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.     

Preschool Children with Gender Normative and Gender Non-Normative Peer Preferences: Psychosocial and Environmental Correlates

We addressed several issues concerning children who show gender non-normative (GNN) patterns of peer play. First, do young children with GNN peer preferences differ from children with gender normative (GN) peer preferences in problem behaviors? Second, do GNN and GN children differ in sociability and isolation and do they have differential socialization opportunities with externalizing, internalizing, and socially competent peers? We employed a Bayesian approach for classifying children as GNN based on their peer preferences as compared to their peers using a sample of Head Start preschool children from a large Southwestern city (N = 257; 53 % boys; M age = 51 months; 66 % Mexican American). To calculate socialization opportunities, we assessed affiliation to each child in the class and weighted that by each peer’s characteristics to determine the exposure that each child had to different kinds of peers. GN children of both sexes interacted more with same-sex peers, which may limit learning of different styles of interaction. As compared to GN children, GNN children exhibited more engagement in other-sex activities and with other-sex play partners and GNN children experienced somewhat fewer peer interactions, but did not differ on problem behaviors or social competence. Boys with GNN peer preferences had increased exposure to peers with problem behaviors. GNN girls experienced little exposure to peers with problem behaviors, but they also had little exposure to socially competent peers, which may reduce learning social skills from peers. Implications of these findings for future socialization and development will be discussed.     

Research with youth of color in low-income communities: Strategies for recruiting and retaining participants

BackgroundYouth of color from low-income urban communities are crucial participants in research, as their involvement can shape effective, culturally responsive interventions and policy to promote youth health and well-being. These young people, however, are an often-neglected research population, due in part to perceived challenges associated with their inclusion as well as marginalized communities’ justifiable mistrust of research.ObjectivesBased on our experience conducting a school-based randomized intervention trial in Baltimore, Maryland, we present strategies for conducting research with low-income, urban youth of color. We discuss strategies in three domains: university-community partnership development, participant recruitment, and participant retention.MethodsWe reviewed partnership building and recruitment strategies employed by our team across four years of trial implementation and evaluated success of participant retention at our final survey timepoint.ResultsPartnership building was facilitated by selection of a study design that maximized benefits for all participants, promotion of capacity building at partner institutions, and attention to research staff hiring and training practices. Effective study recruitment strategies included personal contact with parents and close cooperation between school personnel and study staff. Providing incentives and collecting multiple types of participant contact information contributed to increased retention rates. On average, those who participated in the final survey timepoint were less likely to be male and Latinx and exhibited more favorable baseline mental health than those who did not, suggesting differential attrition based on youth characteristics.ConclusionsLessons learned from this school-based trial can be applied more broadly to research with low-income urban youth of color. Researchers should strive to maximize scientific rigor, minimize harm to vulnerable adolescents and their communities, promote positive research experiences for young people, and provide concrete benefits to those who participate.     

Anxiety and Epilepsy: What Neurologists and Epileptologists Should Know

Although there has been increasing recognition of psychiatric comorbidity in epilepsy, most research and attention in this area has focused on depression. However, comorbid anxiety in epilepsy is highly prevalent, affecting more than 40 % of patients in some reports. Many important outcomes are significantly impacted by anxiety in epilepsy, including quality of life, mortality, and seizure status. Recent evidence from epidemiologic studies suggests a bidirectional association of anxiety and epilepsy, and there is mounting evidence for possible common pathophysiology underlying anxiety and epilepsy. Despite this importance, anxiety is under-recognized and undertreated in clinical practice. A variety of anxiety symptoms are seen in epilepsy, including symptoms exclusively before, during or after seizures (peri-ictal anxiety), symptoms resembling primary anxiety disorders, and anxiety directly related to epilepsy or its treatment. Key therapeutic approaches include pharmacotherapy or cognitive behavioral therapy for most forms of interictal anxiety and better seizure control for peri-ictal anxiety.     

Colorectal Cancer with Potentially Resectable Hepatic Metastases: Optimizing Treatment

Colorectal cancer is a common malignancy and often presents with synchronous or metachronous distant spread. For patients with hepatic metastases, resection is the principal curative option. Liberalization of the indications for hepatic resection has introduced a number of challenges related to the size, distribution, and number of metastases as well as the condition of the future liver remnant. Advances in systemic therapy have solidified its role as both an important adjunct to surgery and also for many patients as a mechanism to facilitate resection. In patients whose disease is marginally resectable as a consequence of the distribution of hepatic lesions that precludes complete resection or out of concern for the future liver remnant, a number of strategies have been advocated, including prehepatectomy systemic therapy, staged surgical approaches, ablative technologies, and preoperative portal vein embolization. It is the purpose of this review to discuss ways in which to optimize the treatment of patients with potentially resectable disease, specifically those who are judged to have “borderline” resectable situations.     

A standard definition of major hepatectomy: resection of four or more liver segments

BackgroundWhile commonly used to describe liver resections at risk for post-operative complications, no standard definition of ‘major hepatectomy’ exists. The objective of the present retrospective study is to specify the extent of hepatic resection that should describe a major hepatectomy.MethodsDemographics, diagnoses, surgical treatments and outcomes from patients who underwent a liver resection at two high-volume centres were reviewed.ResultsFrom 2002 to 2009, 1670 patients underwent a hepatic resection. Post-operative mortality and severe, overall and hepatic-related morbidity occurred in 4.4%, 29.7%, 41.6% and 19.3% of all patients. Mortality (7.4% vs. 2.7% vs. 2.6%) and severe (36.7% vs. 24.7% vs. 24.1%), overall (49.3% vs. 40.6% vs. 35.9%) and hepatic-related (25.6% vs. 16.4% vs. 15.2%) morbidity were more common after resection of four or more liver segments compared with after three or after two or fewer segments (all P < 0.001). There were no significant differences in any post-operative outcome after resection of three and two or fewer segments (all P > 0.05). On multivariable analysis, resection of four or more liver segments was independently associated with post-operative mortality and severe, overall, and hepatic-related morbidity (all P < 0.01).ConclusionsA major hepatectomy should be defined as resection of four or more liver segments.