For Zinc Metal Batteries, How Many Electrons go to Hydrogen Evolution? An Electrochemical Mass Spectrometry Study.

Despite the advantages of aqueous zinc (Zn) metal batteries (AZMB) like high specific capacity (820 mAh g-1 and 5,854 mAh cm-3 ), low redox potential (-0.76 V vs. the standard hydrogen electrode), low cost, water compatibility, and safety, the development of practically relevant batteries is plagued by several issues like unwanted hydrogen evolution reaction (HER), corrosion of Zn substrate (insulating ZnO, Zn(OH)2 , Zn(SO4 )x (OH)y , Zn(ClO4 )x (OH)y etc. passivation layer), and dendrite growth. Controlling and suppressing HER activity strongly correlates with the long-term cyclability of AZMBs. Therefore, a precise quantitative technique is needed to monitor the real-time dynamics of hydrogen evolution during Zn electrodeposition. In this study, we quantify hydrogen evolution using in situ electrochemical mass spectrometry (ECMS). This methodology enables us to determine a correction factor for the faradaic efficiency of this system with unmatched precision. For instance, during the electrodeposition of zinc on a copper substrate at a current density of 1.5 mA/cm2 for 600 seconds, 0.3 % of the total charge is attributed to HER, while the rest contributes to zinc electrodeposition. At first glance, this may seem like a small fraction, but it can be detrimental to the long-term cycling performance of AZMBs. Furthermore, our results provide insights into the correlation between HER and the porous morphology of the electrodeposited zinc, unravelling the presence of trapped H2 and Zn corrosion during the charging process. Overall, this study sets a platform to accurately determine the faradaic efficiency of Zn electrodeposition and provides a powerful tool for evaluating electrolyte additives, salts, and electrode modifications aimed at enhancing long-term stability and suppressing the HER in aqueous Zn batteries.

Surface Hydride Formation on Cu(111) and Its Decomposition to Form H2 in Acid Electrolytes.

Mass spectrometry and Raman vibrational spectroscopy were used to follow competitive dynamics between adsorption and desorption of H and anions during potential cycling of three low-index Cu surfaces in acid electrolytes. Unique to Cu(111) is a redox wave for surface hydride formation coincident with anion desorption, while the reverse reaction of hydride decomposition with anion adsorption yields H2 by recombination rather than oxidation to H3O+. Charge imbalance between the reactions accounts for the asymmetric voltammetry in SO42-, ClO4-, PO43-, and Cl- electrolytes with pH 0.68-4.5. Two-dimensional hydride formation is evidenced by the reduction wave prior to H2 evolution and vibrational bands between 995 and 1130 cm-1. In contrast to Cu(111), no distinct voltammetric signature of surface hydride formation is observed on Cu(110) and Cu(100). The Cu(111) hydride surface phase may serve to catalyze hydrofunctionalization reactions such as CO2 reduction to CH4 and should be broadly useful in electro-organic synthesis.

Electrochemical Conversion of CO2 to Syngas with Palladium-Based Electrocatalysts.

ConspectusElectrocatalytic reduction of CO2 is a desirable method to produce valuable products from CO2. One of the main research challenges for electrocatalysis that produces carbon-containing products from CO2 is avoiding unwanted hydrogen production. Instead of totally eliminating hydrogen, our approach makes use of the readily available protons in aqueous electrolyte to coproduce CO and H2, making synthesis gas (syngas) with a tunable CO:H2 ratio; the resulting syngas can then be used as feedstock for existing thermocatalytic processes such as Fischer-Tropsch and methanol synthesis reactions. We discovered that palladium hydride (PdH), formed under electrocatalytic reaction conditions, is an effective electrocatalyst that enables this unique product distribution. We employed in situ synchrotron techniques to determine the formation of the PdH phase during electrocatalytic reduction of CO2. We also performed density functional theory (DFT) calculations to determine the binding energies of key intermediates on PdH to identify descriptors to correlate experimentally observed trends in activity and selectivity.Since first reporting on the potential application of PdH to produce syngas, our research group has refined control over the syngas product selectivity, improved the activity, and reduced the loading of Pd in electrocatalysts. We achieved this by the following approaches: understanding the structure-function relationship with shape-controlled Pd nanoparticles, determining the cation and isotopic effects of electrolyte, alloying Pd with inexpensive secondary metals, supporting Pd on transition metal carbides and nitrides, and utilizing single atom Pd catalysts. At each step, we monitored the phase transition from Pd to PdH under reaction conditions with in situ synchrotron-based X-ray absorption and X-ray diffraction techniques by identifying the onset potential for the appearance of the characteristic Pd-Pd bond length and diffraction patterns associated with PdH formation. We also identified descriptors for syngas production on PdH, bimetallic PdH, and supported PdH catalysts by correlating DFT calculations of PdH stability in different catalytic systems as well as the effect of PdH formation on the binding strength of reaction intermediates. The research methodology established here is useful not only for continued optimization of Pd-based syngas-producing electrocatalysts but also for enhancing activity while reducing the loading of precious metals for other electrocatalytic applications. Moreover, we feel the advances in electrocatalytic syngas production described here represent a critical step toward sustainable CO2 utilization that should inspire continued efforts.

Isotopic effect on electrochemical CO2 reduction activity and selectivity in H2O- and D2O-based electrolytes over palladium.

The isotopic effect on the electrochemical CO2 reduction reaction (CO2RR) is investigated in this study. A higher CO2RR selectivity over its competing hydrogen evolution reaction was observed in D2O-based electrolytes compared with the H2O-based counterparts, which can be attributed to the lower [D+] concentration than [H+].

Tuning the activity and selectivity of electroreduction of CO2 to synthesis gas using bimetallic catalysts.

The electrochemical carbon dioxide reduction reaction to syngas with controlled CO/H2 ratios has been studied on Pd-based bimetallic hydrides using a combination of in situ characterization and density functional theory calculations. When compared with pure Pd hydride, the bimetallic Pd hydride formation occurs at more negative potentials for Pd-Ag, Pd-Cu, and Pd-Ni. Theoretical calculations show that the choice of the second metal has a more significant effect on the adsorption strength of *H than *HOCO, with the free energies between these two key intermediates (i.e., ΔG(*H)-ΔG(*HOCO)) correlating well with the carbon dioxide reduction reaction activity and selectivity observed in the experiments, and thus can be used as a descriptor to search for other bimetallic catalysts. The results also demonstrate the possibility of alloying Pd with non-precious transition metals to promote the electrochemical conversion of CO2 to syngas.

Enhancing Activity and Reducing Cost for Electrochemical Reduction of CO2 by Supporting Palladium on Metal Carbides.

Electrochemical CO2 reduction reaction (CO2 RR) with renewable electricity is a potentially sustainable method to reduce CO2 emissions. Palladium supported on cost-effective transition-metal carbides (TMCs) are studied to reduce the Pd usage and tune the activity and selectivity of the CO2 RR to produce synthesis gas, using a combined approach of studying thin films and practical powder catalysts, in situ characterization, and density functional theory (DFT) calculations. Notably, Pd/TaC exhibits higher CO2 RR activity, stability and CO Faradaic efficiency than those of commercial Pd/C while significantly reducing the Pd loading. In situ measurements confirm the transformation of Pd into hydride (PdH) under the CO2 RR environment. DFT calculations reveal that the TMC substrates modify the binding energies of key intermediates on supported PdH. This work suggests the prospect of using TMCs as low-cost and stable substrates to support and modify Pd for enhanced CO2 RR activity.

Bimetallic Electrocatalysts for CO2 Reduction.

The increasing concentration of CO2 in the atmosphere has caused various environmental issues. Utilizing CO2 as the carbon feedstock to replace traditional fossil sources in commodity chemical production is a potential solution to reduce CO2 emissions. Electrochemical reduction of CO2 has attracted much attention because it not only converts CO2 into a variety of useful chemicals under mild reaction conditions, but also can be powered by renewable electricity at remote locations. From this review article, we summarize recent literature on the topic of bimetallic electrocatalysts for CO2 reduction. Both selectivity and activity of bimetallic catalysts strongly depend on their compositions and surface structures. Tuning the properties of a bimetallic catalyst could result in a wide range of products, including carbon monoxide, hydrocarbons, carboxylate and liquid oxygenates. By reviewing recent research efforts in the field of bimetallic electrocatalysts for CO2 reduction, we aim to provide the community with a timely overview of the current status of bimetallic CO2 electrocatalysts and to stimulate new ideas to design better catalysts for more efficient CO2 electrolysis processes.

Reactions of water and C1 molecules on carbide and metal-modified carbide surfaces.

The formation of carbides can significantly modify the physical and chemical properties of the parent metals. In the current review, we summarize the general trends in the reactions of water and C1 molecules over transition metal carbide (TMC) and metal-modified TMC surfaces and thin films. Although the primary focus of the current review is on the theoretical and experimental studies of reactions of C1 molecules (CO, CO2, CH3OH, etc.), the reactions of water will also be reviewed because water plays an important role in many of the C1 transformation reactions. This review is organized by discussing separately thermal reactions and electrochemical reactions, which provides insights into the application of TMCs in heterogeneous catalysis and electrocatalysis, respectively. In thermal reactions, we discuss the thermal decomposition of water and methanol, as well as the reactions of CO and CO2 over TMC surfaces. In electrochemical reactions, we summarize recent studies in the hydrogen evolution reaction, electrooxidation of methanol and CO, and electroreduction of CO2. Finally, future research opportunities and challenges associated with using TMCs as catalysts and electrocatalysts are also discussed.