Investigation of Ni catalysed oxygen reduction reaction with the interdigitated array electrodes

Abstract

With the growing concern of the energy crisis and environment pollution, new energy storage and energy utilization methods are demanded for the sustainable development of human being, for example, fuel cells are considered to be a promising clean energy generation technology. All these technologies are based on few fundamental energy conversion reactions, such as hydrogen evolution reaction, oxygen reduction reaction (ORR), and oxygen evolution reaction[1]. Among these, ORR is considered as the “bottleneck” part of the energy storage and utilization due to the sluggish reaction rate and precious metal catalysts are needed.[2] Tremendous effects have been made designing and developing the new catalysts as the cheap and high-efficient substitutes of the precious metal catalysts.[3, 4]

Understanding the specific reaction pathway and mechanism are essential in designing and developing new catalysts. Nowadays, the catalytic abilities of catalysts are evaluated towards the following aspects through the electrochemical methods: (1) The overall catalytic current, as the standard for measuring the catalytic performance, is the main criterion in estimating and selecting the catalysts. (2) The catalytic reaction process/mechanism, information towards the reaction intermediates and reaction pathway draws attention as well. For the ORR, the reaction pathway is determined by monitoring the reaction intermediate of H₂O₂, and is mainly investigated through the rotating ring disk electrodes (RRDE).[5] However, the RRDE performs low detection sensitivity (the collection efficiency is about 20%-30%) along with large noise, which is specially not sensitivity enough in probing reaction intermediates with low concentrations.

The interdigitated array (IDA) electrodes are used as a novel substitution of the conventional RRDE. The IDA electrodes show a collection efficiency of above 90% benefited from the micron size of the electrode digits and the resulted efficient mass transfer mode of diffusion in the quiet environment, which lowers the noise as well.

Here in this work, the IDA electrodes with high collection efficiency are utilized for the in situ detecting of the reaction intermediate and provide information about the reaction pathway. The nonprecious metal catalyst Ni was studied as an example for the investigation of ORR intermediate and catalytic pathway with IDA electrodes. With the decoration of Ni catalyst on the surface of IDA electrodes, as shown in fig. 1A-B, the Ni catalyst shows an ordered array of Ni particles on IDA electrodes, and the energy-dispersive X-ray analysis conforms the chemical composition. Then in order to investigate the reaction process, ORR catalyzed with Ni was carried out with single potential mode and the IDA generator-collector mode. As shown in fig. 1C, the onset potential for the Ni catalyzed ORR is at -0.2 V vs. Ag/AgCl, while the reaction intermediate H₂O₂ is generated and captured by the IDA collector at the same time. Along with the continuous scan of IDA potential to the negative direction, the generation of H₂O₂ was kept at a constant value of around 50%, and the corresponding electron transfer number is about 3 (fig. 1D).

In summary, IDA electrodes which work with the generator-collector mode perform the high collection efficiency of above 90% and is suitable for the investigation of the low concentrated or short-lived reaction intermediates, and therefore probing the reaction process and reaction mechanism. The IDA electrodes could be used as the substitution of the conventional RRDE for the investigating of ORR reaction process. And the nonprecious metal catalyst Ni was studied with the IDA electrodes for the ORR process and the electron transfer number is determined to be 3.

Fig.1. A. Cyclic voltammetry of the deposition of Ni in the 5 mM NiSO₄, 2.3 mM NiCl₂ and 0.72 M H₃BO₃ aqueous solution, Ag/AgCl as the reference electrode, 50 mV/s. B. the EDX of Ni on the IDA surface. C. ORR detection with IDA generator-collector mode in the 0.1 M NaOH aqueous solution. D. the calculation of H₂O₂ intermediate and the corresponding electron transfer number.