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1. Hydrogen evolution reaction catalyzed by ruthenium ion-complexed graphitic carbon nitride nanosheets

   https://doi.org/10.1039/c7ta03826g  

   Peng, Yi; Lu, Bingzhang; Chen, Limei; Wang, Nan; Lu, Jia En; Ping, Yuan; Chen, Shaowei (January 2017, Journal of Materials Chemistry A)

   The development of cost-effective, high-performance electrocatalysts for hydrogen evolution reaction (HER) is urgently needed. In the present study, a new type of HER catalyst was developed where ruthenium ions were embedded into the molecular skeletons of graphitic carbon nitride (C 3 N 4 ) nanosheets of 2.0 ± 0.4 nm in thickness by refluxing C 3 N 4 and RuCl 3 in water. This took advantage of the strong affinity of ruthenium ions to pyridinic nitrogen of the tri- s -triazine units of C 3 N 4 . The formation of C 3 N 4 –Ru nanocomposites was confirmed by optical and X-ray photoelectron spectroscopic measurements, which suggested charge transfer from the C 3 N 4 scaffold to the ruthenium centers. Significantly, the hybrid materials were readily dispersible in water and exhibited apparent electrocatalytic activity towards HER in acid and their activity increased with the loading of ruthenium metal centers in the C 3 N 4 matrix. Within the present experimental context, the sample saturated with ruthenium ion complexation at a ruthenium to pyridinic nitrogen atomic ratio of ca. 1 : 2 displayed the best performance, with an overpotential of only 140 mV to achieve the current density of 10 mA cm −2 , a low Tafel slope of 57 mV dec −1 , and a large exchange current density of 0.072 mA cm −2 . The activity was markedly lower when C 3 N 4 was embedded with other metal ions such as Fe 3+ , Co 3+ , Ni 3+ and Cu 2+ . This suggests minimal contributions from the C 3 N 4 nanosheets to the HER activity, and the activity was most likely due to the formation of Ru–N moieties where the synergistic interactions between the carbon nitride and ruthenium metal centers facilitated the adsorption of hydrogen. This was strongly supported by results from density functional theory calculations. 

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2. Rapid preparation of carbon‐supported ruthenium nanoparticles by magnetic induction heating for efficient hydrogen evolution reaction in both acidic and alkaline media

   https://doi.org/10.1002/sus2.66  

   Liu, Qiming; Lu, Bingzhang; Nichols, Forrest; Ko, Jeffrey; Mercado, Rene; Bridges, Frank; Chen, Shaowei (May 2022, SusMat)

   Abstract Ruthenium has been hailed as a competitive alternative for platinum toward hydrogen evolution reaction (HER), a critical process in electrochemical water splitting. In this study, we successfully prepare metallic Ru nanoparticles supported on carbon paper by utilizing a novel magnetic induction heating (MIH) method. The samples are obtained within seconds, featuring a Cl‐enriched surface that is unattainable via conventional thermal annealing. The best sample within the series shows a remarkable HER activity in both acidic and alkaline media with an overpotential of only ‐23 and ‐12 mV to reach the current density of 10 mA/cm², highly comparable to that of the Pt/C benchmark. Theoretical studies based on density functional theory show that the excellent electrocatalytic activity is accounted by the surface metal‐Cl species that facilitate charge transfer and downshift the d‐band center. Results from this study highlight the unique advantages of MIH in rapid sample preparation, where residual anion ligands play a critical role in manipulating the electronic properties of the metal surfaces and the eventual electrocatalytic activity. 

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3. Platinum-complexed phosphorous-doped carbon nitride for electrocatalytic hydrogen evolution

   https://doi.org/10.1039/d1ta06240a  

   Nichols, Forrest; Liu, Qiming; Sandhu, Jasleen; Azhar, Zahra; Cazares, Rafael; Mercado, Rene; Bridges, Frank; Chen, Shaowei (March 2022, Journal of Materials Chemistry A)

   Sustainable hydrogen gas production is critical for future fuel infrastructure. Here, a series of phosphorous-doped carbon nitride materials were synthesized by thermal annealing of urea and ammonium hexafluorophosphate, and platinum was atomically dispersed within the structural scaffold by thermal refluxing with Zeise's salt forming Pt–N/P/Cl coordination interactions, as manifested in X-ray photoelectron and absorption spectroscopic measurements. The resulting materials were found to exhibit markedly enhanced electrocatalytic activity towards the hydrogen evolution reaction (HER) in acidic media, as compared to the P-free counterpart. This was accounted for by P doping that led to a significantly improved charge carrier density within C 3 N 4 , and the sample with the optimal P content showed an overpotential of only −22 mV to reach the current density of 10 mA cm −2 , lower than that of commercial Pt/C (−26 mV), and a mass activity (7.1 mA μg−1Pt at −70 mV vs. reversible hydrogen electrode) nearly triple that of the latter. Results from the present study highlight the significance of P doping in the manipulation of the electronic structures of metal/carbon nitride nanocomposites for high-performance HER electrocatalysis. 

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4. Identifying Catalytic Active Sites of Trimolybdenum Phosphide (Mo ₃ P) for Electrochemical Hydrogen Evolution

   https://doi.org/10.1002/aenm.201900516  

   Kondori, Alireza; Esmaeilirad, Mohammadreza; Baskin, Artem; Song, Boao; Wei, Jialiang; Chen, Wei; Segre, Carlo U.; Shahbazian‐Yassar, Reza; Prendergast, David; Asadi, Mohammad (May 2019, Advanced Energy Materials)

   Abstract Solid‐state electrocatalysis plays a crucial role in the development of renewable energy to reshape current and future energy needs. However, finding an inexpensive and highly active catalyst to replace precious metals remains a big challenge for this technology. Here, tri‐molybdenum phosphide (Mo₃P) is found as a promising nonprecious metal and earth‐abundant candidate with outstanding catalytic properties that can be used for electrocatalytic processes. The catalytic performance of Mo₃P nanoparticles is tested in the hydrogen evolution reaction (HER). The results indicate an onset potential of as low as 21 mV, H₂formation rate, and exchange current density of 214.7 µmol s⁻¹g⁻¹_cat(at only 100 mV overpotential) and 279.07 µA cm⁻², respectively, which are among the closest values yet observed to platinum. Combined atomic‐scale characterizations and computational studies confirm that high density of molybdenum (Mo) active sites at the surface with superior intrinsic electronic properties are mainly responsible for the remarkable HER performance. The density functional theory calculation results also confirm that the exceptional performance of Mo₃P is due to neutral Gibbs free energy (ΔG_H*) of the hydrogen (H) adsorption at above 1/2 monolayer (ML) coverage of the (110) surface, exceeding the performance of existing non‐noble metal catalysts for HER. 

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5. Mass transport-enhanced electrodeposition of Ni–S–P–O films on nickel foam for electrochemical water splitting

   https://doi.org/10.1039/D0TA12097A  

   Marquez-Montes, Raul A.; Kawashima, Kenta; Son, Yoon Jun; Weeks, Jason A.; Sun, H. Hohyun; Celio, Hugo; Ramos-Sánchez, Víctor H.; Mullins, C. Buddie (March 2021, Journal of Materials Chemistry A)

   null (Ed.)

   Electrochemical water splitting is one of the most promising approaches for sustainable energy conversion and storage toward a future hydrogen society. This demands durable and affordable electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). In this study, we report the preparation of uniform Ni–P–O, Ni–S–O, and Ni–S–P–O electrocatalytic films on nickel foam (NF) substrates via flow cell-assisted electrodeposition. Remarkably, electrodeposition onto 12 cm 2 substrates was optimized by strategically varying critical parameters. The high quality and reproducibility of the materials is attributed to the use of a 3D-printed flow cell with a tailored design. Then, the as-fabricated electrodes were tested for overall water splitting in the same flow cell under alkaline conditions. The best-performing sample, NiSP/NF, required relatively low overpotentials of 93 mV for the HER and 259 mV for the OER to produce a current density of 10 mA cm −2 . Importantly, the electrodeposited films underwent oxidation into amorphous nickel (oxy)hydroxides and oxidized S and P species, improving both HER and OER performance. The superior electrocatalytic performance of the Ni–S–P–O films originates from the unique reconstruction process during the HER/OER. Furthermore, the overall water splitting test using the NiSP/NF couple required a low cell voltage of only 1.85 V to deliver a current density of 100 mA cm −2 . Overall, we demonstrate that high-quality electrocatalysts can be obtained using a simple and reproducible electrodeposition method in a robust 3D-printed flow cell. 

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