Fabrication of nickel-yttria stabilized zirconia 3D micro-pattern by atmospheric plasma spray as a dual-functional electrocatalyst for overall water splitting applications in alkaline medium

Highlights

• A single-step strategy is used to construct 3D micro-pattern catalyst preparation.

• Electrochemically Ni phase is changed to NiOOH, significantly improved the activity.

• Overall water splitting device shows good stability over 200 h.

• Physical parameters from Nyquist plots are derived and checked it changes by ISGP.

• Hydrogen adsorption resistance is changed with Ni content, noted from DFRT.

Abstract

Identification of active and stable dual-functional electrocatalyst is a key obstacle for producing clean hydrogen fuel. Herein, we report a single-step fabrication of nickel-yttria stabilized zirconia (Ni-YSZ) 3D micro-pattern through atmospheric plasma spray as a dual-functional electrocatalyst for overall water splitting. This 3D micro-pattern strategy improves the mass diffusion pathway and active surface area, resulting in good electrocatalytic activity towards oxygen and hydrogen evolution reactions (OER and HER). Although YSZ alone shows little catalytic activity, Ni decorations enhances the activity towards OER and HER in alkaline solution. Using Ni-YSZ as a dual-functional electrocatalyst, it reaches a current density of 10 mA cm⁻² at the potential of 1.70 V, 1.65 V, and 1.67 V after 0 h, 100 h, and 200 h of durability tests. Besides, electrochemical impedance spectroscopy (EIS) spectra analyzed by genetic programming is monitoring the resistance contributed by several relaxations and provides the information about its changes with durability tests and Ni contents. This study opens a new strategy to enhance the water-splitting performance and to analyze the EIS data of electrocatalytic systems.

Introduction

Energy production from renewable energy sources is plentiful, but it is intermittent and unpredictable. It is vital to find compatible energy storage and conversion means [1,2]. In order to advance energy storage technology, water splitting is an alternative way due to its simplicity and eco-friendly nature. The generated electrical energy from sources was stored eco-friendly (no emission of CO₂) ways in the form of chemical energy, like hydrogen and oxygen gas. Usually, the water electrolysis has two half-reactions: OER and HER. Noble metals (Ir, Ru, and Pt) based materials had reported as the benchmark for OER and HER. Despite that, the commercial usage of noble metals was limited by low abundance and high cost. In addition, the water-splitting cell demands greater potential than the theoretical value (ΔE⁰ = 1.23 V versus a reversible hydrogen electrode (RHE)) due to sluggish reaction kinetics. Therefore, it is imperative to decrease the overpotential and ultimately the cell voltage [3].

Many strategies made to improve the catalytic activity towards water electrolysis of the materials, e.g. identification of novel materials [3], atomic composition [4], composite [5], different phases of the compounds [6], interface engineering [7], metal-support interaction [8], direct growth [9], etc. In this context, the nickel-based catalyst is strongly recommended for water splitting due to good catalytic activity and low-cost as compared to noble metals [10]. Practically, nickel exhibits lower electrocatalytic activity than noble metals, which could overcome by increasing intrinsic activity. Even now, there is much room to improve the intrinsic activity of catalysts by simplifying the synthesis protocol and designing electrode architecture [11]. Direct growth of the catalyst on the substrate improves the contact between them and cuts the dead-weights (binder-free) [12,13].

Nickel-based catalysts were prepared by a thermal spraying method, that exhibited good HER activity due to its unique microstructural properties [14,15]. Among various thermal spraying methods, atmospheric plasma spray (APS) method has developed and subjected to coating HER catalyst with patterned microstructure. The APS is a relatively simpler and more cost-effective coating tool than other surface coating techniques like vacuum plasma spray, physical vapor deposition (PVD), chemical vapor deposition (CVD), and electron beam-PVD. In the APS process, injected catalyst powder gets melted and accelerated towards the substrate by the high-temperature plasma jet. Then the molten particles impact the substrate at high velocity and get flattened followed by rapid solidification. Consecutive deposition of the stream of molten particles builds the catalyst's coating microstructure with the desirable thickness. Thermo-physical properties of the feedstock powder determine the in-flight characteristics of the molten particles such as velocity and temperature. This, in turn, influences the flattening degree, solidification and bonding during the coating formation. Nevertheless, submicron and nanosized particles cannot be used directly in the APS due to injection difficulties [[16], [17], [18], [19]].

EIS is a widely used technique to probe the relaxation phenomena occurs at the electrode/electrolyte interfaces. As we know, the electrochemical reactions of OER and HER have consisted of different relaxation processes. Usually, EIS data had analyzed using equivalent circuits, but sometimes it cannot be fitted with the same equivalent circuit at different conditions and durability tests [[20], [21], [22]]. This and other familiar problems of equivalent circuit analysis may lead to miss interpretation of EIS data. Here we use another approach based on finding an analytic form of the distribution function of relaxation times (DFRT), using genetic programming, called ISGP [23].

Ni-YSZ acts as a good electrolyzer at high temperatures. But, it suffers from fast degradation during electrolysis due to crack formation, aggregation of Ni, repositioning of Ni, roughness change, poisoning, deterioration of electronic conductivity, and structural changes [24,25]. We have developed a 3D micro-pattern of Ni-YSZ electrocatalyst for overall water splitting by employing the APS method. The effect of Ni contents on the electrocatalytic performance studied in an alkaline solution. For this purpose, we have tried four different Ni contents i.e. 0, 25, 50, and 75 wt % of Ni with YSZ. Comprehensive electrochemical analysis infers that the YSZ+50 wt % Ni (YSZ+50Ni) shows the best OER and HER catalysts. 75 wt % Ni sample also shows good catalytic activity but suffers from low durability. In addition, YSZ+50Ni exhibits good overall water splitting and excellent durability (in a two-electrode system). Furthermore, we carry out genetic programming approach for analyzing EIS data and witness the variations of solution plus other series resistance, active material resistance, the production rate of intermediates resistance and charge transfer resistance at different time intervals. ISGP suggests that resistances of the solution plus other series resistance, charge transfer, and production rate of intermediates changing with the durability tests. This research provides a novel strategy to enhance the overall water splitting of the catalysts and analyze the EIS data.