Sustainable Energy Economy Research Articles

(Invited) Water-Splitting Electrocatalysts Made By Pulsed-Laser in Liquids Synthesis

Conversion of solar energy into storable fuels is urgently needed to combat climate change. Efficient, robust materials that are exclusively made of non-precious elements are imperative for tomorrow's global sustainable energy economy and necessitate the development of new nanostructured multi-metallic electrocatalysts that turn our most abundant feedstocks, water or carbon dioxide, into carbon-neutral fuels. The sheer number of possible elemental compositions in multi-metallic nanostructures requires a deep understanding of electrocatalytic mechanisms, including identification of reactive intermediates, to rationally design new materials and guide their optimization.We used pulsed-laser in liquids synthesis to realize series of earth-abundant multi-metallic first-row transition metal oxide and hydroxide nanostructures. The pulsed-laser in liquids technique is a flexible synthetic strategy for surfactant-free nanomaterials with independently controlled compositional, morphological, and structural properties, and defect densities. It permits rapid preparation or modification of tailored, complex nanostructures in sufficiently large quantities to study them in bulk. Importantly, metastable materials become accessible as nanoparticles form from a laser-induced plasma that is confined by the liquid and characterized by very high temperatures and pressures, thus enabling access to extreme regions of the material's phase diagram.Our approach enabled unprecedented atomistic-level structural and mechanistic insights into highly active and robust nickel–iron layered double hydroxide nanocatalysts for water oxidation in base; this understanding allowed rapid optimization. Operando spectroscopy data allowed us to identify a cis-dioxo iron(VI) reactive intermediate as the lowest-energy species before O–O bond formation during water oxidation electrocatalysis. Further, we capitalized on the unique advantages of the laser process for integration of our water-splitting electrocatalysts with photoanodes.

Rational electronic control of carbon dioxide reduction over cobalt oxide

The selective reduction of carbon dioxide (CO2) into fuels and chemicals is expected to play a key role in achieving a carbon–neutral sustainable energy economy. Activation of CO2 is the most important step in this process. As the electron transfer to CO2 is the rate-limiting step for CO2 activation, it is significant to modulate the electronic structure of CO2 reduction catalysts to enhance their activities. However, the intrinsic relationships between electronic properties of catalysts and their catalytic activities remains somewhat unclear, which limits the rational design of highly-efficient CO2 reduction catalysts. Herein, a catalyst-insulator–metal system consisting of Al as an electron-donor material was designed to modulate the electronic structure of cobalt oxide (Co3O4) catalyst. The electrons in Al can be efficiently tunneled to Co3O4 across an ultrathin self-formed Al2O3 insulating layer. Experimental and theoretical results undoubtedly confirm that high electron density on Co3O4 favours the adsorption and activation of CO2, while lowers the energy barrier for the formation of COOH* and especially lowers the desorption energy barrier of CO* intermediates, thereby significantly accelerating the reaction kinetics of CO2-to-CO photoreduction. The turnover frequency per Co atom of Co3O4/Al2O3-Al is much higher than that of Co3O4. The apparent quantum yield (AQY) of Co3O4/Al2O3-Al catalyst highly reaches 3.8% at 420 nm, which is superior to those of most reported catalysts. Besides, the increase in electron density on Co3O4 can effectively inhibit the competing hydrogen evolution reaction. The selectivity of CO increases from 57.9% for Co3O4 to 82.4% for Co3O4/Al2O3-Al. Significantly, the catalytic efficiency can be rationally tuned through controlling the content and particle size of Al. This work stablishes the links between the electronic structure of catalysts and their catalytic activities toward CO2 reduction reaction. Moreover, this Al2O3-Al structure reported here, may be used as an unprecedented platform for the electronic effect study of other heterogeneous catalysts.

Luminescent Solar Concentrators for Photoelectrochemical Water Splitting

Solar-driven electrolysis accomplished through photovoltaic (PV) devices is a major candidate for a long-term sustainable energy economy. However, the visually appealing integration of PV cells or ...

Stannites – A New Promising Class of Durable Electrocatalysts for Efficient Water Oxidation

AbstractThe oxygen evolution reaction (OER) through water oxidation is a key process for multiple energy storage technologies required for a sustainable energy economy such as the formation of the fuel hydrogen from water and electricity, or metal‐air batteries. Herein, we investigate the suitability of Cu2FeSnS4 for the OER and demonstrate its superiority over iron sulfide, iron (oxy)hydroxides and benchmark noble‐metal catalysts in alkaline media. Electrodeposited Cu2FeSnS4 yields the current densities of 10 and 1000 mA/cm2 at overpotentials of merely 228 and 330 mV, respectively. State‐of‐the‐art analytical methods are applied before and after electrocatalysis to uncover the fate of the Cu2FeSnS4 precatalyst under OER conditions and to deduce structure‐activity relationships. Cu2FeSnS4 is the first compound reported for OER among the broad class of stannite structure type materials containing multiple members with highly active earth‐abundant transition‐metals for OER.

Carbon supported noble metal nanoparticles as efficient catalysts for electrochemical water splitting.

Due to an increasing requirement of clean and sustainable hydrogen energy economy, it is significant to develop new highly effective catalysts for electrochemical water splitting. In alkaline electrolyte, Platinum (Pt) shows a much slower hydrogen evolution reaction (HER) kinetics relative to acidic condition. Here, we show a versatile synthetic approach for combining different noble metals, such as Rhodium (Rh), RhPt and Pt nanoparticles, with carbon forming noble metal nanoparticles/nanocarbon composites, denoted as Rh(nP)/nC, RhPt(nP)/nC and Pt(nP)/nC, respectively. It was found that in alkaline media these composites exhibited higher performance for the HER than the commercial Pt/C. In particular, Rh(nP)/nC displayed a small overpotential of 44 mV at a current density of 5 mA cm-2 and a low Tafel slope of 50 mV dec-1. Meanwhile, it also showed a comparable activity for the oxygen evolution reaction (OER) to the benchmarking catalyst RuO2. The superior HER and OER performance benefits from the very small size of nanoparticles and synergy between carbon support and nanoparticles.

Photocatalysis: an overview of recent developments and technological advancements

Photocatalysis, which is the catalyzation of redox reactions via the use of energy obtained from light sources, is a topic that has garnered a lot of attention in recent years as a means of addressing the environmental and economic issues plaguing society today. Of particular interest are photosynthesis can potentially mimic a variety of vital reactions, many of which hold the key to develop sustainable energy economy. In light of this, many of the technological and procedural advancements that have recently occurred in the field are discussed in this review, namely those linked to: (1) photocatalysts made from metal oxides, nitride, and sulfides; (2) photocatalysis via polymeric carbon nitride (PCN); and (3) general advances and mechanistic insights related to TiO2-based catalysts. The challenges and opportunities that have arisen over the past few years are discussed in detail. Basic concepts and experimental procedures which could be useful for eventually overcoming the problems associated with photocatalysis are presented herein.

Kinetic‐Oriented Construction of MoS2 Synergistic Interface to Boost pH‐Universal Hydrogen Evolution

AbstractAs a prerequisite for a sustainable energy economy in the future, designing earth‐abundant MoS2 catalysts with a comparable hydrogen evolution catalytic performance in both acidic and alkaline environments is still an urgent challenge. Decreasing the energy barriers could enhance the catalysts' activity but is not often a strategy for doing so. Here, the first kinetic‐oriented design of the MoS2‐based heterostructure is presented for pH‐universal hydrogen evolution catalysis by optimizing the electronic structure based on the simultaneous modulation of the 3d‐band‐offsets of Ni, Co, and Mo near the interface. Benefiting from this desirable electronic structure, the obtained MoS2/CoNi2S4 catalyst achieves an ultralow overpotential of 78 and 81 mV at 10 mA cm−2, and turnover frequency as high as 2.7 and 1.7 s−1 at the overpotential of 200 mV in alkaline and acidic media, respectively. The MoS2/CoNi2S4 catalyst represents one of the best hydrogen evolution reaction performing ones among MoS2‐based catalysts reported to date in both alkaline and acidic environments, and equally important is the remarkable long‐term stability with negligible activity loss after maintaining at 10 mA cm−2 for 48 h in both acid and base. This work highlights the potential to deeply understand and rationally design highly efficient pH‐universal electrocatalysts for future energy storage and delivery.

Experimental Methods for Efficient Solar Hydrogen Production in Microgravity Environment.

Long-term space flights and cis-lunar research platforms require a sustainable and light life-support hardware which can be reliably employed outside the Earth's atmosphere. So-called 'solar fuel' devices, currently developed for terrestrial applications in the quest for realizing a sustainable energy economy on Earth, provide promising alternative systems to existing air-revitalization units employed on the International Space Station (ISS) through photoelectrochemical water-splitting and hydrogen production. One obstacle for water (photo-) electrolysis in reduced gravity environments is the absence of buoyancy and the consequential, hindered gas bubble release from the electrode surface. This causes the formation of gas bubble froth layers in proximity to the electrode surface, leading to an increase in ohmic resistance and cell-efficiency loss due to reduced mass transfer of substrates and products to and from the electrode. Recently, we have demonstrated efficient solar hydrogen production in microgravity environment, using an integrated semiconductor-electrocatalyst system with p-type indium phosphide as the light-absorber and a rhodium electrocatalyst. By nanostructuring the electrocatalyst using shadow nanosphere lithography and thereby creating catalytic 'hot spots' on the photoelectrode surface, we could overcome gas bubble coalescence and mass transfer limitations and demonstrated efficient hydrogen production at high current densities in reduced gravitation. Here, the experimental details are described for the preparations of these nanostructured devices and further on, the procedure for their testing in microgravity environment, realized at the Bremen Drop Tower during 9.3 s of free fall.

Molecular Electrocatalysts for Alcohol Oxidation: Insights and Challenges for Catalyst Design

The implementation of a sustainable energy economy based on renewable but intermittent energy sources necessitates the efficient electrogeneration of chemical fuels and the efficient utilization of...

Engineering Ni2P-NiSe2 heterostructure interface for highly efficient alkaline hydrogen evolution

Developing earth-abundant transition-metal phosphides electrocatalysts with high activity and good durability toward alkaline hydrogen evolution reaction (HER) is crucial for sustainable hydrogen energy economy. However, their intrinsic activity is limited by the inadequate hydrogen adsorption energy. Herein, we reported a novel self-supported heterostructure catalyst in the form of Ni2P-NiSe2, which is obtained by phosphorization of NiSe2 nanosheet arrays grown on the carbon cloth. The heterostructure catalyst exhibits excellent HER performance in 1 M KOH, only requiring an overpotential of 66 mV at a current density of 10 mA cm−2 with a Tafel slope of 72.6 mV dec-1 and showing excellent long-term durability. The experimental and theoretical calculation results demonstrate the strongly electronic interaction between Ni2P and NiSe2, resulting in the optimized Gibbs free energy of hydrogen and water adsorption. This work concerning the regulation of electronic structure through interface engineering may offer a deep insight to explore superior catalysts.

Hydrogen Production and Subsequent Adsorption/Desorption Process within a Modified Unitized Regenerative Fuel Cell

For sustainable and incremental growth, mankind is adopting renewable sources of energy along with storage systems. Storing surplus renewable energy in the form of hydrogen is a viable solution to meet continuous energy demands. In this paper the concept of electrochemical hydrogen storage in a solid multi-walled carbon nanotube (MWCNT) electrode integrated in a modified unitized regenerative fuel cell (URFC) is investigated. The method of solid electrode fabrication from MWCNT powder and egg white as an organic binder is disclosed. The electrochemical testing of a modified URFC with an integrated MWCNT-based hydrogen storage electrode is performed and reported. Galvanostatic charging and discharging was carried out and results analyzed to ascertain the electrochemical hydrogen storage capacity of the fabricated electrode. The electrochemical hydrogen storage capacity of the porous MWCNT electrode is found to be 2.47 wt%, which is comparable with commercially available AB5-based hydrogen storage canisters. The obtained results prove the technical feasibility of a modified URFC with an integrated MWCNT-based hydrogen storage electrode, which is the first of its kind. This is surelya step forward towards building a sustainable energy economy.

Exploiting the non-medical value of waste expired aminophylline for lithium ion battery anode

Expired medicines in the environment may cause the pollution and resource waste if they are not reasonably recycled. Therefore, it is very instructive to exploit the non-medical values of expired medicines. Herein, such an attempt to recycle expired aminophylline was made as the anode active material of lithium ion batteries (LIBs) for the first time. The microstructure and chemical element component of the expired aminophylline were confirmed by using the methods of scanning electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Furthermore, the feasibility of using an expired-aminophylline-based anode in LIBs was evaluated by galvanostatic charge/discharge and cyclic voltammetry. To be satisfactory, the reversible specific discharge capacities of expired-aminophylline-based anode were maintained at 268·8 mAh/g at 50 mA/g for 200 cycles and 140 mAh/g even at 500 mA/g for 1000 cycles. In addition, after coupling with a commercial lithium cobalt (III) oxide (LiCoO2) cathode, the resultant full cell also delivered 130 mAh/g at the 100th cycle at 100 mA/g. These satisfactory results may not only pave a way for the reasonable exploitation of the non-medical values of expired medicines but also offer good inspiration and strategy to develop sustainable energy and circular economy.

Highly dispersed Pt-based catalysts for selective CO2 hydrogenation to methanol at atmospheric pressure

Hydrogenation of CO2 into methanol is promising for achieving the sustainable energy economy, but still has some problems, e.g. low methanol selectivity and high operation pressures (>10 atm). Herein, we prepared a Pt/film hybrid with highly dispersed Pt nanoparticles, and combined Pt/film with In2O3 to form a Pt/film/In2O3 catalyst. By using a dielectric barrier discharge (DBD) plasma reactor, a CO2 conversion of 37.0% and a methanol selectivity of 62.6% are achieved in the hydrogenation of CO2 with H2 on Pt/film/In2O3 at 1 atm and 30 °C. These are higher than those on Pt/In2O3 prepared by the conventional high-temperature H2 reduction (24.9% and 36.5%) and commercial Cu/ZnO/Al2O3 (25.6% and 35.1%). The high-energy electrons of the DBD plasma trigger the CO2 hydrogenation at 1 atm and 30 °C. The higher Pt nanoparticles dispersion, film and In2O3 promote the adsorption of CO2 on Pt/film/In2O3, thus enhancing the hydrogenation of CO2 into methanol. These results are helpful for efficient methanol production from CO2 hydrogenation under atmospheric pressure.

Unified Catalyst for Efficient and Stable Hydrogen Production by Both the Electrolysis of Water and the Hydrolysis of Ammonia Borane

AbstractHydrogen is a promising energy carrier for a future sustainable energy economy. The two most promising routes for its large‐scale production in high purity are electrocatalytic water splitting and chemical release from hydrogen‐storage materials. The rational design and synthesis of robust catalysts for hydrogen evolution are crucial stages in the development of hydrogen as an energy source. This work reports the production of a novel hydrogen evolution catalyst consisting of uniform hollow porous carbon spheres loaded with ruthenium nanoclusters via a simple hydrothermal process at low temperature. The catalyst not only exhibits extraordinary catalytic activity for the hydrogen evolution reaction, with extremely low overpotentials at all pH values, but also shows outstanding activity for hydrolysis of ammonia borane (AB) with a high turnover frequency. Furthermore, the catalyst remains stable during both reactions, thus allowing one unified catalyst to facilitate hydrogen generation by two different methods for highly efficient and superstable hydrogen production. This work may provide a template method for rational design and fabrication of various metals/N‐doped carbon catalysts that promote hydrogen production through both the electrolysis of water and the hydrolysis of AB, which will advance the development of hydrogen as an energy source.

Effects of alkali metal cations on oxygen reduction on N-containing carbons viewed as the interplay between capacitive and electrocatalytic properties: Experiment and theory

The development of new electrocatalysts for the oxygen reduction reaction (ORR) is crucial for the sustainable energy economy. Both fundamental understanding of surface processes under operating conditions and suitable ORR activity measurements are necessary to select the best electrocatalyst candidate. In the present study, to contribute to this matter, we show that both the nature of alkali metal cations (Li+, Na+ and K+), composing supporting aqueous hydroxide solution, as well as the potential sweep rate in rotating disk electrode voltammetry measurements, influence the results of measurements of ORR activities of N-containing nanocarbons. Based on density functional theory calculations, we concluded that the specific interactions of hydrated cations with oxygen functional groups are responsible for such behaviour, leading to a close interplay between the electrode double layer charging and the parallel Faradaic process on carbon surface. From a practical point of view, the presented results indicate that it is necessary to standardize carefully the ORR measurements on different carbon materials.

The Role of Cities in South Africa’s Energy Gridlock

South Africa’s energy sector finds itself in a gridlock situation. The sector is controlled by the state-owned utility Eskom holding the monopoly on the generation and transmission of electricity, which is almost exclusively produced from domestically extracted coal. At the same time, the constitutional mandate enables municipalities to distribute and sell electricity generated by Eskom to local consumers, which constitutes a large part of the cities’ municipal income. This is a strong disincentive for city governments to promote reductions in energy consumption and substantially limits the scope for urban action on energy efficiency and renewable energies. In the present case study, we portray the current development in South Africa’s energy policy and trace how deadlocked legal, financial, and institutional barriers block the transition from a coal-based energy system toward a greener and more sustainable energy economy. We furthermore point to the efforts of major South African cities to introduce low-carbon strategies in their jurisdictions and highlight key challenges for the future development of the country’s energy sector. By engaging with this case study, readers will become familiar with a prime example of the wider phenomenon of national political–economic obstacles to the progress in sustainable urban development.

Exploring the "Goldilocks Zone" of Semiconducting Polymer Photocatalysts by Donor-Acceptor Interactions.

Water splitting using polymer photocatalysts is a key technology to a truly sustainable hydrogen-based energy economy. Synthetic chemists have intuitively tried to enhance photocatalytic activity by tuning the length of π-conjugated domains of their semiconducting polymers, but the increasing flexibility and hydrophobicity of ever-larger organic building blocks leads to adverse effects such as structural collapse and inaccessible catalytic sites. To reach the ideal optical band gap of about 2.3 eV, A library of eight sulfur and nitrogen containing porous polymers (SNPs) with similar geometries but with optical band gaps ranging from 2.07 to 2.60 eV was synthesized using Stille coupling. These polymers combine π-conjugated electron-withdrawing triazine (C3 N3 ) and electron donating, sulfur-containing moieties as covalently bonded donor-acceptor frameworks with permanent porosity. The remarkable optical properties of SNPs enable fluorescence on-off sensing of volatile organic compounds and illustrate intrinsic charge-transfer effects.

High Performance N‐Doped Carbon Electrodes Obtained via Hydrothermal Carbonization of Macroalgae for Supercapacitor Applications

AbstractThe conversion of bio‐waste into useful porous carbons constitutes a very attractive approach to contribute to the development of sustainable energy economy, even more as they can be used in energy storage devices. Here we report the synthesis of N‐doped carbons from hydrothermal carbonization of macroalgae, Enteromorpha prolifera (EP), followed by a mild KOH activation step. The obtained N‐doped carbons exhibited surface areas of up to ∼2000 m2/g with N‐loadings varied in the range of 1.4∼2.9 at %. By modifying activation temperature, we were able to tune the surface chemistry and porosity, achieving excellent control of their properties. The specific capacitance reached values of up to 200 F/g at 1 A/g in 6 M KOH for the sample obtained at activation temperature of 700 °C (AHC‐700). The symmetric supercapacitor using the sample activated at 800 °C (AHC‐800) as electrodes exhibited the highest cycling stability of the samples studied in this work, with capacitance retention of up to 96 % at 10 A/g, even after 10,000 cycles, constituting the highest reported for biomass‐derived carbon electrodes. These results show the great potential of N‐doped carbons as electrodes for supercapacitors and confirm the excellent electrochemical properties of biomass‐derived carbons in energy storage technologies.

Hollow Rh nanoparticles with nanoporous shell as efficient electrocatalyst for hydrogen evolution reaction

Studies on highly effective electrocatalysts for producing hydrogen via electrochemical water splitting is of great significance due to the increasing demand of clean and sustainable hydrogen energy economy. In this work, hollow RhCo nanoparticles with an average diameter of 21 nm were fabricated using the galvanic replacement method in a water-ice bath. Electrochemical evaluation showed that the as-synthesized electrocatalyst exhibited better performance for hydrogen evolution reaction (HER) under acid condition, with a lower overpotential of 28.1 mV at 10 mA cm−2 and a lower Tafel slope of 24 mV dec−1, in comparison with commercial Rh/C. The better HER performance of the as-prepared hollow Rh nanoparticles could be attributed to the larger surface area than commercial Rh nanoparticles, which could provide more active sites and therefore improve the electrocatalytic performance.

Effect of Ti Doping on the Photoelectrochemical Water Splitting Efficiency of WO3 Photoanode

Photocatalytic/photo-electrochemical (PEC) water splitting is a process whereby H2O is split into H2 and O2 using sunlight. This solar to chemical conversion (specifically, solar-energy-driven H2 production) is an ambitious yet crucial objective for futuristic production of eco-friendly energy that is storable and transportable and can be efficiently converted into electricity using fuel cells whenever necessary. Most importantly, this process is believed to lay the foundation for a sustainable hydrogen-based energy economy, which represents a carbon-neutral approach for producing hydrogen gas using the most abundant renewable resources: water and sunlight. WO3, an n-type semiconductor, has been extensively studied as a photoanode for PEC water splitting because of its advantages such as non-toxicity, low cost, and chemical stability in acidic aqueous media. Furthermore, WO3 has a moderate hole diffusion length (~150 nm) compared to other oxides such as Fe2O3 (2–4 nm) and TiO2 (~100 nm), with inherently good electron transport properties. After absorbing a portion of the visible light during illumination, WO3 generates electron–hole pairs and its valence band edge can provide enough potential for the oxidation of water since it is located at approximately 3.0 eV versus NHE. Several strategies have been implemented to enhance the photocatalytic properties of WO3, such as morphological control, transition metal doping, noble metal loading, surface sensitization, and formation of composite materials. In particular, doping and co-doping are possible ways of tailoring the electronic band structure as well as the PEC properties of WO3. Therefore, photo-electrochemical properties of tungsten oxide (WO3) can be tailored by doping foreign atoms from which conduction band (CB)/valence band (VB) edge positions can be altered. Band edge engineering for WO3 is essential since VB maximum of WO3 is more positive more the photo-oxidation potential of H2O but CB minimum of WO3 is lower than the reduction potential of H2O to limit spontaneous solar water reduction. In view of this, here we propose a simple, single step and efficient strategy for band edge tailoring of WO3 by Ti doping for efficient solar water splitting. The physical properties of synthesized Ti doped WO3 thin films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray analysis (EDS), UV-Vis absorption spectra and X-ray photoelectron spectroscopy (XPS). The incorporation of Ti into WO3 reduces the crystallinity and increases the band gap of WO3. From XPS valence band analysis, we confirm that, substitution Ti into WO3 leads to slight shift up the CB maximum upwards more closed to the H+/H2 redox potential without any change in VB maximum. Compared to un-doped WO3, Ti doped WO3 exhibited an unprecedented photocurrent of 1.6 mA cm-2 (at 0.8 V vs Ag/AgCl) under simulated 1.5 AM sunlight without an added water oxidation catalyst due to the improved electrical conductivity and reaction kinetics after Ti doping. The method presented here, demonstrates a simple but systematic and efficient approach for the design and fabrication of band edge tailored WO3 photoanodes via Ti doping efficient for photo-electrochemical water splitting. Figure 1