This is a focused review of recent highlights in the literature in cathode development for low temperature electrochemical carbon dioxide and carbon monoxide reduction to multi-carbon (C2+) products. The major goals for the field are to increase Faradaic efficiency (FE) for specific C2+ products, lower cell voltage for industrially relevant current densities and increase cell lifetime. A key to achieving these goals is the rational design of cathodes through increased understanding of structure-selectivity and structure-activity relationships for catalysts and the influence of catalyst binders and gas diffusion layers (GDLs) on the catalyst microenvironment and subsequent performance.
Bismuth vanadate (BiVO4) is proven to be a promising photocatalyst for water splitting. However, the effect of materials syntheses, electrode preparation and size of photoelectrode on the photocurrent output of BiVO4 photoanodes needs further investigations. In this study, three different BiVO4 nanoparticle synthesis were employed, namely hydrothermal (HT), HT in the presence of ethylene glycol (EG) and HT with the addition of hydrazine hydrate (HH). In addition, two molecular inks (Triton-X and ethyl‐methyl‐imidazole, EMI), were compared for the preparation of BiVO4 photoanodes using a simple doctor-blade technique followed by calcination at 450°C. The photoanodes (9 cm2 active surface) were then compared for their photocurrent density at AM1.5G illumination and 1.2 V (vs . standard hydrogen electrode (SHE)) bias in a specifically designed, three-dimensional (3D)-printed electrochemical cell. The highest photocurrent 0.13 ± 0.1 mA cm–2 was obtained with the EMI ink, whereas tenfold lower photocurrent was obtained with Triton-X due to the higher charge transfer resistance, measured by electric impedance spectroscopy (EIS). The photoresponse was reproducible and relatively stable, with only 8% decrease in five consecutive illumination periods of 1 min.
We review recent research into oxides of platinum group metals (pgms), in particular those of ruthenium and iridium, for use as electrocatalysts for the oxygen evolution reaction (OER). These are used in membrane electrode assemblies (MEAs) in devices such as electrolysers, for water splitting to generate hydrogen as fuel, and in fuel cells where they provide a buffer against carbon corrosion. In these situations, proton exchange membrane (PEM) layers are used, and highly acid-resilient electrocatalyst materials are required. The range of structure types investigated includes perovskites, pyrochlores and hexagonal perovskite-like phases, where the pgm is partnered by base metals in complex chemical compositions. The role of chemical synthesis in the discovery of new oxide compositions is emphasised, particularly to yield powders for processing into MEAs. Part I introduces the electrocatalytic splitting of water to oxygen and hydrogen and provides a survey of ruthenium and iridium oxide structures for oxygen evolution reaction catalysis.
We continue our review of recent research into oxides of platinum group metals (pgms), in particular those of ruthenium and iridium, for use as electrocatalysts for the oxygen evolution reaction (OER). In Part I (1), the electrocatalytic splitting of water to oxygen and hydrogen was introduced as a key process in developing future devices for various energy-related applications. A survey of ruthenium and iridium oxide structures for oxygen evolution reaction catalysis was presented. Part II discusses mechanistic details and acid stability of pgm oxides and presents the conclusions and outlook. We highlight emerging work that shows how leaching of the base metals from the multinary compositions occurs during operation to yield active pgm-oxide phases, and how attempts to correlate stability with crystal structure have been made. Implications of these discoveries for the balance of activity and stability needed for effective electrocatalysis in real devices are discussed.
Traditional microbial synthesis of chemicals and fuels often rely on energy-rich feedstocks such as glucose, raising ethical concerns as they are directly competing with the food supply. Therefore, it is imperative to develop novel processes that rely on cheap, sustainable and abundant resources whilst providing carbon circularity. Microbial electrochemical technologies (MET) offer unique opportunities to facilitate the conversion of chemicals to electrical energy or vice versa, by harnessing the metabolic processes of bacteria to valorise a range of waste products, including greenhouse gases (GHGs). However, the strict growth and nutrient requirements of industrially relevant bacteria, combined with low efficiencies of native extracellular electron transfer (EET) mechanisms, reduce the potential for industrial scalability. In this two-part work, we review the most significant advancements in techniques aimed at improving and modulating the efficiency of microbial EET, giving an objective and balanced view of current controversies surrounding the physiology of microbial electron transfer, alongside the methods used to wire microbial redox centres with the electrodes of bioelectrochemical systems via conductive nanomaterials.
It is imperative to develop novel processes that rely on cheap, sustainable and abundant resources whilst providing carbon circularity. Microbial electrochemical technologies (MET) offer unique opportunities to facilitate the conversion of chemicals to electrical energy or vice versa by harnessing the metabolic processes of bacteria to valorise a range of waste products including greenhouse gases (GHGs). Part I (1) introduced the EET pathways, their limitations and applications. Here in Part II, we outline the strategies researchers have used to modulate microbial electron transfer, through synthetic biology and biohybrid approaches and present the conclusions and future directions.
In recent years, sodium-ion batteries (NIBs) have been explored as an alternative technology to lithium-ion batteries (LIBs) due to their cost-effectiveness and promise in mitigating the energy crisis we currently face. Similarities between both battery systems have enabled fast development of NIBs, however, their full commercialisation has been delayed due to the lack of an appropriate anode material. Hard carbons (HCs) arise as one of the most promising materials and are already used in the first generation of commercial NIBs. Although promising, HCs exhibit lower performance compared to commercial graphite used as an anode in LIBs in terms of reversible specific capacity, operating voltage, initial coulombic efficiency and cycling stability. Nevertheless, these properties vary greatly depending on the HC in question, for example surface area, porosity, degree of graphitisation and defect amount, which in turn are dependent on the synthesis method and precursor used. Optimisation of these properties will bring forward the widespread commercialisation of NIBs at a competitive level with current LIBs. This review aims to provide a brief overview of the current understanding of the underlying reaction mechanisms occurring in the state-of-the-art HC anode material as well as their structure-property interdependence. We expect to bring new insights into the engineering of HC materials to achieve optimal, or at least, comparable electrochemical performance to that of graphite in LIBs.
To combat the global problem of carbon dioxide emissions, hydrogen is the desired energy vector for the transition to environmentally benign fuel cell power. Water electrolysis (WE) is the major technology for sustainable hydrogen production. Despite the use of renewable solar and wind power as sources of electricity, one of the main barriers for the widespread implementation of WE is the scarcity and high cost of platinum group metals (pgms) that are used to catalyse the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER). Hence, the critical pgm-based catalysts must be replaced with more sustainable alternatives for WE technologies to become commercially viable. This critical review describes the state-of-the-art pgm-free materials used in the WE application, with a major focus on phosphides and borides. Several emerging classes of HER and OER catalysts are reviewed and detailed structure–property correlations are comprehensively summarised. The influence of the crystallographic and electronic structures, morphology and bulk and surface chemistry of the catalysts on the activity towards OER and HER is discussed.
