The interest for alkaline fuel cells (AFC), linked to the possibility to use non-carbon fuels in substitute to hydrogen and non-Pt electrocatalysts without performance losses, has been renewed in the last decade, owing to the development of performant alkaline electrolyte membranes. The team of interfacial electrochemistry and processes addresses the topic of AFC by focusing on two aspects: (1) materials and mechanisms of electrooxidation of non-carbon fuels of the boron and nitrogen families (for direct AFC) and (2) non-Pt electrocatalysts for the oxygen reduction reaction (ORR).
(1) Materials and mechanisms of electrooxidation of non-carbon fuels of the boron and nitrogen families (NaBH4, N2H4, N2H4 BH3)
We study the mechanisms of the BH4- oxidation reaction (BOR) on base metals (Pt, Au, Ag, Pd) using a combination of physicochemical (FTIR spectroscopy (Figure 1))1-3, differential electrochemical mass spectrometry: DEMS (Figure 2) 4, 5) and electrochemical techniques (rotating ring-disk electrode (Figure 3) 6, electrochemical impedance spectroscopy (Figure 4) 7-9) at planar and tailored nanostructured electrodes, to understand the elementary steps of the reaction (Figure 5). Doing so, we notably understood that Au is not faradaic efficient to perform the BOR, while Pt is (when BOR is indeed proceeding), despite its large activity for the heterogeneous hydrolysis of the fuel at open-circuit conditions 4, 5. We also demonstrated that the reaction depends on the structuration of the electrocatalysts at stake 10, 11, via mass-transport 10, 12 and poisoning effects 10.
This strategy is presently deployed for other fuels of the boron and nitrogen families (NH3BH3 13, N2H4, N2H4BH3), in collaboration with the Universidade de São Paulo (Fabio H. B. Lima, Edson A. Ticianelli, Brazil), the Université de Montpellier 2 (Umit Demirci, France) and the University of New Mexico (Plamen Atanassov, USA). It shall enable to tailor performant electrocatalysts for practical direct AFC (DAFC).
Figure 1. In situ infrared spectra recorded on a platinum electrode in 1 M NaOH + 1 M NaBH4 solutions at room temperature; Rref = 20 mV vs. RHE. The IR spectra are presented each 100 mV. Reproduced from 3 with permission from the American Chemical Society.
Figure 2. Cyclic voltammogram (black), H2 detection in DEMS (red) and corresponding number of electron exchanged per BH4- anion (black squares) in the course of the BOR at sputtered Pt electrode. Reproduced from 5 with permission from Elsevier.
Figure 3. RRDE voltammograms of 10 mM NaBH4 in 1 M NaOH at 25 mV s−1 and 2500 rpm (positive-going scan). The disk is composed of the different Pt surfaces; the Au-ring is held at +0.2 V vs. RHE to detect BH3OH- intermediates. Reproduced from reference 6 with permission from Elsevier.
Figure 4. (i vs. E) plot and six Nyquist diagrams of the gold electrode impedance obtained at various potentials (a, b, c, d in the region of increasing currents, e, f in the region of the gold deactivation) at 400 rpm. Graphs parametered in Hz. Reproduced from 7 with permission from Elsevier.
Figure 5. Tentative BOR mechanism proposed following the results of the physicochemical and electrochemical techniques detailed above.
(2) Nanostructured manganese oxides for the alkaline ORR In complement to the study of the anode electrocatalysts, DAFC require the development of fuel-tolerant electrocatalysts. Nanostructured manganese oxides supported on carbon black, developed in coll. with the Czech Academy of Science (Jiri Vondràk, Czech Republic) 14-16 are materials of choice for that purpose, should it be for borohydride-tolerant ORR electrocatalysts 17, 18 or ethanol-tolerant ones (Figure 6) 19.
Figure 6. Cell potential and Power density versus current densities for alkaline single cells operating at different temperatures for a) Pt/C and b) NiMnOx/C cathode materials. Reproduced from 19 with permission from Springer.
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