Hydrogen Underpotential Deposition Research Articles

The Influence of Acetone on the Kinetics of Water Electrolysis Examined at Polycrystalline Pt Electrode in Alkaline Solution

This study investigated the impact of acetone on the electrochemical behavior of polycrystalline platinum electrodes in 0.1 M NaOH solution, with respect to the kinetics of hydrogen and oxygen evolution reactions (HER and OER) and indirectly to the underpotential deposition of hydrogen (UPDH). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques were employed to analyze these processes for acetone concentrations ranging from 1.0 × 10−6 to 1.0 × 10−3 M. The addition of (CH3)2C=O enhanced the catalytic efficiency of alkaline water splitting, which was believed to be a result of a significant reduction in the surface tension phenomenon (due to mutual interaction of acetone and water molecules), thus considerably facilitating hydrogen bubble detachment from the Pt electrode. Key findings in this work are described with respect to facilitation of both the HER and the OER reactions’ kinetics by the presence of acetone (also undergoing Pt electroreduction over the potential range for UPDH) in the working solution, without an electrode surface poisoning effect. The latter implies significant opportunities for traces of organic additives into alkaline electrolyte to improve the industrial alkaline water electrolysis process.

Adsorbed Carbon Monoxide-Enabled Self-Terminated Au-Grafting on Pt6 Nanoclusters for Enhanced Methanol Electrooxidation.

The study presents the first example of an adsorbed carbon monoxide (CO) enabled self-terminated Au-grafting on triphenylphosphine (PPh3) stabilized Pt6 nanoclusters (NCs) (Pt6 (PPh3)4Cl5 NCs or Pt6 NCs). Adsorbed PPh3 ligands weaken the Pt-CO bond enabling the self-terminated Au-grafting on Pt6 NCs. The Au-grafted Pt6 NCs exhibit enhanced methanol electrooxidation (MOR) in acidic solutions. The surface is composed of a PtAu ensemble exhibiting enhanced MOR and CO tolerance due to the synergistic interaction of Pt with Au and PPh3. The hydrogen underpotential deposition (H-UPD) signal from a CO-covered surface reveals the existence of free-Pt sites on the PtAu ensemble causing higher MOR reactivity. The Au and PPh3 ensure electrocatalytic activity of the NCs, depriving of them at anodic potentials results in "a death-valley" trend.

Condensed Layer Deposition of Nanoscopic TiO2 Overlayers on High-Surface-Area Electrocatalysts.

Encapsulating an electrocatalytic material with a semipermeable, nanoscopic oxide overlayer offers a promising approach to enhancing its stability, activity, and/or selectivity compared to an unencapsulated electrocatalyst. However, applying nanoscopic oxide encapsulation layers to high-surface-area electrodes such as nanoparticle-supported porous electrodes is a challenging task. This study demonstrates that the recently developed condensed layer deposition (CLD) method can be used for depositing nanoscopic (sub-10 nm thick) titanium dioxide (TiO2) overlayers onto high-surface-area platinized carbon foam electrodes. Characterization of the overlayers by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) showed that the films are amorphous, while X-ray photoelectron spectroscopy confirmed that they exhibit TiO2 stoichiometry. Electrodes were also characterized by hydrogen underpotential deposition (Hupd) and carbon monoxide (CO) stripping, demonstrating that the Pt electrocatalysts remain electrochemically active after encapsulation. Additionally, copper underpotential deposition (Cuupd) measurements revealed that TiO2 overlayers are effective at blocking Cu2+ from reaching the TiO2/Pt buried interface and were used to estimate that between 43 and 98% of Pt surface sites were encapsulated. Overall, this study shows that CLD is a promising approach for depositing nanoscopic protective overlayers on high-surface-area electrodes.

Tailoring iridium-palladium nanoparticles with Ir-rich skin: a highly durable anode electrocatalyst for acidic water electrolysis via a facile microwave-assisted chemical reduction method.

Electrochemical water splitting under acidic conditions is a clean way towards producing hydrogen fuels. The slow kinetics of the oxygen evolution reaction (OER) at the anode is currently a bottleneck for commercial acceptance of this technology. Therefore, arriving at more efficient and sustainable OER electrocatalysts is highly desirable. We here demonstrate the synthesis of iridium-palladium (IrPd) alloy nanoparticles (2-5 nm) with variable average composition (Ir : Pd = 1 : 0, 1 : 1, 1 : 3, 1 : 6, 1 : 9 and 0 : 1) using a facile one-pot microwave-assisted chemical reduction method. The IrPd nanoparticles show structure- and composition-dependent OER performance in acidic media. Utilizing different reduction strengths and precursor ratios, successful alloy catalysts were prepared with Ir-rich skin and sublayers of different Pd compositions. Their structures were revealed using high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and hydrogen underpotential deposition (Hupd) studies. It turned out that (1) the alloy OER catalyst also has a high electrochemically active surface area for hydrogen adsorption/desorption, (2) the OER performance is strongly dependent on the surface Ir contribution and (3) the intact Ir skin is essential for electrocatalyst stability.

(Invited) Extended Thin Film Platinum Group Metal Electrocatalysts with Different Mesoscale Morphologies

The lack of sufficiently active oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts is a bottleneck to the proliferation of PEM fuel cells and water electrolyzers. Commercially available PGM electrocatalysts fail to attain DOE technical targets due to instabilities and activity losses incurred at low loadings. Morphology control of PGM nanostructures is an enticing route towards activity and stability enhancements of electrocatalysts, but not much is known about the structure-property relationships between PGM mesoscale geometry (5 to 100 nm) and their activity and stability.This talk presents our work on model Pt extended surface electrocatalysts of tunable feature sizes and morphologies prepared from self-assembled block copolymer templates. The morphology, either perpendicular lamellae, cylinders, or perforated cylinders, of the extended surface electrocatalyst was controlled by the block copolymer volume fraction. The periodic lateral feature sizes (10 to 40 nm in this work) were controlled by the block copolymer degree of polymerization. The self-assembled block copolymers were transformed into extended surfaces using sequential infiltration synthesis to prepare aluminum oxide followed by reactive ion etching to remove the block copolymer template. Pt was sputtered onto the aluminum oxide supports. ORR data in a RDE with the new extended nanostructured thin films of Pt showed mass activity values of 0.35 A mgPt-1 to 0.46 A mgPt -1 at 0.9 V vs. RHE. The measured electro-chemically active surface area (ECSA) values obtained from hydrogen underpotential deposition were 64 to 70 m2 gPt -1. The lower values hailed from lamellae morphology while the larger values corresponded to a cylindrical morphology. The measured mass activity values, which have no cobalt or nickel additives and are unoptimized and display competitive mass activity values for ORR. We envision that the block copolymer templated extended surfaces could be an alternative to 3M’s state-of-the-art nanostructured thin film technology.

Comparison of methods to determine electrocatalysts’ surface area in gas diffusion electrode setups: a case study on Pt/C and PtRu/C

In recent years, gas diffusion electrode (GDE) half-cell setups have attracted increasing attention, bridging the gap between fundamental and applied fuel cell research. They allow quick and reliable evaluation of fuel cell catalyst layers and provide a unique possibility to screen different electrocatalysts at close to real experimental conditions. However, benchmarking electrocatalysts’ intrinsic activity and stability is impossible without knowing their electrochemical active surface area (ECSA). In this work, we compare and contrast three methods for the determination of the ECSA: (a) underpotential deposition of hydrogen (Hupd); (b) CO-stripping; and (c) underpotential deposition of copper (Cuupd) in acidic and alkaline electrolytes, using representative electrocatalysts for fuel cell applications (Pt and PtRu-alloys supported on carbon). We demonstrate that, while all methods can be used in GDE setups, CO-stripping is the most convenient and reliable. Additionally, the application of Cuupd offers the possibility to derive the atomic surface ratio in PtRu-alloy catalysts. By discussing the advantages of each method, we hope to guide future research in accurately determining surface area and, hence, the intrinsic performance of realistic catalyst layers.

Electrocatalytic Hydrogen Evolution at Full Atomic Utilization over ITO-Supported Sub-nano-Ptn Clusters: High, Size-Dependent Activity Controlled by Fluxional Pt Hydride Species.

A combination of density functional theory (DFT) and experiments with atomically size-selected Ptn clusters deposited on indium-tin oxide (ITO) electrodes was used to examine the effects of applied potential and Ptn size on the electrocatalytic activity of Ptn (n = 1, 4, 7, and 8) for the hydrogen evolution reaction (HER). Activity is found to be negligible for isolated Pt atoms on ITO, increasing rapidly with Ptn size such that Pt7/ITO and Pt8/ITO have roughly double the activity per Pt atom compared to atoms in the surface layer of polycrystalline Pt. Both the DFT and experiment find that hydrogen under-potential deposition (Hupd) results in Ptn/ITO (n = 4, 7, and 8) adsorbing ∼2H atoms/Pt atom at the HER threshold potential, equal to ca. double the Hupd observed for Pt bulk or nanoparticles. The cluster catalysts under electrocatalytic conditions are hence best described as a Pt hydride compound, significantly departing from a metallic Pt cluster. The exception is Pt1/ITO, where H adsorption at the HER threshold potential is energetically unfavorable. The theory combines global optimization with grand canonical approaches for the influence of potential, uncovering the fact that several metastable structures contribute to the HER, changing with the applied potential. It is hence critical to include reactions of the ensemble of energetically accessible PtnHx/ITO structures to correctly predict the activity vs Ptn size and applied potential. For the small clusters, spillover of Hads from the clusters to the ITO support is significant, resulting in a competing channel for loss of Hads, particularly at slow potential scan rates.

The importance of carefully choosing vertex potentials in hydrogen underpotential deposition

Accurate determination of electrochemical surface area (ECSA) is not always possible even for simple monometallic catalysts with well-known techniques. An example is Pt and the determination of its ECSA by hydrogen underpotential deposition (HUPD) used to determine the specific activity. Here, we show an important problem in determining the ECSA of Pt (both wire and foil electrodes) using HUPD which is the changing ECSA with the changing anodic and cathodic vertex potentials (Evertex) set in HUPD analysis. The results confirm that just by varying the Evertex, one could get a wide range of values for ECSA and the ECSA normalized specific activity for the same Pt electrode. This calls for instant remedial action as Pt is not just any catalyst but state-of-the-art in many energy conversion reactions and our study suggests that the Evertex should be chosen based on whether or not to have the effect of Pt oxidation on ECSA.

Review on electrochemical active surface area characterization methods of Pt alloy catalysts

<p indent="0mm">Proton exchange membrane fuel cells (PEMFCs) operating at low temperatures and high-power densities are considered one of the most promising technologies for clean energy. However, some fundamental challenges remains to be addressed before PEMFCs can truly become the top contender, high performance with low Pt usage for example. In order to achieve high-performance and cost-effective in PEMFCs, Pt alloy are currently one of the most promising catalyst candidates. Electrochemical active surface area (ECSA) is a key indicator to gauge the performance of Pt catalysts and an important parameter in studying catalytic kinetic issues in PEMFCs. Therefore, the accurate estimation on the ECSA is exceptionally critical. Known and mature ECSA characterization methods for Pt/C catalysts have been established, which has been widely available for many decades. However, there are challenging issues remaining for ECSA characterization of Pt alloy catalysts due to the significant changes in physical structure (composition, particle size, shape, etc.) and chemical properties (active site, catalytic performance, etc.) compared with Pt catalysts. Therefore, the ECSA characterization approaches for Pt/C catalysts cannot be transplanted to Pt alloy catalysts since the characterization accuracy will no longer meet the requirement. In this review, the present methods of ECSA characterization for Pt alloy catalysts and their inaccuracy sources are described and explained, including physical methods and electrochemical methods. Physical methods mainly cover X-ray diffractometry (XRD) and transmission electron microscopy (TEM). hydrogen underpotential deposition (H-upd), carbon monoxide stripping (CO-stripping) and underpotential deposition of metals are classified as electrochemical methods. In XRD and TEM methods, the geometrical areas calculated based on the average particle size is regard as ECSA. In more cases, it is considered to be theoretical ECSA to evaluate the catalyst utilization. Therefore, the ECSA calculated from XRD and TEM is widely used as a reference. Owing to the changes in surface adsorption properties after alloying, it will be underestimated the ECSA of Pt alloy catalysts by H-upd and CO-stripping, which leads to exaggerate specific activity of when compared with Pt/C catalysts. It is generally accepted that sites from electrochemical methods are catalytically active, but it is still a controversy point. The surface area of different metal components of Pt alloy could be quantified by metal underpotential deposition method. Since the contribution of different metal components to the catalytic activity is different, the actual ECSA cannot be simply calculated by adding the respective metal surface area. The surface active of Pt alloy catalysts is affected by many factors, which there is currently not a universal method to characterize the ECSA accurately. To evaluate the performance of Pt alloy catalysts more objectively, it is necessary to clarify the relationship between the surface structure of different Pt alloys and adsorption energy and coverage of adsorbed species, then modifying the calculated equations of ECSA. In addition, to innovatively develop new electrochemical methods with high accuracy is also essential. Such knowledge is in urgent need in order to develop new high-performance Pt alloy catalysts.

Improved Stability of Platinum Oxygen Reduction Reaction Catalysts by Implementation of Thermally Treated Carbon Supports with High Biomass Content

To improve the lifetime of polymer electrolyte membrane fuel cells (PEMFCs), optimization of the fuel cell components is necessary wherein one research focus is the stability of the platinum catalyst support. Due to the harsh conditions in PEMFCs, e.g. high potentials (≥ 0.85 VRHE) and low pH (≤ 2), the catalyst support, which commonly consists of a high surface area carbon, degrades.[1] This is mainly caused by carbon corrosion, which becomes significant at potentials above 1.1 VRHE.[2] Typically, commercially available and chemically derived raw oil based carbons are used as catalyst supports. The use of multi-walled carbon nanotubes (MWCNTs) as a more stable, graphitic catalyst support with high electrical conductivity is well known in literature.[3, 4] The production of MWCNTs is expensive, which makes upscaling difficult. However, a previous study of our group Schonvogel et al. showed low stability of biomass-based supports, which requires further optimization.[5] Therefore, an approach to increase the stability of the biomass-based carbon support is thermal graphitization to generate a stable and ordered carbon structure. This leads to an increased amount of sp2-hybridized carbon with increased crystallinity and increased graphitic degree. At this point, the use of activated biomasses as carbon support is a cheap and sustainable alternative to MWCNTs and commercially available carbon blacks, for example Vulcan XC-72®.[5, 6] In this work, novel composite supports using thermally treated activated sawdust and MWCNTs will be investigated as Pt-nanoparticle support towards their physical properties and electrochemical ORR activity and stability. Sawdust was steam activated at 750 °C for 1 h in nitrogen atmosphere to create a defined pore system. Afterwards, the activated sawdust (aSD) was mechanically mixed with 20 wt.% MWCNTs. The mixture was treated at high temperatures (1500, 1800 and 2100 °C, under Ar atmosphere) in a furnace. For comparison, MWCNTs only were thermally treated under the same conditions. Physical characterization of the supports was performed using low temperature nitrogen sorption for determination of surface properties and crystallinity powder x-ray diffractometry (PXRD) for analysis of the degree of graphitization. Next, these novel carbon materials were used as support for 40 wt.% platinum nanoparticles. The platinum content was determined using inductive coupled plasma with mass spectrometry (ICP-MS) and the platinum nanoparticle diameter was analyzed with transmission electron microcopy (TEM) resulting in mean diameters ≤ 2 nm. In order to evaluate the activity and stability of the synthesized platinum catalysts, electrochemical characterization was performed for most promising catalysts before and after an accelerated stress test (AST, 0.6 – 1.5 VRHE, 500 mV s-1, 5000 cycles). For electrochemical characterization a three-electrode setup with a rotating ring disc electrode (RRDE) was used. The electrochemical active surface area was obtained by hydrogen underpotential deposition (HUPD) and CO stripping method. In Figure 1 the ORR curves, mass activities (MA) determined at 0.9 VRHE and MA loss of Pt/gaSD_gMWCNTs_2100 and Pt/gMWCNTs_2100 before and after AST are shown.Our study reveals that the mixture of 20 wt.% MWCNTs with sustainable aSD and subsequent thermal treatment results in a catalyst with comparable MA and stability to Pt/MWCNTs_2100. Thus, the use of activated biomass as the main component in catalyst supports constitute a more sustainable and cheaper alternative to MWCNTs. Based on these results, thermal treatment at higher temperatures of ≥ 2500 °C and higher biomass content will be further investigated to increase the stability.[1] F. Ettingshausen, J. Kleemann, A. Marcu, G. Toth, H. Fuess, C. Roth, Fuel Cells 2011, 11, 238-245.[2] V. Bandlamudi, P. Bujlo, C. Sita, S. Pasupathi, Materials Today: Proceedings 2018, 5, 10602-10610.[3] V. N. Popov, Materials Science and Engineering: R: Reports 2004, 43, 61-102.[4] D. Schonvogel, J. Hülstede, P. Wagner, I. Kruusenberg, K. Tammeveski, A. Dyck, C. Agert, M. Wark, J. Electrochem. Soc. 2017, 164, F995-F1004.[5] D. Schonvogel, M. Nowotny, T. Woriescheck, H. Multhaupt, P. Wagner, A. Dyck, C. Agert, M. Wark, Energy Technology 2019, 7, 1900344.[6] M. Iwanow, T. Gartner, V. Sieber, B. Konig, Beilstein J Org Chem 2020, 16, 1188-1202. Figure 1

Investigation of Pt/Fe-N-C Hybrids Towards ORR in Acidic Environment

Polymer electrolyte membrane fuel cells (PEMFCs) are recognized as one of the renewable energy sources in portable, automobile, and stationary applications. Currently, both low temperature (LT) and high temperature (HT) PEMFCs use platinum group metal (PGM) based catalysts for oxygen reduction reaction (ORR) containing usually Pt nanoparticles on carbon black. To reduce the total cost of PEMFC stack, worldwide researchers have considerable attention to find an inexpensive alternative catalyst. Recently, metal-nitrogen-carbon (M-N-C) compounds such as Fe-N-Cs are the most promising PGM-free catalysts for ORR. However, they suffer from insufficient volumetric activity and electrochemical stability in PEMFCs. [1,2] Xiao et al. have proven an improved electrochemical stability of Pt-Fe-N-C electrocatalysts consisting of atomically dispersed Pt and Fe atoms or Pt-Fe alloy nanoparticles in comparison with Fe-N-C only. [3] Mechler et al. have reported that the addition of 1-2 wt.% Pt in hybrid Pt/Fe-N-C catalyst performs well with an increased stability in LT-PEMFCs. [4] Liao et al. have recently shown better ORR activity of Pt/Fe-N-C than Pt/C. [5] With the overriding goal of reducing LT- and HT-PEMFC production costs, Pt/Fe-N-C activity, selectivity and stability with systematically reducing the Pt-content has not been investigated yet. In this study, Pt/Fe-N-C hybrids are synthesized using PMF-011904 from Pajarito Powder (USA) as catalyst support and wet-chemically precipitated Pt nanoparticles with targeted Pt-contents of 40, 5, 1 and 0 wt.%. First, ICP mass spectrometry is used for Pt quantification along all catalysts, and transmission electron microscopy is carried out to investigate morphology and Pt nanoparticle diameter and distribution on the Fe-N-C catalyst. Second, ORR activity and selectivity is investigated by rotating ring disc electrode (RRDE) experiments in 0.1 mol L-1 HClO4, and electrochemical surface area of Pt is calculated by hydrogen underpotential deposition and CO stripping voltammetry. Last, accelerated stress testing (AST) is performed with 5,000 triangle potential cycles between 0.6–1.5 VRHE to evaluate the catalyst stability. Figure 1 shows the initial ORR curves during the RRDE experiments. The mass activities of 40, 5 and 1 wt.% Pt/Fe-N-C determined at 0.9 VRHE are 222.5 ± 41.2 A gPt -1, 170.8 ± 80.4 A gPt -1 and 49.0 ± 8.6 A gPt -1, respectively. Thus, the mass activity depends on Pt-content in the hybrid catalyst strongly. In addition, the other electrochemical characterization results are also going to be discussed in terms of catalyst selectivity, stability and the correlation with morphological aspects during the presentation. Figure 1 ORR curves in O2-saturated 0.1 mol L-1 HClO4 electrolyte with rotation speed of 1,600 rpm and scan rate of 5 mV s-1 (averaged results of three separate electrodes per each catalyst).[1] Y. He, S. Liu, C. Priest, Q.Shi , G. Wu, Chem. Soc. Rev. 2020, 11, 3484–3524.[2] T. Reshetenko, A. Serov, M. Odgaard, G. Randolf, L. Osmieri, A. Kulikovsky, Electrochem. Commun. 2020, 118, 106795.[3] F. Xiao, G.-L. Xu, C.-J. Sun, I. Hwang, M. Xu, H.-W. Wu, Z. Wei, X. Pan, K. Amine, M. Shao, Nano Energy 2020, 77, 10592.[4] A. K. Mechler, N. R. Sahraie, V. Armel, A. Zitolo, M. T. Sougrati, J. N. Schwämmlein, D. J. Jones, F. Jaouen, J. Electrochem. Soc. 2018, 165, F1084.[5] W. Liao, S. Zhou, Z. Wang, F. Liu, H. Pan, T. Xie, Q. Wang, ChemCatChem 2021, 13, 23, 4925-4930. Figure 1

Toward abiotic sugar synthesis from CO2 electrolysis

Toward abiotic sugar synthesis from CO2 electrolysis

Pt Nanoelectrodes Sealed in Quartz Capillaries Modified with Underpotential‐Deposited Bismuth for Formic Acid Electrooxidation

AbstractPlatinum disk nano‐ and micro‐electrodes with radii in the range 40 nm–12.5 μm were fabricated using quartz capillaries and the laser‐puller assisted approach. The hydrogen underpotential deposition (H‐UPD), performed in a 0.5 M H2SO4 aqueous solution, revealed that the nanoelectrodes, with radii of 160 nm and less, displayed extraordinarily large surface areas, which in terms of roughness factors (RFs, i. e., the ratio of the real surface areas to the geometric surface areas) were in the range 1030–3600. This finding was attributed to diffusion of adsorbed species at the Pt/quartz interface along portions of the Pt wires sealed within the glass. RFs between 2 and 3 were instead found at the microelectrodes. Similar results were also obtained with underpotential‐deposited metallic bismuth. In this case, diffusion of Bi adatoms onto the Pt surface was hindered to some extent, providing a RF value of 172. Bi‐modified Pt nano‐ and micro‐electrodes were employed to study the electrooxidation of HCOOH, which is of interest in the field of fuel cells. It was found that the nanoelectrode displayed higher activity towards the electrooxidation of HCOOH and tolerance to CO poisoning, compared to the microelectrode.

Carbon nanotubes decorated with Pt as a viable electrocatalyst system using electrochemical atomic layer deposition

Surface limited redox replacement (SLRR) is an electrochemical deposition method designed to deposit metal thin films, typically onto other metals, that are mere monolayers in thickness, where such low dimensions allows catalyst-support interactions to affect catalyst efficiency. Here we explore the growth of Pt, using iterative Cu-mediated SLRR cycles, directly onto carbon nanotubes (CNTs) which are potentially good candidates as electrocatalyst supports due to their electrical conductivity and chemical resistance. Scanning electron microscope images showed that Pt grew as nanoparticles, and hydrogen underpotential deposition from cyclic voltammetry showed that the active surface area approached an asymptotic value around eight iterations. Catalytic activity, measured using the oxygen reduction reaction, reached a maximum at ten iterations. Both are shown to be influenced by the growth mode, but electrochemical modeling indicates that the trend in activity is also due to a change in activation energy, possibly due to changes in Pt electronic structure due to interactions with the CNTs. Durability cycling showed a greater than 85% retention of surface area for the first 10,000 cycles of accelerated stress testing but decayed steadily to about 50% after 30,000 cycles. This performance may be related to the phenomenon that limits surface area growth.

Balancing CO chemisorption with hydrogen electrochemical adsorption on Pt alloy catalyst for improving direct CO reduction to formaldehyde

Direct CO reduction to formaldehyde (CORTF) provides an alternative, energy-efficient route for production of formaldehyde, a critical chemical commodity as well as promising hydrogen carrier. However, the conventional thermal catalysis approach is challenged with slow reaction kinetics, which can be attributed to imbalanced CO and hydrogen competitive chemisorption to catalyst, with stronger CO adsorption significantly inhibiting hydrogen adsorption that suppresses their reaction towards formaldehyde generation. We report a new, innovative CORTF mechanism to promote the reaction by utilizing hydrogen underpotential deposition to balance the species coverage on catalyst surface. Our study shows that, by separately tuning Pt alloy nanoparticle catalyst composition to control CO chemical adsorption favorability and tuning hydrogen underpotential deposition potential to control hydrogen electrochemical adsorption favorability, the formaldehyde production rate with Pt alloys (866 mg/gcat/h) is 300 times higher compared to conventional thermal catalysis method (2.6 mg/gcat/h) and 50 time higher than previously reported molybdenum phosphide (MoP, 16.3 mg/gcat/h) electrocatalyst. DFT simulations show reduced energy barriers for the rate-limiting steps by means of balancing hydrogen and CO adsorption, thus supporting this new mechanism to achieve more dramatically improved CORTF kinetics. This CORTF mechanism provides a new strategy towards efficient formaldehyde production.

Selective electrocatalytic hydrogenation of α,β-unsaturated ketone on (111)-oriented Pd and Pt electrodes

In this paper, we study the electrocatalytic reduction of methyl vinyl ketone on Pt(111) and Pd-modified Pt(111) electrodes, as well as some of its expected hydrogenation derivatives and isolated functional moieties. The selectivity and Faraday efficiency have been calculated via sampling the catholyte solution after two hours of electrolysis. Furthermore, the adsorbates involved in the deactivation process on both surfaces were studied by means of potential opening experiments and in situ infrared spectroscopy. The Pd-modified Pt(111) electrode is very active (even mass transport limited) for the selective electrochemical hydrogenation of the C=C to 2-butanone, in the hydrogen underpotential deposition potential window (between 0 and 0.2 VRHE), with limited poisoning. Pt(111) is much less active, and poisons rapidly with adsorbed CO. The poison is formed from the C=C bond, not from the C=O moiety, as evidenced by the same poisoning occurring for ethylene. Further hydrogenation to the saturated alcohol happens at more negative potentials, but with 2-butenol as intermediate, not 2-butanone, as the latter species interacts too weakly with the (111) surface.

On the thermodynamics of hydrogen adsorption over Pt(111) in 0.05M NaOH.

The reasons for the sluggish kinetics of the hydrogen adsorption reaction in alkaline media remain a question still to be solved. This information is important to achieve a complete understanding of the mechanistic details that could lead to the production of key catalytic materials necessary for the development of a future hydrogen economy. For a better understanding of this reaction, it is important to acquire information about the thermodynamic parameters characteristic of the different steps in the reaction. Among these, the hydrogen adsorption is a key step in the process of hydrogen evolution. Although some debate still remains about the difference between adsorbed hydrogen in the underpotential deposition (UPD) region and at the overpotential deposition region, there is no doubt that understanding the former can help in the understanding of the latter. Making use of charge density measurements, we report on this paper a thermodynamic study of the hydrogen UPD process on Pt(111) in 0.05M NaOH over the range of temperatures from 283 ≤ T/K ≤ 313. The coulometric features corresponding to HUPD allow for the calculation of the hydrogen coverage and a fit to a Generalized Frumkin isotherm. From these values, different thermodynamic functions for the UPD reaction have been calculated: ΔGads, ΔSads, ΔHads, and the Pt-H bond energy. From extrapolation, a value of ΔSads ◦=-7.5±4Jmol-1K-1 was found, which is very close to 0, much lower than previously reported measurements both in acid and in alkaline solutions. Such value has an effect on the enthalpy and bond energy calculations, the latter having a decreasing tendency with pH and coverage. This tendency is completely different from the acidic systems and implies that the change in the thermodynamic functions due to the formation of the double layer and the reorganization of interfacial water has a strong influence on the process in high pH solutions.

Electrochemical Investigation of Pt/Fe-N-C Catalysts as a Promising Low Pt-Loading Catalyst for Polymer Electrolyte Membrane Fuel Cells

Nowadays polymer electrolyte membrane fuel cells (PEMFC) become commercially established systems used in automobile, stationary and portable power generation industry which provide an improved kinetic, high efficiency, zero emission and high tolerance to the impurities. A standard catalyst used in PEMFC is based on Pt nanoparticles on carbon support materials. However, high cost of this critical raw material stimulates interest in the development of platinum group metals-free electrocatalysts (1). Fe-N-C materials are one of the promising non-precious metal electrocatalysts for these fuel cells which have recently reached outstanding performance in terms of oxygen reduction reaction (ORR) activity (2). However, the volumetric activity and durability of Fe-N-C catalyst is still significantly lower compared to Pt/C in PEMFC mainly due to their lower turnover frequency and active sites density (3).Therefore, in this work, a Black Pearl (BP) based Fe-N-C catalyst is used as ORR active support material for Pt-nanoparticles to investigate synergetic effects of active Pt and Fe-N sites towards stability and volumetric activity. We present a comparative electrochemical characterization of Pt/Fe-N-C and Pt/BP electrocatalysts with loadings of 0.01, 0.1 and 1 wt.% Pt. The measurements were done in terms of rotating ring-disk electrode experiments in 0.1 M HClO4 electrolyte under accelerated stress test (AST) during 5000 cycles in potential range 0.6 – 1.5 V vs RHE in N2-saturated electrolyte, in order to provoke Pt as well as carbon support degradation. The electrochemical surface area (ECSA) of the Pt was determined by hydrogen underpotential deposition (HUPD) and CO stripping methods before and after AST for Pt/BP catalysts. However, for Pt/Fe-N-C the ECSA determination was impossible because for metal-oxide supported Pt catalysts, a straightforward analysis of HUPD and CO methods fails to give meaningful values. The RRDE experiments revealed that ORR mass and specific activities were significantly decreased after AST especially for Pt/BP (Fig. 1) which was attributed to the platinum dissolution as well as carbon support degradation during cycling.1. E. Eren, N. Özkan, Y. Devrim, Int J Hydrogen 2020, 45 (58), 33957-33967.2. Y. He, S. Liu, C. Priest, Q.Shi , G. Wu, Chem. Soc. Rev., 2020, 11, 3484–3524.3. T. Reshetenko, A. Serov, M. Odgaard, G. Randolf, L. Osmieri, A. Kulikovsky, Electrochem Commun 2020, 118, 106795.Figure 1. Cathodic scan of ORR curves of 0.1 wt.% Pt/BP catalyst with 40 µg cm-2 Pt loading in O2-saturated 0.1 M HClO4 at scan rate 5 mV s-1 by 1600 rpm before and after AST. Figure 1

Utilizing Hydrogen Underpotential Deposition for Carbon Monoxide Reduction to Formaldehyde

Formaldehyde is an essential building block for hundreds of chemicals and a promising liquid organic hydrogen carrier (LOHC), yet its indirect energy-intensive synthesis process prohibits it from playing more significant role. Here we report a direct CO reduction to formaldehyde (CORTF) process that hybridizes thermal and electro-catalysis and utilizes hydrogen underpotential deposition property to overcome thermodynamic barrier and scaling relationship restriction. Using molybdenum phosphide as catalyst, formaldehyde can be produced with nearly 100% Faradaic efficiency in aqueous KOH solution, with its formation rate being one order of magnitude higher compared with state-of-the-art thermal catalysis approach. Simultaneous tuning of current density and reaction temperature leads to more selective and productive formaldehyde synthesis, confirming the effectiveness of “hybrid” approach. DFT calculations reveal that desorption of *H2CO intermediate likely serves as rate-limiting step, and the participation of H2O turns the reaction into thermodynamically favorable. Furthermore, a full-cell reaction set-up was demonstrated with CO hydrogenation to HCHO being achieved without any energy input, which shows the spontaneous potential of the reaction. Our study shows the advantage of hybridizing thermal and electro-catalysis in realizing thermodynamics and scaling relation confined reaction, which could serve as a new strategy in future reaction design.

CO2 Electroreduction on Unsupported Pdpt Aerogels: Effects of Alloying and Surface Composition

The prospect of valorizing anthropogenic CO2 emissions by converting this greenhouse gas into added-value products (e.g., CO, formate) through electrochemical conversion in near-neutral aqueous media holds great research interest. While palladium-based catalysts have been repeatedly shown to display a switchable selectivity between CO and formate at high vs. low overpotentials, respectively, a large fraction of the total current is used to convert water into hydrogen as a parasitic byproduct. This is especially true for carbon-supported materials at high overpotentials (i.e., ≤ − 0.7 V vs. the reversible hydrogen electrode), in which the conversion of water to H2 on carbon and of CO2 to CO on Pd occur at comparable rates. Therefore, the removal of this carbon concomitant to the use of unsupported structures that can maintain a high surface area (e.g., as in aerogels) holds the promise of leading to higher selectivities towards CO2 reduction products. Additionally, alloying of metals is a well-researched approach to modify the adsorption kinetics of intermediate species enabling improvement in activity and the tuning of the selectivity towards a certain product. Specifically, alloys of palladium and platinum have been shown to possess high activities for formate generation from CO2 over a wide potential range [1].With this motivation, in this work, unsupported PdPt-aerogels were synthesized in five alloy compositions (from Pd50Pd50 to Pd90Pt10) and as pure metals (Pd100 and Pt100) at a constant web thickness of 6 ± 1 nm using a novel ethanolic synthesis approach [2]. The aerogels’ structure was studied using high-resolution transmission and scanning electron microscopies (HR-TEM, SEM) confirming the homogeneity of the materials. The alloying-degree of the PdPt aerogels was thereafter investigated using ex-situ X-ray absorption spectroscopy (XAS) measurements from which we derived the Cowley short-range order parameter [3], finding all compositions to be random alloys of Pd and Pt with a stoichiometry in good agreement with the Pt:Pd ratios aimed in the synthesis. Furthermore, energy dispersive X-ray spectroscopy (EDX) measurements in line-scan mode were used to investigate the elemental distribution within the structure of the gel (see Fig. 1A-1F), unveiling a slight enrichment of Pt on the surface of the bimetallic PdPt aerogels.These spectroscopic measurements were complemented by electrochemical characterization using CO-stripping as a means to infer the aerogels’ electrochemical surface area (ECSA) and surface composition. While the inferred ECSA values were all ≥70% of what is the expected value based on the aerogels’ web thickness, we observed significant changes in the shape of the hydrogen underpotential deposition (Hupd) that indicate alloy formation on the surface of the aerogels. Additionally, clear trends in the potential position of the CO-oxidation peak as a function of the alloy composition, and in the CO-oxidation charge depending on the CO-adsorption potential (i.e., 0.1 vs. 0.4 V vs. the reversible hydrogen electrode) were found.Lastly, the CO2-reduction activity of these bimetallic PdPt aerogels was tested in our custom, online gas chromatography setup for CO2-electroreduction. Using this approach, the pure palladium aerogel (Pd100) was shown to display Faradaic efficiencies towards CO and formate exceeding 90% at high or low overpotentials, respectively, proving the beneficial effect of the removal of the carbon support on palladium’s CO2 electroreduction selectivity. Meanwhile, the PdPt aerogel alloys featured formate and CO Faradaic efficiencies much lower than those previously reported in Ref. 1, with hydrogen making up the majority of the Faradaic efficiency (see Fig. 1G). Here, the activity towards CO2 electroreduction products was found to decrease with increasing Pt-content, so that on the pure Pt-aerogel (Pt100) hydrogen was the only product. These results are in line with the above-described Pt-enrichment of the alloys’ surfaces, which explains their high activity for the HER reaction and reduced activity for CO2 electroreduction. Figure 1