(Invited) Conventional and Innovative Electrocatalysts for PEM Water Electrolysis
Authors: Millet Pierre 
Published: February 2020

A PEM (Polymer Electrolyte Membrane) water electrolysis cell is a concept of zero-gap cell that uses a thin (100-200 micrometers thick) film of proton-conducting polymer as solid electrolyte [1]. Elevated operating current densities (in the multi A.cm-2 range) can be reached efficiently. Most popular polymer electrolytes operating between 40 and 80°C are perfluoro-sulfonated materials (PFSA like Nafion® or Aquivion® [2]). Attempts to use phosphoric acid doped PBI (polybenzimidazole) electrolytes operating at higher (150-200°C) temperature have also been reported [3]. Platinum group metals (PGM) electrocatalysts are extensively used because only them can sustain the highly acidic environments [4]. Regarding electrocatalysis, conventional PEM water electrolysis cells are using platinum black (unsupported Pt particles) at the cathode for the hydrogen evolution reaction (HER) and unsupported iridium dioxide at the anode for the oxygen evolution reaction (OER). Typical PGM loadings vary from 0.5 – 1.0 mg.cm-2 at the cathode to 1-2 mg.cm-2 at the anode. Whereas the relative cost of platinum group metal (PGM) electrocatalysts in industrial PEM systems is limited to a few percent, costs constraints (especially in view of the development of electrolysers at the multi MW scale, for example for energy storage applications) are calling for cheaper solutions. A first option is to reduce PGM loading. A second option is to develop non-PGM electrocatalysts.
Regarding the reduction of PGMs, carbon-supported Pt nano-particles can be advantageously used to reduce Pt loadings from 1 down to 0.1 mg.cm-2 at the cathode. Reduced IrO2 loadings (down to 0.5 – 1.0 mg.cm-2) have been demonstrated [5] but addition of an inert metal [6] or alloying (to form ternary or quaternary mixed oxides [7]) are also viable alternatives. Regarding the development of non-PGM materials, most advances concern the cathode. Ideally, catalyst containing transition metals (Ni, Co, Fe) that maximize the exchange current density as a function of the M-H bond strength should be used [8]. They are significantly less expensive than PGMs and still adequately electro-active. Whereas in alkaline water electrolysis technology, Ni and Co bulk particles are commonly used, their implementation at the cathode of PEM water electrolyzers is not a straightforward task because they are rapidly corroded. New approaches based on molecular chemistry have been developed to bypass the problem. For example, metallic molecular complexes such as Co, Ni, Fe clathrochelates have been successfully implemented at the surface carbonaceous substrates (carbon powder and fibers) and implemented in PEM water electrolyzers [9]. They offer several advantages compared to nanoparticles: (i) non noble metals can be used as active centres; (ii) only limited amounts of metals are required; (iii) redox properties of active centres can be tuned to desirable values by selecting appropriate organic ligands; (iv) ligands can be used as chemical linkers for efficient surface functionalization.
The purpose of this communication is to review existing and innovative electrocatalysts for PEM water electrolysis applications and to compare their efficiency in relation with microstructural aspects.
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