Oxygen evolution reaction dynamics monitored by an individual nanosheet-based electronic circuit

Self-etching assembly of designed NiFeMOF nanosheet arrays as high-efficient oxygen evolution electrocatalyst for water splitting

2D metal-organic frameworks (MOFs) have emerged as potential candidates for electrocatalytic oxygen evolution reactions (OER) due to their inherent properties like abundant coordination unsaturated active sites and efficient charge transfer. Herein, a versatile and massively synthesizable self-etching assembly strategy wherein nickel-iron foam (NFF) acts as a substrate and a metal ion source. Specifically, by etching the nickel-iron foam (NFF) surface using ligands and solvents, Ni/Fe metal ions are activated and subsequently reacted under hydrothermal conditions, resulting in the formation of self-supporting nanosheet arrays, eliminating the need for external metal salts. The obtained 33 % NiFeMOF/NFF exhibits remarkable OER performance with ultra-low overpotentials of 188/231 mV at 10/100 mA cm-2, respectively, outperforming most recently reported catalysts. Besides, the built 33 % NiFeMOF/NFF(+)||Pt/C(-) electrolyzer presents low cell voltages of 1.55/1.83 V at 10/100 mA cm-2, superior to the benchmark RuO2 (+)||Pt/C(-), implying good industrialization prospects. The excellent catalytic activity stems from the modulation of the electronic spin state of the Ni active site by the introduction of Fe, which facilitates the adsorption process of oxygen-containing intermediates and thus enhances the OER activity. This innovative approach offers a promising pathway for commercial-scale sustainable energy solutions.

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.

The practice of reaction window in an electrocatalytic on-chip microcell

To enhance the efficiency of catalysis, it is crucial to comprehend the behavior of individual nanowires/nanosheets. A developed on-chip microcell facilitates this study by creating a reaction window that exposes the catalyst region of interest. However, this technology's potential application is limited due to frequently-observed variations in data between different cells. In this study, we identify a conductance problem in the reaction windows of non-metallic catalysts as the cause of this issue. We investigate this problem using in-situ electronic/electrochemical measurements and atom-thin nanosheets as model catalysts. Our findings show that a full-open window, which exposes the entire catalyst channel, allows for efficient modulation of conductance, which is ten times higher than a half-open window. This often-overlooked factor has the potential to significantly improve the conductivity of non-metallic catalysts during the reaction process. After examining tens of cells, we develop a vertical microcell strategy to eliminate the conductance issue and enhance measurement reproducibility. Our study offers guidelines for conducting reliable microcell measurements on non-metallic single nanowire/nanosheet catalysts.

On-chip electrocatalytic microdevices

On-chip electrocatalytic microdevices (OCEMs) are an emerging electrochemical platform specialized for investigating nanocatalysts at the microscopic level. The OCEM platform allows high-precision electrochemical measurements at the individual nanomaterial level and, more importantly, offers unique perspectives inaccessible with conventional electrochemical methods. This protocol describes the critical concepts, experimental standardization, operational principles and data analysis of OCEMs. Specifically, standard protocols for the measurement of the electrocatalytic hydrogen evolution reaction of individual 2D nanosheets are introduced with data validation, interpretation and benchmarking. A series of factors (e.g., the exposed area of material, the choice of passivation layer and current leakage) that could have effects on the accuracy and reliability of measurement are discussed. In addition, as an example of the high adaptability of OCEMs, the protocol for in situ electrical transport measurement is detailed. We believe that this protocol will promote the general adoption of the OCEM platform and inspire further development in the near future. This protocol requires essential knowledge in chemical synthesis, device fabrication and electrochemistry.

Ultrafast Synthesis of Mo2C-Based Catalyst by Joule Heating towards Electrocatalytic Hydrogen Evolution Reaction

Developing earth-abundant electrocatalysts useful for hydrogen evolution reactions (HER) is critical for electrocatalytic water splitting driven by renewable energy. Molybdenum carbide (Mo2C) with the crystal structure of hexagonal symmetry has been identified to be an excellent HER catalyst due to its platinum-like electronic structure while the synthesis of Mo2C is generally time consuming and energy intensive. Herein, we demonstrated the ultrafast synthesis of a Mo2C-based electrocatalyst with Joule heating at 1473 K for only 6 s. Benefitting from several advantages including efficient catalytic kinetics, enhanced charge transport kinetics and high intrinsic activity, the as-prepared catalyst exhibited drastically enhanced HER performance compared with commercial Mo2C. It showed an overpotential of 288 mV for achieving a current density of −50 mA cm−2 and good stability, which highlighted the feasibility of the Joule heating method towards preparing efficient electrocatalysts.

On-Chip Investigation of Electrocatalytic Oxygen Reduction Reaction of 2D Materials

The on-chip electrocatalytic microdevice (OCEM) is an emerging platform specialized in the electrochemical investigation of single-entity nanomaterials, which is ideal for probing the intrinsic catalytic properties, optimizing performance, and exploring exotic mechanisms. However, the current catalytic applications of OCEMs are almost exclusively in electrocatalytic hydrogen/oxygen evolution reactions with minimized influence from the mass transfer. Here, an OCEM platform specially tailored to investigate the electrocatalytic oxygen reduction reaction (ORR) at a microscopic level by introducing electrolyte convection through a microfluidic flow cell is reported. The setup is established on gold microelectrodes and later successfully applied to investigate how Ar-plasma treatment affects the ORR activities of 2H MoS₂ . This study finds that Ar-plasma treatment significantly enhances the ORR performance of MoS₂ nanosheets owing to the introduction of surface defects. This study paves the way for highly efficient microscopic investigation of diffusion-controlled electrocatalytic reactions.

Back-Gated van der Waals Heterojunction Manipulates Local Charges toward Fine-Tuning Hydrogen Evolution

Charge redistribution plays a prominent role in interpreting the intrinsic electrocatalytic mechanism. Establishing a quantitative relationship between the local charges and electrochemical performance can fundamentally update the design philosophies beyond conventional methods. We describe exertion of an external electric field in the cobalt phthalocyanine (CoPc)/MoS₂ heterojunction to finely manipulate intermolecular charge transfer. The injected charges (e^- ) from CoPc to MoS₂ migrate to natural S vacancies and enhance Mo-H bonding. Moreover, the band gap of MoS₂ and CoPc can be readily tuned by the electric field, verifying band engineering at the heterointerface. In situ photoluminescence spectra and gate-dependent electrochemical measurement reveal a linear correlation between the charge accumulation and hydrogen evolution reaction (HER) activity. This approach provides a new strategy for the design of catalysts, enabling precise regulation of the electronic configuration to improve catalytic activity.

Interfacial processes in electrochemical energy systems

Electrochemical energy systems such as batteries, water electrolyzers, and fuel cells are considered as promising and sustainable energy storage and conversion devices due to their high energy densities and zero or negative carbon dioxide emission. However, their widespread applications are hindered by many technical challenges, such as the low efficiency and poor long-term cyclability, which are mostly affected by the changes at the reactant/electrode/electrolyte interfaces. These interfacial processes involve ion/electron transfer, molecular/ion adsorption/desorption, and complex interface restructuring, which lead to irreversible modifications to the electrodes and the electrolyte. The understanding of these interfacial processes is thus crucial to provide strategies for solving those problems. In this review, we will discuss different interfacial processes at three representative interfaces, namely, solid-gas, solid-liquid, and solid-solid, in various electrochemical energy systems, and how they could influence the performance of electrochemical systems.

In Situ Construction of Flexible VNi Redox Centers over Ni-Based MOF Nanosheet Arrays for Electrochemical Water Oxidation

Atomic-level design and construction of synergistic active centers are central to develop advanced oxygen electrocatalysts toward efficient energy conversion. Herein, an in situ construction strategy to introduce flexible redox sites of VNi centers onto Ni-based metal-organic framework (MOF) nanosheet arrays (NiV-MOF NAs) as a promising oxygen electrocatalyst is developed. The abundant redox VNi centers with flexible metal valence states of V^+3/+4/+5 and Ni^+3/+2 enable NiV-MOF NAs excellent oxygen evolution reaction (OER) activity and a long-term stability under high current densities, achieving current densities of 10 and 100 mA cm^-2 at recorded overpotentials of 189 and 290 mV, respectively, and showing ignorable decay of initial activity at 100 mA cm^-2 after 100 h OER operation. Operando synchrotron radiation Fourier transform infrared combined with quasi in situ X-ray absorption fine structure spectroscopies reveal at atomic level that the flexible V sites can continuously accept electrons from adjacent active Ni sites to accelerate OER kinetics for NiV-MOF NAs during the reaction process, accompanied by a self-optimized structural distortion of VO₆ octahedron for promoting the electrochemical stability.

Hollow bimetallic selenide derived from a hierarchical MOF-based Prussian blue analogue for urea electrolysis

PBA@MOF–Ni/Se with a nanocube structure grown on a flower-shaped MOF–Ni template exhibits better performance in urea electrolysis.

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.

On-chip electrocatalytic microdevice: an emerging platform for expanding the insight into electrochemical processes

This comprehensive summary of on-chip electrocatalytic microdevices will expand the insight into electrochemical processes, ranging from dynamic exploration to performance optimization.

Trinuclear Ni(ii) oriented highly dense packing and π-conjugation degree of metal–organic frameworks for efficient water oxidation

Trinuclear nickel cluster based MOF shows highly denser crystal packing and π-conjugation degree as well as outstanding OER performance with an overpotential of 270 mV at 10 mA cm⁻².

N,S-Atom-coordinated Co9S8 trinary dopants within a porous graphene framework as efficient catalysts for oxygen reduction/evolution reactions

The intrinsic instability and difficulty in controlling the uniform size distributions of cobalt sulfides greatly restrict their application for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) as a bifunctional electrocatalyst in regenerative fuel cells and rechargeable metal-air batteries. Herein, we synthesize a stable electrocatalyst of N,S-atom-coordinated Co9S8 trinary dopants within a porous graphene framework (Co9S8@NS-3DrGO), in which Co9S8 nanoparticles show uniform sizes and distributions. The stable Co9S8-based composites are fabricated by a facile soft template-assisted strategy, and the attraction of this method is that the intermediate of melamine formaldehyde resin (MFR) plays trifunctional roles, including (i) it acts as the templated bonding material to crosslink GO sheets together, (ii) it facilitates the formation of a core-shell architecture, and (iii) it acts as the N source for doping. Catalyst composition and performance are largely dependent on the pyrolysis temperature. Extensive investigation elucidates that the mechanism of electrocatalytic activity is attributed to: (i) the unique core-shell structure of the composites, as well as uniform particle sizes and distributions of Co9S8, (ii) the active nitrogen (pyridinic N and graphitic N) contents, and (iii) the large surface area and porous architecture. The composite pyrolyzed at 850 °C exhibits the best electrocatalytic performance, which shows a positive ORR half-wave potential (0.826 V), a small OER overpotential (317 mV) at 10 mA cm-2, and high stability, comparable to the commercial noble catalysts Pt/C and RuO2 in alkaline media. Furthermore, when applied in zinc-air batteries, it also displays a comparable performance to a Pt/C + RuO2 mixture catalyst. This work provides an approach to stabilize cobalt sulfides and control their particle sizes and distributions by ingeniously employing the soft template of MFR and pyrolyzing them at various temperatures.

On-Chip in Situ Monitoring of Competitive Interfacial Anionic Chemisorption as a Descriptor for Oxygen Reduction Kinetics

The development of future sustainable energy technologies relies critically on our understanding of electrocatalytic reactions occurring at the electrode-electrolyte interfaces, and the identification of key reaction promoters and inhibitors. Here we present a systematic in situ nanoelectronic measurement of anionic surface adsorptions (sulfates, halides, and cyanides) on ultrathin platinum nanowires during active electrochemical processes, probing their competitive adsorption behavior with oxygenated species and correlating them to the electrokinetics of the oxygen reduction reaction (ORR). The competitive anionic adsorption features obtained from our studies provide fundamental insight into the surface poisoning of Pt-catalyzed ORR kinetics by various anionic species. Particularly, the unique nanoelectronic approach enables highly sensitive characterization of anionic adsorption and opens an efficient pathway to address the practical poisoning issue (at trace level contaminations) from a fundamental perspective. Through the identified nanoelectronic indicators, we further demonstrate that rationally designed competitive anionic adsorption may provide improved poisoning resistance, leading to performance (activity and lifetime) enhancement of energy conversion devices.