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In past two decades electro chemical machining (ECM) and electric discharge machining (EDM) processes have been widely explored for machining of different grades of steel (including AISI 304). Although there is difference in material removal mechanism of EDM (localized heating and vaporization being thermal process) and ECM (ionic dissolution being electrochemical energy based process), yet the user needs broad data base for comparison and selection of particular process. But hitherto little has been reported on relative comparison of EDM and ECM processed AISI steel surface from surface roughness (Ra), surface hardness (HV) and morphological characteristics view point. This paper reports the comparison of AISI 304 steel machined surface by EDM and ECM. The comparison has been made after process optimization of EDM and ECM separately (using Taguchi approach). The results of study suggest that ECM is better process than EDM from Ra view point (3.10 µm for ECM and 6.41 µm for EDM) and porosity% view point, whereas from HV viewpoint no significant difference was observed.

Electrocatalysts play a crucial role in the development of renewable energy conversion and storage nanotechnologies. The unique advantages of heteroatom-doped porous carbon-supported single-atom electrocatalysts (SAC-HDPCs) are clear. These SAC-HDPCs exhibit outstanding activity, selectivity and stability due to their distinct electronic structure, satisfactory conductivity, controllable porosity and heteroatom-doping effect. Rapid and significant developments involving the synthesis, characterization, and structure-property-function relationship of SAC-HDPCs have been made in recent years. In this review, we describe recent research efforts involving advanced (in situ) characterization techniques, innovative synthetic strategies, and electrochemical energy conversion examples of SAC-HDPCs. The electrocatalytic performance of SAC-HDPCs is further considered at an atomic level, and the mechanisms underlying this performance are also discussed in detail. We expect that these analyses and deductions will be useful for the design of new materials and may help to establish a foundation for the design of future SAC-HDPCs.

Metal sulfide arrays can store electrochemical energy owing to their high theoretical capacity, shortened diffusion routes, and rich electrochemical sites. However, their sluggish redox reaction has limited further application in high power density devices. Herein, a novel hybrid and hierarchical nanostructure with a thin layer of NiO deposited on the surface of walnut-like CoS particles is constructed and the NiO layer can greatly improve the capacitive properties of cobalt sulfide. In this hybrid nanostructure, the amount of NiO on the surface of CoS is precisely tuned to boost the electrochemical performance. Consequently, NiO/CoS exhibits a higher capacitance/capacity of 1749 F g−1/243.9 mAh g−1 fueled by the excellent electrical conductivity and appropriate surface structures. When assembled into a hybrid supercapacitor, the NiO/CoS//AC devices deliver a maximum capacitance of 60 C g−1 at 1.0 A g−1. Energy density reaches 45 Wh kg−1 when the power density is at 750 W kg−1. Additionally, the fabricated supercapacitor has good cycling stability with a capacity retention rate of 74% after 5000 cycles.

Owing to their high energy density, lithium-oxygen batteries (LOBs) have been drawn great attention as one of the promising electrochemical energy sources. However, the sluggish kinetics of oxygen reduction/evolution reaction (ORR/OER) hamper the widespread application of LOBs. Herein, an elaborate designed catalysts which are constructed by FeNx moieties dispersed on the network-like hollow dodecahedral carbon and then decorated with Ru nanoparticles (FeNx-HDC@Ru). Since the homogeneously dispersed FeNx moieties could promote ORR performance, and the Ru nanoparticles could facilitate OER capability, the FeNx-HDC@Ru nanocomposites used as cathode catalysts can significantly improve LOBs performance. A lower discharge and charge overpotentials of 0.15 V and 0.78 V can be detected in the first cycle, respectively, and an excellent cycle performance of 90 cycles at 200 mA g−1 and 89 cycles at 500 mA g−1 can be demonstrated. Herein, the charge transfer kinetics has been enhanced with the internal network-like hollow structure and a low impedance Li2O2/catalysts contact interface could be earned by the constructed Ru nanoparticles, these factors would lead to an efficient acceleration to the formation and decomposition of Li2O2 during discharge and charge process.

Electrospinning has been proven as a highly versatile fabrication method for producing nano-structured fibres with controllable morphology, of both the fibres themselves and the void structure of the mats. Additionally, it is possible to use heteroatom doped polymers or to include catalytic precursors in the electrospinning solution to control the surface properties of the fibres. These factors make it an ideal method for the production of electrodes and flow media for a variety of electrochemical devices, enabling reduction in mass transport and activation overpotentials and therefore increasing efficiency. Moreover, the use of biomass as a polymer source has recently gained attention for the ability to embed sustainable principles in the materials of electrochemical devices, complementing their ability to allow an increase in the use of renewable electricity via their application. In this review, the historical and recent developments of electrospun materials for application in redox flow batteries, fuel cells, metal air batteries and supercapacitors are thoroughly reviewed, including an overview of the electrospinning process and a guide to best practice. Finally, we provide an outlook for the emerging use of this process in the field of electrochemical energy devices with the hope that the combination of tailored microstructure, surface functionality and computer modelling will herald a new era of bespoke functional materials that can significantly improve the performance of the devices in which they are used.

Oxygen evolution and oxygen reduction are considered as essential processes in the energy conversion devices. The progress of cost-effective bifunctional catalysts has become a critical issue to be solved. Here, we report that Co(OH)2 @N-doped carbon (NC) was facilely synthesized through the impregnation strategy of metal-organic frameworks derived carbon and cobalt ions. N-doped carbon with porous structure and cobalt hydroxide nanosheets play a synergistic effect role, representing excellent catalytic performance toward oxygen evolution and reduction reactions. The obtained Co(OH)2@NC exhibits remarkable activity in terms of a lower overpotential of 330 mV@10 mA cm−2 for OER and a more positive half-wave potential (E1/2 = 0.84 V) for ORR in alkaline medium, outperforming IrO2 and Pt/C. Due to its superior bifunctional catalytic performance, Co(OH)2@NC catalyst is applied into a promising air electrode of Zn-air battery. This presented strategy of impregnation synthesis in this work provides a new design direction for practical electrochemical energy devices.

Electromagnetic pollution and electrochemical energy consumption have driven enormous endeavors in electromagnetic wave absorption materials and energy storage devices, however, designing versatile composites for addressing the issue remains a huge challenge. Herein, a versatile honeycomb-like N-doped carbon/bimetallic sulfide and oxide (N–CoFe2O4-CoxSy/FexSy@C) composites aerogel was designed via a simple mechanical mixing, freeze-drying and heat treatment method. As-prepared N–CoFe2O4-CoxSy/FexSy@C composites exhibited excellent electromagnetic wave absorption and electrochemical performances. The minimum RL can reach −51.9 dB at 15.5 GHz, with an effective frequency bandwidth (RL

The hybrid battery integrating a typical Zn redox battery and a Zn-air battery is a promising green technology for energy storage, and the cathode integrating the redox reaction and electrocatalytic oxygen reduction is a key point for efficient electrochemical energy conversion. Herein, we report a scalable strategy to fabricate nanoporous Ag2Al intermetallic compound as a self-standing cathode for the hybrid Zn battery. The abundant surface oxygen species, the Ag-Al intermetallic interaction and the np-Ag2Al@AgAlOx interface cooperatively contributed to the catalytic ORR activity. The electrode endows efficient catalytic oxygen reduction (a Tafel slope of 38.0 mV/dec and an onset potential of 0.998 V) and regulated redox activity as compared with Ag. The nanoporous channels allow efficient ion transport, interface charge exchange and gas molecular diffusion. Significantly, the assembled hybrid Zn-Ag2Al/air battery delivers a high capacity of 3.23 mAh/cm2 as compared with recent reports. As far as we know, this is the first exploration for the electrochemical property of Ag2Al, and it would inspire more exploration in developing multifunctional materials and robust hybrid batteries for practical applications.

The development of non-precious metal catalysts to replace scarce and costly Pt-based catalysts is crucial for oxygen reduction reaction (ORR), which involves various electrochemical energy conversions, such as fuel cells and metal-air batteries. However, it is still a challenge to devise a simpler method to obtain high-performance non-noble metal catalyst, which can be used for large-scale production. Herein, Cu and N co-doped mesocellular carbon foam (n-Cu/NC) has been developed by strong electrostatic adsorption (SEA) method, which inherits the original morphology of carbon foam, exhibiting high ORR performance with a half-wave potential of 0.825 V vs. RHE and excellent long-term durability. The outstanding performance of n-Cu/NC is attributed to the high specific surface area, hierarchical porous carbon matrix doped by multiple N atoms and uniform Cu2O/CuO active sites induced by the SEA method. The strategy of SEA method is applied to the preparation of carbon-supported catalysts, which shows great potential in the design and large-scale production of M and N co-doped carbon materials.

The oxygen reduction/evolution reactions (ORR/OER) are a key electrode process in the development of electrochemical energy conversion and storage devices, such as metal-air batteries and reversible fuel cells. The search for low-cost high-performance nanocarbon-based metal-free and non-precious metal bifunctional electrocatalysts for ORR/OER alternatives to the widely-used noble metal-based catalysts is a research focus. This review aims to outline the opportunities and available options for these nanocarbon-based bifunctional electrocatalysts. Through discussion of some current scientific issues, we summarize the development and breakthroughs of these electrocatalysts. Then we provide our perspectives on these issues and suggestions for some areas in the further work. We hope that this review can improve the interest in nanocarbon-based metal-free and non-precious metal bifunctional electrocatalysts for ORR/OER.

Characterization and electrochemical evaluation of novel metal-free electrocatalysts obtained by solar pyrolysis is reported. Carbon-based electrocatalysts were prepared from agave bagasse waste, using a sustainable process based on concentrated solar energy as heat source. Agave was processed in a spherical borosilicate glass solar reactor using a heating rate of 30 °C min−1 to a target of 500, 700, or 900 °C, and maintaining temperature for 1 h under inert atmosphere. The structure and composition of the prepared electrocatalysts were influenced by pyrolysis temperature. In addition, electrocatalytic activity towards the oxygen reduction reaction in 0.1 M KOH solution was explored. The electrocatalyst obtained at 500 °C showed the highest activity among all pyrolyzed samples due to its moderate surface area, but mostly due to its higher oxygen content. The metal-free electrocatalysts reported in this work are promising eco-friendly alternative as cathode materials for anion-exchange membrane fuel cells. This study provides a sustainable approach to use agricultural biomass waste to produce valuable materials for electrochemical energy devices.

Electrochemical energy technologies such as fuel cells, supercapacitors, and batteries are some of the most suitable energy storage and conversion devices to meet our needs proving the future generation’s equitable opportunity to meet their own needs. For this purpose, an earth-abundant precursor such as biomass is the best candidate for the synthesis of the next generation of low-cost and green electrode materials. This review summarizes the most recent progress in biomass-derived carbons for use in fuel cells, supercapacitors and lithium-ion batteries, the physical-chemical properties, desired features, performances, and limitations for electrochemical energy technologies. Several thermochemical treatments such as chemical activation, template methods, doping and hydrothermal treatments have been reviewed. Finally, we provide the reader with comprehensive information of the challenges, future research efforts, advantages, limitations and opportunities which will be a fundamental insight for the future design of biomass-derived carbon electrode materials for electrochemical storage and conversion systems.

The synthesis of nonprecious electrocatalysts for oxygen electrocatalysis is of considerable interest for the development of electrochemical energy devices. Herein, we demonstrate a facile approach for the synthesis of bamboo-like nitrogen-doped carbon nanotube-encapsulated Co0.25Ni0.75 alloy electrocatalyst (Co0.25Ni0.75@NCNT) and its bifunctional oxygen electrocatalytic performance toward oxygen reduction and oxygen evolution reactions. The Co0.25Ni0.75 alloy wrapped with NCNT is obtained by a one-step carbothermal reduction approach using dicyandiamide and NiCo-MOF precursors. Dicyandiamide acts as a nitrogen source, and the in situ generated Co0.25Ni0.75 alloy nanoparticles catalyze the growth of bamboo-like NCNTs. The hollow NiCo-MOF plays a sacrificial role in providing a suitable environment for the controlled growth of Co0.25Ni0.75 alloy and NCNT. Co0.25Ni0.75@NCNT efficiently catalyzes both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) at a favorable overpotential. It shows a low potential gap (ΔE) of ∼0.8 V between the two reactions, and it qualifies for the development of air cathode in metal-air batteries. The enhanced bifunctional activity and excellent durability stem from the chemical composition and the synergistic effect between Co0.25Ni0.75 alloy and encapsulating NCNT. The original phase and morphology of the catalyst is preserved after an extensive durability test. Aqueous rechargeable Zn-air battery (ZAB) is fabricated using a Co0.25Ni0.75@NCNT-based air cathode. The battery has high open-circuit voltage (1.53 V) and a maximum peak power density of 167 mW cm-2 with only 1.6% loss in the voltaic efficiency after 36 h charge-discharge cycles. As a proof-of-concept demonstration, the as-fabricated ZAB is successfully used for the electrochemical water splitting in alkaline solution.

Bipolar membranes (BPMs) are gaining interest in energy conversion technologies. These membranes are composed of cation- and anion-exchange layers, with an interfacial layer in between. This gives the freedom to operate in different conditions (pH, concentration, composition) at both sides. Such membranes are used in two operational modes, forward and reverse bias. BPMs have been implemented in various electrochemical applications, like water and CO2 electrolyzers, fuel cells, and flow batteries, while BPMs are historically designed for acid/base production. Therefore, current commercial BPMs are not optimized, as the conditions change per application. Although the ideal BPM has highly conductive layers, high water dissociation kinetics, long lifetime, and low ion crossover, each application has its own priorities to be competitive in its field. We describe the challenges and requirements for future BPMs, and identify existing developments that can be leveraged to develop BPMs toward the scale of practical applications.

Oxygen evolution reaction (OER), as the primary anodic reaction, plays a critical role in many electrochemical energy conversion processes. As the state-of-the-art OER catalysts, iridium-based materials are largely hindered from practical applications mainly due to the extreme scarcity of iridium. Here we demonstrate the successful fabrication of boron-doped amorphous iridium oxide (IrOx-B) via a facile boric acid-assisted method, which realizes an ultrahigh OER mass activity of 2779 A g Ir −1 at 300 mV overpotential, representing one of the best acidic OER catalysts reported so far. It is found that boric acid can not only facilitate the exposure of Ir, but also dope the amorphous IrOx with a form of metaborate, which could further modify the electronic and local ligand structure of Ir for the improved intrinsic activity. Interestingly, the reported strategy is universal that can be applied to improve other metal oxide OER catalysts, highlighting a versatile strategy for creating high-performance electro-catalysts with ultrahigh mass activity for OER and beyond.

Hierarchical 3-dimensional (3D) carbon nanostructures have potential applicability in several electrochemical energy applications own to their sizeable effective surface, specific energy density, cycling stability and flexibility. Herein, combining electrospinning with in situ chemical vapor deposition (CVD) technologies as an efficient and secure procedure is applied for constructing 3D carbon nanostructure-based on carbon fibers (CFs) and electrospun carbon nanofibers (ECNFs). Affect the catalysis type, immersion solution, carbon sources and treat temperature are experimentally demonstrated. Cactus-like 3D carbon nanomaterials are constructed via growth forest-like carbon nanofibers (CNFs) directly on both CNFs and CFs at 900 °C using Ni as a catalyst, cellulose acetate (CA) as carbon source and ethanol-urea as an immersions solution. The Ni/CNFs/ECNFs mat exhibits a large surface area of 342.3 m2 g−1, while just 19.3 m2 g−1 is recorded to Ni/CNFs/CFs. Based on the nano-nonwoven structure and forest-like grown CNFs nanostructure, Ni/CNFs/ECNFs exhibit a favorable hydrogen evolution reaction (HER) performance in an alkaline medium with a low overpotential of 88 mV to deliver 10 mV cm−2 current density and Tafel slope of 170 mV dec−1. This work proves the synthesis parameters of 3D hierarchical carbon nanostructures and their enticing to apply as an advanced substrate for electrochemical applications.

Engineering of nanostructured materials with a unique and uniform morphological design is considered to be a competent candidate for a diverse range of electrochemical energy applications. However, the construction of ball-in-ball transition metal sulfide via the vaporization process remains a big challenge of today's research community. In this work, carbon encapsulated nickel sulfide (Ni3S2@C) with a complex hollow interior was synthesized and further studied with nitrogen-doped carbon encapsulated nickel sulfide (Ni3S2@NC) for comparison basis. We used metal salt (nickel nitrate) and organic linker (trimesic acid, TMA) as a precursor to synthesize Ni-TMA via a solvothermal method. Next, the sulfidation process was done under 5% H2 balanced argon gas in a tubular furnace to convert into Ni3S2 with encapsulation of carbon. The prepared hybrid material showed ball-in-ball morphology and heazlewoodite mineral phase structure. Furthermore, the prepared materials were examined for electrochemical energy storage properties owing to their well-known faradaic dominant (battery-type) charge storage features and key positive electroactive material for today's growing hybrid electrochemical capacitor. The nitrogen-doped carbon encapsulated nickel sulfide has demonstrated good electrochemical features like a specific capacity of 442.45 F g−1 at 5 mV s−1 of scan rate and 373.52 F g−1 at 2 A g−1 of current density. Moreover, the material showed excellent capacitance retention of 81% and 70% after 5000 and 10,000 cycles run with 100% of coulombic efficiency at 8 A g−1.

Global increasing demand in the need of energy leads to the development of non-conventional, high power energy sources. Supercapacitors (SCs) are one of the typical non-conventional energy storage devices which are based on the principle of electrochemical energy conversion. SCs are promising energy storage devices for better future energy technology. Increasing progress has been made in the development of applied and fundamental aspects of SCs. Manganese oxide electrode materials have been well studied; however, their capacitive performance is still inadequate for practical applications. Recent research is mainly focused on enhancing manganese oxide capacitive performance through the incorporation of electrically conductive materials and by controlling its morphology to reveal a more active surface area for redox reactions. In this review, progress in the applications of manganese oxide carbon-based materials towards the development of highly effective SCs is briefly discussed. In this regard, manganese oxide carbon-based nanocomposites synthesis methods and techniques used to approximate the capacitance of electrode materials are discussed.

Photoelectrochemical properties have been investigated for flexible photoelectrodes containing 310 nm thick ZnO film on spin-coated ITO/PET. The high crystalline structure of ZnO was studied using x-ray diffraction pattern. A value of 3.4 eV has been estimated for optical band gap from its absorption spectra. The flexible ZnO photoelectrode was demonstrated to generate photoelectrochemical current. The photocurrents are enhanced by 4% whereas flat-band potential is shifted by 8 V due to the illumination. Values of 1.022 and 0.714 AW−1 were found to be for photo switching and photoresponsivity, respectively. ZnO/ITO/PET can be used as a substrate for making flexible hybrid PEC devices to generate solar power and solar fuels.

Energy conversion and storage in a Li-ion cell involves multiple closely coupled transport processes, such as species diffusion through solid and solution phases in the electrode. Mathematical modeling of these processes is critical for fully understanding and optimizing the performance of a Li-ion cell. While a number of analytical and numerical models have been presented for solution phase diffusion, most of such work is based on the assumption of a constant current density. This paper presents analytical modeling of solution phase diffusion in a separate–electrode composite for a generalized, time-dependent current density. An analytical solution for the concentration field in a separator–electrode composite in such conditions is derived using the method of eigenfunction expansion. Good agreement with past work as well as numerical simulations is shown. Results for linear, periodic and step-function boundary conditions are discussed. The theoretical analysis presented here may help accurately model realistic processes where the applied current changes over time, for example, cyclic charge and discharge in an electric vehicle, or sudden changes in the battery load. Results presented here contribute towards the fundamental understanding of solution phase diffusion in Li-ion cells, and provide a basis for improving electrochemical energy conversion and storage processes.

Nitrogen-doped carbon dots (N-CDs), derived from the biomass (anthocyanin), are the novel additive to the nanocarbon materials, which is expected to bring a wide spectrum of novel applications. Moreover, metallic oxides are emerging for their unique potential for electrocatalysis. Herein, we report the synthesis of N-CDs for the selective detection of Fe3+ with a limit of detection of 2.57 μM in the range of 5–60 μM using ethylenediamine and H2O2 by a hydrothermal method. The obtained N-CDs displayed a spherical morphology with a particle size range of 2–7 nm and emitted blue luminescence at 394 nm under excitation at 319 nm. Meanwhile, we have demonstrated the fabrication of cost-efficient electrocatalysts for oxygen evolution reaction (OER) in an alkaline medium, employing N-CDs. Owing to the successful incorporation of N-CDs into NiO nanospheres, the resulting N-CDs/NiO with large surface areas, fast charge transfer, and increased conductivity vastly improved the catalytic activity. Remarkably, the optimal of N-CDs/NiO composite requires the overpotential of only 380 mV at a current density of 10 mA cm−2 and a relatively low Tafel slope of 57.96 mV dec−1 compared with pure NiO. These results open up a facile route for the application of N-CDs and offer prospects for CD-metal hybrids as high OER catalysts in electrochemical energy devices.

The conductive metal-organic frameworks (MOFs) are suggested as the ideal electrocatalysts for hydrogen evolution reaction (HER) because of the high utilization of metal atoms. Rational design and facile synthesis of MOFs with large specific surface area, proper metals as center, and tunable chemical components is still full of challenges. Herein, we report the facile synthesis three types of porous MOFs by regulating metal center using benzene-1,3,5-tricarboxylic acid (H3BTC) as organic ligand and have successfully synthesized the rhombic octahedral Cu-BTC, rod-shaped Co-BTC and spherical Ni-BTC materials with large specific surface area ranged in 350-500 m2 g−1. These as-prepared MOFs materials exhibit high performance of HER in 0.5 M H2SO4. Ni-BTC material exhibits the lowest overpotential of 53 mV at 10 mA cm-2 and the smallest Tafel slope of 62 mV dec−1 than those of Cu-BTC (270 mV, 155 mV dec−1) and Co-BTC (123 mV, 100 mV dec−1), which are much superior to these previously reported MOFs catalysts. In addition, the fast catalytic kinetic of Ni-BTC was confirmed by the smaller charge transfer resistance (Rct) value of 0.9 Ω and larger electrochemical active surface area (ECSA) of 35.5 cm2 than those of Cu-BTC (8.2 Ω, 22.5 cm2) and Co-BTC (1.9 Ω, 27.7 cm2). Because of the structural advantage and large ECSA, the turnover frequency (TOF) value of Ni-BTC reaches up to 0.041 s−1 at 120 mV overpotential, which is 20.5 and 2.6 times greater than that of Cu-BTC (0.002 s−1) and Co-BTC (0.016 s−1). Besides, these three types of MOFs exhibited excellent durability over 12 h. This study unfolds diverse insights into the design and facile synthesis of MOFs for electrochemical energy conversion system.

Strain engineering is widely employed to manipulate the intrinsic relationship of activity and the crystal structure, while the mechanism and rational strategy toward high-performance devices are still under investigation. Here straining engineering is utilized to manipulate a series of a typical perovskite structures via introducing different types of heteroions (Bi1-xMxFeO3, M = Ca2+ or Y3+ ion). The space group R3c in BiFeO3 perovskites is found to be maintained with substituting a certain amount of heteroions at Bi3+ sites ( 5%). Such a transformation is linked with the mismatched crystal strain induced by the heteroions substituted at Bi3+ sites, while the activity, stability, and energy storage capability of Bi1-xMxFeO3 have been essentially varied. The results offer a strategy for manipulating stability and activity of perovskites in electrochemical energy conversion and storage.

Major research efforts are being carried out for the technological advancement to an energetically sustainable society. However, for the full commercial integration of electrochemical energy storage devices, not only materials with higher performance should be designed and manufactured but also more competitive production techniques need to be developed. The laser processing technology is well extended at the industrial sector for the versatile and high throughput modification of a wide range of materials. In this work, a method based on laser processing is presented for the fabrication of hybrid electrodes composed of graphene nanowalls (GNWs) coated with different transition-metal oxide nanostructures for electrochemical capacitor (EC) applications. GNW/stainless steel electrodes grown by plasma enhanced chemical vapor deposition were decorated with metal oxide nanostructures by means of their laser surface processing while immersed in aqueous organometallic solutions. The pseudocapacitive nature of the laser-induced crystallized oxide materials prompted an increase of the GNW electrodes’ capacitance by 3 orders of magnitude, up to ca. 28 F/cm3 at 10 mV/s, at both the positive and negative voltages. Finally, asymmetric aqueous and solid-state ECs revealed excellent stability upon tens of thousands of charge–discharge cycles., The authors are grateful for the financial support of the Spanish Ministry of Economy, Industry, and Competitiveness under the projects ENE2017-89210-C2-1-R and ENE2017-89210-C2-2-R and support from AGAUR of Generalitat de Catalunya through projects 2017 SGR 1086. ICMAB acknowledges the financial support from the Spanish Ministry of Economy and Competitiveness, through the ‘Severo Ochoa’ Programme for Centres of Excellence in R&D (CEX2019-000917-S). Y.E.B acknowledges the financial support from CONACyT—Mexico through the postdoctoral scholarship granted 740661. P.G.L thanks the financial support of the Spanish Ministry of Economy, Industry, and Competitiveness through the grant BES-2017-081652 for the formation of scientific researchers. The authors acknowledge the support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).

The unsatisfactory cycle life of nickel-based cathodes hinders the widespread commercial usage of nickel–zinc (Ni–Zn) batteries. The most frequently used methods to improve the cycle life of Ni-based cathodes are usually complicated and/or involve using organic solvents and high energy consumption. A facile process based on the hydrolysis-induced exchange of the cobalt-based metal-organic framework (Co-MOF) was developed to prepare aluminum (Al)-doped cobalt–nickel double hydroxides (Al-CoNiDH) on a carbon cloth (CC). The entire synthesis process is highly efficient, energy-saving, and has a low negative impact on the environment. Compared to undoped cobalt–nickel double hydroxide (Al-CoNiDH-0%), the as-prepared Al-CoNiDH as the electrode material displays a remarkably improved cycling stability because the Al-doping successfully depresses the transition in the crystal phase and microstructure during the long cycling. Benefiting from the improved performance of the optimal Al-CoNiDH electrode (Al-CoNiDH-5% electrode), the as-constructed aqueous Ni–Zn battery with Al-CoNiDH-5% as the cathode (Al-CoNiDH-5%//Zn) displays more than 14% improvement in the cycle life relative to the Al-CoNiDH-0%//Zn battery. Moreover, this Al-CoNiDH-5%//Zn battery achieves a high specific capacity (264 mAh g−1), good rate capability (72.4% retention at a 30-fold higher current), high electrochemical energy conversion efficiency, superior fast-charging ability, and strong capability of reversible switching between fast charging and slow charging. Furthermore, the as-assembled quasi-solid-state Al-CoNiDH-5%//Zn battery exhibits a decent electrochemical performance and satisfactory flexibility.

Developing earth-rich and high-efficiency nonprecious metal catalysts is critical but extremely challenging for oxygen evolution reaction (OER). Herein, a simple room-temperature sulfuration method was developed for in situ synthesis of walnut kernel-like iron-cobalt-nickel sulfide nanosheets on nickel foam (FeCoNiSx/NF). The unique nanosheets exposed abundant active sites and provided large electrochemically active surface area. The as-built FeCoNiSx/NF exhibited high OER performances with the small overpotentials of 231 and 268 mV to afford 10 and 50 mA cm−2 in 1.0 M KOH, respectively, coupled with a small Tafel slope of 55 mV dec−1. Furthermore, the FeCoNiSx/NF acted as the anode towards overall water electrolysis with acceptable results, where commercial Pt/C dropped on the NF worked as the cathode. This study provides some valuable insights for rational construction of nonprecious electrocatalysts in electrochemical energy technologies.

Bi2O3 is a strong candidate as the optimal aqueous battery anode material, but suffers from its poor stability and rate performance (a result of fragile structure and poor conductivity). Herein, two self-derived Bi species, interstitial-Bi liberated from Bi2O3 unit cell and colonized surface nano-Bi, function synergistically in the Bi2O3 by creating a new electronic state in the energy gap and tuning the Bi/Bi2O3 interface coordination. As a result, electronic conductivity, ionic diffusion, and structural stability are fully enhanced, leading to a package of optimal electrochemical performances. Our self-enhanced strategy may thereof serve in design of functional oxide complexes for electrochemical energy conversion and storage.

Temperature and humidity measurements in electrochemical energy devices are essential for maximizing their overall performance under different operating conditions and avoiding hazardous consequences that may arise from the malfunction of these systems. Using sensors for in situ measurements of temperature and relative humidity (RH) is a promising approach for continuous monitoring and management of electrochemical power sources. Here, we report on the feasibility of using thread-based sensors for in situ measurements of temperature and RH in proton exchange membrane fuel cells (PEMFCs) as an example of electrochemical energy devices. Commodity threads are low-cost and flexible materials that hold great promise for the creation of complex three-dimensional (3D) circuits using well-established textile methods such as weaving, braiding, and embroidering. Ex situ and in situ characterization show that threads can be introduced in the gas diffusion layer (GDL) structure to inscribe water highways within the GDL with minimal impact on the GDL microstructure and transport properties. Fluorinated ethylene propylene (FEP) is coated on thread-based sensors to decouple the response to temperature and humidity; the resulting threads achieve a linear change of resistance with temperature (−0.31%/°C), while RH is monitored with a second thread coated with poly(dimethylsiloxane) (PDMS). The combination of both threads allows for minimally invasive and dynamically responsive monitoring of local temperature and RH within the electrode of PEMFCs.

Poly (2-acetyl pyrrole) P (2-APy) has been prepared chemically using ammonium persulfate (APS) as an initiator under high stirring at room temperature and pH (3.5). The formed P(2-APy) has been fully characterized via Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) as well as UV–VIS spectroscopy. The morphology of the formed polymer has shown core–shell of a ball-like structure in the range of nanoscale. The optical energy gap Eopt and the electrochemical energy gap Eg for the resultant polymer have been determined via UV–Vis spectroscopy and cyclic voltammetry, respectively. The ac conductivity measurements have been also investigated. The results showed enhanced optical as well as electrical properties for the formed polymer which can be further exploited for optoelectric devices.

Developing efficient catalysts toward the oxygen evolution reaction(OER) is important for water splitting and rechargeable metal-air batteries. Although NiFe oxides are considered as potentially applicable catalysts in the alkaline media, there are still a limited numbers of researches working on membrane electrode assembly(MEA) fed with pure water due to their poor electrical conductivity. In this work, antimony doped tin oxide(ATO) has been employed as conductive supports where NiFe layered double hydroxide uniformly dispersed[named NiFe-LDH(layered double hydroxide)/ATO]. The catalysts have been synthesized by a one-step co-precipitation method, and then NiFe-LDH/ATO-air plasma was obtained through mild air plasma treatment. According to XPS analysis, binding energies of Ni2p and Fe2p were shifted negatively. Moreover, a new signal of low oxygen coordination appeared on O1s spectrum after air plasma treatment. These XPS results indicated that oxygen vacancies(Ov) were generated after air plasma treatment. Electrochemical measurement indicated that the vacancy-rich NiFe-LDH/ATO-air plasma exhibited better performance than NiFe-LDH/ATO not only in 1 mol/L KOH solutions but also in an alkaline polymer electrolyte water electrolyzer(APEWE) fed with deionized water. This work provides a feasible way to design practical catalysts used in electrochemical energy conversion systems by choosing corrosion resistance supports and defect engineering.

Solid oxide fuel cells (SOFCs) have attracted great interest as an alternative potential way to become the most efficient and cleanest electrochemical energy conversion system. The commercialization of SOFC technology is hindered by the degradation of component materials. The durable and high performing cathode materials is of immense importance in the durability improvement of SOFCs. Cobaltite type perovskite-based oxides have shown remarkable results but cation migration and formation of the insulating phases within and near the interface between cathode and electrolyte is often observed, which impacts greatly on the electrochemical performance and durability. Therefore, the reaction barrier layer (interlayer) typically made of doped ceria is required between cathode and electrolyte. The stability of this layer due to cations cross-migration between cathode and electrolyte and interdiffusion with electrolyte during fabrication and operation is presently one of the foremost issues (motivation) in the SOFC industry. The chemical and structural disparity associated with the cations migration and interdiffusion could affect the stability and functionality of different layers of SOFC. Understanding the formation of secondary phases and their evolution during the operating lifespan is thought-provoking because of the complexity of the system and the occurrence of numerous other processes simultaneously. In this review paper, the recent progress and advancement in this extent are presented, emphasizing the key driving forces, kinetics, analysis techniques at the micro- and nano-scale levels, and cations migration in extensively studied perovskite-based materials. An insightful understanding of the interdiffusion phenomenon taking place in the cathode/electrolyte/interlayer of SOFCs and control measures are then highlighted which is important to achieve the rational design of highly efficient SOFC with outstanding stable performance.

Water splitting via an electrochemical process to generate hydrogen is an economic and green approach to resolve the looming energy and environmental crisis. The rational design of multicomponent materials with seamless interfaces having robust stability, facile scalability, and low-cost electrocatalysts is a grand challenge to produce hydrogen by water electrolysis. Herein, we report a superhydrophilic homogeneous bimetallic phosphide of Ni2P-CuP2 on Ni-foam-graphene-carbon nanotubes (CNTs) heterostructure using facile electrochemical metallization followed by phosphorization without any intervention of metal-oxides/hydroxides. This bimetallic phosphide shows ultralow overpotentials of 12 (HER, hydrogen evolution reaction) and 140 mV (OER, oxygen evolution reaction) at current densities of 10 and 20 mA/cm2 in acidic and alkaline mediums, respectively. The excellent stability lasts for at least for 10 days at a high current density of 500 mA/cm2 without much deviation, inferring the practical utilization of the catalyst toward green fuel production. Undoubtedly, the catalyst is capable enough for overall water splitting at a very low cell voltage of 1.45 V @10 mA/cm2 with an impressive stability of at least 40 h, showing a minimum loss of potential. Theoretical study has been performed to understand the reaction kinetics and d-band shifting among metal atoms in the heterostructure (Ni2P-CuP2) that favor the HER and OER activities, respectively. In addition, the catalyst demonstrates an alternate transformation of solar energy to green H2 production using a standard silicon solar cell. This work unveils a smart design and synthesizes a highly stable electrocatalyst against an attractive paradigm of commercial water electrolysis for renewable electrochemical energy conversion.

Electrode materials with high conductivity and high mass transport rate are highly desirable for a variety of electrochemical energy devices but face a grand challenge to be readily prepared yet. Here, we propose the design and preparation of a nanohybrid of V2O3 nanoparticles embedded in a multichannel carbon nanofiber (V2O3@MCNF) network with high conductivity and high mass transport. We demonstrate the V2O3@MCNF shows superior capability for sodium storage with an excellent capacity of 214.3 mA h g-1 even at 5 A g-1, thanks to its high conductivity for electron transfer and facilitated mass transportation endowed by the one-dimensional conductive multichannel fiber structure. Such favorable structures and properties in V2O3@MCNFs enable them to be applied as high-performance anodes of sodium-ion hybrid capacitors (SIHCs), successfully addressing the critical kinetics imbalance between Faradaic anodes and capacitive cathodes for application of SIHCs, which show impressively high energy/power densities along with impressive cycling performance over 10,000 cycles.

Nickel (Ni) and its (oxy)hydroxide species are an important class of materials that attract significant attention from the scientific community due to their application in alkaline electrochemical energy technologies. Despite their increasing importance, Ni electrochemistry and electrocatalysis are not very well understood. In this article, we contribute to the understanding of the interfacial electrochemical phenomena occurring at polycrystalline Ni electrodes by conducting the first ever systematic analysis of the electrochemical reduction of β-Ni(OH)2. The procedure for the reduction of surface β-Ni(OH)2 is as follows. First, a cyclic voltammetry (CV) measurement is performed in the potential region of β-Ni(OH)2 formation to grow a reproducible hydroxide layer, followed by a linear sweep voltammetry (LSV) measurement conducted between an upper limit potential of Eu = 0.00 V and a lower limit potential (El) ranging between −0.15 V and −0.60 V. The LSV measurement is carried out using very low potential scan rates (s = 0.10, 0.20, 0.50, and 1.0 mV s−1). The analysis of the electrochemical reducibility of the surface β-Ni(OH)2 layer is accomplished by comparing the CV transient of a freshly polished Ni electrode and the CV transient recorded after the LSV experiment that is designed to reduce the β-Ni(OH)2 layer. The influence of different experimental parameters of the above-described procedure is analyzed independently, namely, (i) the lower potential limit of the LSV measurement, (ii) the potential scan rate of the LSV measurement, (iii) the direction of the LSV measurement (anodic or cathodic), (iv) the thickness of the surface layer of β-Ni(OH)2, and (v) the cathodic polarization of the electrode for different polarization times. Our findings demonstrate that, contrary to an existing belief, the reduction of β-Ni(OH)2 can be achieved electrochemically. Features characteristic of the formation and reduction of α-Ni(OH)2 are observed in the CV transients acquired after employing the procedure for the electrochemical reduction of β-Ni(OH)2, indicative that a metallic Ni surface is regenerated. The analysis of the charge density value of the anodic peak in the CV transient also points to the electrochemical reduction of β-Ni(OH)2 to metallic Ni. These findings lead to an updated version of the Bode diagram and might have significant implications for alkaline electrochemical energy technologies using Ni-based materials.

Transition metal hydroxides (TMHs) nanosheets have attracted wide attention in electrochemical energy storage and conversion because of their superior surface area, highly tunable composition, and low cost. Moreover, the self-supported electrode has been extensively studied for electrochemical devices due to its fast electron transfer and mass transport, resulting in enhanced stability and electrode performance. Hence, reviewing the recent advances in self-supported TMHs nanosheets is crucial for developing high-performance electrodes for electrochemical devices. In this review, we first introduce the fundamental properties of TMHs in terms of layered single metal hydroxides (LSHs) and layered double hydroxides (LDHs). Then, we review various synthetic approaches utilized to construct self-supported TMHs nanosheets with tunable compositions and structures. Afterwards, the electrode performance and durability of self-supported TMHs nanosheets in various electrochemical applications (water electrolysis, zinc-air battery and supercapacitor) are comprehensively summarized. Finally, the further perspectives on current challenges and research directions of self-supported TMHs nanosheets towards electrochemical energy storages and conversion applications are proposed.

Nickel (Ni) is an important catalytic metal that finds application in electrochemical energy technologies, such as alkaline water electrolyzers (AWEs) and alkaline fuel cells (AFCs). An essential property that determines nickel’s applicability in AWE and AFC technologies is its stability under operating conditions, i.e., performance at extreme potential (E) conditions and in the presence of reactive gases. However, the electrochemical and electrocatalytic behavior of nickel is not as well understood as that of noble electrocatalytic metals such as platinum. This lack of knowledge needs to be addressed prior to dedicating significant efforts to the design, fabrication, and characterization of Ni-based materials for electrochemical energy technologies. We report experimental data on the catalytic activity and corrosion behavior of polycrystalline Ni in the presence of different dissolved gases. Cyclic voltammetry (CV) and potentiodynamic polarization (PDP) measurements are conducted at room temperature in 0.10 M aqueous NaOH solution saturated with either N2(diss), H2(diss), or O2(diss). CV measurements are performed using different potential scan rates in the regions of α-Ni(OH)2 and NiOOH formation/reduction. PDP measurements are conducted in the same electrolyte and in the presence of different dissolved gases but over a much broader potential range (−0.40 V ≤ E ≤ 2.20 V) and at a very low potential scan rate (s = 0.10 mV s−1) to achieve steady-state conditions. The influence of the state of the electrode’s surface (metallic versus oxidized) on the catalytic activity and corrosion behavior of nickel is also investigated by applying two different types of conditioning. The results demonstrate that the electrochemical behavior of Ni changes depending on the nature of the dissolved gas. The corrosion behavior is shown to depend on the polarization direction, thus the surface state of the electrode, and the nature of the dissolved gas. A graph, which is an extension of the Pourbaix diagram for Ni, summarizes the main interfacial and faradaic processes occurring at the surface of polycrystalline Ni in relation to the potential. The results and their analysis are expected to benefit renewable electrochemical energy technologies, such as alkaline water electrolyzers and fuel cells. In addition, they will serve as standards in electrochemical and electrocatalytic characterization of monocrystalline Ni materials and Ni-based nanomaterials.

The oxygen evolution reaction (OER) is considered an imperative step in the electrocatalytic water splitting process to obtain clean hydrogen fuel. The scarcity of noble metals impedes the practice of such technology at a commercial scale. To replace noble metal electrocatalysts with non-expensive and abundant materials is critical for the practical application of this technology. Herein, we report a novel approach to fabricate 1D nitrogen-doped carbon coated ternary NiCo2O4 nanorods (NiCo2O4@NC) derived from metal–organic frameworks (MOFs), which display a low overpotential (η ≈ 296 mV @10 mA cm−2), a small Tafel slope (53 mV dec−1) and excellent durability for OER in an alkaline medium, which is higher than for a commercially available Ir/C electrocatalyst (η ≈ 350 mV @10 mA cm−2). Such higher electrochemical properties can be primarily attributed to the high electrochemical surface area, the synergistic effect from chemical compositions and the unique mesoporous architecture with a support of nitrogen-doped carbon species. The proposed strategy for the controlled design and fine-tuning of MOF derived functional materials provides projections for producing remarkably active and stable electrocatalysts in electrochemical energy devices. This work presents a meaningful approach to understanding the effect of morphological engineering on electrocatalytic OER activity.

Supercapacitors (SCs) store electrochemical energy with high specific power, faster charge response, and long cycle life at an electrode–electrolyte interface; however, with lower specific energy than commercial batteries. In this article, structural, morphological, surface and electrochemical properties of a polymer–nanocomposite in the PANI–α-MnO2 system has been systematically investigated. The α-MnO2 was synthesized by molten salt methods and the polymeric composite was developed by in-situ polymerization. The materials were characterized by thermal analyses, X-ray and electron diffraction, FTIR spectroscopy, gas adsorption studies, scanning and transmission electron microscopy. The electrochemical properties of the materials before and after PANI modification are studied in 6 M KOH aqueous electrolyte employing cyclic voltammetry, galvanostatic charge–discharge cycling, and electrochemical impedance spectroscopy. A difference in charge storage mechanism from pseudocapacitive type to battery-type was observed upon PANI modification and the corresponding charge storage and charge kinetic parameters have been detailed.

To commercialize fuel cells and metal-air batteries, it is intensely desirable but challenging to design highly active, earth-abundant, low-cost and stable oxygen reduction reaction (ORR) catalysts to replace rare, precious and unstable Pt-based ones. Among them, Fe3N is one of the promising candidates to replace precious and unstable Pt-based ORR catalysis. Herein, to obtain Fe3N nanoparticles encapsulated in carbon frameworks with rich doped nitrogen, Fe2O3 nanoparticles are firstly prepared hydrothermally and converted to Fe3N, and then 3D zeolitic-imidazole frameworks (ZIF) is applied as a scaffold unit, followed by pyrolysis under ammonia gas. The product annealed at 900 °C (ZFN-900) displays a higher half-wave potential (25 mV) than a carbon-supported Pt catalyst in alkaline media. Remarkably, as air electrodes of the zinc-air battery, it exhibits a raised power density peak at 115.8 mW cm−2 with respect to carbon-supported Pt (81.6 mW cm−2) in addition to outstanding durability. Such superior catalytic properties are broadly accredited to the chemical structure synergistic effect, extraordinary surface area, rich active sites, conductive carbon frameworks with highly nitrogen doping. Without a doubt, this method sets out a novel pathway for advantageous and extremely active catalysts with an adapted design for electrochemical energy systems.

Design and fabrication of high-efficiency electrocatalysts are important to generate hydrogen via urea electrocatalysis with the minimum required energy. In this study, in situ growth of a branch-like Mo-doped Ni3S2 nanoforest on Ni foam was achieved through a facile one-pot hydrothermal process. The as-obtained catalyst exhibited excellent activity and robust durability because of its unique hierarchical nanostructures and doping-optimized electronic structural configuration, and required merely 1.33 V (vs. RHE) and 90 mV (overpotential) to attain a current density of 10 mA cm−2 for the urea oxidation reaction (UOR) and hydrogen evolution reaction (HER), without any noticeable loss in activity even after 120 h of operation. On the basis of the experimental results and theoretical calculations, it was confirmed that the incorporation of Mo in Ni3S2 altered the morphology and electronic structure of the catalyst; hence, more active sites were exposed and the Gibbs adsorption energy of the intermediates in the UOR was optimized. Notably, the overall urea electrolysis cell could afford a low voltage of 1.45 V (vs. RHE) to attain a current density of 10 mA cm−2 and showed excellent durability over 120 h. The findings from this study provide new insights into efficient electrocatalysts for urea electrolyzers, which hold great potential in electrochemical energy conversion and sewage treatment applications.

The development of cheap and efficient oxygen reduction and evolution reaction catalysts are important, which not only push the electrochemical energy systems including water electrolyzers, metal-air batteries, and fuel cells nearer to their theoretical limits but also become the substitute for the expensive noble metal catalysts (Pt/C, IrO2 or RuO2 and Pt-Ru/C). In this review, the recently reported potential graphene-based air catalysts such as graphene with non-metals, non-noble metals, metal oxides, nitrides, sulfides, carbides, and other carbon composites are identified in-light-of-their high oxygen reduction reaction/oxygen evolution reaction activity and zinc-air battery performance for the development of high energy density metal-air batteries. Further, the recent progress on the zinc-air batteries including the strategies used to improve the high cycling-performance (stable even up-to 394 cycles), capacity (even up-to 873 mAh g−1), power density (even up-to 350 mW cm−2), and energy density (even up-to 904 W h kg−1) are reviewed. The scientific and engineering knowledge acquired on zinc-air batteries provide conceivable development for practical application in near future.

Persistent magnesium (Mg) dissolution/deposition during cycling is crucial for the practical use of Mg rechargeable batteries, and the alloying-enhanced performance has recently attracted much attention. Nevertheless, the microscopic relationship among the alloys, the defects, and the performance remains under debate. Here, via comprehensive Density Functional Theory calculations, we revealed the effect of alloying-induced grain boundaries (GBs) and demonstrated a microscopic mechanism of how the GBs and alloys affect the performance. Mg atoms at the [0001](100) tilt GB and (110) surface are preferentially stripped during discharge, resulting in a “pit-type” morphology. Surprisingly, alloying does not change Mg's dissolution tendency at GBs. Instead, it can tune the number of tilt GBs, as alloying with Ca or Na can create more GBs than alloying with Li, Al, and Zn, resulting in improved discharge performance. Considering the experimental observation, we also propose a new picture of a GB-dependent electrochemical energy diagram extending from the conventional electrochemical theory.

The interaction between water molecules and surfaces in porous systems is of huge importance in various fields including but not limited to catalysis, adsorption, and the storage or conversion of electrochemical energy. Hydrophilicity of usually non-polar carbon-based materials can be enhanced by the incorporation of heteroatoms like nitrogen but insights into the interaction mechanisms on a molecular level remain hardly achievable due to the lack of porous carbons with a defined and regular atomic construction and a suitable structure model for such substances. This makes theoretical calculations difficult. In order to illuminate the interactions between nitrogen-containing carbon surfaces and water molecules, water adsorption is studied in this work on a series of model materials with different pore structures and atomic construction. In particular, volumetric adsorption measurements and 1H solid-state NMR spectroscopic investigations in combination with theoretical calculations on water in nitrogen-rich C2N materials show a remarkably strong heat of adsorption significantly above 60 kJ mol−1, that is, far beyond what is typically considered physisorption. In such porous materials, water is becoming an integral part of the chemical architecture, and the term “zeocarbons” is therefore well deserved. From a general perspective, this study is intended to set a base for establishing the chemical principles of a new water chemistry confined in strongly interacting nanopores.

Oxygen reduction reaction (ORR) plays a pivotal role in electrochemical energy conversion and commodity chemical production. Oxygen reduction involving a complete four-electron (4e-) transfer is important for the efficient operation of polymer electrolyte fuel cells, whereas the ORR with a partial 2e- transfer can serve as a versatile method for producing industrially important hydrogen peroxide (H2O2). For both the 4e- and 2e- pathway ORR, platinum-group metals (PGMs) have been materials of prevalent choice owing to their high intrinsic activity, but they are costly and scarce. Hence, the development of highly active and selective non-precious metal catalysts is of crucial importance for advancing electrocatalysis of the ORR. Heteroatom-doped carbon-based electrocatalysts have emerged as promising alternatives to PGM catalysts owing to their appreciable activity, tunable selectivity, and facile preparation. This review provides an overview of the design of heteroatom-doped carbon ORR catalysts with tailored 4e- or 2e- selectivities. We highlight catalyst design strategies that promote 4e- or 2e- ORR activity. We also summarise the major active sites and activity descriptors of the respective ORR pathways and describe the catalyst properties controlling the ORR mechanisms. We conclude the review with a summary and suggestions for future research.

The high value-added conversion of biomass lignin has been paramount in the field of lignin utilization, especially for high performance energy conversion and storage devices. A majority of lignin-based supercapacitors generally exhibit inferior electrochemical performance with low capacitance and slow diffusion kinetics due to the poor interfacial compatibility, low conductivity, and uncontrollable morphology. Herein, we designed all-lignin converted graphene quantum dot and graphene sheet (GQD/Gr) hetero-junction for simultaneous fast charging and boosted specific capacitance. The conversion from lignin to GQDs and then refusion into graphene allows the in situ growth of GQDs on graphene, endowing good interfacial compatibility with the GQD/Gr hetero-junction. Furthermore, both GQDs and graphene sheets exhibit highly crystalline structure with obvious graphene lattice, giving GQDs/Gr good conductivity. GQDs play an additive role for avoiding stacks and agglomerates between graphene layers, which endow the assembled GQDs/Gr with massive electron capacitive sites and more hierarchical channels. Therefore, the GQD/Gr hetero-junction gives rise to a high specific capacitance of 404.6 F g−1 and a short charging time constant (τ0) of 0.3 s, 2.5 times higher and 7.5 times faster than that of the unmodified lignin electrode with 162 F g−1 and 2.3 s, respectively. This proposed strategy could offer the opportunity to unblock the critical roadblocks for a superior electrochemical performance lignin-based supercapacitor by composing a 0D/2D GQD/Gr hetero-junction system and also paves a bright way for the high-value industrial lignin conversion into cheap, scalable, and high-performance electrochemical energy devices.

Water splitting is a sustainable method of qualitative as well as quantitative oxygen production to support future energy conversion systems and strictly depends on the nature of the electrocatalyst. Likewise, electrochemical energy storage systems particularly supercapacitors are of importance for future sustainability. Therefore, finding an efficient high-performance electrode material with both electrocatalytic activity and electrochemical energy storage capacity remains an emerging task for sustainability. In view of this challenge, we have derived a composite of partially graphitized carbon with Co2P2O7 (GC-CPP), a non-noble metal, low-cost electroactive material from open-framework cobalt phosphite. The crystal structure, surface morphology and elemental composition of the sample have been analyzed using PXRD, Raman, FESEM, HRTEM and X-ray photoelectron spectroscopy. More importantly, the electrochemical performances have been assessed using cyclic voltammetry, linear sweep voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopic methods. In alkaline electrolyte, GC-CPP catalyzes the OER requiring only 165 mV overpotential to reach 10 mA cm−2 current density with lower Tafel slope (41 mV dec−1). When used as an anode for a supercapacitor, GC-CPP shows higher specific capacitance (900 F g−1) compared to the pyrophosphates studied to date. Further, the high values of energy (31.25 W h kg−1) and power densities (21 kW kg−1) and cyclic durability prove the suitability for practical application. This strategy of deriving highly active electrode material from open frameworks provides an opportunity to design efficient and cost-effective electrode material for renewable energy technologies.

Global demand for energy conversion and storage technologies such as fuel cells, water electrolyzers, batteries, and supercapacitors is increasing, yet their commercial and environmental viability are critically dependent on the performance of their electrode materials and catalysts, which are the indispensable components that drive these systems. Among various materials, layered double hydroxides (LDHs) are considered promising candidates for catalysts and electrodes for electrochemical energy conversion and storage systems. Their diverse range of chemical properties make them highly versatile platforms for developing hybrid nanostructures, including flexible two-dimensional LDH nanostructures with various di-/tri-valent metals. Hybrid LDHs also exhibit unique structural attributes, including 3D hierarchical porous features and heterointerfaces, as well as optimized electrical conductivity and stability, which are crucial to achieving highly efficient multifunctional nanomaterials for electrochemical energy device applications. This review presents recent developments in the design, synthetic routes, structural/chemical modification strategies, and applications of hybrid LDHs as multifunctional nanomaterials for overall water splitting and electrochemical supercapacitors. Recent advances in modification strategies are critically assessed to determine their effect on the physicochemical properties of hybrid LDHs. The hybrid nanostructures' alteration of energy barriers in the electrocatalytic reactions is also discussed. Finally, this review concludes with future outlooks for hybrid LDH nanostructures.

As the world energy crisis remains a long-term challenge, development and access to renewable energy sources are crucial for a sustainable modern society. Electrochemical energy conversion devices are a promising option for green energy supply, although the challenge associated with electrocatalysis have caused increasing complexity in the materials and systems, demanding further research and insights. In this field, scanning probe microscopy (SPM) represents a specific source of knowledge and understanding. Thus, our aim is to present recent findings on electrocatalysts for electrolysers and fuel cells, acquired mainly through scanning electrochemical microscopy (SECM) and other related scanning probe techniques. This review begins with an introduction to the principles of several SPM techniques and then proceeds to the research done on various energy-related reactions, by emphasizing the progress on non-noble electrocatalytic materials., Investigation of electrocatalytic materials with scanning probe techniques (SECM, SICM, SECCM and AFM) for energy storage and conversion devices.

The oxygen evolution reaction (OER) plays a crucial role in many electrochemical energy technologies, and creating multiple beneficial factors for OER catalysis is desirable for achieving high catalytic efficiency. Here, we highlight a new halogen-chlorine (Cl)-anion doping strategy to boost the OER activity of perovskite oxides. As a proof-of-concept, proper Cl doping at the oxygen site of LaFeO3 (LFO) perovskite can induce multiple favorable characteristics for catalyzing the OER, including rich oxygen vacancies, increased electrical conductivity and enhanced Fe-O covalency. Benefiting from these factors, the LaFeO2.9-δCl0.1 (LFOCl) perovskite displays significant intrinsic activity enhancement by a factor of around three relative to its parent LFO. This work uncovers the effect of Cl-anion doping in perovskites on promoting OER performance and paves a new way to design highly efficient electrocatalysts.

This article explains and evaluates contemporary methods for manufacturing bipolar plates (BPPs) for low-temperature polymer electrolyte membrane fuel cells (LT-PEMFC) and highlights the potential of new improved approaches. BPPs are an essential component of fuel cells, responsible for distributing reaction gases to facilitate efficient conversion of gaseous electrochemical energy to electricity. BPPs must balance technical properties such as electrical and thermal conductivities, structural strength and corrosion resistance. Graphitic and metallic materials can meet the required specifications with each material offering distinct advantages and disadvantages. Each materials’ performance is complimented by a comparison of its manufacturability including: the material costs, production rates and required capital investment. These results are contextualised with respect to the target applications to identify the challenges and advantages of manufacturing methods of choice for BPPs. This analysis shows that the optimal choice of BPP manufacturing method depends entirely on the needs of the target application, in particular the relative importance of manufacturing rate, cost and the expected operational life of the bipolar plate to the fuel cell designer.