Council (VR 2019-05577, Flexible metal-air primary batteries).

Reference: 1 United Nations. Transforming our world: The 2030 agenda for sustainable developement. (UN General Assembly, New York, 2015).
2 Yang, Y. et al. Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies. Chem. Rev.
6, 6117–6321 (2022).

Figure 1