The impact of laser-scribing carbon-based supercapacitor electrodes

In highly porous carbon electrodes, a large fraction of pores can be inaccessible to the electrolyte, which translates into lower specific capacitances. This is accentuated at high current densities. To circumvent this, channels can be opened to enhance ionic diffusion. In this work, ionic channels were created using a pulsed laser. Nine sets of laser-scribing parameters (pulse fluence and spot spacing) were applied on two sets of carbon-based supercapacitor electrodes: K-bar hand-coated electrodes (“K”) and screen-printed electrodes (“SP”). Profilometry and scanning electron microscopy revealed that, before laser-scribing, the latter already had several holes and trenches, whilst the former were compact films. Electrochemical measurements in Na2SO4 indicate improvements in the rate capability of the laser-scribed SP electrodes, namely an up to 50% reduction of the rate at which energy density decreases as power densities increase. For laser-scribed K electrodes, the slope of the Ragone plot only decreased by ca. 20% in the best set of conditions. However, for both sets of electrodes, a negative trade-off is observed: laser processed electrodes seem to have a lower specific capacitance. This might be caused by the entrapment of debris in the laser-drilled holes, which could lead to the overestimation of the active mass. Moreover, X-ray Photoelectron Spectroscopy analysis suggests that this may also be explained by the decrease in the oxygen functionalities and by its impact on the electrodes’ wettability. On the other hand, for electrodes tested in an organic electrolyte (tetrabutylammonium perchlorate in acetonitrile), the specific capacitance at 2 A/g was up to 66% higher for laser-scribed electrodes and an energy density of 13 Wh/kg was achieved even at 2.8 kW/kg

Hydrogen technology adoption analysis in Africa using a Doughnut-PESTLE hydrogen model (DPHM)

The hydrogen economy requires the right conditions to produce hydrogen by sustainable routes and provide it to local and international markets for suitable applications. This study evaluated the political, economic, social, technological, legal, and environmental (PESTLE) conditions that can be instrumental in adopting hydrogen technologies most effectively by encapsulating aspects relevant to key stakeholders from hydrogen technology developers through to end-users. For instance, the analysis has shown that countries within a government effectiveness index of 0.5 and −0.5 are leading the planning of hydrogen economies through strategic cooperation with hydrogen technology developers. Furthermore, the combination of a Doughnut and PESTLE analysis created a novel approach to assessing the adoption of hydrogen technologies while evaluating the impact of the hydrogen economy. For instance, the estimated ammonia demand in 2050 and subsequent anthropogenic nitrogen extraction rate will be about two and a half times more than the 2009 extraction rate

Using machine learning to expound energy poverty in the global south: understanding and predicting access to cooking with clean energy

Efforts towards achieving high access to cooking with clean energy have not been transformative due to a limited understanding of the clean-energy drivers and a lack of evidence-based clean-energy policy recommendations. This study addresses this gap by building a high-performing machine learning model to predict and understand the mechanisms driving energy poverty - specifically access to cooking with clean energy. In a first-of-a-kind, the estimated cost of US14.5 to enable universal access to cooking with clean energy encompasses all the intermediate inputs required to build self-sufficient ecosystems by creating value-addition sectors. Unlike previous studies, the data-driven clean-cooking transition pathways provide foundations for shaping policy that can transform the energy and cooking landscape. Developing these pathways is necessary to increase people's financial resilience to tackle energy poverty. The findings also show the absence of a linear relationship between electricity access and clean cooking - evidencing the need for a rapid paradigm shift to address energy poverty. A new fundamental approach that focuses on improving and sustaining the financial capacity of households through a systems approach is required so that they can afford electricity or fuels for cooking.Engineering and Physical Sciences Research Council (EPSRC): EP/S023909/

New insights into water splitting at mesoporous α-Fe₂O₃ films: a study by modulated transmittance and impedance spectroscopies

Closed access. This article was published in the Journal of the American Chemical Society [© American Chemical Society] and the definitive version is available at: http://dx.doi.org/10.1021/ja209530sThin mesoporous films of α-Fe2O3 have been prepared on conducting glass substrates using layer-by-layer self-assembly of ca. 4 nm hydrous oxide nanoparticles followed by calcining. The electrodes were used to study the oxygen evolution reaction (OER) in the dark and under illumination using in situ potential-modulated absorption spectroscopy (PMAS) and light-modulated absorption spectroscopy (LMAS) combined with impedance spectroscopy. Formation of surface-bound higher-valent iron species (or “surface trapped holes”) was deduced from the PMAS spectra measured in the OER onset region. Similar LMAS spectra were obtained at more negative potentials in the onset region of photoelectrochemical OER, indicating involvement of the same intermediates. The impedance response of the mesoporous α-Fe2O3 electrodes exhibits characteristic transmission line behavior that is attributed to slow hopping of holes, probably between surface iron species. Frequency-resolved PMAS and LMAS measurements revealed slow relaxation behavior that can be related to the impedance response and that indicates that the lifetime of the intermediates (or trapped holes) involved in the OER is remarkably long

Superior rate capability of high mass loading supercapacitors fabricated with carbon recovered from methane cracking

High mass loading (ca. 30 mg/cm2) electrodes were prepared with carbon recovered from catalytic methane cracking (MC). As-fabricated supercapacitors displayed 74% of capacitance retention from 6 mA/cm2 to 60 mA/cm2 and a Ragone plot’s slope of −7 Wh/kW (compared to 42% and −31 Wh/kW, respectively, for high mass loading devices fabricated with commercial carbon). The high-rate capability of the MC-recovered carbon is attributed to the presence of carbon black and carbon nanotubes produced during the reaction, which likely increased the electronic and ionic conductivity within the electrode. These results suggest that the by-product of this hydrogen generation route might be a suitable active material for supercapacitors.Engineering and Physical Sciences Research Council (EPSRC): EP/R023662/1; EP/S023909/

Kinetics and mechanism of light-driven oxygen evolution at thin film α-Fe₂O₃ electrodes

Belgium Herbarium image of Meise Botanic Garden

Direct monitoring of the potassium charge carrier in Prussian blue cathodes using potassium K-edge X-ray absorption spectroscopy †

Prussian blue is widely utilized as a cathode material in batteries, due to its ability to intercalate alkaline metal ions, including potassium. However, the exact location of potassium or other cations within the complex structure, and how it changes as a function of cycling, is unclear. Herein, we report direct insight into the nature of potassium speciation within Prussian blue during cyclic voltammetry, via operando potassium K-edge X-ray Absorption Near Edge Structure (XANES) analysis. Clear and identifiable spectra are experimentally differentiated for the fully intercalated (fully reduced Fe2+FeII Prussian white), partially intercalated (Prussian blue; Fe3+FeII), and free KNO3(aq) electrolyte. Comparison of the experiment with simulated XANES of theoretical structures indicates that potassium lies within the channels of the Prussian blue structure, but is displaced towards the periphery of the channels by occluded water and/or structural water present resulting from [Fe(CN)6]4− vacancies. The structural composition from the charge carrier perspective was monitored for two samples of differing crystallite size and electrochemical stability. Reproducible potassium XANES spectral sequences were observed for large crystallites (ca. 100 nm) of Prussian blue, in agreement with retention of capacity; in contrast, the capacity of a sample with small crystallites (ca. 14 nm) declined as the potassium became trapped within the partially intercalated Prussian blue. The cause of degradation could be attributed to a significant loss of [Fe(CN)6]–[Fe(NC)6] ordering and the formation of a potassium-free non-conducting ferrihydrite phase. These findings demonstrate the potential of XANES to directly study the nature and evolution of potassium species during an electrochemical process