In the hydrogen energy industry, penetration of hydrogen into storage and transportation materials is a big problem because the process causes cracks, equipment damage, hydrogen leakage, failure, and even explosion. Hydrogen permeation barriers are, thus, necessary to improve the hydrogen resistance of materials and new coating materials and fabrication techniques are being explored. In this review, recent progress in the research of hydrogen permeation barriers is reviewed in terms of both the materials and preparation technology. The current research progress is first discussed based on traditional barriers such as metals and ceramics. Recent development of two-dimensional materials and composite coatings is described and the hydrogen resistance mechanism is discussed. The advantages and limitations of six common preparation methods: physical vapor deposition, chemical vapor deposition, solgel method, plasma spraying, electrochemical deposition, and plasma electrolytic oxidation are compared. Novel composite coatings with “hydrogen storage and barrier” structures are attractive and constitute one of the future research directions.

MXenes are a new class of two-dimensional (2D) materials with promising applications in many fields because of their layered structure and unique performance. In particular, the physical barrier properties of two-dimensional nanosheets make them suitable as barriers against hydrogen. Herein, MXene coatings were prepared on pipe steel by a simple spin-coating process with a colloidal suspension. The hydrogen resistance was evaluated by electrochemical hydrogen permeation tests and slow strain rate tests, and the corrosion resistance was assessed by potentiodynamic polarization. The results reveal that MXene coatings offer excellent hydrogen resistance and corrosion protection by forming a barrier against diffusion. Experimentally, the hydrogen permeability of the MXene coating is one third of the substrate, and the diffusion coefficient decreases as well. The mechanistic study indicates that the hydrogen resistance of the MXene coatings is affected by the number of spin-coated layers, while the concentration of the d-MXene colloidal suspension determines the thickness of a single coating. However, damage to the sample surface caused by the colloidal suspension that contains H⁺ and F⁻ may limit the improvement of the hydrogen resistance. This paper reveals a new application of 2D MXene materials as a novel efficient barrier against hydrogen permeation and the subsequent alleviation of hydrogen embrittlement in the steel substrate.

High-power impulse magnetron sputtering boasts high ionization, large coating density, and good film adhesion but suffers from drawbacks such as low deposition rates, unstable discharge, and different ionization rates for different materials. Herein, a cylindrical cathode in which the special cathode shape introduces the hollow cathode effect to enhance the discharge is described. To study the discharge performance of the cylindrical cathode, a hollow cathode effect modified time-dependent global model is established to fit the discharge current pulses. The simulation results indicate that the cylindrical cathode has relatively large Hall parameters of 24 (700 V) to 26 (1000 V). Compared to the planar cathode, the cylindrical cathode has a larger plasma density as a result of the hollow cathode effect. In addition, the ionization rate and ion return probability increase by about 3.0% and 4.3%, respectively. Particle transport derived from the plasma diffusion model shows that the magnetic field enables further diffusion of ions than atoms, resulting in nearly pure ion deposition on the substrate. The deposition rate and ion current measured experimentally support the simulation model and results, and this model provides a universal platform to simulate plasma systems with similar structures.

The discharge and deposition characteristics of high-power impulse magnetron sputtering using various target materials with different sputtering yields (Cu, Cr, Ti, and C) were analyzed by a plasma global model. The experimental discharge voltages and currents of various target materials were used as the input parameters of the model. The analysis reveals that the ionization fraction of the sputtered species decreases as increasing the sputtering yield, due to the electron temperature was reduced through the cooling effect of sputtered species. However, as the sputtering yield increases, the plasma density under a given discharge power density is increased and the self-sputtering runaway can be fully developed, resulting in a higher fraction of ion density in the form of metal ions in the ion deposition flux. For some high-sputtering yield materials, such as Cu and Cr, this fraction can be up to 95%.