Mechanism Of Hydrogen Oxidation Reaction Research Articles

Boosting Alkaline Hydrogen Oxidation Kinetics through Interfacial Environments Induced Surface Migration of Adsorbed Hydroxyl

Constructing bifunctional sites through heterojunction engineering to accelerate water formation has become a pivotal strategy to improve the alkaline hydrogen oxidation reaction (HOR) kinetics, which is mainly focused on the synergistic effect of neighboring sites and the energetics of the surface reaction steps. However, the roles of the surface migration of key intermediates that go beyond the bifunctional mechanism limited to neighboring atoms have usually been ignored. Using the heterostructured Ni3C‐Ni catalyst as a model, we found that the rapid surface migration of OHad species from the positively charged Ni3C to the negatively charged Ni component played a decisive role in facilitating water formation. Such unprecedented surface migration of OHad is induced by the large discrepancy between the local surface charge densities and interfacial environments of the Ni3C and Ni components under operating conditions. Benefiting from this, the resultant Ni3C‐Ni exhibited outstanding mass activity for the alkaline HOR, which was approximately 19‐fold and 21‐fold higher than those of Ni and Ni3C, respectively. These findings not only provide novel insights into the alkaline HOR mechanism of heterostructured catalysts but also open new avenues for developing advanced electrocatalysts for alkaline fuel cells.

Boosting Alkaline Hydrogen Oxidation Kinetics through Interfacial Environments Induced Surface Migration of Adsorbed Hydroxyl.

Constructing bifunctional sites through heterojunction engineering to accelerate water formation has become a pivotal strategy to improve the alkaline hydrogen oxidation reaction (HOR) kinetics, which is mainly focused on the synergistic effect of neighboring sites and the energetics of the surface reaction steps. However, the roles of the surface migration of key intermediates that go beyond the bifunctional mechanism limited to neighboring atoms have usually been ignored. Using the heterostructured Ni3C-Ni catalyst as a model, we found that the rapid surface migration of OHad species from the positively charged Ni3C to the negatively charged Ni component played a decisive role in facilitating water formation. Such unprecedented surface migration of OHad is induced by the large discrepancy between the local surface charge densities and interfacial environments of the Ni3C and Ni components under operating conditions. Benefiting from this, the resultant Ni3C-Ni exhibited outstanding mass activity for the alkaline HOR, which was approximately 19-fold and 21-fold higher than those of Ni and Ni3C, respectively. These findings not only provide novel insights into the alkaline HOR mechanism of heterostructured catalysts but also open new avenues for developing advanced electrocatalysts for alkaline fuel cells.

Iridium-Based Alkaline Hydrogen Oxidation Reaction Electrocatalysts.

The hydroxide exchange membrane fuel cells (HEMFCs) are promising but lack of high-performance anode hydrogen oxidation reaction (HOR) electrocatalysts. The platinum group metals (PGMs) have the HOR activity in alkaline medium two to three orders of magnitude lower than those in acid, leading to the high required PGMs amount on anode to achieve high HEMFC performance. The mechanism study demonstrates the hydrogen binding energy of the catalyst determines the alkaline HOR kinetics, and the adsorbed OH and water on the catalyst surface promotes HOR. Iridium (Ir) has a unique advantage for alkaline HOR due to its similar hydrogen binding energy to Pt and enhanced adsorption of OH. However, the HOR activity of Ir/C is still unsatisfied in practical HEMFC applications. Further fine tuning the adsorption of the intermediate on Ir-based catalysts is of great significance to improve their alkaline HOR activity, which can be reasonably realized by structure design and composition regulation. In this concept, we address the current understanding about the alkaline HOR mechanism and summarize recent advances of Ir-based electrocatalysts with enhanced alkaline HOR activity. We also discuss the perspectives and challenges on Ir-based electrocatalysts in the future.

Multidimensional Electrochemistry Decodes the Operando Mechanism of Hydrogen Oxidation

AbstractBeing an efficient approach to the utilization of hydrogen energy, the hydrogen oxidation reaction (HOR) is of particular significance in the current carbon‐neutrality time. Yet the mechanistic picture of the HOR is still blurred, mostly because the elemental steps of this reaction are rapid and highly entangled, especially when deviating from the thermodynamic equilibrium state. Here we report a strategy for decoding the HOR mechanism under operando conditions. In addition to the wide‐potential‐range I–V curves obtained using gas diffusion electrodes, we have applied the AC impedance spectroscopy to provide independent and complementary kinetic information. Combining multidimensional data sources has enabled us to fit, in mathematical rigor, the core kinetic parameter set in a 5‐D data space. The reaction rate of the three elemental steps (Tafel, Heyrovsky, and Volmer reactions), as a function of the overpotential, can thus be distilled individually. Such an undocumented kinetic picture unravels, in detail, how the HOR is controlled by the elemental steps on polarization. For instance, at low polarization region, the Heyrovsky reaction is relatively slow and can be ignored; but at high polarization region, the Heyrovsky reaction will surpass the Tafel reaction. Additionally, the Volmer reaction has been the fastest within overpotentials of interest. Our findings not only offer a better understanding of the HOR mechanism, but also lay the foundation for the development of improved hydrogen energy utilization systems.

Multidimensional Electrochemistry Decodes the Operando Mechanism of Hydrogen Oxidation.

Being an efficient approach to the utilization of hydrogen energy, the hydrogen oxidation reaction (HOR) is of particular significance in the current carbon-neutrality time. Yet the mechanistic picture of the HOR is still blurred, mostly because the elemental steps of this reaction are rapid and highly entangled, especially when deviating from the thermodynamic equilibrium state. Here we report a strategy for decoding the HOR mechanism under operando conditions. In addition to the wide-potential-range I-V curves obtained using gas diffusion electrodes, we have applied the AC impedance spectroscopy to provide independent and complementary kinetic information. Combining multidimensional data sources has enabled us to fit, in mathematical rigor, the core kinetic parameter set in a 5-D data space. The reaction rate of the three elemental steps (Tafel, Heyrovsky, and Volmer reactions), as a function of the overpotential, can thus be distilled individually. Such an undocumented kinetic picture unravels, in detail, how the HOR is controlled by the elemental steps on polarization. For instance, at low polarization region, the Heyrovsky reaction is relatively slow and can be ignored; but at high polarization region, the Heyrovsky reaction will surpass the Tafel reaction. Additionally, the Volmer reaction has been the fastest within overpotentials of interest. Our findings not only offer a better understanding of the HOR mechanism, but also lay the foundation for the development of improved hydrogen energy utilization systems.

Improving the Hydrogen Oxidation Reaction Rate of Ru by Active Hydrogen in the Ultrathin Pd Interlayer.

Enhancing the catalytic activity of Ru metal in the hydrogen oxidation reaction (HOR) potential range, improving the insufficient activity of Ru caused by its oxophilicity, is of great significance for reducing the cost of anion exchange membrane fuel cells (AEMFCs). Here, we use Ru grown on Au@Pd as a model system to understand the underlying mechanism for activity improvement by combining direct in situ surface-enhanced Raman spectroscopy (SERS) evidence of the catalytic reaction intermediate (OHad) with in situ X-ray diffraction (XRD), electrochemical characterization, as well as DFT calculations. The results showed that the Au@Pd@Ru nanocatalyst utilizes the hydrogen storage capacity of the Pd interlayer to "temporarily" store the activated hydrogen enriched at the interface, which spontaneously overflows at the "hydrogen-deficient interface" to react with OHad adsorbed on Ru. It is the essential reason for the enhanced catalytic activity of Ru at anodic potential. This work deepens our understanding of the HOR mechanism and provides new ideas for the rational design of advanced electrocatalysts.

The Role of Discrepant Reactive Intermediates on Ru‐Ru2P Heterostructure for pH‐Universal Hydrogen Oxidation Reaction

AbstractDeveloping highly efficient electrocatalysts for hydrogen oxidation reaction (HOR) under alkaline media is essential for the commercialization of alkaline exchange membrane fuel cell (AEMFC). However, the kinetics of HOR in alkaline media is complicated, resulting in orders of magnitude slower than that in acid, even for the state‐of‐the‐art Pt/C. Here, we find that Ru‐Ru2P/C heterostructure shows HOR performance with a non‐monotonous variation in a whole pH region. Unexpectedly, an inflection point located at pH≈7 is observed, showing an anomalous behavior that HOR activity under alkaline media surpasses acidic media. Combining experimental results and theoretical calculations, we propose the roles of discrepant reactive intermediates for pH‐universal HOR, while H* and H2O* adsorption strengths are responsible for acidic HOR, and OH* adsorption strength is essential for alkaline HOR. This work not only sheds light on fundamentally understanding the mechanism of HOR but also provides new designing principles for pH‐targeted electrocatalysts.

The Role of Discrepant Reactive Intermediates on Ru-Ru2 P Heterostructure for pH-Universal Hydrogen Oxidation Reaction.

Developing highly efficient electrocatalysts for hydrogen oxidation reaction (HOR) under alkaline media is essential for the commercialization of alkaline exchange membrane fuel cell (AEMFC). However, the kinetics of HOR in alkaline media is complicated, resulting in orders of magnitude slower than that in acid, even for the state-of-the-art Pt/C. Here, we find that Ru-Ru2 P/C heterostructure shows HOR performance with a non-monotonous variation in a whole pH region. Unexpectedly, an inflection point located at pH≈7 is observed, showing an anomalous behavior that HOR activity under alkaline media surpasses acidic media. Combining experimental results and theoretical calculations, we propose the roles of discrepant reactive intermediates for pH-universal HOR, while H* and H2 O* adsorption strengths are responsible for acidic HOR, and OH* adsorption strength is essential for alkaline HOR. This work not only sheds light on fundamentally understanding the mechanism of HOR but also provides new designing principles for pH-targeted electrocatalysts.

Platinum-Water Interaction Induced Interfacial Water Orientation That Governs the pH-Dependent Hydrogen Oxidation Reaction.

Understanding the electrode-water interface structure in acid and alkali is crucial to unveiling the underlying mechanism of pH-dependent hydrogen oxidation reaction (HOR) kinetics. In this work, we construct the explicit Pt(111)-H2O interface models in both acid and alkali to investigate the relationship between the HOR mechanism and electrode-electrolyte interface structure using ab initio molecular dynamics and density functional theory. We find that the interfacial water orientation in the outer Helmholtz layer (OHP) induced by the Pt-water interaction governs the pH-dependent HOR kinetics on Pt(111). In alkali, the strong Pt-interfacial water electrostatic interaction behaves as a narrow OHP, which increases the proportion of "H-down" interfacial water and leads to less adsorbed water entering the inner Helmholtz plane (IHP), decreasing the work function of Pt(111). Furthermore, the more "H-down" interfacial water stabilizes the Had adsorption, prevents Had desorption, and suppresses the Volmer step of HOR by forming the solvated [Had···H2O···H2O] complex. Our work provided a visualized molecular-level mechanism to understand the nature of pH-dependent HOR kinetics.

Further optimization of the properties of Pr0.6Sr0.4Fe0.9W0.1O3-δ by Cu-doping as symmetric electrodes for solid oxide fuel cells

Copper-doped Pr0.6Sr0.4Fe0.8Cu0.1W0.1O3-δ (PSFCuW) as a symmetric electrode material is investigated for La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM)-based symmetric solid oxide fuel cells (SSOFCs). The effect of Cu-doping on stability and mechanism of hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) is investigated and analyzed. Fe partially replaced by Cu on the B-site could also keep the phase structure stable after treatment under reducing conditions and improve the electrical conductivity, which reaches 30.3 and 0.78-1 under oxidizing and reducing conditions at 800 oC, respectively. The distribution of relaxation times (DRT) indicates that the rate-limiting step processes under oxidation conditions are oxygen adsorption and diffusion, while hydrogen adsorption and charge transfer have important influences on the rate-limiting step under reduced atmospheres, respectively. Finally, the LSGM-supported SSOFC (PSFCuW|LSGM|PSFCuW) shows excellent stability and reversibility. These results imply that PSFCuW is an up-to-coming symmetric electrode material for application in SSOFCs.

Effect of Lithium Sulfate on the Catalytic Activity of Pt for Hydrogen Oxidation Reaction

The effect of Lithium sulfate on the hydrogen oxidation reaction (HOR) in 0.1 M H2SO4 electrolyte was investigated on flat Pt electrode. The Li+ concentration solutions of 0, 10, 25 and 32 g l−1 were studied using cyclic voltammetry and rotating disk electrode (RDE) techniques. The obtained results demonstrate a good repeatability and confidence in analysis method, to understand the influence of lithium sulfate on HOR for an electrocatalysis system. The electrochemical surface area, limiting current and kinetic parameters were measured and analysed using Koutecky-Levich and Tafel representations to investigates the different types of lithium sulfate interactions on the catalytic properties of Pt. In presence of Li2SO4, the H+ adsorption/desorption process, species mass-transport and kinetic current density are reduced. Furthermore, the Tafel’s slope analyse show a change of the rate-determining steps for HOR mechanism. More detailed results of the kinetic analysis and lithium impact on the studied systems are discussed in this work.

Ultra-Low Pt Loading in PtCo Catalysts for the Hydrogen Oxidation Reaction: What Role Do Co Nanoparticles Play?

The effect of the nature of the catalyst on the performance and mechanism of the hydrogen oxidation reaction (HOR) is discussed for the first time in this work. HOR is an anodic reaction that takes place in anionic exchange membrane fuel cells (AEMFCs) and hydrogen pumps (HPs). Among the investigated catalysts, Pt exhibited the best performance in the HOR. However, the cost and the availability limit the usage. Co is incorporated as a co-catalyst due to its oxophylic nature. Five different PtCo catalysts with different Pt loading values were synthesized in order to decrease Pt loading. The catalytic activities and the reaction mechanism were studied via electrochemical techniques, and it was found that both features are a function of Pt loading; low-Pt-loading catalysts (Pt loading < 2.7%) led to a high half-wave potential in the hydrogen oxidation reaction, which is related to higher activation energy and an intermediate Tafel slope value, related to a mixed HOR mechanism. However, catalysts with moderate Pt loading (Pt loading > 3.1%) exhibited lower E1/2 than the other catalysts and exhibited a mechanism similar to that of commercial Pt catalysts. Our results demonstrate that Co plays an active role in the HOR, facilitating Hads desorption, which is the rate-determining step (RDS) in the mechanism of the HOR.

Bifunctional Palladium-Ceria Catalysts for Hydrogen Oxidation Reaction

In recent years, anion exchange membrane fuel cells (AEMFCs) have drawn much attention as the next-generation cost-effective fuel cells. A major challenge for the AEMFCs is the sluggish kinetics of the hydrogen oxidation reaction (HOR) in alkaline solution, which could be a major hinderance to developing high-performance AEMFC catalysts. Here, we explore the HOR mechanism for palladium-ceria anode catalysts using first-principles theory. This combination of materials has been shown to display excellent HOR performance experimentally. The computations are carried out for multiple palladium concentrations on ceria surfaces. The reaction pathway for HOR is investigated via the Tafel reaction for the dissociation of hydrogen molecules and Volmer reaction for the formation of water molecules. Our findings show that palladium-ceria bifunctional systems have improved HOR activity compared to their individual components. Specifically, an enhanced catalytic activity is predicted for 10 at. % palladium on ceria. We explain this behavior using multiple activity descriptors including hydrogen, OH, and H2O binding energies, and hybridization and charge transfer between the catalyst, the substrate, and adsorbents. The results suggest that the high HOR activity can be attributed to the delicate balance between the H and OH interactions with the palladium-ceria support as well as the interaction between the individual components that make up the heterostructure. The detailed first-principles analysis provides invaluable insights toward electronic, atomistic, and molecular mechanisms of HOR and paves the way for the development of catalysts that use significantly reduced amounts of precious metals. Sahoo et al., ACS Catal. 11, 2561 (2021).

Theoretical study of the alkaline hydrogen oxidation reaction on Ni-Based nanocluster Catalysts: Effects of graphene supports and dopants

The Ni-based catalysts are considered to be the most promising non-noble metal catalysts for the hydrogen oxidation reaction (HOR) in alkaline fuel cells (AFCs). However, their further optimization is restricted by the unclear HOR mechanism. In this work, the density functional theory calculations were carried out to study the alkaline HOR on pure Ni13 and Ni13 supported on graphene (Ni13/G) and B- and N-doped graphene (Ni13/BG and Ni13/NG). The calculations indicate that the HOR proceeds preferentially through the Tafel-Volmer mechanism, in which the adsorption of H* plays a dominant role in the HOR activity. The graphene support and B/N dopants can apparently lower the d-band center of Ni13, thereby weakening the adsorption of H*, which leads to the HOR activity sequence of Ni13 < Ni13/G < Ni13/BG < Ni13/NG. Furthermore, compared to pure Ni13, the oxidation resistance of Ni13/G, Ni13/BG and Ni13/NG is also improved via elevating onset potential of the OH* phase and reducing energy barriers of the OH* reduction with H* and H2*. An efficient way to improve the HOR performance is tuning the adsorption of H* to accelerate the HOR rate and regulating the adsorption of OH* to enhance the antioxidant capacity.

Non‐Platinum Group Metal Electrocatalysts toward Efficient Hydrogen Oxidation Reaction

AbstractDeveloping non‐platinum group metal (non‐PGM) electrocatalysts for the hydrogen oxidation reaction (HOR) represents the efforts towards the more economical use of hydrogen fuel cells and hydrogen energy, which has attracted tremendous attention recently. However, non‐PGM electrocatalysts for the HOR are still in their early development stages as compared with the significant advances in those for the oxygen reduction reaction and hydrogen evolution reaction. Herein, this paper summarizes the recent progresses and highlights the key challenges for the rational design of non‐PGM electrocatalysts, aiming to promote the development of non‐PGM HOR electrocatalysts. Fundamental understandings of the HOR mechanism are firstly reviewed, where theoretical interpretations on the low HOR kinetics in alkaline media, including the hydrogen binding energy theory, the bifunctional mechanism, and the water molecule reorganization, are particularly discussed. Subsequently, progresses of typical non‐PGM HOR electrocatalysts in acid and alkaline media are summarized separately. For the HOR under alkaline conditions, the superiorities and challenges of Ni‐based catalysts are discussed with a particular focus as they are the most promising non‐PGM electrocatalysts. Finally, this paper highlights the challenges and provide perspectives on the future development directions of non‐PGM HOR electrocatalysts.

Spectroscopic Verification of Adsorbed Hydroxy Intermediates in the Bifunctional Mechanism of the Hydrogen Oxidation Reaction

AbstractElucidating hydrogen oxidation reaction (HOR) mechanisms in alkaline conditions is vital for understanding and improving the efficiency of anion‐exchange‐membrane fuel cells. However, uncertainty remains around the alkaline HOR mechanism owing to a lack of direct in situ evidence of intermediates. In this study, in situ electrochemical surface‐enhanced Raman spectroscopy (SERS) and DFT were used to study HOR processes on PtNi alloy and Pt surfaces, respectively. Spectroscopic evidence indicates that adsorbed hydroxy species (OHad) were directly involved in HOR processes in alkaline conditions on the PtNi alloy surface. However, OHad species were not observed on the Pt surface during the HOR. We show that Ni doping promoted hydroxy adsorption on the platinum‐alloy catalytic surface, improving the HOR activity. DFT calculations also suggest that the free energy was decreased by hydroxy adsorption. Consequently, tuning OH adsorption by designing bifunctional catalysts is an efficient method for promoting HOR activity.

Bifunctional mechanism of hydrogen oxidation reaction on atomic level tailored-Ru@Pt core-shell nanoparticles with tunable Pt layers

Anion exchange membrane fuel cells (AEMFCs), as the next generation of cost-effective fuel cells, have attracted renewed attention in recent years. One of the existent challenges to the AEMFCs is the sluggish kinetics of the hydrogen oxidation reaction (HOR) in alkaline solution. The alkaline HOR mechanism is yet in debate, which would be a major barrier to the design of high-performance catalysts. Here, we explore the HOR mechanism on graphene-supported Ru@Pt core-shell nanoparticles. The Pt shell was precisely tailored at atomic monolayer level through controlling Cu underpotential deposition (UPD)-Galvanic displacement (GD) cycle. The relationship between the HOR electrocatalytic activity and the electronic structure of the catalysts has been investigated based on the results from electrochemical rotating disk electrode and X-ray photoelectron spectroscopic characterizations. We find that the hydrogen-binding energy (HBE) is not the sole descriptor of HOR activity, the reactive oxygen species on the catalyst surface also promotes the HOR kinetics through the bifunctional mechanism. This research offers the fundamentals for designing high performance HOR catalysts in alkaline solution and contributes to the commercial application of hydrogen fuel cells.

Mechanistic Insights into the Hydrogen Oxidation Reaction on PtNi Alloys in Alkaline Media: A First-Principles Investigation.

The promising alkaline anion exchange membrane fuel cell suffers from sluggish kinetics of the hydrogen oxidation reaction (HOR). However, the puzzling HOR mechanism hinders the further development of highly active catalysts in alkaline media. In this work, we conducted detailed first-principles calculations to acquire a deep understanding of the alkaline HOR mechanism on PtNi bulk alloys [Pt3Ni(111), Pt2Ni2(111), and PtNi3(111)] and its surface alloy [PtNisurf(111)]. The full free energy profiles suggest that the HOR on PtNi alloys proceeds via the Tafel-Volmer mechanism, that is, the direct decomposition of H2 into two adsorbed H, followed by its reaction with OH- in the electrolyte, as the rate-determining step, to form H2O. Therefore, the HOR activity of PtNi alloys is solely impacted by the adsorption of hydrogen, rather than hydroxyl species, though the oxophilicity is also enhanced by alloying Pt with Ni. Thermodynamically, a moderate H adsorption free energy, ΔGH* ≈ 0.414 eV, is calculated to be an optimal candidate for the HOR at pH = 13. Alloying Pt with Ni can elevate the d-band center (εd), push the value of ΔGH* closer to 0.414 eV, and thus lower the free energy barrier (Ea) of the rate-determining Volmer reaction, leading to the highest HOR activity of PtNi3(111) among all considered PtNi alloys. This situation is further confirmed by both the microkinetic model and the Tafel plot, where PtNi3(111) exhibits the highest reaction rate (r = 9.42 × 103 s-1 site-1) and the largest exchange current density (i0 = 1.42 mA cm-2) for HOR in alkaline media. This work provides a fundamental understanding of the HOR mechanism and theoretical guidance for rational design of electrocatalysts for HOR in alkaline media.

Hydrogen Oxidation Reaction on Pd‐Ni(OH)2 Composite Electrocatalysts in an Alkaline Electrolyte

AbstractExploring the mechanism of hydrogen oxidation reaction (HOR) in alkaline media is essential for the development of alkaline polymer electrolyte fuel cells (APEFCs). In this work, a series of Pd−Ni(OH)2 composites with different Pd/Ni atomic ratios (Pd : Ni=1:0 (without Ni addition), 7 : 1, 5 : 1, 3 : 1, 1 : 1) have been synthesized via wet chemical reduction method, and their corresponding HOR activities have been systematically evaluated with rotating disk electrode (RDE) technique. A good linear relationship between the HOR activities (described as the mass activity and exchange current density) and the surface oxophilicities (assessed by the oxidation current at 0.3 V vs. RHE in CO stripping voltammetry) is observed, indicating a bifunctional mechanism for HOR. The present study offers a direction for further improving the activity of Pd‐based HOR electrocatalyst.

Hydrogen oxidation reaction on modified platinum model electrodes in alkaline media

The mechanism of hydrogen oxidation reaction (HOR) in alkaline media is still under debate. In this work, we have systemically examined the HOR performance on planar Pt electrodes modified with ten different kinds of metal hydroxides/oxides (metal = Mg, Cr, Mn, Fe, Co, Ni, Cu, Ru, La and Ce). The HOR exchange current density (i0) increases monotonically with higher electrode’s oxophilicity, which is determined by the current densities of CO stripping at 0.4 V vs. RHE (iCO@0.4 V). However, the Ru modification exhibits a clear deviation from the monotonic relation and it demonstrates a superior catalytic activity towards HOR, relative to other metal modifications. This unexpected result is ascribed to the reduction in hydrogen binding energy (HBE) owing to the modification of metallic Ru rather than its corresponding hydroxide or oxide. The present study indicates that the electronic effect and the oxophilic effect co-exist in the HOR on Pt-based catalysts in alkaline media, but the former one is of more significance.