Surface‐Guided Formation of Amorphous Mixed‐Metal Oxyhydroxides on Ultrathin MnO2 Nanosheet Arrays for Efficient Electrocatalytic Oxygen Evolution

Earth‐abundant amorphous nanomaterials with rich structural defects are promising alternative catalysts to noble metals for an efficient electrochemical oxygen evolution reaction; however, their inferior electrical conductivity and poor morphological control during synthesis hamper the full realization of their potency in electrocatalysis. Herein, a rapid surface‐guided synthetic approach is proposed to introduce amorphous and mixed‐metal oxyhydroxide overlayers on ultrathin Ni‐doped MnO₂ (Ni? MnO₂) nanosheet arrays via a galvanic replacement mechanism. This method results in a monolithic 3D porous catalyst with a small overpotential of only 232 mV to achieve a current density of 10 mA cm^?2 in 1 m KOH, which is much lower than the corresponding value of 307 mV for the Ni? MnO₂ reference sample. Detailed structural and electrochemical characterization reveal that the unique Ni? MnO₂ ultrathin nanosheet arrays do not only provide a large surface area to guide the formation of active amorphous catalyst layers but also ensure the effective charge transport owing to their high electron conductivity, collectively contributing to the greatly improved catalyst activity. It is envisioned that this highly operable surface‐guide synthetic strategy may open up new avenues for the design and fabrication of novel 3D nanoarchitectures integrated with functional amorphous materials for broadened ranges of applications.     

3D Holey‐Graphene Architecture Expedites Ion Transport Kinetics to Push the OER Performance

The kinetics process of heterogeneous catalysis involves several steps including adsorption, diffusion, and surface chemical reactions. Current studies generally aim at increasing active site amount and improving intrinsic activity. However, the ion diffusion kinetics at the electrode/electrolyte interface as a bottleneck has been rarely directly addressed. Here, a 3D holey‐graphene framework is demonstrated as a catalyst‐loading platform, with nanoscale holes that can be elaborately tuned via facile aqueous‐phase chemical etching. This enables the ions to be efficiently transported to deeply buried active sites to mitigate their insufficient supply. With systematical electrochemical investigations tuned by varied pore structures, a series of models from a simplified equivalent circuit to complicate realistic one are proposed to figure out the modulation rules of weakened electrochemical diffusion domination and identify the ion transport resistance as well. Moreover, given the inevitable negative effect on the conductivity of graphene skeleton by introducing nanoscale holes, the balance between the outside ion transport and the inside charge transport of electrode is highlighted. Such a protocol represents a synergistic modulation of catalytic performance from both the supply side (reactive ion transport) and the consuming side (active site), and provides striking information for the precise design of catalyst electrodes toward further pushing the oxygen evolution reaction performance limit.     

Molecular assembly of redox-conductive ferrocene-streptavidin conjugates--towards bio-electrochemical devices

Peptidic spacers, 0.4 and 2 nm in length, were used to couple ferrocene moieties to streptavidin. The resulting conjugates were immobilised on electrode surfaces using biotin binding. The electron transfer through multilayers of the conjugates is strongly dependent on the length of the spacer between the protein and the attached ferrocene. A monolayer of the long-linker conjugate immobilised on interdigitated microelectrode arrays was found to electrochemically bridge the 2 microm wide non-conductive gap between the electrodes. The redox current through the layer is dependent on external parameters such as the applied voltage difference between the two electrode arrays or the temperature. The long-range electrochemical conductivity in combination with the biotin binding capability is a prerequisite for the application of the conjugates in future bio-electrochemical devices.     

Carbon Nitride/Reduced Graphene Oxide Film with Enhanced Electron Diffusion Length: An Efficient Photo‐Electrochemical Cell for Hydrogen Generation

Polymeric carbon nitride (CN) has emerged as a promising semiconductor for energy‐related applications. However, its utilization in photo‐electrochemical cells is still very limited owing to poor electron–hole separation efficiency, short electron diffusion length, and low absorption coefficient. Here the synthesis of a highly porous carbon nitride/reduced graphene oxide (CN‐rGO) film with good photo‐electrochemical properties is reported. The CN‐rGO film exhibits long electron diffusion length and high electrochemical active surface area, good charge separation, and enhanced light‐harvesting properties. The film displays a 20‐fold enhancement of photocurrent density over pristine CN, reaching up to 75 µA cm^?2 at 1.23 V versus reversible hydrogen electrode (RHE) in an alkaline solution, as well as stability over a wide pH range. Photocurrent measurements with a hole scavenger reveal a photocurrent density of 660 µA cm^?2 at 1.23 V versus RHE and a quantum efficiency of 60% at 400 nm, resulting in the production of 0.8 mol h^?1 g^?1 of hydrogen. The substantial photo‐electrochemical activity enhancement and hydrogen production together with the low price, high electrochemical surface area, long electron diffusion length, stability under harsh condition, and tunable photophysical properties of CN materials open many possibilities for their utilization in (photo)electrochemical and electronic devices.     

Concurrently Approaching Volumetric and Specific Capacity Limits of Lithium Battery Cathodes via Conformal Pickering Emulsion Graphene Coatings

To achieve the high energy densities demanded by emerging technologies, lithium battery electrodes need to approach the volumetric and specific capacity limits of their electrochemically active constituents, which requires minimization of the inactive components of the electrode. However, a reduction in the percentage of inactive conductive additives limits charge transport within the battery electrode, which results in compromised electrochemical performance. Here, an electrode design that achieves efficient electron and lithium‐ion transport kinetics at exceptionally low conductive additive levels and industrially relevant active material areal loadings is introduced. Using a scalable Pickering emulsion approach, Ni‐rich LiNi_0.8Co_0.15Al_0.05O₂ (NCA) cathode powders are conformally coated using only 0.5 wt% of solution‐processed graphene, resulting in an electrical conductivity that is comparable to 5 wt% carbon black. Moreover, the conformal graphene coating mitigates degradation at the cathode surface, thus providing improved electrochemical cycle life. The morphology of the electrodes also facilitates rapid lithium‐ion transport kinetics, which provides superlative rate capability. Overall, this electrode design concurrently approaches theoretical volumetric and specific capacity limits without tradeoffs in cycle life, rate capability, or active material areal loading.     

Integration of FeOOH and Zeolitic Imidazolate Framework‐Derived Nanoporous Carbon as an Efficient Electrocatalyst for Water Oxidation

As a cost‐effective catalyst for the oxygen evolution reaction (OER), the potential use of FeOOH is hindered by its intrinsic poor electron conductivity. Here, the significant enhancement of OER activity and long‐term stability of electrodeposited FeOOH on zeolitic imidazolate framework‐derived N‐doped porous carbons (NPCs) are reported. In alkaline media, FeOOH/NPC supported on nickel foam as a 3D electrode delivers a current density of 100 mA cm^?2 at a small overpotential of 230 mV and exhibits a low Tafel slope of 33.8 mV dec^?1 as well as excellent durability, making it one of the most active OER catalysts. Such high performance is attributed to a combined effect of the excellent electron conductivity of NPC and the synergy between FeOOH and NiO derived from Ni substrate.     

Exploiting the Condensation Reactions of Acetophenone to Engineer Carbon‐Encapsulated Nb2O5 Nanocrystals for High‐Performance Li and Na Energy Storage Systems

Efficient synthetic methods to produce high‐performance electrode‐active materials are crucial for developing energy storage devices for large‐scale applications, such as hybrid supercapacitors (HSCs). Here, an effective approach to obtain controllable carbon‐encapsulated T‐Nb₂O₅ nanocrystals (NCs) is presented, based on the solvothermal treatment of NbCl₅ in acetophenone. Two separate condensation reactions of acetophenone generate an intimate and homogeneous mixture of Nb₂O₅ particles and 1,3,5‐triphenylbenzene (TPB), which acts as a unique carbon precursor. The electrochemical performance of the resulting composites as anode electrode materials can be tuned by varying the Nb₂O₅/TPB ratio. Remarkable performances are achieved for Li‐ion and Na‐ion energy storage systems at high charge–discharge rates (specific capacities of ≈90 mAh g^?1 at 100 C rate for lithium and ≈125 mAh g^?1 at 20 C for sodium). High energy and power densities are also achieved with Li‐ and Na‐ion HSC devices constructed by using the Nb₂O₅/C composites as anode and activated carbon (YPF‐50) as cathode, demonstrating the excellent electrochemical properties of the materials synthesized with this approach.     

A Conductive Molecular Framework Derived Li2S/N,P‐Codoped Carbon Cathode for Advanced Lithium–Sulfur Batteries

Li₂S is one of the most promising cathode materials for Li‐ion batteries because of its high theoretical capacity and compatibility with Li‐metal‐free anode materials. However, the poor conductivity and electrochemical reactivity lead to low initial capacity and severe capacity decay. In this communication, a nitrogen and phosphorus codoped carbon (N,P–C) framework derived from phytic acid doped polyaniline hydrogel is designed to support Li₂S nanoparticles as a binder‐free cathode for Li–S battery. The porous 3D architecture of N and P codoped carbon provides continuous electron pathways and hierarchically porous channels for Li ion transport. Phosphorus doping can also suppress the shuttle effect through strong interaction between sulfur and the carbon framework, resulting in high Coulombic efficiency. Meanwhile, P doping in the carbon framework plays an important role in improving the reaction kinetics, as it may help catalyze the redox reactions of sulfur species to reduce electrochemical polarization, and enhance the ionic conductivity of Li₂S. As a result, the Li₂S/N,P–C composite electrode delivers a stable capacity of 700 mA h g^?1 with average Coulombic efficiency of 99.4% over 100 cycles at 0.1C and an areal capacity as high as 2 mA h cm^?2 at 0.5C.     

Nanoconfined Carbon‐Coated Na3V2(PO4)3 Particles in Mesoporous Carbon Enabling Ultralong Cycle Life for Sodium‐Ion Batteries

Na₃V₂(PO₄)₃ (denoted as NVP) has been considered as a promising cathode material for room temperature sodium ion batteries. Nevertheless, NVP suffers from poor rate capability resulting from the low electronic conductivity. Here, the feasibility to approach high rate capability by designing carbon‐coated NVP nanoparticles confined into highly ordered mesoporous carbon CMK‐3 matrix (NVP@C@CMK‐3) is reported. The NVP@C@CMK‐3 is prepared by a simple nanocasting technique. The electrode exhibits superior rate capability and ultralong cyclability (78 mA h g^?1 at 5 C after 2000 cycles) compared to carbon‐coated NVP and pure NVP cathode. The improved electrochemical performance is attributed to double carbon coating design that combines a variety of advantages: very short diffusion length of Na⁺/e^? in NVP, easy access of electrolyte, and short transport path of Na⁺ through carbon toward the NVP nanoparticle, high conductivity transport of electrons through the 3D interconnected channels of carbon host. The optimum design of the core–shell nanostructures with double carbon coating permits fast kinetics for both transported Na⁺ ions and electrons, enabling high‐power performance.     

Holey 2D Nanomaterials for Electrochemical Energy Storage

2D nanomaterials provide numerous fascinating properties, such as abundant active surfaces and open ion diffusion channels, which enable fast transport and storage of lithium ions and beyond. However, decreased active surfaces, prolonged ion transport pathway, and sluggish ion transport kinetics caused by self‐restacking of 2D nanomaterials during electrode assembly remain a major challenge to build high‐performance energy storage devices with simultaneously maximized energy and power density as well as long cycle life. To address the above challenge, porosity (or hole) engineering in 2D nanomaterials has become a promising strategy to enable porous 2D nanomaterials with synergetic features combining both 2D nanomaterials and porous architectures. Herein, recent important progress on porous/holey 2D nanomaterials for electrochemical energy storage is reviewed, starting with the introduction of synthetic strategies of porous/holey 2D nanomaterials, followed by critical discussion of design rule and their advantageous features. Thereafter, representative work on porous/holey 2D nanomaterials for electrochemical capacitors, lithium‐ion and sodium‐ion batteries, and other emerging battery technologies (lithium‐sulfur and metal‐air batteries) are presented. The article concludes with perspectives on the future directions for porous/holey 2D nanomaterial in energy storage and conversion applications.     

Carbon‐Tailored Semimetal MoP as an Efficient Hydrogen Evolution Electrocatalyst in Both Alkaline and Acid Media

The electrolysis processes such as hydrogen evolution reaction (HER) require high efficient catalysts with robust surface stability. A high conductivity is also necessary to speed up the charge transport between the catalyst and the electrolyte. Recently, the observation of exceedingly high conductivity in the topological semimetal MoP, has provided a model catalyst to investigate the correlation between the electrical transport and the electrocatalytic activity for the HER. Thus, MoP is encapsulated in a Mo, P codoped carbon layer (MoP@C). This composite material exhibits outstanding HER performance, with an extremely low overpotential of 49 mV at a current density of 10 mA cm^?2 and a Tafel slope of 54 mV dec^?1 in an alkaline medium. In addition, electron transport analysis indicates that MoP exhibits high conductivity and mobility due to the existence of triple‐point fermions and a complex Fermi surface. Furthermore, the presence of P? C and Mo? C bonds at the interface between the carbon layer and the MoP particles modulates the band structure of MoP@C and facilitates fast electron transfer, accumulation, and subsequent delocalization, which are in turn responsible for the excellent HER activity.     

In Situ “Chainmail Catalyst” Assembly in Low‐Tortuosity,Hierarchical Carbon Frameworks for Efficient and Stable Hydrogen Generation

The chainmail catalysts (transition metals or metal alloys encapsulated in carbon) are regarded as stable and efficient electrocatalysts for hydrogen generation. However, the fabrication of chainmail catalysts usually involves complex chemical vapor deposition (CVD) or prolonged calcination in a furnace, and the slurry‐based electrode assembly of the chainmail catalysts often suffers from inferior mass transfer and an underutilized active surface. In this work, a freestanding wood‐based open carbon framework is designed embedded with nitrogen (N) doped, few‐graphene‐layer‐encapsulated nickel iron (NiFe) alloy nanoparticles (N‐C‐NiFe). 3D wood‐derived carbon framework with numerous open and low‐tortuosity lumens, which are decorated with carbon nanotubes (CNTs) “villi”, can facilitate electrolyte permeation and hydrogen gas removal. The chainmail catalysts of the N‐C‐NiFe are uniformly in situ assembled on the CNT “villi” using a rapid heat shock treatment. The high heating and quenching rates of the heat shock method lead to formation of the well‐dispersed ultrafine nanoparticles. The self‐supported wood‐based carbon framework decorated with the chainmail catalyst displays high electrocatalytic activity and superior cycling durability for hydrogen evolution. The unique heat shock method offers a promising strategy to rapidly synthesize well‐dispersed binary and polynary metallic nanoparticles in porous matrices for high‐efficiency electrochemical energy storage and conversion.     

In Situ Transmission Electron Microscopy on Energy‐Related Catalysis

Energy‐related catalytic reactions have been extensively investigated in past years in order to efficiently utilize clean and environmentally benign energy sources. In these systems, the catalysts and the catalyst/active medium interfaces play a crucial role in determining their performance. Thus, understanding the physical and chemical properties of catalysts during reactions is of importance to provide fundamental insights for designing the devices. Transmission electron microscopy can provide tremendous information regarding materials' morphology, microstructure, and chemical properties at nanoscales. With in situ electron microscopy, dynamic processes of catalytic reactions in both gas and liquid environments have been investigated in real time. In this paper, the recent research progress of in situ and operando electron microscopy techniques are introduced with the representative works in energy‐related reactions, including electrochemical catalysis in liquid media and heterogeneous catalysis in gas media. The uniqueness of the specific electron microscopy methods and scientific merits of each work are highlighted. Finally, outlooks on emerging research opportunities are discussed.     

Highly Active Trimetallic NiFeCr Layered Double Hydroxide Electrocatalysts for Oxygen Evolution Reaction

The development of efficient and robust earth‐abundant electrocatalysts for the oxygen evolution reaction (OER) is an ongoing challenge. Here, a novel and stable trimetallic NiFeCr layered double hydroxide (LDH) electrocatalyst for improving OER kinetics is rationally designed and synthesized. Electrochemical testing of a series of trimetallic NiFeCr LDH materials at similar catalyst loading and electrochemical surface area shows that the molar ratio Ni:Fe:Cr = 6:2:1 exhibits the best intrinsic OER catalytic activity compared to other NiFeCr LDH compositions. Furthermore, these nanostructures are directly grown on conductive carbon paper for a high surface area 3D electrode that can achieve a catalytic current density of 25 mA cm^?2 at an overpotential as low as 225 mV and a small Tafel slope of 69 mV dec^?1 in alkaline electrolyte. The optimized NiFeCr catalyst is stable under OER conditions and X‐ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy, and elemental analysis confirm the stability of trimetallic NiFeCr LDH after electrochemical testing. Due to the synergistic interactions among the metal centers, trimetallic NiFeCr LDH is significantly more active than NiFe LDH and among the most active OER catalysts to date. This work also presents general strategies to design more efficient metal oxide/hydroxide OER electrocatalysts.     

Doped‐MoSe2 Nanoflakes/3d Metal Oxide–Hydr(Oxy)Oxides Hybrid Catalysts for pH‐Universal Electrochemical Hydrogen Evolution Reaction

Clean hydrogen production is highly promising to meet future global energy demands. The design of earth‐abundant materials with both high activity for hydrogen evolution reaction (HER) and electrochemical stability in both acidic and alkaline environments is needed, in order to enable practical applications. Here, the authors report a non‐noble 3d metal Cl‐chemical doping of liquid phase exfoliated single‐/few‐layer flakes of MoSe₂ for creating MoSe₂/3d metal oxide–hydr(oxy)oxide hybrid HER‐catalysts. It is proposed that the electron‐transfer from MoSe₂ nanoflakes to metal cations and the chlorine complexation‐induced neutralization, as well as the in situ formation of metal oxide–hydr(oxy)oxides on the MoSe₂ nanoflakes' surface, tailor the proton affinity of the catalysts, increasing the number and HER‐kinetics of their active sites in both acidic and alkaline electrolytes. The electrochemical coupling between doped‐MoSe₂/metal oxide–hydr(oxy)oxide hybrids and single‐walled carbon nanotubes heterostructures further accelerates the HER process. Lastly, monolithic stacking of multiple heterostructures is reported as a facile electrode assembly strategy to achieve overpotential for a cathodic current density of 10 mA cm^?2 of 0.081 and 0.064 V in 0.5 m H₂SO₄ and 1 m KOH, respectively. This opens up new opportunities to address the current density versus overpotential requirements targeted in pH‐universal hydrogen production.     

NiS2/FeS Holey Film as Freestanding Electrode for High‐Performance Lithium Battery

In this work, a freestanding NiS₂/FeS holey film (HF) is prepared after electrochemical anodic and chemical vapor deposition treatments. With the combination of good electrical conductivity and holey structure, the NiS₂/FeS HF presents superior electrochemical performance, due to the following reasons: (i) Porous structure of HF provides a large surface area and more active sites/channels/pathways to enhance the ion/mass diffusion. Moreover, the porous structure can reduce the damage from the volumetric expansion. (ii) The as‐prepared electrode combines the current collector (residual NiFe alloy) and active materials (sulfides) together, thus reducing the resistance of the electrode. Additionally, the good conductivity of HF can improve electron transport. (iii) Sulfides are more stable as active materials than sulfur, showing only a small capacity decay while retaining high cyclability performance. This work provides a promising way to develop high energy and stable electrode for Li‐S battery.     

Surface Engineering of 3D Gas Diffusion Electrodes for High‐Performance H2 Production with Nonprecious Metal Catalysts

In this work, a methodology is demonstrated to engineer gas diffusion electrodes for nonprecious metal catalysts. Highly active transition metal phosphides are prepared on carbon‐based gas diffusion electrodes with low catalyst loadings by modifying the O/C ratio at the surface of the electrode. These nonprecious metal catalysts yield extraordinary performance as measured by low overpotentials (51 mV at ?10 mA cm^?2), unprecedented mass activities (>800 A g^?1 at 100 mV overpotential), high turnover frequencies (6.96 H₂ s^?1 at 100 mV overpotential), and high durability for a precious metal‐free catalyst in acidic media. It is found that a high O/C ratio induces a more hydrophilic surface directly impacting the morphology of the CoP catalyst. The improved hydrophilicity, stemming from introduced oxyl groups on the carbon electrode, creates an electrode surface that yields a well‐distributed growth of cobalt electrodeposits and thus a well‐dispersed catalyst layer with high surface area upon phosphidation. This report demonstrates the high‐performance achievable by CoP at low loadings which facilitates further cost reduction, an important part of enabling the large‐scale commercialization of non‐platinum group metal catalysts. The fabrication strategies described herein offer a pathway to lower catalyst loading while achieving high efficiency and promising stability on a 3D electrode.     

Highly Stable Oxygen Electrodes Enabled by Catalyst Redistribution through an In Situ Electrochemical Method

In this work, for the first time an in situ electrochemical pretreatment approach to fabricate a highly reversible oxygen electrode with redistributed ultrafine RuO₂ catalysts on a carbon nanotube (CNT) matrix is reported. The optimally pretreated RuO₂/CNT oxygen electrodes demonstrate an extremely stable cycling life, 800 times with non‐Li metal anode, under a capacity‐limited protocol of 800 mAh g^?1 in an ether‐based electrolyte in an O₂ environment. The highly stable activity of ultrafine RuO₂ catalysts in oxygen reduction and evolution processes originates from the synergetic effect of greatly reduced size of catalyst (<2 nm) and uniform redistribution of catalyst particles after the in situ electrochemical pretreatment process. The pretreatment method discovered in this work can not only significantly enhance the activity/efficiency of the catalysts used for air electrodes but can also be widely applied to other electro‐catalysis systems.     

The Effects of Catalyst Layer Deposition Methodology on Electrode Performance

The catalyst layer of the cathode is arguably the most critical component of low‐temperature fuel cells and carbon dioxide (CO₂) electrolysis cells because their performance is typically limited by slow oxygen (O₂) and CO₂ reduction kinetics. While significant efforts have focused on developing cathode catalysts with improved activity and stability, fewer efforts have focused on engineering the catalyst layer structure to maximize catalyst utilization and overall electrode and system performance. Here, we study the performance of cathodes for O₂ reduction and CO₂ reduction as a function of three common catalyst layer preparation methods: hand‐painting, air‐brushing, and screen‐printing. We employed ex‐situ X‐ray micro‐computed tomography (MicroCT) to visualize the catalyst layer structure and established data processing procedures to quantify catalyst uniformity. By coupling structural analysis with in‐situ electrochemical characterization, we directly correlate variation in catalyst layer morphology to electrode performance. MicroCT and SEM analyses indicate that, as expected, more uniform catalyst distribution and less particle agglomeration, lead to better performance. Most importantly, the analyses reported here allow for the observed differences over a large geometric volume as a function of preparation methods to be quantified and explained for the first time. Depositing catalyst layers via a fully‐automated air‐brushing method led to a 56% improvement in fuel cell performance and a significant reduction in electrode‐to‐electrode variability. Furthermore, air‐brushing catalyst layers for CO₂ reduction led to a 3‐fold increase in partial CO current density and enhanced product selectivity (94% CO) at similar cathode potential but a 10‐fold decrease in catalyst loading as compared to previous reports.     

Effect of Electron‐Transport Material on Light‐Induced Degradation of Inverted Planar Junction Perovskite Solar Cells

This paper presents a systematic study of the influence of electron‐transport materials on the operation stability of the inverted perovskite solar cells under both laboratory indoor and the natural outdoor conditions in the Negev desert. It is shown that all devices incorporating a Phenyl C₆₁ Butyric Acid Methyl ester ([60]PCBM) layer undergo rapid degradation under illumination without exposure to oxygen and moisture. Time‐of‐flight secondary ion mass spectrometry depth profiling reveals that volatile products from the decomposition of methylammonium lead iodide (MAPbI₃) films diffuse through the [60]PCBM layer, go all the way toward the top metal electrode, and induce its severe corrosion with the formation of an interfacial AgI layer. On the contrary, alternative electron‐transport material based on the perylendiimide derivative provides good isolation for the MAPbI₃ films preventing their decomposition and resulting in significantly improved device operation stability. The obtained results strongly suggest that the current approach to design inverted perovskite solar cells should evolve with respect to the replacement of the commonly used fullerene‐based electron‐transport layers with other types of materials (e.g., functionalized perylene diimides). It is believed that these findings pave a way toward substantial improvements in the stability of the perovskite solar cells, which are essential for successful commercialization of this photovoltaic technology.