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 Impact of Comprehensive Care for Joint Replacement Bundled Payment Program on Care Delivery

Background

Comprehensive Care for Joint Replacement (CJR) is a Medicare initiative to test the impact of holding a hospital accountable for services provided during an episode of care for a lower extremity joint arthroplasty on costs and quality. This study examines whether hospital participation in CJR is associated with having programs focused on improving posthospitalization care or reducing costs using a survey of orthopedic surgeons.

Treatment Experiences,Information Needs,Pain and Quality of Life in Men with Metastatic Castrate-resistant Prostate Cancer: Results from the EXTREQOL Study

Aims

Delaying progression, ameliorating symptoms and maintaining quality of life (QoL) are primary aims of treatment for metastatic castrate-resistant prostate cancer (mCRPC). Real-world rather than clinical trial data about symptoms and side-effects are sparse. In EXTREQOL, patients' QoL, pain and information needs were recorded during treatment.

Differentiated Thyroid Cancer in Children: A UK Multicentre Review and Review of the Literature

Aims

To obtain an overview of the management and outcomes of children aged 18 years or younger diagnosed with differentiated thyroid carcinoma of follicular cell origin across the UK, by collecting and analysing data from the limited number of centres treating these patients. This multicentre data might provide a more realistic perspective than single-institution series.

A phase 1/2 trial of ibrutinib in combination with pembrolizumab in patients with mismatch repair proficient metastatic colorectal cancer

Background MMR proficient (pMMR) colorectal cancer (CRC) is usually unresponsive to immunotherapy. Recent data suggest that ibrutinib may enhance the anti-tumour activity of anti-PD-1 immunotherapy. In this study, we evaluated the safety and efficacy of ibrutinib plus pembrolizumab in refractory metastatic CRC.Methods This was a phase 1/2 study in patients with refractory metastatic pMMR CRC. The primary endpoints for phases 1 and 2 were maximum tolerated dose (MTD) and disease control rate, respectively. The secondary endpoints were safety, progression-free survival (PFS) and overall survival (OS).Results A total of 40 patients were enrolled. No dose-limiting toxicity was observed, and MTD was not identified. The highest tested dose of ibrutinib, 560 mg once daily, was combined with a fixed dose of pembrolizumab 200 mg every 3 weeks for the phase 2 portion. The most common grade 3/4 treatment-related adverse events were anaemia (21%), fatigue (8%) and elevated alkaline phosphatase (8%). Among 31 evaluable patients, 8 (26%) achieved stable disease, and no objective response was observed. The median PFS and OS were 1.4 and 6.6 months, respectively.Conclusion Ibrutinib 560 mg daily plus pembrolizumab 200 mg every 3 weeks appears to be well tolerated with limited anti-cancer activity in metastatic CRC.ClinicalTrials.gov identifier {"type":"clinical-trial","attrs":{"text":"NCT03332498","term_id":"NCT03332498"}}NCT03332498.Subject terms: Cancer immunotherapy, Colorectal cancer     

Association of Multiple Plasma Biomarker Concentrations with Progression of Prevalent Diabetic Kidney Disease: Findings from the Chronic Renal Insufficiency Cohort (CRIC) Study

Open in a separate window