A comprehensive review on the electrochemical parameters and recent material development of electrochemical water splitting electrocatalysts

Electrochemical splitting of water is an appealing solution for energy storage and conversion to overcome the reliance on depleting fossil fuel reserves and prevent severe deterioration of the global climate. Though there are several fuel cells, hydrogen (H2) and oxygen (O2) fuel cells have zero carbon emissions, and water is the only by-product. Countless researchers worldwide are working on the fundamentals, i.e. the parameters affecting the electrocatalysis of water splitting and electrocatalysts that could improve the performance of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) and overall simplify the water electrolysis process. Noble metals like platinum for HER and ruthenium and iridium for OER were used earlier; however, being expensive, there are more feasible options than employing these metals for all commercialization. The review discusses the recent developments in metal and metalloid HER and OER electrocatalysts from the s, p and d block elements. The evaluation perspectives for electrocatalysts of electrochemical water splitting are also highlighted.

Advanced Spatiotemporal Voltammetric Techniques for Kinetic Analysis and Active Site Determination in the Electrochemical Reduction of CO2

ConspectusElectrochemical reduction of the greenhouse gas CO₂ offers prospects for the sustainable generation of fuels and industrially useful chemicals when powered by renewable electricity. However, this electrochemical process requires the use of highly stable, selective, and active catalysts. The development of such catalysts should be based on a detailed kinetic and mechanistic understanding of the electrochemical CO₂ reduction reaction (eCO₂RR), ideally through the resolution of active catalytic sites in both time (i.e., temporally) and space (i.e., spatially). In this Account, we highlight two advanced spatiotemporal voltammetric techniques for electrocatalytic studies and describe the considerable insights they provide on the eCO₂RR. First, Fourier transformed large-amplitude alternating current voltammetry (FT ac voltammetry), as applied by the Monash Electrochemistry Group, enables the resolution of rapid underlying electron-transfer processes in complex reactions, free from competing processes, such as the background double-layer charging current, slow catalytic reactions, and solvent/electrolyte electrolysis, which often mask conventional voltammetric measurements of the eCO₂RR. Crucially, FT ac voltammetry allows details of the catalytically active sites or the rate-determining step to be revealed under catalytic turnover conditions. This is well illustrated in investigations of the eCO₂RR catalyzed by Bi where formate is the main product. Second, developments in scanning electrochemical cell microscopy (SECCM) by the Warwick Electrochemistry and Interfaces Group provide powerful methods for obtaining high-resolution activity maps and potentiodynamic movies of the heterogeneous surface of a catalyst. For example, by coupling SECCM data with colocated microscopy from electron backscatter diffraction (EBSD) or atomic force microscopy, it is possible to develop compelling correlations of (precatalyst) structure-activity at the nanoscale level. This correlative electrochemical multimicroscopy strategy allows the catalytically more active region of a catalyst, such as the edge plane of two-dimensional materials and the grain boundaries between facets in a polycrystalline metal, to be highlighted. The attributes of SECCM-EBSD are well-illustrated by detailed studies of the eCO₂RR on polycrystalline gold, where carbon monoxide is the main product. Comparing SECCM maps and movies with EBSD images of the same region reveals unambiguously that the eCO₂RR is enhanced at surface-terminating dislocations, which accumulate at grain boundaries and slip bands. Both FT ac voltammetry and SECCM techniques greatly enhance our understanding of the eCO₂RR, significantly boosting the electrochemical toolbox and the information available for the development and testing of theoretical models and rational catalyst design. In the future, it may be possible to further enhance insights provided by both techniques through their integration with in situ and in operando spectroscopy and microscopy methods.

3D Self-Supported Nitrogen-Doped Carbon Nanofiber Electrodes Incorporated Co/CoOx Nanoparticles: Application to Dyes Degradation by Electro-Fenton-Based Process

We developed free-standing nitrogen-doped carbon nanofiber (CNF) electrodes incorporating Co/CoO_x nanoparticles (NPs) as a new cathode material for removing Acid Orange 7 (AO7; a dye for wool) from wastewater by the heterogeneous electro-Fenton reaction. We produced the free-standing N-doped CNF electrodes by electrospinning a polyacrylonitrile (PAN) and cobalt acetate solution followed by thermal carbonation of the cobalt acetate/PAN nanofibers under a nitrogen atmosphere. We then investigated electro-Fenton-based removal of AO7 from wastewater with the free-standing N-doped-CNFs-Co/CoO_x electrodes, in the presence or not of Fe²⁺ ions as a co-catalyst. The electrochemical analysis showed the high stability of the prepared N-doped-CNF-Co/CoO_x electrodes in electrochemical oxidation experiments with excellent degradation of AO7 (20 mM) at acidic to near neutral pH values (3 and 6). Electro-Fenton oxidation at 10 mA/cm² direct current for 40 min using the N-doped-CNF-Co/CoO_x electrodes loaded with 25 wt% of Co/CoO_x NPs led to complete AO7 solution decolorization with total organic carbon (TOC) removal values of 92.4% at pH 3 and 93.3% at pH 6. The newly developed N-doped-CNF-Co/CoO_x electrodes are an effective alternative technique for wastewater pre-treatment before the biological treatment.

Two-Dimensional Electrocatalysts for Efficient Reduction of Carbon Dioxide

Two-dimensional (2D) materials are attractive catalysts for the electrochemical reduction of carbon dioxide reaction (eCO₂ RR) by virtue of their tunable atomic structures, abundant active sites, enhanced conductivity, suitable binding affinity to carbon dioxide and/or reaction intermediates, and intrinsic scalability. Herein, recent advances in 2D catalysts for the eCO₂ RR are reviewed. Structural features and properties of 2D materials that contribute to their advanced electrocatalytic properties are summarized, and strategies for enhancing their activity and selectivity for the eCO₂ RR are reviewed. Prospects and challenges of applications of 2D catalysts for the eCO₂ RR on an industrial scale are highlighted.

How many surface atoms in Co3O4 take part in oxygen evolution? Isotope labeling together with differential electrochemical mass spectrometry

Understanding the mechanism underlying the oxygen evolution reaction (OER) on oxides is crucial for the development of many energy storage systems. Here, the mechanism of OER on a Co₃O₄ spinel catalyst is investigated in alkaline media using ¹⁸O-labeling combined with differential electrochemical mass spectrometry (DEMS). This work unravels the role of surface oxygen of the oxide in the OER. It is shown that in H₂¹⁸O-containing electrolyte the amount of ¹⁸O¹⁶O evolved increases from cycle to cycle together with a concomitant decrease of the amount of ¹⁶O₂ with each cycle before reaching a steady-state value. ¹⁸O¹⁶O is also evolved from a H₂¹⁶O solution on a Co₃O₄ electrode pre-treated in H₂¹⁸O-containing solution, indicating the formation of the ¹⁸O-labeled oxide in the previous step. Therefore, the oxide layer takes part in OER via an oxygen exchange mechanism. The total number of oxygen atoms of the oxide participating in OER is 0.1 to 0.2% of the total oxide loading, corresponding to about 10-30% of the surface atoms; these represent the catalytically active sites. Moreover, the real surface area of the catalyst is estimated using different methods (namely the ball model, double layer capacitance method, redox peak method, isotope exchange), and compared to the BET data. The surface areas calculated from the BET data, ball model and redox peak method are similar for small particles, which indicates their smooth surface; however they are smaller than that estimated from double-layer capacitance. For larger particles, the much larger surface area estimated from the redox peak in comparison to that expected from the ball model seems to be due to their roughness. Thus, this work highlights the importance of probing the mechanism when investigating the OER activity of a catalyst.

Petal-like hierarchical array of ultrathin Ni(OH)2 nanosheets decorated with Ni(OH)2 nanoburls: a highly efficient OER electrocatalyst

A solvothermally synthesized petal-like 3D hierarchical array of β-Ni(OH)₂ nanosheets and nanoburls has displayed an abnormal enhancement in activity in the OER as a consequence of surface faceting of planes from (001) to (101) and (202) within a minimum number of potential sweeps in addition to the formation of oxyhydroxide.

Transition-Metal (Co, Ni, and Fe)-Based Electrocatalysts for the Water Oxidation Reaction

Increasing energy demands and environment awareness have promoted extensive research on the development of alternative energy conversion and storage technologies with high efficiency and environmental friendliness. Among them, water splitting is very appealing, and is receiving more and more attention. The critical challenge of this renewable-energy technology is to expedite the oxygen evolution reaction (OER) because of its slow kinetics and large overpotential. Therefore, developing efficient electrocatalysts with high catalytic activities is of great importance for high-performance water splitting. In the past few years, much effort has been devoted to the development of alternative OER electrocatalysts based on transition-metal elements that are low-cost, highly efficient, and have excellent stability. Here, recent progress on the design, synthesis, and application of OER electrocatalysts based on transition-metal elements, including Co, Ni, and Fe, is summarized, and some invigorating perspectives on the future developments are provided.

Novel enzymatic glucose biosensor based on distributed electrodes covered with a solvothermal synthesized graphene material and platinum nanoparticles

Series connected distributed sensing electrode patterned with TRGO decorated platinum nanoparticles is a good platform for enzymatic glucose sensor.

Lindqvist Polyoxoniobate Ion-Assisted Electrodeposition of Cobalt and Nickel Water Oxidation Catalysts

A method has been developed for the efficient electrodeposition of cobalt and nickel nanostructures with the assistance of the Lindqvist ion [Nb6O19](8-). Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectrometry, and a range of electrochemical techniques have been used to characterize the morphology, composition, catalytic water oxidation activity and stability of the films in alkaline solution. SEM images show that films consisting of nanoparticles with diameters of ca. 30 to 40 nm are formed after 40-50 potential cycles of deposition. Nb and Co/Ni are detected in the films by EDX. ICP-MS results show an elemental ratio of 1:1 for Co:Nb and 1:3 for Ni:Nb, respectively. Raman spectra reveal the presence of both [Nb6O19](8-) and Co(OH)2/Ni(OH)2. The films exhibit excellent stability and efficiency for electrocatalytic water oxidation in alkaline solution. Turnover frequencies of 12.9 and 13.2 s(-1) were determined by rotating ring disk electrode voltammetry at an overpotential of 480 mV for Co and Ni films, respectively. Fourier transformed large amplitude alternating current (FTAC) voltammetry reveals an additional underlying oxidation process for Co under catalytic turnover conditions, which indicates that a Co(IV) species is involved in the efficient catalytic water oxidation reactions. FTAC voltammetric data also suggest that the Ni films undergoes a clear phase transformation upon aging in aqueous 1 M NaOH and the electrogenerated higher oxidation state Ni from β-NiOOH is the more active form of the catalyst.

Reduction of graphene oxide – a comprehensive electrochemical investigation in alkaline and acidic electrolytes

Electrochemical characterization to investigate the extent of reduction and the nature of reminiscent oxygen moieties in GO-based materials.