Abstract
H
2
O
2
is a sacrificial reductant that is often used as a hole scavenger to gain insight into photoanode properties. Here we show a distinct mechanism of H
2
O
2
photo-oxidation on ...haematite (
α
-Fe
2
O
3
) photoanodes. We found that the photocurrent voltammograms display non-monotonous behaviour upon varying the H
2
O
2
concentration, which is not in accord with a linear surface reaction mechanism that involves a single reaction site as in Eley–Rideal reactions. We postulate a nonlinear kinetic mechanism that involves concerted interaction between adions induced by H
2
O
2
deprotonation in the alkaline solution with adjacent intermediate species of the water photo-oxidation reaction, thereby involving two reaction sites as in Langmuir–Hinshelwood reactions. The devised kinetic model reproduces our main observations and predicts coexistence of two surface reaction paths (bi-stability) in a certain range of potentials and H
2
O
2
concentrations. This prediction is confirmed experimentally by observing a hysteresis loop in the photocurrent voltammogram measured in the predicted coexistence range.
In this paper we review some of the considerations and potential sources of error when conducting Incident Photon to Current Efficiency (IPCE) measurements, with focus on photoelectrochemical (PEC) ...cells for water splitting. The PEC aspect introduces challenges for accurate measurements often not encountered in dry PV cells. These can include slow charge transfer dynamics and, depending on conditions (such as a white light bias, which is important for samples with non-linear response to light intensity), possible composition changes, mostly at the surface, that a sample may gradually undergo as a result of chemical interactions with the aqueous electrolyte. These can introduce often-overlooked dependencies related to the timing of the measurement, such as a slower measurement requirement in the case of slow charge transfer dynamics, to accurately capture the steady-state response of the system. Fluctuations of the probe beam can be particularly acute when a Xe lamp with monochromator is used, and longer scanning times also allow for appreciable changes in the sample environment, especially when the sample is under realistically strong white light bias. The IPCE measurement system and procedure need to be capable of providing accurate measurements under specific conditions, according to sample and operating requirements. To illustrate these issues, complications, and solution options, we present example measurements of hematite photoanodes, leading to the use of a motorized rotating mirror stage to solve the inherent fluctuation and drift-related problems. For an example of potential pitfalls in IPCE measurements of metastable samples, we present measurements of BiVO
4
photoanodes, which had changing IPCE spectral shapes under white-light bias.
Thylakoid membranes contain the redox active complexes catalyzing the light-dependent reactions of photosynthesis in cyanobacteria, algae and plants. Crude thylakoid membranes or purified ...photosystems from different organisms have previously been utilized for generation of electrical power and/or fuels. Here we investigate the electron transferability from thylakoid preparations from plants or the cyanobacterium Synechocystis. We show that upon illumination, crude Synechocystis thylakoids can reduce cytochrome c. In addition, this crude preparation can transfer electrons to a graphite electrode, producing an unmediated photocurrent of 15 μA/cm2. Photocurrent could be obtained in the presence of the PSII inhibitor DCMU, indicating that the source of electrons is QA, the primary Photosystem II acceptor. In contrast, thylakoids purified from plants could not reduce cyt c, nor produced a photocurrent in the photocell in the presence of DCMU. The production of significant photocurrent (100 μA/cm2) from plant thylakoids required the addition of the soluble electron mediator DCBQ. Furthermore, we demonstrate that use of crude thylakoids from the D1-K238E mutant in Synechocystis resulted in improved electron transferability, increasing the direct photocurrent to 35 μA/cm2. Applying the analogous mutation to tobacco plants did not achieve an equivalent effect. While electron abstraction from crude thylakoids of cyanobacteria or plants is feasible, we conclude that the site of the abstraction of the electrons from the thylakoids, the architecture of the thylakoid preparations influence the site of the electron abstraction, as well as the transfer pathway to the electrode. This dictates the use of different strategies for production of sustainable electrical current from photosynthetic thylakoid membranes of cyanobacteria or higher plants.
The conversion of solar energy (SEC) to storable chemical energy by photosynthesis has been performed by photosynthetic organisms, including oxygenic cyanobacteria for over 3 billion years. We have ...previously shown that crude thylakoid membranes from the cyanobacterium Synechocytis sp. PCC 6803 can reduce the electron transfer (ET) protein cytochrome c even in the presence of the PSII inhibitor DCMU. Mutation of lysine 238 of the Photosystem II D1 protein to glutamic acid increased the cytochrome reduction rates, indicating the possible position of this unknown ET pathway. In this contribution, we show that D1-K238E is rather unique, as other mutations to K238, or to other residues in the same vicinity, are not as successful in cytochrome c reduction. This observation indicates the sensitivity of ET reactions to minor changes. As the next step in obtaining useful SEC from biological material, we describe the use of crude Synechocystis membranes in a bio-photovoltaic cell containing an N-acetyl cysteine-modified gold electrode. We show the production of significant current for prolonged time durations, in the presence of DCMU. Surprisingly, the presence of cytochrome c was not found to be necessary for ET to the bio-voltaic cell.
Transpiration is the process by which water is carried in plants from the roots to the leaves where evaporation takes place. Here, we report a transpiration driven electrokinetic power generator ...(TEPG) that exploits capillary flow of water in an asymmetrically wetted cotton fabric coated with carbon black. Accumulation of protons induced by the electrical double layer formed at the solid (carbon black)/liquid (water) interface gives rise to potential difference between the wet and dry sides. The conductive carbon black coating channels electrical current driven by the pseudostreaming mechanism. A TEPG of 90 mm × 30 mm × 0.12 mm yields a maximum voltage of 0.53 V, maximum current of 3.91 μA, and maximum energy density of 1.14 mWh cm–3, depending on the loading of the carbon black. Multiple TEPGs generate enough power to light up a light-emitting diode (20 mA × 2.2 V) or charge a 1 F supercapacitor.
In the drive to achieve economically viable solid oxide fuel cells, efforts have been directed towards substantially decreasing their operating temperature. Unfortunately, these efforts have been ...hindered by extremely sluggish electrode kinetics at reduced temperatures. In this report, we show that silicon impurities on the surface of the electrolyte play a critical role in influencing electrode kinetics. More specifically, improvements by as much as three orders of magnitude are reported for the performance of platinum electrodes on yttria-stabilized zirconia electrolytes prepared as high purity thin films with a largely Si-free surface. These improvements in performance are estimated to enable operation of a solid oxide fuel cell down to approximately 400 °C.
Gas sensors are employed in many applications including detection of toxic and combustible gases, monitoring emissions from vehicles and other combustion processes, breath analysis for medical ...diagnosis, and quality control in the chemicals, food and cosmetics industries. Many of these applications employ miniaturized solid-state devices, whose electrical properties change in response to the introduction of chemical analytes into the surrounding gas phase. Key challenges remain as to how to optimize sensor sensitivity, selectivity, speed of response and stability. The principles of operation of such devices vary and a brief review of operating principles based on potentiometric/amperometric, chemisorptive, redox, field effect and nanobalance approaches is presented. Due to simplicity of design and ability to stand up to harsh environments, metal oxide-based chemoresistive devices are commonly selected for these purposes and are therefore the focus of this review. While many studies have been published on the operation of such devices, an understanding of the underlying physicochemical principles behind their operation have trailed behind their technological development. In this article, a detailed review is provided which serves to update progress made along these lines. The introduction of nanodimensioned materials has had a particularly striking impact on the field over the past decade. Advances in materials processing has enabled the fabrication of tailored structures and morphologies offering, at times, orders of magnitude improvements in sensitivity, while high-resolution analytical methods have enabled a much improved examination of the structure and chemistry of these materials. Selected examples, illustrating the type of nanostructured devices being fabricated and tested, are discussed. This review concludes by highlighting trends suggesting directions for future progress.
Gas sensors produced by electrospinning demonstrate excellent performance in terms of sensitivity, reversibility, response and recovery time. These merits are formally attributed to their ...nanostructured fibrillar morphology and the relationship between microstructure and gas sensing properties. Thus, careful investigation is required to optimize the microstructure in order to achieve high gas sensing performance. In this work we study the microstructure evolution of nanostructured TiO2 gas sensors produced by electrospinning and examine the correlation between processing conditions, microstructure and gas sensing properties. Two distinguished TiO2 mesoporous morphologies were obtained, one with smaller grain size (29nm) and higher specific surface area (71.4m2/g) and the other with larger grain size (42nm) and lower specific surface area (13.3m2/g). The sensors displayed stable and reversible response to CO and NO2, with response and recovery time of several min. Two contributions to the overall impedance were observed, corresponding to the surface depletion region and the remaining bulk region. The surface impedance was found to be sensitive to CO and NO2 unlike the bulk impedance that displayed very little sensitivity to these gases. These results give new insight on the correlation between microstructure and gas sensing properties of electrospun TiO2 gas sensors.
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