An official website of the United States government
Biomass is abundant, inexpensive and renewable, therefore, it is highly expected to play a significant role in our future energy and chemical landscapes. The US DOE has identified furanic compounds (furfural and 5-(hydroxymethyl)furfural (HMF)) as the top platform building-block chemicals that can be readily derived from biomass sources [1]. Electrocatalytic conversion of these furanic compounds is an emerging route for the sustainable production of valuable chemicals [2, 3]. In my presentation, I will first present our recent mechanistic study of electrochemical reduction of furfural through a combination of voltammetry, preparative electrolysis, thiol-electrode modifications, and kinetic isotope studies [4]. It is demonstrated that two distinct mechanisms are operable on metallic Cu electrodes in acidic electrolytes: (i) electrocatalytic hydrogenation (ECH) and (ii) direct electroreduction. The contributions of each mechanism to the observed product distribution are clarified by evaluating the requirement for direct chemical interactions with the electrode surface and the role of adsorbed hydrogen. Further analysis reveals that hydrogenation and hydrogenolysis products are generated by parallel ECH pathways. Understanding the underlying mechanisms enables the manipulation of furfural reduction by rationally tuning the electrode potential, electrolyte pH, and furfural concentration to promote selective formation of important bio-based polymer precursors and fuels. Next, I will present electrocatalytic conversion of HMF to two biobased monomers in an H-type electrochemical cell [5]. HMF reduction (hydrogenation) to 2,5-bis(hydroxymethyl)furan (BHMF) was achieved under mild electrolyte conditions and ambient temperature using a Ag/C cathode. Meanwhile, HMF oxidation to 2,5-furandicarboxylic acid (FDCA) with ~100% efficiency was facilitated under the same conditions by a homogeneous nitroxyl radical redox mediator, together with an inexpensive carbon felt anode. The selectivity and efficiency for Ag-catalyzed BHMF formation were sensitive to cathode potential, owing to competition from HMF hydrodimerization reactions and water reduction (hydrogen evolution). Moreover, the carbon support of Ag/C was active for HMF reduction and contributed to undesired dimer/oligomer formation at strongly cathodic potentials. As a result, high BHMF selectivity and efficiency were only achieved within a narrow potential range near –1.3 V. Fortunately, the selectivity of redox-mediated HMF oxidation was insensitive to anode potential, thus allowing HMF hydrogenation and oxidation half reactions to be performed together in a single cathode-potential-controlled cell. Electrocatalytic HMF conversion in a paired cell achieved high molar yields of BHMF and FDCA, and nearly doubled electron efficiency compared to the unpaired cell. Finally, I will briefly introduce our recent work on the development of a three-electrode flow cell with an oxide-derived Ag (OD-Ag) cathode and catbon felt anode for paring elecatalytic oxidation and reduction of HMF. The flow cell has a remarkable cell voltage reduction from ~7.5 V to ~2.0 V by transferring the electrolysis from the H-type cell to the flow cell [6]. This represents a more than fourfold increase in the energy efficiency at the 10 mA operation. A combined faradaic efficiency of 163% was obtained to BHMF and FDCA. Alternatively, the anodic hydrogen oxidation reaction on platinum further reduced the cell voltage to only ~0.85 V at 10 mA operation. These paired processes have shown potential for integrating renewable electricity and carbon for distributed chemical manufacturing in the future.
more »
« less
Evidence of Radical Intermediate Generated in the Electrochemical Oxidation of Iodide
We present evidence of the generation of radical ion formation during the oxidation of iodide on a fluorine doped tin oxide (FTO) electrode in acetonitrile. The cyclic voltammograms for the oxidation of iodide and triiodide on FTO are significantly different as in the case of the oxidation of Pt electrode. These differences are assigned to kinetic differences on the FTO surface that require significant over potentials to drive the oxidation of iodide and triiodide. We propose that at the highly positive potentials the iodine radical intermediate, I·, becomes thermodynamically stable at FTO. The radical nature of the intermediate was verified by the formation of radicals of the usual traps of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and 2,2,5,5 tetramethyl-1-pyrroline N-oxide (TMPO) when these were added to an electrolyzed solution. Irradiation of an iodine solution causes the homolytic cleavage of I2 and yields the same radical intermediate with TMPO as in the electrolysis experiment. Similar results were obtained from the electrolysis of bromide solutions upon addition of TMPO. Short term electrolysis (< 1 h) gives triiodide as a final product while long-term electrolysis (> 17 h) yields additional byproducts. Byproducts were determined to be organoiodines by gas chromatography-mass spectrometry (GC-MS). Overall, our results are consistent with iodine atoms reacting with the electrolyte during electrolysis at the FTO electrode and with a sequential two-electron transfer process.
more »
« less
- Award ID(s):
- 1726897
- PAR ID:
- 10216970
- Date Published:
- Journal Name:
- Journal of the Mexican Chemical Society
- Volume:
- 63
- Issue:
- 3
- ISSN:
- 1870-249X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The oxygen electrode in a proton-conductor based solid oxide cells is often a triple-conducting material that enables the transport and exchange of electrons (e-), oxygen ions (O2-), and protons (H+), thus expanding active areas to enhance the oxygen electrode activity. In this work, a theoretical model was developed to understand stability of tri-conducting oxygen electrode by studying chemical potentials of neutral species (i.e., μ_(O_2)^ , μ_(H_2)^ , and μ_(H_2 O)^ ) as functions of transport properties, operating parameters, and cell geometry. Our theoretical understanding shows that: (1) In a conventional oxygen-ion based solid oxide cell, a high μ_(O_2)^ (thus high oxygen partial pressure) exists in the oxygen electrode during the electrolysis mode, which may lead to the formation of cracks at the electrode/electrolyte interface. While in a proton-conductor based solid oxide cell, the μ_(O_2)^ is reduced significantly, suppressing the crack formation, and resulting in improved performance stability. (2) In a typical proton-conductor based solid oxide electrolyzer, the dependence of μ_(O_2)^ on the Faradaic efficiency is negligible. Hence, approaches to block the electronic current can improve the electrolysis efficiency while achieving stability. (3) The difference of the μ_(O_2)^ (thus p_(O_2)^ ) between the oxygen electrode and gas phase can be reduced by using higher ionic conducting components and improving electrode kinetics, which lead to further improvement of electrode stability.more » « less
-
Electrochemical dehalogenation of polyhalogenated compounds is an inefficient process as the working electrode is passivated by the deposition of short-chain polymers that form during the early stages of electrolysis. Herein, we report the use of 1, 1, 1, 3, 3, 3-hexaflouroisopropanol (HFIP) as an efficient reagent to control C–H formation over the radical association. Debromination of 1,6-dibromohexane was examined in the presence of Ni(II) salen and HFIP as the electrocatalyst and hydrogen atom source, respectively. Electrolysis of 10 mM 1,6-dibromohexane and 2 mM Ni(II) salen in the absence of HFIP yields 50% unreacted 1,6-dibromohexane and ∼40% unaccounted for starting material, whereas electrolysis with 50 mM HFIP affords 65%n-hexane. The mechanism of hydrogen atom incorporation was examined via deuterium incorporation coupled with high-resolution mass spectrometry, and density functional theory (DFT) calculations. Deuterium incorporation analysis revealed that the hydrogen atom originated from the secondary carbon of HFIP. DFT calculations showed that the deprotonation of hydroxyl moiety of HFIP, prior to the hydrogen atom transfer, is a key step for C–H formation. The scope of electrochemical dehalogenation was examined by electrolysis of 10 halogenated compounds. Our results indicate that through the use of HFIP, the formation of short-chain polymers is no longer observed, and monomer formation is the dominant product.more » « less
-
Cyclic voltammetry and controlled-potential (bulk) electrolysis have been employed to investigate the direct electrochemical reduction of acetochlor (1) at carbon and silver cathodes in dimethylformamide. Voltammograms of1exhibit a single irreversible cathodic peak at both cathode materials. Catalytic properties of silver towards carbon–halogen bond cleavage are evidenced by a positive shift in the reduction of acetochlor as compared to the more inert glassy carbon electrode. Voltammograms in the presence of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), and comparisons of calculated relative interaction energies between acetochlor, possible intermediates, and deschloroacetochlor in the presence of different proton donors, suggest strong hydrogen-bonding interactions between HFIP and a carbanion intermediate. Addition of HFIP to electrolysis conditions promotes complete reduction at both cathode materials, with formation of deschloroacetochlor in high yields. In deuterium labelling studies, the use of DMF-d7led to no evidence for deuterium atom incorporation. However, when HFIP-OD or D2O were employed as a proton source, substantial amounts of deuterated deschloroacetochlor were observed. A mechanism for the reduction of acetochlor is proposed, in which radical intermediates do not play a significant role in reduction, rather a carbanion intermediate pathway is followed.more » « less
-
In this work, a robust solid oxide electrolysis cell with Sr 2 Fe 1.5 Mo 0.5 O 6−δ –Ce 0.8 Sm 0.2 O 1.9 (SFM–SDC) based electrodes has been utilized to verify the conceptual process of partial oxidation of methane (POM) assisted steam electrolysis, which can produce syngas and hydrogen simultaneously. When the cathode is fed with 74%H 2 –26%H 2 O and operated at 850 °C, the open circuit voltage (OCV), the minimum energy barrier required to overcome the oxygen partial gradient, is remarkably reduced from 0.940 to −0.012 V after changing the feed gas in the anode chamber from air to methane, indicating that the electricity consumption of the steam electrolysis process could be significantly reduced and compensated by the use of low grade thermal energy from external heat sources. It is found that after ruthenium (Ru) impregnation, the electrolysis current density of the electrolyzer is effectively enhanced from −0.54 to −1.06 A cm −2 at 0.6 V and 850 °C, while the electrode polarization resistance under OCV conditions and 850 °C is significantly decreased from 0.516 to 0.367 Ω cm 2 . Long-term durability testing demonstrates that no obvious degradation but a slight improvement is observed for the electrolyzer, which is possibly due to the activation of the SFM–SDC electrode during operation. These results indicate that the robust Ru infiltrated solid oxide electrolyzer is a very promising candidate for POM assisted steam electrolysis applications. Our result will provide insight to improve the electrode catalysts used in POM assisted steam electrolysis.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.29356/jmcs.v63i3.529
