The decarbonization of the energy chain is one of the main challenges of our time. The European Union has placed this challenge at the heart of its research agenda, with Horizon Europe's Cluster 5 specifically targeting advances in clean energy technologies, including energy storage and green hydrogen production. Meeting these targets demands not only the scaling up of renewable energy generation, but equally the development of efficient, high-performance technologies capable of storing and converting that energy on demand. In this regard, electrochemical processes are a prime solution as they convert the obtained electrical energy into chemical energy, which can be stored in either batteries/supercapacitors or by production of green hydrogen via electrocatalysis. Electrochemical processes take place at the interface between an electrode and a surrounding liquid, called the electrolyte. For application electrochemical technologies rely critically on a large interfacial area, while keeping mass and volume small. In practice, this means making electrode materials porous, upon which the area increases when the pore size decreases, as more pores per volume are possible. However, in recent years the porosity has reached the length scale where pore sizes are only on the order of a few nanometres. At this lengths cale electrochemical processes happening on opposing sites of the pores can interfere with each other. A good measure for the range that is occupied by electrochemical processes is the extension of the electric double-layer (EDL) λ. The EDL is a layer that builds up in front of every surface because the charging of the electrode attracts ions from the solution of opposite charge. Once the pore size is smaller than 2 λ the opposing EDLs interact with each other. This is what we call “confinement of the double-layer”. At even smaller length scale the ions and reactants of the electrolyte, that in aqueous solution are typically encapsulated in a layer of water, called the solvation shell, cannot enter the pores without shedding this solvation shell anymore. Upon desolvation these ions and reactants change their chemical behaviour leading to changes in the stored energy, as well as the catalytic activity. This is what we call “confinement of ions”. Understanding the effect of these two types of nanoconfinement on charge storage and hydrogen production are the focus of NanoC3.
To study the different confinement effects NanoC3 relied on two different model systems that tackle the different length scales involved in the project:
a) Design of a novel nanofabrication routine to fabricate gold slit electrodes with tuneable slit size based on previous results of one of the collaborators in the project (O1 + O2)
b) Usage of molecularly pillared MoS2 provided by a second collaborator of the project, whose interlayer spacing can be precisely engineered by changing the pillar length (O3 + O4)
For both model materials the first objective (O1+O3) was an initial assessment of their charging behaviour, especially the time necessary to charge the respective electrode and the influence of different electrolyte solutions, e.g. by changing the size of the ions or λ, which scales with concentration. Afterwards, their investigation under reaction conditions for the hydrogen evolution was planned (O2+O4).
The pathway from these fundamental scientific objectives to broader societal and technological impact operates on several levels. At the most direct level, the insights generated by NanoC3 regarding how nanoconfinement modifies the EDL structure, ion transport, and reaction kinetics provide the mechanistic understanding needed to rationally engineer next-generation porous electrode materials. Understanding why and how confinement enhances or suppresses specific electrochemical processes allows materials scientists and engineers to move beyond trial-and-error optimization toward knowledge-guided design. Further, the mechanistic insights gained are directly transferable to other electrochemical reactions of high relevance to the energy transition, including the oxygen evolution reaction and the electrochemical reduction of CO2. At the strategic level, NanoC3 is aligned with and contributes to the objectives of Horizon Europe's key research area on energy storage and to the EU's broader hydrogen strategy. The project addresses fundamental knowledge gaps whose resolution is a recognized prerequisite for scaling up both electrolyser and supercapacitor technologies to the level required by the European Green Deal.
