Visualization of Hydrogen Evolution at Individual Platinum Nanoparticles at a Buried Interface

Revealing the diverse electrochemistry of nanoparticles with scanning electrochemical cell microscopy

The next generation of electroactive materials will depend on advanced nanomaterials, such as nanoparticles (NPs), for improved function and reduced cost. As such, the development of structure-function relationships for these NPs has become a prime focus for researchers from many fields, including materials science, catalysis, energy storage, photovoltaics, environmental/biomedical sensing, etc. The technique of scanning electrochemical cell microscopy (SECCM) has naturally positioned itself as a premier experimental methodology for the investigation of electroactive NPs, due to its unique capability to encapsulate individual, spatially distinct entities, and to apply a potential to (and measure the resulting current of) single-NPs. Over the course of conducting these single-NP investigations, a number of unexpected (i.e. rarely-reported) results have been collected, including fluctuating current responses, and carrying of the NP by the SECCM probe, hypothesised to be due to insufficient NP-surface interaction. Additionally, locations with measurable electrochemical activity have been found to contain no associated NP, and conversely locations with no activity have been found to contain NPs. Through presenting and discussing these findings, this article seeks to highlight complications in single-NP SECCM experiments, particularly those arising from issues with sample preparation.

Gas Evolution in Water Electrolysis

Gas bubbles generated by the hydrogen evolution reaction and oxygen evolution reaction during water electrolysis influence the energy conversion efficiency of hydrogen production. Here, we survey what is known about the interaction of gas bubbles and electrode surfaces and the influence of gas evolution on practicable devices used for water electrolysis. We outline the physical processes occurring during the life cycle of a bubble, summarize techniques used to characterize gas evolution phenomena in situ and in practical device environments, and discuss ways that electrodes can be tailored to facilitate gas removal at high current densities. Lastly, we review efforts to model the behavior of individual gas bubbles and multiphase flows produced at gas-evolving electrodes. We conclude our review with a short summary of outstanding questions that could be answered by future efforts to characterize gas evolution in electrochemical device environments or by improved simulations of multiphase flows.

Modulating Surface Cation Concentration via Tuning the Molecular Structures of Ethylene Glycol-Functionalized PEDOT for Improved Alkaline Hydrogen Evolution Reaction

The sluggish catalytic kinetics of nonprecious metal-based electrocatalysts often hinder them from achieving efficient hydrogen evolution reactions (HERs). Poly(3,4-ethylenedioxythiophene) (PEDOT) and its derivatives have been promising materials for various electrochemical applications. Nevertheless, previous studies have demonstrated that PEDOT coatings can be detrimental to HER performance. In this study, we investigated the alkaline HER efficiency of nickel foam coated with three types of ethylene glycol (EG)-functionalized EDOT. Specifically, EDOT derivatives bearing hydroxyl (-OH) and methoxy (-OCH3) end groups on the EG side chain and molecules containing two EDOT units are interconnected via EG moieties. EG groups are selected due to their strong interaction with alkali metal cations. Intriguingly, improved HER performance is observed on all electrodes coated with EG-functionalized EDOTs. Electrochemical impedance spectroscopy, electrochemical quartz crystal microbalance with dissipation, and XPS analysis are employed to explore the origin of enhanced HER efficiency. The results suggest the EG moieties can induce locally concentrated ions near the electrode surface and facilitate water dissociation through noncovalent interactions. The influence of EG chain length is systematically investigated by synthesizing molecules with di-EG, tetra-EG, and hexa-EG functionalities. This study highlights the importance of molecular design in modifying electrode surface properties to promote alkaline HER.

Influence of Particle Size on Mass Transport during the Oxygen Reduction Reaction at Single Silver Particles Using Scanning Electrochemical Cell Microscopy

Single entity electrochemical measurements enable insight into the electrocatalytic activity of individual particles based on composition, shape, and crystallographic orientation. In addition to structural effects, particle size can further influence electrocatalytic activity and reaction mechanisms through mass transport effects. In this work, electrodeposition was used to grow well-separated silver particles of varying sizes from 100 to 500 nm in radius. Using a multimicroscopy approach of scanning electrochemical cell microscopy combined with scanning electron microscopy, the electrocatalytic current of individual silver particles toward the oxygen reduction reaction was evaluated as a function of their size. It was found that the current density increased with decreasing particle radius, which was correlated to the mass transport of oxygen to the silver particle, demonstrating the importance of size dependent mass transport effects that can occur at the single particle level using scanning electrochemical cell microscopy and opening new opportunities for quantitative electrocatalysis measurements.

Single Entity Electrocatalysis

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.

Stereoscopic Imaging of Single Molecules at Plasma Membrane of Single Cell Using Photoreduction-Assisted Electrochemistry

Stereoscopic imaging of single molecules at the plasma membrane of single cell requires spatial resolutions in 3 dimensions (x-y-z) at 10-nm level, which is rarely achieved using most optical super-resolution microscopies. Here, electrochemical stereoscopic microscopy with a detection limit down to a single molecule is achieved using a photoreduction-assisted cycle inside a 20-nm gel electrolyte nanoball at the tip of a nanopipette. On the basis of the electrochemical oxidation of Ru(bpy)3 2+ into Ru(bpy)3 3+ followed by the reduction of Ru(bpy)3 3+ into Ru(bpy)3 2+ by photogenerated isopropanol radicals, a charge of 1.5 fC is obtained from the cycling electron transfers involving one Ru(bpy)3 2+/3+ molecule. By using the nanopipette to scan the cellular membrane modified with Ru(bpy)3 2+-tagged antibody, the morphology of the cell membrane and the distribution of carcinoembryonic antigen (CEA) on the membrane are electrochemically visualized with a spatial resolution of 14 nm. The resultant stereoscopic image reveals more CEA on membrane protrusions, providing direct evidence to support easy access of membrane CEA to intravenous antibodies. The breakthrough in single-molecule electrochemistry at the cellular level leads to the establishment of high-resolution 3-dimensional single-cell electrochemical microscopy, offering an alternative strategy to remedy the imperfection of stereoscopic visualization in optical microscopes.

Spatially Confined Microcells: A Path toward TMD Catalyst Design

With the ability to maximize the exposure of nearly all active sites to reactions, two-dimensional transition metal dichalcogenide (TMD) has become a fascinating new class of materials for electrocatalysis. Recently, electrochemical microcells have been developed, and their unique spatial-confined capability enables understanding of catalytic behaviors at a single material level, significantly promoting this field. This Review provides an overview of the recent progress in microcell-based TMD electrocatalyst studies. We first introduced the structural characteristics of TMD materials and discussed their site engineering strategies for electrocatalysis. Later, we comprehensively described two distinct types of microcells: the window-confined on-chip electrochemical microcell (OCEM) and the droplet-confined scanning electrochemical cell microscopy (SECCM). Their setups, working principles, and instrumentation were elucidated in detail, respectively. Furthermore, we summarized recent advances of OCEM and SECCM obtained in TMD catalysts, such as active site identification and imaging, site monitoring, modulation of charge injection and transport, and electrostatic field gating. Finally, we discussed the current challenges and provided personal perspectives on electrochemical microcell research.

Investigation of the Electrocatalytic Reduction of Peroxydisulfate Using Scanning Electrochemical Microscopy

The elementary steps of the electrocatalytic reduction of S2O82- using the Ru(NH3)63+/2+ redox couple were investigated using scanning electrochemical microscopy (SECM) and steady-state voltammetry (SSV). SECM investigations were carried out in a 0.1 M KCl solution using a 3.5 μm radius carbon ultramicroelectrode (UME) as the SECM tip and a 25 μm radius platinum UME as the substrate electrode. Approach curves were recorded in the positive feedback mode of SECM by reducing Ru(NH3)63+ at the tip electrode and oxidizing Ru(NH3)62+ at the substrate electrode, as a function of the tip-substrate separation and S2O82- concentration. The one-electron reaction between electrogenerated Ru(NH3)62+ and S2O82- yields the unstable S2O83•-, which rapidly dissociates to produce highly oxidizing SO4•-. Because SO4•- is such a strongly oxidizing species, it can be further reduced at both the tip and the substrate, or it can react with Ru(NH3)62+ to regenerate Ru(NH3)63+. SECM approach curves display a complex dependence on the tip-substrate distance, d, due to redox mediation reactions at both the tip and the substrate. Finite element method (FEM) simulations of both SECM approach curves and SSV confirm a previously proposed mechanism for the mediated reduction of S2O82- using the Ru(NH3)63+/2+ redox couple. Our results provide a lower limit for dissociation rate constant of S2O83•- (∼1 × 106 s-1), as well as the rate constants for electron transfer between SO4•- and Ru(NH3)62+ (∼1 × 109 M-1 s-1) and between S2O82- and Ru(NH3)62+ (∼7 × 105 M-1 s-1).

Electrochemical Visualization of Membrane Proteins in Single Cells at a Nanoscale Using Scanning Electrochemical Cell Microscopy

The electrochemical visualization of proteins in the plasma membrane of single fixed cells was achieved with a spatial resolution of 160 nm using scanning electrochemical cell microscopy. The model protein, the carcinoembryonic antigen (CEA), is linked with a ruthenium complex (Ru(bpy)₃²⁺)-tagged antibody, which exhibits redox peaks in its cyclic voltammetry curves after a nanopipette tip contacts the cellular membrane. Based on the potential-resolved oxidation or reduction currents, an uneven distribution of membrane CEAs on the cells is electrochemically visualized, which could only be achieved previously using super-resolution optical microscopy. Compared with current electrochemical microscopy, the single-cell scanning electrochemical cell microscopy (SECCM) strategy not only improves the spatial resolution but also utilizes the potential-resolved current from the antibody-antigen complex to increase electrochemical imaging accuracy. Eventually, the electrochemical visualization of cellular proteins at the nanoscale enables the super-resolution study of cells to provide more biological information.

Evaluating Analytical Expressions for Scanning Electrochemical Cell Microscopy (SECCM)

Scanning electrochemical cell microscopy (SECCM) maps the electrochemical activity of a surface with nanoscale resolution using an electrolyte-filled nanopipette. The meniscus at the end of the pipet is sequentially placed at an array of locations across the surface, forming a series of nanometric electrochemical cells where the current-voltage response is measured. Quantitative interpretation of these responses typically employs numerical modeling to solve the coupled equations of transport and electron transfer, which require costly software or self-written code. Expertise and time are required to build and solve numerical models, which must be rerun for each new experiment. In contrast, algebraic expressions directly relate the current response to physical parameters. They are simpler to use, faster to calculate, and can provide greater insight but frequently require simplifying assumptions. In this work, we provide algebraic expressions for current and concentration distributions in SECCM experiments, which are formulated by approximating the pipet and meniscus using 1-D spherical coordinates. Expressions for the current and concentration distributions as a function of experimental parameters and in various conditions (steady state and time dependent, diffusion limited, and including migration) all show excellent agreement with numerical simulations employing a full geometry. Uses of the analytical expressions include determination of expected currents in experiments and quantifying electron-transfer rate constants in SECCM experiments.

Operando Scanning Electrochemical Probe Microscopy during Electrocatalysis

Scanning electrochemical probe microscopy (SEPM) techniques can disclose the local electrochemical reactivity of interfaces in single-entity and sub-entity studies. Operando SEPM measurements consist of using a SEPM tip to investigate the performance of electrocatalysts, while the reactivity of the interface is simultaneously modulated. This powerful combination can correlate electrochemical activity with changes in surface properties, e.g., topography and structure, as well as provide insight into reaction mechanisms. The focus of this review is to reveal the recent progress in local SEPM measurements of the catalytic activity of a surface toward the reduction and evolution of O₂ and H₂ and electrochemical conversion of CO₂. The capabilities of SEPMs are showcased, and the possibility of coupling other techniques to SEPMs is presented. Emphasis is given to scanning electrochemical microscopy (SECM), scanning ion conductance microscopy (SICM), electrochemical scanning tunneling microscopy (EC-STM), and scanning electrochemical cell microscopy (SECCM).

Voltammetric Mapping of Hydrogen Evolution Reaction on Pt Locally via Scanning Electrochemical Cell Microscopy

The advancement in nanoscale electrochemical tools has offered the opportunity to better understand heterogeneity at electrochemical interfaces. Scanning electrochemical cell microscopy (SECCM) has been increasingly used for revealing local kinetics and the distribution of active sites in electrocatalysis. Constant-contact scanning and hopping scanning are the two commonly used modes. The former is intrinsically faster, whereas the latter enables full voltammetry at individual locations. Herein, we revisit a less used mode that combines the advantages of hopping and constant-contact scan, resulting in a faster voltammetric mapping. In this mode, the nanodroplet cell in SECCM maintains contact with the surface during the scanning and makes intermittent pauses for local voltammetry. The elimination of frequent retraction and approach greatly increases the speed of mapping. In addition, iR correction can be readily applied to the voltammetry, resulting in more accurate measurements of the electrode kinetics. This scanning mode is demonstrated in the oxidation of a ferrocene derivative on HOPG and hydrogen evolution reaction (HER) on polycrystalline Pt, serving as model systems for outer-sphere and inner-sphere electron transfer reactions, respectively. While the kinetics of the inner-sphere reaction is consistent spatially, heterogeneity is observed for the kinetics of HER, which is correlated with the crystal orientation of Pt.

Interfaces Decrease the Alkaline Hydrogen-Evolution Kinetics Energy Barrier on NiCoP/Ti3C2Tx MXene

Heterointerfaces can adjust the adsorption energy with intermediates in the transition state for a much decreased kinetics energy barrier (E_a). One typical transition metal phosphide, NiCoP grains (∼5 nm in size), was anchored on a Ti₃C₂T_x MXene monolayer (∼1 nm in thickness) to boost the kinetics toward alkaline hydrogen evolution reaction (HER). General electrochemical experiments at different temperatures give a small E_a of 31.4 kJ mol^-1, showing a 22.1% decrease compared to its counterpart NiCoP nanoparticles (40.3 kJ mol^-1). Impressively, the overpotential of NiCoP@MXene dramatically decreases from 71 mV to 4 mV at 10 mA cm^-2 when the temperature increases from 25 °C to 65 °C. On a single NiCoP@MXene sheet, scanning electrochemical microscopy (SECM) tests also give a very close value of E_a = 31.9 kJ mol^-1, with a relative error of ∼1.6%. Density functional theory (DFT) calculations confirm the interface between NiCoP and MXene can effectively decrease the energy barrier of water dissociation by 16.0%. The three kinds of studies on macro, micro/nano, and atomic scales disclose the interfaces can reduce the kinetics energy barrier about 16.0-22.1%. Besides, the photothermal effect of MXenes can easily raise the catalyst temperature under vis-NIR light, which has been applied in practical scenarios under sunlight for energy savings.

Direct measuring of single-heterogeneous bubble nucleation mediated by surface topology

Heterogeneous bubble nucleation is one of the most fundamental interfacial processes ranging from nature to technology. There is excellent evidence that surface topology is important in directing heterogeneous nucleation; however, deep understanding of the energetics by which nanoscale architectures promote nucleation is still challenging. Herein, we report a direct and quantitative measurement of single-bubble nucleation on a single silica nanoparticle within a microsized droplet using scanning electrochemical cell microscopy. Local gas concentration at nucleation is determined from finite element simulation at the corresponding faradaic current of the peak-featured voltammogram. It is demonstrated that the criteria gas concentration for nucleation first drops and then rises with increasing nanoparticle radius. An optimum nanoparticle radius around 10 nm prominently expedites the nucleation by facilitating the special topological nanoconfinements that consequently catalyze the nucleation. Moreover, the experimental result is corroborated by our theoretical calculations of free energy change based on the classic nucleation theory. This study offers insights into the impact of surface topology on heterogenous nucleation that have not been previously observed.

Single-Entity Electrochemistry of Nano- and Microbubbles in Electrolytic Gas Evolution

Gas bubbles are found in diverse electrochemical processes, ranging from electrolytic water splitting to chlor-alkali electrolysis, as well as photoelectrochemical processes. Understanding the intricate influence of bubble evolution on the electrode processes and mass transport is key to the rational design of efficient devices for electrolytic energy conversion and thus requires precise measurement and analysis of individual gas bubbles. In this Perspective, we review the latest advances in single-entity measurement of gas bubbles on electrodes, covering the approaches of voltammetric and galvanostatic studies based on nanoelectrodes, probing bubble evolution using scanning probe electrochemistry with spatial information, and monitoring the transient nature of nanobubble formation and dynamics with opto-electrochemical imaging. We emphasize the intrinsic and quantitative physicochemical interpretation of single gas bubbles from electrochemical data, highlighting the fundamental understanding of the heterogeneous nucleation, dynamic state of the three-phase boundary, and the correlation between electrolytic bubble dynamics and nanocatalyst activities. In addition, a brief discussion of future perspectives is presented.

Surface Engineering of Defective and Porous Ir Metallene with Polyallylamine for Hydrogen Evolution Electrocatalysis

The design of defects and porous structures into metallene with functional surfaces is highly desired to improve its permeability, surface area, and active sites, but remains a great challenge. In this work, polyallylamine-encapsulated Ir metallene with defects and porous structure (Ir@PAH metallene) is easily fabricated by a one-step wet chemical reduction method. The Ir@PAH metallene exhibits excellent hydrogen evolution reaction (HER) performance with an overpotential of only 14 mV at 10 mA cm^-2 , a low Tafel slope of 31.2 mV dec^-1 , and almost no activity decay after stability test. The abundant defects and pores as well as several-atomic-layer nanosheet structures of Ir@PAH metallene provide a large specific surface area, high conductivity, and efficient mass transport/diffusion. In addition, surface-functionalized PAH molecules can modulate the electronic structure through strong Ir-N interaction and act as proton carriers to capture hydrogen ions, which is very beneficial for the HER in acidic media. This work provides a useful strategy for the synthesis of the defective and porous metallene with functionalized surfaces for various catalytic applications.

Topography Mapping with Scanning Electrochemical Cell Microscopy

High-resolution scanning electrochemical cell microscopy (SECCM), synchronously visualizing the topography and electrochemical activity, could be used to directly correlate the structure and activity of materials nanoscopically. However, its topographical measurement is largely restricted by the size and stability of the meniscus droplet formed at the end of the nanopipette. In this paper, we report a scheme that could reliably gain several tens nanometer resolution (≥65 nm) of SECCM using homemade ∼50 nm inner diameter probes. Furthermore, the topography and hydrogen evolution reaction (HER) activity of ∼45 nm self-assembled Au nanoparticles monolayer were simultaneously recorded successfully. This scheme could make mapping of both topologic and chemical properties of samples in the nanometer regime with SECCM routinely, which potentially can largely expand the field of SECCM applications.

Heterogeneity between and within Single Hematite Nanorods as Electrocatalysts for Oxygen Evolution Reaction

Understanding the structural nature of the active sites in electrocatalysis is central to discovering general design rules for better catalysts in fuel cells and electrolyzers. Nanostructures are widely used as electrocatalysts, but the location and structure of the active sites within the nanostructure are often unknown. This information is hidden in conventional bulk measurements due to ensemble averaging, hindering direct structure-activity correlation. Herein, we use a single-entity electrochemical approach to reveal the heterogeneity in electrocatalysts via scanning electrochemical cell microscopy (SECCM). Using hematite (α-Fe₂O₃) nanorods as the model catalyst for oxygen evolution reaction (OER), the electrocatalytic activity is measured at individual nanorods. Finer mapping within a single nanorod shows that the OER activity is consistently higher at the body portion vs the tip of the nanorod. Our results directly suggest the benefit of synthesizing longer hematite nanorods for better OER performance. The origin of the enhanced local activity is explained by the larger fraction of {001} facet exposed on the body compared to the tip. The finding goes beyond OER on hematite nanorods, highlighting the critical role of single-entity activity mapping and colocalized structural characterization in revealing active sites in electrocatalysis.

Highly spatial imaging of electrochemical activity on the wrinkles of graphene using all-solid scanning electrochemical cell microscopy

Here, all-solid scanning electrochemical cell microscopy (SECCM) is first established by filling polyacrylamide (PAM) into nanocapillaries as a solid electrolyte. A solid PAM nanoball at the tip of a nanocapillary contacts graphene and behaves as an electrochemical cell for simultaneously measuring the morphology and electrochemical activity. Compared with liquid droplet-based SECCM, this solid nanoball is stable and does not leave any electrolyte at the contact regions, which permits accurate and continuous scanning of the surface without any intervals. Accordingly, the resolutions in the lateral (x-y) and vertical (z) directions are improved to ∼10 nm. The complete scanning of the wrinkles on graphene records low currents at the two sidewalls of the wrinkles and a relatively high current at the center of the wrinkles. The heterogeneity in the electrochemical activity of the wrinkle illustrates different electron transfer features on surfaces with varied curvatures, which is hardly observed by the current electrochemical or optical methods. The successful establishment of this high spatial electrochemical microscopy overcomes the current challenges in investigating the electrochemical activity of materials at the nanoscale, which is significant for a better understanding of electron transfer in materials.

Characterization of the Depth of Discharge-Dependent Charge Transfer Resistance of a Single LiFePO4 Particle

The discharged state affects the charge transfer resistance of lithium-ion secondary batteries (LIBs), which is referred to as the depth of discharge (DOD). To understand the intrinsic charge/discharge property of LIBs, the DOD-dependent charge transfer resistance at the solid-liquid interface is required. However, in a general composite electrode, the conductive additive and organic polymeric binder are unevenly distributed, resulting in a complicated electron conduction/ion conduction path. As a result, estimating the DOD-dependent rate-determining factor of LIBs is difficult. In contrast, in micro/nanoscale electrochemical measurements, the primary or secondary particle is evaluated without using a conductive additive and providing an ideal mass transport condition. To control the DOD state of a single LiFePO₄ active material and evaluate the DOD-dependent charge transfer kinetic parameters, we use scanning electrochemical cell microscopy (SECCM), which uses a micropipette to form an electrochemical cell on a sample surface. The difference in charge transfer resistance at the solid-liquid interface depending on the DOD state and electrolyte solution could be confirmed using SECCM.

Unravelling the last milliseconds of an individual graphene nanoplatelet before impact with a Pt surface by bipolar electrochemistry

Contactless interactions of micro/nano-particles near electrochemically or chemically active interfaces are ubiquitous in chemistry and biochemistry. Forces arising from a convective field, an electric field or chemical gradients act on different scales ranging from few microns down to few nanometers making their study difficult. Here, we correlated optical microscopy and electrochemical measurements to track at the millisecond timescale the dynamics of individual two-dimensional particles, graphene nanoplatelets (GNPs), when approaching an electrified Pt micro-interface. Our original approach takes advantage of the bipolar feedback current recorded when a conducting particle approaches an electrified surface without electrical contact and numerical simulations to access the velocity of individual GNPs. We evidenced a strong deceleration of GNPs from few tens of μm s-1 down to few μm s-1 within the last μm above the surface. This observation reveals the existence of strongly non-uniform forces between tens of and a thousand nanometers from the surface.

Large-Scale Synthesis of Functionalized Nanowires to Construct Nanoelectrodes for Intracellular Sensing

A strategy for one-pot and large-scale synthesis of functionalized core-shell nanowires (NWs) to high-efficiently construct single nanowire electrodes is proposed. Based on the polymerization reaction between 3,4-ethylenedioxythiophene (EDOT) and noble metal cations, manifold noble metal nanoparticles-polyEDOT (PEDOT) nanocomposites can be uniformly modified on the surface of any nonconductive NWs. This provides a facile and versatile approach to produce massive number of core-shell NWs with excellent conductivity, adjustable size, and well-designed properties. Nanoelectrodes manufactured with such core-shell NWs exhibit excellent electrochemical performance and mechanical stability as well as favorable antifouling properties, which are demonstrated by in situ intracellular monitoring of biological molecules (nitric oxide) and unraveling its relevant unclear signaling pathway inside single living cells.

Reversed Charge Transfer and Enhanced Hydrogen Spillover in Platinum Nanoclusters Anchored on Titanium Oxide with Rich Oxygen Vacancies Boost Hydrogen Evolution Reaction

The catalytic activity of metal clusters is closely related with the support; however, knowledge on the influence of the support on the catalytic activity is scarce. We demonstrate that Pt nanoclusters (NCs) anchored on porous TiO₂ nanosheets with rich oxygen vacancies (V_O -rich Pt/TiO₂ ) and deficient oxygen vacancies (V_O -deficient Pt/TiO₂ ), display significantly different catalytic activity for the hydrogen evolution reaction (HER), in which V_O -rich Pt/TiO₂ shows a mass activity of 45.28 A mg_Pt ^-1 at -0.1 V vs. RHE, which is 16.7 and 58.8 times higher than those of V_O -deficient Pt/TiO₂ and commercial Pt/C, respectively. DFT calculations and in situ Raman spectra suggest that porous TiO₂ with rich oxygen vacancies can simultaneously achieve reversed charge transfer (electrons transfer from TiO₂ to Pt NCs) and enhanced hydrogen spillover from Pt NCs to the TiO₂ support, which leads to electron-rich Pt NCs being amenable to proton reduction of absorbed H*, as well as the acceleration of hydrogen desorption at Pt catalytic sites-both promoting the HER. Our work provides a new strategy for rational design of highly efficient HER catalysts.