ENERGY APPLICATIONS

Enabling Precision: ALD in Energy Storage and Conversion

Article co-written by Abhishekkumar Thakur (MSc.)

Atomic layer deposition (ALD), a powerful technique to deposit material atomic layer-by-layer conformally on a geometrically demanding surface, has enhanced advanced electronic chip manufacturing that power modern smartphones and PCs. However, it is not limited to only chip manufacturing. In this article, we will walk you through the applications of ALD in energy storage and conversion.

In 2017, the world used 13.5 billion tonnes of oil equivalent (TOE) energy, which is expected to rise to 21 billion TOE by 2050. Meeting this demand and addressing environmental and economic concerns has driven the development of sustainable technologies. These focus on capturing energy (e.g. solar energy), storing that energy (e.g. in batteries), and enhancing energy efficiency through technologies like LEDs and fuel cells. Advanced energy technologies require nanoscale materials, and ALD enables precise synthesis at the atomic level for nanostructure and composition control.

Figure 1. The mechanism, properties, large-scale potentials, and the applications for energy storage and conversion, of ALD/MLD technology (Image Source)

Let’s go through some of the emerging applications of ALD in energy capture, storage, and efficient consumption.

ALD in Solar Cells

ALD is a versatile technique for depositing materials that absorb solar energy. It has been applied to metal-sulfides like Sb₂S₃, CuSbS2, and CuZnSnS to enhance solar power conversion efficiency.

Additionally, ALD is used to create perovskite absorbers in solar cells, improving stability and preventing degradation. Thin ALD barrier layers, such as a 6 nm bilayer of tin oxide and zinc tin oxide, significantly extend the operational stability of perovskite solar cells.

The use of ALD or MLD based hybrid organic/inorganic films shows promise for flexible solar cell encapsulation, where MLD stands for molecular layer deposition, which is a kind of ALD to deposit organic thin-films.

In solar cells, ALD is also employed to grow transparent conducting oxides (TCOs) like indium, tin, and zinc oxide, often doped with metals or non-metals. The composition of TCO films can be easily adjusted using ALD, impacting conductivity and transparency for higher energy conversion.

Figure 2. Typical structure of perovskite solar cell (Image Source)

Advantages of using ALD in solar cells:

Precise control over composition (thereby, material properties) of ternary/quaternary or doped absorber materials or TCOs
Pin-hole free high quality layers
Angstrom level thickness control

ALD in Batteries

ALD is actively used to manufacture batteries as well, like lithium-ion batteries (LIBs). During battery use, the electrodes undergo mechanical stress (induced from volume change of electrodes) and chemical reactions, leading to degradation and eventual failure of batteries. ALD based protective layer helps stabilize electrodes and prevent issues. For example, a group of researchers coated lithium metal anodes with a self-healing polymer and ALD LiPON layer. This combo accommodated volume changes and prevented electrolyte degradation, resulting in more stable and longer-lasting batteries. This protective approach is not limited to LIBs and can be applied to various batteries.

Figure 3. The basic structure of lithium-ion battery (Image Source)

Similarly, ALD is used in sodium-ion batteries (SIBs) to enhance electrode stability, making them a promising alternative to LIBs. ALD alumina serves as a protective layer to improve the stability of SIB anodes.

Advantages of using ALD in batteries:

Pin-hole free anode stabilization layer with excellent physical and chemical barrier properties to prevent electrolyte degradation
Angstrom level thickness control for thin effective stabilization layer without impacting charge transport
Excellent conformality of the stabilization layer all around the anode

ALD in Fuel Cells

As an alternative to batteries, micro-solid oxide fuel cells (µSOFCs) are gaining attention as efficient energy storage for portable electronics. Unlike traditional SOFCs (>700°C), µSOFCs operate at lower temperatures, requiring new electrolyte materials. Gadolinium-doped ceria (GDC) is promising but faces challenges like reduction at low oxygen levels and oxygen gas permeation. ALD helps by depositing a thin layer to block chemical and physical damage, improving device performance. For example, a 40 nm ALD-deposited yttria-stabilized zirconia (YSZ) barrier layer effectively protected the GDC electrolyte, leading to significantly enhanced electrochemical performance in µSOFCs.

Figure 4. Schematic diagram of thin-film SOFC (Image Source)

Advantages of using ALD in Fuel Cells:

Pin-hole free thin chemical and physical blocking layer of YSZ to protect GDC against the reduction of ceria
Excellent conformality of YSZ barrier layer on textured substrates

ALD in Supercapacitors

ALD is a valuable tool for exploring nanostructured electrodes in batteries and supercapacitors, enhancing performance through geometric effects. For instance, as shown in figure 2, TiN deposited by ALD on a sacrificial alumina membrane created TiN nanotubes. After removing alumina, MnO2 was electrochemically deposited on the nanotubes, resulting in a mixed TiN/MnO2 nanotube structure. These nanotubes exhibited high and stable capacitance as a supercapacitor electrode due to improved electron flow and a large surface area from the high aspect ratio structure.

Figure 5. Application of ALD in a mixed TiN/MnO2 nanotube supercapacitors (Image Source)

Advantages of using ALD in Supercapacitors:

Conformal deposition of TiN inside the high aspect ratio hollow channels in the alumina membrane for TiN nanotubes formation

This highlights how a material’s structure significantly influences its energy conversion performance, and ALD is instrumental in achieving desired nanostructures. The controlled ALD process is crucial for understanding how material morphology affects performance, guiding the synthesis of higher-performing energy conversion materials.

ALD in Electrocatalytic Water Splitting

Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen:

2 H₂O → 2 H₂ + O₂

Efficient and economical water splitting would be a technological breakthrough that could underpin a hydrogen economy, based on green hydrogen.

Highly conformal ALD has proven critical to optimize cell efficiencies as latest achievements have focused on the use of nanoparticles and thin film catalysts to split water at lower reaction temperatures. ALD helps create 3D nanostructures for efficient electrocatalysis.

One example involves using MoS2 as a cost-effective alternative to platinum in hydrogen evolution reactions (HER). Traditional MoS2 has limitations, but a hybrid ALD/MLD process improves its nanostructure, enhancing HER activity without the need for time-consuming steps.

Another application involves synthesizing Pt and CoOx nanoparticles on TiO₂ nanotubes using ALD. This complex structure boosts photoelectrochemical (PEC) water splitting performance due to the high dispersion of Pt and CoOx, large TiO₂ surface area, and synergistic effects of the dual catalyst. ALD enables precise control over the TiO₂ thickness, optimizing charge separation for efficient water splitting.

Figure 6. (above) Structure of hybrid molybdenum thiolate film consisting of alternating MoS2-like domains and organic linkers. (below) Schematic of the synthesis of CoOx/TiO2/Pt catalysts. CNCs refer to carbon nanocoils.