bluuebunny/crossref_metadata_2025 huggingface dataset:
CO2 Electrocatalytic Reduction at Gold and Copper Electrodes: Role of Particle Size and Surface Chemistry
Authors: Andrews Evan Michael , Flake John , Fang Yuxin
Published: February 2020

Electrochemical CO2 reduction holds the potential to produce energy-dense liquid fuels using renewable energy sources. Previous works have shown that Cu provides some of the highest yields for valuable products such as methane or ethylene, yet the mechanisms for selectivity are not well understood.  Of course, nature performs similar reactions using enzymes including nanocluster metal alloys and ligands with extraordinary selectivity.   Here we discuss the effects of various ligands at Cu, Au and Au-Cu alloy electrocatalysts.

From the literature, results of CO2 electroreduction on bulk Cu, Au and Au-Cu alloys are known (1-3) as are the size effects of Cu and Au nanoparticles (4, 5), but the effect of chemical surface modification is still unknown, as is the size effect of gold/copper alloy nanoparticles. 

In this work we demonstrate the effect of chemical surface modification and the size effect of Au-Cu alloy nanoparticles with the aim of combining the two effects to improve CO conversion and hydrogenation.  The effect of surface chemistry is studied using planar electrodes modified by organic compounds.  Copper and gold foils are exposed to strongly adsorbed ligands such as alkylthiols and glutathione (GSH) and subsequently used as electrocatalysts.  As shown in Figure 1, where copper foil was modified with GSH, the adsorbed ligands strongly affect the product yields and selectivity.  At the optimal CO yield potential, the yield of CO on the GSH-modified copper was 2.4 times that of blank bulk copper. Although the yield of CH4 has been decreased by a factor of 0.5 in exchange, the overall yield of gas product has been increased by a factor of 2 on the surface engineered electrode. 

The size effect of Au-Cu nanoparticles is investigated using nanoparticles synthesized via the Brust-Schiffrin method immobilized in various binder inks as electrocatalysts for CO2reduction.  Ultimately, we consider the potential to break scaling relationships associated with conventional metal electrodes through the use of size control, alloys and ligands. 

References

1.            Y. Hori, A. Murata, K. Kikuchi and S. Suzuki, J. Chem. Soc., Chem. Commun., 728 (1987).

2.            Y. Hori, K. Kikuchi and S. Suzuki, Chemistry Letters, 1695 (1985).

3.            J. Christophe, T. Doneux and C. Buess-Herman, Electrocatalysis, 3, 139 (2012).

4.            D. R. Kauffman, D. Alfonso, C. Matranga, H. Qian and R. Jin, Journal of the American Chemical Society, 134, 10237 (2012).

5.           R. Reske, H. Mistry, F. Behafarid, B. Roldan Cuenya and P. Strasser, Journal of the American Chemical Society (2014).

Figure 1
Enhancing the Electrochemical Surface Area of Tin Based Porous Electrodes for Carbon Dioxide Reduction to Formate
Authors: Agarwal Arun S , Gautam Dushyant , Rode Edward , Jasinski Jacek B , Nguyen Tu Quang , Sunkara Mahendra Kumar , Sridhar Narasi
Published: February 2020

Conversion of CO2 to value added chemicals can provide an attractive alternate pathway for making petrochemicals without petroleum based raw materials, storing renewable energy in chemical form, as well as providing greater acceptance to CO2 as a valuable resource of chemical manufacture. This work deals with the electrochemical reduction of CO2 to formate / formic acid, with specific focus on economic feasibility of this process. The current density or rate of production of product directly affects the overall capital expenditure of a large scale process by affecting the reactor requirements. In this paper, the various approaches studied to enhance the electrochemical surface area of the electrodes will be presented. Electroplating of tin of carbon fiber substrates provided a 5-10 times area enhancement. Nano particles of tin (~ 5 nm size) were formed on high surface area activated carbon black (Vulcan XC-72).  The porous electrode structure is optimized based on the binder content and loading on the base substrate. Studies were also performed to ascertain the effects of tin oxide on the reduction of CO2 to the desired product. The time evolution of the performance of such electrodes with respect to both, the overall current density as well as the selectivity towards formate product will be presented. Characterization of the electrode and particle morphology based on ‘before’ and ‘after’ experiment SEM and TEM analysis will also be presented.
Effect of Acid-Leaching on the Electrochemical Performance of Carbon-Supported Copper Phthalocyanine Tetrasulfonic Acid Tetrasodium Salt (CuTSPc/C) Catalys
Authors: Qiao Jinli , Qing Xin
Published: February 2020

Since the strong O=O bond (498kJ mol-1) with very slow oxygen reduction reaction (ORR) rate at the cathode has a much larger overpotential (or polarization) than that of hydrogen oxidation reaction (HOR) at the anode, the cathodic ORR has become the most challenging step in a fuel cell1. Up to now, Pt is still the best catalyst candidate for the ORR due to its high catalytic activity. As yet, there are still two major challenges for the Pt-based catalysts, one is high cost, and the other is insufficient electrochemical durability2-3. Therefore, the major effort in fuel cell research and development have been put on reducing Pt loading by exploring more effective synthesis technologies, and/or replacing Pt metal using other non-precious metals such as Fe, Co and Cu.  

In this regard, carbon-based transition metal macrocycle complexes catalysts have become one of the most promising candidates to replace Pt/C catalyst, especially these catalysts synthesized after thermal treatment show high catalytic activity3,4. For revealing the mechanism of ORR and improving the ORR activity, one should know the active site in these catalysts. So far, there is a consensus that carbon, nitrogen and transition metals are all considered to be indispensable elements for the ORR activity5. However, for the transition metals, there is few report clearly point out whether it is a part of the active sites in these macrocycle complexes. In this work, with carbon-supported copper phthalocyanine tetrasulfonic acid tetrasodium salt, CuTSPc/C, as a target, the role of transition metal in macrocycle complexe was investigated by acid-leaching procedures.
The Electrochemical Conversion of Carbon Dioxide to Fuels on a Nanoporous Copper/M Catalyst
Authors: Billy Joshua , Co Anne C.
Published: February 2020

Introduction

The electrochemical conversion of CO2 has been studied for many years, and copper is the only metal catalyst found to produce hydrocarbons1. This work examines the behavior of a nanoporous copper/M electrode when used as a catalyst for the electroreduction of CO2.

Materials and Methods

A copper/aluminum alloy was used as the starting material for producing a high surface area copper catalyst. The aluminum was removed through an etching procedure in strong base. The resulting nanoporous copper was crushed and mixed with Nafion, a binding agent, then casted over a copper foil substrate. After drying, transition metal (M) was galvanically displaced onto the copper to form nanoporous copper/M. Carbon dioxide electroreduction was performed by placing the catalyst in a specially designed flow cell. A bubbler in the cell was used to deliver CO2 to saturate the electrolyte, 0.1 M KHCO3, at a rate of 10 mL/min. The catalyst acts as the working electrode in this electrochemical cell. The product distribution from the electroreduction process was collected at a series of potentials. Gaseous products were identified and quantified using gas chromatography coupled with both mass spectrometry and a thermal conductivity detector. Liquid products were analyzed using nuclear magnetic resonance.

The surface morphology and composition of the catalyst was characterized using scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS). The SEM images show a rough surface that retains a porous structure before and after experiments. Depth profiling studies with XPS show that the transition metal (M) from the surface of the material migrates to the bulk after electrolysis.

Results and Discussion

Identification of reduction products reveal that the catalyst is selective toward C2 species such as ethane and ethanol. This is a phenomenon that is also observed on nanoporous copper without the transition metal, while the addition of a transition metal changes the product selectivity. The only C1 species produced were carbon monoxide and formic acid, while no methane and methanol was detected. The most interesting product observed was n-propanol, a C3 species.

This work provides further insight into how selectivity of the CO2 reaction can be altered by tuning the catalyst properties. The use of a nanoporous copper catalyst provides an increased surface area and introduces many step edges, which are sites that are considered to be especially electrochemically active2. The role of the transition metal has yet to be determined, however future work will focus on optimizing the selectivity as a function of controlling the Cu/M (where M = transition metals) composition in the nanoporous copper framework.

References

1] Hori, Y.; Kikuchi, K.; Suzuki, S. Chem. Lett. 1985, 1695.

2] Billy, Coleman, Walz, Co, US 62/058,121.

Figure 1
High Activity of Ru@CoSe2/C Electrocatalyst Toward Oxygen Reduction Reaction
Authors: Li Hengyi , Cheng Xuan , Zhang Ying
Published: February 2020

High electrocatalytic activity toward oxygen reduction reaction (ORR) of carbon supported Ru@CoSe2 catalysts (Ru@CoSe2/C) were prepared through a simple and facile two-step route. The CoSe2 nanoparticles were first obtained by microwave method, and then followed by galvanic substitution of Ru on CoSe2. The introduction of a small amount of Ru into CoSe2 led to a core-satellite structure in which the disordered Ru nanoparticles were adsorbed on the CoSe2 nanocrystallites. Compared with Ru85Se15/C[1] and CoSe2/C[2], the Ru@CoSe2/C exhibited higher ORR activity due to the enhanced Ru utilization through the interaction between Ru and CoSe2. Accordingly, the half-wave potential of 0.548 V and the cathodic current density of 3.37 mA cm-2 at 0.6 V were achieved for Ru@CoSe2/C in an acidic solution.

[1]       Q. Zheng, X. Cheng, T. C. Jao, F. B. Weng, A. Su, Y. C. Chiang. Fuel cell performances at optimized Nafion and Ru85Se15 loadings in cathode catalyst layer.
Journal of Power Sources
201 (2012) 151-158

[2]       H. Li, D. Gao, X. Cheng. Simple microwave preparation of high activity Se-rich CoSe2/C for oxygen reduction reaction.
Electrochimica Acta

138 (2014) 232-239
Synthesis and Electrochemical Properties of IrO2 Nanosheets
Authors: Shimizu Wataru , Ishida Takanobu , Miyasaka Syu , Ayato Yusuke , Sugimoto Wataru
Published: February 2020

1. Introduction

IrO2 is known as one of the most highly active and stable OER catalysts. It is essential that high surface area materials are used, either by preparing porous architectures or dispersing nanoparticles on conducting supports since Ir is a precious metal. Two-dimensional nanosheets have recently attracted increased interest as building blocks for high surface area porous materials. Nanosheets with electronic conductivity, for example, RuO2 nanosheets find application in transparent conductive electrodes as well as electrochemical energy storage and conversion.1,2) IrO2 is analogous to RuO2in many aspects; both possess the rutile structure and are room-temperature metallic conductors. Here we report the successful synthesis of an ion exchangeable layered iridium oxide which can be exfoliated into elemental nanosheets and subsequently used as an electrocatalyst.

2. Experimental

Layered K0.30Ir0.93O2•nH2O was prepared by solid-state reaction of a 1:2 mixture of K2CO3 and IrO2 under inert atmosphere. Proton exchange was conducted by reaction in 1 M HCl for 3 days. Chemical exfoliation was conducted by shaking H0.30Ir0.93O2•nH2O in an aqueous solution of tetrabutylammonium hydroxide for 1 week. Electrochemical properties were studied in H2SO4 and HClO4.

Results and discussion

The Ir0.93O2 nanosheets (typical AFM image given in Fig. 1 inset) have an average thickness of c.a. 1.5 nm. The sharp edges suggest that the nanosheets are highly crystalline. Cyclic voltammograms of the Ir0.93O2 nanosheet electrode in 0.5 M H2SO4 is shown in Fig. 1. Three distinct redox peaks are observed. The E
1/2=0.95 and 1.19 peaks are typical of IrO2
3) and assigned to the Ir3+/Ir4+ and Ir4+/Ir5+ couple. The peaks at E
1/2=0.83 is observed only for IrO2 nanosheet, and shifts in HClO4; this peak is attributed to ad/desorption of anion. The onset of OER is observed at a potential similar to IrO2 nanoparticles.4) The specific capacitance between 0.2 and 1.2 V vs RHE at 2 mV s-1 was 93 kF mol-1, or 432 F g-1, which is the highest reported value for IrO2-based materials. It also closely matches the value of 88 kF mol-1 for RuO2nanosheet. Thus, almost all of the nanosheets surface is electrochemically accessible, and thus should provide unprecedented high utilization of the precious metal.

1) W. Sugimoto, H. Iwata, Y. Yasunaga, Y. Murakami, Y. Takasu, Angew. Chem. Int. Ed., 42, 4092 (2003). 

2) K. Fukuda, T. Saida, J. Sato, M. Yonezawa, Y. Takasu, W. Sugimoto, Inorg. Chem., 49, 4391 (2010).

3) H. Andreas, H. Elzanowska, I. Serebrennikova, V.J. Birss. J. Electrochem. Soc., 147, 4598 (2000).

4) Y. Lee, J. Suntivich, K.J. May, E.E. Perry, Y. Shao-Horn, J. Phys. Chem. Lett. 3, 399 (2012).

Figure 1. Cyclic voltammograms of IrO2 nanosheet electrodes in 0.5 M H2SO4 at 50 mV -1 (inset shows a typical AFM image of nanosheets on Si).

Figure 1
Synthesis and Characterization of Cu, Fe, Co Based Non-Precious Metal Catalysts for ORR in Alkaline Fuel Cells
Authors: Goenaga Gabriel A. , Roy Asa Logan , Cantillo Nelly M. , Foister Shane , Zawodzinski Thomas A.
Published: February 2020

The sluggish oxygen reduction reaction (ORR) in both proton exchange membrane and anion exchange membrane fuel cells, imposes the need for a catalyst to increase the reaction rate and overall cell efficiency.  Pt, due to its high catalytic activity and relative stability, is currently the most used ORR catalyst.  However, as a precious metal, Pt is expensive and a limited resource, greatly impacting the fuel cell’s cost and, as a consequence limiting its mass commercialization.

An advantage of alkaline fuel cells (AFCs) over proton exchange membrane fuel cells (PEMFCs) is that non-precious metals catalysts (NPMC) that are not very efficient for the ORR in acidic environment present higher catalytic activity and stability in alkaline media, even comparable to Pt.  Synthesis of NPMCs is often inspired by natural systems. Enzymes, such as laccase, are very efficient ORR catalysts and are known to reduce oxygen at approximately 1.2 V vs. the reversible hydrogen electrode (RHE) under mild pH conditions.  

We have synthesized a series of pyrolyzed NPMCs based on Cu, Co, Fe and their bimetallic combinations, and a phthalocyanine-like ligand supported on a carbon black.  The as prepared catalysts are then pyrolyzed at temperatures ranging from 600 oC to 1000 oC.    Various methods were used to optimize the catalysts activity; we evaluated the impact of different solvents during synthesis and removal of excess metal after pyrolysis.  Catalyst ORR activity and stability in 0.1 M KOH were tested by rotating ring disk electrode (RRDE) experiment.  Half wave potentials, number of electrons transferred and reaction rate were also calculated.

The bimetallic catalysts based on CoFe and CuFe surpassed the ORR activity of commercial Pt/C, reaching half wave potentials E1/2of 0.832 V and 0.815 V respectively, compared to 0.808 V vs. RHE of Pt/C.  See figure 1.

The samples were characterized using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), and x-ray powder diffraction (XRD).  RRDE experiments were used to study the effect of catalyst loading, to determine the reaction order and catalyst stability.  Ongoing experiments study the performance of the catalysts on a single cell using various anion exchange membranes.

Figure 1.  RDE plots for Cu, Co, Fe based catalysts and their comparison to commercial 30% Pt/C.

Acknowledgments

We gratefully acknowledge the support of this work by the NSF-funded TN-SCORE program, NSF EPS-1004083, under Thrust 2.

References:

X. Li, B. N. Popov, T. Kawahara, H. Yanagi. J. Power Sources 196 (2011) 1717-1722.

H. Peng, F. Liu, X. Liu, S. Liao, C. You, X. Tian, H. Nan, F. Luo, H. Song, Z. Fu and P. Huang.  ACS Catalysis 4 (2014) 3797−3805.

Figure 1
Inhibiting Effects of Nafion on ORR Catalyzed By Precious and Non-Precious Electrocatalysts Supported on Carbon
Authors: Chlistunoff Jerzy , Sansiñena José-María
Published: February 2020

Nafion has been an ionomer of choice in polymer electrolyte fuel cells (PEFCs) for three decades.1 However, recent research studies demonstrate that the polymer has a number of undesired properties. One of such properties is its ability to restructure at the catalyst surface, which may lead to a partial blocking of the catalyst surface and thus an inhibition of the desired electrochemical reactions, i.e., oxygen reduction and hydrogen oxidation. While a simple evidence for the blocking was presented for ideal Pt/Nafion interfaces,2-5quantitative evaluation of such effects is much more difficult for highly dispersed catalysts, such as carbon supported Pt.

                In this work, we will present results of our recent rotating ring disk electrode study on the inhibiting effects of Nafion on the electrocatalysis of oxygen reduction by carbon supported Pt and transition metal macrocycles. We will demonstrate that carbon plays an active role in the ORR inhibition and that the effects are especially strong for the non-precious catalysts (Fig. 1).

Acknowledgement

The financial support from the University of California (grant 12-LR-237440) is gratefully acknowledged.

References

1)       G. H. McCain, M. J. Covitch, J. Electrochem. Soc., 131, 1350-1352 (1984).

2)       J. Chlistunoff, F. Uribe, B. Pivovar, ECS Trans., 1,137-146 (2006).

3)       J. Chlistunoff, F. Uribe, B. Pivovar, ECS Trans. 2, 37-46, (2006).

4)       J. Chlistunoff, B. Pivovar, ECS Trans., 11, 1115-1125 (2007).

5)       D. L. Wood, J. Chlistunoff, J. Majewski, R. L. Borup, J. Am. Chem. Soc., 131, 18096-18104 (2009).

Fig. 1. The effect of Nafion on ORR catalyzed by carbon (XC72) supported cobalt phthalocyanine (CoPC). Scan rate 10 mV s-1. Rotation rate 400 rpm. Catalyst ink composition: 46.5 mg XC72; 0.8 mg CoPC; 8 cm3 isopropanol; 41.9 μl 5% Nafion (solid line); 83.9 μl 5% Nafion (dotted line). Catalyst loading 0.1 mg (for XC72).

Figure 1
Catalyst Layer Architectures with Supportless Pt Hollow Spheres for Polymer Electrolyte Fuel Cells (PEFCs)
Authors: Cilingir Dogan Didem , Cho Seong Hun , Park Gu-Gon , Yang Tae-Hyun , Yim Sung-Dae
Published: February 2020

Supportless Pt black is an attractive oxygen reduction reaction (ORR) catalyst for polymer electrolyte fuel cells (PEFCs) due to its merits such as no concern of support corrosion at high potentials and high durability against Pt dissolution at frequent potential change conditions. However, when the Pt black catalysts were fabricated as MEAs the performance is still lower than the current Pt/C catalysts and they have difficulty in reducing Pt content in the MEAs. In the present study, we have prepared sub-micron size Pt hollow spheres as an alternative catalyst layer architecture for PEFC MEAs. The big Pt spheres were employed to make 3-dimensional catalyst layer architectures and to enhance oxygen transport in catalyst layers, and hollow type Pt was made to reduce Pt content and to facilitate water management. 200 nm Pt hollow spheres with 10 nm shell thickness were prepared using carbon sphere templates. The Pt hollow spheres revealed comparable ORR performance with commercial Pt/C catalyst in a half cell test. Catalyst layer fabrication was also performed using the catalysts by optimizing the catalyst slurries and its fuel cell performance at different operating conditions and durability will be discussed based on single cell test results.
Platinum-Dysprosium Alloys for Oxygen Reduction in Alkaline Media
Authors: Sljukic Biljana , Santos Diogo M.F. , Martins Marta , Sequeira Cesar A.C. , Macciò Daniele , Saccone Adriana
Published: February 2020

Oxygen reduction reaction (ORR) has become one of the most studied electrochemical reactions due to its fundamental complexity and great importance in practical applications [1]. Specifically, different renewable energy technologies, including metal-air batteries and fuel cells (FCs) [2], are based on this reaction. Direct borohydride fuel cells use alkaline sodium borohydride solution as fuel and oxygen as the oxidant.

The mechanism of the ORR is primarily determined by the electrode material and electrolyte solution. Recently, bimetallic alloys of Pt or Pd with rare earth (RE) metals scandium (Sc) or yttrium (Y) were suggested as novel electrocatalysts for the ORR based on density functional theory calculations [3]. Electrochemical measurements have confirmed improved activity of polycrystalline Pt3Sc and Pt3Y electrodes in comparison with pure Pt by a factor of 1.5 - 1.8 and 6 - 10, respectively, in the range 0.9 - 0.87 V.

Herein, platinum-dysprosium (Pt-Dy) alloys, with different amounts of RE metal (50 to 60 at.%) were prepared by arc-melting starting from the stoichiometric amounts of the two metals and their composition and structure were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy.

Subsequently, electrocatalytic activity for the ORR of Pt-Dy alloy electrodes was studied in alkaline solution by cyclic voltammetry (CV) and chronoamperometry (CA). Main kinetic parameters, namely charge transfer coefficient and number of electrons exchanged, were evaluated for temperatures ranging from 25 to 65 ºC.

Furthermore, electrocatalytic activity for the ORR was studied in the presence of sodium borohydride in the supporting electrolyte in order to simulate conditions in direct borohydride fuel cells where borohydride crossover is possible to occur.

References

[1] Lee, J.; Jeong, B.; Ocon, J. D. Oxygen Elecrocatalysis in Chemical Energy Conversion and Storage Technologies. Current Applied Physics
2013, 13, 309.

[2] Mulder, G. Fuel Cells – Alkaline Fuel Cells | Overview. Encyclopedia of Electrochemical Power Sources
2009, 321.

[3] Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Nørskov, J. K. Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts. Nature Chemistry
2009, 1, 552.
(Invited) Oxygen Reduction Reaction at Cathodes on Proton Conducting Oxide Electrolytes: Contribution from Three Phase Boundary Compared to Bulk Path
Authors: Merkle Rotraut , Poetzsch Daniel , Maier Joachim
Published: February 2020

So far, the oxygen reduction reaction for cathodes on proton conducting electrolytes such as acceptor-doped BaZrO3 is less well understood than for cathodes on oxide ion conductors. Two groups of reaction pathways can be hypothesized: (i) without involvement of oxygen vacancies (which are, in contrast to cathodes on oxide conducting electrolytes, not required for the overall reaction on proton conducting electrolytes), (ii) with involvement of oxygen vacancies as catalytically active center, where the vacancies are restored at the end of the reaction by water desorption. Reaction rates for different steps being rate determining are derived, considering also the dependence of point defect concentrations on both pO2 and pH2O. These predictions are compared to experimental findings.

By thermogravimetry, a proton concentration in the range of 0.1-1mol% could be detected in Ba0.5Sr0.5Fe0.8Zn0.2O3-
d (BSFZ) under humid conditions.[1] Impedance measurements on pore-free BSFZ and Ba0.5Sr0.5Co0.8Fe0.2O3-
d (BSCF) microelectrodes  show that their proton conductivity suffices for the oxygen reduction reaction to proceed by the "bulk path" on the whole surface area.[2] In the analysis of these measurements on gas-symmetrical cells, the influence of the perceptible hole conductivity of the BaZr1-xYxO3-x/2 proton conducting electrolyte in high pO2must properly be accounted for.[3]

The measured pO2 and pH2O dependencies indicate that molecular oxygen species and oxygen vacancies are involved in the rate determining step. Thus, it appears that at least for perovskites such as BSFZ and BSCF with high oxygen vacancy concentration, the early steps of oxygen reduction are similar for cathodes on oxide conducting and proton conducting electrolytes, and differ only in the later stages (oxygen incorporation and transfer to the electrolyte vs. water desorption).

[1] D. Poetzsch, R. Merkle, J. Maier, Phys. Chem. Chem. Phys. 16 (2014) 16446

[2] D. Poetzsch, R. Merkle, J. Maier, in preparation

[3] D. Poetzsch, R. Merkle, J. Maier, J. Power Src. 242 (2013) 784
(Invited) Microreactors for Characterization and Benchmarking of Photocatalysts
Authors: Vesborg Peter C. K. , Dionigi Fabio , Bøndergaard Daniel , Pedersen Thomas , Domen Kazunari , Maeda Kazuhiko , Dahl Søren , Hansen Ole , Chorkendorff Ib
Published: February 2020

In the field of photocatalysis the batch-nature of the typical benchmarking experiment makes it very laborious to obtain good kinetic data as a function of parameters such as illumination wavelength, irradiance, catalyst temperature, reactant composition, etc.  Microreactors with on-line mass spectrometry, on the other hand, allow fast and automated acquisition of quantitative kinetic data. [1,2] As an example, we show how microreactor experiments on water splitting using Pt- or Rh-loaded GaN:ZnO photocatalysts quickly rank different catalysts according to their activity for gas-phase water splitting - but also how the activity scales with relative humidity and the crucial role of CrOx "capping" of the Pt- or Rh-co catalyst in order to prevent the loss of H2/O2 product via backward reaction on the precious metal. [3,4] The data suggests that protons transfer via the catalyst surface between the oxygen-evolving sites and the hydrogen evolving co-catalyst sites.  Recently, the microreactor experimental platform is being developed to support in-situ UV-VIS-IR spectroscopy [5] and even the introduction of liquid aqueous electrolyte and electrodes - all while retaining high sensitivity time resolved mass spectrometric product detection. [6] [1] Vesborg et al. Chemical Engineering Journal, 160, p. 738-741 (2010) [2] Vesborg et al. J. Phys. Chem. C, 114, p. 11162-11168 (2010) [3] Dionigi et al. Energy & Env. Sci., 4, p. 2937-2942 (2011) [4] Dionigi et al. J. Catal., 292, p. 26-31 (2012) [5] Dionigi et al. Rev. Sci. Instr., 84, p. 103910 (2013) [6] Bøndergaard et al. "Fast and sensitive method for detecting volatile species in liquids", submitted
Impedance Spectroscopy Analysis of Ni/YSZ Interfaces Prepared by Liquid Precursor Deposition
Authors: Buyukaksoy Aligul , Birss Viola
Published: February 2020

The state-of-the-art material for SOFC anodes is a Ni-yttria-stabilized zirconia (Ni-YSZ) based cermet, due primarily to the excellent electrocatalytic activity of Ni towards H2oxidation. To design and construct Ni-YSZ anodes with maximum performance, it is important to understand the reaction mechanism at the Ni/YSZ interface. Therefore, model anode designs, in which dense Ni films were deposited on dense YSZ electrolytes in the form of regularly spaced strips, have been developed by multiple groups [1, 2] to control the Ni/YSZ interfacial length, with impedance spectroscopy (EIS) used to evaluate performance. However, a consensus has not been reached yet on the exact reaction steps and how they change with temperature. Charge transfer at the Ni/YSZ interface, hydrogen diffusion on Ni surface and hydrogen adsorption/desorption reaction, have all been suggested as the rate-limiting steps in the literature [1, 2]. Thus, a clear identification of the rate limiting step at the Ni/YSZ interface and its correlation with the Ni-YSZ composite anodes is still required.

In this work, solution precursor deposited Ni thin films were deposited on dense Zr oxide discs for comparison with Ni-YSZ composite anodes that were prepared by the infiltration of the Ni solution precursor into porous YSZ scaffolds. EIS analysis was carried out, and a transmission line model, which allows the separation of the porous YSZ scaffold ionic resistance and the Ni/YSZ interfacial impedance, was used to fit the impedance data. The dominant impedance arcs in the thin film Ni anodes and the Ni/YSZ interfacial impedance, extracted from the transmission line modelling of the Ni-YSZ composites, both exhibited an activation energy of ~1.3 eV. The same activation energy obtained for the Ni/YSZ interfacial impedance obtained from both Ni thin film anodes and infiltrated Ni-YSZ anodes validates the transmission line fitting approach while the obtained activation energy value of ~1.3 eV indicates that the charge transfer process occurring at the triple phase boundary is the rate determining reaction step in Ni-YSZ anodes.

Acknowledgements:

The authors gratefully acknowledge the Eyes High PDF Program at the University of Calgary and Alberta Innovates – Technology Futures (AITF) for the support of AB, as well as the Natural Sciences and Engineering Research Council of Canada (NSERC) for the overall financial support of this work.

References:

1. A. Bieberle, L. P. Meier and L. J. Gauckler,  J. Electrochem. Soc., 148 (2001) A646.

2. A. Utz, H. Stormer, D. Gerthsen, A. Weber, E and Ivers-Tiffee, Solid State Ionics, 192 (2011) 565.
Investigation of Infiltration via Multi-Physics Simulation Tool with Realistic Microstructure Properties
Authors: Yang Tao , Celik Ismail Bektas , Sezer Hayri , Lee Shiwoo , Gerdes Kirk
Published: February 2020

Infiltration is a potential method to improve the performance of SOFCs. The purpose of this study is to investigate the effects of infiltration via multi-physics simulation tools with realistic microstructure properties. The physics-based simulation tool is developed to extract the properties of fuel cells based on the measured polarization curves and impedance behavior for different fuel/air utilization cases, which has been verified in our previous work. Furthermore, we incorporate the microstructure properties distribution within the electrodes of realistic SOFC. These structural properties are either obtained from experiments (e.g. volume fraction) or calculated based on percolation theory (e.g. interface area, length of triple phase boundary, etc.). In this study, we apply the developed simulation tool to analyze both the baseline button cell and the one with infiltrated cathode. The performances of both cells are predicted with the multi-physics numerical simulation and compared against experimental measurements. In addition, the performances of baseline cell and cathode-infiltrated cell with working loads are predicted. Finally, the properties of both cells are extracted and compared to show the effects of infiltration. The results will help us better understand and improve the infiltration processes.
Stabilization of Ni-YSZ Nanocomposite Anodes by Deposition of a Thin YSZ Overlayer
Authors: Buyukaksoy Aligul , Birss Viola
Published: February 2020

Micro solid oxide fuel cells (μ-SOFCs) are energy conversion devices that are being considered for portable devices, such as laptop computers and mobile phones.  As the low operating temperatures of μ-SOFCs (600 °C) result in high electrode resistances, one solution to this is to increase the triple phase boundary (TPB) length.  In order to achieve this goal, ethylene glycol-based polymeric precursors of the Ni and YSZ components, which allow atomic level mixing of ions, were used to form nanocomposite Ni-YSZ thin films in this work. This, in turn, resulted in Ni-YSZ anode layers with an average grain size of ca. 25 nm, producing a very high TPB length (in comparison to conventional Ni-YSZ anodes) and hence a low electrode polarization resistance of 0.64 Ω.cm2 at 550 °C in humidified H2.

It has been reported that Ni-YSZ thin films, prepared by pulsed laser deposition (PLD), exhibit Ni grain growth at operating temperatures (400-600 °C), resulting in a deleterious effect on the long-term stability of the electrode performance [1]. Although the YSZ network within the composite films constrained Ni grain growth to some extent, significant grain growth at the outer surface of the film could not be inhibited [1]. To overcome this problem for the polymeric precursor-deposited Ni-YSZ films developed here, we deposited a thin, porous YSZ layer (ca. 100 nm in thickness) on top of the Ni-YSZ film. This led to a decrease in the polarization resistance degradation rate, from 3.7% per hour at 550 °C in Ni-YSZ thin films to only 0.39 % per hour at the same temperature.The results obtained in this study show that atomic level mixing, resulting from the use of polymeric precursors, yields very good electrode performances that can be stabilized by the application of additional layers. This will allow for significantly lower μ-SOFC cost as a result of the increase in power density and improved robustness. 

Acknowledgements:

The authors gratefully acknowledge the Eyes High PDF Program at the University of Calgary and Alberta Innovates – Technology Futures (AITF) for the support of AB, as well as the Natural Sciences and Engineering Research Council of Canada (NSERC) for the overall financial support of this work.

References:

U. P. Muecke, K. Akiba, A. Infortuna, T. Salkus, N. V. Stus and L. J. Gauckler, Solid State Ionics, 178 (2008) 1762.
(Invited) Recent Topics on x-Ray Analysis of Fuel-Cell Catalysts
Authors: Imai Hideto
Published: February 2020

X-ray analysis utilizing interaction between x-rays and electrons in catalysts provides versatile information on the fuel-cell-catalyst development from basic structural and electronic structural details to reaction and degradation mechanism in in-situ environments.

Quick overview of x-ray analysis methods using various physical phenomena, such as, diffraction, scattering, x-ray absorption, x-ray photo-emission, and latest challenges of their application to the catalyst development for both platinum-based and non-platinum catalysts will be presented.
(Invited) Understanding Fuel Cell Materials Degradation Via Advanced Electron Microscopy Techniques
Authors: More Karren L.
Published: February 2020

Polymer electrolyte membrane (PEM) fuel cell performance degradation, particularly associated with the cathode catalyst layer, can be directly attributed to the stability and durability of individual material constituents comprising the membrane electrode assemblies (MEAs), including the electrocatalyst, catalyst support, recast ionomer, polymer membrane, and gas diffusion-microporous layers.  The structural and chemical changes of these MEA constituents are being quantified via advanced electron microscopy methods to elucidate the specific degradation mechanisms contributing to performance loss, with the ultimate goal being microstructural and compositional improvements that will enhance MEA durability.  Research efforts at Oak Ridge National Laboratory have been focused on the high-resolution microstructural and microchemical characterization of as-fabricated fuel cell materials (e.g., the individual materials constituents and the same materials incorporated in fresh MEAs), these materials/MEAs subjected to accelerated stress tests (ASTs) designed to degrade specific MEA components, and field-aged MEAs.  These studies are used to establish critical processing-microstructure-performance correlations and to elucidate the individual materials changes contributing to measured MEA degradation, performance loss, and failure.  Analytical microscopy methods have been used to mechanistically explain the degradation of various carbon support structures, to quantify Pt loss due to migration, catalyst coalescence, and to understand the structure, distribution, and composition of thin ionomer films within catalyst layers.  Understanding and quantifying the structural and compositional changes of the materials comprising the MEA during electrochemical-aging will allow for the implementation of processing changes and critical materials development that are required for optimizing PEM fuel cell durability and performance.

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Research sponsored by (1) the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy and (2) Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences (CNMS), which is an Office of Science User Facility.
Improvement of the Process Model for the Ohmic Loss of the Proton Exchange Membrane Fuel Cell
Authors: Rezaei Niya Seyed Mohammad , Phillips Ryan K. , Hoorfar Mina
Published: February 2020

The process model is defined as an analytical solution for the governing equations of an electrochemical system which is used to predict the impedance characteristics of the system and identify the relation between different parts of the measured impedances and physicochemical properties and operating conditions [1, 2]. Despite various attempts made towards presenting a process model for proton exchange membrane (PEM) fuel cells, no complete process model is introduced yet [1, 3]. We have focused on developing such a process model by separately modeling the three losses: ohmic, activation, and mass transport. Our first process model of the ohmic loss [3] required an estimation of the water concentration in the catalyst layer which cannot be readily determined experimentally. Moreover, while the water transport in the catalyst layer was considered in the governing equations of the membrane, the water flux from the membrane towards the catalyst layer was not considered in the water flux equation of the catalyst layer. In this study, the ohmic-loss model presented before is improved by calculating the water concentration in the catalyst layer from other inputs as well as considering the water flux from the membrane in the catalyst layer equation. In the new model, the equivalent circuit extracted from the theoretical relation determined for the impedance of the ohmic loss still has the same format as the model presented before [3] (see Figure 1). However, the elements of the equivalent circuit have different theoretical definitions.

In the following equations, subscripts m, cl, and GDL represent the membrane, catalyst layer and cathode gas diffusion layer properties, respectively. l, D, M, F, T and j denote the thickness, diffusion coefficient, molecular mass, Faraday’s constant, temperature and current density, respectively. ρ, σ and λpresent the density, electrical conductivity and water content, respectively. Also,

a = 0.5193 exp(1268(1/303-1/T))

 
b = 0.326 exp(1268(1/303-1/T))

 α = 2.5/(22F)

 β = ρmDm/Mm

The model is compared against the measured impedances obtained for a 7.98-cm2cell operated at the 90% relative humidities for the anode and cathode, three different temperatures (65 °C, 70 °C and 75 °C), and two potentials (0.7 V and 0.75 V). The measured impedances are reported in the frequency span of 10-0.01 Hz as the ohmic loss is dominant in this range [3]. The results in Figure 2 show an excellent agreement between the theoretical predictions and the measured impedances.

References

S.M. Rezaei Niya, M. Hoorfar, J. Power Sources, 240, 281 (2013).

P. Agarwal, M. Orazem, J. Electrochem. Soc., 139, 1917 (1992)

S.M. Rezaei Niya, M. Hoorfar, Electrochimica Acta, 120, 193 (2014). 

Figure 1
Electrochemical Investigation of Benzil/Aluminum Ion Interactions
Authors: Cheek Graham T.
Published: February 2020

Introduction

       Benzil has been investigated as a model diketone in various electrochemical studies (1-4 ).  Previous work in this laboratory has dealt with the effect of metal ion addition on the electrochemical behavior of 9-fluorenone (5).  In the present work, the effect of aluminum ion additions to acetonitrile solutions of benzil has been studied.  The presence of adjacent carbonyl groups in benzil suggests that benzil reduction products may be effective ligands toward added metal ions.  The goal of this study is to determine the effects of these metal/ligand interactions upon the cyclic voltammetric characteristics of the benzil system.  

 Experimental

       Acetonitrile (AN), benzil, and aluminum trifluoromethansulfonate [aluminum triflate, Al(TfO)3] were obtained from Aldrich Chemical Co. Tetraethylammonium tetrafluoroborate [TEA BF4] was purchased from Southwestern Analytical Chemicals (SACHEM). Voltammograms were acquired with a PAR283 potentiostat using PowerSuiteTM  software. Potentials are reported with respect to a Ag/AgCl (0.1M EMICl in EMI BF4) reference electrode (Cypress Systems).  Vitreous carbon electrodes were obtained from Cypress Systems. All experiments were carried out in a Vacuum Atmospheres drybox.

  Results and Discussion

     Benzil  undergoes two successive electron-transfers in acetonitrile/TEA BF4 (Figure 1a), although the second process is somewhat irreversible due to proton donation from acetonitrile (2-4).  The addition of Al(TfO)3 at the 1:1 Al(TfO)3 : benzil  level produces a new reduction peak at -0.70V, which is assigned to the reduction of a benzil:Al3+ complex. The second reduction process has become shifted to slightly more positive potentials, suggesting interaction of the benzil radical anion with Al3+.  The reduction process at -1.80 in Figure 1a is due to the reduction of Al3+ in the solution.   Addition of further  Al(TfO)3 to give 2:1 Al(TfO)3 : benzil (Figure 1b) gives the -0.7 V reduction process as the predominant response. This general behavior supports a strong interaction of Al3+ with benzil reduction products, ultimately giving the benzil dianion with each oxygen complexed with one Al3+. 

       

References

H. E. Stapelfeldt and S. P. Perone,  Analytical Chemistry,  41(4), 623 (1969).

M. D. Ryan and D. H. Evans,  J. Electroanal. Chem.,  67, 333 (1976).

M. Chandrasekaran,  M. Noel and V. Krishnan, 
J. Chem. Soc., Perkin Trans.2, 979 (1992).

G. T. Cheek, ECS Transactions, 6(6), 47 (2007).

D. Canby, E. Sanders,and G. T. Cheek, J. Electrochem. Soc., 160(7), G3159 (2013).

Figure 1
Electrochemistry Vs. Photochemistry: A Mechanistic Comparison of the Nucleophilic Trapping of Radical Cations from Electron-Rich Olefin
Authors: Graaf Matthew D. , Moeller Kevin D
Published: February 2020

The reactivity of electrochemically generated radical cations towards various nucleophiles have been investigated extensively.1 Complementary, the area of photoredox catalysis has made a significant contribution to the use of radical ions in organic synthesis.2 The use of these photoredox catalysts, in comparison with our electrochemical methods, has provided great insight into the behavior of radical cations. Although both of these methods efficiently produce radical cations, the mechanistic differences can have dramatic effects. As a result, the products obtained from identical substrates can vary immensely. Herein we report the mechanistic comparison of radical cations generated electrochemically and those generated photochemically.

1 a) Campbell, J. M.; Xu, H. C.; Moeller, K. D. JACS, 2012 , 134, 18338 b) Xu, H. C.; Campbell, J. M.; Moeller, K. D. JOC,
2014 , 79, 379

2 Schultz, D. M. and Yoon, T. P. Science, 2014 , 343, 985

Figure 1
Pathways to Low Cost Electrochemical Energy Storage: A Comparison of Aqueous and Nonaqueous Flow Batteries
Authors: Darling Robert M. , Gallagher Kevin G. , Brushett Fikile , Ha Seungbum , Kowalski Jeffrey
Published: February 2020

Energy storage is increasingly viewed as a valuable asset for electricity grids composed of high fractions of intermittent sources, like wind turbines, or unreliable transmission and distribution services.  However, the likelihood that batteries will meet the stringent cost and durability requirements of grid applications is largely unquantified.  In this work, we investigate conceptual and actual aqueous and nonaqueous flow batteries designed to store energy for hours and analyze the relationships among technological performance characteristics, component cost factors, and system price.  The benefit of high voltage associated with the nonaqueous approach must be balanced against the burden of high solubility of active materials.  Requirements in harmony with economically effective energy storage are derived for both systems to facilitate comparison.
Acetylacetonate Complexes for Non-Aqueous Vanadium(III) Acetylacetonate Redox Flow Batterie Applications
Authors: Kucharyson Jonathan F. , Suttil James A. , Escalante-Garcia Ismailia L. , Thompson Levi T
Published: February 2020

An ester functionalized vanadium acetylacetonate (acac) complex was synthesized and characterized for use in redox flow batteries (RFBs). The resulting complex has a solubility of 1.32M, a significant improvement over other metal complexes that have been considered for non-aqueous RFB applications, and a theoretical energy density of 37 Wh/L. While short term flow cell cycling of the complex shows high coulombic efficiencies (95%), capacity fade limits long term cycling. To better understand the causes of capacity fade, a stability study utilizing bulk electrolysis was conducted on both the parent vanadium acetylacetonate, and the ester functionalized complex. The results were used to determine the shelf life, solubility at each oxidation state, and cycle life for each half reaction. The shelf life is greater than the cycling experiment timelines and should not result in a loss of capacity. The solubilities at each oxidation state do not change by more than 25% of the solubility of the parent species. Vanadyl formation is evident during oxidation, and ligand detachment occurrs during reduction. The cycle life for the vanadium acetylacetonate is limited by vanadyl formation and reaches 80% capacity within ten cycles. The ester functionalized system shows rapid vanadyl formation and reaches 80% capacity after just the first cycle. Work is ongoing to better characterize side products and better understand the mechanism of the reactions.
Probabilistic Analysis for the Safety of Battery Based Energy Storage Systems
Authors: Gully Benjamin , Agarwal Arun S , Hill Davion , Sridhar Narasi
Published: February 2020

The adoption of lithium ion batteries in multi megawatt systems for grid use or vehicles on the road around us leads to an ever increasing need for assessing the safety in these systems. Furthermore, the industry has great interest in preventing any such event in deployed systems to maintain a positive public opinion of the hugely needed technology, and facilitate rapid deployment. This paper presents a framework for representing and assessing these aspects. Fundamentally, the work focuses on development of a probabilistic framework for analyzing risk, populated with laboratory data. This allows comparisons of relative risks, areas of key interest, and a more intuitive understanding of how these probabilities proliferate. We can thus aggregate risks into useful categories to help define requisite design criteria and performance thresholds. The paper concludes with initial suggestions of metrics that can be used to better illustrate battery system safety issues, and help customers select systems that best meet their needs.