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Hydrogen peroxide can be produced electrochemically by selective oxygen reduction reaction or selective water oxidation reaction in various electrolytes. A very promising, sustainable, and efficient method to produce hydrogen peroxide is the so-called electrochemical “lab-on-a-chip” technology, where microfluidic electrochemical flow cells can be used. The main advantage of such a system is “on demand” and “on site” production. If these systems are to be commercialized, suitable electrocatalysts (anode and cathode), sensors, and a device design must be developed and interconnected. Such technology could then eventually be deployed in an industrial environment with internal/external numbering-up approach.

Compared to aqueous electrolytes, the fundamental understanding of the chemical and electrochemical processes occurring in non-aqueous electrolytes in general is far less developed. This is no different for Li-ion battery (LiB) electrolytes, where many questions regarding the solid electrolyte interphase (SEI) on the anode side remain unanswered, including its chemical composition, the mechanism of formation and the impact on LiB performance. Here, we present a detailed experimental and theoretical study of the electrochemistry of ethylene carbonate (EC) and its chemical relationship with trace amounts of water and HF across a vast range of electrode materials, from well-ordered single crystals to realistic graphite electrodes. We reveal the electrocatalytic nature of EC, HF and water electroreduction at all interfaces. Moreover, we show that these reactions are connected in a closed cycle by chemical reactions, that take place either at the interface or in the bulk of the electrolyte. For the first time, we unveil the catalytic role of water in EC electroreduction and demonstrate that the composition of the SEI depends predominantly on the balance between the (electro)chemistry of EC, water and HF.

Perovskite oxides are an important class of oxygen evolution reaction (OER) catalysts in alkaline media, despite the elusive nature of their active sites. Here, we demonstrate that the origin of the OER activity in a La1-xSrxCoO3 model perovskite arises from a thin surface layer of Co hydr(oxy)oxide (CoOxHy) that interacts with trace-level Fe species present in the electrolyte, creating dynamically stable active sites. Generation of the hydr(oxy)oxide layer is a consequence of a surface evolution process driven by the A-site dissolution and O-vacancy creation. In turn, this imparts a 10-fold improvement in stability against Co dissolution and a 3-fold increase in the activity-stability factor for CoOxHy/LSCO when compared to nanoscale Co-hydr(oxy)oxides clusters. Our results suggest new design rules for active and stable perovskite oxide-based OER materials.

Development of high-performance hydrogen oxidation catalysts with ultralow precious metal loading is critical to the development of next-generation anion-exchange membrane fuel cells. Here, a novel Ru-rich Pt–RuO2 heterojunction catalyst was synthesized via a solvothermal process followed by thermal treatment. The Pt–RuO2 catalyst has ultrafine Pt clusters and a heterojunction interface between Pt and RuO2, which facilitates high hydrogen oxidation activity while minimizing adverse adsorption of the phenyl group in the polymer electrolyte. The performance of a membrane electrode assembly employing the Pt–RuO2/C reached a peak power density of 0.77 W cm−2 with an anode Pt loading of 25 μgPt cm−2, achieving a specific power of 31 W mgPt−1 under H2/O2 conditions. The combined analysis of electrode performance and cost indicates that Pt–RuO2/C is one of the most promising catalysts that is approaching the U.S. DOE 2020 performance and cost targets for transportation applications.

Ability to utilize structure-function relationship in the design of electrocatalysts relies on fundamental understanding of physicochemical processes at the topmost surface. It has been demonstrated that reaction rate for majority of electrochemical reactions exhibit notable dependence on geometric arrangement of surface atoms due to varying levels of binding strength between different atomic structures and adsorbate molecules. Control of surface structure at nanoscale is being investigated here through a thin film approach. The surface characteristics of electrochemically deposited Pt thin films were probed for electrooxidation of small organic molecules such as formic acid and methanol. The low-index Pt single crystal electrodes were used for reactivity benchmarking which later guided thermal annealing processes, and ultimately, fine tuning of the ratio between (1 1 1) and (1 0 0) surface facets on Pt thin films. The tailored thin film structure is responsible for significant activation of both formic acid and methanol oxidation reactions, which justifies the approach.

In order to probe the hydroxyl (OHad) adsorption energy as one of the possible descriptors for hydrogen evolution reaction (HER) in alkaline media, we performed studies on polycrystalline copper (Cu(Poly)), a well-known poor catalyst for HER. By employing three different preparation methods for Cu(Poly), we were able to establish links between surface oxidation extent, oxophilicity, hydrophilicity and their catalytic activity for the HER. Ultimately, we propose that the formation of Cu(I)-OHad‐‐‐OH2 activated complex is essential for lowering the energy barrier for water dissociation, which leads to higher activity for the HER in alkaline media. Keywords: Hydrogen evolution reaction, Electrocatalysis, Surface science, Copper, Structure-function relationships

The feasible commercialization of alkaline, phosphoric acid and polymer electrolyte membrane fuel cells depends on the development of oxygen reduction reaction (ORR) electrocatalysts with improved activity, stability, and selectivity. The rational design of surfaces to ensure these improved ORR catalytic requirements relies on the so-called "descriptors" (e.g., the role of covalent and noncovalent interactions on platinum surface active sites for ORR). Here, we demonstrate that through the molecular adsorption of melamine onto the Pt(111) surface [Pt(111)-Mad], the activity can be improved by a factor of 20 compared to bare Pt(111) for the ORR in a strongly adsorbing sulfuric acid solution. The Mad moieties act as "surface-blocking bodies," selectively hindering the adsorption of (bi)sulfate anions (well-known poisoning spectator of the Pt(111) active sites) while the ORR is unhindered. This modified surface is further demonstrated to exhibit improved chemical stability relative to Pt(111) patterned with cyanide species (CNad), previously shown by our group to have a similar ORR activity increase compared to bare Pt(111) in a sulfuric acid electrolyte, with Pt(111)-Mad retaining a greater than ninefold higher ORR activity relative to bare Pt(111) after extensive potential cycling as compared to a greater than threefold higher activity retained on a CNad-covered Pt(111) surface. We suggest that the higher stability of the Pt(111)-Mad interface stems from melamine's ability to form intermolecular hydrogen bonds, which effectively turns the melamine molecules into larger macromolecular entities with multiple anchoring sites and thus more difficult to remove.

Developing next-generation battery chemistries that move beyond traditional Li-ion systems is critical to enabling transformative advances in electrified transportation and grid-level energy storage. In this work, we provide the first evidence for common descriptors for improved reversibility of metal plating/stripping in nonaqueous electrolytes for multivalent ion batteries. Focusing first on the specific role of chloride (Cl-) in promoting electrochemical reversibility in multivalent systems, rotating disk (RDE) and ring-disk electrode (RRDE) investigations were performed utilizing a variety of divalent cations (Mg2+, Zn2+, and Cu2+) and the bis-(trifluoromethane sulfonyl) imide (TFSI-) anion. By introducing varying concentrations of Cl-, a cooperative effect is observed between TFSI- and Cl- that yields the more reversible behavior of mixed electrolytes relative to electrolytes containing only TFSI-. This effect is shown to be general for Mg, Zn, and Cu electrodeposition, and mechanistic understanding of the role of Cl- in improving reversibility of TFSI-based electrolytes is obtained through the combination of R(R)DE experimental results and density functional theory (DFT) evaluation of the redox activity and thermodynamic stability of various TFSI- and Cl-based solution complexes of metal ions. The cooperative anion effect is further generalized to other mixed-anion systems, where systematic variations in anion association strength predicted from DFT (i.e., Cl- > OTf- ≈ TFSI- > BF4- > PF6-) yield corresponding trends in redox potentials and improvements of ≥200 mV in the reversibility of metal deposition/dissolution. These results identify anion association strength as a common descriptor for the reversibility of divalent metal anodes and suggest a set of general design principles for developing new electrolytes with improved activity and stability.

A remaining challenge for the deployment of proton-exchange membrane fuel cells is the limited durability of platinum (Pt) nanoscale materials that operate at high voltages during the cathodic oxygen reduction reaction. In this work, atomic-scale insight into well-defined single-crystalline, thin-film and nanoscale surfaces exposed Pt dissolution trends that governed the design and synthesis of durable materials. A newly defined metric, intrinsic dissolution, is essential to understanding the correlation between the measured Pt loss, surface structure, size and ratio of Pt nanoparticles in a carbon (C) support. It was found that the utilization of a gold (Au) underlayer promotes ordering of Pt surface atoms towards a (111) structure, whereas Au on the surface selectively protects low-coordinated Pt sites. This mitigation strategy was applied towards 3 nm Pt3Au/C nanoparticles and resulted in the elimination of Pt dissolution in the liquid electrolyte, which included a 30-fold durability improvement versus 3 nm Pt/C over an extended potential range up to 1.2 V. Deployment of proton-exchange membrane fuel cells is limited by the durability of Pt-nanoscale catalysts during cathodic oxygen reduction reactions. Dissolution processes on single crystalline and thin film surfaces are now correlated leading to the design of PtAu catalysts with suppressed dissolution.

By combining idealized experiments with realistic quantum mechanical simulations of an interface, we investigate electro-reduction reactions of HF, water and methanesulfonic acid (MSA) on the single crystal (111) facets of Au, Pt, Ir and Cu in organic aprotic electrolytes, 1 M LiPF6 in EC/EMC 3:7W (LP57), the aprotic electrolyte commonly used in Li-ion batteries, 1 M LiClO4 in EC/EMC 3:7W and 0.2 M TBAPF6 in 3 : 7 EC/EMC. In our previous work, we have established that LiF formation, accompanied by H2 evolution, is caused by a reduction of HF impurities and requires the presence of Li at the interface, which catalyzes the HF dissociation. In the present paper, we find that the measured potential of the electrochemical response for these reduction reactions correlates with the work function of the electrode surfaces and that the work function determines the potential for Li+ adsorption. The reaction path is investigated further by electrochemical simulations suggesting that the overpotential of the reaction is related to stabilizing the active structure of the interface having adsorbed Li+. Li+ is needed to facilitate the dissociation of HF which is the source of protons. Further experiments on other proton sources, water and methanesulfonic acid, show that if the hydrogen evolution involves negatively charged intermediates, F− or HO−, a cation at the interface can stabilize them and facilitate the reaction kinetics. When the proton source is already significantly dissociated (in the case of a strong acid), there is no negatively charged intermediate and thus the hydrogen evolution can proceed at much lower overpotentials. This reveals a situation where the overpotential for electrocatalysis is related to stabilizing the active structure of the interface, facilitating the reaction rather than providing the reaction energy., By combining idealized experiments with realistic quantum mechanical simulations of an interface, we investigate electroreduction reactions of HF, water and methanesulfonic acid on the single crystal (111) facets of Au, Pt, Ir and Cu in a variety of aprotic electrolytes.

The poor activity and stability of electrode materials for the oxygen evolution reaction are the main bottlenecks in the water-splitting reaction for H2 production. Here, by studying the activity–stability trends for the oxygen evolution reaction on conductive M1OxHy, Fe–M1OxHy and Fe–M1M2OxHy hydr(oxy)oxide clusters (M1 = Ni, Co, Fe; M2 = Mn, Co, Cu), we show that balancing the rates of Fe dissolution and redeposition over a MOxHy host establishes dynamically stable Fe active sites. Together with tuning the Fe content of the electrolyte, the strong interaction of Fe with the MOxHy host is the key to controlling the average number of Fe active sites present at the solid/liquid interface. We suggest that the Fe–M adsorption energy can therefore serve as a reaction descriptor that unifies oxygen evolution reaction catalysis on 3d transition-metal hydr(oxy)oxides in alkaline media. Thus, the introduction of dynamically stable active sites extends the design rules for creating active and stable interfaces. Understanding what underpins the activity and stability of oxygen evolution catalysts is an ongoing issue in the field of water splitting. Now, researchers show that balancing the rate of Fe dissolution and deposition over a metal hydr(oxy)oxide host yields dynamically stable Fe active sites, with the Fe–host interaction key to the performance.

Understanding and controlling electrochemical interfaces at atomic and molecular levels have transformed electrochemistry into a science with clearly defined fundamental principles leading to significant impact on various electrochemical systems and devices. Although the principles guiding the activity of electrochemical reactions are quite well established, the driving forces that control stability are still poorly understood. Here we utilize in situ monitoring of the early stages of Pt dissolution using the stationary probe rotating disk electrode technique coupled to inductively coupled plasma mass spectrometry (SPRDE-ICPMS). Our unique SPRDE-ICPMS method provides picogram sensitivity levels that, in combination with STM, provide otherwise inaccessible information about the dissolution and redeposition of Pt(111) in acidic environments. We propose two distinct dissolution mechanisms that are active during oxide formation and subsequent oxide reduction. Whereas an electrochemical dissolution mechanism is observed during anodic Pt dissolution (Pt → Pt2 + + 2e−), a combination of electrochemical (PtO + 2H+ + 2e−→Pt0 + H2O) and chemical (PtO∗ + 2H+→Pt2 + + H2O) steps control the dissolution of Pt during the cathodic scan. The redeposition of Pt (Pt2 + + 2e−→Pt) observed on the cathodic scan is controlled by a delicate balance between the diffusion of Pt2 + from the double layer and redeposition of Pt2 + on Pt oxide-free sites.

The formation of solid electrolyte interphase on graphite anodes plays a key role in the efficiency of Li-ion batteries. However, to date, fundamental understanding of the formation of LiF as one of the main solid electrolyte interphase components in hexafluorophosphate-based electrolytes remains elusive. Here, we present experimental and theoretical evidence that LiF formation is an electrocatalytic process that is controlled by the electrochemical transformation of HF impurity to LiF and H2. Although the kinetics of HF dissociation and the concomitant production of LiF and H2 is dependent on the structure and nature of surface atoms, the underlying electrochemistry is the same. The morphology, and thus the role, of the LiF formed is strongly dependent on the nature of the substrate and HF inventory, leading to either complete or partial passivation of the interface. Our finding is of general importance and may lead to new opportunities for the improvement of existing, and design of new, Li-ion technologies.

p { margin-bottom: 0.1in; direction: ltr; color: rgb(0, 0, 10); line-height: 120%; text-align: left; }p.western { font-size: 12pt; }p.cjk { font-size: 12pt; }a:link { color: rgb(5, 99, 193); } By combining idealized experiments with realistic quantum mechanical simulations of the interface, we investigate electro-reduction reactions of HF and water impurities on the single crystal (111) facets of Au, Pt, Ir and Cu in an organic aprotic electrolyte, 1M LiPF6 in EC/EMC 3:7w (LP57), which are common reactions happening during the formation of the SEI on graphite. In our previous work, we have established that the LiF formation, accompanied with H2 evolution, is caused by a reduction of HF impurities and requires the presence of Li at the interface, which catalyzes the HF dissociation. In the present paper, we find that the measured potential of the electrochemical response for these reduction reactions correlates with the work function of the electrode surfaces and that the work function determines the potential for Li+ adsorption. The reaction path is investigated further by electrochemical simulations suggesting that the overpotential of the reaction is related to stabilizing the active structure of the interface having Li+ adsorbed. The Li+ is needed to facilitate the dissociation of HF which is the source of proton. Further experiments on the other proton sources, water and methanesulfonic acid, show that if the hydrogen evolution involves negatively charged intermediates, F- or HO-, a cation at the interface can stabilize them and facilitate the reaction kinetics. When the proton source is already significantly dissociated (in the case of a strong acid), there is no negatively charged intermediate and thus the hydrogen evolution can proceed at much lower overpotentials. This reveals a situation where the overpotential for electrocatalysis is related to stabilizing the active structure of the interface, facilitating the reaction rather than providing the reaction energy. This has implications for the SEI layer formation in Li-ion batteries and for reduction reactions in alkaline environment as well as for design principles for better electrodes.

A significant challenge facing the development of new chemistries for multivalent batteries is the fact that many promising electrolytes are incompatible with either metallic anodes or high voltage cathodes, both of which are required in order for such systems to compete with existing Li-ion batteries. Furthermore, the specific species responsible for delivering ions to the active electrode are generally not known, as solvation structure may change significantly relative to the bulk as solution complexes approach the electrified interface. Many Mg electrolytes in particular rely on the presence of significant concentrations of Cl- to enable reversible metal plating/stripping at the anode, which preclude their use with oxide cathodes due to parasitic reactions that take place at high voltage. Beyond this limitation, however, it is also unclear what the specific role of Cl- is in promoting reversibility in Mg and other multivalent systems, making it challenging to develop general design rules for synthesizing new electrolytes that can avoid the use of Cl- altogether. We have recently revealed that a cooperative effect between Cl- and bis-(trifluoromethane sulfonyl) imide (TFSI-) anions yields the overall more reversible behavior of mixed TFSI-/Cl- electrolytes relative to electrolytes containing only TFSI- – an effect that is general across multiple multivalent systems (i.e., Mg, Zn and Cu). Mechanistic understanding of the role of Cl- in improving reversibility of TFSI-based electrolytes is provided through the combination of electrochemical measurments with computational evaluation of the activity and stability of various TFSI- and Cl-based solution complexes. This cooperative anion effect is generalizable to dual anion electrolytes without Cl- as well, with the strength of anion association to the working cation systematically modifying the electrochemical response in direct agreement with trends predicted by theory. These results further indicate that interfacial speciation indeed deviates from that expected in the bulk, as even nominally weakly-coordinating anions appear to participate strongly in the plating/stripping reactions. Overall, this work points to the existence of a common descriptor for divalent metal plating/stripping – anion association strength – and suggests a set of general design principles for developing new multivalent electrolytes.

In the past decade, polymer electrolyte membrane fuels (PEMFCs) have been evaluated for both automotive and stationary applications. One of the main obstacles for large scale commercialization of this technology is related to the sluggish oxygen reduction reaction that takes place on the cathode side of fuel cell. Consequently, ongoing research efforts are focused on the design of cathode materials that could improve the kinetics and durability. Majority of these efforts rely on novel synthetic approaches that provide control over the structure, size, shape and composition of catalytically active materials. This article highlights the most recent advances that have been made to tailor critical parameters of the nanoscale materials in order to achieve more efficient performance of the oxygen reduction reaction (ORR).

Developing a new generation of battery chemistries is a critical challenge to moving beyond current Li-ion technologies. In this work, we introduce a surface-science-based approach for understanding the complex phenomena controlling the reversibility of Mg anodes for Mg-ion batteries. We identify the profound impact of trace levels of H2O (≤3 ppm) on the kinetics of Mg deposition and determine that passive films of MgO and Mg(OH)2 are formed only after Mg deposition ceases, rather than continuously during Mg reduction. We also find that Cl– inhibits passivation through the formation of adsorbed Cl– (Mg–Cl(ad)) and/or MgCl2 on the surface, as well as through a dynamic competition with H2O in the double layer. This surface-science-based approach goes well beyond Mg anodes, highlighting the need for more in-depth understanding of electrolyte chemistries before a new generation of efficient and reversible battery technologies can be realized.

The development of alternative energy systems for the clean production, storage, and conversion of energy is strongly dependent on our ability to understand, at atomic molecular levels, the functional links between the activity and stability of electrochemical interfaces. Whereas structure–activity relationships are rapidly evolving, the corresponding structure–stability relationships are still missing. This is primarily because there is no adequate experimental approach capable of monitoring the stability of well-defined single crystals in situ. Here, by utilizing the power of inductively coupled plasma mass spectrometry (ICP-MS) connected to a stationary probe and coupling this technique to the rotating disk electrode method, it was possible to simultaneously measure the dissolution rates of surface atoms (as low as 0.4 pg cm–2 s–1) and correlate them with the kinetic rates of electrochemical reactions in real time. Making use of this unique probe, it was possible to establish almost “atom by atom” stru...

Oxygen reduction and ethanol oxidation reactions were studied on Au(1 1 1), Pt(1 1 1) and Ir(1 1 1) in alkaline solutions containing sodium and/or lithium cations. By keeping the same (1 1 1) surface orientation and exploring oxophilicity trends and non-covalent interactions between OHad and alkali metal cations (AMCn+), we were able to gain deep insights into the multiple roles that OHad plays in these important electrocatalytic reactions. Cyclic voltammetry experiments revealed that OHad formation initiates at distinct electrode potentials, governed by the oxophilicity of the specific metal surface, with further OHad adlayer stabilization by non-covalent alkali-cation interactions and affecting the formation of a “true oxide” layer at higher electrode potentials. Although OHad is a simple spectator for the ORR, it promotes the ethanol oxidation reaction (EOR) at lower potentials and act as spectator at high OHad coverages. By changing the alkali metal cation at the interface (Li+) on more oxophilic surfaces, it was possible to promote the EOR even more, relative to Na+, without changing the product distribution for the reaction. This cation effect suggests that OHad–Li+(H2O)x clusters can stabilize the ethoxide adlayer, thus improving the EOR activity. Our results indicate the importance of the entire electrochemical interface in determining the electrocatalytic activity during reaction.

In spite of large improvements in RDE, shaped nanoparticles do not exhibit substantial improvement in electrochemical devices, which requiresadditional steps in research aimed to development of more active nanoscale materials. The effect of nanoparticle size, surface mophology, composition and near-surface composition, shape, architecture, nature and porosity of the support will be thoroughly addressed. The main emphasis will be placed on the atomic precision approach that offers a new direction in the catalyst development. While hollow structures greatly diminish buried non-functional precious metal atoms, their uncommon geometry provides a pathway for tailoring physical and chemical properties. These structures can address some of the major design criteria for advanced nanoscale electrocatalysts, namely, high surface-to-volume ratio, 3D surface molecular accessibility, and optimal precious metal utilization, including durability and selectivity.

A significant challenge facing the development of new chemistries for multivalent batteries is the fact that many of the most promising electrolytes are incompatible with either metallic anodes or high voltage cathodes, both of which are required in order for such systems to compete with existing Li-ion batteries. For example, many Mg electrolytes rely on the presence of significant concentrations of Cl- to enable reversible metal plating/stripping at the anode, which is incompatible with oxide cathode materials due to parasitic reactions (e.g. oxide dissolution, Cl2 gas evolution) that take place at high voltage. More broadly speaking, investigations of multivalent anode chemistry are generally focused on a single working cation, making it difficult to identify and understand trends that may extend beyond individual systems and point to general design principles for developing new multivalent electrolyte chemistries. Focusing on the specific role of Cl- in promoting reversibility in multivalent systems, systematic rotating disk and ring-disk electrode (RDE and RRDE) investigations of metal plating/stripping were performed utilizing a variety of divalent cations (i.e., Mg2+, Zn2+ and Cu2+) and the bis-(trifluoromethane sulfonyl) imide (TFSI-) anion. By introducing varying concentrations of Cl-, a cooperative effect is observed between TFSI- and Cl- that yields the overall more reversible behavior of mixed electrolytes relative to electrolytes containing only TFSI-. This effect is shown to be general for Mg, Zn and Cu electrodeposition, and mechanistic understanding of the role of Cl- in improving reversibility of TFSI-based electrolytes is provided through the combination of R(R)DE experimental results with computational evaluation of the activity and stability of various TFSI- and Cl-based solution complexes. The cooperative anion effect can be further generalized to other electrolyte systems, pointing to the presence of common descriptors for the activity of divalent metal anodes and suggesting a set of general design principles for developing new multivalent electrolytes.

Surface electrochemistry aimed towards materials with advanced catalytic processes will be thoroughly discussed in a view of footprint, contributions and approach that was pursued by Prof. Andrzej Wieckowski. A number of well-defined single crystalline surfaces will be brought to the audience to recall past time when many groups were focused on CO electrochemistry and design of new real world catalysts that would meet strict CO-tolerant requirements. Andrzej was pioneering a difficult pathway in executing the most complex in-situ measurements and he was always challenging others in order to push the field towards new frontiers. That time created a number of highly visible articles and research groups were competing who would be the first in proposing the new reaction mechanism, novel material or to dispute work of others who made questionable claims. That time is known as golden era of electrochemistry, where scientific knowledge did not have alternative in front highly critical audience. This field is missing those times, and for that reason, this symposium is an outstanding opportunity to recall past, and to remember Prof. Wieckowski as a pillar who was setting the new limits.

The future of high-voltage rechargeable batteries is closely tied to the fundamental understanding of the processes that lead to the potential-dependent degradation of electrode materials and organic electrolytes. To date, however, there have been no methods able to provide quantitative, in situ and in real time information about the electrode dissolution kinetics and concomitant electrolyte decomposition during charge/discharge. We describe the development of such a method, which is of both fundamental and technological significance. Our novel approach enables simultaneous and independent measurements of transition-metal cation dissolution rates from different oxide hosts (Co3+/4+ or Cr3+/4+), deintercalation kinetics of working cations (Mg2+), and the relative rate of electrolyte decomposition.

Made available in DSpace on 2018-12-11T16:39:00Z (GMT). No. of bitstreams: 0 Previous issue date: 2015-08-12 U.S. Department of Energy Abstract We use the rotating ring disk (RRDE) method to study activity-selectivity relationships for the oxygen reduction reaction (ORR) on Pt(111) modified by various surface coverages of adsorbed CNad (ΘCNad). The results demonstrate that small variations in ΘCNad have dramatic effect on the ORR activity and peroxide production, resulting in volcano-like dependence with an optimal surface coverage of ΘCNad = 0.3 ML. These relationships can be simply explained by balancing electronic and ensemble effects of co-adsorbed CNad and adsorbed spectator species from the supporting electrolytes, without the need for intermediate adsorption energy arguments. Although this study has focused on the Pt(111)-CNad/H2SO4 interface, the results and insight gained here are invaluable for controlling another dimension in the properties of electrochemical interfaces. Materials Science Division, Argonne National Laboratory Faculdade de Engenharia, UNESP - Univ. Estadual Paulista Instituto de Quimica de Sao Carlos, Universidade de Sao Paulo Faculdade de Engenharia, UNESP - Univ. Estadual Paulista U.S. Department of Energy: DE-AC02-06CH11357

Water and oxygen electrochemistry lies at the heart of interfacial processes controlling energy transformations in fuel cells, electrolyzers, and batteries. Here, by comparing results for the ORR obtained in alkaline aqueous media to those obtained in ultradry organic electrolytes with known amounts of H2O added intentionally, we propose a new rationale in which water itself plays an important role in determining the reaction kinetics. This effect derives from the formation of HOad···H2O (aqueous solutions) and LiO2···H2O (organic solvents) complexes that place water in a configurationally favorable position for proton transfer to weakly adsorbed intermediates. We also find that, even at low concentrations (

Improvements in the fundamental understanding of electrocatalysis have started to revolutionize the development of electrochemical interfaces for the efficient conversion of chemical energy into electricity, as well as for the utilization of electrons to produce new chemicals that then can be re-used in energy conversion systems. Here, some facets of the role of trace level of impurities (from 10−7 to 10−6 M) in electrocatalysis of the oxygen reduction reaction, hydrogen oxidation and evolution reactions, and CO oxidation reactions are explored on well-characterized platinum single crystal surfaces and high surface area materials in alkaline and acidic environments. Of particular interest is the effect of anions (e.g., Cl−, $$ {\text{NO}}_{ 3}^{ - } $$ ) and cations (i.e., Cu2+) present in the supporting electrolytes as well as surface defects (i.e., ad-islands) that are present on metal surfaces. The examples presented are chosen to demonstrate that a small level of impurities may play a crucial role in governing the reactivity of electrochemical interfaces.

The role of non-covalent interaction between both non-specifically adsorbed hydrated K+, Li+ and Ba2+ cations (M+) and hydroxyl species (M+-OHad) as well as underpotentially deposited Cu adatoms and OHad (Cuupd-OHad) on bulk CO oxidation (COb) is investigated on Pt(111) and Pt(100) in alkaline solutions. The effect of Cuupd-OHad on COb oxidation is observed almost at the reversible potential for CO oxidation, signaling that Cuupd-induced adsorption of OHad activates COb oxidation at very low overpotentials. In contrast, the effect of M+-OHad was found to be strongly dependent on the surface coverage of OHad on Pt; e.g., while no effect is found when the surface is pre-covered by adsorbed CO (COad), a discernable effect is observed on OHad pre-covered Pt surface.

The methods used to improve catalytic activity are well-established, however elucidating the factors that simultaneously control activity and stability is still lacking, especially for oxygen evolution reaction (OER) catalysts. Here, by studying fundamental links between the activity and stability of well-characterized monometallic and bimetallic oxides, we found that there is generally an inverse relationship between activity and stability. To overcome this limitation, we developed a new synthesis strategy that is based on tuning the near-surface composition of Ru and Ir elements by surface segregation, thereby resulting in the formation of a nanosegregated domain that balances the stability and activity of surface atoms. We demonstrate that a Ru0.5Ir0.5 alloy synthesized by using this method exhibits four-times higher stability than the best Ru-Ir oxygen evolution reaction materials, while still preserving the same activity.

Design of active and stable Pt-based nanoscale electrocatalysts for the oxygen reduction reaction (ORR) will be the key to improving the efficiency of fuel cells that are needed to deliver reliable, affordable and environmentally friendly energy. Here, by exploring the ORR on Pt single crystals, cubo-octahedral (polyhedral) Pt NPs with different sizes (ranging from 2 to 7 nm), and 7–8 nm Pt NPs with different shapes (cubo-octahedral vs. cube vs. octahedral), we presented a surface science approach capable of rationalizing, and ultimately understanding, fundamental relationships between stability of Pt NPs and activity of the ORR in acidic media. By exploring the potential induced dissolution/re-deposition of Pt between 0.05 and 1.3 V, we found that concomitant variations in morphology of Pt(111) and Pt(100) lead to narrowing differences in activity between Pt single crystal surfaces. We also found that regardless of an initial size or shape, NPs are metastable and easily evolve to thermodynamically equilibrated shape and size with very similar activity for the ORR. We concluded that while initially clearly observed, the particle size and shape effects diminish as the particles age to the point that it may appear that the ORR depends neither on the particle size nor particle shape.

The design of catalysts for Polymer Electrolyte Membrane Fuel Cells is guided by two important principles, improving specific catalytic activity and increasing lifetime [1]. Over the past 3 decades, there was a tremendous effort to understand at the atomic and molecular levels the key parameters that control the catalytic activity of the interface for the oxygen reduction reaction (ORR), the limiting reaction for high performance devices. Understanding the functional links between structure-activity relationships from Pt single crystalline surfaces to PtNi alloy single crystals established the importance of controlling structure, composition in both bulk and particularly at the surface, leading to real catalytic reactivity enhancement of almost 2 orders of magnitude [2]. These studies paved the way to synthesis of advanced nanostructures with well-defined parameters such as particle size, distribution, and alloy composition, culminating with PtNi nanoframes, displaying one of the highest specific activities measured for the ORR [3]. While there are room for improvement with increasing precision in nanostructure synthesis, the corresponding understanding of durability of Pt-based is far less developed. One of the limitations is that measuring durability has always been linked to a particular experimental protocol, leading to the development of accelerated stress tests (AST). These protocols measure the changes in surface area, as well as catalytic activity as a function of cycle number, but provide very little information about the mechanism of degradation. Transmission Electron Microscopy helped shed light on particle size evolution as a function of cycle number, where small particles (ca. 3nm) often coalesce and grow to large sizes, ranging from 5 to 7nm [4]. Carbon corrosion can also contribute to Pt degradation especially under start-stop conditions. However, only recently after the development of an in situ that is sensitive enough to measure rates of Pt dissolution as a function of electrode potential we were able to begin establishing structure-stability relationships that goes beyond simple changes to surface area. In this talk, we will present how the stability trends observed from well-defined single crystal Pt surfaces can help understand durability of nanoparticle systems. First, we reveal how the mechanism of Pt-oxide induced dissolution is highly sensitive on surface structure, where dissolution from (110) surface is almost 10 times higher than on (111) surfaces [5]. The effect anions and other species present at the interface is also relevant for stability of Pt surface atoms. Beyond Pt dissolution, we discuss the effects of redeposition processes that can occur after fast changes in electrode potential, leading to a clear pathway to understand the kinetics of nanoparticle evolution. Further comparison between the rates of Pt dissolution measured from nanoparticles to extended surfaces (single crystal and thin films), helped elucidate the importance of controlling particle size, distribution and support loading. Overall, establishing the functional link between structure-stability can provide a deeper understanding on how to control the durability of fuel cell catalysts, together with advances in catalytic activity that can bring fuel cell technology one step closer to a cost–effective widespread commercialization. [1] Stamekovic, V.; Strmcnik, D.; Lopes, P. P.; Markovic, N. M., Nature Materials, 16, 57-69, 2017 [2] Stamenkovic, V. et al., Science 315, 493-497, 2007 [3] Chen, C. et al., Science 343, 1339-1343, 2014 [4] Borup, R. et al., Chemical Reviews, 107, 3904-3951, 2007 [5] Lopes, P. P. et al., ACS Catalysis, 6, 2536-2544, 2016

Hydrogen evolution reaction (HER) has been at the core of the development of fundamentals of electrochemistry and electrocatalysis. In acidic electrolytes, it has been generally accepted that the differences in reaction rates/activity are closely related to the differences in hydrogen adsorption free energy on different materials. In recent years, the fundamental investigation of the HER has been enjoying a renewed vigor, but this time in alkaline electrolytes, where there is much less consensus on the underlying activity descriptors. While some groups insist on the single descriptor of hydrogen binding energy, our group has continuously pointed out to the bifunctional nature of the HER in alkaline environments, i.e. a good catalyst needs both a beneficial OHad energetics and beneficial Had energetics. In most cases, the OHad-metal energetics is by far the more unfavorable of the two and OHad adsorption therefore represents the rate determining step. In some cases (e.g. polycrystalline iridium), the OHad-metal energetics no longer represents a significant obstacle in the HER process; hence the catalyst's rate determining step becomes Had adsorption and the catalyst starts behaving as it would in acid – as a pseudo mono-functional catalyst. For most materials, however, the HER rates in alkaline environments are significantly lower than in acid. One such example is copper (Cu), where reported values for HER activity in alkaline are indeed lower but are interestingly scattered over a wide range of values. In a manuscript recently published [1], we were interested in the underlying cause of these discrepancies. We explore how the different history the polycrystalline copper surfaces experienced changes their electrochemical properties (e.g. the HER activity). By using detailed spectroscopic surface analysis, we were able to find different extents of oxidation on various copper surfaces and link them to different hydrophilicity of the surfaces. With the established trends we propose that the formation of Cu(I)-OHad‐‐‐OH2 activated complex is essential in lowering the energy barrier for water dissociation, which leads to higher activity for the HER in alkali environments. [1] Farinazzo Bergamo Dias Martins, P. et al. Hydrogen evolution reaction on copper: Promoting water dissociation by tuning the surface oxophilicity. Electrochem. commun. 100, 30-33 (2019).

Understanding of processes that are behind the mechanism for electrochemical evolution of hydrogen and oxygen from water is considered critical in for further advancement of materials that could be implemented in electrolyzers. Atomic scale insight at well-defined interfaces provides invaluable guidance to overcome limitations in operating efficiency of these devices. All of that points towards the need for development of more active, durable and cost effective oxygen evolution and hydrogen evolution electrocatalysts. Metal oxides are emerging as promising group of materials that could deliver desirable performance. While majority of research is placed on the catalyst design and synthesis aiming to improve their efficiency, not too much was done to resolve and understand the role of liquid phase including trace level of impurities or dopants. Molecular species from the electrolyte and the nature of their interaction with the catalyst surface will also be thoroughly discussed. The knowledge acquired from two-dimensional surfaces has been used to create real-world catalysts with superior properties.

Due to the vigorous demanding of renewable energy, it is important to develop efficient electrocatalyst with high stability and activity .The importance of Water electrolysis, energy storage concept from electricity to hydrogen and oxygen, is highly accentuated to achieve hydrogen-based energy cycles. In oxygen evolution reaction (OER), which is anode reaction of water electrolysis, low stability and sluggish kinetics are considered as major problems. Various candidates have recently been proposed including transition metal analogues (oxide, hydroxide, phosphide and nitride, etc.), however in-detail analysis of reaction mechanism and activity-stability relations are still under debate. In this talk, we will discuss about activity-stability trend of OER catalyst and suggest which factor is important for designing durable and active electrocatalyst.

Dihydrogen, the chemical fuel with the highest specific energy, is the ideal energy carrier for our transition to renewable energy society. The successful implementation of the so-called "Hydrogen Economy" relies on cost-effective, high performance production of H2 in water electrolyzers and its energy efficient consumption in fuel cells. While fuel cell technology have developed tremendously over the past 20 years, with fuel cell automobiles commercially available nowadays, such as the Toyota Mirai and the Honda Clarity, electrolyzer technology is still heavily based on cell and stack designs dating back 40 years ago. This proven H2 technology does not provide a cost effective solution for large scale deployment due extensive use of noble metals such as Ir and Pt that are required to drive both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), respectively. Together with kinetic limitations, the challenges facing water electrolyzers also include poor electrode stability and losses associated with limited electronic conductivity of metal oxides in the anodes. Overall, then, the design of materials for water electrolysis requires understanding the at atomic scale what are the fundamental limitations behind catalytic activity for both OER and HER, how does activity and stability are interconnected, and based on this knowledge, how can we effectively design new electrochemical interfaces and tailored 3D electrode morphology that will be at the core of the next generation water electrolyzer technology.

The progress in platinum-based catalysts with the goal of improving their activity and durability in a proton exchange membrane fuel cell will be presented. The studies of the oxygen reduction reaction on well-defined surfaces have guided the approach to design of the real world nanostructured catalysts. Insights from single crystalline systems and thin film surfaces have yielded atomically precise information related to surface structure and alloying with other metals in the quest to improve their intrinsic catalytic activity. These experimental efforts are also producing information necessary for the design of catalysts with enhancing stability. Atomic level control of the positioning of elements within catalyst structure enables the design of shapes and surfaces that mimic the key descriptors discovered through research on well-defined surfaces. The meticulous characterization of nanoscale materials using extremely high-purity conditions and cutting-edge techniques provides greater understanding of their performance and degradation pathways. This knowledge enables implementation of these materials in membrane electrode assemblies (MEA) for demonstration of targeted performance in a fuel cell vehicle. The catalyst, its support, water, ionomer, and gaseous reactants create challenging environment to control and engineer at the level that would be capable to address all technical targets that are necessary for wide deployment of this technology. The transition from high-purity liquid electrolytes into MEA still needs to be addressed before stepping into mass production of fuel cell stacks. The overall research strategy presented here exhibits the merits of a continuous pipeline from fundamental to applied research in the development of fuel cell technology.

Implementation of single-crystal metal electrodes into standard electrochemical procedures has provided invaluable insight into the structure of and processes occurring at the metal–electrolyte interface. However, the accuracy required in their manufacture to provide ideally cut crystals with the lowest possible degree of miscut in conjunction with the amount of material required, especially in the case of precious metals, can make their use highly restrictive. We present here fundamental insight into a general procedure for producing thin metal films containing large, atomically flat (111) terraces without the use of an epitaxial template. Thermal annealing in a controlled atmosphere induces long-range ordering in magnetron sputtered thin metal films deposited on an amorphous substrate. The ordering transition in these thin metal films yields characteristic (111) electrochemical signatures with minimal amount of material and provides an adequate replacement for oriented bulk single crystals. Moreover, th...

Trends in the HER are studied on selected metals (M= Cu, Ag, Au, Pt, Ru, Ir, Ti) in acid and alkaline environments. We found that with the exception of Pt, Ir and Au, due to high coverage by spectator species on non-noble metal catalysts, experimentally established positions of Cu , Ag, Ru and Ti in the observed volcano relations are still uncertain. We also found that while in acidic solutions the M-Hupd binding energy most likely is controlling the activity trends, the trends in activity in alkaline solutions are controlled by a delicate balance between two descriptors: the M-Had interaction as well as the energetics required to dissociate water molecules. The importance of the second descriptor is confirmed by introducing bifunctional catalysts such as M modified by Ni(OH); e.g. while the latter serves to enhance catalytic decomposition of water, the metal sites are required for collecting and recombining the produced hydrogen intermediates.

Advances in electrocatalysis at solid-liquid interfaces are vital for driving the technological innovations that are needed to deliver reliable, affordable and environmentally friendly energy. Here, we highlight the key achievements in the development of new materials for efficient hydrogen and oxygen production in electrolysers and, in reverse, their use in fuel cells. A key issue addressed here is the degree to which the fundamental understanding of the synergy between covalent and non-covalent interactions can form the basis for any predictive ability in tailor-making real-world catalysts. Common descriptors such as the substrate-hydroxide binding energy and the interactions in the double layer between hydroxide-oxides and H---OH are found to control individual parts of the hydrogen and oxygen electrochemistry that govern the efficiency of water-based energy conversion and storage systems. Links between aqueous- and organic-based environments are also established, encouraging the 'fuel cell' and 'battery' communities to move forward together.

Design and synthesis of active, stable and cost-effective materials for efficient hydrogen production (hydrogen evolution reaction, HER) is of paramount importance for the successful deployment of hydrogen-based alternative energy technologies. The HER, seemingly one of the simplest electrochemical reactions, has served for decades to bridge the gap between fundamental electrocatalysis and practical catalyst design. However, there are still many open questions that need to be answered before it would be possible to claim that design principles of catalyst materials are fully developed for the efficient hydrogen production. In this review, by summarizing key results for the HER on well-characterized electrochemical interfaces in acidic and alkaline media, we have broadened our understanding of the HER in the whole range of pH by considering three main parameters: the nature of the proton donor (H3O+ in acid and H2O in alkaline), the energy of adsorption of Had and OHad, and the presence of spectator species. Simply by considering these three parameters we show that great deal has already been learned and new trends are beginning to emerge, giving some predictive ability with respect to the nature of electrochemical interface and electrocatalytic activity of the HER.

Efficient chemical transformations in energy conversion and storage systems depend on understanding superoxide anion (O2–) electrochemistry at atomic and molecular levels. Here, a combination of experimental and theoretical techniques are used for rationalizing, and ultimately understanding, the complexity of superoxide anion (electro)chemistry in organic environments. By exploring the O2 + e– ↔ O2– reaction on well-characterized metal single crystals (Au, Pt, Ir), Pt single crystal modified with a single layer of graphene (Graphene@Pt(111)), and glassy carbon (GC) in 1,2 dimethoxyethane (DME) electrolytes, we demonstrate that (i) the reaction is an outer-sphere process; (ii) the reaction product O2– can “attack” any part of the DME molecule, i.e., the C–O bond via nucleophilic reaction and the C–H bond via radical hydrogen abstraction; (iii) the adsorption of carbon-based decomposition products and the extent of formation of a “solid electrolyte interface” (“SEI”) increases in the same order as the react...

Active in alkaline environment: The activity of nickel, silver, and copper catalysts for the electrochemical transformation of water to molecular hydrogen in alkaline solutions was enhanced by modification of the metal surfaces by Ni(OH)(2) (see picture; I = current density and η = overpotential). The hydrogen evolution reaction rate on a Ni electrode modified by Ni(OH)(2) nanoclusters is about four times higher than on a bare Ni surface.

Pt extended surfaces and nanoparticle electrodes are used to understand the origin of anomalous activities for electrocatalytic reactions in alkaline electrolytes as a function of cycling/time. Scanning tunneling microscopy (STM) of the surfaces before and after cycling in alkaline electrolytes was used to understand the morphology of the impurities and their impact on the catalytic sites. The nature of the contaminant species is identified as 3d-transition metal cations, and the formation of hydr(oxy)oxides of these elements is established as the main reason for the observed behavior. We find that, while for the oxygen reduction reaction (ORR) and the hydrogen oxidation reaction (HOR) the blocking of the sites by the undesired 3d-transition metal hydr(oxy)oxide species leads to deactivation of the reaction activities, the CO oxidation reaction and the hydrogen evolution reaction (HER) can have beneficial effects from the same impurities, the latter being dependent on the exact nature of the adsorbing spe...

The role of noncovalent interactions in the hydrogen oxidation reaction (HOR), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER) on Ru, Ir, and Ru0.5Ir0.5 electrodes is studied in alkaline solution containing K+, Li+, and Ba2+ cations. We found that noncovalent interaction between hydrated cations and covalently bonded OHad increases in the same order as specific charge density of the corresponding cation (K+

Design and synthesis of materials for efficient electrochemical transformation of water to molecular hydrogen and of hydroxyl ions to oxygen in alkaline environments is of paramount importance in reducing energy losses in water–alkali electrolysers. Here, using 3d-M hydr(oxy)oxides, with distinct stoichiometries and morphologies in the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) regions, we establish the overall catalytic activities for these reaction as a function of a more fundamental property, a descriptor, OH–M2+δ bond strength (0 ≤ δ ≤ 1.5). This relationship exhibits trends in reactivity (Mn < Fe < Co < Ni), which is governed by the strength of the OH–M2+δ energetic (Ni < Co < Fe < Mn). These trends are found to be independent of the source of the OH, either the supporting electrolyte (for the OER) or the water dissociation product (for the HER). The successful identification of these electrocatalytic trends provides the foundation for rational design of ‘active sites’ for practical alkaline HER and OER electrocatalysts. Efficient electrochemical transformation of water to molecular hydrogen and of hydroxyl ions to oxygen in alkaline environments is important for reducing energy losses in water–alkali electrolysers. Insight into the activities of hydr(oxy)oxides on platinum catalyst surfaces for hydrogen and oxygen evolution reactions should prove significant for designing practical alkaline electrocatalysts.