Long-Term Stability Challenges and Opportunities in Acidic Oxygen Evolution Electrocatalysis.

Polymer electrolyte membrane water electrolysis (PEMWE) has been regarded as a promising technology for renewable hydrogen production. However, acidic oxygen evolution reaction (OER) catalysts with long-term stability impose a grand challenge in its large-scale industrialization. In this review, critical factors that may lead to catalyst's instability in couple with potential solutions are comprehensively discussed, including mechanical peeling, substrate corrosion, active-site over-oxidation/dissolution, reconstruction, oxide crystal structure collapse through the lattice oxygen-participated reaction pathway, etc. Last but not least, personal prospects are provided in terms of rigorous stability evaluation criteria, in situ/operando characterizations, economic feasibility and practical electrolyzer consideration, highlighting the ternary relationship of structure evolution, industrial-relevant activity and stability to serve as a roadmap towards the ultimate application of PEMWE.

In Situ Precise Tuning of Bimetallic Electronic Effect for Boosting Oxygen Reduction Catalysis.

Tuning intermediate adsorption energy by shifting the d-band center offers a powerful strategy to tailor the reactivity of metal catalysts. Here we report a potential sweep method to grow Pd layer-by-layer on Au with the capability to in situ measure the surface structure through an ethanol oxidation reaction. Spectroscopic characterizations reveal charge-transfer induced valence band restructuring in the Pd overlayer, which shifts the d-band center away from the Fermi level compared to bulk Pd. Precise overlayer control gives the optimal bimetallic surface of two monolayers (ML) Pd on Au, which exhibits more than 370-fold mass activity enhancement in oxygen reduction reaction (at 0.9 V vs. reversible hydrogen electrode) and 40 mV increase in half-wave potential compared to the Pt/C. Tested in a homemade Zn-air battery, the 2-ML-Pd/Au/C exhibits a maximum power density of 296 mW/cm2 and specific activity of 804 mAh/gZn, much higher than Pt/C with the same catalyst loading amount.

Amorphous versus Crystalline in Water Oxidation Catalysis: A Case Study of NiFe Alloy.

Catalytic water splitting driven by renewable electricity offers a promising strategy to produce molecular hydrogen, but its efficiency is severely restricted by the sluggish kinetics of the anodic water oxidation reaction. Amorphous catalysts are reported to show better activities of water oxidation than their crystalline counterparts, but little is known about the underlying origin, which retards the development of high-performance amorphous oxygen evolution reaction catalysts. Herein, on the basis of cyclic voltammetry, electrochemical impedance spectroscopy, isotope labeling, and in situ X-ray absorption spectroscopy studies, we demonstrate that an amorphous catalyst can be electrochemically activated to expose active sites in the bulk thanks to the short-range order of the amorphous structure, which greatly increases the number of active sites and thus improves the electrocatalytic activity of the amorphous catalyst in water oxidation.

Revealing Energetics of Surface Oxygen Redox from Kinetic Fingerprint in Oxygen Electrocatalysis.

The key step for rational catalyst design in heterogeneous electrocatalysis is to reveal the distinctive energy profile of redox reactions of a catalyst that give rise to specific activity. However, it is challenging to experimentally obtain the energetics of oxygen redox in oxygen electrocatalysis because of the liquid reaction environment. Here we develop a kinetic model that constructs a quantitative relation between the energy profile of oxygen redox and electrochemical kinetic fingerprints. The detailed study here demonstrates that the kinetic fingerprints observed from experiments can be well described by different energetics of oxygen redox. On the basis of the model, a feasible methodology is demonstrated to derive binding energies of the oxygen intermediates from electrochemical data. The surface property of different catalysts derived from our model well rationalizes the experimental trends and predicts potential directions for catalyst design.

Nanostructuring Confinement for Controllable Interfacial Charge Transfer.

Carbon nanostructures supported semiconductors are common in photocatalytic and photoelectrochemical applications, as it is expected that the nanoconductors can improve the spatial separation and transport of photogenerated charge carriers. Transfer of charge carriers through the carbon-semiconductor interface is the key electronic process, which determines the role of charge separation channels, and is sensitively influenced by band structures of the semiconductor near the contacts. Usually, this electronic process suffers from excessive energy dissipation by thermionic emission, which will undesirably prevent the interfacial charge transfer and eventually aggravate the recombination of photogenerated charge carriers. Unfortunately, this critical issue has hardly been consciously considered. Here, ultrathin dopant-free tunneling interlayers coated on the surface of graphene and sandwiched between the carbon sheets and the semiconductor nanostructures are adopted as a model system to demonstrate energy saving for the interfacial charge transfer. The nanostructuring confinement of band bending within the ultrathin interlayers in contact with the graphene sheets effectively narrows the width of the potential barriers, which enables tunneling of a substantial number of photogenerated electrons to the co-catalysts without unduly consuming energy. Besides, the dopant-free tunneling interlayers simultaneously block the transferred electrons in the sandwiched graphene sheets from leakage.

Identification of Surface Reactivity Descriptor for Transition Metal Oxides in Oxygen Evolution Reaction.

A number of important reactions such as the oxygen evolution reaction (OER) are catalyzed by transition metal oxides (TMOs), the surface reactivity of which is rather elusive. Therefore, rationally tailoring adsorption energy of intermediates on TMOs to achieve desirable catalytic performance still remains a great challenge. Here we show the identification of a general and tunable surface structure, coordinatively unsaturated metal cation (MCUS), as a good surface reactivity descriptor for TMOs in OER. Surface reactivity of a given TMO increases monotonically with the density of MCUS, and thus the increase in MCUS improves the catalytic activity for weak-binding TMOs but impairs that for strong-binding ones. The electronic origin of the surface reactivity can be well explained by a new model proposed in this work, wherein the energy of the highest-occupied d-states relative to the Fermi level determines the intermediates' bonding strength by affecting the filling of the antibonding states. Our model for the first time well describes the reactivity trends among TMOs, and would initiate viable design principles for, but not limited to, OER catalysts.

Tunneling Interlayer for Efficient Transport of Charges in Metal Oxide Electrodes.

Due to the limited electronic conductivity, the application of many metal oxides that may have attractive (photo)-electrochemical properties has been limited. Regarding these issues, incorporating low-dimensional conducting scaffolds into the electrodes or supporting the metal oxides onto the conducting networks are common approaches. However, some key electronic processes like interfacial charge transfer are far from being consciously concerned. Here we use a carbon-TiO2 contact as a model system to demonstrate the electronic processes occurring at the metal-semiconductor interface. To minimize the energy dissipation for fast transfer of electrons from semiconductor to carbon scaffolds, facilitating electron tunneling while avoiding high energy-consuming thermionic emission is desired, according to our theoretical simulation of the voltammetric behaviors. To validate this, we manage to sandwich ultrathin TiO2 interlayers with heavy electronic doping between the carbon conductors and dopant-free TiO2. The radially graded distribution of the electronic doping along the cross-sectional direction of carbon conductor realized by immobilizing the dopant species on the carbon surface can minimize the energy consumption for contacts to both the carbon and the dopant-free TiO2. Our strategy provides an important requirement for metal oxide electrode design.

One-dimensional hybrid nanostructures for heterogeneous photocatalysis and photoelectrocatalysis.

Semiconductor-based photocatalysis and photoelectrocatalysis have received considerable attention as alternative approaches for solar energy harvesting and storage. The photocatalytic or photoelectrocatalytic performance of a semiconductor is closely related to the design of the semiconductor at the nanoscale. Among various nanostructures, one-dimensional (1D) nanostructured photocatalysts and photoelectrodes have attracted increasing interest owing to their unique optical, structural, and electronic advantages. In this article, a comprehensive review of the current research efforts towards the development of 1D semiconductor nanomaterials for heterogeneous photocatalysis and photoelectrocatalysis is provided and, in particular, a discussion of how to overcome the challenges for achieving full potential of 1D nanostructures is presented. It is anticipated that this review will afford enriched information on the rational exploration of the structural and electronic properties of 1D semiconductor nanostructures for achieving more efficient 1D nanostructure-based photocatalysts and photoelectrodes for high-efficiency solar energy conversion.

Efficient and Stable Proton Exchange Membrane Water Electrolysis Enabled by Stress Optimization.

Proton exchange membrane water electrolysis (PEMWE) is a promising solution for the conversion and storage of fluctuating renewable energy sources. Although tremendously efficient materials have been developed, commercial PEMWE products still cannot fulfill industrial demands regarding efficiency and stability. In this work, we demonstrate that the stress distribution, a purely mechanical parameter in electrolyzer assembly, plays a critical role in overall efficiency and stability. The conventional cell structure, which usually adopts a serpentine flow channel (S-FC) to deliver and distribute reactants and products, resulted in highly uneven stress distribution. Consequently, the anode catalyst layer (ACL) under the high stress region was severely deformed, whereas the low stress region was not as active due to poor electrical contact. To address these issues, we proposed a Ti mesh flow channel (TM-FC) with gradient pores to reduce the stress inhomogeneity. Consequently, the ACL with TM-FC exhibited 27 mV lower voltage initially and an 8-fold reduction in voltage degradation rate compared to that with S-FC at 2.0 A/cm2. Additionally, the applicability of the TM-FC was demonstrated in cross-scale electrolyzers up to 100 kW, showing a voltage increase of only 20 mV (accounting for less than 2% of overall voltage) after three orders of magnitude scaleup.

Optimizing Ionomer Distribution in Anode Catalyst Layer for Stable Proton Exchange Membrane Water Electrolysis.

The high cost of proton exchange membrane water electrolysis (PEMWE) originates from the usage of precious materials, insufficient efficiency, and lifetime. In this work, an important degradation mechanism of PEMWE caused by dynamics of ionomers over time in anode catalyst layer (ACL), which is a purely mechanical degradation of microstructure, is identified. Contrary to conventional understanding that the microstructure of ACL is static, the micropores are inclined to be occupied by ionomers due to the localized swelling/creep/migration, especially near the ACL/PTL (porous transport layer) interface, where they form transport channels of reactant/product couples. Consequently, the ACL with increased ionomers at PTL/ACL interface exhibit rapid and continuous degradation. In addition, a close correlation between the microstructure of ACL and the catalyst ink is discovered. Specifically, if more ionomers migrate to the top layer of the ink, more ionomers accumulate at the ACL/PEM interface, leaving fewer ionomers at the ACL/PTL interface. Therefore, the ionomer distribution in ACL is successfully optimized, which exhibits reduced ionomers at the ACL/PTL interface and enriches ionomers at the ACL/PEM interface, reducing the decay rate by a factor of three when operated at 2.0 A cm-2 and 80 °C. The findings provide a general way to achieve low-cost hydrogen production.

Constructing Robust 3D Ionomer Networks in the Catalyst Layer to Achieve Stable Water Electrolysis for Green Hydrogen Production.

The widespread application of proton exchange membrane water electrolyzers (PEMWEs) is hampered by insufficient lifetime caused by degradation of the anode catalyst layer (ACL). Here, an important degradation mechanism has been identified, attributed to poor mechanical stability causing the mass transfer channels to be blocked by ionomers under operating conditions. By using liquid-phase atomic force microscopy, we directly observed that the ionomers were randomly distributed (RD) in the ACL, which occupied the mass transfer channels due to swelling, creeping, and migration properties. Interestingly, we found that alternating treatments of the ACL in different water/temperature environments resulted in forming three-dimensional ionomer networks (3D INs) in the ACL, which increased the mechanical strength of microstructures by 3 times. Benefitting from the efficient and stable mass transfer channels, the lifetime was improved by 19 times. A low degradation rate of approximately 3.0 µV/h at 80 °C and a high current density of 2.0 A/cm2 was achieved on a 50 cm2 electrolyzer. These data demonstrated a forecasted lifetime of 80 000 h, approaching the 2026 DOE lifetime target. This work emphasizes the importance of the mechanical stability of the ACL and offers a general strategy for designing and developing a durable PEMWE.

A Solid Electrolyte RHE for Electrode Diagnosis of Proton Exchange Membrane Water Electrolyzers.

Reference electrode is the foundation of electrochemical study; thus, most electrode materials are tested in a three-electrode mode to acquire potential-dependent kinetics. However, it is difficult to directly use conventional reference electrodes to detect potential information in solid electrolyte devices due to their compact assembly structure. Therefore, the kinetic study of an electrochemical device faces challenges in precise identification of specific problems originating from the anode or cathode. Here, focusing on proton exchange membrane water electrolysis, we design a solid electrolyte reversible hydrogen electrode (SE-RHE), which can be used for electrode diagnosis under various operating conditions. Compared to the reference electrodes reported in the literature, which are mainly based on liquid electrolyte, the SE-RHE is highly sensitive and compatible, as well as easy to assemble. The potential deviation is less than ±0.5 mV, and the cell voltage derived from the electrode potential well reproduces the value that was directly measured with a deviation less than 0.2%. The reference electrode developed in this work enables the kinetic study of a specific electrode rather than the entire cell. For instance, an interesting observation is that the cathode shows distinct stability under stable and fluctuating operations. Differing from the high stability under stable operation, the cathode degrades significantly under fluctuating operations.

Kinetic Insights of Proton Exchange Membrane Water Electrolyzer Obtained by Operando Characterization Methods.

Benefiting from the merits of short response time, high current density, and differential pressure, the proton exchange membrane water electrolyzer (PEMWE) is attracting increasing attention from both academic and industry researchers. A limiting factor that impedes the widespread application of the PEMWE is its reliance on the rarest elements, such as iridium and platinum. In order to optimize the device performance as well as to reduce the usage of rare elements, it is important but difficult to directly observe the reaction within the electrolyzer under working conditions. Thus, operando characterization methods are urgently needed to probe in real time the water electrolysis process during operation. In this perspective, we highlight the important role and summarize the recent advances of operando characterization methods in obtaining kinetic insights about PEMWEs. Based on the demands of kinetic optimization, an outlook of future characterization methods is given at the end.

Noble metal nanowire arrays as an ethanol oxidation electrocatalyst.

Vertically aligned noble metal nanowire arrays were grown on conductive electrodes based on a solution growth method. They show significant improvement of electrocatalytic activity in ethanol oxidation, from a re-deposited sample of the same detached nanowires. The unusual morphology provides open diffusion channels and direct charge transport pathways, in addition to the high electrochemically active surface from the ultrathin nanowires. Our best nanowire arrays exhibited much enhanced electrocatalytic activity, achieving a 38.0 fold increase in specific activity over that of commercial catalysts for ethanol electrooxidation. The structural design provides a new direction to enhance the electrocatalytic activity and reduce the size of electrodes for miniaturization of portable electrochemical devices.

Ordered clustering of single atomic Te vacancies in atomically thin PtTe2 promotes hydrogen evolution catalysis.

Exposing and stabilizing undercoordinated platinum (Pt) sites and therefore optimizing their adsorption to reactive intermediates offers a desirable strategy to develop highly efficient Pt-based electrocatalysts. However, preparation of atomically controllable Pt-based model catalysts to understand the correlation between electronic structure, adsorption energy, and catalytic properties of atomic Pt sites is still challenging. Herein we report the atomically thin two-dimensional PtTe2 nanosheets with well-dispersed single atomic Te vacancies (Te-SAVs) and atomically well-defined undercoordinated Pt sites as a model electrocatalyst. A controlled thermal treatment drives the migration of the Te-SAVs to form thermodynamically stabilized, ordered Te-SAV clusters, which decreases both the density of states of undercoordinated Pt sites around the Fermi level and the interacting orbital volume of Pt sites. As a result, the binding strength of atomically defined Pt active sites to H intermediates is effectively reduced, which renders PtTe2 nanosheets highly active and stable in hydrogen evolution reaction.

An essential descriptor for the oxygen evolution reaction on reducible metal oxide surfaces.

The development of a universal activity descriptor like the d-band model for transition metal catalysts is of great importance to catalyst design. However, due to the complicated electronic structures of metal oxides, the correlation of the binding energies of reaction intermediates (*OH, *O, and *OOH) in the oxygen evolution reaction (OER) with experimentally controllable properties of metal oxides has not been well established. Here we demonstrate that excess electrons are the essential factor that governs the binding properties of intermediates on the surfaces of reducible metal oxides. We propose that the number of excess electrons (NEE) is an essential activity descriptor toward the OER activities of these oxides, which perfectly reproduces the volcano curve plotted using the descriptor ΔG O - ΔG OH, so that tuning NEE can effectively tailor the OER activities of reducible metal oxide based catalysts. Guided by this descriptor, we predict a novel non-precious catalyst with an overpotential of 0.54 eV, which could be a potential alternative to current Ru or Ir based catalysts.

Identification of catalytic sites for oxygen reduction and oxygen evolution in N-doped graphene materials: Development of highly efficient metal-free bifunctional electrocatalyst.

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are critical to renewable energy conversion and storage technologies. Heteroatom-doped carbon nanomaterials have been reported to be efficient metal-free electrocatalysts for ORR in fuel cells for energy conversion, as well as ORR and OER in metal-air batteries for energy storage. We reported that metal-free three-dimensional (3D) graphene nanoribbon networks (N-GRW) doped with nitrogen exhibited superb bifunctional electrocatalytic activities for both ORR and OER, with an excellent stability in alkaline electrolytes (for example, KOH). For the first time, it was experimentally demonstrated that the electron-donating quaternary N sites were responsible for ORR, whereas the electron-withdrawing pyridinic N moieties in N-GRW served as active sites for OER. The unique 3D nanoarchitecture provided a high density of the ORR and OER active sites and facilitated the electrolyte and electron transports. As a result, the as-prepared N-GRW holds great potential as a low-cost, highly efficient air cathode in rechargeable metal-air batteries. Rechargeable zinc-air batteries with the N-GRW air electrode in a two-electrode configuration exhibited an open-circuit voltage of 1.46 V, a specific capacity of 873 mAh g(-1), and a peak power density of 65 mW cm(-2), which could be continuously charged and discharged with an excellent cycling stability. Our work should open up new avenues for the development of various carbon-based metal-free bifunctional electrocatalysts of practical significance.

Biomolecule-assisted synthesis of carbon nitride and sulfur-doped carbon nitride heterojunction nanosheets: An efficient heterojunction photocatalyst for photoelectrochemical applications.

A biomolecule-assisted pyrolysis method has been developed to synthesize sulfur-doped graphitic carbon nitride (CNS) nanosheets. During the synthesis, sulfur could be introduced as a dopant into the lattice of carbon nitride (CN). Sulfur doping changed the texture as well as relative band positions of CN. By growing CN on preformed sulfur-doped CN nanosheets, composite CN/CNS heterojunction nanosheets were constructed, which significantly enhanced the photoelectrochemical performance as compared with various control counterparts including CN, CNS and physically mixed CN and CNS (CN+CNS). The enhanced photoelectrochemical performance of CN/CNS heterojunction nanosheets could be ascribed to the efficient separation of photoexcited charge carriers across the heterojunction interface. The strategy of designing and preparing CN/CNS heterojunction photocatalysts in this work can open up new directions for the construction of all CN-based heterojunction photocatalysts.

Spatially branched hierarchical ZnO nanorod-TiO2 nanotube array heterostructures for versatile photocatalytic and photoelectrocatalytic applications: towards intimate integration of 1D-1D hybrid nanostructures.

Hierarchically ordered ZnO nanorods (NRs) decorated nanoporous-layer-covered TiO2 nanotube array (ZnO NRs/NP-TNTAs) nanocomposites have been prepared by an efficient, two-step anodization route combined with an electrochemical deposition strategy, by which monodispersed one-dimensional (1D) ZnO NRs were uniformly grown on the framework of NP-TNTAs. The crystal phases, morphologies, optical properties, photocatalytic as well as photoelectrocatalytic performances of the well-defined ZnO NRs/NP-TNTAs heterostructures were systematically explored to clarify the structure-property correlation. It was found that the ZnO NRs/NP-TNTAs heterostructure exhibits significantly enhanced photocatalytic and photoelectrocatalytic performances, along with favorable photostability toward degradation of organic pollutants under UV light irradiation, as compared to the single component counterparts. The remarkably enhanced photoactivity of ZnO NRs/NP-TNTAs heterostructure is ascribed to the intimate interfacial integration between ZnO NRs and NP-TNTAs substrate imparted by the unique spatially branched hierarchical structure, thereby contributing to the efficient transfer and separation of photogenerated electron-hole charge carriers. Moreover, the specific active species during the photocatalytic process was unambiguously determined and photocatalytic mechanism was tentatively presented. It is anticipated that our work could provide new insights for the construction of various hierarchical 1D-1D hybrid nanocomposites for extensive photocatalytic applications.