bluuebunny/crossref_metadata_2025 huggingface dataset:
Stimuli-Responsive Hydrogel Films Based on Crosslinked Chitosan
Authors: Zhang Yongchao
Published: February 2020

Electrochemical research has been particularly difficult at primarily undergraduate institutions (PUIs) because students entering a research lab in their freshman and sophomore years have no or little knowledge of analytical chemistry or physical chemistry, let alone electrochemistry. However, considering the limited resources and amount of time faced by students at PUIs, electrochemistry presents a great opportunity to expose the students to multi-disciplinary research that encompasses chemistry, biology, material science and engineering.  Here we report the development of a stimuli-responsive hydrogel film based on crosslinked chitosan on the surface of an electrode. The film shows a pH-responsive, switch-like behavior. The negatively charged redox probe, Fe(CN)6
3-/4-, is found to be trapped and enriched in the catechol-crosslinked chitosan film under acidic conditions, and released into solution by either increasing pH or applying a negative voltage. The release of negative species from the film can be controlled by the amplitude and duration of the applied voltage. This pH- and voltage-responsive material has promising applications in biofabrications, biosensors and drug delivery systems. Future studies and incorporation of similar research in undergraduate curriculum will also be discussed.

Figure 1
Probing the Film Formation Mechanisms at the Slurry/Substrate Interface Relevant to the Chemical Mechanical Planarization Process
Authors: Janes Lisa M. , Keleher Jason J
Published: February 2020

Chemical Mechanical polishing (CMP) has emerged as a critical technology for achieving global planarization of substrates used in advanced technologies such as integrated circuits and data storage media. CMP has been described as a kinetic/thermodynamic balance between the formation of a chemically modified layer and its subsequent abrasion via nanoparticles.  While the nanoparticle in the slurry plays a key role it has been proposed that the chemically modified surface layer is the key activation barrier to achieving desired removal and surface quality. Therefore it is necessary to optimize and balance the rates of chemical modification and mechanical abrasion in the system, which is dependent on the nature of the complexing/passivation chemistry in the slurry.   This presentation will provide insight into the key reaction mechanisms that occur at the slurry/substrate interface relevant to integrated circuit (copper) and data storage substrate (nickel phosphorous) CMP processes.  This worked explored the use of electrochemical techniques (static/dynamic open circuit potential (OCP) and Tafel analysis) to probe the impact of organic additives on the surface redox and film forming mechanisms. More specifically, through monitoring the OCP (under both abrasive and non-abrasive conditions) versus time the film removal rate and subsequent passivation film formation kinetics can be determined as a function of slurry additives (complexing agents).  These results can be correlated to macroscopic performance metrics such as static/dynamic etch rates and overall polishing rates.  Additionally, the surface quality will be measured using atomic force microscopy (AFM) and a correlation between surface roughness and the film formation kinetics will be presented.
Structure and Electroactivity of Tungsten Oxides in Acid Medium: Enhancement of Charge Propagation Rates through Nanostructuring and Formation of Mixed-Metal Oxide Systems
Authors: Rutkowska Iwona Agnieszka , Wadas Anna , Kulesza Pawel J
Published: February 2020

Electrochromism in tungsten oxides and related systems has been recognized for long time and has been still of interest for display applications. Redox transitions and related reversible coloration of thin WO3 films are believed to be controlled by simultaneous injection or removal of electrons and protons. The dynamics of partial reduction (in acid medium) of transparent tungsten(VI) oxides to blue hydrogen tungsten(VI,V) oxide bronzes or lower tungsten(VI,IV) oxides is largely dependent of the oxide morphology, nanostructuring, degree of hydration, or population of colored W(VI,V) or W(VI,IV) sites. For example, by depositing nanorods of tungsten oxide, rather than conventional tungsten oxide, on the ITO conducting glass (or  the inert glassy carbon) substrate, the enhancement in the degree of the system's voltammetric reduction in acid medium (0.5 M H2SO4) has been observed. Tungsten oxide nanorods seem to be partially and reversibly reduced not only to nonstoichiometric hydrogen tungsten(VI,V) oxide bronzes (HxWO3, 0<x<1) but also substoichiometric lower tungsten (VI,IV) oxides (WO3-y, 0<y<1).

            Another important issue is the mobility and availability of protons within the tungsten oxide film. We have found that by controlled admixing of tungsten oxide with zirconia nanoparticles both nanostructured morphology and high mobility of protons can be adjusted. The resulting mixed (hybrid or composite) system is well-behaved: redox reactions (which are characteristics of WO3) are fast and reversible despite the presence of semiconducting zirconia (ZrO­2). While ZrO2 is not electroative, WO3 undergoes redox reactions leading to formation of partially reduced hydrogen bronzes (H
x
WO3) in which mobility of H+ is coupled to electron transfers. The composite material is stabilized through electrostatic attraction between positively charged zirconium oxo cations (ZrO2+, ZrOH3+, [Zr3(OH)4]3+) and anionic tungstates (WO4
2- in WO3*H2O or H2WO4). Among important issues are: high population of hydroxyl groups and high mobility of protons at surfaces of zirconia (ZrO2) nanostructures in addition to the existence of fast electron transfers coupled to proton displacements in partially reduced tungsten oxides (HxWO3). In a different series of chronocoulometric experiments, diffusion coefficients (for charge propagation) have been determined, and they have been found typically higher for hybrid mixed metal (tungsten(VI)/zirconium(IV) oxide films (close to 10-7 cm s-1).  Mechanistic considerations and the results of spectroelectrochemical and diagnostic spectroscopic (e.g. FTIR) experiments will also be provided.

We acknowledge collaboration with Christian Perruchot and Mohamed Jouini from ITODYS, Université Paris Diderot, France.
The Preparation and Electrochromic Properties of the Polyurethanes Containing Triphenylamine Moiety
Authors: Niu Haijun
Published: February 2020

Electrochromism is a phenomenon that optical changes of chemical materials occur in a reversible electrochemical process. For electrochromic polymers, the electrochromism is related to the changing of band gaps during the doping-dedoping process. The color changes are commonly between a transparent state, where the chromophore only absorbs in UV region and colored state, or between two colored states in a given electrolyte solution.

Two series of novel triphenylamine-containing aromatic polyurethanes (PUs) were synthesized via polycondensation of N, N-bis (4 - diisocyanate) -4 -N'- carbazolyl-1,4-aniline, and N, N-bis (4 - diisocyanate) -4 -N'- carbazolyl-1,4-phenylenediamine with various dihydroxy such as phenolphthalein, bisphenol A, 4,4'-dihydroxybiphenyl, 4,4'-bishydroxybenzophenone, bisphenolfluorene. The variation of the backbone structure significantly affected the dihedral angles and changed the electronic properties, solubility, molecular weight and thermal stability of PUs. The obtained PUs exhibited stable and reversible electrochromic properties. The color of PUs can be varied from initial colorless to green, then to blue via electro-oxidation. Considering the sensitive chromic properties of PUs, they will have potential applications for eletrochomic materials.

C.J. Chen, Y.C. Hu and G.S. Liou, Polym.Chem, 2013, 4,4162-4171.

H.J. Niu, J.W. Cai and P Zhao, Dyes Pigments, 2013, 96, 158-169.

H.M. Wang and S.H. Hsiao, Polymer, 2009, 50, 1692-1697.

Figure 1
Sunlight Responsive Thermochromics - Dynamic Windows Made Simple
Authors: Byker Harlan
Published: February 2020

Dynamic windows offer an opportunity to save energy while enhancing a building’s environment and the occupants’ sense of wellbeing. A thermochromic interlayer with a continuously variable light transmission has been developed for windows that are sunlight responsive. Thus the heat of the sun can provide all the energy required to tint the window and block the much of the visible portion of the energy of the sun when there is sun shining directly on the window. The windows clear to a high visible light transmission state when the sun is not shining directly on the windows. The thermochromic system involves a temperature change induced thermodynamic shift in equilibrium of a reversible reaction. The chemistry that allows this to take place will be described and a status update on commercialization will be given.
In Situ Optical Studies of Carbon Accumulation with Different Molecular Weight Alkanes on SOFC Ni Anodes
Authors: McIntyre Melissa D. , Walker Robert A
Published: February 2020

The addition of steam to carbon containing fuels is a common method for minimizing carbon accumulation on anodes in solid oxide fuel cells (SOFCs).  The required steam to fuel ratio depends on fuel type where higher molecular weight hydrocarbons require higher steam concentrations.  The mechanism(s) responsible for carbon suppression with humidified fuels remain speculative, however, and motivate the need for direct observation of the surface chemistry on SOFC anodes during operation. Experiments presented in this work utilize in situ Raman spectroscopy coupled with voltammetry measurements to examine the extent of carbon accumulation on Ni-based anodes of electrolyte supported SOFCs exposed to dry and humidified fuels (methane, propane and n-hexane) at 700ᵒC.  Vibrational Raman spectra show that highly ordered graphite forms on the anodes exposed to methane and propane while disordered graphite forms during exposure to hexane.  The carbon structure was unaffected when 3% steam was added to the fuel and the amount of carbon accumulation was reduced for humidified methane (figure 1).  Electrochemical data showed rapidly diminishing performance and continued cell degradation for anodes exposed to propane and hexane compared to experiments with methane.  From these data, we quantify carbon induced degradation of SOFC anodes exposed to the various fuels.

Figure 1. Representative Raman spectra collected on SOFC anodes exposed to (a) dry and (b) humidified methane (red), propane (green) and n-hexane (blue) at 700ᵒC.  The G and 2D peaks at 1561 cm-1 and 2695 cm-1, respectively, are associated with the vibrational modes of highly ordered graphite.  The D peak at 1346 cm-1 is associated with the vibrational mode of disordered graphite.  The Raman feature at 2915 cm-1 corresponds to the C-H vibrational mode and is present for spectra recorded during fuel exposure with methane.  The spectrum recorded on a pristine anode that has been fully reduced with hydrogen (black) is shown as a reference.   Spectra have been off set on the y-axis for clarity.

Figure 1
(Invited) Toward a Distributed Renewable Electrochemical Energy and Mobility System: Non-Precious Metal Hydrogen and Oxygen Evolution Catalysts in Base
Authors: Yan Yushan
Published: February 2020

One of the grand challenges facing humanity today is the development of an alternative energy system that is safe, clean, and sustainable and where combustion of fossil fuels no longer dominates. A distributed renewable electrochemical energy and mobility system (DREEMS) could meet this challenge. At the foundation of this new energy system are a number of electrochemical devices including fuel cells, electrolyzers, and flow batteries. For all these devices electrocatalysis plays a critical role in controlling their performance and cost, and thus their commercial viability. In this presentation, I will focus on our recent work on non-precious metal based oxygen and hydrogen evolution catalysts in base.
(Invited) Electrochemistry at Metal-Oxide Interfaces: At the Crossroads of Activity and Stability
Authors: Markovic Nenad M
Published: February 2020

The identification of materials needed for efficient energy conversion and fuel production in electrochemical systems must be guided by two equally important fundamental properties: optimization of their catalytic behavior and their long-term stability in hostile electrochemical environments. This is especially true for the oxygen evolution reaction (OER), the anodic-half cell reaction that takes place at high overpotentials on oxidized metal surfaces in hydrogen-oxygen electrolyzers, metal air batteries and electrometallurgy. The kinetics of the OER have, for the most part, been closely tied to the concept of the volcano plot, which generally expresses the rate of the OER as a function of more fundamental properties of the oxide materials, known as descriptors. These analyses showed that the interaction between the substrate and the reactants, intermediates, and the products has to be optimized for the reaction to proceed efficiently. Although concepts resulting from volcano plot analyses have led to the establishment of important catalytic trends, many fundamental questions still remain open. One key question is what relationship exists between the kinetics of the OER and the stability of oxide materials. The lack of understanding of such stability-activity relationships derives mainly from the fact that research directed to the development of anode materials for the OER has been strongly “activity-centric”, while almost completely ignoring the stability of active components during the OER.

Here, we show that this disparity in focus has masked the inherently close ties that exist between the stability and activity of monometallic and complex oxide catalysts. By studying the stability-activity relationships of well-characterized oxide surfaces, we demonstrate that there is a fundamental link between the stability of catalysts and their reactivity for the OER. This trend is observed for many oxide catalysts in both acidic and alkaline electrolytes, indicating that the stability-activity relationship is independent of the specific ionic/molecular species mediating the OER (i.e. OH- vs. H2O). We found that the degree of stability is always inversely proportional to activity and that stable surfaces are, in fact, not reactive.
Catalyst Design of Delafossite Oxides for Water Electrolysis Anode Using Theoretical Calculations
Authors: Toyoda Kenji , Hinogami Reiko , Miyata Nobuhiro , Aizawa Masato
Published: February 2020

Understanding the oxygen evolution reaction (OER) is crucial for improving the performance of water electrolysis. Delafossite oxides (ABO2) were investigated for their potential as OER catalysts using density functional theory (DFT) calculations. The relationships between the calculated eg or t2g occupancy of the B site and the experimentally-determined OER activity were examined. Our examination suggests that t2g electrons of the B site can be significant for the OER process. The calculated t2g occupancy is approximately linearly related to OER activity. Thus, we propose that t2g occupancy can be employed as a simple descriptor of the OER activity of delafossite oxide catalysts.
Electrochemical Deposition of Pt Nanoclusters on WC for Hydrogen Evolution Reaction in Acid
Authors: Liu Yihua , Kimmel Yannick C. , Esposito Daniel , Chen Jingguang , Moffat Thomas , You Hoydoo
Published: February 2020

Water splitting by electrolysis is an effective and renewable way to produce hydrogen fuel from acid or alkaline electrolytes.  Presently, alkaline water electrolyzers are being the most widely employed, largely due to the use of low-cost catalysts.  Yet, there is strong interest for expanded use of acid water electrolyzers in future because of better device performance than the alkaline.  In this work, we explore and develop new fabrication techniques that could minimize the use of the Pt catalyst that is highly active, but too expensive to be used for hydrogen evolution reaction (HER) in acid water electrolyzers.  Of our particular interest is to grow Pt monolayer films and nanostructures on tungsten carbide (WC) supports by electrodeposition. 

Fig. 1 shows Pt deposition as a function of applied potential from a NaCl-supporting electrolyte.  An interesting finding is that the deposition of Pt is prevented at potentials below -0.5 V, in spite of high driving forces for Pt clusters to form and grow.  This talk will present ways in which this anomalous deposition behavior can be exploited for preparing novel Pt catalysts. A brief summary will be given for Pt deposition on Au [[1]] and Ni [[2]] electrodes where a monolayer-thick Pt film is deposited at -0.8 V but additional growth is quenched.  For WC substrates, we will discuss challenges associated with the lack of Pt depositing at -0.80 V, before introducing to a new approach that is developed to facilitate the formation of Pt nuclei.   

The proposed method involves cycling the potential between -0.8 V and -0.45 V at a fast rate of 50 mV/s.  As shown in Fig. 2, Pt nanoclusters with high particle density could be grown on WC, and their average size is about 5 nm. To gain mechanistic insights on Pt nanostructure nucleation and growth, the deposits were analyzed using scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), and underpotentially deposited (UPD) hydrogen and copper.  Correlations between deposition potential cycle and particle growth and particle density will be discussed.  

Finally, Pt deposits were examined as catalysts for HER in 0.5 M sulfuric acid.  The dependence of HER activity on the number of deposition cycles is shown in Fig. 3 where the HER activity approaches that of bulk platinum as the deposition cycles increase even though the expected total amount of platinum is far less than a complete coverage. This work illustrates that electrodeposition provides a simple and scalable way for preparing Pt nanocluster/WC catalysts, which exhibit high activity and durability toward HER in acid media.    

[1] Y. Liu, D. Gokcen, U. Bertocci, T. P. Moffat, Science, 338, 1327 (2012).

[2] Y. Liu, C. M. Hangarter, D. Garcia, T. P. Moffat, Surf. Sci. (2014)  dx.doi.org/10.1016/j.susc.2014.06.002

Figure 1
Photoelectrochemical Phenomena at Nano-V2O5 Films Impregnated with Bi3+
Authors: Mascaro Lucia Helena , Gromboni Murilo Fernando , Marken Frank , Downeey Elisabeth
Published: February 2020

In the last two decades, the development of nanoscience and technology developed several nanostructures at different forms, such as nanoparticles, nanowires, nanotubes and nanoribbons. These materials offer a high potential for improving properties and can be used in many fields such as microelectronics, batteries, photovoltaic and photocatalytic devices. Vanadium oxides belong to the important class of 3d transition metals, with various electric, magnetic and structural properties, making these materials attractive for many industrial applications. This study aimed to obtain and characterize vanadium oxide films on titanium from ammonium metavanadate and then modify it by impregnation with Bi (III). The vanadium pentoxide was synthesized on Ti substrates by painting the surface with a suspension of 250 mg ammonium metavanadate in 5 ml of polyethylene glycol (PEG-300). The coated Ti plate was then heated at 400, 500 or 600 °C for one hour. Subsequently, the film obtained was impregnated with Bi (III) for 120 minutes under constant stirring, using 50 mL of a solution 10 mmol L-1 of bismuth nitrate at nitric acid. Then the samples were again annealed for one hour at the same temperature. The films were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), UV-Visible spectroscopy and photoelectrochemical. From SEM analysis was possible to observe, after impregnation of Bi(III), the growing of long crystals on V2O5 films at 400 and 500 °C, while for the sample to 600 °C a globular structure was preferentially deposited on the tip of V2O5 crystals. Through EDS analysis it was observed that the sample obtained at 500 °C is that has the highest amount of Bi in the composition. The mapping by EDS showed that the elements were distributed over all surface. From XRD analysis the presence of V2O5 orthorhombic phase and monoclinic BiVO4 was detected. Band-gap calculated for the samples before and after impregnation also proved that V2O5 was converted to BiVO4, since the value before impregnation was 2.20 eV and after impregnation showed a value of 2.42 eV. The photocurrent value obtained for nano-V2O5 films impregnated with Bi3+ at 500 °C was 5 times higher than pure BiVO4 films. The results showed that it has been possible obtain a nanostructured V2O5 / BiVO4film on Ti substrate which is photoactive and could be used both in photoelectrocatalytic water as oxidation of organic molecules.

Acknowledgements: FAPESP 2012/23422-5 and CAPES.
Selective Oxygen Reduction to Hydrogen Peroxide Using Earth-Abundant Metal Pyrites As High-Performance Electrocatalysts
Authors: Ying Diwen , Ding Qi , Jin Song
Published: February 2020

It is important to develop alternative catalysts made of inexpensive and earth-abundant elements with high catalytic activity and stability to replace the noble metal catalysts in various environmental1 and renewable energy applications.2 Here we report earth-abundant metal pyrites MS2 (M as Fe, Ni, and Co) as high performance electrocatalysts3,4 for oxygen reduction reaction (ORR) that is selective for generating hydrogen peroxide. In neutral solution of 0.1 M sodium perchlorate and acidic solution of 0.1 M perchloric acid, all three MS2 show high ORR catalytic activity with excellent Tafel slopes. Based on Koutecky-Levich (K-L) analysis, all MS2 and noble metal show similar potential-dependent selectivity of the two-electron peroxide pathway in neutral condition, and CoS2 is the best among metal pyrites in terms of the positive potential for 50% H2O2selectivity. The stability of metal pyrites is quite good in under ORR operation in both neutral and acidic solutions. These inexpensive metal pyrites are excellent ORR catalysts for selective peroxide generation and could find promising applications in environmental and renewable energy technologies.

1. X.-W. Liu; W.-W. Li; H.-Q. Yu; “Cathodic catalysts in bioelectrochemical systems for energy recovery from wastewater” Chem. Soc. Rev., 2014, 43, 7718-7745.

2. Matthew S. F.; Song Jin;  “Earth-abundant inorganic electrocatalysts and their nanostructures for energy conversion applications” Energy Environ. Sci., 2014, 7, 3519-3542.

3. Matthew S. F.; Mark A. L.; Qi Ding; Nicolas S. K.; Song Jin; “Earth-Abundant Metal Pyrites (FeS2, CoS2, NiS2, and Their Alloys) for Highly Efficient Hydrogen Evolution and Polysulfide Reduction” J. Phys. Chem. C, 2014, 118 (37), 21347-21356.

4. Matthew S. F.; Rafal D.; Mark A. L.; Nicholas S. K.; Qi Ding; Song Jin; “High-Performance Electrocatalysis Using Metallic Cobalt Pyrite (CoS2) Micro- and Nanostructures” J. Am. Chem. Soc., 2014, 136 (28), 10053-100061.

Figure 1
Metal-Metal Bonding Dynamics of Anode Materials for Oxygen Evolution Reaction (OER) in Basic Electrolyte Studied By Using in Situ Surface Stress Measurements
Authors: Hoang Thao Thi Huong , Cohen Yair , Gewirth Andrew A.
Published: February 2020

The electrochemical splitting of water offers an attractive way to provide a carbon-free source of hydrogen, however the efficiency is limited primarily by the overpotential of the anodic oxygen evolution reaction (OER, 4OH-→ 2H2O + 4e- + O2).1

Knowledge of oxide formation prior and during the OER is essential for understanding the mechanism and electrocatalysts. Extensive studies on the composition and morphology of metal and oxide electrodes in the OER by Raman,2 XAS,3 and QCM4have been reported. However, there is no reported technique to directly interrogate the oxide film during potential cycling or its evolution over time and multiple electrochemical cycles.            

In this work, in situ electrochemical stress measurements are used to interrogate changes in oxide structure before and during the oxygen evolution reaction (OER) from Ir, Ni, Co, Au, and Pt electrodes in alkaline electrolyte. Stress evolution during potential cycling reports on changes in oxidation state and oxide forms. Hysteresis observed in the potential - dependent stress from Ir, Au, and Pt electrodes is associated with chemical irreversibility in electrode composition and roughness. Alternatively, Ni and Co exhibit reversible conversion between hydroxide and oxyhydroxide forms during cycling. From the experimentally determined stress, charge passed during electrode oxidation, and Young’s modulus, the change in strain exhibited by Ni and Co electrodes during hydroxide-oxyhydroxide conversion is calculated to be 7.0% and 8.4%, respectively. We also show that the magnitude of change in stress is proportional to the amount of material that is further oxidized. The similarity between processes yielding higher oxides and those involved with the OER mechanism yields are a rough correlation between film thickness and OER onset.5

Finally, we report the effect of electrodeposition additives to modify electrodeposited OER-active films to achieve high electrocatalyst efficiency for this reaction.

References

(1) Matsumoto, Y.; Sato, E. Materials Chemistry and Physics
1986, 14, 397-426.

(2) Desilvestro, J.; Corrigan, D. A.; Weaver, M. J. J Electrochem Soc
1988, 135, 885-892.

(3) Totir, D.; Mo, Y.; Kim, S.; Antonio, M. R.; Scherson, D. A. J Electrochem Soc
2000, 147, 4594-4597.

(4) Mo, Y.; Hwang, E.; Scherson, D. A. J Electrochem Soc
1996, 143, 37-43.

(5) Hoang, T. T. H.; Cohen, Y.; Gewirth, A. A. Analytical Chemistry
2014, 86, 11290-11297.
In Situ Reactive Imaging of Photoassisted Water Splitting Reaction Intermediates on n-Doped Strontium Titanate Using Surface Interrogation Scanning Electrochemical Microscopy
Authors: Simpson Burton H , Zhou Xuan , Rodríguez-López Joaquín
Published: February 2020

Scanning electrochemical microscopy (SECM) gives spatially resolved information about electrochemical reactions occurring on surfaces, and easily probes reaction mechanisms. The in situ surface study of photoelectrochemical processes, such as water splitting at semiconductor electrodes, is challenging due to its fast kinetics and the multiple reactive intermediates formed. By combining the surface interrogation mode (SI-) of SECM [1] with transport simulations, the reactivity of multiple adsorbed intermediates resulting from water splitting on gently n-doped strontium titanate (STO) was investigated.  Using this technique, we addressed the in situ detection of long lived holes and photogenerated hydroxyl radical on STO. Combined with the measurements on the local production of oxygen using the collection mode of SECM, our methodology provides an integrated view of reactivity at photocatalytic surfaces.

In our SI-SECM measurements, a micro- or nano-electrode used as SECM tip was positioned close to the STO surface and activated to generate a redox titrant that provided information about the surface coverage and reaction kinetics of adsorbed intermediates. By addressing different parts on the electrode surface, it is possible to obtain information about different surface condition son the STO electrode. This combined methodology brings spatial resolution to SI-SECM, and measured different adsorbate coverage on pristine and defective STO (100) surfaces. For the first time, SI-SECM was adapted to the reactive imaging of adsorbates to show how these affect photocatalytic performance on a heterogeneous surface.

[1] Rodríguez-López, J. The Surface Interrogation Mode of Scanning Electrochemical Microscopy (SI-SECM): an approach to the study of adsorption and (electro)catalysis at electrodes. In Electroanalytical Chemistry, a series of advances. Vol. 24. Bard, A.J. and Zoski, C.G., Eds. 2012, CRC Press, pp. 287-352.
A New Family of Catalysts for Production of Solar Fuels
Authors: Hupp Joseph T.
Published: February 2020

Nature uses a variety of transition-metal containing, cluster-based cofactors to catalyze water oxidation, carbon dioxide reduction, and hydrogen evolution. Efforts to mimic Nature by constructing artificial catalytic clusters are often complicated either by catalyst aggregation and inactivation, or poor catalytic activity due to blocking of catylst active-sites by ligands needed for cluster stabilization or immobilization. We have developed a route to catalytic clusters of well-defined and pre-determined size and composition that is based on deploying atomic layer deposition on electrode-supported arrays of metalloporphyrins, or within appropriately designed, mesoporous metal-organic framework materials. This approach avoids the complications of both catalyst aggregation and ligand inactivation, while setting the stage for systematic, hypothesis-driven investigations of catalyst size/activity and composition/activity relationships.

Our focus thus far has been chiefly on transition-metal oxides and transition-metal sulfides. This talk will present the general approach to catalyst synthesis, as well as some representative findings for water oxidation,water reduction, or CO2 reduction.
Enhanced Low Bias Performance in Textured BiVO4 Photoanodes through Halide and Phosphate Treatments
Authors: Nair Vineet Vijayakrishnan , Perkins Craig L , Law Matt
Published: February 2020

Over the last couple of years, BiVO4 has merged as a promising photoanode material owing to its earth abundance, suitable band gap (2.4eV), high absorption coefficient and a reasonable hole diffusion length (100nm)[1]. The intrinsic electron mobility has been shown to be a bottleneck[2] in achieving high photocurrents with a standalone BiVO4 photoanode. Recently photocurrents as high as 5mA/cm2 at OER have been demonstrated using a heterojunction with WO3 rods[3]. This however does not solve the intrinsic electron transport problems of BiVO4 and hence enhancing the photoresponse at relatively low biases close to the flatband potential still remains a challenge and limits its application in a high efficiency tandem device. Here we report photocurrents in excess of 3mA/cm2 at applied biases as low as 0.6V vs RHE with an onset potential of 0.19V vs RHE for sulfite oxidation.

A common route to improving the elctronic performance of BiVO4 is via addition of W or Mo as dopants. However, the addition of both W and Mo always results in an anodic shift in the onset potential, limiting the photoresponse at lower biases. In the this study, phosphate-based treatments of the fluorine-doped tin oxide are shown to eliminate the small rectifying junction existing at the back contact with BiVO4. Simultaneously, halide treatments were used to improve electron transport in undoped –BiVO4. Using results from our previous work[4], we have been able to successfully texture 300nm thin films with the previously mentioned phosphate and halide treatments to unity carrier separation at OER and over 80% at 0.6V RHE. Owing to the {004} texturing, these devices also demonstrate a record high catalytic activity for a standalone BiVO4 photoanode towards oxygen evolution. Such high carrier separation was achieved via halide treatment as well as surface texturing which reduces the density of in-gap surface defects states. UPS and XPS measurements were used to ascertain the nature and magnitude of these in-gap states. Impedance spectroscopy shows that the enhanced charge transfer on the surface of the textured photoelectrodes is the origin for the catalytic efficiency of these devices.

This study hopes to pave the way towards a rational improvement in the electronic and catalytic properties of standalone undoped- BiVO4 as well as a new approach in designing highly efficient photoanodes for oxygen evolution.

[1] Abdi F.F., Savenije T.J., May M.M., Dan B., Krol R., J.Phys.Chem.Lett., 2013,4,2752-2757

[2] Park Y., McDonald K.J., Choi K.S., Chem.Soc. Rev., 2013,422321-2337.

[3] Shi X., Choi Y., Zhang K., Kwon J., Kim D.Y., Lee J., Ho L.S., Kim J.K., Park J.H., Nat. Comm., 5, 2014, 4775.

[4] Nair V.V., Perkins C.L., Law M., Submitted.

Figure 1
Engineering of Oxide Materials for Improved Photoelectrochemical Water Oxidation
Authors: Lin Yan-Gu , Hsu Yu-Kuei
Published: February 2020

Global climate warming and environment pollution have spurred scientists to develop new high-efficient and environmental-friendly energy technologies. Hydrogen is an ideal fuel for fuel cell applications. Hydrogen has to be produced from renewable and carbon-free resources using nature energies such as sunlight if one thinks of clean energy and environmental issues. In this regard, a photoelectrochemical (PEC) cell consisting of semiconductor photoelectrodes that can harvest light and use this energy directly for splitting water is a more promising way for hydrogen generation. Abundant and inexpensive oxide semiconductor such as ZnO has been recognized as a promising photoelectrode, but the photoconversion efficiency is substantially limited by its large band gap and rapid charge recombination. Recently, doping with 4d transition metal, such as Mo, has been carried out to remarkably enhance the PEC performance of many photoanodes including TiO2, BiVO4, and Fe2O3. Nevertheless, 1-D Mo-doped ZnO nanostructures have not been reported for PEC water splitting. We report the first demonstration of cobalt phosphate (Co-Pi) assisted Zn1-xMoxO nanorods (NRs) as visible-light-sensitive photofunctional electrodes to fundamentally improve the performance of ZnO NRs for PEC water splitting. The maximum photoconversion efficiency could be successfully achieved as high as 1.05%, with the significant photocurrent density of 1.4 mA cm-2. More importantly, in addition to achieve the maximum incident photon to current conversion efficiency (IPCE) value of 86%, it could be noted that the IPCE of Zn1-xMoxO photoanodes at the monochromatic wavelength of 450 nm is up to 12%. Our PEC performances are comparable to those of many oxide-based photoanodes in recent reports. The improvement in photoactivity of PEC water splitting may be attributed to the enhanced visible-light absorption, increased charge-carrier densities, and improved interfacial charge-transfer kinetics due to the synergistic effects of Mo incorporation and Co-Pi modification, thus contributing to photocatalysis. The new design of constructing highly photoactive Co-Pi assisted Zn1-xMoxO photoanodes enriches the doping community and sheds light on developing high efficiency photoelectrodes for solar-hydrogen field.

Reference:

[1] Y.G. Lin, Y.K. Hsu, Y.C. Chen, S.B. Wang, J.T. Miller, L.C. Chen, and K.H. Chen, Energy Environ. Sci., 5, 8917 (2012).

[2] Y.G. Lin, Y.K. Hsu, Y.C. Chen, L.C. Chen, S.Y. Chen, and K.H. Chen, Nanoscale, 4,6515 (2012).

[3] Y.K. Hsu, Y.C. Chen, Y.G. Lin, L.C. Chen, and K.H. Chen, J. Mater. Chem., 22, 2733 (2012).

[4] Y.G. Lin, Y.K. Hsu, A.M. Basilio, Y.T. Chen, K.H. Chen, and L.C. Chen, Optics Express, 22, A21 (2014).

[5] Y.K. Hsu, S.Y. Fu, M.H. Chen, Y.C. Chen, and Y.G. Lin, Electrochimica Acta, 120, 1 (2014).

[6] Y.G. Lin, Y.C. Chen, J.T. Miller, L.C. Chen, K.H. Chen, and Y.K. Hsu, ChemCatChem, 6, 1684 (2014).

[7] Y.G. Lin, Y.K. Hsu, Y.C. Chen, B.W. Lee, J.S. Hwang, L.C. Chen, and K.H. Chen, ChemSusChem, 7, 2748 (2014).
Near-Isothermal Doped-Hercynite Redox Cycle for Solar-Thermal Water Splitting
Authors: Muhich Christopher , Ehrhart Brian , Alshankiti Ibraheam , Ward Barbara , Musgrave Charles , Weimer Alan
Published: February 2020

Hydrogen productivity exceeding 350 micromoles H2/g total redox material has been demonstrated for near-isothermal processing using the doped-hercynite cycle for oxidation with steam carried out at 1350oC following 1500oC reduction.  This temperature difference driving the redox is quite narrow compared to standard 500oC temperature swing (T-swing) redox processing.   Such near-isothermal processing substantially reduces the difficult solid/solid heat recuperation, the energy required to heat back up to reduction temperatures following oxidation and the thermal stresses associated with heating/cooling active materials associated with standard T-swing redox cycling.  Evidence is provided indicating that the cycle occurs via an oxygen-vacancy formation process.  Efficiency analysis identifies isothermal processing with perfect steam/steam heat exchange as providing for the highest theoretically possible efficiency.  Although it may be impossible to operate isothermally with perfect gas/gas heat exchange in practice, it does provide for an optimal “stretch” target to aim for.   Near-isothermal operation seems achievable in practice and may provide for the most realistic efficient mode of operation.
Using Soft X-Rays and Electrons to Determine the Chemical and Electronic Structure of Semiconductors for Solar Water Splitting
Authors: Heske Clemens
Published: February 2020

In order to maximize the contribution of solar energy to the future global energy portfolio, it is important to optimize the performance, stability, and cost of the devices involved in solar energy conversion (in particular solar water splitting). For that purpose, novel material classes are being exploited, invariably leading to the investigation and use of complex, often ternary or even multinary compounds.

The purpose of this talk is to demonstrate how a tool chest of soft x-ray and electron spectroscopies (in particular using high-brilliance synchrotron radiation) is uniquely suited to unravel the electronic and chemical properties of surfaces and interfaces of such materials, and how an understanding on such fundamental level can lead to increased performance. It will be shown how photoelectron spectroscopy (PES), x-ray-excited Auger electron spectroscopy (XAES), inverse photoemission (IPES), x-ray emission spectroscopy (XES), and x-ray absorption spectroscopy (XAS) can be suitably combined to derive band gaps, study local chemical bonding and electronic level alignment, and derive insights into chemical stability.

As examples, WO3- and GaInP2-based water-splitting devices will be discussed, and Cu(In,Ga)(S,Se)2 solar materials will be used to further illustrate pertinent aspects of the experimental tool chest.
Artificial Photosynthesis Using Iron Foam Photoanodes Coated with Catalytic Graphene-Manganese Porphyrin Self-Assembled Structures
Authors: Kaplan Amir , Korin Eli , Bettlehiem Armand
Published: February 2020

Photoelectrochemical (PEC) water splitting, also called artificial photosynthesis, is one way to produce clean hydrogen using solar energy to split water into oxygen and hydrogen gases. The main challenge in PEC field deals with low current densities of the photoanode. Today's conventional approaches to increase current densities are directed towards improving semiconductor (e.g. hematite) properties, increasing the active surface area of the electrode and adding known water splitting co-catalysts.

            The present work deals with the characterization of porous iron foams as photoanodes, instead of the mostly used planar transparent electrodes. The iron foams can be coated by a thin hematite layer by a simple heat treatment procedure. The performance of these electrodes is significantly increased by doping with silicon, Co(II) ions, and macrocyclic catalysts. The iron foams electrodes yield efficient photocurrents for water splitting in the range of ~ 0.2-0.4 mA/cm2 at 1.23 V vs. NHE.

            Anionic carboxylic-rich graphene oxide (CGO) and cationic Mn(III) tetrakis(N-Methyl-4-Pyridinium) porphyrin (MnTMPyP) form self-assembled leaf-like structures. These are obtained through electrostatic and π-π stacking interactions, as indicated by the red shift of the metalloporphyrin Soret band throughout the 6-12 pH range. When exploited for the photoelectrochemical oxidation of water, CGO-MnTMPyP films on iron/hematite foams exhibit substantial activity as evidenced by the current density (0.54 mA/cm2 at 1.23 V vs. NHE) and IPCE (9.0 % at 1.43 V vs. NHE) measured at pH 10.
Electrophoretic Deposition of NiCo2S4 Thin Film As a Catalytic Material for Dye-Sensitized Solar Cells
Authors: Chou Shu-Wei , Lin Jeng-Yu
Published: February 2020

Dye-sensitized solar cells (DSCs) have become one of the most promising photovoltaic devices due to their low cost, simple fabrication, and relatively high powerconversion efficiency.To reduce the cost of DSCs, it is necessary to find out high-efficiency and low-cost alternatives to replace the conventional noble Pt counter electrode (CE). Among inorganic compounds, transition metal sulfides, such as CoS, NiS,MoS2 and WS2, have attracted extensive attention dueto their excellent electrocatalytic activity for I3
− reduction.

In this study, NiCo2S4 was firstly synthesized via a facile solvothermal method and then subjected to electrophoresis onto a fluorinated tin oxide (FTO) glass substrate. The resultant NiCo2S4 CE demonstrated both high transmittance (>75%) at visible wavelengths and improved electrocatalytic activity, while the DSC based on the transparent CE still achieved an impassive cell efficiency of 6.14%.Moreover, the NiCo2S4 films deposited by the facile electrophoretic deposition has the following benefits,including low temperature, high depositionrate, controllable thickness, great uniformity, scalable production, and without using any organic binders.Therefore, the DSC assembled withthe transparent NiCo2S4 CE could be a promising candidate as Pt-free CE for DSCs application.
Novel Photoelectrodes and Noble Metal-Free Catalysts for Light-Driven Water Electrolysis
Authors: Fiechter Sebastian
Published: February 2020

The generation of fuels from sunlight and water is considered as a task of paramount importance for a sustainable energy supply in the future. Decomposition of water by a photoelectrochemical process is a possibility to harvest solar energy in the form of hydrogen in a large scale. For this purpose, a membrane can be used which -when immersed in an aqueous electrolyte and illuminated by sunlight- will be able to split water into hydrogen and oxygen. Such a device is frequently addressed as artificial leaf. From an eco­nomic and environmental point of view this device should comprise of cheap, abundant and non-toxic elements featuring a Solar-to-Hydro­gen (STH) efficiency > 10% and generating hydrogen at a price of ~3 €/kg H2.

In our lab two approaches have been pursued to realize artificial leaf type structures:

o  triple junction thin film a-Si/µc-Si solar cells in superstrate geometry with integrated catalysts [1],

o  tandem junction solar cells combined with novel semiconducting ternary metal oxide anodes [2].

As photoanode materials Fe2WO6 and BiVO4 are under investigation. Their performances have shown considerable improvement with respect to their photovoltage and their photocurrent densities when synthesized using sol-gel and spray pyrolysis. To lower the costs of the device, platinum was replaced by carbon supported MoS2 nanoparticles as hydrogen evolving catalyst. Deposited as a blend with PEDOT:PSS on the backside of the solar cell, a SHE yield of 3.7% was achieved. RuO2 was replaced by manganese and cobalt oxide thin films, respectively.

References:

[1]      Diana Stellmach, Peter Bogdanoff, Onno Gabriel, Bernd Stannowski, Rutger Schlatmann,

          Roel van de Krol, Sebastian Fiechter; FORMATEX 2013, 880-886.

[2]      Fatwa F. Abdi, Lihao Han, Arno H.M. Smets, Miro Zeman, Bernard Dam, Roel van de Krol,          NATURE COMM. 2013, 4: 2195, DOI: 10.1038/ncomms3195.

[3]      Eric L. Miller, Björn Marsen, Daniela Paluselli, Richard Rocheleau, Electrochemical and Solid-      State Letters 2005, 8: A247.

Figure 1
Pt- and TCO-Free Flexible Counter Electrodes for Dye-Sensitized Solar Cells
Authors: Lin Jeng-Yu , Wang Wei-Yan , Chou Shu-Wei
Published: February 2020

Dye-sensitized solar cells (DSSCs) have attracted extensiveattention as low cost, easy manufacturingprocess and acceptedphotovoltaic performance. Typically, a DSSC includes a dye-sensitizedsemiconductor photoanode, an I−/I3
− redox electrolyte and a Pt counter electrode (CE). As a crucial component in DSSCs, the basicfunctions of a CE are to increase the transfer of the electrons from the external circuit back to the electrolyte and accelerate the reduction reaction from I3
−to I−.Nevertheless, limited resources and high prices would restrict the application of Pt thin film coated on a transparent conducting oxide (TCO).To solve this issue, several alternatives have been proposed to serve as TCO-freeand Pt-free CEs for DSSCs.

In this study, Ni3S2/Ni−P hybrid film coated on a flexible polyimide (PI) substrate was employed as a TCO-free and Pt-free flexible CE for DSSCs. The bilayer Ni3S2/Ni−P hybrid film was deposited on a polyimide (PI) plastic substrate by using a series of chemical/electrochemical treatments/processes. As a result, the DSSC assembled with the Ni3S2/Ni−P coated PI flexible CE displayed an impressive photovoltaic conversion efficiency of 6.28%.
Double-Wall TiO2 Nanotubes for Dye-Sensitized Solar Cells: A Study of Growth Mechanism
Authors: Lin Lu-Yin , Chen WeiChieh , Yeh Min-Hsin , Ramamurthy Vittal , Ho KuoChuan
Published: February 2020

One–dimensional TiO2 nanotubes (TNTs) are promising semiconductor photoanodes for dye–sensitized solar cells (DSSCs) due to their rapid electron transportation property, however, the lack of high surface area restricts their application. Double–walled TiO2 nanotubes (DWTNTs) were synthesized to compensate for the smaller surface area of single–walled TiO2 nanotubes (SWTNTs) and the comprehensive growth mechanism of DWTNTs is clearly demonstrated in this study. The evolution of pores and pits in three dimensions causes the formation of DWTNTs. After optimizing the thickness of DWTNTs, the DSSC with DWTNTs on the flexible Ti substrate showed a higher power conversion efficiency (η) of 6.85%, as compared to that of the cell with SWTNTs (4.63%), owing to a significant increase in the short–circuit photocurrent density (J
SC) caused by the higher surface area for dye adsorption for the former case. Electrochemical impedance spectroscopy (EIS), incident photon–to–electron conversion efficiency (IPCE) curves, and UV–vis absorption spectra were conducted to sustain the explanations.
Photovoltaic Effect in Ferroelectric Pt/(Bi0.9La0.1)(Fe0.97Ta0.03)O3/Graphene Hetrostructures
Authors: Katiyar Rajesh K. , Sharma Yogesh , Barrionuevo Diestra Danilo G. , Mendoza Frank , Pavunny Shojan P. , Morell Gerardo , Weiner Brad R. , Katiyar Ram S.
Published: February 2020

Recently, observation of switchable polarization-induced ferroelectric photovoltaic effect (FEPV) in BiFeO3 (BFO) has attracted great interest. In order to improve its FEPV properties, co-substituted [Bi0.9La0.1][Fe0.97Ta0.03]O3 (BLFTO) films were fabricated on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition (PLD). The phase formation of the films was confirmed by X-ray diffraction, Raman spectroscopy, and XPS studies. The FEPV properties in La and Ta co-doped BiFeO3 (BFO) thin films were evaluated under illumination using sandwich configuration with a transparent conducting electrode (TCE) graphene as top electrode. The TCE electrode was transferred by chemical method on BLFTO thin films. The band gap of the films was determined to be 2.66 eV from spectrophotometric measurements, which is very close to that of pure BFO (2.7 eV). The ferroelectric nature of the films was characterized by P-E loop measurements, which indicated that leakage was still the prominent factor for not getting good polarization hysteresis. The piezo-force microscopy (PFM) studies however confirmed the ferroelectric nature of the films. The photovoltaic effect of the films was studied in both geometries, namely top-bottom and planar electrodes configurations. We will present aforementioned investigations and discuss possible photovoltaic applications of the above structure.