ALD/ALE 2025 Session AA1-WeM: Catalyst and Fuel Cell Applications

Electrochemical reduction of CO₂ is a promising strategy for sustainable production of high-value chemicals like formate, a valuable feedstock in the textiles, food and chemicals industries. In CO₂ electrolysers, selective catalysts that suppress the competing hydrogen evolution reaction (HER) are essential, along with a well-defined triple-phase boundary (TPB) at the gas diffusion electrode (GDE)/catalyst/electrolyte interface to enhance CO₂ mass transfer. Beyond intrinsic catalytic properties, interfacial factors such as surface roughness and hydrophobicity impact the TPB region and performance. To investigate these effects, we employed plasma-enhanced (PE-) and thermal (T-)ALD of In₂S₃ on GDEs, creating model catalysts without ionomers or binders [1].

The ALD processes used In(acac)₃ and H₂S (plasma) precursors at a substrate temperature of 180 °C. Saturated growth was confirmed on Si substrate, achieving 0.28 and 0.17 Å/cycle for PE- and T-ALD, respectively. Both films were crystalline, with the PE-In₂S₃ film displaying a tetragonal structure and the T- In₂S₃ film a cubic structure. Next, In₂S₃ films were successfully deposited on GDEs, as confirmed by XPS and XRF mapping. Cross-sectional SEM-EDS revealed the penetration of In and S 100 µm deep inside the GDE, in contrast to superficial deposits by conventional coating methods. The PE-In₂S₃ and T-In₂S₃ electrocatalysts showed a comparable surface roughness and electrochemical active surface area. The hydrophobicity was studied via water contact angle (CA) measurements. Compared to the bare GDE (151.0°), the CA of T-In₂S₃ exhibited a limited decrease to 128.8°, whereas PE-In₂S₃ displayed a significantly diminished value of 76.9°, indicative of plasma-induced modification of the underlying PTFE-rich GDE surface. Thus, the hydrophobicity decreased drastically for PE-In₂S₃.

The T-In₂S₃ films achieved a formate Faradaic efficiency of more than 90% at 1 A cm⁻² in a flow-by electrolyzer. In contrast, PE-In₂S₃ exhibited reduced electrochemical activity and selectivity, attributed to its higher wettability. The increased hydrophilicity caused excessive penetration of the electrolyte into the GDE, resulting in a shift in the TPB, limiting CO₂ diffusion to the catalyst layer and promoting HER over formate production. On the other hand, the conformal coverage provided by T-ALD together with its favourable hydrophobic interfacial properties stabilized the TPB region, allowing for long-term stability of up to 30 h. These findings contribute to the understanding of how catalyst interfacial properties promote activity and stability in CO₂ electroreduction.

[1] Van den Hoek et al. Adv. Energy Mater. 2025, 2404178.

Wind and solar have in recent years become the cheapest sources of electricity; however, their intermittency is still an obstacle to further capacity expansion. Hydrogen generation through proton exchange membrane water electrolysis (PEMWE) is a promising solution to this problem but requires scarce platinum group metal catalysts to facilitate the half-reactions at the electrodes, which may soon become a bottleneck for scale-up. Hence, the catalyst is typically deposited sparsely onto a support material (e.g., Pt-loaded carbon, Pt/C), thereby increasing surface area of the catalytically active metal. With incipient wet impregnation as a scalable and thus industrially accepted method, morphology and dispersion of Pt is difficult to control. In contrast, ALD enables Pt/C fabrication at atomic level precision, but is so far only employed in lab scale experiments¹.

In this contribution, we demonstrate the manufacturing of highly active Pt/C catalysts for PEMWE through ALD on particles in a fluidized bed reactor (FBR-ALD), and investigate the upscaling potential of the process². The process is carried out at atmospheric pressure, such that no expensive and difficult to scale vacuum systems are required from the outset. First, we tailor catalysts with a nanometer scale dispersion of Pt on carbon using sub-gram scale batches of catalyst. The catalyst is analyzed for its morphology via TEM imaging and screened for electrochemical performance via cyclic voltammetry measurements to obtain the electrochemically active surface area. Second, to evaluate the performance of the most promising ALD-made Pt/C catalysts under realistic conditions, we manufacture cathode catalyst layers and characterize their performance and durability in a laboratory-scale PEMWE system. Finally, we transfer the FBR-ALD process to a larger scale reactor (>10 g batches of catalyst), where we investigate saturation behavior and utilization of the MeCpPtMe₃ precursor through residual gas analysis. In addition to fundamental insights, we thereby gain an indication of economic efficiency of the process. In conclusion, we demonstrate that ALD manufacturing of high performance Pt/C catalyst is a scalable process.

(1) Yan, H.; Lin, Y.; Wu, H.; Zhang, W.; Sun, Z.; Cheng, H.; Liu, W.; Wang, C.; Li, J.; Huang, X.; Yao, T.; Yang, J.; Wei, S.; Lu J.; Nat. Commun. 2017, 8 (1), 1070. https://doi.org/10.1038/s41467-017-01259-z.

(2) Grillo, F.; Van Bui, H.; Moulijn, J. A.; Kreutzer, M. T.; van Ommen, J. R.; J. Phys. Chem. Lett. 2017, 8 (5), 975–983. https://doi.org/10.1021/acs.jpclett.6b02978.

The Power‐to‐Liquids (PtL) approach can produce sustainable fuels by utilizing Fischer‐Tropsch (FT) processes to convert green hydrogen and carbon dioxide. While FT is a promising technology for producing sustainable fuels, high-performance catalysts are crucial to its success. Thin film catalysis offers opportunities to tailor the catalysts properties by design that are pre-adapted to specific chemical environment for their targeted application. FT benefits from thin film catalysts by reducing the amount of active materials, streamlining activation phase and enhancing selectivity toward desired end products. We report a cobalt-based thin-film FT catalyst synthesized by a combination of different deposition techniques including atomic layer deposition (ALD), plasma enhanced chemical vapour deposition (PECVD), magnetron sputtering and successive ionic layer adsorption and reaction (SILAR). Mn/MnO_x and Pt are studied as promoters and enhancers toward FT while considering cobalt as the FT active material. The size of platinum particles is controlled by number of ALD cycles. An ultra-low amount of Pt grown by 5ALD cycles is loaded onto the stack of cobalt based heterostructures (Mn-Co-SiO_x/Al) to study the effect of platinum as reduction promotor. The reduction behaviour of cobalt in the presence of Mn and Mn-Pt in oxidation-reduction conditions is investigated using in-situ synchrotron grazing incident X-ray diffraction. It is revealed that platinum promotes the reduction of cobalt species (spinel-Co₃O₄ to Co⁰) as indicated by a significant lowering of the reduction temperature to 350 °C while the addition of Mn retards the reduction of CoO to Co⁰. Temperature-programmed reduction (TPR) studies further confirm the effect of Pt on promoting the reduction of Co₃O₄ at lower temperatures to the FT active phase. These well-defined Pt-Mn-Co-SiO_x/Al hierarchical heterostructures represent a promising thin film FT catalyst system with significantly enhanced FT activity. Clear structure−property correlations required to insight-driven scale up the process, will be presented.

CO impurities in hydrogen derived from fossil fuel reforming present a substantial barrier to efficiency and environmental impact. This challenge underscores the importance of the water-gas shift (WGS) reaction, a critical process for purifying hydrogen for fuel cell applications. Au catalysts have shown outstanding low temperature catalytic activity toward WGS reaction, but they are struggling with the durability concerns.We fabricate a depth-controlled TiO₂ nanotrap via area-selective atomic layer deposition (ALD) method to balance the activity and anti-sintering properties of Au nanoparticles.^[1] It exhibits enhanced sintering resistance while retaining an activity similar to that of pure Au catalyst. The average size of sintered Au (8.4 ± 2.7 nm) is three times smaller than that of pure Au catalyst (21.1 ± 7.1 nm) after calcination in air at 700 ºC. Based on the area-selective ALD method, Pt clusters are selectively located on Au/CeO₂ to activate the interfacial active sites poisoned by intermediate species.^[2] The selective method effectively avoids the coverage of Au surface sites by Pt atoms and maintains the low temperature catalytic activity of Au/CeO₂ catalyst. Durability test indicates that the Pt-activated Au/CeO₂ catalyst shows superior durability in WGS atmosphere (200 ºC), which only attenuates 7.9% (from 78.0% to 70.1%) after 150 h. The analysis of surface intermediates and density functional theory calculations reveal that the introduction of Pt ensures the regenerative capacity of Au/CeO₂ active sites by facilitating the decomposition of intermediate species and the desorption of CO₂.

References

[1] Tang, Y. T.; Ma, X. Y.; Du, X. D.; Liu, X.; Chen, R.; Shan, B. Breaking the activity-stability trade-off of Au catalysts by depth-controlled TiO₂ nanotraps. J. Catal. 2023, 423, 145-153.

The outstanding performance of noble metals such as Pt, Ru, Pd, Ir, etc., for different catalytic applications has been widely demonstrated.^[1,2] However, due to their scarcity, efforts have been made to reduce or substitute these noble metals. Atomic Layer Deposition (ALD) is one of the best technique to facilitate loading mass reduction on a support of interest.^[3,4] Moreover, ALD is the most suitable technology to decorate with noble metal nanoparticles, high aspect ratio and high surface area substrate architectures.^[5]

Surface energy variations between the noble metals and the support surfaces cause the ALD growth to initiate as single atoms, then nanoclusters and as the number of ALD cycles increases nanoparticles (NPs), the agglomeration between NPs dominates over the individual NP size increase, thus developing thin films of relatively higher thickness. These surface energy variations considerably increase the nucleation delay of noble metals. In this sense, our efforts focused on improving the functionality with pretreatments on carbonaceous supports that showed promise in reducing the nucleation delay of Ir deposited by ALD.⁶

It is highly important to choose the right substrates for electrocatalytic applications. Among the available substrates, TiO₂ nanotube (TNT) layers and carbon papers (CP) are the best options considering their physiochemical properties, availability, extensive literature and the low costs incurred when using them as support substrates in electrocatalysis. Variations in the morphological aspects of TNT layers and several surface modifications for CPs have received great attention from applied fields due to their enhanced surface area, conductivity and stability.^[7–12]

The presentation will introduce and describe the synthesis of Iridium nanostructures by ALD on TNT layers and CP substrates, including the corresponding physico-chemical and electrochemical characterization and the encouraging results obtained for alkaline Hydrogen Evolution Reaction (HER).

References:

1. Huang, Z. F. et al. Advanced Energy Materials. 7 (2017) 1700544.
2. Wang, Q. et al. Nature. commun. 11, (2020) 4246.
3. Yoo, J. E. et al. Electrochem. commun. 86, (2018) 6.
4. Anitha, V. C. et al. J. Catal. 365, (2018) 86.
5. Zazpe, R. et al. Langmuir 32, (2016) 10551.
6. Rodriguez-Pereira, J. et. al. Manuscript in preparation
7. Sopha, H. et al. Appl. Mater. Today 9, (2017) 104.
8. Macak, J. M., Zlamal, M., Krysa, J. & Schmuki, P. Small 3, (2007) 300.
9. Liu, C., Sun, C., Gao, Y., Lan, W. & Chen, S. ACS Omega 6, (2021) 19153.
10. Thalluri, S. M. et al. Small (2023) 2300974.
11. Thalluri, S.M. et al. Energy & Environmental Materials (2024) e12864.
12. Bawab, B et.al. Manuscript submitted.