Atoms vs. Ions: Intermediates in Reversible Electrochemical Hydrogen Evolution Reaction

We present a critical analysis of the mechanism of reversible hydrogen evolution reaction based on thermodynamics of hydrogen processes considering atomic and ionic species as intermediates. Clear distinction between molecular hydrogen evolution/oxidation (H2ER and H2OR) and atomic hydrogen evolution/oxidation (HER and HOR) reactions is made. It is suggested that the main reaction describing reversible H2ER and H2OR in acidic and basic solutions is: H3O++2e−⇌(H2+)adH2+OH− and its standard potential is E0 = −0.413 V (vs. standard hydrogen electrode, SHE). We analyse experimentally reported data with models which provide a quantitative match (R.J.Kriek et al., Electrochem. Sci. Adv. e2100041 (2021)). Presented analysis implies that reversible H2 evolution is a two-electron transfer process which proceeds via the stage of adsorbed hydrogen molecular ion H2+ as intermediate, rather than Had as postulated in the Volmer-Heyrovsky-Tafel mechanism. We demonstrate that in theory, two slopes of potential vs. lg(current) plots are feasible in the discussed reversible region of H2 evolution: 2.3RT/F≈60 mV and 2.3RT/2F≈30 mV, which is corroborated by the results of electrocatalytic hydrogen evolution studies reported in the literature. Upon transition to irreversible H2ER, slowdown of H2+ formation in the first electron transfer stage manifests, and the slope increases to 2.3RT/0.5F≈120 mV; R,F,T are the universal gas, Faraday constants and absolute temperature, respectively.

Liquid Hydrogen: A Review on Liquefaction, Storage, Transportation, and Safety

Decarbonization plays an important role in future energy systems for reducing greenhouse gas emissions and establishing a zero-carbon society. Hydrogen is believed to be a promising secondary energy source (energy carrier) that can be converted, stored, and utilized efficiently, leading to a broad range of possibilities for future applications. Moreover, hydrogen and electricity are mutually converted, creating high energy security and broad economic opportunities toward high energy resilience. Hydrogen can be stored in various forms, including compressed gas, liquid hydrogen, hydrides, adsorbed hydrogen, and reformed fuels. Among these, liquid hydrogen has advantages, including high gravimetric and volumetric hydrogen densities and hydrogen purity. However, liquid hydrogen is garnering increasing attention owing to the demand for long storage periods, long transportation distances, and economic performance. This paper reviews the characteristics of liquid hydrogen, liquefaction technology, storage and transportation methods, and safety standards to handle liquid hydrogen. The main challenges in utilizing liquid hydrogen are its extremely low temperature and ortho- to para-hydrogen conversion. These two characteristics have led to the urgent development of hydrogen liquefaction, storage, and transportation. In addition, safety standards for handling liquid hydrogen must be updated regularly, especially to facilitate massive and large-scale hydrogen liquefaction, storage, and transportation.

Molybdenum nitrides from structures to industrial applications

Owing to their remarkable characteristics, refractory molybdenum nitride (MoN x )-based compounds have been deployed in a wide range of strategic industrial applications. This review reports the electronic and structural properties that render MoN x materials as potent catalytic surfaces for numerous chemical reactions and surveys the syntheses, procedures, and catalytic applications in pertinent industries such as the petroleum industry. In particular, hydrogenation, hydrodesulfurization, and hydrodeoxygenation are essential processes in the refinement of oil segments and their conversions into commodity fuels and platform chemicals. N-vacant sites over a catalyst’s surface are a significant driver of diverse chemical phenomena. Studies on various reaction routes have emphasized that the transfer of adsorbed hydrogen atoms from the N-vacant sites reduces the activation barriers for bond breaking at key structural linkages. Density functional theory has recently provided an atomic-level understanding of Mo–N systems as active ingredients in hydrotreating processes. These Mo–N systems are potentially extendible to the hydrogenation of more complex molecules, most notably, oxygenated aromatic compounds.

Adsorption-Based Hydrogen Storage in Activated Carbons and Model Carbon Structures

The experimental data on hydrogen adsorption on five nanoporous activated carbons (ACs) of various origins measured over the temperature range of 303–363 K and pressures up to 20 MPa were compared with the predictions of hydrogen density in the slit-like pores of model carbon structures calculated by the Dubinin theory of volume filling of micropores. The highest amount of adsorbed hydrogen was found for the AC sample (ACS) prepared from a polymer mixture by KOH thermochemical activation, characterized by a biporous structure: 11.0 mmol/g at 16 MPa and 303 K. The greatest volumetric capacity over the entire range of temperature and pressure was demonstrated by the densest carbon adsorbent prepared from silicon carbide. The calculations of hydrogen density in the slit-like model pores revealed that the optimal hydrogen storage depended on the pore size, temperature, and pressure. The hydrogen adsorption capacity of the model structures exceeded the US Department of Energy (DOE) target value of 6.5 wt.% starting from 200 K and 20 MPa, whereas the most efficient carbon adsorbent ACS could achieve 7.5 wt.% only at extremely low temperatures. The initial differential molar isosteric heats of hydrogen adsorption in the studied activated carbons were in the range of 2.8–14 kJ/mol and varied during adsorption in a manner specific for each adsorbent.

How Stable Are 2H-MoS2 Edges Under Hydrogen Evolution Reaction Conditions?

MoS₂, have emerged as a promising class of electrocatalysts for the production of H₂ via the hydrogen evolution reaction (HER) in acidic conditions.<div>The edges of MoS₂ are known for their HER activity, but their precise atomistic nature and stability under HER conditions is not yet known. In contrast to other typical uses of MoS₂ as a catalyst, under HER there is no external source of sulfur. Therefore, the sulfidation of the edges can only decrease under operating conditions and the thermodynamics of the process are somewhat ill-defined. Our results suggest that the 50%S S-edge may be active for HER via the Volmer-Tafel mechanism and is, despite a high H coverage, stable with respect to H₂S release. </div><div>At the 50%S Mo-edge, the adsorbed hydrogen opens the way for H₂S release, leading to the 0%S Mo-edge, which was previously investigated and found to be HER active. HER being a water-based process, we also considered the effect of the presence of H₂O and the in-situ formation of OH. For the 50%S Mo-edge, H₂O is only very weakly adsorbed and OH formation is unfavorable. Nevertheless, OH assists the loss of sulfur coverage, leading to OH-based HER active sites. In contrast, OH is strongly adsorbed on the 50%S S-edge. By explicitly considering the electrochemical potential using grand-canonical density functional theory, we unveil that the Volmer-Heyrovsky mechanism on sulfur sites is still accessible in the presence of surface OH at the 50%S S-edge. However, the 50%S S-edge is found to be mildly unstable with respect to H₂S in the presence of water/OH. Hence, we suggest that the 50%S S-edge evolves over time towards a 0%S S-edge, covered by surface OH that will block permanently the active sites. </div>

How Stable Are 2H-MoS2 Edges Under Hydrogen Evolution Reaction Conditions?

MoS₂, have emerged as a promising class of electrocatalysts for the production of H₂ via the hydrogen evolution reaction (HER) in acidic conditions.<div>The edges of MoS₂ are known for their HER activity, but their precise atomistic nature and stability under HER conditions is not yet known. In contrast to other typical uses of MoS₂ as a catalyst, under HER there is no external source of sulfur. Therefore, the sulfidation of the edges can only decrease under operating conditions and the thermodynamics of the process are somewhat ill-defined. Our results suggest that the 50%S S-edge may be active for HER via the Volmer-Tafel mechanism and is, despite a high H coverage, stable with respect to H₂S release. </div><div>At the 50%S Mo-edge, the adsorbed hydrogen opens the way for H₂S release, leading to the 0%S Mo-edge, which was previously investigated and found to be HER active. HER being a water-based process, we also considered the effect of the presence of H₂O and the in-situ formation of OH. For the 50%S Mo-edge, H₂O is only very weakly adsorbed and OH formation is unfavorable. Nevertheless, OH assists the loss of sulfur coverage, leading to OH-based HER active sites. In contrast, OH is strongly adsorbed on the 50%S S-edge. By explicitly considering the electrochemical potential using grand-canonical density functional theory, we unveil that the Volmer-Heyrovsky mechanism on sulfur sites is still accessible in the presence of surface OH at the 50%S S-edge. However, the 50%S S-edge is found to be mildly unstable with respect to H₂S in the presence of water/OH. Hence, we suggest that the 50%S S-edge evolves over time towards a 0%S S-edge, covered by surface OH that will block permanently the active sites. </div>

Electrokinetic and in situ spectroscopic investigations of CO electrochemical reduction on copper

Rigorous electrokinetic results are key to understanding the reaction mechanisms in the electrochemical CO reduction reaction (CORR), however, most reported results are compromised by the CO mass transport limitation. In this work, we determined mass transport-free CORR kinetics by employing a gas-diffusion type electrode and identified dependence of catalyst surface speciation on the electrolyte pH using in-situ surface enhanced vibrational spectroscopies. Based on the measured Tafel slopes and reaction orders, we demonstrate that the formation rates of C2+ products are most likely limited by the dimerization of CO adsorbate. CH4 production is limited by the CO hydrogenation step via a proton coupled electron transfer and a chemical hydrogenation step of CO by adsorbed hydrogen atom in weakly (7 < pH < 11) and strongly (pH > 11) alkaline electrolytes, respectively. Further, CH4 and C2+ products are likely formed on distinct types of active sites.

Insights into Hydrogen Evolution Reaction on 2D Transition Metal Dichalcogenides

<p>Understanding the hydrogen evolution reaction (HER) behaviors over 2D transition metal dichalcogenides (2D-TMDs) is critical for the development of non-precious HER electrocatalysts with better activity. In this work, by combining density functional theory calculations with microkinetic modelling, we thoroughly investigated the HER mechanism on 2D-TMDs. We find there is an important dependence of simulated cell size on the calculated hydrogen adsorption energy and the activation barrier for MoS₂. Distinct from previous “H migration” mechanisms proposed for the Heyrovsky reaction − the rate-determining step for MoS₂, we propose the Mo site only serves as the stabilized transition state rather than H adsorption. In comparison to transition metal electrocatalysts, we find that the activation barrier of the Heyrovsky reaction on 2D-TMDs scales with the hydrogen adsorption energy exactly as for transition metals except that all activation energies are displaced upwards by <i>ca.</i> 0.4 eV. This higher Heyrovsky activation barrier is responsible for the substantially lower activity of 2D-TMDs. We further show that this higher activation barrier stems from the more positively charged adsorbed hydrogen on the chalcogenides interacting repulsively with the incoming proton. Based on these insights, we discuss potential strategies for the design of non-precious HER catalysts with activity comparable to Pt.</p>

Insights into Hydrogen Evolution Reaction on 2D Transition Metal Dichalcogenides

<p>Understanding the hydrogen evolution reaction (HER) behaviors over 2D transition metal dichalcogenides (2D-TMDs) is critical for the development of non-precious HER electrocatalysts with better activity. In this work, by combining density functional theory calculations with microkinetic modelling, we thoroughly investigated the HER mechanism on 2D-TMDs. We find there is an important dependence of simulated cell size on the calculated hydrogen adsorption energy and the activation barrier for MoS₂. Distinct from previous “H migration” mechanisms proposed for the Heyrovsky reaction − the rate-determining step for MoS₂, we propose the Mo site only serves as the stabilized transition state rather than H adsorption. In comparison to transition metal electrocatalysts, we find that the activation barrier of the Heyrovsky reaction on 2D-TMDs scales with the hydrogen adsorption energy exactly as for transition metals except that all activation energies are displaced upwards by <i>ca.</i> 0.4 eV. This higher Heyrovsky activation barrier is responsible for the substantially lower activity of 2D-TMDs. We further show that this higher activation barrier stems from the more positively charged adsorbed hydrogen on the chalcogenides interacting repulsively with the incoming proton. Based on these insights, we discuss potential strategies for the design of non-precious HER catalysts with activity comparable to Pt.</p>