Cathodic H2 Evolution Reaction Research Articles

Electrified hydrocarbon-to-oxygenates coupled to hydrogen evolution for efficient greenhouse gas mitigation

Chemicals manufacture is among the top greenhouse gas contributors. More than half of the associated emissions are attributable to the sum of ammonia plus oxygenates such as methanol, ethylene glycol and terephthalic acid. Here we explore the impact of electrolyzer systems that couple electrically-powered anodic hydrocarbon-to-oxygenate conversion with cathodic H2 evolution reaction from water. We find that, once anodic hydrocarbon-to-oxygenate conversion is developed with high selectivities, greenhouse gas emissions associated with fossil-based NH3 and oxygenates manufacture can be reduced by up to 88%. We report that low-carbon electricity is not mandatory to enable a net reduction in greenhouse gas emissions: global chemical industry emissions can be reduced by up to 39% even with electricity having the carbon footprint per MWh available in the United States or China today. We conclude with considerations and recommendations for researchers who wish to embark on this research direction.

Architecting the AuPt alloys for hydrazine oxidation as an anolyte in fuel cell: Comparative analysis of hydrazine splitting and water splitting for energy-saving H2 generation

Replacing the kinetically sluggish anodic oxygen evolution reaction (OER) with hydrazine (N2H4) oxidation reaction (HzOR) could be the effective approach for achieving energy-saving hydrogen (H2) fuel production in a water electrolyzer system. Thus, developing the efficient HzOR electrocatalysts, combined with the cathodic H2 evolution reaction (HER) is of vital importance for the high-rate H2 fuel generation as well as for the advancement of a N2H4 fuel cell. Herein, we utilized a facile integrated process of pulsed laser irradiation and sonochemical process to synthesize AuPt alloys by the irradiation of laser to a mixture of Au/Pt solution in methanol/DI water in varied proportions. The AuPt alloy plays a key role in the chemisorption of N2H4 on its surface, forming a dative bond involving electrons of the lone pair of nitrogen in N2H4 and empty orbitals of Pt in the alloy, indicating its high intrinsic activity against HzOR. The optimal composition of Au1Pt8 electrode demonstrates outstanding characteristics of HER with an ultralow overpotential of 26 mV at 10 mA cm−2 in alkaline medium while requiring 502 mV to attain 10 mA cm−2 for HzOR in 0.5 M N2H4/1.0 M KOH electrolyte. In addition, the assembled overall N2H4 splitting electrolyzer cell using Au1Pt8 alloys as both anode and cathode requires cell voltage of only ~0.172 V at 10 mA cm−2 with tremendous stability over 10 h, which is much lower than the voltage of 1.773 V required for the overall water splitting electrolyzer. The present study validates the feasibility of AuPt alloys for stimulating N2H4 fuel cells in the future to achieve both electrical energy generation and high-rate H2 fuel production.

A clean and membrane-free chlor-alkali process with decoupled Cl2 and H2/NaOH production

Existing chlor-alkali processes generally use asbestos, mercury or fluorine-containing ion-exchange membranes to separate the simultaneous chlorine production on the anode and hydrogen production on the cathode, and form sodium hydroxide in the electrolyte. Here, using the Na+ de-intercalation/intercalation of a Na0.44MnO2 electrode as a redox mediator, we decouple the chlor-alkali process into two independent steps: a H2 production step with the NaOH formation in the electrolyte and a Cl2 production step. The first step involves a cathodic H2 evolution reaction (H2O → H2) and an anodic Na+ de-intercalation reaction (Na0.44MnO2 → Na0.44−xMnO2), during which NaOH is produced in the electrolyte solution. The second step depends on a cathodic Na+ intercalation reaction (Na0.44−xMnO2 → Na0.44MnO2) and an anodic Cl2 production (Cl → Cl2). The cycle of the two steps provides a membrane-free process, which is potentially a promising direction for developing clean chlor-alkali technology.

The phase-shift method for determining adsorption isotherms of hydrogen in electrochemical systems

A linear relationship between the behavior ( - ϕ vs. E) of the phase shift ( 0 ∘ ⩽ - ϕ ⩽ 90 ∘ ) for the optimum intermediate frequency and that ( θ vs. E) of the fractional surface coverage ( 1 ⩾ θ ⩾ 0 ) of H for the cathodic H 2 evolution reaction (HER), i.e., the phase-shift method, in electrochemical systems has been suggested and verified using cyclic voltametric, linear sweep voltametric or differential pulse voltametric, and ac impedance techniques. The Langmuir and Frumkin adsorption isotherms of under-potentially deposited hydrogen (UPD H) or over-potentially deposited hydrogen (OPD H) for the cathodic HER at noble and transition-metal (Pt, Ir, Rh, Re, Pd, Au, Pt–Rh alloy, Ni)/aqueous electrolyte interfaces have been studied using the phase-shift method. At the interfaces, the fractional surface coverage ( θ ), equilibrium constant ( K), and standard free energy ( Δ G ads 0 , Δ G θ 0 ) of H (UPD H, OPD H) for the cathodic HER are determined using the phase-shift method. The applicability of the Temkin adsorption isotherm ( θ vs. E) of OPD H for the cathodic HER at noble and transition-metal (Ir, Re, Ni)/aqueous electrolyte interfaces also has been studied using the phase-shift method. The phase-shift method is a simple and efficient tool for determining the adsorption, electrode kinetic, and thermodynamic parameters ( θ , K, Δ G ads 0 , Δ G θ 0 ) of H (UPD H, OPD H) for the cathodic HER in electrochemical systems. The phase-shift method can be effectively used as a new electrochemical method to determine adsorption isotherms (Langmuir, Frumkin, Temkin) of intermediates for sequential reactions in electrochemical systems.

The phase-shift method for determining Langmuir and Temkin adsorption isotherms of over-potentially deposited hydrogen for the cathodic [formula omitted] evolution reaction at the poly- [formula omitted] aqueous electrolyte interface

A linear relationship between the behavior ( - ϕ vs. E ) of the phase shift ( 0 ∘ ⩽ - ϕ ⩽ 90 ∘ ) for the optimum intermediate frequency and that ( θ vs. E ) of the fractional surface coverage ( 1 ⩾ θ ⩾ 0 ) of over-potentially deposited hydrogen (OPD H) for the cathodic H 2 evolution reaction (HER), i.e., the phase-shift method, at the poly-Pt/0.5 M H 2 SO 4 aqueous electrolyte interface has been verified using cyclic voltammetric, differential pulse voltammetric, and ac impedance techniques. The phase-shift method for determining the suitable adsorption isotherm (Langmuir, Frumkin, Temkin) of OPD H for the cathodic HER at the interface also has been proposed. At the poly-Pt/0.5 M H 2 SO 4 aqueous electrolyte interface, the Langmuir adsorption isotherm ( θ vs. E ) of OPD H, the equilibrium constant ( K = 1.3 × 10 - 4 ) for OPD H and the standard free energy ( Δ G ads 0 = 22.2 kJ / mol ) of OPD H are determined using the phase-shift method. At the same interface, the Temkin adsorption isotherm ( θ vs. E ) of OPD H, the equilibrium constant ( 1.3 × 10 - 3 ⩾ K ⩾ 1.3 × 10 - 5 with θ , i.e., 0 ⩽ θ ⩽ 1 ) for OPD H, and the standard free energy ( 16.5 ⩽ Δ G θ 0 ⩽ 27.9 kJ / mol with θ , i.e., 0 ⩽ θ ⩽ 1 ) of OPD H are also determined using the phase-shift method. At the intermediate values of θ , i.e., 0.2 < θ < 0.8 , the Langmuir and Temkin adsorption isotherms of OPD H for the cathodic HER at the interface are converted to each other. The equilibrium constant ( K 0 ) for the Temkin adsorption isotherm ( θ vs. E ) is ca. 10 times greater than that ( K ) for the corresponding Langmuir adsorption isotherm ( θ vs. E ). The interaction parameter ( g ) for the Temkin adsorption isotherm ( θ vs. E ) is ca. 4.6 greater than that ( g ) for the corresponding Langmuir adsorption isotherm ( θ vs. E ). These numbers (10 times and 4.6) can be taken as constant conversion factors between the corresponding adsorption isotherms (Temkin, Langmuir, Frumkin). The Temkin adsorption isotherm corresponding to the Langmuir or the Frumkin adsorption isotherm, and vice versa, can be effectively converted using the constant conversion factors. Both the phase-shift method and constant conversion factors can be effectively used as a new electrochemical method to determine the suitable adsorption isotherms (Langmuir, Frumkin, Temkin) of H for the cathodic HER in electrochemical systems.

Constant Correlation Factors between Temkin and Langmuir or Frumkin Adsorption Isotherms at Poly-Pt, Re, and Ni/Aqueous Electrolyte Interfaces

The constant correlation factors between the Temkin and the Langmuir or the Frumkin adsorption isotherms of over-potentially deposited hydrogen (OPD H) for the cathodic H2 evolution reaction (HER) at poly-Pt and Re/0.5M <TEX>$H_2SO_4$</TEX> and poly-Ni/0.05 M KOH aqueous electrolyte interfaces have been experimentally and consistently found using the phase-shift method. At intermediate values of the fractional surface coverage <TEX>$(\theta),\;i.e.,\;02<{\theta}<0.8$</TEX>, the Langmuir and Temkin adsorption isotherms of OPD H for the cathodic HER are correlated to each other even though the adsorption conditions or processes are different from each other. At the same range of <TEX>$\theta$</TEX>, correspondingly, the Frumkin and Temkin adsorption isotherms of OPD H for the cathodic HER are correlated to each other. The equilibrium constants <TEX>$(K_o)$</TEX> for the Temkin adsorption isotherms <TEX>$({\theta}\;vs.\; E)$</TEX> are consistently ca. 10 times greater than those (K, Ko) for the corresponding Langmuir or Frumkin adsorption isotherms (<TEX>$({\theta}\;vs.\; E)$</TEX>. The interaction parameters (g) for the Temkin adsorption isotherms <TEX>$({\theta}\;vs.\; E)$</TEX> are consistently ra. 4.6 greater than those (g) for the corresponding Langmuir or Frumkin adsorption isotherms <TEX>$({\theta}\;vs.\; E)$</TEX>. These numbers (10 times and 4.6) can be taken as constant correlation factors between the corresponding adsolftion isotherms (Temkin, Langmuir, Frumkin) at the interfaces. The Temkin adsorption isotherm corresponding to the Langmuir or the Frumkin adsorption isotherm, and vice versa, can be effectively verified or confirmed using the constant correlation factors. Both the phase-shift methodand constant correlation factors are useful and effective for determining or confirming the suitable adsorption isotherms (Temkin, Langmuir, Frumkin) of intermediates for sequential reactions in electrochemical systems.

The phase-shift method for determining Langmuir adsorption isotherms of over-potentially deposited hydrogen for the cathodic H 2 evolution reaction at poly-Re/aqueous electrolyte interfaces

A linear relationship between the behavior (− ϕ vs. E) of the phase shift (0°⩽− ϕ⩽90°) for the optimum intermediate frequency and that ( θ vs. E) of the fractional surface coverage (1⩾ θ⩾0) of over-potentially deposited hydrogen (OPD H) for the cathodic H 2 evolution reaction (HER), i.e., the phase-shift method, at poly-Re/ 0.5 M H 2 SO 4 and 0.1 M KOH aqueous electrolyte interfaces has been suggested and verified using cyclic voltammetric, differential pulse voltammetric, and AC impedance techniques. At the poly-Re/ 0.5 M H 2 SO 4 aqueous electrolyte interface, the Langmuir adsorption isotherm ( θ vs. E) of OPD H, the equilibrium constant ( K=4.5×10 −7) for OPD H, and the standard free energy ( ΔG ads o =36.2 kJ/ mol) of OPD H are determined using the phase-shift method. At the poly-Re/ 0.1 M KOH aqueous electrolyte interface, θ vs. E of OPD H, K=1.9×10 −6 for OPD H, and ΔG ads o =32.6 kJ/ mol of OPD H are also determined using the phase-shift method. The applicability of the Frumkin and Temkin adsorption isotherms ( θ vs. E) of OPD H for the cathodic HER at the interfaces also has been studied using the phase-shift method. The phase-shift method experimentally provides the link between the microscopic system of the adsorption sites and process of OPD H on the atoms and the macroscopic system of the electrode kinetics and thermodynamics for OPD H at the interfaces. The phase-shift method can be effectively used as a new electrochemical method to determine or estimate adsorption isotherms of intermediates for sequential reactions in electrochemical systems.

Determination of the Langmuir adsorption isotherms of under- and over-potentially deposited hydrogen for the cathodic H 2 evolution reaction at poly-Ir/aqueous electrolyte interfaces using the phase-shift method

The Langmuir adsorption isotherms of under-potentially deposited hydrogen (UPD H) and over-potentially deposited hydrogen (OPD H) for the cathodic H 2 evolution reaction (HER) at poly-Ir/0.5 M H 2SO 4 and 0.05 M KOH aqueous electrolyte interfaces have been studied using cyclic voltammetric, differential pulse voltammetric, and ac impedance techniques. The behavior of the lagged phase shift (0°⩽− φ⩽90°) for the optimum intermediate frequency can be linearly related to that of the fractional surface coverage (1⩾ θ⩾0) of H (UPD H, OPD H) for the cathodic HER at the interfaces. A linear relationship between the phase-shift profile (− φ vs. E) for the optimum intermediate frequency and the Langmuir adsorption isotherm ( θ vs. E) of H (UPD H, OPD H), i.e., the phase-shift method, can be used as a new electrochemical method to determine or estimate the fractional surface coverage ( θ), equilibrium constant ( K), and standard free energy (Δ G ads 0) of H (UPD H, OPD H) for the cathodic HER at the interfaces. At the poly-Ir/0.5 M H 2SO 4 aqueous electrolyte interface, K and Δ G ads 0 of the OPD H for the cathodic HER are 9.9×10 −5 and 22.8 kJ/mol , respectively. At the poly-Ir/0.05 M KOH aqueous electrolyte interface, K and Δ G ads 0 of the UPD H for the cathodic HER are 3.9 and −3.4 kJ/mol , respectively. At the poly-Ir/0.05 M KOH aqueous electrolyte interface, K and Δ G ads 0 of the OPD H for the cathodic HER are 5.8×10 −4 and 18.5 kJ/mol , respectively. The two different Langmuir adsorption isotherms for H (UPD H, OPD H) correspond to two different adsorption sites of H (UPD H, OPD H) on the poly-Ir electrode surface. The two different adsorption sites of UPD H and OPD H act as two distinguishable electroadsorbed H species. The phase-shift method is a simple and efficient tool for determining the adsorption, electrode kinetic, and thermodynamic parameters ( θ, K, Δ G ads 0) of H (UPD H, OPD H) for the cathodic HER at the interfaces.

Determination of the equilibrium constant and standard free energy of the over-potentially deposited hydrogen for the cathodic H 2 evolution reaction at the Pt–Rh alloy electrode interface using the phase-shift method

The Langmuir adsorption isotherm of the over-potentially deposited hydrogen (OPD H) for the cathodic H 2 evolution reaction (HER) at the Pt–Rh (Pt:Rh; 80: 20 wt% ) alloy/0.5 M H 2 SO 4 aqueous electrolyte interface has been studied using cyclic voltammetric and ac impedance techniques. The behavior of the phase shift (0°⩽− φ⩽90°) for the optimum intermediate frequency can be linearly related to that of the fractional surface coverage (1⩾ θ⩾0) of the OPD H for the cathodic HER at the interface. The phase-shift profile (− φ vs. E) for the optimum intermediate frequency, i.e., the phase-shift method, can be used as a new electrochemical method to determine the Langmuir adsorption isotherm ( θ vs. E) of the OPD H for the cathodic HER at the interface. At the Pt–Rh alloy electrode interface, the equilibrium constant ( K) and the standard free energy (Δ G ads) of the OPD H are 2.2×10 −4 and 20.9 kJ/ mol , respectively. At the steady state, the behaviors of the cyclic voltammogram and the Langmuir adsorption isotherm of the OPD H for the cathodic HER at the Pt–Rh alloy electrode interface are similar to those of the pure Pt electrode interfaces. At the steady state, the effect of Rh on the OPD H for the cathodic HER can be neglected at the Pt–Rh (Pt:Rh; 80: 20 wt% ) alloy/0.5 M H 2 SO 4 aqueous electrolyte interface.

Electrochemical sorption of H into Fe and mild-steel: kinetic conditions for enhancement or inhibition by adsorbed HS-

Sulfide species, HS-, S2- and H2S, are often the most common and important poisons that arise in practical situations of embrittlement of ferruginous metals by sorbed H. The absorption and permeation of H can be either enhanced or inhibited under conditions of cathodic polarization in the presence of HS-. The effect of chemisorption of S on H transfer into and through Fe and mild-steel in aqueous NaOH solution has been studied comparatively in relation to poisoning by species derived from AsO2- using a kinetic equation-fitting procedure applied to H permeation currents in relation to simultaneously measured polarization behaviour on the H2-evolution side of a bielectrode membrane. A kinetic-fitting model is applied to all the constituent partial reactions involved at the electrode, including processes involving chemisorbed poison (P) species. The important relation of surface coverage by H to the H concentration in a sub-surface state is examined in relation to the kinetics of steps in the cathodic H2 evolution reaction (HER) and the step of interfacial transfer of H into the metal host. The rates of absorption and desorption of H, and its diffusion from the sub-surface layer into the bulk metal, are important in determining the effects of the poisons on the sub-surface concentration of H and therefore the H permeation current.

In situ determination of the ‘real are factor’ in H2 evolution electrocatalysis at porous NiFe composite electrodes

Abstract Composite cathode materials, such as electrodeposited NiFe, NiMo or NiMoCd, are of considerable practical interest as possible electrocatalysts for H2 production in water electrolysers and for cathodes in the chlor-alkali process. The present work extends the examination of the real electrode surface area factor in electrocatalysis at materials havinhg porous or irregular surfaces in relation to evaluation of the role of true electrocatalytic behaviour of a 65 at.% Fe + 35 at.% Ni electrodeposited material. The results also enable the behaviour of the adsorbed H intermediate in the cathodic H2 evolution reaction to be examined. Two distinct regions of steady state behaviour, exhibiting different Tafel slopes over the temperature range 298 to 348 K, are observed. The real to apparent electrode surface area ratio was evaluated from the double-layer capacitance values that were determined by means of a recently published method based on a curve-fitting extrapolation procedure applied to the initial, short-time, regions of potential-relaxation transients recorded after current interruptions. The double-layer capacitance values are derived under conditions of sufficient electrolyte conductivity that the effects of distributed ohmic resistance in the pores of the electrode matrix are minimized. The adsorption behaviour of the H species was also studied by means of a.c. impedance and potential-relaxation measurements.

Comparison of entropic and enthalpic components of the barrier symmetry factor, β, for proton discharge at liquid and solid Hg in relation to the variation of Tafel slopes and β with temperature

In order to provide information from a new direction on the temperature variation of the barrier symmetry factor, β, and the corresponding Tafel slope, b=RT/βF, for the step of proton discharge at the Hg electrode, the kinetics of the cathodic H2 evolution reaction have been studied over a wide temperature range through the melting point of Hg from several proton donors, in a series of suitable non-aqueous solvents that enable low-temperature experiments to be performed. The potential dependence of both the heat and entropy of activation has been determined through the melting point of Hg, enabling the corresponding enthalpic (βH) and entropic (βS) components of β in the relation β=βH+TβS to be evaluated. Substantial differences in βH and βs for solid and liquid Hg surfaces are found including, in some cases, a difference of sign of the βs factor. The temperature-dependence of β, which is associated with the non-conventional variation of b with temperature, now demonstrated for many systems, arises in a complex way which is depnendent on the state and structure of the electrode metal's surface, in relation probably to solvent adsorption and orientation at the interface. The usually assumed value of 0.5 for the barrier symmetry factor is entirely coincidental when this quantity is identified with βH, derived for 298 K. Only when βs= 0, is βH near 0.5, otherwise there is a compensatory variation of βH with Tβs at a given T.

ChemInform Abstract: State of Adsorption and Coverage by Overpotential‐Deposited (OPD) H in the H2 Evolution Reaction at Au and Pt.

AbstractFrom comparison of Tafel polarization lines and potential decay data for the kinetic behavior of the cathodic H2 evolution reaction (HER) at Au in acid and alkaline solution, it is concluded that there is very little coverage of the OPD Hintermediate involved in the steady‐state process of HER of appreciable current densities.