Electrodeposition and pyrolysis of Mn/polypyrrole nanocomposites: a study based on soft X-ray absorption, fluorescence and photoelectron microspectroscopies

Abstract

Electrodeposition of manganese/polypyrrole (Mn/PPy) nanocomposites has been recently shown to be a technologically-relevant synthesis method for the fabrication of Oxygen Reduction Reaction (ORR) electrocatalysis. In this study we have grown such composites with a potentiostatic anodic/cathodic pulse-plating procedure and characterised them by a multi-technique approach, combining a suite of in situ and ex situ spectroscopic methods with electrochemical measurements.We have thus achieved a sound degree of molecular-level understanding of the hybrid coelectrodeposition process consisting in the electropolymerisation of polypyrrole with incorporation of Mn. By in situ Raman we followed the formation of MnO x and polymer by monitoring the buildup and development of the relevant vibrational bands. The compositional and chemical-state distribution of the as-deposited material has been investigated ex situ by soft X-ray fluorescence (XRF) mapping and micro-absorption spectroscopy (micro-XAS). XRF shows that the spatial distribution of Mn is consistent in a rather wide range of current densities (c.d.), while micro-XAS reveals a mixture of Mn valencies, with higher oxidation states prevailing at higher c.d.s. The pyrolysis of the electrodeposits, desirable for obtaining more durable and active catalysts, has been followed in situ by photoelectron microspectroscopy, allowing to assess the evolution of: (i) the electrodeposit morphology, resulting in a uniform distribution of nanoparticles; (ii) the chemical state of manganese, changing from a mixture of valences to a final state consisting of Mn(III) andMn(IV) oxides and (iii) the bonding nature of nitrogen, from initially N-pyrrolic to a combination of pyridinic and Mn-N/graphitic. 21 form. As far as the electrocatalytic performance is concerned, the onset and halfwave potentials for the pyrolysed material are shifted with respect to the values measured for the corresponding aselectrodeposited system, denoting considerably improved activity. In terms of the same electrocatalytic figures of merit, our electrocatalyst outperforms MnO x supported on a range of carbons, such as: C-powders, 127 vulcan, 31,128 and nanocarbon, 129 while mesoporous C-N supports 130 and appropriately nanostructured MnO 2 (nanospheres and nanowires, 131 microsphere/nanosheet core−corona hierarchical architectures, nanorods, and nanotubes 132 ) yield better ORR capabilities.

Accurate Assessment of the Oxygen Reduction Electrocatalytic Activity of Mn/Polypyrrole Nanocomposites Based on Rotating Disk Electrode Measurements, Complemented with Multitechnique Structural Characterizations

“…The concomitant presence of the four features at 315, 368, 499, and 656 cm −1 is usually ascribed to Mn 3 O 4 [71, 72], but some of these bands can also appear in MnO 2 [71, 73], Mn 2 O 3 [72], or MnO [72, 74]. Even though the positions of Raman bands alone are not enough to support a conclusive speciation of Mn oxides not only owing to the intrinsic complexity of these vibrational spectra [68] but also owing to inconsistent literature in the field [75, 76], our Raman results, in combination with previous micro-XAS data [51], definitively prove that the electrodeposition process yields a mixture of Mn(II, III, IV) oxides.…”

“…By comparing Figures 6(a) and 6(b) one can straightforwardly conclude that the top regions of the particles are not conductive while current preferentially flows at the border of the particles. This finding suggests that the grain can be composed of poorly conductive manganese oxides, with PPy in the reduced state, coherently with XRF mapping and micro-XAS analysis [51] as well as Raman Spectroscopy (see next section). Moreover, our results are in keeping with a previous study, showing that the top of PPy globules is preferentially reduced during electrodeposition [60].…”

“…The morphological, compositional, and chemical-state distribution of the as-deposited Mn/PPy material, using the same electrodeposition procedure employed for this investigation, has been studied in detail by soft X-ray fluorescence (XRF) mapping and microabsorption spectroscopy (micro-XAS) in a previous paper of ours [51]. This analysis disclosed a rather homogeneous Mn spatial distribution that is consistent in a rather wide range of current densities, while micro-XAS has revealed that the potentiostatic pulsed codeposition process—alternating anodic and cathodic polarization intervals—leads to a PPy deposit containing manganese (II, III, IV) oxyhydroxides and traces of Mn(0).…”

“…The Mn/PPy catalysts were synthesized in deaerated electrodeposition baths. Electrodeposition was carried out with the pulsed potential procedure described in previous works [47, 48, 50, 51] optimized in view of growing a composite consisting of two constituents that are formed anodically (PPy and Mn(III, IV) compounds) and cathodically (Mn(0) and/or precipitated Mn(II) containing species). The initial step (0.971 V/RHE) does not lead to faradaic reactions, but it is required to relax the compositional double layer.…”

Parameter estimation for a morphochemical reaction-diffusion model of electrochemical pattern formation

“…The synthetic observation is generated from the model field at time T=20, with unbiased, random, Gaussian noise added. In practice we expect the standard deviation of the noise on the experimental data to be of the order of a few percent [58], [59]. Here, for the synthetic normalized data, we present results where the standard deviation of the noise is taken to be 0.1, representing 10% of the maximum value of the field.…”