Electrochemical Active Surface Area Research Articles

Insights of the efficient hydrogen evolution reaction performance in bimetallic Au4Cu2 nanoclusters.

The design of efficient electrocatalysts for improving hydrogen evolution reaction (HER) performance using atomically precise metal nanoclusters (NCs) is an emerging area of research. Here, we have studied the HER electrocatalytic performance of monometallic Cu6 and Au6 nanoclusters and bimetallic Au4Cu2 nanoclusters. A bimetallic Au4Cu2/MoS2 composite exhibits excellent HER catalytic activity with an overpotential (η10) of 155 mV vs. reversible hydrogen electrode observed at 10 mA cm-2 current density. The improved HER performance in Au4Cu2 is due to the increased electrochemically active surface area (ECSA), and Au4Cu2 NCs exhibits better stability than Cu6 and Au6 systems and bare MoS2. This augmentation offers a greater number of active sites for the favorable adsorption of reaction intermediates. Furthermore, by employing X-ray photoelectron spectroscopy (XPS) and Raman analysis, the kinetics of HER in the Au4Cu2/MoS2 composite were elucidated, attributing the favorable performance to better electronic interactions occurring at the interface between Au4Cu2 NCs and the MoS2 substrate. Theoretical analysis reveals that the inherent catalytic enhancement in Au4Cu2/MoS2 is due to favorable H atom adsorption over it and the smallest ΔGH* value. The downshift in the d-band of the Au4Cu2/MoS2 composite influences the binding energy of intermediate catalytic species. This new catalyst sheds light on the structure-property relationship for improving electrocatalytic performance at the atomic level.

Estimation of the electrochemical active site density of a metal-free carbon-based catalyst using phosphomolybdate (PMo12) as an adsorbate.

A method to estimate the electrochemical active site density (SD) of carbon (C) and nitrogen-doped carbon (N/C-900) using phosphomolybdate (PMo12) as a probe molecule is proposed. The complete coverage of the active sites by the probe molecules is established irrespective of the adsorbate concentration (1, 5, or 10 mM), potential cycling (1 or 10 cycles) and cleaning time (2, 5, or 10 min). A conversion factor derived from a smooth and polished glassy carbon disk of known geometrical area is used to estimate the electrochemical active surface area (ECSA) of the carbon catalyst from the SD. The relatively higher SD values estimated from DC voltammetry than from large-amplitude Fourier-transform alternating-current voltammetry (FTacV) is indicative of the contribution of capacitive charge in the former. Adsorbed probe molecules (PMo12) can readily be desorbed from the catalyst surface by cycling the electrode to lower potentials. The active site density of N/C-900 (∼0.36 × 1019 sites g-1) is higher than that of C (∼0.17 × 1019 sites g-1).

Investigation on the performance of proton exchange membrane water electrolyzer coupled with a catalyst layer pore network model

Improvement of performance and cost reduction are urgent issues for proton exchange membrane water electrolyzers (PEMWEs). This study explored ways to improve electrolyzer performance from the perspective of the catalyst layer (CL) microstructures. The pore network model of the CL was established, and the obtained structural, mass transfer and electrochemical parameters were coupled with the macroscale performance model of PEMWE to get the multi-scale performance model. The mass transfer, heat transfer, and electrochemical processes inside the electrolyzer were considered to improve the model's accuracy. The maximum error between the simulated results by this multi-scale performance model and the experimental data was verified to be only 3.1%. The verified model was then used to predict the electrolyzer performance with various cathode/anode catalyst layer (CCL/ACL) microstructures. The predicted results showed that the electrochemical activity surface area affects the performance of PEMWE to a much greater extent than the conductivity. The performance of PEMWE gradually deteriorated as the catalyst particle radius or pore diameter of the CCL/ACL increased by the same degree, and the catalyst particle radius had a relatively more significant impact on the performance of the electrolyzer. Overall, the variation of ACL structure had a more substantial effect on the electrolyzer performance than that of CCL.

Enhanced Electrocatalytic Nitrate Reduction to Ammonia Using Functionalized Multi-Walled Carbon Nanotube-Supported Cobalt Catalyst.

Ammonia (NH3) is vital in modern agriculture and industry as a potential energy carrier. The electrocatalytic reduction of nitrate (NO3-) to ammonia under ambient conditions offers a sustainable alternative to the energy-intensive Haber-Bosch process. However, achieving high selectivity in this conversion poses significant challenges due to the multi-step electron and proton transfer processes and the low proton adsorption capacity of transition metal electrocatalysts. Herein, we introduce a novel approach by employing functionalized multi-walled carbon nanotubes (MWCNTs) as carriers for active cobalt catalysts. The exceptional conductivity of MWCNTs significantly reduces charge transfer resistance. Their unique hollow structure increases the electrochemical active surface area of the electrocatalyst. Additionally, the one-dimensional hollow tube structure and graphite-like layers within MWCNTs enhance adsorption properties, thus mitigating the diffusion of intermediate and stabilizing active cobalt species during nitrate reduction reaction (NitRR). Using the MWCNT-supported cobalt catalyst, we achieved a notable NH3 yield rate of 4.03 mg h-1 cm-2 and a high Faradaic efficiency of 84.72% in 0.1 M KOH with 0.1 M NO3-. This study demonstrates the potential of MWCNTs as advanced carriers in constructing electrocatalysts for efficient nitrate reduction.

How microstructures, oxide layers, and charge transfer reactions influence double layer capacitances. Part 1: impedance spectroscopy and cyclic voltammetry to estimate electrochemically active surface areas (ECSAs).

Varying the electrode potential rearranges the charges in the double layer (DL) of an electrochemical interface by a resistive-capacitive current response. The capacitances of such charge relocations are frequently used in the research community to estimate electrochemical active surface areas (ECSAs), yet the reliability of this methodology is insufficiently examined. Here, the relation of capacitances and ECSAs is critically assessed with electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) data on polished (Au, Ti, Ru, Pt, Ni, glassy carbon, graphite plate) and porous (carbon fleeces) electrodes. By investigating this variety of electrodes, the frequency-dependencies observed in the measured capacitances are shown to arise from the inherent resistive-capacitive DL response, charge transfer reactions, and resistively damped capacitive currents in microstructures (such as pores, pinholes, or cracks). These frequency-dependencies are typically overlooked when capacitances are related to ECSAs. The capacitance at the specimen-characteristic relaxation frequency of the resistive-capacitive DL response is proposed as a standardized capacitance-metric to estimate ECSAs. In 1 M perchloric acid, the polished gold electrode and the high-surface area carbon fleeces show ratios of capacitance-metric over surface-area of around 3.7 μF cm-2. Resistively damped currents in microstructures and low-conducting oxide layers are shown to complicate trustworthy capacitance-based estimations of ECSAs. In the second part of this study, advanced equivalent circuits models to describe the measured EIS and CV responses are presented.

Sn-based film electrodeposited on Ag foil for selective electrochemical CO2 reduction to CO.

Electrochemical CO2 reduction (ECR) to valuable chemicals and fuels using renewable energy is a promising way to reduce carbon emission. Herein, Sn-based films were electrodeposited on Ag foil surfaces (Sn/Ag-y) for selective ECR to CO, where y represented the concentration of SnCl2 in the electrodeposition bath. The Sn/Ag-20 electrode achieved a high CO faradaic efficiency of 96.0% with a current density of 69.3 mA cm-2. The enhanced catalytic performance could be attributed to appropriate superficial properties, large electrochemical active surface areas, low charge transfer resistance, efficient stabilization capacity of the CO2˙- intermediates, and suitable combination with electrolytes.

Electrochemical preparation and features of newly cross-stacking multi-layered reduced graphene oxide (rGO) and polyaniline (PANI) modified carbon-based electrode

Carbon-based electrode as an economically benign choice is widely used in electrochemical or bio-electrochemical systems. However, its poor electrical conductivity that leading high overpotential and energy loss, especially in large scale facilities, remains a bottleneck for its application. Herein, a unique cross-stacking multi-layered reduced graphene oxide (rGO) and polyaniline (PANI) modified carbon brush is fabricated via one-step electro-depositing. In particular, the top-most layer of the modified electrode is orientated controlled to be rGO or PANI layer by adjusting the initial CV scanning in the positive or negative direction. As was indicated by cyclic voltammetry, the improved electrochemical activity is achieved by coupling the advantages of the highly conductive network offered by graphene with desirable stability provided by the well-dispersed deposition of nanoscale PANI particles. In comparison, the modified electrode with rGO on the top-most layer (LrGO) showed higher degree of sp2-hybridized -C-C- ordered structure in Raman profile, lower O/C ratio in XPS analysis, higher Zeta potential (−2.05 mV) and more hydrophilic than unmodified one. Moreover, benefiting from the unique cross-stacking multi-layered matrix of rGO and PANI, the best electrochemical performance was achieved on the electrode LrGO with high exerted electrochemical active surface area (ECSA) of 0.85 mF cm−2, and the charge transfer resistance as low as 0.32 Ω. The findings of this study provide a guidance for the modification and application of carbon-based electrode using rGO and PANI, which potentially enables the scaling-up of carbon-based electrode in various (bio-) electrochemical systems with high electrochemical performance.

Electrodeposited alloy electrodes in the Co–W–Mo system as highly efficient catalysts for hydrogen production from alkaline water electrolysis

In the presented paper the non-precious electrode materials containing Co, Mo, and W demonstrating high catalytic activity towards hydrogen evolution reaction were characterized. The studied alloy electrodes were electrodeposited from electrolytes with a simple composition, containing CoSO4, Na2WO4, Na2MoO4, and Na3C6H5O7. The structure, surface morphology, and composition of the prepared deposits were analyzed using scanning microscopy, X-ray diffraction, and transmission electron microscopy techniques. Cathodic polarization and electrochemical impedance spectroscopy were applied to assess the electrocatalytic activity and elucidate the HER mechanism in 1 M KOH solution. The nanocrystalline structure, fine grain size, and presence of microcracks on the surface of the studied alloys contributed to their notably high electrochemically active surface area (ECSA) (65.7e2, 11.7e4, and 99.7e3 cm2 for the Co–W, Co–Mo, and Co–W–Mo, respectively), a crucial factor driving their superior catalytic activity. The intrinsic catalytic activity evaluated at the overpotential of 150 mV was 9.2, 3.3, 0.78 and 1.4 μA cm-2 for the Co, Co–W, Co–Mo and Co–W–Mo electrodes, respectively. This indicates that the superior electrocatalytic characteristics observed for the alloy coatings, compared to Co resulted from the substantial increase in their ECSA. Among the investigated electrodes, the ternary Co–W–Mo alloy was characterized by the highest electrocatalytic performance confirmed by the lowest overpotential (42 mV) required to attain a current density of 10 mA cm−2.

Anion-tuning in cobalt chalcogenides for a comparative study on electro-oxidation of glucose

Electronic structure modulation is of great important to enhance the electrocatalytic ability for glucose oxidation and follow up the electrochemical sensing performance. In this work, Co3X4 (X = O, S, Se) synthesized by oxidation, sulfurization and selenylation of ZIF-67 precursor, respectively, are served as electrocatalysts and then modified on conductive indium tin oxide (ITO) to construct sensing electrodes. The Co3S4/ITO electrode shows a highest sensitivity of 1109.9 μA−1 mM−1 cm−2 when compared with those of the Co3Se4/ITO electrode (515.9 μA−1 mM−1 cm−2), Co3O4/ITO electrode (227.7 μA−1 mM−1 cm−2), and even the relative reported sensing electrodes, enabling more efficient electro-oxidation of glucose by S substitution in Co3X4. The experimental characterization and theoretical calculation elucidate that tuning the anion components in cobalt chalcogenides can modulate their electrochemical active surface area, hydrophilicity, electrical conductivity and binding energies for glucose adsorption, in turn, the electrocatalytic ability. This work provides not only an effective strategy for performance improvement but also a significant understanding of how the anion component of electrocatalyst affect their glucose oxidation ability.

Preparation of Ni-doped ferrous oxalate nanorods electrocatalysts for oxygen evolution reaction

The development of new, efficient, and low-cost OER electrocatalysts with advanced structures is of great significance for hydrogen production by electrolysis of water. A Ni-doped ferrous oxalate nanorod electrocatalyst (Ni-FeC2O4) was synthesized using a one-step solvothermal method, which showed an outstanding OER activity with a low overpotential of 296 mV at 10 mA cm−2. The Ni-FeC2O4 electrocatalyst exhibited excellent OER stability in the 300 mV constant potential test, which was mainly attributed to the fact that Ni doping can increase the electrochemical active surface areas and the interfacial charge transfer abilities of the FeC2O4 electrocatalyst, optimize the electronic structure of the real active species ferric hydroxide, promote the formation of the decisive step *OOH of ferric hydroxide, and improve its intrinsic activity.

Nickel oxide doped ceria nanoparticles (NiO@CeO2) for boosting oxygen evolution reaction and enhancing stability

Electrochemical water electrolysis for the production of H2 and O2 is an extremely important and attractive technology that mainly depends on efficient and affordable electrocatalysts. However, the sluggish kinetics of OER is the main obstacle in water electrolysis. In this work, we have fabricated hybrid electrocatalysts based on CeO2 and NiO nanomaterials by varying the concentration of NiO to boost OER activity and stability. CeO2 and NiO nanomaterials were synthesized using [(NH4)2 Ce(NO3)6] and [Ni(NO3)2·6H2O] as precursors, respectively. All the prepared electrocatalysts namely NiO, CeO2, 10-NiO@CeO2, 20-NiO@CeO2, 30-NiO@CeO2, and 40-NiO@CeO2, were well characterized by various analytical tools to evaluate their morphologies, phases, crystallinities, compositions and electrochemical activities. Among the series of fabricated electrocatalysts, 20-NiO@CeO2 appeared as an efficient electrocatalyst with a very low overpotential of 392 mV to reach 50 mAcm−2 current density. In addition, a lower value of the Tafel slope, as measured from the curve of linear sweep voltammetry (LSV), indicates better OER kinetics. Furthermore, the charge transfer resistance (Rct) and electrochemical active surface area (ECSA) were measured, which further supports the excellent OER performance of 20-NiO@CeO2. These findings pave the way toward the rational design of bimetallic electrode materials for excellent OER activity.

Nanostructure-performance relationships in titania-only electrodes for the selective electrocatalytic hydrogenation of oxalic acid

The preparation of robust and selective hydrogenation electrodes not based on critical raw materials is a relevant goal for the development of sustainable electrocatalytic processes. Oxalic acid is an emerging intermediate produced from CO2 that can be used to produce valuable, added-value monomers for high-performance polymers by selective electrocatalytic reduction. We demonstrated that with the rapid breakdown anodization (RBA) method, superior-performance TiO2-only electrodes can be directly prepared with a fast and low-cost procedure. The electrodes were based on a conductive Ti substrate, used to generate a roughened titania layer by RBA characterized by an ordered array of vertically aligned TiO2 nanotubes (NTs) on top of them and partially reduced TiO2-x nanoparticles stabilized by the interaction with the NTs layer. In addition, the preparation method leads to highly nanoporous NTs. The electrodes obtained by RBA at 60 V led to the best Faradaic efficiency (FE) to glycolic acid (86 %) and oxalic conversion (48 %) at −0.8 V vs. RHE, superior to the existing literature on TiO2-only electrodes at room temperature. The NTs structural features, such as length and surface roughness, were calculated by AFM and linearly correlated with the oxalic acid conversion and the electrochemical active surface area (ECSA). Additionally, the ECSA was found to be correlated with selectivity, indicating the complex interrelations between the NT nanostructure and performance. This finding emphasizes the importance of controlling the nanostructure as a critical factor in tuning the selectivity toward glyoxylic acid or glycolic acid.

Numerical analysis of PEMFC stack performance degradation using an empirical approach

This paper proposes a performance degradation model based on empirical approaches, which can break down the power loss and quantify the influence of each aging phenomena from the overall performance degradation. With the presence of primary degradation mechanisms, the approach can be used to analyze the performance degradation of fuel cells under different degradation conditions. The aging parameters that represent hydrogen crossover, membrane resistance increase, and electrochemical active surface area (ECSA) loss are introduced to characterize the performance degradation. Based on the polarization behavior of proton exchange membrane fuel cells (PEMFCs), the variations of degradation stressors are determinated by fitting the aging parameters to the measuring polarization curves. The empirical approach is validated by a 150 h durability experiment and agrees well with all polarization curves. The results show that ECSA loss always maintains an apparent influence and hydrogen crossover accounts for 56% of the performance degradation in OCV.

In Situ Construction of a Co2P/CoP Heterojunction Embedded on N-Doped Carbon as an Efficient Electrocatalyst for a Hydrogen Evolution Reaction.

Noble metal-free electrocatalysts have received widespread attention in a hydrogen evolution reaction (HER) due to the importance of renewable energy development. Herein, a Co2P/CoP heterojunction embedded on an N-doped carbon (Co2P/CoP/NC) electrocatalyst was prepared via an in situ pyrolysis method. The as-prepared electrocatalyst exhibited efficient electrocatalytic activity for HER in an acidic solution. The Co2P/CoP/NC catalyst displayed an overpotential of 184 mV at 10 mA cm-2 and a low Tafel slope of 82 mV dec-1, which could be attributed to the tight Co2P/CoP heterojunction and the synergetic effect of Co2P/CoP and N-doped carbon. In addition, the electrochemical active surface area of Co2P/CoP/NC was 75.2 m2 g-1, which indicated that more active regions can be applied for the HER process. This report may pave a new way for the design of efficient and low-cost N-doped-carbon-supported 3d transition metal phosphide electrocatalysts.

Manufacturing Strategies for Engineering Carbon Electrodes Via Non-Solvent Induced Phase Separation

Porous electrodes are at the core of emerging electrochemical systems and tuning the electrode properties is critical to overcome system-specific limitations. To meet the stringent performance and cost requirements, there is a need for tailored microstructures generated through inexpensive and scalable synthesis methods with precursors originating from reliably-sourced, globally-abundant feedstocks. To this end, we recently introduced a manufacturing route using non-solvent induced phase separation (NIPS), yielding porous carbon electrodes with multimodal and interconnected porosity. The synthesized NIPS electrodes demonstrated a beneficial trade-off between transport properties (permeability, mass transfer) and electrochemically active surface area (ECSA) resulting in high performance in iron and all-vanadium redox flow batteries (RFBs)[1,2]. Moreover, NIPS manufacturing offered control over electrode characteristics (pore size, surface area) which are critical parameters for broader electrochemical devices. Besides its use for RFB application, we hypothesize that the electrode morphologies resulting from NIPS could be applied to a broader range of electrochemical systems employing porous electrodes. Consequently, here we focus on extending the library of electrode microstructures achievable via NIPS through the alteration of simple process parameters.In this presentation, I will discuss our latest advances on the bottom-up manufacturing of porous electrodes using NIPS targeting porosity gradients and high ECSA materials. For the former, we introduce the use of a control layer, casted on top of the NIPS polymer solution at the interface with the coagulation bath, thereby altering the rate of the phase separation process. We investigate various process parameters such as the composition, viscosity, and thickness of the control layer and assess their effect onto the resulting microstructures. The NIPS method assisted by a control layer generates porosity gradients, resulting in a slight increase of the electrode ECSA at the cost of a higher pressure drop. To further increase the ECSA of the prepared electrodes, we also propose nano-patterning approaches to significantly enhance ECSA through addition of nano-scale porosity without compromising the pressure drop through the porous structure. Finally, both porosity gradient and nano-patterned NIPS electrodes are tested in a all-vanadium redox flow cell to extract microstructure-performance relationships. Despite being in its early technological stage, this work illustrates the versatility of NIPS to manufacture porous electrodes through a very broad microstructural design space.

Accelerated Stress Tests to Project PEM Fuel Cell Durability

One of the major degradation mechanisms limiting the long-term durability of proton exchange membrane fuel cells (PEMFCs) is the loss of platinum electrochemically active surface area (ECSA) of the carbon-supported platinum (Pt/C) cathode catalyst, caused by Pt dissolution that is followed by both Ostwald ripening of the Pt nanoparticles and loss of Pt into the ionomer phase [1]. The Pt ECSA loss is accelerated when subjecting PEMFCs to extended load-cycling inducing concomitant cycling of the cathode potential. To this end, accelerated stress tests (ASTs) can be conducted either by controlling the cell/stack current (“load-cycling” AST) under H2/air (anode/cathode) or the potential (“voltage-cycling” AST) under H2/N2 (anode/cathode).Most of the experiments studying the effect of load-cycling on catalyst durability have been based on voltage-cycling in a H2/N2 configuration, showing that Pt ECSA loss is aggravated with increasing upper potential limit (UPL), temperature, and relative humidity (RH) [2, 3, 4]. Comparing voltage-cycling induced degradation under H2/N2 versus H2/air, a recent study has found an essentially identical Pt ECSA loss, but a slightly higher H2/air performance decay when cycling under H2/N2 [4]. This study by General Motors Corporation indicates that Pt ECSA loss may not be a unique descriptor for H2/air performance loss, contrary to what was observed in recent work by Toyota Motor Corporation and Kyushu University [5] as well as in our own studies [6].In this talk, we will discuss the correlation between H2/air performance loss and Pt ECSA loss during voltage-cycling ASTs at different conditions (UPL, RH, and gas-feed) conducted in 5 cm2 active area single-cell PEMFCs, complemented by a voltage-loss analysis to deconvolute oxygen reduction reaction (ORR) activity losses and mass transport losses (due to oxygen mass transport and proton conduction in the cathode catalyst layer). We will also discuss whether the H2/air performance loss is governed by the Pt ECSA loss (independent of catalyst loading) or, as we had proposed previously, by the cathode electrode roughness factor (rf) loss (in cm2 Pt/cm2 cathode, i.e., the product of ECSA and Pt loading) [7]. As roughly 100,000 [8] or even more voltage-cycles are expected for heavy-duty applications, requiring very long measurement times, an approach to relate the degradation under harsh AST conditions with those under application-relevant conditions will be discussed. Experiments are conducted with cathode catalysts based on different carbon supports (Vulcan, Ketjenblack, or so-called accessible carbon supports [9, 10]), on catalysts with different initial Pt ECSAs (i.e., different Pt nanoparticle sizes) and with different initial Pt-loadings (i.e. different initial rf). Finally, exploratory experiments to evaluate the effect of start-up/shut-down on the correlation between electrode rf and H2/air performance decay will be discussed.

Investigating the Stability of Electrodeposited NiMo Alloy for Hydrogen Evolution Reaction in Alkaline Media

A lot of efforts have been devoted to develop highly active noble-metal-free catalysts for hydrogen production by water electrolysis.1,2 Nevertheless, the stability of such catalysts, yet equally important, is rarely touched. NiMo alloy is an emerging catalyst for hydrogen evolution reaction (HER) in alkaline media.3 In the present work, we studied the stability of NiMo alloy for HER under alkaline (1M KOH, Fe-free) conditions as a model catalyst. Using electrodeposition at room temperature, we deposited NiMo alloy onto nickel foam substrates. A long-term (20h) stability test at a fixed current density of -100 mA cm-2 was carried out using chronopotentiometry. Electrochemical active surface area (ECSA) and HER polarization curves were determined prior and after this stability test. Here, obvious activity loss is observed. The ECSA of NiMo catalyst after chronopotentiometry test increases by 10%. Thus, an obvious increase in the overpotential is observed upon normalizing the catalytic current with ECSA (Figure 1a). As further stability test 100 CV cycles at a scan rate of 100 mV s-1 from 0 to –350 mV vs. RHE were carried out, the catalysts then hold for 30 min at open circuit potential and again 100 CV cycles carried out. A clear drop in activity during the second 100 cycles is also observed here and especially at the higher current density of 60 mA cm-2 (Figure 1b). In summary, we found that NiMo, though active initially, loses its activity with time under HER conditions in alkaline media.References Vesborg, P. C. K.; Seger, B.; Chorkendorff, I. Phys. Chem. Lett. 2015, 6, 951– 957.Zou, X. X.; Zhang, Y. Soc. Rev. 2015, 44, 5148– 5180.Kuznetsov, V. V.; Gamburg, Y. D.; Zhulikov, V. V.; Krutskikh, V. M.; Filatova, E. A.; Trigub, A. L.; Belyakova, O. A. Electrochim. Acta 2020, 354, 136610. Figure 1

(Poster Award - Honorable Mention) Probing the Catalyst-Ionomer Interface in Catalyst Layers with Sulfo-Phenylated Polyphenylene Ionomers Using CO-Displacement and Stripping Techniques

Sulfo-phenylated polyphenylene ionomers (sPPP) are a class of hydrocarbon Ionomer being explored as a replacement for the perfluorinated sulfonic acid (PFSA) ionomer in fuel cell membranes and catalyst layers because they do not contain fluorinated compounds. Integration of sPPP ionomers into catalyst layers is challenging, as they have significantly different morphology compared to PFSA-based ionomers. Furthermore, little is known about the resulting ionomer-catalyst interface and whether characterization techniques developed for catalysts layers with PFSA-based ionomers will perform well for catalyst layers containing an sPPP ionomer. In this work, we examine the usefulness of CO displacement and stripping techniques to probe both the electrochemically active surface area (ECSA) of Pt/C in the catalyst layer and adsorbed species on the Pt catalyst sites. For catalyst layers containing PFSA, we measure a strong CO displacement reduction peak at potentials above potential of zero charge, when CO is introduced to the system, corresponding to sulfonic acid groups being displaced from active sites. The range of sulfonic acid groups adsorbed on the surface is 15- 25 % depending on the carbon support used. When the catalyst layers contain an sPPP ionomer instead of PFSA, the CO displacement peak is almost entirely suppressed, resulting in less than 10 % of sulfonic acid group adsorption. This is indicative that sPPP ionomer orientation near the Pt surface is very different compared to PFSA-based ionomer and that sulfonic acid groups do not “poison” Pt when in contact with the sPPP ionomer. However, mass activity increase is not observed for the sPPP ionomer-based catalyst layers, indicating there might be different “poisoning” mechanisms present, perhaps associated with the phenyl rings adsorption [1]. ECSA measurements for MEAs containing the non-fluorinated ionomer catalyst layers align with those measured for PFSA MEAs with the same catalysts type, loading, and comparable ionomer content, indicating that CO stripping is still a useful tool for assessing the ECSA for MEAs.[1] Zhai, Y., Baturina, O., Ramaker, D., Farquhar, E., St-Pierre, J., & Swider-Lyons, K. (2015). Chlorobenzene Poisoning and Recovery of Platinum-Based Cathodes in Proton Exchange Membrane Fuel Cells. Journal of Physical Chemistry C, 119(35), 20328–20338. https://doi.org/10.1021/acs.jpcc.5b06362

Regeneration Strategies for Ruthenium-Poisoned ORR Catalysts in Reformate PEM Fuel Cells

Reformate polymer electrolyte membrane (PEM) fuel cells have great potential to transform the global maritime transportation in a more environmentally friendly and CO2-neutral manner. Platinum (Pt)-ruthenium (Ru) alloy nanoparticles (NPs) supported on carbon (PtRu/C) are the most commonly used electrocatalyst material to accelerate the hydrogen oxidation reaction (HOR) of the reformate gas (H2/CO2/CO) at the anode [1]. The high CO tolerance of PtRu/C catalysts is based on the ensemble effect [2]. However, these materials suffer extremely from the Ru dissolution and Ruz+ crossover to the cathode, which are the main degradation processes in reformate PEM fuel cells. At the cathode, the Ruz+ species immediately redeposit on the Pt/C catalyst, resulting in a drastic decay of the entire performance of the reformate PEM fuel cell. Even if the Ru coverage on the Pt/C catalyst is less than 20 at. %, there is an 8-fold decrease in kinetics of the oxygen reduction reaction (ORR) [3]. Therefore, it is necessary to develop a regeneration protocols to mitigate the Ru poisoning at the cathode.In this study, we developed and tested different regeneration protocols to recover the Ru-poisoned ORR catalysts using a rotating disc electrode (RDE) set-up. To simulate the worst situation for poisoned ORR catalysts, commercially available Pt-Ru alloy NPs supported on carbon were used and characterized by TEM, XRD, XAS, and XPS. In general, our regeneration protocols can be classified into dynamic and steady-state conditions. All protocols included a pre-activation step by holding the potential at 1.4 VRHE for 5 minutes in 0.1 M HClO4. During the dynamic regeneration process, the potential was switched between 1.4 VRHE and 1.6 VRHE with a duration time of 10 s by 10 times using a pulse voltammetry technique. In contrast, the steady-state regeneration protocol consisted of only one pulse at 1.6 VRHE for 100 s. For the recovery procedure of the poisoned ORR catalysts, the experimental parameters were correlated with the changes in ORR performance and electrochemically active surface area (ECSA) via CO stripping as well as the loss of Ru content and particle structure established from μ-XRF, ICP-OES, and TEM.After the dynamic regeneration process, the ORR mass activity (MA) and specific activity (SA) of PtRu2/Vulcan increase initially from 0.07 ± 0.01 to 0.21 ± 0.02 A mgPt+Ru -1 and 62 ± 9 to 190 ± 16 μA cmPt+Ru -2 at 0.85 VRHE, iR-free, respectively. Hence, the dynamic regeneration protocol leads to an improvement of the MA by a factor of 2 and of the SA by 2.1 for PtRu2/Vulcan. In comparison, the steady-state regeneration experiment shows an enhancement by 2.2 (0.26 ± 0.03 A mgPt+Ru -1) for MA and by 3.5 (276 ± 16 μA cmPt+Ru -2) for SA, respectively. Therefore, the steady-state regeneration process is more effective compared to the dynamic regeneration protocol. Based on the µ-XRF data, we observed that the Ru content of the PtRu2/Vulcan decreases from 62 ± 2 at. % to either 5 ± 1 at. % or 8 ± 1 at. % after the steady-state or dynamic regeneration process. The loss of Ru from the particle surface can also be monitored by CO stripping, where the CO oxidation peak is shifted positively from 0.58 VRHE to 0.67 VRHE and 0.63 VRHE after steady-state and dynamic regeneration protocol, respectively. As the particle size of the PtRu2 NPs is around ~ 5.3 ± 1.2 nm, the ECSA is mainly unchanged after both regeneration protocols.Based on our results, the steady-state regeneration process with 1.6 VRHE for 100 s is more effective than the dynamic one with pulses from 1.4 to 1.6 VRHE each with 10 s by 10 times to recover the ORR activity of the PtRu2 alloy NPs. The proposed regeneration protocols can improve the performance and life-time of the reformate PEM fuel cell.

Numerical Simulations for in-Plane Distribution of Platinum Degradation in Dynamic Operating Conditions of Polymer Electrolyte Membrane Fuel Cells

Predicting degradation of polymer electrolyte membrane fuel cells (PEMFCs) is an important issue from the viewpoint of durability and cost. In fact, in technical roadmaps regarding fuel cell of each country, high targets of durability and performance are set for transport applications especially for heavy duty vehicles. One of important degradation process takes place in catalyst layer. PEMFC catalysts are typically platinum or platinum alloys and consist of nanoparticles to increase the electrochemically active surface area (ECSA). However, these particles are not stable in dynamic operating conditions of the transport applications. A typical degradation mechanism of the catalyst is the electrochemical Ostwald ripening mechanism, and it is known that the platinum particle size distribution shifts to larger particle diameter, resulting in lower ECSA and lower performance. Therefore, predicting platinum catalyst degradation is important to enhance durability of the products.Various numerical models for catalyst degradation of PEMFC have been proposed and used to explore degradation mechanisms. Many of these are zero or one-dimensional models, but factors that affect catalyst degradation, such as temperature, potential, and humidity, have non-uniform distributions in the active area of large-size (several hundred cm2) cells. Furthermore, these factors change unsteadily due to the dynamic operating conditions. In order to predict the degradation of cells, it is necessary to analyze the dynamic behavior of large-area cells, but there are few spatially resolved catalyst degradation predictions [1].In this analysis, transient analysis of a large-area cell for a dynamic operating conditions is performed to predict in-plane distribution of catalyst degradation. Furthermore, we will analyze changes in the power generation performance of the cells when these degradations progress. A three-dimensional numerical simulation model based on our original methodology [2] was used as the performance prediction model for the PEMFC. Catalyst degradation considers the mechanisms of platinum dissolution, oxidation, and platinum oxide dissolution [3]. The in-plane distribution of platinum degradation is predicted from the time-series results of in-plane distributions of temperature, potential, humidity, etc., obtained by the performance prediction model. The performance of the PEMFC at the time of progressive degradation is analyzed by reflecting the effect of ECSA decrease due to platinum degradation in the performance prediction model. Although this method considers only a part of the catalyst degradation process, a path is presented to analyze the distribution of catalyst degradation under dynamic operating conditions in large-area cells.