The Paris agreement, signed December 12th, 2015, prompts the European Union and 194 states to lower their greenhouse gas emissions, increase energy efficiency and the share of renewables in their energy mix. Unfortunately, renewable energy is, by essence, intermittent and thus hardly meets the very high variability of demand for power consumption.
Batteries, electrolysers and fuel cells play an essential role in this new energy landscape. Metal-air batteries are investigated as potential alternatives to the widest deployed Li-batteries, which are limited in energy density (< 200 Wh kg-1) 1. Complementary to batteries are water electrolysers and fuel cells. Water electrolysers split water molecules into molecular hydrogen (H2) and oxygen (O2), and proton-exchange membrane fuel cells (PEMFC) recombine them efficiently to produce electricity. The two technologies are investigated in the Electrocatalysis group of EIP, independently on the chemical nature of the electrolyte (acid or alkaline). The focus is also on model Pd nanofilms for H2 storage.
H2 can be produced from water electrolysis in alkaline, neutral or acid electrolytes, and each system has its own advantages and drawbacks. At high pH, the Nernst potential of the oxygen evolution reaction (OER) is considerably lowered, which enables to use non-noble metals (KOH electrolysers use nickel and stainless steel electrodes). However, alkaline electrolytes are highly corrosive, easily poisoned by carbon dioxide, and feature low ion conductivity. At low pH, water electrolysers operating with an acid solid polymer electrolyte – also referred to as polymer electrolyte membrane (PEM) water electrolysers (PEMWE) – feature several advantages compared to alkaline electrolyzers: (i) larger energy efficiency at high current density, (ii) versatility (PEMWE may be adapted to different working regimes), (iii) faster start-up, (iv) use of similar polymer electrolyte technology than in PEMFC, for which considerable advances have been obtained during the last decade.
PEMFC, thanks to their potentially unlimited energy density, are progressively implemented in new portable, automotive or remote stationary devices. However, several hurdles remain before widespread commercialization of PEMFCs: (i) improving the rate of the cathode reaction (the oxygen reduction reaction, ORR) kinetics, (ii) decreasing the cost of the catalytic layers, and (iii) improving the durability of the cathode material. Despite the extensive efforts in research and development made during the last 15 years, Pt-based nanoparticles remain the only – but unstable – electrocatalyst able to accelerate efficiently the rate of the ORR in PEMFC operating conditions. Improving the ORR activity (x 2-3) and decreasing the catalyst cost is currently best achieved using homogeneous Pt-M/C nanoalloys (M being a transition metal, M = Co, Ni, Cu or Fe) or core-shell nanoparticles composed of a Pt-enriched shell and a metallic or alloyed core.
In the HOLLOW project, we investigate a novel class of nanomaterials that are able fulfil cost, performance and stability requirements of PEMFC cathodes. Hollow Pt-M/C nanoparticles are synthesized in water at room temperature by a one-pot simple method that uses the nanoscale Kirkendall effect and the galvanic replacement of a non-noble metal element M (M = Co, Ni, Cu) atoms by Pt atoms. The best porous hollow PtNi/C nanocatalyst achieved 6-fold and 9-fold enhancement in mass and specific activity for the ORR, respectively over standard solid Pt/C nanocrystallites of the same size. The catalytic enhancement is 4-fold and 3-fold in mass and specific activity, respectively over solid PtNi/C nanocrystallites with similar chemical composition, Pt lattice contraction and crystallite size. To know more about this.
In the SURICAT project, we develop viable alternatives to conventional carbon blacks used as catalysts supports. Metal-oxide substrates must fulfill three criteria: be electron-conducting, resist corrosion and possess an opened porous structure compatible with facile ionomer insertion and efficient mass-transport properties. To know more about this.
In the frame of the H2E project (and others before), we also studied the degradation mechanisms of PEMFC materials. The work conducted over the past 7 years has shown that Pt/C or Pt-Co/C nanoparticles used in the cathode of a PEMFC are not stable. Corrosion of the carbon support leads to the detachment of the metal nanoparticles (electrical disconnection) and increases mass-transport losses (it ruins the structure of the catalytic layer). Pt and Pt3Co nanoparticles are also corroded at the cathode and form Ptz+ (z = 2, 4) and Co2+ cations, which redistribute themselves throughout the whole membrane electrode assembly (MEA) and impact both the performance of the catalytic layer (surface area loss) and the proton-exchange membrane fuel cell (change of mechanical/electrochemical properties). To know more about this.
We also evidenced on real membrane electrode assemblies provided by Axane/Air Liquide that the PEMFC cathode materials age heterogeneously during on site operation. To know more about this.
Alkaline fuel cells (AFC) enable to use non-carbon fuels in substitute to hydrogen and non-Pt electrocatalysts without performance losses. 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). To know more about this.
The reactivity in heterogeneous catalysis and especially in electrocatalysis is very sensitive to the surface structure. To bridge surface reactivity and structure, it is mandatory to work on well-defined surfaces. Single crystal electrodes, for which a single orientation can be put in contact with the electrolyte, represent a very interesting model catalyst for the study of the crystallographic orientation on catalytic activity. Our team has extensive experience in this field. In recent years, we have developed a unique approach to link the macroscopic properties to the structure at the atomic level of the electrodes. We have designed and built cells that enable electrochemical measurements coupled with the use of synchrotron radiation for structural characterization (in situ experiments). In particular, we have successfully employed a thin electrolyte layer cell adapted to surface X-ray diffraction measurements. We employed it on the French-CRG beamline D2AM at ESRF (Grenoble) for study of the electro-insertion of the hydrogen in Pd nanofilms.
We are also investigating the basic mechanisms of metal nanofilms growth. Although electrochemical metal deposition is wildely used in many industrial processes, few fundamental studies are conducted to elucidate the elementary steps and most of the work focus on homo-epitaxial growth. We investigate hetero-metallic growth (Cu/Pt, Pd/Au, Pd/Pt, ...), to understand the initial steps both for underpotential deposition (UPD) and for the next few layers that are still highly correlated with the substrate.
Finally, we are using Pd nanofilms to try to decorrelate the different aspects that are influencing reaction kinetics. For instance, NaBH4 electro-oxidation exhibits an original activity on Pd materials, compared to Au and Pt. By using Pd nanofilms of several thicknesses on Pt(111), we are able to examine the influence of hydride formation, electronic and geometric effects on the reactivity. The electro-oxidation of light organic molecules (CH3OH, CH2O…) is performed on similar surfaces.
Metal-air batteries also are electrochemical generators of large energy density. We recently started to work on Li-air batteries, in industrial collaborations. With EDF, we tackled the issues of the oxygen electrode in aqueous Li-air batteries. With Hutchinson, we are presently working on the ORR/OER mechanisms in non-aqueous electrolytes, a topic in which we initiated a collaboration with Northeastern University (Sanjeev Mukerjee, USA).