CHEMICAL SCIENCE

Wuhan University reports new views on the catalytic reaction mechanism of hydrogen


On September 29, 2022, the team of Professor Chen Shengli of the School of Chemistry and Molecular Sciences of Wuhan University published an article in the journal Nature Catalysis entitled “Hydrogen bond network connectivity in the electric double layer dominates the kinetic pH effect in hydrogen.” Electrocatalysis on Pt’s research results.

In this study, combined with denominative molecular dynamics simulation, in situ surface enhancement infrared spectroscopy and computational spectroscopy, the microstructure of the electric double layer, the kinetics of the primitive reaction and the adsorption thermodynamics of key intermediates under the reaction conditions of acid and alkali hydrogen electrodes were systematically studied, revealing that the huge difference between the hydrogen bond network connectivity in the acid and alkali double electrolayers is the root cause of the catalytic pH effect of hydrogen. The corresponding author of the paper is Professor Chen Shengli; The first authors are Li Peng and Jiang Yaling.

Hydrogen electrode reaction, including hydroxide reaction (HOR) and hydrogen precipitation reaction (HER), is the basic reaction of fuel cell and water electrolysis technology, and is also the two cornerstones of promoting the development of hydrogen energy industry and achieving carbon neutrality goals. In addition, the hydrogen electrode reaction has also been a model reaction for basic research in electrochemistry. For a long time, it has been difficult to agree on the reasons for the significant slowdown of the reaction kinetics of hydrogen electrodes under alkaline conditions. This seriously restricts the development of alkaline fuel cells and water electrolysis technology, and is also a major problem in basic electrochemistry.

So far, the study of this kinetic pH effect has mainly considered the effect of adsorption of intermediates such as *H and *OH on the energy of the surface reaction step, while ignoring the dependence of the interface electric layer structure on pH. Therefore, accurately obtaining the interfacial electrodynamic microstructure characteristics of hydrogen electrocatalytic systems under different pH conditions will be a breakthrough point to solve this problem. However, at present, the analysis of the interface structure and process of electrocatalytic architecture at the atomic and molecular scales is still a challenge, and any single technology is insufficient.

In this work, Professor Chen Shengli’s team combined denominative molecular dynamics (AIMD) simulation and in situ surface augmented infrared spectroscopy (in situ SEIRAS) technology, through the acidic and basic interface electric double layer structure of AIMD simulation, as well as the computational vibration spectroscopy of interface water molecules and SEIRAS experimental spectroscopy, proposed that the significant difference in hydrogen bond network connectivity in the interface double electric layer is the root cause of the huge kinetic pH effect of hydrogen-electric catalysis. It provides new perspectives and new horizons for understanding the mechanism of electrocatalytic reactions.

Figure 1: Comparison of AIMD simulation results of atomic structure of the electrodouble layer at the acid and base interface under hydrogen electrode reaction conditions.

Based on the AIMD simulation, the atomic structure characteristics of the acidic and alkaline interface electric double layer under the reaction conditions of hydrogen electrode are first analyzed and compared. The results show that the crowded alkali metal cations in the alkaline electric double layer form a region with significant lack of water molecules at the interface. At the same time, the strong interaction between alkali metal cations and solvated water molecules greatly reduces the ability of water molecules in the alkaline double layer to form hydrogen bonds between each other, thereby greatly reducing the connectivity of the hydrogen bond network in the alkaline electric double layer. In contrast, the distribution of water molecules in the acidic interface electric double layer and the discontinuity of the hydrogen bond network are negligible. It is well known that hydrogen-electrocatalytic reactions are essentially a hydrogen transfer and transfer process, including hydrogen transfer between the electrode surface and its adjacent interface species (that is, the formation and desorption of surface Had intermediates) and hydrogen transfer between electrolyte phases and interface species through the electric double layer. It is conceivable that this discontinuity of the hydrogen bond network in the alkaline electric double layer will severely inhibit the hydrogen-electrocatalytic reaction, because the hydrogen bond network constitutes a high-speed channel for protons to and from the electrode surface, which is crucial in electrocatalysis.

For the hydrogen transfer process between the electrode surface and its adjacent interface species, it has traditionally been believed that the dissociation process of water molecules and H3O+ at the alkaline and acidic interfaces is respectively. However, the microscopic structure of the electric double layer at the acid-base interface simulated by AIMD shows that the water molecules closest to the electrode surface are directly involved in the hydrogen transfer process, whether at the acidic or alkaline interface. This is different from what has traditionally been understood. Subsequently, the free energy barrier of the dissociation process of the water molecules at the interface is compared, and the binding energy of Had on the electrode surface. The results show that the dissociation barrier of the interface water molecules and the adsorption strength of the Had are not the reasons for the significant decrease in the kinetics of the hydrogen electrocatalytic reaction in the alkaline medium, because the alkaline interface has a more moderate hydrogen adsorption strength and a lower water dissociation reaction barrier. These results mean that the hydrogen transfer process in the electric double layer plays a key role in the kinetics of hydrogen-electrocatalytic reactions. Therefore, it can be concluded that the discontinuity of the hydrogen bond network in the alkaline interface electric double layer is the root cause of the slow kinetics of the hydrogen-electrocatalytic reaction.

Figure 2: In situ surface-enhanced infrared spectroscopy and computational spectroscopy results verify the discontinuity of the hydrogen bonding network of the electric double layer.

The in-situ surface enhanced infrared spectroscopy technology was applied to detect the hydrogen electrocatalytic interface on the Pt electrode in acid and alkali media, and the infrared experimental spectroscopic signal at the interface was analyzed in combination with computational spectroscopic analysis. The results show that at the alkaline interface, the difference between the expansion and contraction vibration frequency of water molecules located in the discontinuous region of the hydrogen bond network and the expansion vibration frequency of water molecules in the region close to the bulk phase is much higher than that at the acidic interface. At the same time, the proportion of water molecules in the discontinuous region of the hydrogen bond network at the alkaline interface is also much lower than that of the acidic interface. Therefore, it can be inferred that for the actual hydrogen-electrocatalytic system, there is indeed a significant water molecule distribution and hydrogen bond network discontinuity in the alkaline interface electric double layer.

Figure 3: Improvement of OHad species on Pt3Ru catalyst for alkaline interface hydrogen bond network connectivity.

Finally, using the Pt-Ru alloy as the model catalyst further reveals the essence of OHad adsorption intermediates at the Ru site in improving the kinetics of alkaline hydrogen-electrocatalytic reactions, that is, increasing the connectivity of the hydrogen bond network in the electric double layer, rather than only affecting the surface reaction path and its energy as previously thought.

This study highlights the critical role of interface electric layer structures in electrocatalysis and provides a research paradigm for the study of electrochemical interface atomic structures in conjunction with AIMD simulation, experimental spectroscopy, and computational spectroscopy. The work was supported by the National Natural Science Foundation of China (21832004) and the surface project (21673163), and the research process was discussed and supported by Professor Cai Wenbin of Fudan University and Professor Guo Cunlan of Wuhan University, and the Supercomputing Center of Wuhan University provided computing resources support for the computational simulation part. (Source: Science Network)

Related Paper Information:https://doi.org/10.1038/s41929-022-00846-8



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