What Makes On-Chip Microdevices Stand Out in Electrocatalysis?

Clean and sustainable energy conversion and storage through electrochemistry shows great promise as an alternative to traditional fuel or fossil-consumption energy systems. With regards to practical and high-efficient electrochemistry application, the rational design of active sites and the accurate description of mechanism remain a challenge. Toward this end, in this Perspective, a unique on-chip micro/nano device coupling nanofabrication and low-dimensional electrochemical materials is presented, in which material structure analysis, field-effect regulation, in situ monitoring, and simulation modeling are highlighted. The critical mechanisms that influence electrochemical response are discussed, and how on-chip micro/nano device distinguishes itself is emphasized. The key challenges and opportunities of on-chip electrochemical platforms are also provided through the Perspective.

Ultra-High Proportion of Grain Boundaries in Zinc Metal Anode Spontaneously Inhibiting Dendrites Growth.

Aqueous Zn-ion batteries are an attractive electrochemical energy storage solution for their budget and safe properties. However, dendrites and uncontrolled side reactions in anodes detract the cycle life and energy density of the batteries. Grain boundaries in metals are generally considered as the source of the above problems but we present a diverse result. This study introduces an ultra-high proportion of grain boundaries on zinc electrodes through femtosecond laser bombardment to enhance stability of zinc metal/electrolyte interface. The ultra-high proportion of grain boundaries promotes the homogenization of zinc growth potential, to achieve uniform nucleation and growth, thereby suppressing dendrite formation. Additionally, the abundant active sites mitigate the side reactions during the electrochemical process. Consequently, the 15 µm Fs-Zn||MnO2 pouch cell achieves an energy density of 249.4 Wh kg-1 and operates for over 60 cycles at a depth-of-discharge of 23 %. The recognition of the favorable influence exerted by UP-GBs paves a new way for other metal batteries.

New Insights into Phase-Mechanism Relationship of Mgx MnO2 Nanowires in Aqueous Zinc-Ion Batteries.

In response to the call for safer energy storage systems, rechargeable aqueous manganese-based zinc-ion (Zn-ion) batteries using mild electrolyte have attracted extensive attention. However, the charge-storage mechanism and structure change of manganese-based cathode remain controversial topics. Herein, a systematic study to understand the electrochemical behavior and charge storage mechanism based on a 3 × 3 tunnel-structured Mgx MnO2 as well as the correspondence between different tunnel structures and reaction mechanisms are reported. The energy storage mechanism of the different tunnel structure is surface faradaic dissolution/deposition coupled with an intercalation mechanism of cations in aqueous electrolyte, which is confirmed by in situ X-ray diffraction, in situ Raman and ex situ extended X-ray absorption fine structure. The deposition process at the cathode is partially reversible due to the accumulation of a birnessite layer on the surface. Compared to smaller tunnels, the 3 × 3 tunnel structure is more conducive to deposit new active materials from the electrolyte. Therefore, pristine Mgx MnO2 nanowires with large tunnels display an excellent cycling performance. This work sheds light on the relationship between the tunnel structure and Mn2+ deposition and provides a promising cathode material design for aqueous Zn-ion batteries.

Regulating Lattice-Water-Adsorbed Ions to Optimize Intercalation Potential in 3D Prussian Blue Based Multi-Ion Microbattery.

Miniaturized energy storage device (MESD) is the core module in microscale electronic equipment, yet its electrochemical performance is far away from the actual requirements. The extensive research efforts have improved the performance of MESD via the fabrication techniques and material construction, while ignoring the expansion of optimization strategy in the combination of energy storage mechanism. Herein, the Prussian blue/Zn microbattery is reported with the regulation of lattice-water-adsorbed intercalated ion. The optimal charge transport of cathode is achieved via the optimization of 3D structure of microelectrode to maximize the electrochemical performance. Also, lattice-water-adsorbed ion storage mechanism is further investigated to guide the design of differential energy storage for cathode and anode. The Cu3 (Fe(CN)6 )2 /Zn microbattery, with K+ inter/deintercalation in the cathode and Zn2+ deplating/plating in the anode, displays high capacity (0.281 mAh cm-2 at 2.5 mA cm-2 ), rate performance (0.181 mAh cm-2 at 25 mA cm-2 ), and cycling stability (77.6% capacity retention after 1500 cycles) enhanced by Cu2+ in the electrolyte. This highly efficient combination of fabrication process, active material, and multi-ion storage for microelectrode shows a high tolerance for optimization strategies, expanding the compatibility of optimization path for high-performance MESD.

Langmuir-Blodgett Nanowire Devices for In Situ Probing of Zinc-Ion Batteries.

In situ monitoring the evolution of electrode materials in micro/nano scale is crucial to understand the intrinsic mechanism of rechargeable batteries. Here a novel on-chip Langmuir-Blodgett nanowire (LBNW) microdevice is designed based on aligned and assembled MnO2 nanowire quasimonolayer films for directly probing Zn-ion batteries (ZIBs) in real-time. With an interdigital device configuration, a splendid Ohmic contact between MnO2 LBNWs and pyrolytic carbon current collector is demonstrated here, enabling a small polarization voltage. In addition, this work reveals, for the first time, that the conductance of MnO2 LBNWs monotonically increases/decreases when the ZIBs are charged/discharged. Multistep phase transition is mainly responsible for the mechanism of the ZIBs, as evidenced by combined high-resolution transmission electron microscopy and in situ Raman spectroscopy. This work provides a new and adaptable platform for microchip-based in situ simultaneous electrochemical and physical detection of batteries, which would promote the fundamental and practical research of nanowire electrode materials in energy storage applications.

Superior Hydrogen Evolution Reaction Performance in 2H-MoS2 to that of 1T Phase.

In the hydrogen evolution reaction (HER), energy-level matching is a prerequisite for excellent electrocatalytic activity. Conventional strategies such as chemical doping and the incorporation of defects underscore the complicated process of controlling the doping species and the defect concentration, which obstructs the understanding of the function of band structure in HER catalysis. Accordingly, 2H-MoS2 and 1T-MoS2 are used to create electrocatalytic nanodevices to address the function of band structure in HER catalysis. Interestingly, it is found that the 2H-MoS2 with modulated Fermi level under the application of a vertical electric field exhibits excellent electrocatalytic activity (as evidenced by an overpotential of 74 mV at 10 mA cm-2 and a Tafel slope of 99 mV per decade), which is superior to 1T-MoS2 . This unexpected excellent HER performance is ascribed to the fact that electrons are injected into the conduction band under the condition of back-gate voltage, which leads to the increased Fermi level of 2H-MoS2 and a shorter Debye screen length. Hence, the required energy to drive electrons from the electrocatalyst surface to reactant will decrease, which activates the 2H-MoS2 thermodynamically.

Field-Effect Tuned Adsorption Dynamics of VSe2 Nanosheets for Enhanced Hydrogen Evolution Reaction.

Transition metal dichalcogenides, such as MoS2 and VSe2 have emerged as promising catalysts for the hydrogen evolution reaction (HER). Substantial work has been devoted to optimizing the catalytic performance by constructing materials with specific phases and morphologies. However, the optimization of adsorption/desorption process in HER is rare. Herein, we concentrate on tuning the dynamics of the adsorption process in HER by applying a back gate voltage to the pristine VSe2 nanosheet. The back gate voltage induces the redistribution of the ions at the electrolyte-VSe2 nanosheet interface, which realizes the enhanced electron transport process and facilitates the rate-limiting step (discharge process) under HER conditions. A considerable low onset overpotential of 70 mV is achieved in VSe2 nanosheets without any chemical treatment. Such unexpected improvement is attributed to the field tuned adsorption-dynamics of VSe2 nanosheet, which is demonstrated by the greatly optimized charge transfer resistance (from 1.03 to 0.15 MΩ) and time constant of the adsorption process (from 2.5 × 10-3 to 5.0 × 10-4 s). Our results demonstrate enhanced catalysis performance in the VSe2 nanosheet by tuning the adsorption dynamics with a back gate, which provides new directions for improving the catalytic activity of non-noble materials.

Electric-field-assisted proton coupling enhanced oxygen evolution reaction.

The discovery of Mn-Ca complex in photosystem II stimulates research of manganese-based catalysts for oxygen evolution reaction (OER). However, conventional chemical strategies face challenges in regulating the four electron-proton processes of OER. Herein, we investigate alpha-manganese dioxide (α-MnO2) with typical MnIV-O-MnIII-HxO motifs as a model for adjusting proton coupling. We reveal that pre-equilibrium proton-coupled redox transition provides an adjustable energy profile for OER, paving the way for in-situ enhancing proton coupling through a new "reagent"- external electric field. Based on the α-MnO2 single-nanowire device, gate voltage induces a 4-fold increase in OER current density at 1.7 V versus reversible hydrogen electrode. Moreover, the proof-of-principle external electric field-assisted flow cell for water splitting demonstrates a 34% increase in current density and a 44.7 mW/cm² increase in net output power. These findings indicate an in-depth understanding of the role of proton-incorporated redox transition and develop practical approach for high-efficiency electrocatalysis.

Stabilization of Ni-containing Keggin-type polyoxometalates with variable oxidation states as novel catalysts for electrochemical water oxidation.

The development of new recyclable and inexpensive electrochemically active species for water oxidation catalysis is the most crucial step for future utilization of renewables. Particularly, transition metal complexes containing internal multiple, cooperative metal centers to couple with redox catalysts in the inorganic Keggin-type polyoxometalate (POM) framework at high potential or under extreme pH conditions would be promising candidates. However, most reported Ni-containing POMs have been highly unstable towards hydrolytic decomposition, which precludes them from application as water oxidation catalysts (WOCs). Here, we have prepared new tri-Ni-containing POMs with variable oxidation states by charge tailored synthetic strategies for the first time and developed them as recyclable POMs for water oxidation catalysts. In addition, by implanting corresponding POM anions into the positively charged MIL-101(Cr) metal-organic framework (MOF), the entrapped Ni2+/Ni3+ species can show complete recyclability for water oxidation catalysis without encountering uncontrolled hydrolysis of the POM framework. As a result, a low onset potential of approximately 1.46 V vs. NHE for water oxidation with stable WOC performance is recorded. Based on this study, rational design and stabilization of other POM-electrocatalysts containing different multiple transition metal centres could be made possible.

Surface passivation for highly active, selective, stable, and scalable CO2 electroreduction.

Electrochemical conversion of CO2 to formic acid using Bismuth catalysts is one the most promising pathways for industrialization. However, it is still difficult to achieve high formic acid production at wide voltage intervals and industrial current densities because the Bi catalysts are often poisoned by oxygenated species. Herein, we report a Bi3S2 nanowire-ascorbic acid hybrid catalyst that simultaneously improves formic acid selectivity, activity, and stability at high applied voltages. Specifically, a more than 95% faraday efficiency was achieved for the formate formation over a wide potential range above 1.0 V and at ampere-level current densities. The observed excellent catalytic performance was attributable to a unique reconstruction mechanism to form more defective sites while the ascorbic acid layer further stabilized the defective sites by trapping the poisoning hydroxyl groups. When used in an all-solid-state reactor system, the newly developed catalyst achieved efficient production of pure formic acid over 120 hours at 50 mA cm-2 (200 mA cell current).

Serrated lithium fluoride nanofibers-woven interlayer enables uniform lithium deposition for lithium-metal batteries.

The uncontrollable formation of Li dendrites has become the biggest obstacle to the practical application of Li-metal anodes in high-energy rechargeable Li batteries. Herein, a unique LiF interlayer woven by millimeter-level, single-crystal and serrated LiF nanofibers (NFs) was designed to enable dendrite-free and highly efficient Li-metal deposition. This high-conductivity LiF interlayer can increase the Li+ transference number and induce the formation of 'LiF-NFs-rich' solid-electrolyte interface (SEI). In the 'LiF-NFs-rich' SEI, the ultra-long LiF nanofibers provide a continuously interfacial Li+ transport path. Moreover, the formed Li-LiF interface between Li-metal and SEI film renders low Li nucleation and high Li+ migration energy barriers, leading to uniform Li plating and stripping processes. As a result, steady charge-discharge in a Li//Li symmetrical cell for 1600 h under 4 mAh cm-2 and 400 stable cycles under a high area capacity of 5.65 mAh cm-2 in a high-loading Li//rGO-S cell at 17.9 mA cm-2 could be achieved. The free-standing LiF-NFs interlayer exhibits superior advantages for commercial Li batteries and displays significant potential for expanding the applications in solid Li batteries.

Composition Engineering of Amorphous Nickel Boride Nanoarchitectures Enabling Highly Efficient Electrosynthesis of Hydrogen Peroxide.

Developing advanced electrocatalysts with exceptional two electron (2e- ) selectivity, activity, and stability is crucial for driving the oxygen reduction reaction (ORR) to produce hydrogen peroxide (H2 O2 ). Herein, a composition engineering strategy is proposed to flexibly regulate the intrinsic activity of amorphous nickel boride nanoarchitectures for efficient 2e- ORR by oriented reduction of Ni2+ with different amounts of BH4 - . Among borides, the amorphous NiB2 delivers the 2e- selectivity close to 99% at 0.4 V and over 93% in a wide potential range, together with a negligible activity decay under prolonged time. Notably, an ultrahigh H2 O2 production rate of 4.753 mol gcat -1 h-1 is achieved upon assembling NiB2 in the practical gas diffusion electrode. The combination of X-ray absorption and in situ Raman spectroscopy, as well as transient photovoltage measurements with density functional theory, unequivocally reveal that the atomic ratio between Ni and B induces the local electronic structure diversity, allowing optimization of the adsorption energy of Ni toward *OOH and reducing of the interfacial charge-transfer kinetics to preserve the OO bond.

Ultra-Fast and In-Depth Reconstruction of Transition Metal Fluorides in Electrocatalytic Hydrogen Evolution Processes.

Hitherto, there are almost no reports on the complete reconstruction in hydrogen evolution reaction (HER). Herein, the authors develop a new type of reconfigurable fluoride (such as CoF2 ) pre-catalysts, with ultra-fast and in-depth self-reconstruction, substantially promoting HER activity. By experiments and density functional theory (DFT) calculations, the unique surface structure of fluorides, alkaline electrolyte and bias voltage are identified as key factors for complete reconstruction during HER. The enrichment of F atoms on surface of fluorides provides the feasibility of spontaneous and continuous reconstruction. The alkaline electrolyte triggers rapid F- leaching and supplies an immediate complement of OH- to form amorphous α-Co(OH)2 which rapidly transforms into ß-Co(OH)2 . The bias voltage promotes amorphous crystallization and accelerates the reconstruction process. These endow the generation of mono-component and crystalline ß-Co(OH)2 with a loose and defective structure, leading to an ultra-low overpotential of 54 mV at 10 mA cm-2 and super long-term stability exceeding that of Pt/C. Moreover, DFT calculations confirm that F- leaching optimizes hydrogen and water adsorption energies, boosting HER kinetics. Impressively, the self-reconstruction is also applicable to other non-noble transition metal fluorides. The work builds the fundamental comprehension of complete self-reconstruction during HER and provides a new perspective to conceive advanced catalysts.

Efficient and stable noble-metal-free catalyst for acidic water oxidation.

Developing non-noble catalysts with superior activity and durability for oxygen evolution reaction (OER) in acidic media is paramount for hydrogen production from water. Still, challenges remain due to the inadequate activity and stability of the OER catalyst. Here, we report a cost-effective and stable manganese oxybromide (Mn7.5O10Br3) catalyst exhibiting an excellent OER activity in acidic electrolytes, with an overpotential of as low as 295 ± 5 mV at a current density of 10 mA cm-2. Mn7.5O10Br3 maintains good stability under operating conditions for at least 500 h. In situ Raman spectroscopy, X ray absorption near edge spectroscopy, and density functional theory calculations confirm that a self-oxidized surface with enhanced electronic transmission capacity forms on Mn7.5O10Br3 and is responsible for both the high catalytic activity and long-term stability during catalysis. The development of Mn7.5O10Br3 as an OER catalyst provides crucial insights into the design of non-noble metal electrocatalysts for water oxidation.

Constructing Three-Dimensional Macroporous TiO2 Microspheres with Enhanced Pseudocapacitive Lithium Storage under Deep Discharging/Charging Conditions.

TiO2 has been intensively investigated as an anode material for lithium-ion batteries (LIBs) in 1.0-3.0 V (vs Li+/Li). However, it is a challenge to realize its theoretical capacity (336 mAh g-1) in this limited potential range. Extending the potential range below 1.0 V would increase its capacity but usually at the expense of its cyclic stability owing to the sluggish ionic diffusion and unsatisfactory structural stability. Here, three-dimensional (3D) macroporous TiO2 microspheres with interconnected pores and nanocrystalline thin walls have been constructed through a scalable template-assisted spray drying method to overcome these obstacles. When applied to LIBs, high and stable discharge capacity (300 mAh g-1 at 0.1 A g-1) as well as superior cyclic stability (242 mAh g-1 after 1000 cycles at 1.0 A g-1) can be achieved under deep discharging/charging conditions (0.01-3.0 V vs Li+/Li). Furthermore, the 3D macroporous structure is well preserved under deep discharging/charging and the in situ X-ray diffraction (XRD) patterns and Raman spectra reveal the dominant pseudocapacitive contribution at low potentials (0.01-1.0 V). This work not only develops a facile method to synthesize macroporous metal oxides but also provides insight into the lithium storage mechanism of TiO2 under deep discharging/charging conditions.

In situ monitoring of the electrochemically induced phase transition of thermodynamically metastable 1T-MoS2 at nanoscale.

1T-MoS2 is widely used in the hydrogen evolution reaction (HER) due to its abundant active sites and good conductivity. However, 1T-MoS2 is thermodynamically metastable due to the distorted crystal structure. Recently, researchers have detected the J1 and A1g Raman peaks after the HER process and confirmed that the 2H-1T phase possesses good stability. Therefore, continuous HER is likely to transform 1T-MoS2 into a stable 2H-1T mixed phase. The in situ characterization of 1T-MoS2 individual nanosheets in the HER process is important to understand the intrinsic electrocatalytic behaviour at confined nanoscale, which has rarely been investigated. Herein, we built an individual 1T-MoS2 nanosheet micro-nano device by the intercalation of N-butyllithium into 2H-MoS2. Then, the device was kept at an overpotential (η) of 450 mV, which was much lower than the onset potential, for 20 minutes to ensure continuous HER. Through this electrochemical treatment, we successfully obtained a mixed phase of 2H-1T and monitored the electrochemical phase transition by in situ Raman mapping and atomic force microscopy (AFM). The HER performance of the 2H-1T phase was superior to that of 1T-MoS2 and 2H-MoS2. Additionally, computational simulations demonstrated that the 2H-1T phase exhibited optimal hydrogen adsorption energy. The presented work displays the excellent catalysis of the mixed phase obtained by the electrochemical phase transition, which provides new directions for improving the catalytic activity of TMDs.

High Energy Density Micro-Supercapacitor Based on a Three-Dimensional Bicontinuous Porous Carbon with Interconnected Hierarchical Pores.

On-chip micro-supercapacitors (MSCs) have attracted great attention recently. However, the performance of MSCs is usually unsatisfactory because of the unreasonable pore structure. The construction of a three-dimensional (3D) interconnected porous carbon-based MSC by controllable activation is proposed. The porous monolithic carbon microelectrode activated by ZnO nanowires provides electron/ion bicontinuous conduction path. The fabricated MSC with this microelectrode rendered a high areal specific capacitance of 10.01 mF cm-2, 6 times higher than that of pure pyrolyzed carbon-based MSC, 1.6-5 times higher than that of the MSC with porous carbon activated by ZnO nanoparticles because of its cross-linking macropore-mesopore-micropore structure and considerable areal atomic ratio. The optimization mechanism of the hierarchical channel pore for the electrochemical performance of MSCs is investigated in detail. Four kinds of electrolytes, including H2SO4, redox additive KI/H2SO4, LiCl, and LiTFSi, are employed for constructing MSCs. The voltage window of water in a salt electrolyte assembled LiTFSi-MSC is expanded to 2.5 V. The energy density of LiTFSi-MSC is 6 times higher than that of H2SO4-MSC, which can drive light-emitting diodes without serial or parallel connection. This high-performance 3D interconnected porous carbon-based MSC shows a great potential in applications for large-scale integration of micro-/nanodevices.

Co-Electrodeposited porous PEDOT-CNT microelectrodes for integrated micro-supercapacitors with high energy density, high rate capability, and long cycling life.

Recently, conducting polymers (CPs) have gained significant attention for their potential applications in micro-supercapacitors (MSCs). Prior to actualizing this potential, however, several critical issues should be resolved, notably their low cycling stability and comparatively low capacitance and energy density. Concurrently, challenges remain in improving the performance of CPs for use in MSCs in terms of their electrical conductivity, energy density, and cycling stability. For this investigation, we fabricated a high-performance MSC based on poly(3,4-ethylenedioxythiophene) (PEDOT)-coated multi-walled carbon nanotube (MWCNT) nanoporous network microelectrodes by photolithography combined with electrochemical co-deposition on micro-current collectors. We then sought to confirm the proposed higher electrochemical performance of this hybrid MSC with the synergetic effect of PEDOT as a pseudo-capacitive material and MWCNTs as electric double-layer capacitive material. As reported herein, the hybrid MSC delivers a maximum specific capacitance of 20.6 mF cm-2 (82.4 F cm-3) and, consequently, a comparatively high areal energy density of 2.82 µW h cm-2 (11.4 mW h cm-3) in a wide voltage window of 1.0 V at a current density of 0.1 mA cm-2, and a maximum power density of 18.55 W cm-3 at an energy density of 8.1 mW h cm-3. Furthermore, the MSC displays remarkable long-term cycling stability, retaining 99.9% of its initial capacitance after 20 000 CV and GCD cycles with a coulombic efficiency of 100%. Additionally, two PEDOT-CNT MSCs are coupled in series to power a red light emitting diode. The results provided herein confirm that the PEDOT-CNT MSCs exhibit improved performance over other CP based MSCs.

Field Effect Enhanced Hydrogen Evolution Reaction of MoS2 Nanosheets.

Hydrogen evolution reaction performance of MoS2 can be enhanced through electric-field-facilitated electron transport. The best catalytic performance of a MoS2 nanosheet can achieve an overpotential of 38 mV (100 mA cm-2 ) at gate voltage of 5 V, the strategy of utilizing the electric field can be used in other semiconductor materials to improve their electrochemical catalysis for future relevant research.