Comparison of the Spinels Co 3 O 4 and NiCo 2 O 4 as Bifunctional Oxygen Catalysts in Alkaline Media

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Optimized Nanopores Opened on N-Doped Carbon Nanohorns Filled with Fe/Fe2O3 Nanoparticles as Advanced Electrocatalysts for the Oxygen Evolution Reaction

N-doped carbon nanohorns filled with Fe nanoparticles (Fe-N-CNHs) were produced by one-step positive pressure-assisted arc discharge in the Ar and N₂ mixture. After oxidation treatments in air, Fe was converted into Fe₂O₃, and nanopores were opened on CNHs from 1 to 5 nm controlled by oxidation temperature. Fe-N-CNHs oxidized in O₂ at 550 °C (Fe₂O₃-N-CNH550ox) show 245 mV at 20 mA cm^-1, which is much smaller than that of the ones oxidized at 500 °C (Fe₂O₃-N-CNH500ox), contributing to the larger pore size on CNHs (3-5 nm vs 2-3 nm) and a larger number of nanopores caused by the enhanced sidewall nanopores. However, the stability of Fe₂O₃-N-CNH550ox becomes much poorer than that of Fe₂O₃-N-CNH500ox after 2000 cycles. The unique relationship between the overpotential and long-term stability can be explained by the consideration of the size of Fe₂O₃ nanoparticles and nanopores on CNHs. Furthermore, the stability for Fe₂O₃-N-CNH550ox can be rapidly increased after heat treatment in Ar for 1 h caused by shrinking the size of tip nanopores. Herein, we first reveal that the performance of OER is related to the nanopore size of carbon carriers and the catalyst of nanometal particles. The optimization of pore-opening conditions in carbon carriers can be achieved a superior electrocatalytic OER performance, including a low overpotential at high current density and long-term stability.

Gas diffusion electrodes for H2O2 production and their applications for electrochemical degradation of organic pollutants in water: A review

Nowadays, it is a great challenge to minimize the negative impact of hazardous organic compounds in the environment. Highly efficient hydrogen peroxide (H₂O₂) production through electrochemical methods with gas diffusion electrodes (GDEs) is greatly demand for degradation of organic pollutants that present in drinking water and industrial wastewater. The GDEs as cathodic electrocatalyst manifest more cost-effective, lower energy consumption and higher oxygen utilization efficiency for H₂O₂ production as compared to other carbonaceous cathodes due to its worthy chemical and physical characteristics. In recent years, the crucial research and practical application of GDE for degradation of organic pollutants have been well developed. In this review, we focus on the novel design, fundamental aspects, influence factors, and electrochemical properties of GDEs. Furthermore, we investigate the generation of H₂O₂ through electrocatalytic processes and degradation mechanisms of refractory organic pollutants on GDEs. We describe the advanced methodologies towards electrochemical kinetics, which include the enhancement of GDEs electrochemical catalytic activity and mass transfer process. More importantly, we also highlight the other technologies assisted electrochemical process with GDEs to enlarge the practical application for water treatment. In addition, the developmental prospective and the existing research challenges of GDE-based electrocatalytic materials for real applications in H₂O₂ production and wastewater treatment are forecasted. According to our best knowledge, only few review articles have discussed GDEs in detail for H₂O₂ production and their applications for degradation of organic pollutants in water.

Metal free-covalent triazine frameworks as oxygen reduction reaction catalysts – structure–electrochemical activity relationship

A covalent triazine framework coated on glassy carbon electrode performs high catalytic activity towards the ORR.

An Electrically Rechargeable Zinc/Air Cell with an Aqueous Choline Acetate Electrolyte

Due to the feasibility of an electrically rechargeable zinc/air cell made of a zinc foil as active material, an aqueous choline acetate (ChAcO) mixture as an electrolyte and a spinel MnCo₂O₄ (MCO) and NiCo₂O₄ (NCO) as a bi-functional oxygen catalyst was investigated in this work. The 30 wt.% water-containing aqueous ChAcO solution showed high contact angles close to those of KOH favoring triple-phase boundary formation in the gas diffusion electrode. Conductivity and pH value of 30 wt.% H₂O/ChAcO amounted to 5.9 mS cm⁻¹ and 10.8, respectively. Best results in terms of reversible capacity and longevity of zinc/air cell were yielded during 100 h charge/discharge with the MnCo₂O₄ (MCO) air electrode polarization procedure at 100 µA cm⁻² (2.8 mA g⁻¹_zinc). The corresponding reversible capacity amounted to 25.4 mAh (28% depth of discharge (DOD)) and the energy efficiency ranged from 29–54% during the first and seventh cycle within a 1500 h polarization period. Maximum active material utilization of zinc foil at 100 µA cm⁻² was determined to 38.1 mAh (42% DOD) whereas a further charging step was not possible due to irreversible passivation of the zinc foil surface. A special side-by-side optical cell was used to identify reaction products of the zinc/air system during a single discharge step in aqueous ChAcO that were identified as Zn(OH)₂ and ZnO by Raman analysis while no carbonate was detected.

Directly Electrospun Carbon Nanofibers Incorporated with Mn3O4 Nanoparticles as Bending-Resistant Cathode for Flexible Al-Air Batteries

Al-air batteries are regarded as potential power source for flexible and wearable devices. However, the traditional cathodes of Al-air batteries are easy to be broken after continuous bending. This is why few Al-air batteries have been tested under the state of dynamic bending so far. Herein, carbon nanofibers incorporated with Mn₃O₄ catalyst have been prepared as bending-resistant cathodes through direct electrospinning. The cathode assembled in Al-air battery showed excellent electrochemical and mechanical stability. A high specific capacity of 1021 mAh/cm² was achieved after bending 1000 times, which is 81.7% of that in platform state. This work will facilitate the progress of using Al-air battery in flexible electronics.

Recent Progresses in Oxygen Reduction Reaction Electrocatalysts for Electrochemical Energy Applications

Abstract

Electrochemical energy storage systems such as fuel cells and metal–air batteries can be used as clean power sources for electric vehicles. In these systems, one necessary reaction at the cathode is the catalysis of oxygen reduction reaction (ORR), which is the rate-determining factor affecting overall system performance. Therefore, to increase the rate of ORR for enhanced system performances, efficient electrocatalysts are essential. And although ORR electrocatalysts have been intensively explored and developed, significant breakthroughs have yet been achieved in terms of catalytic activity, stability, cost and associated electrochemical system performance. Based on this, this review will comprehensively present the recent progresses of ORR electrocatalysts, including precious metal catalysts, non-precious metal catalysts, single-atom catalysts and metal-free catalysts. In addition, major technical challenges are analyzed and possible future research directions to overcome these challenges are proposed to facilitate further research and development toward practical application.

Graphic Abstract

Morphology Control of Carbon-Free Spinel NiCo2O4 Catalysts for Enhanced Bifunctional Oxygen Reduction and Evolution in Alkaline Media

Spinel NiCo₂O₄ is considered a promising precious metal-free catalyst that is also carbon-free for oxygen electrocatalysis. Current efforts mainly focus on optimal chemical doping and substituent to tune its electronic structures for enhanced activity. Here, we study its morphology control and elucidate the morphology-dependent catalyst performance for bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Three types of NiCo₂O₄ catalysts with significantly distinct morphologies were prepared using temple-free, Pluronic-123 (P-123) soft, and SiO₂ hard templates, respectively, via hydrothermal methods followed by calcination. Whereas the hard-template yields spherelike dense structures, soft-template assists the formation of a unique nanoneedle cluster assembly containing abundant meso- and macropores. Furthermore, the effect of morphology of NiCo₂O₄ on their corresponding bifunctional catalytic performance was systematically investigated. The flowerlike nanoneedle assembly NiCo₂O₄ catalyst via the soft-template method exhibited the highest catalytic activity and stability for both ORR and OER. In particular, it exhibited an onset and half-wave potentials of 0.94 and 0.82 V versus reversible hydrogen electrode, respectively, for the ORR in alkaline media. Although it is still inferior to Pt, the NiCo₂O₄ represents one of the best ORR catalyst compared to other reported carbon-free oxides. Meanwhile, remarkable OER activity and stability were achieved with an onset potential of 1.48 V and a current density of 15 mA/cm² at 1.6 V, showing no activity loss after 20 000 potential cycles (0-1.9 V). The demonstrated stability is even superior to Ir for the OER. The morphology-controlled approach provides an effective solution to create a robust three-dimensional architecture with increased surface areas and enhanced mass transfer. Importantly, the soft template can yield a high degree of spinel crystallinity with ideal stoichiometric ratios between Ni and Co, thus promoting structural integrity with enhanced electrical conductivity and catalytic properties.