The effect of pH on the hydrogen oxidation and evolution reaction (HOR/HER) rates is addressed for the first time for the three most active monometallic surfaces: Pt, Ir, and Pd carbon-supported catalysts. Kinetic data were obtained for a proton exchange membrane fuel cell (PEMFC; pH z 0) using the H 2 -pump mode and with a rotating disk electrode (RDE) in 0.1 M NaOH. Our findings point toward: (i) a similar z100-fold activity decrease on all these surfaces when going from low to high pH; (ii) a reaction rate controlled by the Volmer step on Pt/C; and (iii) the H-binding energy being the unique and sole descriptor for the HOR/HER in alkaline electrolytes. Based on a detailed discussion of our data, we propose a new mechanism for the HOR/HER on Pt-metals in alkaline electrolytes.Fuel cells and electrolyzers are important for renewable energy conversion and storage. They are currently based on protonexchange membranes (PEMs) operating at low pH (pH z 0), which offer high power densities, but require large amounts of platinum for the oxygen reduction reaction (ORR) in fuel cells 1 and of Ir for the oxygen evolution reaction (OER) in electrolyzers. 2 For the hydrogen oxidation/evolution reaction (HOR/HER) only very small amounts of Pt are required due to its extremely high activity for the HOR/HER. 3 The H 2 anode performance in PEMFCs suggested exchange current densities (i 0 ) in the order of 10 2 mA cm Pt À2 , 4 which was conrmed by mass-transport-free fuel cell measurements 3,5 and microelectrode data. 6 Until then, 100-fold lower i 0 -values for Pt in acid were reported erroneously, generally based on rotating disk electrode (RDE) measurements 7,8 from which, however, the kinetics of reactions with i 0 -values much above the diffusion limited RDE current density (z2-3 mA cm disk À2 ) cannot be quantied. 9In an alkaline electrolyte, non-noble metal catalysts are very active for the ORR 10,11 and for the OER, 12,13 so that in conjunction with alkaline membranes (OH À -exchange membranes 14,15 ) a replacement of the noble-metal intensive PEM technology by alkaline membrane technology seems promising. Unfortunately, for yet unclear reasons, the HOR/ HER kinetics on Pt are much slower in alkaline than in acid electrolytes, and large amounts of Pt are needed to catalyze the HOR/HER in an alkaline environment. 9 Therefore, it is critical to develop alternative HOR/HER catalysts for alkaline electrolytes and -to guide this search -to elucidate the reasons for the poor HOR/HER activity of Pt in alkaline electrolytes.Traditionally, the overall reactions have been written either with protons in acid or with hydroxide ions in alkaline media: 16 in acid:in base:The future of electromobility relies on the development of cost effective and durable energy conversion systems such as fuel cells and electrolyzers. These devices, based on proton-exchange membranes (PEMs), operating at pH 0, offer high power densities, but require large amounts of noble metal for the oxygen reduction reaction (ORR) in fuel cells and the oxygen evo...

The hydrogen oxidation and evolution reaction (HOR/HER) behavior of carbon supported metal (Pt, Ir, Rh, Pd) nanoparticle electrocatalysts is studied using the H 2 pump approach, in a proton exchange membrane fuel cell (PEMFC) setup. After describing the best method for normalizing the net faradaic currents to the active surface area of the electrodes, we measure the HOR/HER kinetic parameters (exchange current densities and transfer coefficients) in a temperature range from 313 K to 353 K and calculate the activation energy for the HOR/HER process. We compare the measured kinetic parameters with those extracted from different mass-transport limitation free setups in literature, to evaluate the hydrogen electrocatalysis on these most active surfaces. The HOR/HER activity scales with the following: Pt > Ir Rh > Pd. The anodic and cathodic transfer coefficients are similar for all metals (ca. 0.5), leading to Tafel In the current view of energy conversion based on the use of fuel cells and electrolyzers, the hydrogen electrocatalysis plays a central role. H 2 is used as a fuel in proton exchange membrane fuel cells (PEMFCs), where it is electrochemically oxidized at the anode electrode according to:In PEMFC anode electrode, only small amounts of Pt (ca. 0.05 mg Pt /cm 2 geo ) are required to catalyze the hydrogen oxidation, without contributing to any efficiency loss of the overall fuel cell performance.1 The same would hold true for the hydrogen evolution reaction -HER -at the cathode side of water electrolyzer systems. Moreover, except when considering contamination issues e.g. due to the presence of CO in reformate hydrogen, or stability issues e.g. due to hydrogen starvation events mode, a replacement of the current carbon supported platinum (Pt/C) based electrode technology is not contemplated in PEMFC. 2 The drawback of both PEM-based fuel cells and electrolyzer systems arise from the large amounts of noble metal (ca. ≈0.4 mg metal /cm 2 geo ) required to catalyze at acceptable rates the sluggish oxygen reduction reaction (ORR) in fuel cell cathodes, and the oxygen evolution reaction (OER) in electrolyzer anodes. [3][4][5] Contrary to the acidic PEM-based technologies, anion exchange membrane (AEM) based devices, [6][7][8] which are operating at high pH, offer the use of cost-effective non-noble metal electrodes to catalyze the ORR 9,10 and OER 11-13 at almost similar rates than on noble metal electrodes in acidic electrolytes. As a result, a replacement of PEMbased devices by AEM ones will be advantageous 14 if and only if AEM conductivities will be further increased to the level of PEM, 15,16 and their sensitivity to CO 2 significantly reduced. 17 However, getting rid of the noble metal contents in AEM-device electrodes would be feasible only in the case of similar hydrogen oxidation and evolution reaction (HOR/HER) rates in AEMs versus PEM-based conversion devices. Unfortunately, recent studies have shown that the HOR/HER rates of noble metal electrodes in the alkaline environment are much slower than in ...

The oxygen and water transport through various microporous layers (MPLs) is investigated by fuel cell tests in a 5 cm 2 active area cell under differential-flow conditions, analyzing polarization curves, the associated high-frequency resistance, and the oxygen transport resistance extracted from limiting current density measurements. In this study, MPLs with two different carbon blacks are prepared and compared to a commercial material, all coated on the same GDL-substrate (Freudenberg); furthermore, perforated MPLs with large pores produced by a thermally decomposable polymeric pore former with a particle diameter of ≈30 μm are examined. The materials are characterized by mercury porosimetry, nitrogen adsorption and scanning electron microscopy. While at dry conditions (T cell = 80 • C, RH = 70%, p abs = 170 kPa) the performance of all materials is similar, at conditions of high water saturation (T cell = 50 • C, RH = 120%, p abs = 300 kPa), MPLs with larger pores or perforations exhibit a performance improvement due to a ≈30% reduction in oxygen transport resistance. The results indicate that liquid water is transported exclusively through these large pores, while the oxygen transport occurs in the small pores defined by the carbon black structure.

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