Tunable crystal structure and proton conductivity of lanthanide nitrilotrismethylphosphonates

Metal phosphonates are multifunctional solids with remarkable stability and proton conducting properties owing to their structure is usually composed of extended hydrogen-bond networks that favor proton transfer pathways [1]. Moreover, these properties can be enhanced by appropriate modification of the synthesis conditions [2, 3]. In this communication, a new family of isostructural 2D layered compounds based on lanthanide nitrilotris-methylphosphonates is reported. These compounds have been isolated at room temperature and have the general formula Ln[N(CH2)3(PO3H2)2(PO3H)(H2O)]SO4·2H2O (Ln= Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Yb). The coordination environment of Ln3+ is composed by eight oxygen atoms from three different ligands and two oxygens from bound waters. This connectivity creates positive charged layers connected to sulfate ions through hydrogen-bonds. These compounds show promising proton conductivity with values ranging between 7.6·10-2 and 3.8·10-2 S·cm-1 at 80 °C and 95% RH and low activation energy corresponding to Grotthuss-type proton transfer mechanism. In addition, a structural transformation occurs at T > 70 °C accompanied by a remarkable enhanced conductivity. Studies on the structure-properties relationships will be discussed.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. MINECO: MAT2016-77648-R Junta Andalucía: P12-FQM-1656 y FQM-11

Exploiting Multifunctionality of M2+ (M=Co2+, Ni2+) Phosphides for Electrocatalysis toward HER, OER and ORR

INTRODUCTION The scarcity and high cost of Pt and Ru/Ir-based noble metal electrocatalysts forces to design alternative low-cost and efficient materials for sustainable energy storage and conversion technologies1. Among them, phosphorus-containing coordination polymers, such as metal phosphonates and phosphinates, have emerged as potential precursors of transition-metal phosphide (TMP) electrocatalysts2. EXPERIMENTAL STUDY Pyrolysis under 5%H2-Ar atmosphere at different temperatures. Electrocatalytic performances were investigated toward Oxygen Evolution Reaction (OER), Oxygen Reduction Reaction (ORR) and Hydrogen Evolution Reaction (HER). RESULTS AND DISCUSSION In this research-work, we report the synthesis and crystal structure of several families of divalent metal (Co2+, Ni2+) derived from the (2-carboxyethyl)(phenyl)phosphinic acid (CEPPA)3 and etidronic acid4. These solids were used as precursor of metal phosphides (M2P/MP) by thermal reduction under 5%H2-Ar atmosphere at different temperatures and their electrocatalytic performances were investigated toward OER, ORR and HER. The relationship between M/P molar ratios and/or the M2+ coordination environment in the precursor structures and the electrocatalytic activity of the prepared metal phosphides will be discussed. The presence/absence of N-doped carbon graphitic matrix in the final materials will be also studie

Tuning the activity of cobalt 2-hydroxyphosphonoacetates-derived electrocatalyst for water splitting and oxygen reduction: insights into the local order by pair distribution funcion analysis

Electrode materials generally exhibit complex heterostructures wherein both amorphous and crystalline phases may coexist [1]. Advanced techniques, such as Pair Distribution Function (PDF) analysis, may offer valuable structural insight into the local order and domain sizes of the active phase present in these materials. Herein, cobalt pyrophosphate- and phosphide-based electrocatalysts were prepared from the corresponding metal (R,S)-2-hydroxyphosphonoacetates by pyrolysis in N2 and 5%-H2/Ar at different temperatures (500 – 800 ºC) [2]. Their electrocatalytic performances toward the hydrogen and oxygen evolution reactions (HER, OER) and oxygen reduction reaction (ORR) were compared with those resulting from the N-doping and P-enrichment of the electrocatalysts. In addition, differential PDF (d-PDF) analysis of the spent electrocatalysts revealed that, regardless the amorphous or crystalline nature of the metal pyrophosphate/phosphides obtained, all of them transformed into biphasic cobalt oxy(hydroxydes) under tested OER conditions. Conversely, d-PDF demonstrated that most effective electrocatalysts for HER and ORR maintained their stability during the experimental conditions.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech; PID2019-110249RB-I00/AEI/10.13039/501100011033; TED2021–129836B-I00/AEI/10.13039/501100011033/Unión Europea NextGenerationEU/PRTR (MICIU, Spain); P20-00416 (Junta de Andalucia, Spain/FEDER

New multifunctional sulfonato-containing metal phosphonates proton conductors

Anchoring of acidic functional groups to organic linkers acting as ligands in metal phosphonates has been demonstrate to be a valid strategy to develop new proton conductor materials, which exhibit tunable properties and are potentially applicable to proton exchange membranes, such as those used in PEMFCs [1,2]. In this work, the structural and proton conductivity properties of several families of divalent and trivalent metal amino-sulfophosphonates are presented. The chosen ligand, (H2O3PCH2)2-N-(CH2)2-SO3H, was reacted with the appropriate metal salt using highthrough-put screening and/or microwave-assisted synthesis. Different crystal structures haven been solved displaying a variety of metal ligand coordination modes, in whose frameworks acidic groups contribute to create strong H-bond networks; together with lattice and bound water molecules. Proton conductivity values oscillate between 10-4 and 10-2 S.cm-1, at 80 ºC and 95 % relative humidity, most of them showing activation energies characteristic of a Grotthuss-type proton transport mechanism.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. MINECO: MAT2016-77648-R Junta de Andalucía: P-12-FQM-1656 y FQM-11

Electrocatalytic properties of spray-drying-synthesized cobalt or nickel phosphonate-derived materials.

As a class of coordination polymers (CPs), metal phosphonates (MPs) are constructed by coordination bonds connecting metal sites and phosphonate (RPO32−) ligands, where the metal sites are dispersed uniformly at the atomic level. This feature facilitates the construction of OER/HER transition metal phosphide (TMP) pre-catalysts, making them very attractive precursors of Non-Precious Metal Electrocatalysts (NPMCs) [1, 2] In this work, we report the synthesis and crystal structures of several transition-metal phosphonates derived from the phosphonopropionic acid (PPA), [FexM1-x(HO3PCH2CH2COO)2(H2O)2; M=Co2+ or Ni2+; x= 0, 0.2]. These solids have been prepared for the first time by spray-drying synthesis directly on carbon paper (CP) (Scheme 1). Pyrolysis of spray-dried materials in 5%-H2/Ar led to TMP-based energy-conversion electrocatalysts. As compared with other conventional procedures, this synthetic methodology allowed to improve the water-splitting activity.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. PID2019-110249RB-I00/AEI/10.13039/501100011033 TED2021–129836B-I00/AEI/10.13039/501100011033/Uni´on Europea NextGenerationEU/PRTR (MICIU, Spain) P20-00416 (Junta de Andalucia, Spain/FEDER

Implementing Pair Distribution Function analysis to rationalize the water-splitting activity of Co-phosphonate-derived electrocatalysts.

Pair Distribution Function analysis (PDF) is a total X-ray scattering technique, including the diffuse scattering and the Bragg diffraction. Thus, PDF can be used to characterize structural domains of amorphous solids to investigate local order/properties correlations [1]. Herein, a follow-up of the chemical evolution of pyrophosphate- or phosphide-based Fe/Co electrocatalysts is carried out by synchrotron PDF analysis. The catalysts were prepared from the metal (R,S) 2-hydroxyphosphonoacetates by pyrolysis in N2 (500 ºC and 700 ºC) or 5%-H2/Ar (800 ºC) and studied toward the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Comparison of PDF patterns of the amorphous (500 ºC) and the semicrystalline Fe/Co pyrophosphates (700 ºC) showed that the local order of the amorphous solid is composed of nanoclusters of ~ 7 Å (Figure 1). In contrast, the PDF pattern of the Fe/Co phosphide (800 ºC) is formed by a mixed of the crystalline phases o-Co2P and o-CoP. Differential PDF (d-PDF) analysis of the spent catalysts revealed that, irrespectively of the amorphous or crystalline nature, all pyrolyzed solids transformed under OER operation into biphasic CoO(OH), composed of discrete clusters with size ≤ 20 ÅUniversidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Zinc(II), cobalt(II) and manganese(II) networks with phosphoserine ligand: Synthesis, crystal structures magnetic and conduction properties

A series of zinc(II), cobalt(II) and manganese(II) coordination networks with the phosphoserine ligand (Pser) is synthesized and characterized. Whereas in compounds 1 and 2 with the general formula [M(Pser)]n [M = Zn (1) and Co(2)], the metal(II) ion presents a tetrahedral geometry, in [Co(Pser)(H2O)2]n (3) and [Mn(Pser)(H2O)]n (4)], the metal(II) ions are in a distorted octahedral geometry. The 3D frameworks are formed by inorganic layers built up from MO4 or MO6 polyhedra and phosphate groups. These layers are linked by the carboxylate groups of the phosphoserine ligand. The presence of extended hydrogen bonding stabilizes the 3D network and favors the proton transfer leading to moderate proton conductors. The highest proton conductivity, 2.70·10−5 S cm−1 (at 80 ºC and 95% RH), is obtained for compound 3. Temperature dependent magnetic susceptibility measurements for 2−4 reveal predominant antiferromagnetic interactions between the paramagnetic metal(II) ions.Proyectos MAT2013-41836-R y MAT2016-77648-R del MINECO, y proyecto (P12-FQM-1656) de la Junta de Andalucía

Synthesis and properties of novel lanthanide carboxyphosphonates

Metal phosphonates are essentially acidic solids featured by groups such as P-OH, -COOH, etc. The presence of bound and lattice water favors the formation of H-bond networks, which make these compounds appropriate as proton conductors, attractive for proton exchange membranes (PEMs) of Fuel Cells. Moreover, these properties can be enhanced by appropriate modification of the synthesis conditions. We report here, general characteristics of three new series of isostructural compounds resulting from the combination of the polyfunctional 5-(dihydroxyphosphoryl) isophthalate acid with lanthanide ions. All compounds were synthesized under hydrothermal conditions and their crystal structures were solved from powder X-ray diffraction data using synchrotron radiation. In contrast with Series III compounds, which exhibit a layered structure, Series I and II present pillared frameworks. All these compounds contain water molecules that contribute to the formation of H-bond networks. Upon exposure to ammonia vapour, from an aqueous solution, solid state transformations are observed which are accompanied of an enhancement of their proton conductivity properties.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

Preparation of N-doped Carbon/Metal Phosphides as Promising Trifunctional Electrocatalysts Toward the OER, ORR and HER

Sustainable energy storage and conversion technologies, such as electrochemical water splitting and fuel cells, attracts increasing attention as alternative processes to advance toward a global decarbonation. However, the high cost, scarcity, and poor stability of the most active electrocatalysts, mainly based on noble metal (Pt, RuO2, IrO2,…), difficult severely their large-scale production and use. In this regard, the development of earth-abundant electrocatalysts, with high activity for the different processes, is needed. Several imidazole-containing metal etidronates, MLIm-n (M2+= Fe, Co, Ni and solid solutions; L= ETID; Im= Imidazole; n= [0, 3]), were prepared using different synthetic procedures. Thermal reduction of MLIm-n in 5%H2–Ar at different temperatures resulted on core–shell N-doped carbon/TMPs with variable content of MP and MxP as crystalline phases. Their electrocatalytic activities have been widely studied by cyclic and linear sweep voltammetry, impedance spectroscopy, transmission electron microscopy, and XPS analysis. Preliminary results reveal that factors such as the presence of the imidazole and the metal coordination environment in the precursor samples determine the final composition and electrochemical properties of the resulting pyrolyzed derivatives. So, the Co2+- derivative, CoLIm-0@800, with a composition CoP/Co2P= 80/20 wt. %, exhibited the best electrocatalytic properties toward OER/ORR/HER as well as good capabilities as anode for overall water splitting in comparison to the expensive reference RuO2 electrocatalyst

Applications of N,N-bis(phosphonomethyl)glycine-derived Sn4+ or Co2+ phosphonates as proton conductors or energy-conversion electrocatalysts.

Metal phosphonates (MPs), a subclass of coordination polymers, may exhibit acidic groups such as POH, SO3H, COOH, N+-H, etc. Combining these features with electrocatalytically active transition metals, make them highly appealing in the field of fuel cells and electrolysers, as potential proton conductors and/or precursors of electrocatalysts [1,2]. Herein, we investigate the synthesis, characterization and applications of a series of Co2+ and Sn4+ phosphonates derived from glycine-N,N-bis(methylenenphosphonic acid) (BPMGLY). In the case of the tin derivative, an amorphous compound, Sn(C4H11O8NP2)0.75Cl2.5(H2O)2.5 (Sn4+- BPMGLY), was obtained by hydrothermal synthesis. Its pyrolytic treatment at 700 ºC in air led to an amorphous pyrophosphate, (Sn4+- BPMGLY@700). Regarding cobalt phosphonates, three crystalline phases with composition [Co(C4H9O8NP2(H2O)2]·nH2O (n=0, 2) were obtained and their crystal structure were solved. All families were extensively studied as proton conductors across a wide range of temperature and humidity conditions, displaying the Sn4+ derivatives the highest conductivity values of 7.99·10-4 and 6.63·10-3 S·cm-1 for Sn4+-BPMGLY and Sn4+-BPMGLY@700, respectively, at 95 °C and 95% relative humidity (RH) (Figure 1a). Furthermore, the cobalt phosphonates were utilized as precursors for non-precious metal catalysts (NPMCs), by pyrolysis in 5%-H2/Ar at different temperatures and studied as electrocatalysts towards the oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR)Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech