Increasing the rate of the hydrogen evolution reaction in neutral water with protic buffer electrolytes

Electrocatalytic generation of H₂ is challenging in neutral pH water, where high catalytic currents for the hydrogen evolution reaction (HER) are particularly sensitive to the proton source and solution characteristics. A tris(hydroxymethyl)aminomethane (TRIS) solution at pH 7 with a [2Fe-2S]-metallopolymer electrocatalyst gave catalytic current densities around two orders of magnitude greater than either a more conventional sodium phosphate solution or a potassium chloride (KCl) electrolyte solution. For a planar polycrystalline Pt disk electrode, a TRIS solution at pH 7 increased the catalytic current densities for H₂ generation by 50 mA/cm² at current densities over 100 mA/cm² compared to a sodium phosphate solution. As a special feature of this study, TRIS is acting not only as the primary source of protons and the buffer of the pH, but the protonated TRIS ([TRIS-H]⁺) is also the sole cation of the electrolyte. A species that is simultaneously the proton source, buffer, and sole electrolyte is termed a protic buffer electrolyte (PBE). The structure–activity relationships of the TRIS PBE that increase the HER rate of the metallopolymer and platinum catalysts are discussed. These results suggest that appropriately designed PBEs can improve HER rates of any homogeneous or heterogeneous electrocatalyst system. General guidelines for selecting a PBE to improve the catalytic current density of HER systems are offered.

Molecular hydrogen (H₂), a clean-burning and energy-dense fuel source, has been widely discussed as an attractive way to store intermittent energy from solar and wind through water electrolysis (1, 2). Current commercial electrolyzers can be separated into two categories based on their operating pH. The first are acidic polymer electrolyte membrane electrolyzers that work best with rare and expensive platinum-based electrocatalysts for the hydrogen evolution reaction (HER) (3). The second are strongly alkaline electrolyzers that suffer from caustic basic reaction conditions (4). Neutral pH conditions with inexpensive catalysts composed of Earth-abundant elements are a target for practical solar-to-hydrogen fuel devices due to lower cost and fewer safety concerns (5), but achieving fast rates with mild overpotentials under neutral conditions remains a challenge (6–12). In the pH range from 5 to 9, the electrocatalytic activity of platinum (Pt) itself does not conform to the expected thermodynamic potential shift with pH dependence of −59 mV/pH (13). This is due to the low concentration of the hydronium ion in this pH range and a transition to water as the primary reactant, which has a higher thermodynamic requirement for hydrogen evolution (13). Studies of electrocatalysts using buffers to maintain the pH in this range and ionic salts such as potassium chloride (KCl) to provide ionic strength to ensure high solution conductivity have shown that the buffer can aid the HER activity, presumably by acting as a proton donor (6, 14–18). To extend the scope of water-soluble electrocatalysts, biopolymers and bioinspired metallopolymer catalysts have also been studied (7, 12, 17–26). Bren and coworkers recently reported particularly enlightening studies of the effects of buffer pK_a and structure on the mechanism of the hydrogen evolution reaction for cobalt minienzymes (17, 18).We recently reported a new metallopolymer catalyst system built around a customized [2Fe-2S] catalyst site with a bridging aryldithiolato ligand which exhibits remarkable catalytic activity, air stability, and chemical stability (21). The electrocatalytic mechanism of the [2Fe-2S] catalysts with aryldithiolato ligands is known from previous studies and these catalysts operate at rates of 10⁵ s⁻¹ and faster (27–30). The readily synthesized and water-soluble metallopolymer composed of tertiary amine side-chain groups, PDMAEMA-g-[2Fe-2S] (Fig. 1), approached the current density of Pt operating in neutral water under the same conditions and matched the Faradaic yield (97 ± 3%) (21). Although the detailed structural and mechanistic causality of these profound improvements for these metallopolymer electrocatalysts remain subjects of study, the nature of this molecular system is ideal for studying solution effects on the HER reaction at neutral pH for complexes that are normally insoluble in water. In the course of characterizing these electrocatalysts, solutions containing tris(hydroxymethyl)aminomethane (TRIS) at pH 7 were discovered to be exceptionally advantageous to the catalytic rate. In contrast to the few previous studies of TRIS buffer with electrocatalysts (14, 15, 18), we utilized TRIS at a high concentration. At pH 7, TRIS is sufficiently in the cationic protonated form that additional electrolyte such as KCl is not needed for conductance. This important distinction from conventional studies allows TRIS to simultaneously play the roles of pH buffer, proton source, and sole electrolyte. There is precedence in employing buffers in a manner in which they are the sole electrolyte (7, 31–34). Referring to such species simply as a “buffer” or as an “electrolyte” is inadequate in representing the three functions including proton source. For the purposes of this paper we term a species that serves all three functions a protic buffer electrolyte (PBE). In the following discussion, a TRIS PBE solution is one in which [TRIS-H]⁺Cl⁻ is the sole electrolyte and the cation is a proton source, and a sodium phosphate PBE solution is one in which Na⁺[H₂PO₄]⁻ is the sole electrolyte and the anion is a proton source.Open in a separate windowFig. 1.(A) Depiction of the 2e⁻ electrocatalytic HER with POEGMA-g-[2Fe-2S] and/or PDMAEMA-g-[2Fe-2S] metallopolymers using TRIS or sodium phosphate protic buffer electrolytes at pH 7. (B) Image of POEGMA-g-[2Fe-2S] with MW = 14,216 grown in silico. The [2Fe-2S] active site is in the center of the polymer, blue represents the polymer backbone, and the rest are the oligo(ethylene glycol) side chains. See SI Appendix for the details of modeling and a larger image.One of the key unanswered questions for these new catalyst systems is whether the metallopolymer composition (i.e., amine side-chain groups) or the PBEs are more important to afford this outstanding catalytic activity. Herein we study the effects of PBEs by comparing the HER performances of a standard platinum catalyst and a [2Fe-2S] metallopolymer catalyst in TRIS PBE solutions, sodium phosphate PBE solutions, and a KCl electrolyte solution without a PBE. For this study, nonionic water-soluble metallopolymers were used, which were made using oligo(ethylene glycol) side-chain groups on the polymer to avoid the possibility of contributing effects of the protonated amino groups of PDMAEMA-g-[2Fe-2S] referred to earlier. The metallopolymer catalyst used in this work is designated as POEGMA-g-[2Fe-2S] (Fig. 1). We previously reported that this water-soluble metallopolymer was largely inactive for H₂ electrocatalysis at neutral pH in phosphate buffer (22). The current findings suggest that the use of electrolytes composed of inexpensive cationic organic proton donors can be readily applied to any homogeneous or heterogeneous electrocatalyst system as a facile means to enhance HER activity.     

The Effect of Sacrificing the Posterior Cruciate Ligament in Total Knee Arthroplasties That Use a Highly Congruent Polyethylene Component

Background

There is continued debate regarding retention versus sacrificing of the posterior cruciate ligament (PCL) in total knee arthroplasty (TKA). We sought to determine if there was a difference in range of motion (ROM) after TKA between patients with PCL sacrifice versus PCL retention when using a highly congruent polyethylene insert.

Mortality Rate and Mid-Term Outcomes of Total Hip Arthroplasty Using Dual Mobility Cups for the Treatment of Femoral Neck Fractures in a Middle Eastern Population

Background

The dual mobility cups (DMCs) were shown to reduce dislocation rate following total hip arthroplasty for any etiology, including femoral neck fractures. No reported studies evaluating DMC results for femoral neck fracture in a Middle Eastern population were found in the literature.

Liver Transplant From Controlled Cardiac Death Donors Using Normothermic Regional Perfusion: Comparison With Liver Transplants From Brain Dead Donors

Background

Liver transplantation from donors after either controlled or uncontrolled cardiac death (DCD) is associated with considerable rates of primary nonfunction (PNF) and ischemic cholangiopathy (IC). Normothermic regional perfusion (NRP) could significantly reduce such rates.

Association Between Patient and Physician Sex and Physician-Estimated Stroke and Bleeding Risks in Atrial Fibrillation

Background

Physicians treating nonvalvular atrial fibrillation (AF) assess stroke and bleeding risks when deciding on anticoagulation. The agreement between empirical and physician-estimated risks is unclear. Furthermore, the association between patient and physician sex and anticoagulation decision-making is uncertain.

Non alcoholic fatty liver disease and risk of incident diabetes in subjects who are not obese

Background and aims

It is not known whether non alcoholic fatty liver disease (NAFLD) is a risk factor for diabetes in non obese, non centrally-obese subjects. Our aim was to investigate relationships between fatty liver, insulin resistance and a biomarker score for liver fibrosis with incident diabetes at follow up, in subjects who were neither obese nor centrally-obese.

Maternal reproductive performance and fetal development of the Wistar Audiogenic Rat (WAR) strain

Wistar Audiogenic Rat (WAR) strain is an animal model for epilepsy studies, the chronic multifactorial disease that affects millions of people worldwide. The animals of this strain are genetically predisposed to sound-induced seizures, called audiogenic seizures, and have been used for many years in studies to understand the mechanisms involved in the epilepsies and their neuropsychiatric comorbidities, as well as the screening of potential anti-convulsant agents. Nevertheless, little is known about the reproductive characteristics of these animals. The main goal of this study was to characterize the female reproductive performance and the fetal growth of WARs in comparison to the Wistar rats, obtaining important information for physiology and behavioral studies, as well as for the preservation of the strain. The results indicated few differences between WAR and Wistar regarding the female reproductive performance. There was no significant difference in the number of pregnant females by mating, number of live births per female, number of cells per blastocyst, and several characteristics related to reproductive performance, such as pre- and post-implantation losses. However, significant differences were observed in birth weight and weight gain until weaning, with WAR animals presenting a body weight below Wistar at birth and reduced body weight gain during the lactation period. In addition, the WAR females showed lower body weight on the day 20 of pregnancy and a larger number of corpora lutea, when compared with those of Wistar animals. Thus, we conclude that although Wistar and WAR strains have few differences in their reproductive performance, which might impact future physiological life challenges or others experimentally induced procedures, it still is a very viable strain regarding reproduction.

Abbreviations: CONCEA: National Council for the Control of Animal Experimentation; GEPR: genetically epilepsy-prone rats; WAR: Wistar Audiogenic Rat.     

Ticagrelor Versus Clopidogrel in Patients With STEMI Treated With Fibrinolysis: TREAT Trial

BackgroundThe efficacy of ticagrelor in the long-term post–ST-segment elevation myocardial infarction (STEMI) treated with fibrinolytic therapy remains uncertain.ObjectivesThe purpose of this study was to evaluate the efficacy of ticagrelor when compared with clopidogrel in STEMI patients treated with fibrinolytic therapy.MethodsThis international, multicenter, randomized, open-label with blinded endpoint adjudication trial enrolled 3,799 patients (age <75 years) with STEMI receiving fibrinolytic therapy. Patients were randomized to ticagrelor (180-mg loading dose, 90 mg twice daily thereafter) or clopidogrel (300- to 600-mg loading dose, 75 mg daily thereafter). The key outcomes were cardiovascular mortality, myocardial infarction, or stroke, and the same composite outcome with the addition of severe recurrent ischemia, transient ischemic attack, or other arterial thrombotic events at 12 months.ResultsThe combined outcome of cardiovascular mortality, myocardial infarction, or stroke occurred in 129 of 1,913 patients (6.7%) receiving ticagrelor and in 137 of 1,886 patients (7.3%) receiving clopidogrel (hazard ratio: 0.93; 95% confidence interval: 0.73 to 1.18; p = 0.53). The composite of cardiovascular mortality, myocardial infarction, stroke, severe recurrent ischemia, transient ischemic attack, or other arterial thrombotic events occurred in 153 of 1,913 patients (8.0%) treated with ticagrelor and in 171 of 1,886 patients (9.1%) receiving clopidogrel (hazard ratio: 0.88; 95% confidence interval: 0.71 to 1.09; p = 0.25). The rates of major, fatal, and intracranial bleeding were similar between the ticagrelor and clopidogrel groups.ConclusionAmong patients age <75 years with STEMI, administration of ticagrelor after fibrinolytic therapy did not significantly reduce the frequency of cardiovascular events when compared with clopidogrel. (Ticagrelor in Patients With ST Elevation Myocardial Infarction Treated With Pharmacological Thrombolysis [TREAT]; NCT02298088)     

Ultra-low-dose unenhanced chest CT: Prospective comparison of high kV/low mA versus low kV/high mA protocols

Purpose

To qualitatively and quantitatively compare unenhanced ultra-low-dose chest computed tomography (ULD-CT) acquired at 80 kVp and 135 kVp.