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 ...

Mesoporous nitrogen-doped carbon derived from the ionic liquid N-butyl-3-methylpyridinium dicyanamide is a highly active, cheap, and selective metal-free catalyst for the electrochemical synthesis of hydrogen peroxide that has the potential for use in a safe, sustainable, and cheap flow-reactor-based method for H(2)O(2) production.

Pt-based core–shell nanoparticles have emerged as a promising generation of highly active electrocatalysts to accelerate the sluggish kinetics of oxygen reduction reaction (ORR) in fuel cell systems. Their electronic and structural properties can be easily tailored by modifying the Pt shell thickness, core composition, diameter, and shape; this results in significant improvements of activity and durability over state-of-the-art pure Pt catalysts. Prompted by the relevance of efficient and robust ORR catalysts for electrochemical energy conversion, this Perspective reviews several concepts and selected recent developments in the exploration of the structure and composition of core–shell nanoparticles. Addressing current achievements and challenges in the preparation as well as microscopic and spectroscopic characterization of core–shell nanocatalysts, a concise account of our understanding is provided on how the surface and subsurface structure of multimetallic core–shell nanoparticles affect their reactivity. Finally, perspectives for the large-scale implementation of core–shell catalysts in polymer exchange membrane fuel cells are discussed.

Dealloying of Pt bimetallic nanoparticles is a promising synthesis method to prepare highly active electrocatalysts for oxygen reduction reaction (ORR) in alkaline and acidic PEM fuel cells. We present here a structural, compositional and electrochemical characterization linked with ORR activity for carbon supported PtCu 3 , PtCu, and Pt 3 Cu alloy nanoparticles in different electrolytes and pH values. The effects of electrolyte and pH are systematically examined on the ECSA and Pt mass based activity (j mass ) for various Pt-Cu alloys. We observed the formation of Cu oxide species and redissolution/redeposition of Cu species during the voltage cycling up to 1.0 V/RHE in 0.1 M KOH. In contrast, the voltage cycling in 0.1 M HClO 4 immediately causes the dissolution of Cu and results in Pt-enriched particle surface. We have correlated the ECSA and mass activity with the as-synthesized composition in dependence on both electrolytes. In summary, after voltage cycling in 0.1 M HClO 4 the values of j mass increase according: Pt 3 Cu < PtCu < PtCu 3 . However, after voltage cycling in 0.1 M KOH the values of j mass increase in the following trend: PtCu 3 < PtCu < Pt 3 Cu. Only after activation process, PtCu 3 core-shell catalyst shows significantly enhanced ORR activity in 0.1 M KOH compared to pure Pt.

Understanding and improving durability of fuel cell catalysts are currently one of the major goals in fuel cell research. Here, we present a comparative stability study of multi walled carbon nanotube (MWCNT) and conventional carbon supported platinum nanoparticle electrocatalysts for the oxygen reduction reaction (ORR). The aim of this study was to obtain insight into the mechanisms controlling degradation, in particular the role of nanoparticle coarsening and support corrosion effects. A MWCNT-supported 20 wt.% Pt catalyst and a Vulcan XC 72R-supported 20 wt.% Pt catalyst with a BET surface area of around 150 m(2) g(-1) and with a comparable Pt mean particle size were subjected to electrode potential cycling in a "lifetime" stability regime (voltage cycles between 0.5 to 1.0 V vs. RHE) and a "start-up" stability regime (cycles between 0.5 to 1.5 V vs. RHE). Before, during and after potential cycling, the ORR activity and structural/morphological (XRD, TEM) characteristics were recorded and analyzed. Our results did not indicate any activity benefit of MWCNT support for the kinetic rate of ORR. In the "lifetime" regime, the MWCNT supported Pt catalyst showed clearly smaller electrochemically active surface area (ECSA) and mass activity losses compared to the Vulcan XC 72R supported Pt catalyst. In the "start-up" regime, Pt on MWCNT exhibited a reduced relative ECSA loss compared to Pt on Vulcan XC 72R. We directly imaged the trace of a migrating platinum particle inside a MWCNT suggesting enhanced adhesion between Pt atoms and the graphene tube walls. Our data suggests that the ECSA loss differences between the two catalysts are not controlled by particle growth. We rather conclude that over the time scale of our stability tests (10,000 potential cycles and beyond), the macroscopic ECSA loss is primarily controlled by carbon corrosion associated with Pt particle detachment and loss of electrical contact.

Pt-Co alloy nanoparticles have emerged as one of the most promising electrocatalysts for the oxygen reduction reaction (ORR) in hydrogen fuel cells. Our study presents a comprehensive structural, compositional and electrochemical characterization linked with ORR activity for carbon supported PtCo 3 , PtCo, and Pt 3 Co alloy nanoparticle catalysts in 0.1 M HClO 4 and 0.1 M KOH. Surface-sensitive cyclic voltammetry was used to investigate the changes of composition of outermost atomic layers of Pt-Co alloys. Our electrochemical results in alkaline media clearly show the stability and voltage-induced accumulation of Co on the particle surface, whereas in 0.1 M HClO 4 the voltage cycling initiates the rapid dissolution of Co to form a Pt-enriched surface surrounding by alloy core. We correlated the ECSA and ORR activity with the as-synthesized chemical composition of Pt-Co alloys. In results, after electrochemical treatment in 0.1 M HClO 4 the Pt mass based activities (j mass ) increase according: Pt(HT) < PtCo < Pt 3 Co < PtCo 3 at comparable particle size. Unlike to acid, after voltage cycling in 0.1 M KOH j mass increase according: PtCo 3 < Pt(HT) < PtCo < Pt 3 Co. However, in 0.1 M KOH activated PtCo 3 core-shell catalyst shows 4-5 fold higher mass activity compared to pure Pt and Pt(HT).

A key challenge in today’s fuel cell research is the understanding and maintaining the durability of the structure and performance of initially highly active Pt fuel cell electrocatalysts, such as dealloyed Pt or Pt monolayer catalysts. Here, we present a comparative long‐term stability and activity study of supported dealloyed PtCu3 and PtCo3 nanoparticle fuel cell catalysts for the oxygen reduction reaction (ORR) and benchmark them to a commercial Pt catalyst. PtCu3 and PtCo3 were subjected to two distinctly different voltage cycling tests: the “lifetime” regime [10 000 cycles, 0.5–1.0 V vs. RHE (reversible hydrogen electrode), 50 mV s−1] and the corrosive “start‐up” regime (2000 cycles, 0.5–1.5 V vs. RHE, 50 mV s−1). Our results highlight significant activity and stability benefits of dealloyed PtCu3 and PtCo3 for the ORR compared with those of pure Pt. In particular, after testing in the “lifetime” regime, the Pt‐surface‐area‐based activity of the Pt alloy catalysts is still two times higher than that of pure Pt. From our electrochemical, morphological, and compositional results, we provide a general picture of the temporal sequence of dominant degradation mechanisms of a Pt alloy catalyst during its life cycle.