The processing and characterisation of novel self-assembled DNA platinum electrocatalysts for the Hydrogen Evolution Reaction (HER)

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Hendi, Ruba ORCID: https://orcid.org/0000-0002-3259-9076 (2021). The processing and characterisation of novel self-assembled DNA platinum electrocatalysts for the Hydrogen Evolution Reaction (HER). University of Birmingham. Ph.D.

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Abstract

Fuel cell technology is a sustainable and green means of power generation and can help to tackle the global demands of energy and power. Fuel cells can have a wide range of applications, from use in small portable devices such as mobile phones or laptops to more energy demanding applications like electric vehicles or industrially for heat and power generation. However, their high expense has hindered their commercialization and widespread adoption. The major contribution to these high costs arises from the use of expensive raw materials, in particular platinum metal which is commonly used as a catalyst however is highly prone to poisoning from impurities in the fuel. With the aim of reducing platinum metal loadings further in catalytic materials associated with clean fuel and energy production, a novel synthetic biology approach to platinum nanostructure design involving the use of DNA as a scaffold for the accurate placement of metal atoms is suggested. The design of the material considers the spacing of the individual platinum metal atoms along the backbone of DNA, in order to reduce and control undesirable metal clustering and consequently expose more of the metal active sites, thereby raising the catalytic efficiency of the resultant material. The use of DNA as a versatile tool for self-assembly is emerging in the field of DNA nanotechnology and particularly methods such as DNA origami, where the fabrication of two-dimensional nanostructures is possible. These methods will be employed to achieve the catalyst nanostructure design of well distributed platinum metal on a DNA scaffold. The platinating agent in this study involves the use of the anticancer drug cisplatin (cisPt), [Pt (NH3)2Cl2], due to its extensively studied and well understood covalent interactions with the DNA nucleobases. The ability of this platinum metal complex to form such strong interactions with the DNA is pivotal in ensuring the formation of a robust catalyst precursor material. Lastly, through the combination of the well understood Watson-Crick specific base pairings of the DNA and the interactions between cisPt and DNA nucleobases reported in the literature, the aim is to specifically control the assembly of the platinum on the DNA template to produce localised and highly distributed platinum active sites suited for catalysing fuel cell reactions like the hydrogen evolution reaction (HER). In this study, the novel DNA-cisPt based nano-material (synthesised in-house) is characterised to measure its electrochemical properties in order to assess its feasibility for its use as an electrocatalyst for fuel cell applications. The initial work, involving the complex formation between salmon milt DNA and cisPt, focuses on investigating the ideal and optimum platinum metal loading on the DNA scaffold for the HER in an acidic environment (pH 3). The studies reveal interesting trends in catalytic behaviour as a function of platinum loading for the HER and the influence of this on the resultant surface morphology of the material via STEM imaging characterisation. This includes increased electrocatalytic performance with increased metal loadings although simultaneously we observed increased clustering of the metal atoms and poorer distribution of it on the DNA support. However, the non-conductive nature of the DNA material greatly hindered the attainable electrocatalytic performance at all loadings of the platinum metal. Thus, an integral study focusing on different processing treatments of the DNA was carried out to improve the conductive and electronic properties of the DNA material as an electrocatalyst support, which would both improve the electrochemical activity and durability of the nanomaterial. The resulting effect of three different methods has been detailed throughout the thesis including the method of: functionalization of single walled carbon nanotubes (SWCNT’s) with the DNA-cisPt material, the graphitization of the DNA through the evacuation of the hydrogen and oxygen atoms upon high dose exposure to an electron beam, and finally carbonisation of the DNA via a low temperature pseudo pyrolysis process (255 °C, 30 minutes). Experimental work presented in this thesis focuses on assessing the surface morphology of the nanomaterial using Scanning Transmission Electron Microscopy (STEM) and the electrocatalytic performance for the HER via electrochemical studies. X-ray Photoelectron Spectroscopy (XPS) is used to identify the chemical composition of the material, while Atomic Force Microscopy (AFM) is occasionally used to observe the surface topography of the DNA. The experimental results reveal that interesting trends in catalytic behaviour as a function of platinum loading in the DNA-cisPt nanomaterial can be attained for the HER reaction (pH 3). It is clear from the experimental findings that the functionalisation of the SWCNTs with cisPt gives rise to exceptional improvements in the electrocatalytic performance of the nanotubes towards the HER (pH 3), compared to the unmodified SWCNT’s. Insights into the mechanism of the HER reaction, as revealed by the Tafel slopes, show that the Volmer-Heyrovsky reaction mechanism dominates for the HER over the surface of the fabricated SWCNT-DNA-cisPt structures and that the cisPt loading correlates strongly with the calculated transfer coefficient (α) and the Tafel slope. On the other hand, the treatment process of the DNA via its projection to a flood electron beam (e-beam) led to significant improvements in the electrochemical performance of the material for the electrocatalysis of the HER (pH 3) compared to the untreated counterpart. This supports the hypothesis made to suggest that the conductive properties of the DNA can be improved through this processing technique. The electrocatalytic performance of the e-beam treated DNA-cisPt material could be correlated with the platinum (Pt) loading. An optimum loading was found, which was intermediate of all the Pt loadings studied. Lastly, carbonisation of the DNA/glassy carbon (GC) and DNA-cisPt /GC modified electrode via a heat treatment process at the optimum experimental pseudo pyrolysis conditions found (255 °C for a duration of 30 minutes) was also proven to be successful in significantly improving the electrocatalytic performance of the material for the HER (pH 3), compared to the untreated counterpart. Carbonization of the virgin DNA via a pseudo pyrolysis process can significantly improve the electrochemical performance for the catalysis of the HER (pH 3) and the performance attained is equivalent to that measured in the presence of the platinum metal i.e. carbonised DNA-cisPt. It can be proposed that the carbonised DNA is best described as being equivalent to a nitrogen, phosphorus doped (N, P-doped) (graphitic) carbon, which justifies the improved electrocatalytic response measured. The experimental results have shown that cisPt is an effective source of platinum metal and its combination with DNA is ideal for establishing controlled placement of the platinum metal atoms on the scaffold, at an atomic precision level via covalent bonding. This enables improved distribution of the platinum metal on the DNA catalyst support, thus increasing the utilisation of the expensive metal and consequently reducing its costs. To achieve this distribution, self-assembled nanostructures such as triangular DNA origami structures and Holliday Junction (HJ) arrays have been employed in the final part of this work. The experimental results have shown that electrocatalysts of reduced platinum metal loadings can be fabricated without compromising the fuel cell performance as demonstrated through the electrochemical studies of the HER (pH 3). The surface morphological studies via STEM imaging have shown how highly distributed platinum metal atoms on the DNA nanostructures can be achieved as opposed to the non-assembled DNA scaffold i.e. salmon milt DNA. As a result of this, there is increased availability and exposure of the catalyst active sites. This is a stepping stone towards improving the utilisation of the ‘true’ available surface area, consequently leading to more effective and efficient use of the platinum metal and consequently reducing costs.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):