How do iridium-based nanomaterials contribute to a greener future?

The Metrohm Poster Award was initially introduced 29 years ago during the Conference for Electroanalytical Chemistry (ELACH) and has become a longstanding tradition. The most recent edition of this award was given to two winners at Electrochemistry 2022 in Berlin, Germany. The conference, themed «At the Interface between Chemistry and Physics», attracted over 600 scientists specializing in diverse electrochemistry disciplines. Electrochemistry 2022 served as a post-pandemic platform for participants to explore cutting-edge trends and applications, and to share advancements in vital areas such as sensor technology, energy storage, CO₂ reduction, photoelectrochemistry, bioelectrochemistry, electrosynthesis, corrosion, electrochemical analytics, and electrocatalysis.

Metrohm Poster Award 2022 winners

More than 300 poster presentations took place, and the poster committee (members of the scientific panel) carefully chose the top two. The winners were then honored with a prize of €500 each at the award ceremony.

The research of Marko Malinović is presented in this article. His poster was titled: «Size controlled synthesis of crystalline IrO₂ nanoparticles for oxygen evolution reaction in acidic environment».

CO₂, climate change, and cars

Implementing strategies to mitigate climate change is of the utmost importance. The consequences of excessive CO₂ emissions and subsequent influence on regional climates is already being felt, resulting in the more frequent occurrence of natural disasters with inevitable human casualties.

Heavy reliance of the transport sector on fossil fuels consequently generated 37% of total CO₂ emissions in 2021 [1]. Despite the increasing number of electric cars on the road, additional environmentally friendly technological solutions are needed to tackle the challenge of CO₂ emission reduction. 

Recently, more attention has been given to hydrogen-powered cars as a partial solution. This class of vehicles is based on fuel cell technology where hydrogen (in a reaction with oxygen) generates the electricity needed to power the vehicle, with only water and heat as the side products. Even though this sounds ideal, hydrogen can only be considered climate neutral if it is produced using renewable energy sources. In 2020, a total of 57 TWh of hydrogen was produced in Germany, a third of which resulting from the steam reforming of fossil fuels, and therefore directly linked to high CO₂ emissions [2]. The global share of hydrogen derived from renewable energy sources known as «green hydrogen» less than 1%, alarmingly indicating where the focus must be shifted to make an impact.

Water electrolysis

The proposed solution to circumvent excessive CO₂ emission during the production of hydrogen is via electrochemical water splitting. The electrical energy required for the endothermic reaction of water splitting is provided from renewable sources, resulting in the production of green hydrogen.

Among different electrolysis technologies available for industrial scale production, alkaline water electrolyzers and polymer electrolyte membrane (PEM) water electrolyzers are most commonly used. Of these two, the latter provides up to four times higher current density and is more adaptable to the sometimes rather unpredictable electrical input from renewable power sources [3]. When compared to the actual price expansion of fossil fuels, green hydrogen has become fully cost-competitive, and in some parts of the world even cheaper than hydrogen derived from fossil fuel sources.

This brings up the following question: What is keeping this technology from holding a larger share of global hydrogen production?

Can green hydrogen decarbonize the mobility sector in the future?

To answer this question, we will focus on PEM water electrolyzers (PEM-WE). These electrolyzers can conveniently operate under dynamic conditions, enabling their coupling with renewable energy sources. Ultimately, excess electricity can be stored in the form of hydrogen.

In order to make this happen, two electrochemical reactions must take place in the PEM cell. At the anode, water is oxidized to generate oxygen, electrons, and protons in the reaction known as the oxygen evolution reaction (OER). Consequently, protons are conducted through the membrane and reduced at the cathode to form hydrogen (Figure 1). 

Despite hydrogen being the desired product, the bottleneck of this process is the sluggish OER which directly influences the overall efficiency of the water electrolyzer. High potentials are applied to overcome the kinetic problem of OER that, together with the acidic environment originating from a polymer electrolyte membrane, creates rather harsh conditions in the cell, thus limiting the choice of catalysts for this reaction mostly to noble metals. 

Iridium to the rescue, at a cost

Out of several researched materials, iridium-based catalysts have offered the best compromise between catalytic activity and durability [4]. However, this is also where the main problem lies regarding the successful scale-up of PEM water electrolysis. The estimated availability of iridium is approximately seven tons per year, making it one of the scarcest metals in the world [5]. Low amounts of available iridium, together with volatile supply-demand trends and force majeure factors related to the main mining sites, are reflected in its price which has skyrocketed in 2023 to approximately €150,000 per kg (down from a high point of €172,200 per kg at the end of April 2022) [6].

Keeping in mind the high and unpredictable cost and availability of iridium, a major scientific challenge is to find a way to reduce the loading of iridium-based catalyst used in PEM-WE while maintaining high performance and durability. Intriguingly, Bernt et al. [7] calculated that if the transportation sector was to be decarbonized by 2100 using hydrogen-powered vehicles, the iridium-specific power density must be reduced by a factor of 50 compared to the current state. 

Nanomaterials for sustainable energy conversion

The gravity of this challenge is the driving force behind Marko Malinović’s research conducted in the group of Prof. Dr. Marc Ledendecker. Designing an efficient and durable iridium-based catalyst with a reduced amount of noble metal is not a trivial task. Numerous catalyst designs (Figure 1) are reported in the literature addressing this challenge, comprising bare metal iridium, metal oxides, mixed metal oxides, core-shell structures, leached oxides, and nanostructured materials [8]. Marko’s research is focused on iridium oxide materials as they can potentially offer metal-like conductivity but also enhanced durability compared to their metallic counterparts.

To ensure maximized utilization of the catalyst, Marko’s research aims to synthesize nanomaterials that possess high surface-to-volume ratios as only the surface of the catalyst actively participates in catalysis. Even though amorphous iridium oxide (IrO₂) is well known for its superior activity towards OER, the durability is still not sufficient to ensure longer operation times [9]. Crystalline iridium oxide obtained at temperatures ≥400 °C has a positive influence on the stability of the catalyst [10]. However, high calcination temperatures inevitably lead to a decrease in catalytically active surface area.

The novel synthesis route developed in the Ledendecker research group offers the possibility to synthesize IrO₂ nanoparticles with preserved particle size and morphology even after the thermal treatment at high temperatures [11]. What makes this method unique is the fact that improvement in durability is not traded at the expense of catalytically active surface area. Thus, the primary goal of maximized utilization of the catalyst is ensured.

The next steps

Further reduction of the amount of this precious noble metal is called for. This could be secured through the introduction of an earth-abundant material as a core material which is consequently coated with a thin layer of IrO₂, creating a structure known as «core-shell» (Figure 1) [12].

Selecting the right core material could have a crucial impact on the final electrochemical properties of the active iridium oxide shell. Besides thermodynamic compatibility between core and shell material, the main preconditions that core materials must fulfill in order to be considered are their metallic conductivity and resistance to corrosion in an acidic medium [13]. Bearing in mind that corrosion resistance of the non-noble metals under the operational conditions of PEM-WE is questionable, this task is of major importance and will have special attention in Marko’s future research plans.