Mass-transport-limited oxidation of formic acid on a Pd ML Pt(100) electrode in perchloric acid

A hybrid FeOx/CoOx/Pt ternary nanocatalyst for augmented catalysis of formic acid electro-oxidation

Platinum-based catalysts that have long been used as the anodes for the formic acid electro-oxidation (FAO) in the direct formic acid fuel cells (DFAFCs) were susceptible to retrogradation in performance due to CO poisoning that impaired the technology transfer in industry. This work is designed to overcome this challenge by amending the Pt surface sequentially with nanosized cobalt (nano-CoOx, fibril texture of ca. 200 nm in particle size) and iron (nano-FeOx, nanorods of particle size and length of 80 and 253 nm, respectively) oxides. This enriched the Pt surface with oxygenated groups that boosted FAO and mitigated the CO poisoning. The unfilled d-orbitals of the transition metals and their tendency to vary their oxidations states presumed their participation in a faster mechanism of FAO. Engineering the Pt surface in this FeOx/CoOx/Pt hierarchy resulted in a remarkable activity toward FAO, that exceeded four times that of the Pt catalyst with up to ca. 2.5 times improvement in the catalytic tolerance against CO poisoning. This associated a ca. - 32 mV shift in the onset potential of FAO which increased to - 40 mV with a post-activation of the same catalyst at - 0.5  in 0.2 mol L-1 NaOH, displaying the catalyst's competitiveness in reducing overpotentials in DFAFCs. It also exhibited a favorable amelioration in the catalytic durability in long-termed chronoamperometric electrolysis. The electrochemical impedance spectroscopy and the CO stripping voltammetry were employed to elucidate the origin of enhancement.

Subsurface Hydride Formation Leads to Slow Surface Adsorption Processes on a Pd(111) Single-Crystal Electrode in Acidic Electrolytes

Palladium is one of the most important catalysts due to its widespread use in heterogeneous catalysis and electrochemistry. However, an understanding of the electrochemical processes and interfacial phenomena at Pd single-crystal electrodes/electrolytes is still scarce. In this work, the electrochemical behavior of the Pd(111) electrode was studied by the combination of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in different acidic electrolytes, namely, sulfuric acid, perchlorate acid, methane sulfonic acid, and hydrofluoric acid. An analysis of CV profiles shows the strong adsorption of all anions at low electrode potential, partially overlapping with underpotential deposited hydrogen (UPD-H), leading to the appearance of a pair of sharp peaks in what would be considered the "hydrogen region". All anions studied (HSO4-, ClO4-, CH3SO3-, and F-) adsorb specifically and interact with (or effectively block) the surface-adsorbed hydroxyl phase formed on the Pd(111) terrace at higher potentials. Strikingly, the scan rate-dependent results show that the process of anion adsorption and desorption is a kinetically rather slow step. EIS measurements show that the exact mechanism of this slow anion ad/desorption process actually stems from (sub)surface phenomena: the direct hydrogen insertion into Pd lattice (hydrogen subsurface absorption) commences from ca. 0.40 V and leads to the formation of (subsurface) Pd hydrides (PdHx). We argue that the subsurface hydrogen phase significantly alters the work function and thereby the kinetics of the anion adsorption and desorption processes, leading to irreversible peaks in the voltammetry. This precise understanding is important in guiding further fundamental work on Pd single crystals and will be crucial to advancing the eventual design of optimized Pd electrocatalysts.

Influence of Cations on HCOOH and CO Formation during CO2 Reduction on a PdMLPt(111) Electrode

Understanding the role of cations in the electrochemical CO2 reduction (CO2RR) process is of fundamental importance for practical application. In this work, we investigate how cations influence HCOOH and CO formation on PdMLPt(111) in pH 3 electrolytes. While only (a small amount of adsorbed) CO forms on PdMLPt(111) in the absence of metal cations, the onset potential of HCOOH and CO decreases with increasing cation concentrations. The cation effect is stronger on HCOOH formation than that on CO formation on PdMLPt(111). Density functional theory simulations indicate that cations facilitate both hydride formation and CO2 activation by polarizing the electronic density at the surface and stabilizing *CO2-. Although the upshift of the metal work function caused by high coverage of adsorbates limits hydride formation, the cation-induced electric field counterbalances this effect in the case of *H species, sustaining HCOOH production at mild negative potentials. Instead, at the high *CO coverages observed at very negative potentials, surface hydrides do not form, preventing the HCOOH route both in the absence and presence of cations. Our results open the way for a consistent evaluation of cationic electrolyte effects on both activity and selectivity in CO2RR on Pd-Pt catalysts.

How palladium inhibits CO poisoning during electrocatalytic formic acid oxidation and carbon dioxide reduction

Development of reversible and stable catalysts for the electrochemical reduction of CO₂ is of great interest. Here, we elucidate the atomistic details of how a palladium electrocatalyst inhibits CO poisoning during both formic acid oxidation to carbon dioxide and carbon dioxide reduction to formic acid. We compare results obtained with a platinum single-crystal electrode modified with and without a single monolayer of palladium. We combine (high-scan-rate) cyclic voltammetry with density functional theory to explain the absence of CO poisoning on the palladium-modified electrode. We show how the high formate coverage on the palladium-modified electrode protects the surface from poisoning during formic acid oxidation, and how the adsorption of CO precursor dictates the delayed poisoning during CO₂ reduction. The nature of the hydrogen adsorbed on the palladium-modified electrode is considerably different from platinum, supporting a model to explain the reversibility of this reaction. Our results help in designing catalysts for which CO poisoning needs to be avoided.

Influence of crystalline phase on electrocatalytic behaviour for Sm2-xSrxNiO4-δ (x = 0.4 to 1.0) Ruddlesden Popper based system : A comparative study of bulk and thin electrocatalysts

Here, with the motive of understanding influence of compositional engineering with active site alternation on catalytic behaviour is studied for Ruddlesden Popper based system Sm2-xSrxNiO4-δ. A phase change from orthorhombic...

Tunable Periodically Ordered Mesoporosity in Palladium Membranes Enables Exceptional Enhancement of Intrinsic Electrocatalytic Activity for Formic Acid Oxidation

Developing superior electrocatalysts for formic acid oxidation (FAO) is the most crucial step in commercializing direct formic acid fuel cells. Herein, we electrodeposited palladium membranes with periodically ordered mesoporosity obtained by asymmetrically replicating the bicontinuous cubic phase structure of a lyotropic liquid-crystal template. The Pd membrane with the largest periodicity and highest degree of order delivered up to 90.5 m²  g^-1 of electrochemical active surface area and 3.34 A mg^-1 electrocatalysis capability towards FAO, 3.8 and 7.8 times the values of the commercial Pd/C catalyst, respectively. By controlling the temperature and potential of the electrodeposition procedure, the periodicity area and order degree of the mesoporosity are highly tunable. These Pd membranes gave prototype formic acid fueled cells with 4.3 and 2.4 times the maximum current and power density of the commercial Pd/C catalyst.

Adsorption processes on a Pd monolayer-modified Pt(111) electrode

Specific adsorption of anions is an important aspect in surface electrochemistry for its influence on reaction kinetics in either a promoted or inhibited fashion. Perchloric acid is typically considered as an ideal electrolyte for investigating electrocatalytic reactions due to the lack of specific adsorption of the perchlorate anion on several metal electrodes. In this work, cyclic voltammetry and computational methods are combined to investigate the interfacial processes on a Pd monolayer deposited on Pt(111) single crystal electrode in perchloric acid solution. The “hydrogen region” of this Pd_MLPt(111) surface exhibits two voltammetric peaks: the first “hydrogen peak” at 0.246 V_RHE actually involves the replacement of hydrogen by hydroxyl, and the second “hydrogen peak” H_II at 0.306 V_RHE appears to be the replacement of adsorbed hydroxyl by specific perchlorate adsorption. The two peaks merge into a single peak when a more strongly adsorbed anion, such as sulfate, is involved. Our density functional theory calculations qualitatively support the peak assignment and show that anions generally bind more strongly to the Pd_MLPt(111) surface than to Pt(111).

Specific adsorption of anions is an important aspect in surface electrochemistry for its influence on reaction kinetics in either a promoted or inhibited fashion.