Progress in non-precious metal oxide-based cathode for polymer electrolyte fuel cells

457

367

This study provides an analysis of the technological barriers for all-electric vehicles, either based on batteries (BEVs) or on H 2 -powered proton exchange membrane (PEM) fuel cells (FCEVs). After an initial comparison of the two technologies, we examine the likely limits for lithium ion batteries for BEV applications, and compare the projected cell-and system-level energy densities with those which could be expected from lithium-air and lithium-sulfur batteries. Subsequently, we will review the current development status of H 2 PEM fuel cells, with particular attention to their viability with regards to the required amount of platinum and the resulting cost and availability constraints. It is widely accepted that global warming is caused by CO 2 emissions and that they must be substantially reduced in order to prevent climate change. Since ≈23% of the world-wide CO 2 emissions are due to transportation, ≈75% of which are contributed by the road sector (numbers from 2012 1 ), a reduction of CO 2 emissions from vehicles is imperative to combat global warming. Towards this goal, many countries have passed legislature to lower passenger vehicle emissions over the long term like, e.g., the European Union mandate for 95 g CO2 /km fleet average emissions by 2020.2 The analysis by Eberle et al. 3 in Figure 1 suggests that this rather ambitious goal can only be met by means of extended range electric vehicles or all-electric vehicles in combination with the integration of renewable energy (e.g., wind and solar). Without increased integration of renewable energy sources and basing the calculations on the current European electricity generation mix, the only vehicle concept which could meet the 95 g CO2 /km target are pure battery electric vehicles (BEV100 in Fig. 1). However, for electricity produced entirely by renewable energy sources, the 95 g CO2 /km target could also be met by extended range electric vehicles with 40 miles all-electric range (E-REV40 in Fig. 1), if 50% of driving is powered by the battery (i.e., the average driving range would have to be below 80 miles), or by fuel cell electric vehicles (FECVs), with hydrogen produced by water electrolysis. While these propulsion concepts look promising, their contribution to CO 2 emission savings in the transportation sector would only be meaningful if their market penetration were substantial. In the absence of government regulations, the latter largely hinges on consumer acceptance, which in turn strongly depends on cost. In addition, in the case of BEVs, recent studies clearly showed that BEV driving range (closely followed by cost) are the predominant variables determining consumer acceptance. 4 In the following we will thus focus on the two vehicle types, which would be capable to meet and exceed the CO 2 emission targets of 95 g CO2 /km on the long-term, viz., pure BEVs and hydrogen powered FCEVs. For both vehicle types, but particularly for the latter, meaningful CO 2 emission reductions require the predominant use of renewable energy, which in turn necess...

Section: Hydrogen Fuel Cells -Materials Requirements and Durabilitymentioning

confidence: 99%

Review—Electromobility: Batteries or Fuel Cells?

Gröger

Gasteiger

Suchsland³

2015

457

367

“…In another study of our group, X-Ray photoelectron spectroscopy (XPS) analysis could not provide any evidence for a possible N-doping of ZrO 2 . 24 An assessment of the presence as well as of the possible influence of oxide defects on the ORR activity has been reported in the literature, 3,4,9,10,14-16,22,23 but the conclusions are still rather ambiguous. ORR mechanistic studies by RRDE.-The most active sample of all the catalysts (ZrOPc/KB 1000 PO) was additionally examined in alkaline and acid environment by RRDE using a Pt ring to quantify the extent of hydrogen peroxide formation.…”

mentioning

confidence: 99%

“…Without further distinction, their promises and drawbacks are that they offer reasonable activities, but that they generally lack long-term stability in the acidic environment of a proton exchange membrane fuel cell (PEMFC). 1 In an approach to improve the stability of non-noble-metal ORR electrocatalysts, valve-metal oxide based analogues that exhibit high stability against chemical dissolution in acidic environment have been explored [2][3][4][5][6][7] mostly by Ota and co-workers, who studied the ORR activity of sputtered oxides based on Nb, Ta, Ti and Zr, [8][9][10] with Zr based oxides exhibiting the highest ORR activity so far. Surprisingly, beyond their work, only very few studies on this class of catalysts have been published, [11][12][13] although a benefit of this class of materials was found in their robustness against methanol poisoning.…”

mentioning

confidence: 99%

ZrO₂Based Oxygen Reduction Catalysts for PEMFCs: Towards a Better Understanding

Mittermeier

Madkikar

Wang

et al. 2016

The oxygen reduction reaction (ORR) activity of ZrO 2 based, carbon-supported nanoparticles is not conclusively reported in literature. This study examines the dependence of the ORR activity on the used precursors as well as on the heat-treatment atmosphere and temperature. We further determine the ORR activation energy and the ORR mechanism. Various precursors containing Zr and/or N were employed in the synthesis, and the ORR activity was measured by rotating (ring) disk electrode (R(R)DE) voltammetry in both acidic and alkaline electrolyte as well as by measurements in a single-cell polymer electrolyte membrane fuel cell (PEMFC) configuration. We show that even the most active ZrO 2 based ORR catalysts exhibit an activity gap of ca. two orders of magnitude compared to the DOE target of 300 A/cm 3 for PGM-free ORR catalysts, thus requiring further development. Our RRDE analysis suggests a primarily 2-electron ORR mechanism in the case of the tested catalysts in acid, which in turn provides a consistent temperature dependence between RDE and PEMFC experiments, allowing also for a mechanistic (re-) interpretation of experimental results in the literature.

“…evaluate the influence of conductivity and defects on the surface of the oxynitride, as the addition of defects simultaneously modifies both conductivity and microstructure (and charge density 53 ). Another complication arises from the role of the δ Ta-O phase, in particular, how it may disrupt the connectivity or surface active sites for the ORR.…”

Section: Resultsmentioning

confidence: 99%

Influence of Aliovalent Substitutions on Oxygen Reduction on Tantalum Oxynitrides

Pasqualeti

Padgett

Kuo

et al. 2017

The development of active and stable oxygen reduction reaction (ORR) electrocatalysts using low-cost, abundant elements is of critical importance to the widespread adoption of proton-exchanged membrane fuel cells (PEMFCs). Tantalum-containing compounds, such as tantalum oxynitrides, can catalyze ORR in acid, with the degree of oxidation proposed to play a central role in controlling the ORR activity. Herein, we report the incorporation of aliovalent ions in tantalum oxynitrides (Ta-ON) and their effect on the ORR activity in acid. We synthesize Ta-ON by utilizing urea as the nitrogen source and use aliovalent (Ti 4+ ) and isovalent (Nb 5+ ) substitutions to evaluate the influence of the aliovalent substitution on the ORR activity. We found that an aliovalent Ti 4+ substitution increases the ORR activity by 3x, which we attribute to improved electrical conductivity and increased ORR active sites. In comparison, isovalent Nb 5+ substitution or carbon inclusion as oxycarbonitride did not positively affect the ORR activity. Our finding points to the important role of defects and conductivity in the Ta Active, acid-stable, and cost-effective cathodes for the oxygen reduction reaction (ORR) play an enabling role in the commercialization of proton-exchanged membrane fuel cells (PEMFCs). Platinum nanoparticles supported on high surface-area carbon supports (Pt/C) are currently the most active ORR electrocatalyst; 1 however, there are concerns with the scalability and stability 2 of Pt. 3,4 To identify alternatives, many groups have examined non-Pt ORR electrocatalysts made of inorganic, organic, and hybrid organic-inorganic materials. Examples include Fe-N-C, 5 and polyaniline (PANI)-Fe/Co-C, 6,7 some of which have ORR kinetics almost already comparable to Pt/C. 8,9 However, these electrocatalysts are not yet stable under longterm operations.3 While many efforts have been made to improve the stability 10 and identify the active sites, 11 long-term stability of these carbon-based electrocatalysts remains a challenge.Valve metal compounds such as tantalum-based compounds are among a few classes of non-precious-metal materials that are stable in acid.12 Their exceptional stability has spurred investigations of how they can be used as electrocatalysts. Examples include tantalum oxynitride (TaON) and tantalum nitride (Ta 3 N 5 ), which have also been investigated for solar-driven water splitting. [13][14][15][16] However, most valvemetal oxides are not active toward the ORR. As a result, significant efforts have been devoted to activating these materials for the ORR. Ota et al. have shown that controlling the degree of oxidation can improve the ORR in tantalum oxynitrides.17,18 By surveying TaC x N y , 17Ta-CNO 17,19 and TaO x N y /Ti, 20 they have proposed that an ideal interaction between the oxygen molecule and the surface tantalum must be optimized via this degree of oxidation to improve the ORR activity. 21The same group has also demonstrated correlations between the ORR and oxygen vacancies, which they proposed as the ...