Kaiqi Xu

• Grandcolas, Mathieu; Wabende, Brian; Yang, Juan; Mei, Sen; Xu, Kaiqi; Norby, Truls Eivind & Chatzitakis, Athanasios Eleftherios (2019). Preparation of TiO2 rutile nanorods decorated with cobalt oxide nanoparticles for solar photoelectrochemical activity. Materials letters (General ed.).  ISSN 0167-577X.  247, s 1- 3 . doi: 10.1016/j.matlet.2019.03.087
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Jensen, Ingvild Julie Thue; Grandcolas, Mathieu & Norby, Truls Eivind (2019). Ta3N5/Co(OH)x composites as photocatalysts for photoelectrochemical water splitting. Photochemical and Photobiological Sciences.  ISSN 1474-905X.  18(4), s 837- 844 . doi: 10.1039/c8pp00312b
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Vøllestad, Einar; Ruan, Qiushi; Tang, Junwang & Norby, Truls Eivind (2019). Hydrogen from wet air and sunlight in a tandem photoelectrochemical cell. International journal of hydrogen energy.  ISSN 0360-3199.  44(2), s 587- 593 . doi: 10.1016/j.ijhydene.2018.11.030
• Sun, Xinwei; Xu, Kaiqi; Fleischer, Christian; Liu, Xin; Grandcolas, Mathieu; Strandbakke, Ragnar; Bjørheim, Tor Svendsen; Norby, Truls Eivind & Chatzitakis, Athanasios Eleftherios (2018). Earth-Abundant Electrocatalysts in Proton Exchange Membrane Electrolyzers. Catalysts.  ISSN 2073-4344.  8(12) . doi: 10.3390/catal8120657 Full text in Research Archive. Show summary

  In order to adopt water electrolyzers as a main hydrogen production system, it is critical to develop inexpensive and earth-abundant catalysts. Currently, both half-reactions in water splitting depend heavily on noble metal catalysts. This review discusses the proton exchange membrane (PEM) water electrolysis (WE) and the progress in replacing the noble-metal catalysts with earth-abundant ones. The efforts within this field for the discovery of efficient and stable earth-abundant catalysts (EACs) have increased exponentially the last few years. The development of EACs for the oxygen evolution reaction (OER) in acidic media is particularly important, as the only stable and efficient catalysts until now are noble-metal oxides, such as IrOx and RuOx. On the hydrogen evolution reaction (HER) side, there is significant progress on EACs under acidic conditions, but there are very few reports of these EACs employed in full PEM WE cells. These two main issues are reviewed, and we conclude with prospects for innovation in EACs for the OER in acidic environments, as well as with a critical assessment of the few full PEM WE cells assembled with EACs.

• Chatzitakis, Athanasios Eleftherios; Grandcolas, Mathieu; Xu, Kaiqi; Mei, Sen; Yang, Juan; Jensen, Ingvild Julie Thue; Simon, Christian & Norby, Truls Eivind (2017). Assessing the photoelectrochemical properties of C, N, F codoped TiO2 nanotubes of different lengths. Catalysis Today.  ISSN 0920-5861.  287, s 161- 168 . doi: 10.1016/j.cattod.2016.11.040 Full text in Research Archive. Show summary

  The aim of this work has been the photoelectrochemical (PEC) study of nanostructured photoanodes based on TiO2. Highly ordered and well adhered TiO2 nanotubes (TNTs) of different lengths (∼2–20 μm) were prepared in a two-step process in ethylene glycol solutions containing fluorides, and detailed XPS analysis showed that they have become co-doped with C, N and F. PEC measurements revealed that increasing surface area is not followed by increase in the photoconversion efficiency, but rather that an optimal balance between electroactive surface area (ESA) and charge carrier concentration exists. TNTs of around 10 μm show the optimum incident photon-to-current efficiency (IPCE) of ∼33% and an overall photoconversion efficiency of ∼6.3% under UV illumination of 4 mW cm−2 light intensity. Finally, Mott-Schottky analysis revealed significant frequency dispersion of the estimated space charge layer capacitance, which renders the accurate estimation of the flatband position and charge carrier concentration unreliable. On the other hand, more realistic charge carrier concentrations can be obtained by normalizing the capacitance per ESA.

• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios & Norby, Truls (2017). Solid-state photoelectrochemical cell with TiO2 nanotubes for water splitting. Photochemical and Photobiological Sciences.  ISSN 1474-905X.  16(1), s 10- 16 . doi: 10.1039/c6pp00217j Full text in Research Archive.

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• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Norby, Truls Eivind & Bjørheim, Tor Svendsen (2019). Ta3N5 nanotubes loaded with Co(OH)x/Co-Pi-high photocurrent densities and improved photocorrosion resistance.
• Xu, Kaiqi; Grandcolas, Mathieu; Yang, Juan; Mei, Sen; Chatzitakis, Athanasios Eleftherios & Norby, Truls Eivind (2019). Photoelectrochemical water splitting on Ta3N5 nanotubes for H2 production.
• Grandcolas, Mathieu; Wabende, Brian; Yang, Juan; Mei, Sen; Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios & Norby, Truls Eivind (2018). TiO2 Nanorods Decorated With Cobalt oxide as Photoanodes for a Bio-Hybrid PEC Cell.
• Sun, Xinwei; Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios & Norby, Truls Eivind (2018). Can hydroxyl radicals travel far? Gas phase transport and detection after photocatalytic generation at TiO2 nanorods.
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Bjørheim, Tor Svendsen & Norby, Truls Eivind (2018). Surface modification of Ta3N5 nanotubes as photocatalyst for photoelectrochemical water splitting.
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Norby, Truls Eivind; Grandcolas, Mathieu; Yang, Juan & Mei, Sen (2018). Ta3N5 / Co(OH)x composite as the photocatalyst for photoelectrochemical water splitting.
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Vøllestad, Einar; Ruan, Qiushi; Tang, Junwang & Norby, Truls Eivind (2018). Solid-state photoelectrochemical cell for wet air electrolysis and hydrogen production.
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Vøllestad, Einar; Ruan, Qiushi; Tang, Junwang & Norby, Truls Eivind (2018). Solid-state tandem photoelectrochemical cell for wet air electrolysis and hydrogen production.
• Chatzitakis, Athanasios Eleftherios; Xu, Kaiqi & Norby, Truls Eivind (2017). TiO2 nanotubes as photoanode electrodes in solid-state photoelectrochemical cells.
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Bjørheim, Tor Svendsen & Norby, Truls Eivind (2017). Fuels from the sun, sea and air-by solid-state photoelectrochemical cell.
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Bjørheim, Tor Svendsen & Norby, Truls Eivind (2017). Tantalum oxynitride for photoelectrochemical water splitting.
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Norby, Truls Eivind; Mei, Sen; Juan, Yang; Grandcolas, Mathieu & Simen, Christian (2017). Solid-state photoelectrochemical Cell with Tantalum Nitride Nanotubes as Photoanode.
• Yang, Juan; Mei, Sen; Grandcolas, Mathieu; Simon, Christian; Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Norby, Truls Eivind; Yang, Mingyi; Backe, Paul Hoff & Bjørås, Magnar (2017). Synthesis of Nanosized Cobalt Oxides and Application in a Bio-hybrid Photoelectrochemical Cell for CO2 Utilization. Show summary

  CO2 capture and utilization (CCU) can alleviate the disastrous effects of greenhouse gas (GHG) emissions form the use of fossil fuels. The use of CO2 emission as feedstock for direct conversion into valuable chemicals has attracted increasing research interest worldwide. A number of approaches, such as chemical, electrochemical, photochemical and biological routes have used CO2 as a sustainable carbon resource for the production of chemicals. In the present CO2BioPEC project, a novel hybrid photoelectrochemical concept, based on solar energy and formate dehydrogenase biocatalyst is applied, as illustrated in Figure 1 (left). This multidisciplinary project aims at demonstrating a bio-hybrid photoelectrochemical cell, in which solar energy is efficiently captured and utilized to convert CO2 into energy-rich compounds, using formate dehydrogenase enzymes as biocatalysts. Cobalt oxide is a low-cost material with a band gap that can absorb visible light. It is also a promising co-catalyst that has been widely used for oxygen evolution route. In the present work, nanosized cobalt oxide was synthesized and used as a co-catalyst on the photoanode. The results showed that the formation of cobalt oxides (Co3O4, CoOOH and Co(OH)2) is strongly dependent on the temperature and reaction time. The photochemical efficiency will be further investigated by impregnating nanosized cobalt oxides onto Ta3N4 /TaON nanotubes.

• Chatzitakis, Athanasios Eleftherios; Grandcolas, Mathieu; Xu, Kaiqi; Mei, Sen; Yang, Juan; Simon, Christian & Norby, Truls (2016). Assessing the photoelectrocatalytic activity of C, N, F codoped TiO2 nanotubes of different lengths for oxygen evolution.
• Fleischer, Christian; Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios & Norby, Truls Eivind (2016). Development of high surface area TiO2 photo-electrodes and the significance of solid-state proton conductor in water splitting photoelectrochemical cell..
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios & Norby, Truls (2016). An All-Solid-State Photoelectrochemical Cell for Water Splitting.
• Xu, Kaiqi & Norby, Truls Eivind (2016). All-Solid-State Photoelectrochemical Water Splitting for Hydrogen Generation. Show summary

  The combination of solar energy and water splitting hydrogen production results in a feasible, prospective energy conversion and storage process—photoelectrochemical (PEC) water splitting. In this work, an all-solid-state PEC cell has been fabricated and its performance with different photoanodes has been studied. An attempt to move one step further to fabricate an even more compact solid-state PEC cell using a proton-electron mixed conducting membrane has also been carried out later. In the photoanodes preparation stage, titania with different morphologies, e.g., drop-cast P25 titania nanoparticles, thermally treated Ti foil and highly organized titania nanotubes (TNT), are prepared and tested for their intrinsic properties, such as the donor density, flatband potential, etc. In particular, TNTs are synthesized by a 2-step anodization method, with immersing pretreatment, long and stable TNTs can be grown on a thin Ti substrate. Three TNT samples synthesized for 5, 20 and 30 min in the 2nd anodization step have been studied, and later applied in the PEC cell. The Type 1 solid-state PEC cell is fabricated by simply attaching the photoanode and the cathode onto the two sides of a Nafion proton conducting membrane. In this way, water oxidation reaction and proton reduction reaction will take place in each side of the membrane, hence gases are separated as soon as they are generated. The cathode consists of the carbon paper as the substrate, and the platinum coated carbon nanoparticles (Pt-C) as the electrocatalyst, which is connected with the photoanode through an external circuit. Regarding the photo-to-current performance, PEC cells employed with TNT as their photoanodes perform better than the ones with other photoanodes. Furthermore, the ionic conductivity around the TNT photoanodes has a significant impact on the overall cell performance. With deionized water as the liquid environment, the cell employed with the TNT 5 min photoanode gives the highest efficiency, while replacing the deionized water with a 0.5 M sodium sulphate solution makes the cell with the TNT 30 min sample perform the best. The hydrogen production of the Type 1 solid-state PEC cell was confirmed by GC measurements. When it comes to the Type 2 cell, the external circuit was removed, and the Nafion membrane was integrated with the carbon paper to form a protonelectron mixed conducting membrane (MCM). Electrons and protons generated by the photoanode during the oxidation half reaction were expected to transport through the integrated membrane simultaneously. The electrocatalyst—Pt-C was deposited directly on one side of the MCM, so that proton and hydrogen can easily recombine into hydrogen molecules that side. The concept of producing hydrogen by this type of cell was confirmed by the detection of a hydrogen peak from GC measurements, in which a basic solution was introduced to the photoanode in order to enlarge the chemical potential difference between the two sides of the MCM. However, involving alkalies leads to the carbon corrosion, which results in the formation of carbonate ions. Consequently, a relatively large methane production was observed, since the reaction from carbonate ions to methane is energetically more favorable than hydrogen production. In conclusion, a compact, robust, solid-state PEC cell for water splitting hydrogen generation can be built through a simple process, and a novel nanostructure modification of titania as the photoanode can enhance the overall cell performance. Protons and electrons generated during the water oxidation can pass through a proton-electron MCM simultaneously, and get recombined into hydrogen where the electrocatalyst is present, which results in a more compact solid-state PEC cell.

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