bluuebunny/crossref_metadata_2025 huggingface dataset:
Intermetallic PtCo Catalysts for Fuel Cell Applications
Authors: Wang Chenyu , Li Dongguo , Kim Yu Seung , Cullen David A. , More Karren L. , Spendelow Jacob S
Published: February 2020

Conventional catalysts for the cathode reaction in polymer electrolyte fuel cells (PEFCs) are based on Pt and random alloys of Pt with base metals. Addition of base metals such as Co to nanostructured Pt catalysts is known to enhance oxygen reduction reaction (ORR) activity, but poor stability of Co under the aggressive conditions of high potential and low pH found in the cathode of PEFCs limits the enhancement provided by Co. Recently, we have developed several varieties of L10 ordered PtCo nanoparticle catalysts that exhibit remarkably enhanced performance and durability. Several factors, including particle size, Co content, degree of ordering, Pt shell thickness, and nanoparticle-support interactions are critical in determining performance and durability. Synthetic procedures developed at LANL have enabled preparation of L10-PtCo with small particle size, high electrochemical surface area, good dispersion on the carbon-based support, and high performance in PEFC cathode testing under H2/air. These intermetallic particles retain high Co content with L10 ordering even after 30,000 square wave potential cycles between 0.6 and 0.95 V in membrane electrode assemblies (MEAs). Content of Co is a key factor controlling ORR kinetic performance. Furthermore, leaching of Co into the membrane and electrode ionomer can result in performance degradation due to cationic poisoning. Therefore, any reduction in Co leaching from PtCo catalysts would be helpful. The L10-PtCo structures developed at LANL retain Co content approximately 50% higher than a random PtCo alloy after 30,000 potential cycles in MEA. This excellent resistance to Co leaching is the most important contribution of the L10 structure to providing high durability. Testing of new PtCo catalysts in MEAs is particularly critical to validate their performance and durability under real-world conditions. In addition to discussing the L10-PtCo synthesis, this talk will also describe work at LANL on optimizing the integration of these catalysts into high performance, durable MEAs.
Miniaturized Electrochemical Supercapacitor with Patterned Horizontally Aligned Carbon Nanotube Sheets
Authors: Dousti Behnoush , Choi Ye Il , Lee Gil Sik
Published: February 2020

Introduction: Miniaturized energy storage systems have emerged as a promising approach to address the growing demands to power electronic devices on small footprint area. Microsupercapacitors have become a crucial part of this advancement and have drawn much attention in the research communities. Designing a microstructure and choosing the electroactive materials are two significant factors that greatly affect the performance of the device. Here, we report for the first time the successful fabrication and characterization of a microsupercapacitor with interdigitated structure based on carbon nanotube sheets (CNT sheet). Carbon nanotube sheet is a novel structure which is spun directly from the as-grown carbon nanotube forest. Horizontally aligned structure of CNT sheet offering excellent conductivity and large surface area enabled us to develop a light micro-device with higher electrochemical performance than other CNT-based counterparts in aqueous electrolyte.

Materials and process: Spinnable carbon nanotube was grown via chemical vapor deposition (CVD) in a horizontal furnace at temperature of 780°C using acetylene as hydrocarbon and iron as metal catalyst. Multi-sheets of 5, 10, 20, 30 and 50 layers on two different substrates (glass slide and silicon wafer) and were patterned with reactive ion etch (RIE). CNT sheet was used as both active material and current collector. Interdigital structure (IDE) with gap size of 145 um, electrode width of 300um and the length of 5mm were fabricated. The effect of active material thickness on the device performance including specific capacitance and rate capability was studied. Furthermore, long-term stability of the devices was investigated using cyclic voltammetry technique at scan rate of 100mV/s for 10,000 cycles.

Results: Thickness dependent specific capacitance of 5 mF/cm2
at the scan rate of 50 mV/s per footprint area of the device was obtained for multi-sheet of 50 layers. Stability and performance of the device was tested using an aqueous PVA-H3PO4 gel electrolyte that also offers desirable electrochemical capacitive properties. High Coulombic efficiency (around 95%), great rate capability (1.2 mF/cm2 at scan rate of 5V/s) and excellent capacitance retention (less than 12%) over 10,000 cycles were obtained.

Conclusion: We have fabricated a long-term stable planar microsupercapacitor using horizontally aligned carbon nanotube sheets and achieved a noticeable specific capacitance and high rate capability which demonstrates the potential of CNT sheet for future thin film micro scale energy storage devices.

Figure 1
Cu-Pt Bi-Metallic Catalysts By Continuous Electroless Deposition for Methanol Oxidation Reaction
Authors: Tavakoli Mehrabadi Bahareh Alsadat , Xiong Wen , Caldwell Gabe , Regalbuto John R. , Weidner John W. , Monnier John R.
Published: February 2020

Direct methanol fuel cells (DMFCs) are one promising alternative energy storage solution that circumvents one of the chief challenges of current hydrogen fuel cell technology, the elimination of the hydrogen storage step (liquid methanol storage being far more easily handled and transported than compressed H2). However, a severe limitation to DMFCs is the deactivation of Pt catalysts from reaction byproducts, specifically CO from the oxidation of methanol. CO strongly chemisorbs onto Pt sites, reducing the operating efficiency of the fuel cell. As a result, more research is being done into the use of another metal alongside Pt to improve its catalytic activity. Cu is a promising candidate to be incorporated into these catalysts. As bimetallic Cu-Pt catalysts have been shown to have increased activity towards methanol oxidation reaction (MOR) [1, 2]. Cu drastically improves the oxidation of CO, protecting the Pt sites from poisoning [3], and additionally, the presence of Cu increases the amount of surface defects, improving the number of active sites present [4]. In this work, a series of carbon supported Pt-Cu catalysts were prepared by using continuous electroless deposition method. This method was used to produce highly-controlled compositions of Pt-Cu bimetallic catalysts that enhance the catalytic activity of pure Pt. The series of carbon supported Pt-Cu then characterized and evaluated for MOR. The XRD, chemisorption, STEM analysis in addition to cyclic voltammetry in acidic environment have been used for evaluation of these sets of catalysts. XRD and STEM analysis indicated the alloy formation, and the cyclic voltammetry results have shown higher activity towards methanol oxidation in comparison to the commercial supported Pt.

References

Zhunze Su, et al., Journal of Power Sources, 2015, 279 (1), 2015, pp 334-342.

Chatwarin Poochai, et al., Materials Chemistry and Physics, 2015, 163 (1), 2015, pp 317-3303.

Lin Han, et al., Journal of Power Sources, 2015, 286, pp 488-494

Eric J. Coleman, et al., ACS Catal., 2015, 5 (2), pp 1245–1253
Electrocatalyst Evaluation of 3D Printed Microfluidic Membraneless Glucose Biofuel Cell with Integrated Pencil Graphite Electrodes
Authors: Bandapati Madhavi , Rewatkar Prakash , Krishnamurthy Balaji , Goel Sanket
Published: February 2020

This work demonstrates for the first time the application of Pencil Graphite (PG) based enzyme nano biocomposite electrode substrates for a membraneless glucose biofuel Cell (GBFC) in a microfluidic environment [1]. PG is non-toxic, user friendly and prone to the environment with good mechanical rigidity and chemical inertness making it biocompatible. Low background currents, strong adsorption properties, wide potential window, good sensitivity and reproducibility, adjustable electro active surface area and ease of miniaturization make it an excellent candidate to employ as a working electrode [2]. A time-efficient and cost-effective process is proposed involving simple and easy steps for the fabrication of the PG electrodes [3].

After screening various grades of pencil leads, bioanode was fabricated from B grade PG by substantially increasing its surface area by coating carboxylated multiwalled carbon nanotube (MWCNT) through dipping method followed by the covalent immobilization of the anodic catalyst, glucose oxidase enzyme [4]. Likewise, the biocathode was fabricated from 5H grade PG by coating carboxylated multiwalled carbon nanotube through dipping method and electrodeposition of polyaniline (PANI) coating followed by the covalent immobilization of the cathodic catalyst, Laccase enzyme [5]. These electrodes were then inserted into a microfluidic co-laminar flow based membraneless GBFC as shown in Fig. 1a., designed and fabricated using a 3D printer. Glucose was used as fuel and air as oxidant with p-Benzoquinone (PBQ) and 2, 2- azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) in solubilized form as anodic and cathodic mediators respectively and the eletrochemical charecteristics were measured using polarization studies under pysiological glucose concentation conditions (40 mM, 37ºC, pH 7.0) by varying the flow rates, dimensions of the device and distance between electrodes. This portable microfluidic set up generated maximum power density of 10 μWcm-2, OCP of 500±5 mV and current density 64 μA cm-2 at a flow rate of 40 μlmin-1 which could sustain for more than 100 hours. Hence, this work successfully demonstrates the strong potential of using readily available and biocompatible PGs as low cost alternatives to carbon based electrodes to power miniaturized low temperature microelectronic devices and sensors.

Figure 1
Electrochemical Synthesis of Cu-Cu2o Composite Layers and Its Catalytic Properties Towards Ethylene Generation
Authors: Mech Krzysztof , Bisztyga-Szklarz Magdalena , Szacilowski Konrad
Published: February 2020

Recently a number of articles concerning subjects related to limitation of greenhouse gasses emission or neutralization of greenhouse gasses present in atmosphere significantly increased. Global warming negatively influences on non-failure operation of ecosystem and due to glaciers melting resulting in water volume increase causing raising of water level in seas and oceans. Large number of negative factors resulting from global warming effect creating necessity of performance of intensified investigations going to development of technologies enabling limitation of emission or neutralization of greenhouses gasses through its conversion to chemical compounds which can be applied in synthesis of other useful chemical compounds or functional materials. Other important problem beyond limitation of CO2 amount in atmosphere is related to possibilities of its conversion to chemical compounds which can be used as energy carriers. It creates possibilities for its applications in renewable energy storage oriented systems. The proposed project is focused on photoelectrochemical conversion of one of main greenhouse gas, CO2 to ethylene.

The main scientific objective of performed studies was the development of methodology for electrochemical synthesis of binary composite Cu-Cu2O material. Copper is the only metal that exhibits very high selectivity for electrochemical conversion of CO2 to ethylene (1). Ren et al. observed similar effect in case of p-type Cu2O (2). Reported effects allow assumption that combination of both materials can be great solution in the frame of light supported electrochemical conversion of carbon dioxide to ethylene. Authors describe optimization of electrochemical synthesis process towards synthesis of composite coatings of wide composition range. Synthesis conditions were investigated starting from the thermodynamic analysis of the electrolyte. Conditions for synthesis were determined based on the results of voltammetric tests and electrochemical quartz crystal microbalance analyses. Synthesized coatings were characterized towards their composition, structure, morphology and catalytic properties towards electrochemical reduction of CO2 to ethylene.

References

Y. Kwon, Y. Lum, E. L. Clark, J. W. Ager and A. T. Bell, ChemElectroChem, 3, 1012 (2016).

D. Ren, Y. Deng, A. D. Handoko, C. S. Chen, S. Malkhandi and B. S. Yeo, ACS Catalysis, 5, 2814 (2015).

P. D. Tran, S. K. Batabyal, S. S. Pramana, J. Barber, L. H. Wong and S. C. J. Loo, Nanoscale, 4, 3875 (2012).
Hydrogen Oxidation Reaction on Metal@Pt Core-Shell Catalysts in Alkaline Electrolyte
Authors: Hu Jue , Zhang Chengxu
Published: February 2020

Hydrogen oxidation reaction (HOR) as the key reaction in hydrogen-oxygen fuel cell has been widely investigated, and ultra low loading Pt (≤0.05 mg cm–2) is required for HOR in proton exchange membrane fuel cells.(1, 2) However, the HOR kinetics becomes sluggish in base, which is at least two orders of magnitude slower than that in acid.(3) The sluggish HOR kinetics in alkaline electrolyte hinders the development of alkaline exchange membrane fuel cell, which is more commercially viable because of the potential to forego noble metal catalysts and less serious corrosion.(4) To obtain the catalysts with high HOR activity in alkaline environment, it is essential to understand the reason for this puzzling difference of HOR kinetics in acid and base. The understanding of the sluggish hydrogen oxidation reaction (HOR) kinetics in alkaline electrolyte is crucial for designing high performance electrocatalysts. Here, we report a clear and convincing result on this problem by investigating the relationship between the HOR kinetics and Pt-adsorbate energetics. Using electrochemical analysis for well-modified Pt surfaces with distinct Pt-adsorbate interactions, we establish a clear trend in activity for HOR in alkaline electrolyte, that is, the activity changes in the order Au@Pt < Pt < Pd@Pt < Ru@Pt (Figure 1a). A decisive role of Pt－Had energetics in the HOR kinetics on Pt surfaces is determined, while no favorable effects of Pt－OHad energetics in the HOR kinetics were found (Figure 1b). The in-situ XANES analysis demonstrated a higher Had coverage on the pure Pt surface than that on the Pt surface with Ru core due to more strongly bonded Had on pure Pt surface (Figure 1c,d). These findings provide us a novel aspect to understand the HOR kinetics on Pt surfaces and also provides fundamental insights into the influence of the Pt－Had interaction on the HOR mechanism.

Figure Caption

Figure 1. (a) HOR polarization curves normalized to the limiting current density. (b) Trend in the exchange current density for HOR in alkaline solution as a function of the Hupd coverage. Error bars stand for the variation of at least two sets of experimental repeats. (c) Magnified XANES from Pt L
3 edge of commercial Pt/C in 0.1 M KOH solution at different potentials. (d) Comparison of the relative changes in the intensity of absorption peaks for Ru@Pt/C and commercial Pt/C shown in the XANES spectra.

References

M. K. Debe, Electrocatalyst approaches and challenges for automotive fuel cells. Nature
486, 43-51 (2012).

H. A. Gasteiger, S. S. Kocha, B. Sompalli, F. T. Wagner, Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. Appl. Catal. B: Environ.
56, 9-35 (2005).

J. Durst et al., New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. Energy Environ. Sci.
7, 2255-2260 (2014).

J. R. Varcoe, R. C. T. Slade, Prospects for alkaline anion-exchange membranes in low temperature fuel cells. Fuel Cells
5, 187-200 (2005).

Figure 1
Bipolar Membrane Development for Fuel Cells and Electrolyzers
Authors: Chen Yingying , Klein Ellis , Deutsch Todd G , Neyerlin K C
Published: February 2020

Bipolar membranes (BPMs) are composed of a cation exchange layer (CEL), an anion exchange layer (AEL), and a nanometer scale bipolar junction (Fig 1). At reverse bias polarization the potential drop within both the AEL and CEL are almost negligible when compared to the potential drop within the junction. This large potential drop within the bipolar junction results in a high electric field, which, in combination with water dissociation catalyst, allow BPMs to facilitate water dissociation (into H+ and OH- ions) at a rate seven orders of magnitude greater than the rate in bulk water [1].

BPMs have been studied at the laboratory and pilot scale for a wide variety of applications, including: electrodialysis for water treatment [2], CO2 reduction [3], solar water splitting [4], and fuel cells [5]. Despite the commercial availability of BPMs, significant hurdles exist both in the fundamental understanding of limiting processes and their subsequent incorporation into electrochemical devices. The main technological challenges for BPMs at present include: i) low membrane stability, ii) insufficient water transport at high current densities, and iii) high membrane resistance.

Here we will discuss the optimization of BPM junctions and related water management issues. An example of the improved interfacial charge transfer for BPMs fabricated using 3-D electrospun junctions is shown in Fig 2. The AC impedance spectra shown in Fig 2, highlights both the similarity in the high frequency resistance (membrane + electrolyte resistance) of the two samples, along with the significantly different low frequency (membrane + electrolyte + interfacial charge transfer) resistance. The experimental results from both 4-electrode and membrane electrode assembly (MEA) level operando testing of BPM junctions fabricated using 2-D and 3-D approaches both with and without water dissociation promoters will be shown.

Reference

[1] Strathmann, H., Krol, J. J., Rapp, H. J., & Eigenberger, G. (1997). Limiting current density and water dissociation in bipolar membranes. Journal of Membrane Science, 125(1), 123-142.

[2] Reig, M., Valderrama, C., Gibert, O., & Cortina, J. L. (2016). Selectrodialysis and bipolar membrane electrodialysis combination for industrial process brines treatment: Monovalent-divalent ions separation and acid and base production. Desalination, 399, 88-95.

[3] Li, Y. C., Zhou, D., Yan, Z., Gonçalves, R. H., Salvatore, D. A., Berlinguette, C. P., & Mallouk, T. E. (2016). Electrolysis of CO2 to syngas in bipolar membrane-based electrochemical cells. ACS Energy Letters, 1(6), 1149-1153.

[4] Luo, J., Vermaas, D. A., Bi, D., Hagfeldt, A., Smith, W. A., & Grätzel, M. (2016). Bipolar Membrane‐Assisted Solar Water Splitting in Optimal pH. Advanced Energy Materials, 6(13), 1600100.

[5] Peng, S., Xu, X., Lu, S., Sui, P. C., Djilali, N., & Xiang, Y. (2015). A self-humidifying acidic–alkaline bipolar membrane fuel cell. Journal of Power Sources, 299, 273-279.

Figure 1
Proton Transport within Acid Aggregates in a Hydrated Precise Sulfophenylated Polyethylene
Authors: Paren Benjamin , Picard Lionel , Rannou Patrice , Marechal Manuel , Neary William , Kendrick Aaron , Kennemur Justin , Frischknecht Amalie L. , Winey Karen
Published: February 2020

Hydrated acid aggregates in a precise sulfophenylated polyethylene exhibit high proton conductivity. This study focuses on a new precise polymer synthesized by ring-opening polymerization, p5PhSA, that has a polyethylene backbone with a sulfonated phenyl group pendant on every 5th carbon. The structure of p5PhSA is characterized with X-ray scattering and the proton conductivity is measured by electrical impedance spectroscopy. Experiments are performed as a function of relative humidity and temperature. Water uptake in p5PhSA is determined by sorption measurements as a function of humidity. Atomistic molecular dynamics simulations are used to elucidate the structure of the acid aggregates in the amorphous polymer matrix and are directly compared to absolute X-ray scattering data. At 40°C and 90% relative humidity, the proton conductivity of p5PhSA is 0.23 S/cm, exceeding that of Nafion at the same conditions. At room temperature, the interaggregate distance nearly doubles from 1.8 nm at 0% relative humidity to 3.4 nm at 100% relative humidity. The swelling of these acid aggregates with water is reversible and facilitates the proton transport through p5PhSA.

Figure 1
Studies on Proton Transmission across Graphene in Proton-Exchange Membrane Structures
Authors: Bukola Saheed , Silva Liyanage Mayura , Korzeniewski Carol , Harris Joel M. , Creager Stephen E
Published: February 2020

This talk will cover a wide range of recent studies involving proton transmission across CVD graphene layers incorporated into proton-exchange-membrane (PEM) sandwich structures. Many low-temperature electrochemical systems including fuel cells and water electrolysis cells involve proton transmission across proton-exchange membranes integrated with electrodes. A key to optimizing such systems is the accomplishment of selective proton transmission at high rates, while simultaneously inhibiting crossover transmission of other molecular species such as water, hydrogen / oxygen, methanol, and other species. Single-layer CVD graphene has been shown to accomplish this. Proton transmission across graphene in Nafion | graphene | Nafion sandwich structures has been shown to occur with an area-normalized resistance of less than 0.05 ohm cm2 at room temperature, with high selectivity relative to other ions. The graphene layer is nearly defect free and should be an excellent barrier to other species besides protons. The talk will include studies of proton transmission rates using hydrogen pump cells, water electrolysis using PEM cells, and related work. Graphene characterization studies using confocal Raman spectroscopy and X-ray photoelectron spectroscopy, and graphene defect characterization using chemical etching of graphene on copper, will also be covered. Effects of the graphene structure (single vs double layer) and the nature of the 2D material (graphene vs. hexagonal boron nitride) on proton transmission rates, and the rates for other ions besides proton, will also be discussed.
Caveat of High Temperature Accelerated Stability Test for Anion Exchange Membranes
Authors: Park Eun Joo , Maurya Sandip , Hibbs Michael , Fujimoto Cy , Kim Yu Seung
Published: February 2020

Alkaline stability of hydroxide-conducting anion exchange membranes (AEMs) is a significant requirement for practical applications of alkaline membrane fuel cells. Recently, quaternized aryl ether-free polyaromatics with alkaline stable polymer backbone structure have been receiving much attention as promising materials for AEMs.1 Since a prolonged stability test (≥1000 h) is time-consuming and costly, a harsher test condition at more elevated temperature and higher alkaline concentration to push the limit of AEM stability was advised for degradation and durability studies of such membranes.

In this study, we investigate alkaline stability of aryl ether-free Diels-Alder polyphenylene (DAPP) with a trimethylammonium cation attached to an electron-donating hexamethylene spacer2 under low and high temperature (80, 120 and 160 °C) alkaline stability test conditions. Although high temperature stability testing may seem to be a useful tool to measure alkaline stability in a short time period, we observed different phenomena when DAPP AEM was subjected to the extreme test conditions in a pressure vessel. Even though DAPP AEM did not show any structural change or significant decrease in ion exchange capacity or hydroxide conductivity in 4 M NaOH at 80 °C after 3000 h, at the elevated temperature of 160 °C, the membrane underwent instant crosslinking resulted in insolubility of the membrane and sharp decrease in water uptake and swelling degree. The source of crosslinking is the low concentration of residual alkyl bromide from the incomplete quaternization process, which often causes initial conductivity decrease of AEMs. The crosslinked membrane did not show any further change in their properties, exhibiting consistent ion exchange capacity and hydroxide conductivity during the follow-up alkaline stability test (4 M NaOH, 80 °C, 1000 h). In this presentation, structural analysis and membrane properties including ion exchange capacity, water uptake, swelling degree, solubility, mechanical properties and hydroxide conductivity of DAPP AEMs after different temperature alkaline stability testing will be discussed to elucidate the unexpected consequence of high temperature accelerated stability test.

References

E. J. Park, Y. S. Kim. J. Mater. Chem. A, 2018, 6, 15456–15477.

M. R. Hibbs. J. Polym. Sci., Part B: Polym. Phys., 2013, 51, 1736–1742.
The Role of Mesostructure for CO2 Reduction on Dendritic Copper Electrocatalysts
Authors: Mehrabi Hamed , Coridan Robert
Published: February 2020

Electrochemical reduction of CO2 for production of fuels has been a promising field of research for simultaneously decreasing carbon emissions and storing solar energy in dense chemical bonds. Copper is an important electrocatalyst for this process because it catalyzes a broad range of hydrocarbons and alcohol CO2 reduction products. The structure of copper catalysts can affect product selectivity through several known strategies. Hierarchically structured copper foams incorporate several strategies for the production of high value products, including the diffusive trapping of products such as CO and HCOOH into further reduced products. During fabrication of the electrocatalyst, hydrogen evolution (HER) is responsible for the formation of the large-scale morphology of the electrode, but also promotes the formation of electrode areas that are particularly effective at HER. Here, we explore alternative fabrication strategies for dendritic Cu electrocatalysts by modifying the pH of the solution to control hydrogen bubbling during synthesis. We describe the synthesis and structural characterization of the catalyst surfaces as a function of pH. We also show that the branching ratios and Faradaic efficiency of CO2 reduction can be controlled through the conditions of electrode synthesis. Based on these results, we consider the relationship between electrode mesostructure and electrolytic branching ratios in order to tailor the selectivity of Cu electrocatalysts for the CO2 reduction reaction.
Modelling Anion-Exchange Membrane-Electrode Assembly Systems for CO2 Reduction
Authors: Weng Lien-Chun , Bell Alexis T. , Weber Adam Z.
Published: February 2020

Gas diffusion electrodes (GDEs) have drastically increased the achievable CO2 reduction (CO2R) current density by overcoming the significant mass transport limitations observed in aqueous planar electrodes.1-3 The thin diffusion layer in GDEs, typically in the order of 10-100 nm, also allows operation of CO2R in alkaline conditions, which have been shown to suppress the side reaction, hydrogen evolution reaction (HER), and reduce the onset potentials of CO2R products such as CO and C2H4.4-7

With this increase in total current density, conventional cell designs for planar electrodes become severely limited by the ohmic drop across the cell, making membrane-electrode assemblies (MEAs) an attractive alternative. MEAs have smaller cell resistances as they do not have aqueous electrolyte compartments and can greatly reduce the distance between the two electrodes.

In this talk, we explore the mechanisms and limitations through multiphysics modeling of two MEA designs: one with gaseous feeds at both the anode and cathode (full-MEA); the other with an aqueous anode feed (KHCO­3 or KOH exchange solution) and a gaseous cathode feed (exchange-MEA). We discuss important cell design considerations such as the distribution of the applied voltage, water management, and CO2 utilization efficiencies. Finally, we examine the effects of temperature and the trade-off between CO2 solubility and water supply for a full-MEA.

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(Invited) Electronic Tuning of Photo-Electro-Catalytic Processes By Intermediate-Band Oxides
Authors: Hu Shu
Published: February 2020

Intermediate-band oxides possess tunable transport energy levels: they can serve as chemically-stable, transparent conductive protective layers for durable photo- and electro-catalytic processes. They have initially shown to promote charge separation, practically interfacing the otherwise unstable light absorbers and protecting them against corrosion. The initial re-discovery of “leaky” TiO2 coatings utilize their Ti3+- defect bands to stabilize an entire class of Si, GaP, GaAs, CdTe, BiVO4 semiconductors, as well as structured Si pillars and GaAs wires for 1000s-hour operation under sunlight. Recently, a ternary protective coating of manganese-alloyed titanium oxide, (Ti,Mn)Ox, had achieved the incorporation of transition metal cation impurities, e.g. Mn3+, into an otherwise insulating TiO2 coating. Here, Mn compositions can reach as high as 70% for a 150°C growth, which cannot be achieved by any method other than ALD.

The individual redox of the Mn3+ composes a continuous Mn-impurity band. On the one hand, this intermediate band induced electronic interactions between ALD coatings and underlying electro-catalysts, for boosting the hydrogen evolution activity of such buried interfaces. On the other hand, these oxide coatings have further shown to confine charge transport to specific electrochemical potentials and confine charge transfer to specific surface-attached intermediates, thus promoting product selectivity. To summarize, the tunable electronic structures of intermediate-band oxides enabled new strategies for controlling charge separation and charge transfer at liquid interfaces.
(Invited) Efficient Charge Ggeneration Dynamics in Hematite Photoanodes Decorated with Gold Nanostructures
Authors: Furube Akihiro , Okazaki Masahiro , Chen Liang-Yih
Published: February 2020

Photochemical water splitting offers a useful solution for efficient energy conversion into hydrogen gas. Hematite has been focused on this purpose due to the advantage of low-cost, chemical stability, and suitable band gap. This photocatalytic ability, however, is limited by the short-lived carriers, the poor conductivity, and the short hole diffusion length. As one solution, combination with noble metal can enhance the photocatalytic performance. In particular, gold nanoparticles (AuNP) indicate characteristic absorption in the visible region and injection of electrons into the semiconductor. In this study, a hybrid material of hematite photoanodes with gold nanostructures was fabricated and the carrier dynamics was elucidated by femtosecond transient absorption (TA) spectroscopy.

The hybrid material was prepared by immersing the photoanode into the AuNP solution for 16 h, followed by heating at 400℃ for 4 h to remove the organic matter. Femtosecond pump-probe TA spectroscopy was carried out with a Ti:sapphire laser (800 nm wavelength, 130 fs fwhm pulse width, 0.7 mJ/pulse intensity, 1 kHz repetition; Spectra Physics, Hurricane) using pump pulses at 400 nm and probe pulses in the Vis/NIR region.

FE-SEM images of hematite and hematite decorated with AuNP (Au/Hematite) indicated that 40 nm AuNPs were decorated on the hematite surface. TA signal of hematite exhibited the positive absorption peak at the 585 nm, while the signal of Au/Hematite did bleaching at 580-620 nm overlapping with broad absorption whose apparent peak shifts from 670 nm towards 580 nm with time. The negative signal at the plasmon band indicates heat generation within the time resolution and cooling in the picosecond timescale. The observed strong positive absorption is induced by the increased electron density in the hematite due to electron transfer from AuNP, demonstrating efficient charge carrier generation in hematite by the decorated gold nanostructure.
(Invited) Single-Source Precursors for Gas Phase Deposition of Catalytic Coatings for Water Splitting Applications
Authors: Mathur Sanjay , Jürgensen Lasse , Fischer Thomas , Leduc Jennifer , Möllmann Alexander
Published: February 2020

Keywords:
Single-source precursor, CVD, nanostructured films, catalytic water splitting

Investigation of the interplay of metal-organic chemistry will enrich the state-of-the-art of CVD and ALD technology and open new possibilities for the applications of new Ir-based materials. Therefore new heteroleptic Janus-typed compounds exhibiting high volatility and defined thermal decomposition under CVD and ALD conditions are reported to elaborate the precursor chemistry – materials synthesis – functional property chain. The new precursors unify both reactivity and sufficient stability through its heteroleptic constitution to provide a precise control over compositional purity in CVD and ALD deposits. CVD- and ALD-grown materials were tested towards their (electro)catalytic applications, particular in the oxygen evolution reactions. In this work functional characterization of deposited materials will be reported and their catalytic behavior is examined. The deposition on various substrate materials without the need of additional reactant gases underlines the potential of heteroleptic precursor class for CVD and ALD of metallic thin films. The presented CVD data opens new possibilities in the vapor phase synthesis of materials facilitating the application of such films, for example, as electro- or photocatalyst in OER and ORR.

Figure 1
Production of C4 Species from the Electrochemical Reduction of CO on Nanostructured Cu/CuOx Catalyst
Authors: Spurgeon Joshua M , Strain Jacob
Published: February 2020

The electrolytic conversion of CO2 to useful fuels and chemicals is a promising technology for greenhouse gas reduction and waste utilization, but converting CO2 to larger multicarbon products with any selectivity is a major challenge. However, commercial electrolyzers for the conversion of CO2 to CO at high current density and high yield are in development. Subsequent electrochemical reduction of carbon monoxide affords the possibility of promoting a different reaction pathway for higher multicarbon product selectivity. The reaction conditions for the aqueous conversion of CO on Cu/CuOx catalyst have been explored including applied bias, gas flow rate, and oxide thickness. Significant faradaic efficiencies for C4 products (> 5% FE), including gamma-hydroxybutryic acid and succinic acid, were observed and quantified via NMR. The gas flow rate and resulting hydrodynamics of CO near the catalyst surface play a crucial role, with little C4 species observed at lower flow rates. A proposed reaction pathway for the formation of these C4 products will be presented as well.
The Interplay between Quantum Efficiency, Light Flux, and Turnover Frequency in Molecular-Modified Photocathodes
Authors: Moore Gary F. , Wadsworth Brian L. , Beiler Anna M. , Khusnutdinova Diana , Reyes Cruz Edgar A. , Nanyangwe Sylvia K.
Published: February 2020

Chemical modification of semiconductor surfaces with electrocatalysts provides a strategy for developing integrated materials capable of converting sunlight to fuels and other value-added products, but their development is hampered by an incomplete understanding of the factors limiting their performance. Although kinetic models have been separately developed to describe photoelectrochemical or homogeneous electrocatalytic reactions, related modeling for molecular-modified photoelectrodes has not been as extensively elaborated. This work addresses kinetic parameters pertinent to heterogeneous-homogeneous catalysis at molecular-modified semiconductors. Photoelectrosynthetic hydrogen evolution using a cobalt porphyrin-modified gallium phosphide cathode [1-2] is analyzed under variable scan rates, pH values, and light intensity, yielding information on the relationship between the external quantum efficiency, illumination conditions, and turnover frequency.

B. L. Wadsworth, D. Khusnutdinova, G. F. Moore, J. Mater. Chem. A., 6, 21654–21665 (2018); DOI:10.1039/C8TA05805A

A. M. Beiler, D. Khusnutdinova, B. L. Wadsworth, G. F. Moore, Inorg. Chem., 56, 12178–12185 (2017); DOI:10.1021/acs.inorgchem.7b01509
(Invited) New Mixed-Anion Photocatalysts for Water Splitting and CO2 Fixation
Authors: Maeda Kazuhiko
Published: February 2020

Water splitting and CO2 fixation using semiconductor photocatalysts are important reactions from the viewpoint of solar-to-fuel energy conversion toward artificial photosynthesis. Mixed-anion compounds containing more than one anionic species in a single-phase have attracted attention as visible-light-driven photocatalysts, since as compared to oxygen 2p orbital, p orbitals of less electronegative anion can form a valence band that possesses more negative potential. In this talk, recent progress on the development of new mixed-anion photocatalysts that are applicable to water splitting and CO2 fixation will be given.

We developed Ta and N-codoped rutile TiO2 for visible-light-driven water oxidation. TiO2:Ta,N powders modified with a RuO2 cocatalyst were active under visible light up to 540 nm for water oxidation to produce O2 in the presence of reversible electron acceptors (IO3
– or Fe3+), while TiO2:N exhibited negligible activity. Results of time-resolved infrared absorption spectroscopy revealed that the codoping Ta with N into TiO2 prolonged the lifetime of photogenerated free electrons, leading to high photocatalytic activity. Overall water splitting into H2 and O2 was achieved using TiO2:Ta,N, in combination with a H2-evolution photocatalyst of SrTiO3:Rh and an Fe3+/Fe2+ or IO3
–/I– redox couple even under simulated sunlight (AM1.5G). A similar result was obtained with rutile TiO2 codoped with N and F.

TiO2:Ta,N also served as a very stable photoanode to oxidize water under visible light, and could be coupled to not only a H2 evolution cathode but also a molecular photocathode for CO2 reduction. A photoelectrochemical cell constructed with RuO2/TiO2:Ta,N photoanode and Ru(II)–Re(I) supramolecular photocathode was capable of reducing CO2 into CO using water as the electron source upon visible light, with stoichiometric production of CO and O2.

Another interesting example of mixed-anion photocatalyst is an oxyfluoride pyrochlore Pb2Ti2O5.4F1.2. It had been believed that oxyfluorides are unsuitable as visible-light-responsive photocatalysts because of the highest electronegativity of fluorine. Surprisingly, Pb2Ti2O5.4F1.2 had an unprecedented small band gap of ca. 2.4 eV, and worked as a stable photocatalyst for visible-light H2 evolution and CO2 reduction when modified with suitable promoters. Density functional theory calculations indicated that the unprecedented visible-light-response of Pb2Ti2O5.4F1.2 originates from strong interaction between Pb-6s and O-2p orbitals, which is enabled by a short Pb–O bond in the pyrochlore lattice due to the fluorine substitution.
(Invited) Mechanistic Studies of the Direct Electrochemical CO2 Reduction on Non-Metallic MNC Single Site and MNC/Cu Hybrid Catalysts
Authors: Strasser Peter
Published: February 2020

In this talk, I will highlight some of our recent advances in the field of direct electrochemical reduction of CO2 into value-added fuels and chemicals on non-metallic carbon-based MNC electrocatalysts with molecularly-defined coordinative MNx metal-nitrogen centers as active sites. Special focus will be placed on efficient production of CO at lab-scale and industrial current densities with emphasis on a fundamental understanding of between selectivity and the nature of the MNx moiety. Furthermore, the design of hybrid electrocatalysts consisting of CO-efficient MNC backbones decorated with CuOx nanoparticles will be addressed and the molecular mechanisms of their enhanced ethylene production discussed.
Charge Carrier Dynamics in Metal-Oxide/Nanocarbon Hybrid Photoelectrodes
Authors: Kormányos Attila , Honarfar Alireza , Pullerits Tönu , Janáky Csaba
Published: February 2020

One of the most critical challenges of the 21st century is the shift in energy use from fossil fuels to renewable sources. Photoelectrochemical solar energy conversion, as a potentially attractive candidate, has gained an increased momentum during the recent years. Although both water-splitting (hydrogen evolution) and carbon dioxide reduction are promising avenues, they both have substantial shortcomings to be addressed. A common virtue of these two processes is that a good photoelectrode has to concurrently fulfill many attributes to drive these reactions efficiently. One of the most important is the efficient charge carrier transport, which ensures that most of the photogenerated charge carriers can be extracted both toward the electrode/electrolyte junction and toward the back contact. Numerous promising photoelectrode material showed poor performance due to extensive charge carrier recombination both in the bulk and at the surface. A well-known example is α-Fe2O3 (hematite), in which the hole-diffusion length is only 2-4 nm.1 One possible way to improve charge carrier transport properties is to deposit the metal-oxide on an interconnected nanocarbon network (e.g., carbon nanotubes or graphene). These highly conductive nanoscaffolds can improve the transport of the photogenerated charge carriers, which in turn, results in an enhanced photoelectrochemical performance.2

In my presentation, I am going to discuss how charge carrier dynamics of various hematite and hematite/nanocarbon composites is affected by the composition of the nanohybrids and by the applied bias. Hematite/nanocarbon photoelectrodes were synthesized by a two-step method: first the nanocarbon layer was spray-coated on the surface of the substrate electrode, and the nanostructured metal-oxide component was electrodeposited in a subsequent step. The metal-oxide/nanocarbon ratio was precisely controlled by the deposition charge density. To investigate charge carrier dynamics on a timescale ranging from seconds to femtoseconds three different techniques were employed: (i) transient absorption spectroscopy, employing a fs pump-probe setup, (ii) intensity-modulated photocurrent spectroscopy, and finally (iii) fast chronoamperometry. By covering this broad timescale, it was possible to study charge carrier transport, trapping and recombination processes, which were greatly affected by both the applied potential and the presence of the nanocarbons.

1 Energy Environ. Sci., 2016, 9, 2744—2775.

2 J. Am. Chem. Soc. 2017, 139, 6682−6692.
(Invited) Electrochemistry of the N-H Bond: Synthesis and Oxidation of Ammonia
Authors: Manthiram Karthish
Published: February 2020

Both reductively synthesizing and oxidatively breaking the N-H bond is essential to the chemistry of life. Electrochemical routes for N-H bond formation and dissociation are less developed than their enzymatic and thermochemical counterparts. Our group is working on lithium-mediated electrochemical ammonia synthesis at ambient conditions. We have found in-depth mechanistic understanding of the reaction network through which ammonia is synthesized beginning from nitrogen gas and hydrogen donors. This molecular-level understanding has contributed towards uncovering reaction conditions which lead to higher rates and Faradaic efficiencies for ammonia synthesis. Complementary efforts in our group have examined electrochemical N-H bond dissociation, including fundamental understanding of why this bond is difficult to break. Altogether, these studies provide an increasingly detailed picture of how interfacial electron and proton transfers contribute towards making and breaking the N-H bond.
Thermochemical and Techno-Economic Evaluation of Ammonia Synthesis Via Lithium-Molten Salt Processes
Authors: Garzon Fernando H , Matanovic Ivana , Gomez Jamie
Published: February 2020

Ammonia is an attractive carbon-free energy carrier as it is easily liquified, transported and stored at a lower energy penalty than other non-carbon containing fuels. Anhydrous ammonia production occurs today in large Haber-Bosch (H-B) plants fueled from natural gas. Industrial (H-B) plant techno-economics are poorly suited for remote small-scale ammonia production from renewable energy. Highly efficient ammonia synthesis in small-scale H-B systems does not exist, thus stimulating research towards a low temperature and pressure processes. The thermodynamics of low temperature ammonia synthesis are quite favorable however, the high activation energy for nitrogen reduction requires efficient catalysts and high temperatures. Electrochemistry offers a means to activate the process at lower temperatures however this has proven to be very challenging [1]. Direct electrochemical protonation of nitrogen at low temperatures has produced rates on the order of 10-9 to 10-10 moles/cm2-sec, many orders of magnitude too low for industrial production [2]. Nitrogen reduction on lithium metal occurs spontaneously; hence molten lithium salt processes may prove to be attractive for ammonia generation [3,4,5]. We have evaluated the thermodynamics and techno-economics of intermediate temperature NH3 synthesis processes, based on the spontaneous electro-reduction of nitrogen in lithium ion systems where the potential for lithium is close to metal deposition and the subsequent formation of lithium nitride. The advantage of the molten salt lithium processes is the reaction is autocatalytic however the direct protonation of Li3N with hydrogen yields lithium amide. Protonation with water does yield ammonia however the reaction is very exothermic. The electrolysis of the LiOH product is also a major obstacle as the reaction between LiOH and Li produces Li2O[6]. To overcome this difficulty, we have also evaluated alternative iodide and bromide cycles were lithium electrowinning is known to be viable. The techno-economics of the processes were also considered in a preliminary study.

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