ICMCTF 2025 Session TS1-2-MoA: Coatings for Batteries and Hydrogen Applications II

The electrolytic energy conversion has become one of the main technologies considered to deliver actions on reducing CO₂ emissions in the energy, heavy-duty transportation and industrial processes sectors. The electrolytic cells (fuel cells and electrolysis cells) can be utilised in energy conversion, generation, and storage, which has been demonstrated at scale in many regions around the world already. Solid oxide cells are typically composed of porous ceramic matrix and Ni metal catalyst fuel electrodes, dense ceramic electrolytes, such as YSZ or GDC, and perovskite air electrodes. These cells operate at relatively high temperatures (typically 600-850°C) and, therefore do not require precious metal group catalysts to drive the reaction forward in both electrolysis (H₂ generation) and fuel cell (energy conversion) modes. Moreover, they are more versatile in terms of required fuel type allowing utilisation of hydrogen as well as alternative fuels, such as methanol, ammonia, or biogas.

In this work, thin (~3 µm) nanostructured cermet anodes consisting of YSZ-Ni and GDC-Ni were doped with transition metals to study their influence on coatings microstructure and electrochemical performance. The anodes were deposited onto commercial YSZ electrolyte support cells using oblique angle pulsed DC reactive magnetron sputtering. The coating microstructure was evaluated using FIB-SEM and TEM and focused on the triple-phase boundary evolution in relation to the amount of the added dopant (0-5 wt.%). The chemical composition of the coatings was assessed using EDX, XPS and XRD analysis. The polarization curves were obtained from SOFC single stack assemblies under hydrogen and air flows for anode and cathode, respectively, at operating temperatures of 850°C to evaluate the electrochemical performance of the deposited films.

In this study, operando Raman spectroscopy was employed to investigate the mechanism of the oxygen reduction reaction (ORR) on a cobalt single-atom catalyst (Co-SAC). The Co-SAC was synthesized and utilized as a cathode catalyst for an alkaline anion exchange membrane fuel cell (AAEMFC). The results demonstrate excellent ORR activity, with an electron transfer number of 3.96 and J_iming is 5.5 mAcm^-2.

X-ray absorption spectroscopy (XAS) revealed that the Co-SAC features a Co-N5 coordination structure, with cobalt in the +3 oxidation state. Furthermore, wavelet transform (WT) analysis confirmed the presence of isolated cobalt single atoms.

To minimize interference from the electrolyte during Raman laser measurements, the catalyst-coated membrane (CCM) method was adopted. This approach effectively prevents direct interaction between the laser and the electrolyte while ensuring efficient OH⁻ group transfer to the catalyst. Additionally, in the operando setup, electrochemical impedance spectroscopy (EIS) was integrated with Raman spectroscopy. This combination enabled a detailed observation of the ORR mechanism and the evolution of surface phenomena under different applied biases.

This study represents a significant breakthrough in unveiling the ORR mechanism, particularly for Co-based single-atom catalysts.

In recent years, materials such as transition metal oxides and nitrides have been popular in catalyst research. Compared to scarce noble metals, molybdenum-based materials not only have more abundant resources but also exhibit excellent activity[1], making them highly suitable to replace the costly noble metal catalysts (Ru, Ir, RuO₂, IrO₂). Molybdenum nitride (MoN) possesses outstanding corrosion resistance and electronic conductivity[2], allowing it to perform the hydrogen evolution reaction (HER) in acidic media. If different elements like Ti, Co, Ni, and V are doped into MoₓN as a substrate, a synergistic effect is expected to further enhance HER performance. Additionally, when attaching catalysts to electrodes, one must consider the uniformity of surface coverage and adhesion on electrodes of various shapes. As is well known, sputtering offers advantages such as uniform coating, easy control of film thickness, and excellent adhesion. Therefore, using this method to prepare catalyst thin films on electrodes is the optimal choice.

In this study, we employed RF sputtering and High Power Impulse Magnetron Sputtering (HiPIMS) techniques to deposit Mo_xN thin films for comparison. Initially, we deposited Mo_xN thin films using HiPIMS and found in preliminary results that the HiPIMS-deposited Mo_xN thin films exhibited an overpotential of 415 mV at η=10 during hydrogen evolution reaction (HER) tests in 0.5M H₂SO₄. Energy-dispersive X-ray spectroscopy (EDS) analysis revealed that the Mo and N contents in the Mo_xN thin films were 60 at.% and 40 at.%, respectively. Grazing-incidence X-ray diffraction (GIXRD) results indicated that the thin films have a face-centered cubic (FCC) structure similar to Mo₃N₂. Building on these Mo_xN results, we will further explore the impact of varying the N/Mo ratio on HER performance. Additionally, we plan to attempt doping other elements into Mo_xN to observe the changes induced by doping.

As a consequence of the depletion of fossil fuels, the escalating energy crisis has driven researchers to explore innovative energy. To address this problem, exploring hydrogen energy generation via water splitting emerges as a promising solution. This process involves the hydrogen evolution reaction (HER), a multi-electron transfer process necessitating catalysts to facilitate efficient rates. Despite noble metals have conventionally served this purpose due to their favorable Gibbs free energy, their prohibitive costs pose challenges for widespread adoption and commercialization. In response, our investigation focuses on HER within alkaline electrolytes, aiming to engineer alternative electrocatalysts that are both cost-effective and efficient.

Nitrogen-doped carbon quantum dots (N-CQDs) incorporated into highly conductivetransition metal nitride offer enhanced electrochemical performance, delivering high energy density and outstanding electrochemical stability. The present study reports a high-performance supercapacitor electrode consisting of electrophoretic anchoredzero-dimensional N-CQDs with reactively co-sputtered titanium chromium nitride nanopyramids (Ti-Cr-N) thin film on flexible stainless-steel mesh (SSM) substrates. The nanopyramids of N-CQDs/Ti-Cr-N offer remarkable electrochemical performance through Li⁺ storage, ascribed to the abundant electroactive sites and enhanced synergism between the high specific surface area of N-CQDs and higher conductivity of Ti-Cr-N.Subsequently, the N-CQDs/Ti-Cr-N/SSM electrode in 1M Li₂SO₄ aqueous electrolyte exhibits an excellent gravimetric capacitance of 393.8 F.g^-1 at a specific current density of 0.32 A.g^-1. Further, the N-CQDs/Ti-Cr-N/SSM heterostructure outperforms other multi-cationic-based supercapacitors with a maximum energy density of 41.41 Wh.kg^-1 and a superior power density of 7.0 kW.kg^-1. Impressive electrochemical stability of ~88.6% is retained by the heterostructure even after 5000 continuous charge-discharge cycles. Insights into charge-storage mechanisms highlight the dominance of surface-limited capacitive and pseudocapacitive kinetics, with fewer contributions from diffusion-controlled faradaic processes. Furthermore, an exemplary mechanical stability of ~99.98% over 1200 bending cycles demonstrates the N-CQDs/Ti-Cr-N/SSM heterojunction’s excellent resilient structural strength, validating the present electrode potential for high-performance flexible supercapacitor application.

Hydrogen fuel is an alternative green energy to neutralize the carbon emission and to fulfill the global energy demand. Electrochemical water splitting is a technique to produce hydrogen comprising of both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), where OER is a bottleneck reaction in water splitting for efficient performance. Thus, it is necessary to prepare a suitable catalyst with an excellent water splitting performance. Though RuO₂ is a benchmark catalyst for OER performance, scarcity and cost of resources are limited to the use of RuO₂ in water splitting performance. Transition-metal based electrocatalyst already garners attention due to its excellent redox properties, earth abundant and cost-effective, which makes it suitable for alternative catalyst.

In this study, FeNiMoWNbx high entropy alloy (HEA) coatings with different Nb contents were fabricated on carbon cloth (CC) at high inclined angle using magnetron sputtering technique. The composition of Nb is altered with changing the RF power from 0 to 100 W. Chemical composition of FeNiMoWNbx was analyzed by the field emission electron probe microanalyzer (FE-EPMA). Phase analysis and crystallinity of FeNiMoWNbx were analyzed with glancing angle X-ray diffractometer (GA-XRD). Surface roughness and cross-section morphologies of FeNiMoWNbx films were examined using atomic force microscope (AFM) and field emission scanning electrochemical microscopy (FE-SEM). The OER performance of FeNiMoWNbx films was investigated with the polarization curve obtained in 1 M KOH and 1 M KOH + 3.5 wt.% NaCl aqueous solution. FeNiMoWNbx thin films exhibited a superior OER performance with low overpotential in 1 M KOH and 1 M KOH + 3.5 wt.% NaCl solution. The effects of Nb concentration in FeNiMoWNbx film and the sputtering inclined angle on the kinetics of OER performance was discussed. This works provides the strategy for the fabrication of cost effective FeNiMoWNbx HEA thin film catalyst for efficient water splitting performance.

Exploiting pseudocapacitance in rationally engineered nanomaterials offers greater energy storage capacities at faster rates. The present research reports a high-performance Molybdenum Oxynitride (MoON) nanostructured material deposited directly over stainless-steel mesh (SSM) via reactive magnetron sputtering technique for flexible symmetric supercapacitor (FSSC) application. The MoON/SSM flexible electrode manifests remarkable Na⁺-ion pseudocapacitive kinetics, delivering exceptional ~881.83 F.g^-1 capacitance, thanks to the synergistically coupled interfaces and junctions between nanostructures of Mo₂N, MoO₂, and MoO₃ co-existing phases, resulting in enhanced specific surface area, increased electroactive sites, improved ionic and electronic conductivity. Employing 3D Bode plots, b-value, and Dunn’s analysis, a comprehensive insight into the charge-storage mechanism has been presented, revealing the superiority of surface-controlled capacitive and pseudocapacitive kinetics. Utilizing PVA-Na₂SO₄ gel electrolyte, the assembled all-solid-state FSSC (MoON/SSM||MoON/SSM) exhibits impressive cell capacitance of 30.7 mF.cm^-2 (438.59 F.g^-1) at 0.125 mA.cm^-2. Moreover, the FSSC device outputs superior energy density of 4.26 μWh.cm^-2 (60.92 Wh.kg^-1) and high power density of 2.5 mW.cm^-2 (35.71 kW.kg^-1). The device manifests remarkable flexibility and excellent electrochemical cyclability of ~91.94% over 10,000 continuous charge-discharge cycles. These intriguing pseudocapacitive performances combined with lightweight, cost-effective, industry-feasible, and environmentally sustainable attributes make the present MoON-based FSSC a potential candidate for energy-storage applications in flexible electronics.

References:

1. Bhanu Ranjan and Davinder Kaur, Small, 20 (20), 2307723 (2024). https://doi.org/10.1002/smll.202307723
   
2. Bhanu Ranjan and Davinder Kaur, ACS Applied Materials & Interfaces, 16 (12), 14890-14901 (2024). https://doi.org/10.1021/acsami.4c00067
3. Bhanu Ranjan and Davinder Kaur, ACS Applied Energy Materials, 7 (10), 4513-4527(2024). https://doi.org/10.1021/acsaem.4c00563
4. Bhanu Ranjan and Davinder Kaur, Journal of Energy Storage, 71, 108122. (2023). https://doi.org/10.1016/j.est.2023.108122
5. Bhanu Ranjan, Gagan Kumar Sharma, and Davinder Kaur, Applied Surface Science, 588, 152925 (2022). https://doi.org/10.1016/j.apsusc.2022.152925
6. Bhanu Ranjan, Gagan Kumar Sharma, and Davinder Kaur, Applied Physics Letters, 118 (22), 223902 (2021). https://doi.org/10.1063/5.0048272
   
7. Bhanu Ranjan, Gagan Kumar Sharma, Gaurav Malik, Ashwani Kumar, and Davinder Kaur, Nanotechnology, 32 (45), 455402 (2021). https://doi.org/10.1088/1361-6528/ac1bdf

The increasing demand for microelectronics has significantly driven the advancement of thin film energy storage devices, specifically batteries. Till now, a range of materials have been investigated for lithium-ion batteries, with vanadium oxide emerging as a promising material. Vanadium oxides are known for multiple oxidation-reduction states during electrochemical reaction, hence, can promote multiple diffusion of Lithium ions resulting in high energy and power density. In this work, binder-free Vanadium oxide (V₂O₅) has been synthesized by reactive DC magnetron sputtering on aluminium foil substrate. Vanadium target (99.99% pure) is bombarded with high-energy Ar⁺ ions which dislodge atoms from the vanadium surface. These ejected atoms then react with oxygen ions to form highly pure V₂O₅ and deposit onto a substrate, forming a thin film. The deposited layers were analyzed for their structural and surface morphology using XRD and SEM techniques. Highly ordered brick like nanostructures were observed during SEM analysis. X-ray photoelectron spectroscopy measurements were carried out to understand the chemical bonding of the cathode. The surface of vanadium oxide obtained from this binder-free approach helps us to create a high-quality cathode-electrolyte interface with high wettability (33.3^o). Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) cycling, and electrochemical impedance measurements were used to investigate the capacity and cycling stability of the V₂O₅ cathode in 1 M lithium hexa-flouro-phosphate. As a result, it can be interpreted that this binder-free technology can be used to fabricate efficient lithium free cathode for new generation thin film batteries.

With the advancement of technology, lithium-ion batteries have emerged as a future energy storage technology with the gradual development of electric vehicles. Silicon-based materials, due to their high theoretical capacity, energy density (~4200mAh/g), and natural abundance, are considered as candidates for negative electrode materials in lithium-ion batteries.

In this study, our research team successfully prepared reduced graphene oxide (rGO) using the Hummer method and incorporated commercial SiO_x micron-sized powder to synthesize SiO_x@rGO composite material as an anode for lithium-ion batteries. The initial charge capacity was measured at 1487 mAh/g, with a discharge capacity of 1060 mAh/g, yielding an initial coulombic efficiency of 71%. After 40 cycles, the capacity retention remained at 91%. However, there are currently no theoretical studies addressing the lithiation process and lithium ion insertion/extraction mechanisms in SiOx materials.

Therefore, in our study, we not only explore the use of SiO_x@rGO composite material as an anode to improve the theoretical capacity and energy density of lithium-ion batteries but also aim to enhance the electrical conductivity and electrochemical performance of the battery. Conductive materials such as copper, gold, and platinum will be deposited on the prepared SiO_x@rGO anode material. These conductive coatings will provide additional electrons, creating a driving force for lithium ion diffusion into the anode material during discharge to achieve charge conservation. This process is expected to enhance the capacity and cycling stability of lithium-ion batteries. We will characterize the materials using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). Furthermore, charge-discharge and cycling performance tests will be conducted on the lithium-ion batteries to investigate the effect of the conductive materials on their performance. Cyclic voltammetry will be employed to observe the electrochemical reactions during charge and discharge cycles.

This study conducts a theoretical analysis of SiO_x@rGO composite material as an anode for lithium-ion batteries by coating it with conductive materials, aiming to provide valuable reference data for both commercial and academic purposes.