| 1. |
- Ekspong, Joakim, et al.
(författare)
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Solar-driven water splitting at 13.8 % solar-to-hydrogen efficiency by an earth-abundant PV-electrolyzer
- 2021
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Ingår i: ACS Sustainable Chemistry and Engineering. - : American Chemical Society (ACS). - 2168-0485. ; 9:42, s. 14070-14078
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Tidskriftsartikel (refereegranskat)abstract
- We present the synthesis and characterization of an efficient and low cost solar-driven electrolyzer consisting of Earth-abundant materials. The trimetallic NiFeMo electrocatalyst takes the shape of nanometer-sized flakes anchored to a fully carbon-based current collector comprising a nitrogen-doped carbon nanotube network, which in turn is grown on a carbon fiber paper support. This catalyst electrode contains solely Earth-abundant materials, and the carbon fiber support renders it effective despite a low metal content. Notably, a bifunctional catalyst–electrode pair exhibits a low total overpotential of 450 mV to drive a full water-splitting reaction at a current density of 10 mA cm–2 and a measured hydrogen Faradaic efficiency of ∼100%. We combine the catalyst–electrode pair with solution-processed perovskite solar cells to form a lightweight solar-driven water-splitting device with a high peak solar-to-fuel conversion efficiency of 13.8%.
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| 2. |
- Kawde, Anurag, et al.
(författare)
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Photo-electrochemical hydrogen production from neutral phosphate buffer and seawater using micro-structured p-Si photo-electrodes functionalized by solution-based methods
- 2018
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Ingår i: Sustainable Energy & Fuels. - : Royal Society of Chemistry (RSC). - 2398-4902. ; 2:10, s. 2215-2223
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Tidskriftsartikel (refereegranskat)abstract
- Solar fuels such as H2 generated from sunlight and seawater using earth-abundant materials are expected to be a crucial component of a next generation renewable energy mix. We herein report a systematic analysis of the photo-electrochemical performance of TiO2 coated, microstructured p-Si photoelectrodes (p-Si/TiO2) that were functionalized with CoOx and NiOx for H2 generation. These photocathodes were synthesized from commercial p-Si wafers employing wet chemical methods. In neutral phosphate buffer and standard 1 sun illumination, the p-Si/TiO2/NiOx photoelectrode showed a photocurrent density of 1.48 mA cm2 at zero bias (0 VRHE), which was three times and 15 times better than the photocurrent densities of p-Si/TiO2/CoOx and p-Si/TiO2, respectively. No decline in activity was observed over a five hour test period, yielding a Faradaic efficiency of 96% for H2 production. Based on the electrochemical characterizations and the high energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) and emission spectroscopy measurements performed at the Ti Ka1 fluorescence line, the superior performance of the p-Si/TiO2/ NiOx photoelectrode was attributed to improved charge transfer properties induced by the NiOx coating on the protective TiO2 layer, in combination with a higher catalytic activity of NiOx for H2-evolution. Moreover, we report here an excellent photo-electrochemical performance of p-Si/TiO2/NiOx photoelectrode in corrosive artificial seawater (pH 8.4) with an unprecedented photocurrent density of 10 mA cm2 at an applied potential of 0.7 VRHE, and of 20 mA cm2 at 0.9 VRHE. The applied bias photon-to-current conversion efficiency (ABPE) at 0.7 VRHE and 10 mA cm2 was found to be 5.1%
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| 3. |
- Kwong, Wai Ling, et al.
(författare)
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Cationic Vacancy Defects in Iron Phosphide : A Promising Route toward Efficient and Stable Hydrogen Evolution by Electrochemical Water Splitting
- 2017
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Ingår i: ChemSusChem. - : Wiley-Blackwell. - 1864-5631 .- 1864-564X. ; 10:22, s. 4544-4551
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Tidskriftsartikel (refereegranskat)abstract
- Engineering the electronic properties of transition metal phosphides has shown great effectiveness in improving their intrinsic catalytic activity for the hydrogen evolution reaction (HER) in water splitting applications. Herein, we report for the first time, the creation of Fe vacancies as an approach to modulate the electronic structure of iron phosphide (FeP). The Fe vacancies were produced by chemical leaching of Mg that was introduced into FeP as "sacrificial dopant". The obtained Fe-vacancy-rich FeP nanoparticulate films, which were deposited on Ti foil, show excellent HER activity compared to pristine FeP and Mg-doped FeP, achieving a current density of 10 mAcm(-2) at overpotentials of 108 mV in 1 m KOH and 65 mV in 0.5 m H2SO4, with a near-100% Faradaic efficiency. Our theoretical and experimental analyses reveal that the improved HER activity originates from the presence of Fe vacancies, which lead to a synergistic modulation of the structural and electronic properties that result in a near-optimal hydrogen adsorption free energy and enhanced proton trapping. The success in catalytic improvement through the introduction of cationic vacancy defects has not only demonstrated the potential of Fe-vacancy-rich FeP as highly efficient, earth abundant HER catalyst, but also opens up an exciting pathway for activating other promising catalysts for electrochemical water splitting.
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| 4. |
- Kwong, Wai Ling, et al.
(författare)
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Cobalt- doped hematite thin films for electrocatalytic water oxidation in highly acidic media
- 2019
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Ingår i: Chemical Communications. - : The Royal Society of Chemistry. - 1359-7345 .- 1364-548X. ; 55:34, s. 5017-5020
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Tidskriftsartikel (refereegranskat)abstract
- Earth-abundant cobalt-doped hematite thin-film electrocatalysts were explored for acidic water oxidation. The strategically doped hematite produced a stable geometric current density of 10 mA cm(-2) for up to 50 h at pH 0.3, as a result of Co-enhanced intrinsic catalytic activity and charge transport properties across the film matrix.
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| 5. |
- Kwong, Wai Ling, et al.
(författare)
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Critical Role of {002} Preferred Orientation on Electronic Band Structure of Electrodeposited Monoclinic WO3 Thin Films
- 2018
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Ingår i: Sustainable Energy & Fuels. - 2398-4902. ; 2:10, s. 2224-2236
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Tidskriftsartikel (refereegranskat)abstract
- Monoclinic WO3 films with tunable (002) and (200) orientations parallel to fluorinedoped tin oxide substrates were electrodeposited from peroxotungstic acid (PTA) solutions and subjected to annealing (≤500°C). It is concluded that the crystallographic orientations of the films are dependent on the nucleation density and the thermal-induced recrystallisation kinetics, which can be controlled respectively by the tungsten concentration of the PTA solution and the annealing conditions. This approach allows the fabrication of films with preferred (002), equally mixed (002) and (200), and (200) orientations, which were characterised in terms of the associated mineralogical, nanostructural, optical, chemical, photoelectrochemical, and photocatalytic properties. The present work decouples the concurrent effects of crystallographic orientation and trend in oxygen vacancy concentration on the optical indirect band gap of WO3 thin films, where the linear correlation between these three variables can be used as a diagnostic tool to engineer the electronic band structure of films for applications requiring photosensitivities in different regions of the solar spectrum. In effect, a low valence band maximum, low oxygen vacancy concentration, and resultant low band gap associated with (002) preferred orientation results in the highest photoelectrochemical performance, which is relevant to applications under solar illumination. In contrast, a high concentration of oxygen vacancies, which act as surface reaction sites associated with (200) preferred orientation, results in the highest efficiency in the photodegradation of methylene blue, which is relevant to applications under UV illumination.
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| 6. |
- Kwong, Wai Ling, et al.
(författare)
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Electrochemical N2 reduction at ambient condition - Overcoming the selectivity issue via control of reactants' availabilities
- 2021
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Ingår i: International journal of hydrogen energy. - : Elsevier. - 0360-3199 .- 1879-3487. ; 46:59, s. 30366-30372
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Tidskriftsartikel (refereegranskat)abstract
- Ammonia production via the electrochemical N2 reduction reaction (NRR) at ambient conditions is highly desired as an alternative to the Haber-Bosch process, but remains a great challenge due to the low efficiency and selectivity caused by the competing hydrogen evolution reaction (HER). Herein we investigate the effect of availabilities of reactants (protons, electrons and N2) on NRR using a FeOx-coated carbon fiber paper cathode in various electrochemical configurations. NRR is found viable only under the conditions of low proton-and high N2 availabilities, which are achieved using 0.12 vol% water in LiClO4- ethyl acetate electrolyte and gaseous N2 supplied to the membrane-electrode assembly cathode. This results in an NRR rate of 29 +/- 19 pmolNH3 s(-1) cm(-2) at a Faradaic efficiency of 70 +/- 24% at the applied potential of-0.1 V vs. NHE. Other conditions (high proton-, or low N2-availability, or both) yield a lower or negligible amount of ammonia due to the competing HER. Our work shows that promoting NRR by suppressing the HER requires optimization of the operational variables, which serves as a complementary strategy to the development of NRR catalysts.
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| 7. |
- Kwong, Wai Ling, et al.
(författare)
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High-performance iron (III) oxide electrocatalyst for water oxidation in strongly acidic media
- 2018
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Ingår i: Journal of Catalysis. - : Academic Press. - 0021-9517 .- 1090-2694. ; 365, s. 29-35
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Tidskriftsartikel (refereegranskat)abstract
- Stable and efficient oxygen evolution reaction (OER) catalysts for the oxidation of water to dioxygen in highly acidic media are currently limited to expensive noble metal (Ir and Ru) oxides since presently known OER catalysts made of inexpensive earth-abundant materials generally suffer anodic corrosion at low pH. In this study, we report that a mixed-polymorph film comprising maghemite and hematite, prepared using spray pyrolysis deposition followed by low-temperature annealing, showed a sustained OER rate (>24 h) corresponding to a current density of 10 mA cm−2 at an initial overpotential of 650 mV, with a Tafel slope of only 56 mV dec−1 and near-100% Faradaic efficiency in 0.5 M H2SO4 (pH 0.3). This performance is remarkable, since iron (III) oxide films comprising only maghemite were found to exhibit a comparable intrinsic activity, but considerably lower stability for OER, while films of pure hematite were OER-inactive. These results are explained by the differences in the polymorph crystal structures, which cause different electrical conductivity and surface interactions with water molecules and protons. Our findings not only reveal the potential of iron (III) oxide as acid-stable OER catalyst, but also highlight the important yet hitherto largely unexplored effect of crystal polymorphism on electrocatalytic OER performance.
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| 8. |
- Kwong, Wai Ling, et al.
(författare)
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Scalable Two-Step Synthesis of Nickel Iron Phosphide Electrodes for Stable and Efficient Electrocatalytic Hydrogen Evolution
- 2017
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 121:1, s. 284-292
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Tidskriftsartikel (refereegranskat)abstract
- The development of efficient, durable, and inexpensive hydrogen evolution electrodes remains a key challenge for realizing a sustainable H-2 fuel production via electrocatalytic water splitting. Herein, nickel-iron phosphide porous films with precisely controlled metal content were synthesized on Ti foil using a simple and scalable two-step strategy of spray-pyrolysis deposition followed by low-temperature phosphidation. The nickel-iron phosphide of an optimized Ni:Fe ratio of 1:4 demonstrated excellent overall catalytic activity for hydrogen evolution reaction (HER) in 0.5 M H2SO4, achieving current densities of -10 and -30 mA cm(-2) at overpoteritials of 101 and 123 mV, respectively, with a Tafel slope of 43 mV dec(-1). Detailed analysis obtained by X-ray diffraction, electron microscopy, electrochemistry, and X-ray photoelectron spectroscopy revealed that the superior overall HER activity of nickel iron phosphide as compared to nickel phosphide and iron phosphide was a combined effect of differences in the morphology (real surface area) and the intrinsic catalytic properties (electronic structure). Together with a long-term stability and a near-100% Faradaic efficiency, the nickel-iron phosphide electrodes produced in this study provide blueprints for large-scale H-2 production.
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| 9. |
- Kwong, Wai Ling, et al.
(författare)
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Transparent Nanoparticulate FeOOH Improves the Performance of a WO3 Photoanode in a Tandem Water-Splitting Device
- 2016
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:20, s. 10941-10950
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Tidskriftsartikel (refereegranskat)abstract
- Oxygen evolution catalysts (OEC) are often employed on the surface of photoactive, semiconducting photoanodes to boost their kinetics and stability during photoelectrochemical water oxidation. However, the necessity of using optically transparent OEC to avoid parasitic light absorption by the OEC under front-side illumination is often neglected. Here, we show that furnishing the surface of a WO3 photoanode with suitable loading of FeOOH as a transparent OEC improved the photocurrent density by 300% at 1 V versus RHE and the initial photocurrent-to-O-2 Faradaic efficiency from similar to 70 to similar to 100%. The data from the photo-voltammetry, electrochemical impedance, and gas evolution measurements these improvements were a combined result of reduced hole-transfer resistance for water oxidation, minimized surface recombination of charge carriers, and improved stability against photocorrosion of WO3. We demonstrate the utility of transparent FeOOH-coated W(O)3 in a solar-powered, tandem water-splitting device by combining it with a double-junction Si solar cell and a Ni-Mo hydrogen evolution catalyst. This device performed at a solar-to-hydrogen conversion efficiency of 1.8% in near-neutral K2SO4 electrolyte.
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| 10. |
- Melder, Jens, et al.
(författare)
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Electrocatalytic Water Oxidation by MnOx/C : In Situ Catalyst Formation, Carbon Substrate Variations, and Direct O2/CO2 Monitoring by Membrane-Inlet Mass Spectrometry
- 2017
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Ingår i: ChemSusChem. - : John Wiley & Sons. - 1864-5631 .- 1864-564X. ; 10:22, s. 4491-4502
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Tidskriftsartikel (refereegranskat)abstract
- Layers of amorphous manganese oxides were directly formed on the surfaces of different carbon materials by exposing the carbon to aqueous solutions of permanganate (MnO4- ) followed by sintering at 100-400 °C. During electrochemical measurements in neutral aqueous buffer, nearly all of the MnOx /C electrodes show significant oxidation currents at potentials relevant for the oxygen evolution reaction (OER). However, by combining electrolysis with product detection by using mass spectrometry, it was found that these currents were only strictly linked to water oxidation if MnOx was deposited on graphitic carbon materials (faradaic O2 yields >90 %). On the contrary, supports containing sp3 -C were found to be unsuitable as the OER is accompanied by carbon corrosion to CO2 . Thus, choosing the "right" carbon material is crucial for the preparation of stable and efficient MnOx /C anodes for water oxidation catalysis. For MnOx on graphitic substrates, current densities of >1 mA cm-2 at η=540 mV could be maintained for at least 16 h of continuous operation at pH 7 (very good values for electrodes containing only abundant elements such as C, O, and Mn) and post-operando measurements proved the integrity of both the catalyst coating and the underlying carbon at OER conditions.
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