Hydrogen Evolution Reaction Process Research Articles

Lattice Strain and Schottky Junction Dual Regulation Boosts Ultrafine Ruthenium Nanoparticles Anchored on a N-Modified Carbon Catalyst for H2 Production.

Ruthenium-based materials are considered great promising candidates to replace Pt-based catalysts for hydrogen production in alkaline conditions. Herein, we adopt a facile method to rationally design a neoteric Schottky catalyst in which uniform ultrafine ruthenium nanoparticles featuring lattice compressive stress are supported on nitrogen-modified carbon nanosheets (Ru NPs/NC) for efficient hydrogen evolution reaction (HER). Lattice strain and Schottky junction dual regulation ensures that the Ru NPs/NC catalyst with an appropriate nitrogen content displays superb H2 evolution in alkaline media. Particularly, Ru NPs/NC-900 with 1.3% lattice compressive strain displays attractive activity and durability for the HER with a low overpotential of 19 mV at 10 mA cm-2 in 1.0 M KOH electrolyte. The in situ X-ray absorption fine structure measurements indicate that the low-valence Ru nanoparticle with shrinking Ru-Ru bond acts as catalytic active site during the HER process. Furthermore, multiple spectroscopy analysis and density functional theory calculations demonstrate that the lattice strain and Schottky junction dual regulation tunes the electron density and hydrogen adsorption of the active center, thus enhancing the HER activity. This strategy provides a novel concept for the design of advanced electrocatalysts for H2 production.

Bimetal-organic framework-derived nanorod-like FexNi1−x@C, P composites for electrochemical hydrogen evolution

Electrocatalytic decomposition of water using bifunctional catalysts is a potential method for clean hydrogen energy production. In order to solve the weakness of present catalysts, this study rationally designed a novel iron-nickel bifunctional electrocatalyst of FexNi1−x @C, P composites prepared by pyrolysis and phosphorus doping of a FeNi-bimetal-organic framework (FeNi-MOF). The innovative combination of FeNi bimetallic co-doping, low-temperature curing and phosphating has bred an excellent electrocatalyst for the hydrogen evolution reaction (HER) in alkaline solution. When the mass ratio of phosphate to FexNi1−x @C250–800 was 10:1, FexNi1−x @C250–800, P(10:1) showed the highest hydrogen evolution activity, obtaining a low 189 mV hydrogen evolution overpotential with a Tafel slope of 91.9 mV·dec−1 at 10 mA·cm−2 current density. Meanwhile, FexNi1−x @C250–800, P exhibited good stability during HER process. This work would open up a novel pathway for designing efficient and stable bifunctional electrocatalysts for water electrolysis as well as sustainable chemistry applications.

(Digital Presentation) Tracking Local pH Changes of Electrodeposited Co-Mo-Based Titania Composites for HER

Cobalt-molybdenum and cobalt-molybdenum-based phosphides are known hydrogen evolution electrocatalysts in alkaline electrolytes.1 Recently it has been shown that the addition of titania particles to create composite Co-Mo-TiO2 and Co-Mo-P-TiO2 enhances the hydrogen evolution reaction (HER).2 In order to better understand the role of TiO2 in the these composites, the local pH change was characterized during both electrodeposition and HER. The composites were electrodeposited onto a copper mesh working electrode in close vicinity to a flat-bottom pH probe. During electrodeposition from a citrate-boric acid electrolyte, there in an initial drop of pH followed by a pH rise with time that was not influenced by the presence of TiO2. However, there was a significant change in the local pH when TiO2 was present in Co-Mo-based electrocatalyst during the HER process. Without TiO2 particles present in the solid state, the local pH change during electrolysis follows an expected increase with time, while with embedded TiO2 particles in the metal matrix, the local pH initially decreases slightly and remains constant with time. The results suggest that the enhanced HER performance with TiO2 in the alloy composite electrocatalyst is a consequence of the buffering of the local pH. References Fan, D. Piron, A. Sleb, P. Paradis, J. Electrochem. Soc. 141, 382-387 (1994); P.R. Zabinski, H. Nemoto, S. Meguro, K. Asami K., Hashimoto K., J. Electrochem. Soc., 150, C717 (2003); Y.-Y. Ma, C.-X. Wu, X.-J. Feng, H.-Q. Tan, L.-K. Yan, Y. Liu, Z.-H. Kang, E.-B. Wang, Y.-G. Li, Energy Environ. Sci. 10, 788-798 (2017); A. C. Thenuwara, L. Dheer, N. H. Attanayake, Q. Yan, U. V. Waghmare, D. R. Strongin, ChemCatChem. 10, 4832-4837 (2018); X. Chen, Q. Li, Q. Che, Y. Chen, X. Xu, ACS Sustain. Chem. Eng. 7, 2437-2445 (2018).Wang, H.K. Bilan, E.J. Podlaha, J. Electrochem. Soc. 166, F661-F669 (2109); C. Wang, E. J. Podlaha, J. Electrochem. Soc, 167, 132502 (1~4) (2020).

Re nanoflower-decorated carbon cloth for pH-universal hydrogen evolution reaction: Unveiling the intrinsic electrocatalytic activity of metallic Re

The hydrogen evolution reaction (HER) is one of the primary routes for clean hydrogen energy production to replace fossil fuels. However, the lack of cost-effective electrocatalysts hinders its practical implementation. Re, which is considerably less expensive than Pt, has recently emerged as an electrocatalyst owing to its high HER activity over a wide pH range. However, HER processes of Re have been studied primarily through spectroscopy, with little theoretical interpretation to explain the intrinsic HER activity of Re. Additionally, it is necessary to develop antioxidative Re electrocatalysts to prevent hydrogen intercalation, and to suppress catalyst aggregation to ensure long-term stability. In this work, a theoretical analysis was first conducted to investigate the free energy of each HER intermediate and demonstrate high performance of Re. Then, a novel free-standing electrocatalyst consisting of Re nanoflowers (NFs) grown in-situ on a carbon cloth (CC) was synthesized (ReNF@CC) via a hydrothermal method followed by thermal annealing. The Re NFs homogeneously grown on the CC surface effectively suppressed self-aggregation to preserve abundant active sites, and the direct coupling of Re NFs enhanced the conductivity and physical/electrochemical stability to prevent their detachment from the CC during operation. A high HER performance was obtained with low overpotentials of 103, 115, and 75 mV at 10 mA cm−2 in 0.5 M H2SO4, 1 M PBS, and 1 M KOH, respectively; additionally, a superior durability with respect to commercial Pt/C was achieved. Overall, this study offers a versatile and universal strategy for highly efficient water electrolysis in different media.

Anion substitution induced vacancy regulating of cobalt sulfoselenide toward electrocatalytic overall water splitting

Anion substitution is a valid strategy to modulate the active sites of the transition metal dichalcogenides (TMDs). Herein, a series of cobalt sulfoselenide nanomeshes (CoS2(1-x)Se2x@NC) were synthesized by calcining S/Se power with ultrathin metal–organic framework (MOFs) nanosheets. The vacancy concentration of CoS2(1-x)Se2x@NC could be adjusted through changing the ratio of S/Se precursor. Interestingly, CoS1.25Se0.75@NC electrocatalyst possesses the largest vacancy concentration as well as the optimal electrocatalytic performance. CoS1.25Se0.75@NC delivers an overpotential as low as 134 mV for hydrogen evolution reaction (HER) and 270 mV for oxygen evolution reaction (OER) at the current density of 10 mA cm -2, respectively. Furthermore, CoS1.25Se0.75@NC affords a low cell voltage of 1.67 V (at 10 mA cm−2) and outstanding cycling stability for overall water splitting reaction (more than 55 h). For HER process, theoretical calculations prove that anion vacancy not only lower the free energy barrier of H2O dissociation step but also favor the desorption step of intermediate H*. For OER process, the anion vacancies could modulate the adsorption/desorption free energy of oxygen-containing intermediates. The present work demonstrates a practical approach to modulate the vacancy concentration of cobalt sulfoselenide and provides new ideas for design of efficient non-metal electrocatalysts.

Design and application of g-C3N4-based materials for fuels photosynthesis from CO2 or H2O based on reaction pathway insights

Photocatalytic CO2 reduction reaction (CRR) and hydrogen evolution reaction (HER) based on graphitic carbon nitride (g-C3N4) that is regarded as the metal-free “holy grail” photocatalyst, provide promising strategies for producing next-generation fuels, contributing to achieving carbon neutrality, alleviating energy and environment crisis. However, the activity of CRR and HER over g-C3N4 leaves much to be desired. Therefore, numerous studies have sprung up to enhance photoactivity. A comprehensive understanding of the CRR and HER reaction pathways is crucial for designing g-C3N4-based materials, further promoting efficient fuel production. Different from previous reviews that focus on g-C3N4 modification from the viewpoint of material science. In this review, we divided the multistep processes of CRR and HER into five reaction pathways and summarized the latest advances for improving each pathway of fuels synthesis through CRR or HER. Meanwhile, the existing bottleneck issues of each step were also discussed. Finally, comprehensive conclusions, including the remaining challenges, outlooks, etc., for CRR and HER over g-C3N4 were put forward. We are sure that this review will conduce to the understanding of the structure–activity relationship between CRR, HER processes, and g-C3N4 structure, which can provide the reference for developing high-powered photocatalysts, not confined to g-C3N4.

Strong Cr–O Hybridization Prevents Active Cu(II) from Cathodic Reconstruction for Alkaline Hydrogen Evolution Catalysis

The fully filled d10 orbital structure of metallic Cu is inert for alkaline hydrogen evolution reaction (HER), whereas its oxidized state, CuO with platinum (Pt)‐like d9 configuration has been predicted to benefit catalysis. However, the weak Cu(II)–O hybridization undergoes thermodynamically favorable cathodic reconstruction to metallic state during HER process. Herein, Cu(II) is successfully stabilized via introducing strong Cr–O hybridization and its potential for HER is unfolded. The Cr‐modified Cu(II) oxide surface delivers an overpotential of 247 mV at 100 mA cm−2, which is substantially better than the pristine metallic Cu (549 mV). Refined structural analysis confirms the existence of abundant Cu(II) with Cr incorporation. Theoretical calculations unravel the strong covalent Cr–O interaction that confines lattice O nearby Cu sites, leaving Cu(II) for the catalysis. The concepts of employing strong hybridization to secure active yet unstable catalytic sites provide a new vision for catalyst design.

Boosting electrochemical nitrogen reduction performance through water-in-salt electrolyte

While electrochemical nitrogen reduction reaction (NRR) stands as a promising technique for sustainable NH3 fabrication due to its mild operation conditions, it has long been suffered from low efficiency and poor selectivity owing to the inertness of N2 and competition hydrogen evolution reaction (HER). Herein, we report the use of oxygen vacancy rich TiO2 loaded with Au nanoparticles as NRR catalyst in super concentrated lithium bis(trifluoromethane sulfonyl) imide (Li TFSI) “water-in-salt” system (20 M electrolyte) and obtain the Faradaic efficiency of 37.2 % with an NH3 yield rate of 20.3 μg h−1 cm−2 at − 0.4 V vs RHE, accompanied by the significant decay of both current density and onset potential of hydrogen evolution reaction. Electrochemical experimental and theoretical calculation results indicate that the HER process can be greatly hindered both kinetically and energetically. The unique effect of this “water-in-salt” electrolyte would offer a new idea to improve the NRR selectivity via HER suppression.

Accelerating electrochemical hydrogen production on binder-free electrodeposited V- doped Ni-Mo-P nanospheres

• V-doped Ni-Mo-P nanostructure was fabricated using electrochemical deposition. • Fabricated electrode exhibited outstanding electrocatalytic HER and OER activity. • Synergistic effect and high surface area was main reason for excellent performance. • Deposited electrode represented outstanding electrocatalytic durability. The emergence and fabrication of cost-effective and active electrodes in the overall water splitting process are one of the most critical challenges for hydrogen production. Here, firstly Ni-P electrodes were generated at different scan rates using the electrochemical deposition method, and then molybdenum was doped in Ni-P structure using the same method and vanadium using hydrothermal deposition method, and V-doped Ni-Mo-P electrode was generated. It was revealed that the synthesized electrode displays outstanding electrochemical properties for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). For reaching a current density of 10 mA cm −2 , the optimum sample needed only 65 mV in the HER process and only 261 mV of overpotential in the OER process. In addition, the electrocatalytic stability investigation disclosed that this electrode had excellent durability at the 100 mA cm −2 in HER and OER process after 10 h of electrolysis in which only less than 2 % increase in overpotential was observed, which indicates excellent electrocatalytic stability. Such outstanding electrocatalytic properties were mainly because of increasing the active surface area and the synergistic effect due to doping of molybdenum and vanadium in the electrode structure .

Regulating surface composition of platinum-copper nanotubes for enhanced hydrogen evolution reaction in all pH values

Regulating catalyst composition is one of the efficient approaches to boost intrinsic activity of electrocatalysts for water splitting. Herein, four different hollow porous platinum-copper (PtCu) nanotubes (NTs) with controllable compositions were precisely fabricated by a facile wet-chemistry method. Importantly, Pt5Cu2 NTs display the best hydrogen evolution reaction (HER) performance in all pH conditions compared to other samples, which just require overpotentials of 34 ± 2, 32 ± 2, and 284 ± 2 mV at 10 mA cm−2 in basic, acidic, and neutral solutions, respectively. Moreover, Pt5Cu2 NTs also exhibit outstanding stability and corrosion resistance in all pH ranges. Then, mechanism analysis reveals that the electronic structure of Pt sites is regulated by changing the ratio of Pt and Cu, which directly optimizes the binding energy of reaction intermediates and promotes electron transfer during the HER process. In addition, a porous nanotube structure with countless nanoparticles on the surface provides a large number of active sites, enhancing the adsorption/desorption of reactants. This work emphasizes the importance of catalyst composition and provides a highly active potential HER catalyst for practical hydrogen production.

Hybrid Ni2P/CoP Nanosheets as Efficient and Robust Electrocatalysts for Domestic Wastewater Splitting

The development of low‐cost, robust and efficient non‐noble metal electrocatalysts is still a pursuit for the hydrogen evolution reaction (HER). Herein, a self‐standing electrocatalyst, Ni2P/CoP nanosheet, was fabricated directly on three‐dimensional Ni foams by two facile steps, which illustrated both high activity and stability for HER in different electrolytes. Benefiting from the porous structures of nanosheets with large specific surface area and the hybrid Ni2P/CoP, the as‐prepared electrocatalyst presented remarkable HER with overpotentials of 65.2 and 87.8 mV to reach a current density of −10 mA cm−2 in neutral and alkaline media, respectively. Density function theory calculations revealed a lower activation energy of water dissociation and efficient HER steps of hybrid Ni2P/CoP nanosheets compared with mono CoP. The self‐standing electrocatalyst maintained excellent chemical stability. Additionally, the HER process in domestic wastewater was realized with more impressive performance by using Ni2P/CoP nanosheets compared with commercial Pt/C. Hydrogen was continuously generated for 20 h in mildly alkaline dishwashing wastewater. This work provides a feasible way to fabricate non‐noble metal and self‐standing hybrid bimetallic phosphides for HER in neutral and alkaline media, showing great potential for efficient hydrogen production by re‐utilizing wastewater resources.

NiMoO4 @Co3S4 nanorods with core-shell structure for efficient hydrogen evolution reactions in electrocatalysts

High-efficiency electrocatalysts are required for the hydrogen evolution reaction processes used in clean-energy applications. Single-component materials have limited physicochemical properties and do not meet the requirements of high-performance material applications. More complex materials must therefore be developed for these purposes. This study involves growing flower cluster-like NiMoO4 nanorod arrays on reduced graphene oxide-modified nickel foam (NF/rGO) substrates by a simple two-step hydrothermal method, followed by NiMoO4 nanorod modification with Co3S4 nanoparticles, to achieve an efficient hydrogen evolution reaction (HER) strategy for NF/rGO/NiMoO4 @Co3S4 core-shell structure composite catalysts. The proposed vertically aligned NiMoO4 nanorods served as the core framework, providing abundant specific surface area and fast charge transfer channels. The Co3S4 particles grown on the NiMoO4 nanorods provided sufficient electrochemical active sites for full contact with the electrolyte solution. In the 1.0 M KOH solution, the NF/rGO/NiMoO4 @Co3S4 core-shell structure exhibited a low overpotential of 40 mV for HER at a current density of 10 mA cm−2. These properties indicate that the proposed material could be used in HER processes in energy applications.

Competitive‐Reduction Chemistry on Ni‐Doped FeS2 Catalyst Reinforcing Acid‐Corrosion Resistance for Stable Hydrogen Evolution Reactions

Transition‐metal chalcogenides are promising low‐cost hydrogen evolution reaction (HER) catalysts; however, they suffer from serious electrochemical corrosion in acidic environment. Herein, it is proposed that Ni‐doped FeS2 nanowire arrays (Ni‐FeS2 NWs) fabricated through a simple doping strategy show significantly enhanced antiacid stability during HER process relative to its FeS2 counterpart enabled by a S‐site competitive‐reduction chemistry. Transition‐state energy calculations indicate that Ni‐FeS2 and FeS2 show different HER catalytic active center at S and Fe sites, respectively. For Ni‐FeS2, this means more H+ preferentially adsorb on S sites for enhanced HER which simultaneously suppresses the competitive reduction of acid corrosion of FeS2 occurred at the same S sites. However, for FeS2 the corrosion reaction and HER occurred, respectively, at S and Fe sites are paralleled. Besides, density functional theory calculations also show that the Ni‐FeS2 exhibits the enhanced structural stability during the HER process after Ni2+ doping into FeS2.

Unique three-dimensional heterostructure of MoS2@Co-MOF decorated with Co-Al layered double hydroxide: An effective synergistic alkaline hydrogen evolution electrocatalyst

Development of non-platinum based electrocatalysts with optimized structure is significantly important to realize the blueprint of "hydrogen economy". Herein, a three-dimensional (3D) nanoflower heterostructure composite electrocatalyst for alkaline hydrogen evolution reaction (HER) is constructed (MoS2@Co-MOF@CoAl-LDHs/NF), which is composed of CoAl layered double hydroxides (CoAl-LDHs), in-situ Co-MOF and activated molybdenum disulfide (MoS2). Benefiting from the coordination effect between the components, act outs outstanding electrocatalytic activity for HER process, requiring only an overpotential of 96 mV to achieve 10 mA cm−2, a Tafel plot of 73.53 mV dec−1 in 1.0 M potassium hydroxide (KOH) solution, of which the overpotential and the Tafel plot compare favorably to the majority of reported non-noble-metal HER electrocatalyst material. Given the above description, this work breaks new prospects for the development of high-performance electrocatalysts in media that are not conducive to traditional catalyst design.

Copper Incorporated Molybdenum Trioxide Nanosheet Realizing High-Efficient Performance for Hydrogen Production

The development of highly active non-precious metal electrocatalysts is crucial for advancing the practical application of hydrogen evolution reaction (HER). Doping engineering is one of the important strategies to optimize the electrocatalytic activity of electrocatalysts. Herein, we put forward a simple strategy to optimize the catalytic activity of MoO3 material by incorporating the Cu atoms into the interlayer (denoted as Cu-MoO3). The prepared Cu-MoO3 nanosheet has a larger surface area, higher conductivity, and strong electron interactions, which contributes to optimal reaction kinetics of the HER process. As a result, the Cu-MoO3 nanosheet only needs a small overpotential of 106 mV to reach the geometric current density of 10 mA cm−2. In addition, it also delivers a low Tafel slope of 83 mV dec−1, as well as high stability and Faraday efficiency. Notably, when using the Cu-MoO3 as a cathode to construct the water electrolyzer, it only needs 1.55 V to reach the 10 mA cm−2, indicating its promising application in hydrogen generation. This work provides a novel type of design strategy for a highly active electrocatalyst for an energy conversion system.

Molybdenum‐based electrocatalysts with nanostructured supports for hydrogen evolution reaction

AbstractElectrochemical water splitting is one of the most efficient ways to yield H2 fuel due to its renewability and environmental friendliness. For minimizing the energy loss during the electrocatalytic hydrogen evolution reaction (HER), as a crucial half‐cell reaction of electrochemical water splitting, researchers are seeking for suitable electrocatalysts, which can significantly decrease the overpotential and consequently promote the reaction rate and total cell efficiency. Among the noble‐metal free HER catalysts, Mo‐based electrocatalysts have been reported to possess excellent electrocatalytic activity and long‐term durability in terms of the HER process. Recently, in order to further improve the reaction rate of HER, nanostructured supports for Mo‐based catalysts were designed, which not only increase active sites but also contribute to alleviating the overpotential. Herein, an overview is presented of recent research progress on Mo‐based electrocatalysts with nanostructured supports for the HER. It starts with summarizing the fundamentals of HER and recent progress of Mo‐based electrocatalysts. Subsequently, the design and synthesis of nanostructures, electrochemical performance, and the related mechanisms for Mo‐based electrocatalysts with nanostructured supports are discussed. Finally, the future opportunities and challenges are highlighted for the development of the Mo‐based electrocatalysts from the points of view of both fundamental understanding and practical application.

Hydrogen spillover effect enhanced by carbon quantum dots activated MoS2

The weak hydrogen adsorption and slow catalytic kinetics are unfavorable for MoS2 as a catalyst for hydrogen evolution reaction (HER) in alkaline solution. Herein, carbon quantum dots (CQDs) with high proton adsorption energy were expected to enhance the proton adsorption of MoS2 during the HER process. The CQDs activated MoS2 can establish a suitable hydrogen adsorption gradient due to the surface charge redistribution to promote the hydrogen spillover pathway. After the CQDs were loaded on MoS2, the charge distribution would gradually transfer from the C sites of CQDs to the S sites on MoS2 surface, showing a charge gradient distribution calculated by density functional theory predictions. The adsorbed hydrogen can spill over from hydrogen-enriched CQDs to hydrogen-deficient MoS2 in HER. The process could be monitored by the quasi in-situ electrochemical impedance spectroscopy. The enhanced hydrogen spillover effect by CQDs activated MoS2 can pave the way for the design and preparation of efficient and low-cost catalysts.

Hierarchical heterostructure consists of Al3+ doped NiFe2O4 nanocubes on NiCo2O4 nanosheets for efficient electrocatalytic water splitting

Spinels are advanced electrocatalysts for oxygen evolution reaction (OER) due to oxophilic properties, however, their research in the field of hydrogen evolution reaction (HER) or overall water splitting (OWS) is rarely reported. In this paper, a hierarchical heterostructure electrocatalyst consists of Al3+ doped NiFe2O4 nanocubes on NiCo2O4 nanosheet arrays (A-NCO@NFO/NF) was successfully prepared for electrocatalytic OWS. The preparing strategy is conducting [Fe(CN)6]3− anion exchange and Al3+ cation doping of NiCo(OH)x grown on nickel foam (NF) accompanied by subsequent air calcination. HRTEM confirmed that the successful formation of NCO/NFO heterointerface and the incorporation of Al3+ into the lattices of NCO and NFO. XPS indicated that the NCO/NFO heterojunction promotes the electron transfer from Ni to Fe, which is favorable for the adsorption of H+ during the HER process, and the Al3+ facilitates the electron transfer from Fe to Co, which is favorable for the formation of CoOOH active species during the OER process. The HER and OER overpotentials of the electrocatalyst are only 178 mV and 159 mV, respectively, and a current density of 10 mA cm−2 can be achieved with only 1.61 V applied during the OWS process, also with excellent stability for 25 h. This study provides a new idea for designing spinel-based green composite electrocatalysts.

FeNi@N‐Doped Graphene Core–Shell Nanoparticles on Carbon Matrix Coupled with MoS2 Nanosheets as a Competent Electrocatalyst for Efficient Hydrogen Evolution Reaction

AbstractSynthesis of noble‐metal‐free electrocatalysts for green hydrogen production is crucial to overcoming the energy demand of modern society. One of the most competitive and alternative noble‐metal‐free electrocatalysts for hydrogen evolution reaction (HER) is molybdenum disulfide (MoS2)‐based composites. Herein, it is shown that MoS2 nanosheets grow on FeNi@N‐doped graphene nanoparticles/N‐doped carbon matrix (FeNi@NG/NCM@MoS2), using the hydrothermal method. FeNi@NG/NCM@MoS2 hybrid displays outstanding HER performance with a low overpotential of 79 mV at 10 mA cm−2, a small Tafel slope of 40.2 mV dec−1, and high durability. First‐principles density functional theory simulations confirm the electron transformation from FeNi alloy to NG surface of FeNi@NG particle and subsequently further transfer to MoS2 nanosheets, which decrease the Gibbs free energy (ΔGH* ≈ −0.08 eV) and local work function for enhanced HER activities. This work highlights the understanding of electron transfer in demonstrating the kinetic reaction of the HER process and offers a new avenue for constructing efficient MoS2‐based electrocatalysts.

(Digital Presentation) Investigating the Origin of the Large HER Overpotential of Ti3C2 Using in-Situ/Operando Raman Spectroelectrochemistry

MXenes have shown great promise as electrocatalysts for the hydrogen evolution reaction (HER), but their mechanism is still poorly understood. Currently, the benchmark Ti3C2 MXene suffers from a large overpotential. To reduce this overpotential, modifications must be made to the structure through mechanistic studies to increase the reaction rate of the proton/electron coupled transfer steps. Here, we use in-situ/operando Raman spectroelectrochemistry to probe the HER mechanism and environmental changes of the Ti3C2 electrocatalyst during HER. In acidic media, we observed a shift in the termination group from –O– to –OH, consistent with a protonation step followed by a deprotonation step to form hydrogen gas. This mechanism was further corroborated by density functional theory (DFT) calculations. DFT calculations show that protonation of the –O– functional groups followed by binding of a Ti–O–H* with a Ti–H* is the rate determining step, resulting in a large HER overpotential. In neutral media, we observe an overcharging of the structure allowing for the adsorption of water prior to the HER process, which is rate limiting and causes a large HER overpotential. Through DFT analyses, we found that the reduction process in both electrolytic systems substantially increases the Ti–C bond strength for the outer Ti atoms accompanied by a shift of the Ti d-band center away from the Fermi level indicating a much lower reactivity during HER. By combining the experimental and computational results, reaction mechanisms were found for both electrolytic systems. These fundamental insights enable for proper modifications to be made for MXene catalysts to enhance their electrocatalytic performance.