High-Performance Bipolar Membrane Development for Improved Water Dissociation.

Bipolar membranes (BPMs) are the enabling component of many promising electrochemical devices used for separation and energy conversion. Here, we describe the development of high-performance BPMs, including two-dimensional BPMs (2D BPMs) prepared by hot-pressing two preformed membranes and three-dimensional BPMs (3D BPMs) prepared by electrospinning ionomer solutions and polyethylene oxide. Graphene oxide (GOx) was introduced into the BPM junction as a water-dissociation catalyst. We assessed electrochemical performance of the prepared BPMs by voltage-current (V-I) curves and galvanostatic electrochemical impedance spectroscopy. We found the optimal GOx loading in 2D BPMs to be 100 µg cm-2, which led to complete coverage of GOx at the interface. The integration of GOx beyond this loading moderately improved electrochemical performance but significantly compromised mechanical strength. GOx-catalyzed 2D BPMs showed comparable performance with a commercially available Fumasep BPM at current densities up to 500 mA cm-2. The 3D BPMs exhibited even better performance: lower resistance and higher efficiency for water dissociation and substantially higher stability under repeated cycling up to high current densities. The improved electrochemical performance and mechanical stability of the 3D BPMs make them suitable for incorporation into CO2 electrolysis devices where high current densities are necessary.

Elucidating the Dynamic Nature of Fuel Cell Electrodes as a Function of Conditioning: An ex Situ Material Characterization and in Situ Electrochemical Diagnostic Study.

To increase the commercialization of fuel cell electric vehicles, it is imperative to improve the activity and performance of electrocatalysts through combined efforts focused on material characterization and device-level integration. Obtaining fundamental insights into the ongoing structural and compositional changes of electrocatalysts is crucial for not only transitioning an electrode from its as-prepared to functional state, also known as "conditioning", but also for establishing intrinsic electrochemical performances. Here, we investigated several oxygen reduction reaction (ORR) electrocatalysts via in situ and ex situ characterization techniques to provide fundamental insights into the interfacial phenomena that enable peak ORR mass activity and high current density performance. A mechanistic understanding of a fuel cell conditioning procedure is described, which encompasses voltage cycling and subsequent voltage recovery (VR) steps at low potential. In particular, ex situ membrane electrode assembly characterization using transmission electron microscopy and ultra-small angle X-ray scattering were performed to determine changes in carbon and Pt particle size and morphology, while in situ electrochemical diagnostics were performed either during or after specific points in the testing protocol to determine the electrochemical and interfacial changes occurring on the catalyst surface responsible for oxygen transport resistances through ionomer films. The results demonstrate that the voltage cycling (break-in) step aids in the removal of sulfate and fluoride and concomitantly reduces non-Fickian oxygen transport resistances, especially for catalysts where Pt is located within the pores of the carbon support. Subsequent low voltage holds at low temperature and oversaturated conditions, i.e., VR cycles, serve to improve mass activities by a factor of two to three, through a combined removal of contaminants, surface-blocking species (e.g., oxides), and rearrangement of the catalyst atomic structure (e.g., Pt-Pt and Pt-Co coordination).

Enhancement of microbial fuel cell performance by introducing a nano-composite cathode catalyst.

Iron aminoantipyrine (Fe-AAPyr), graphene nanosheets (GNSs) derived catalysts and their physical mixture Fe-AAPyr-GNS were synthesized and investigated as cathode catalysts for oxygen reduction reaction (ORR) with the activated carbon (AC) as a baseline. Fe-AAPyr catalyst was prepared by Sacrificial Support Method (SSM) with silica as a template and aminoantipyrine (AAPyr) as the organic precursor. 3D-GNS was prepared using modified Hummers method technique. The Oxygen Reduction Reaction (ORR) activity of these catalysts at different loadings was investigated by using rotating ring disk (RRDE) electrode setup in the neutral electrolyte. The performance of the catalysts integrated into air-breathing cathode was also investigated. The co-presence of GNS (2 mg cm-2) and Fe-AAPyr (2 mg cm-2) catalyst within the air-breathing cathode resulted in the higher power generation recorded in MFC of 235 ±â€¯1 µW cm-2. Fe-AAPyr catalyst itself showed high performance (217 ±â€¯1 µW cm-2), higher compared to GNS (150 ±â€¯5 µW cm-2) while AC generated power of roughly 104 µW cm-2.

Role of Nitrogen Moieties in N-Doped 3D-Graphene Nanosheets for Oxygen Electroreduction in Acidic and Alkaline Media.

This study elucidates the synthesis-structure-property correlations of nitrogen moieties present in nitrogen-functionalized graphene nanomaterials toward oxygen reduction reactions (ORRs) and their electrochemical pathways in acidic and alkaline electrolytes. Porous three-dimensional nitrogen-doped graphene nanosheets (N/3D-GNSs) were fabricated using the sacrificial support method and doped with nitrogen using 10 atom % NH3 under thermal pyrolysis at T = 650, 850, and 1050 °C for evaluating the nitrogen species formed under different temperatures. The abundances of the various nitrogen species formed under pyrolytic conditions were evaluated with X-ray photoelectron spectroscopy. Using rotating ring-disk electrode, we analyzed the role played by the nitrogen moieties influencing the electrochemical activity of the N/3D-GNS supports for oxygen reduction reactions (ORRs) in both acidic and alkaline media. It was demonstrated that the concentrations of the nitrogen moieties: graphitic-N, quaternary, hydrogenated-N (hydrogenated nitrogen combined pyrrolic nitrogen and hydrogenated pyridine) and pyridinic-N varied considerably with pyrolysis temperatures. A decrease in graphitic-N content and an increase in the ratio of hydrogenated-N/pyridinic-N significantly improved the activity of the material. The half-wave and onset potentials as well as the current densities and hydrogen peroxide (H2O2)/(HO2-) yields of the N/3D-GNS materials also varied between acidic and alkaline electrolytes but followed the general trend in terms of pyrolysis temperatures and abundance of the nitrogen moieties. Among the synthesized materials, the 3D-graphene nanosheets that were doped with nitrogen at 850 °C, optimized to have the highest hydrogenated-N and lowest pyridinic-N as well as better catalyst-ionomer integration, showed the highest ORR performance. This strategy for the tunable synthesis of nitrogen-doped graphene materials with controlled nitrogen functionalization offers a platform for developing active supports or catalytic nanomaterials for fuel cell applications.

Three-dimensional graphene nanosheets as cathode catalysts in standard and supercapacitive microbial fuel cell.

Three-dimensional graphene nanosheets (3D-GNS) were used as cathode catalysts for microbial fuel cells (MFCs) operating in neutral conditions. 3D-GNS catalysts showed high performance towards oxygen electroreduction in neutral media with high current densities and low hydrogen peroxide generation compared to activated carbon (AC). 3D-GNS was incorporated into air-breathing cathodes based on AC with three different loadings (2, 6 and 10 mgcm-2). Performances in MFCs showed that 3D-GNS had the highest performances with power densities of 2.059 ± 0.003 Wm-2, 1.855 ± 0.007 Wm-2 and 1.503 ± 0.005 Wm-2 for loading of 10, 6 and 2 mgcm-2 respectively. Plain AC had the lowest performances (1.017 ± 0.009 Wm-2). The different cathodes were also investigated in supercapacitive MFCs (SC-MFCs). The addition of 3D-GNS decreased the ohmic losses by 14-25%. The decrease in ohmic losses allowed the SC-MFC with 3D-GNS (loading 10 mgcm-2) to have the maximum power (Pmax) of 5.746 ± 0.186 Wm-2. At 5 mA, the SC-MFC featured an "apparent" capacitive response that increased from 0.027 ± 0.007 F with AC to 0.213 ± 0.026 F with 3D-GNS (loading 2 mgcm-2) and further to 1.817 ± 0.040 F with 3D-GNS (loading 10 mgcm-2).

Air Breathing Cathodes for Microbial Fuel Cell using Mn-, Fe-, Co- and Ni-containing Platinum Group Metal-free Catalysts.

The oxygen reduction reaction (ORR) is one of the major factors that is limiting the overall performance output of microbial fuel cells (MFC). In this study, Platinum Group Metal-free (PGM-free) ORR catalysts based on Fe, Co, Ni, Mn and the same precursor (Aminoantipyrine, AAPyr) were synthesized using identical sacrificial support method (SSM). The catalysts were investigated for their electrochemical performance, and then integrated into an air-breathing cathode to be tested in "clean" environment and in a working microbial fuel cell (MFC). Their performances were also compared to activated carbon (AC) based cathode under similar conditions. Results showed that the addition of Mn, Fe, Co and Ni to AAPyr increased the performances compared to AC. Fe-AAPyr showed the highest open circuit potential (OCP) that was 0.307 ± 0.001 V (vs. Ag/AgCl) and the highest electrocatalytic activity at pH 7.5. On the contrary, AC had an OCP of 0.203 ± 0.002 V (vs. Ag/AgCl) and had the lowest electrochemical activity. In MFC, Fe-AAPyr also had the highest output of 251 ± 2.3 µWcm-2, followed by Co-AAPyr with 196 ± 1.5 µWcm-2, Ni-AAPyr with 171 ± 3.6 µWcm-2, Mn-AAPyr with 160 ± 2.8 µWcm-2 and AC 129 ± 4.2 µWcm-2. The best performing catalyst (Fe-AAPyr) was then tested in MFC with increasing solution conductivity from 12.4 mScm-1 to 63.1 mScm-1. A maximum power density of 482 ± 5 µWcm-2 was obtained with increasing solution conductivity, which is one of the highest values reported in the field.

Co-generation of hydrogen and power/current pulses from supercapacitive MFCs using novel HER iron-based catalysts.

In this work, four different supercapacitive microbial fuel cells (SC-MFCs) with carbon brush as the anode and an air-breathing cathode with Fe-Aminoantipyrine (Fe-AAPyr) as the catalyst have been investigated using galvanostatic discharges. The maximum power (Pmax) obtained was in the range from 1.7 mW to 1.9 mW for each SC-MFC. This in-series connection of four SC-MFCs almost quadrupled Pmax to an operating voltage of 3025 mV and a Pmax of 8.1 mW, one of the highest power outputs reported in the literature. An additional electrode (AdHER) connected to the anode of the first SC-MFC and placed in the fourth SC-MFC evolved hydrogen. The hydrogen evolution reaction (HER) taking place at the electrode was studied on Pt and two novel platinum group metal-free (PGM-free) catalysts: Fe-Aminoantipyrine (Fe-AAPyr) and Fe-Mebendazole (Fe-MBZ). The amount of H2 produced was estimated using the Faraday law as 0.86 mMd-1cm-2 (0.132 L day-1) for Pt, 0.83 mMd-1cm-2 (0.127 L day-1) for Fe-AAPyr and 0.8 mMd-1cm-2 (0.123 L day-1) for Fe-MBZ. Hydrogen evolution was also detected using gas chromatography. While HER was taking place, galvanostatic discharges were also performed showing simultaneous H2 production and pulsed power generation with no need of external power sources.

Computational and experimental evidence for a new TM-N3/C moiety family in non-PGM electrocatalysts.

In the present work, we demonstrate that the formation energies of previously unexplored single and double carbon vacancy based TM-N3/C defect moieties are favourable. This prediction suggests that these defect motifs, in particular DV-Fe-N3/C can form during high-temperature catalyst synthesis. Defect specific N 1s core-level shifts were computed from first-principles for the deconvolution of XPS observations.