1 3
4.1 N‑Containing Proteins
ORR-based electrocatalysts are considered as essential components of metal–air batteries and fuel cells [51, 52]. The most common ORR electrocatalysts are mainly
platinum and its alloys due to their higher current density and relatively lower overpotential. But the higher cost, availability, anode crossover and severe intermediate tolerance, etc. of platinum impose restrictions on the advancement of fuel cell
technologies. Therefore, the development of low-cost, as well as high-performance
ORR catalysts, has being investigated in recent times [53]. In this regard, low-cost
and metal-free carbon-based electrocatalysts have drawn immense attention owing
to their superior durability and excellent electrocatalytic activity. There are different
types of carbon materials, including porous carbon, graphene, activated carbon and
carbon nanotubes, which have been used to improve the catalytic activity and durability of electrocatalysts by maximizing their electroactive surface area. Recently,
it was found that the electrocatalytic performance of the ORRs can be enhanced by
nitrogen doping of carbon materials [28]. The presence of abundant and free-flowing sp
2
hybridized pi electrons are mainly responsible for this significant enhancement in performance. Lone pair electrons of nitrogen atoms also help to connect the
delocalized conjugated systems, thereby resulting in additional improvement in electrocatalytic activity. Nitrogen doping of carbon nanostructures can be accomplished
by two conventional procedures such as ‘in situ doping,’ which is done during the
synthesis of carbon materials, and ‘post doping,’ which is achieved through the
post-treatment of carbon nanostructures. But these methods usually require expensive hardware, multi-step processes and high energy consumption, which eventually
restricts their practical applicability. To overcome these problems, exploitation of
N-containing proteins can be beneficial because of their easy availability as well as
the simplicity of preparing N-doped carbonaceous materials.
Blood proteins (BPs) from animals, which can be sourced from the meat industry, contain various amino acids such as proline, glutamic acid and tyrosine, and an
abundance of hemoprotein. These substances can successfully produce active electrocatalysts for ORR in both acidic and alkaline environments. Therefore, BP could
be used as a potential precursor material for making highly active electrocatalysts.
Guo et al. [54] reported a novel approach for designing nitrogen-enriched carbonbased electrocatalysts for ORR through the co-pyrolysis of a carbon black support
and BP. At first, the BP was decomposed at 350 °C for 5 h under a constant flow
of nitrogen. The pyropolymer obtained was then mixed with the carbon black support by ball milling. The sample yielded was again treated at 1000 °C for 2 h in a
nitrogen atmosphere (BP350C1000) and then used as an electrocatalyst for ORR.
As a control, the BP was also carbonized for 2 h at 1000 °C and the sample was
denoted as BP1000. Furthermore, BP3501000 was prepared by the continuous heat
treatment of BP for 5 h at 350 °C and 2 h at 1000 °C without adding the carbon
black into it. The results demonstrated that the nitrogen present in the as-synthesized electrocatalyst from BP is mainly in the form of pyrrolic- and pyridinic-type
nitrogen species. The electrocatalyst comprising higher amounts of pyrrolic-type
nitrogen species exhibited better electrocatalytic performance towards the ORR in
terms of limited current density, half-wave potential, and onset potential. In this
244
Reprinted from the journal
Topics in Current Chemistry (2020) 378:43
4.1 N‑Containing Proteins
ORR-based electrocatalysts are considered as essential components of metal–air batteries and fuel cells [51, 52]. The most common ORR electrocatalysts are mainly
platinum and its alloys due to their higher current density and relatively lower overpotential. But the higher cost, availability, anode crossover and severe intermediate tolerance, etc. of platinum impose restrictions on the advancement of fuel cell
technologies. Therefore, the development of low-cost, as well as high-performance
ORR catalysts, has being investigated in recent times [53]. In this regard, low-cost
and metal-free carbon-based electrocatalysts have drawn immense attention owing
to their superior durability and excellent electrocatalytic activity. There are different
types of carbon materials, including porous carbon, graphene, activated carbon and
carbon nanotubes, which have been used to improve the catalytic activity and durability of electrocatalysts by maximizing their electroactive surface area. Recently,
it was found that the electrocatalytic performance of the ORRs can be enhanced by
nitrogen doping of carbon materials [28]. The presence of abundant and free-flowing sp
2
hybridized pi electrons are mainly responsible for this significant enhancement in performance. Lone pair electrons of nitrogen atoms also help to connect the
delocalized conjugated systems, thereby resulting in additional improvement in electrocatalytic activity. Nitrogen doping of carbon nanostructures can be accomplished
by two conventional procedures such as ‘in situ doping,’ which is done during the
synthesis of carbon materials, and ‘post doping,’ which is achieved through the
post-treatment of carbon nanostructures. But these methods usually require expensive hardware, multi-step processes and high energy consumption, which eventually
restricts their practical applicability. To overcome these problems, exploitation of
N-containing proteins can be beneficial because of their easy availability as well as
the simplicity of preparing N-doped carbonaceous materials.
Blood proteins (BPs) from animals, which can be sourced from the meat industry, contain various amino acids such as proline, glutamic acid and tyrosine, and an
abundance of hemoprotein. These substances can successfully produce active electrocatalysts for ORR in both acidic and alkaline environments. Therefore, BP could
be used as a potential precursor material for making highly active electrocatalysts.
Guo et al. [54] reported a novel approach for designing nitrogen-enriched carbonbased electrocatalysts for ORR through the co-pyrolysis of a carbon black support
and BP. At first, the BP was decomposed at 350 °C for 5 h under a constant flow
of nitrogen. The pyropolymer obtained was then mixed with the carbon black support by ball milling. The sample yielded was again treated at 1000 °C for 2 h in a
nitrogen atmosphere (BP350C1000) and then used as an electrocatalyst for ORR.
As a control, the BP was also carbonized for 2 h at 1000 °C and the sample was
denoted as BP1000. Furthermore, BP3501000 was prepared by the continuous heat
treatment of BP for 5 h at 350 °C and 2 h at 1000 °C without adding the carbon
black into it. The results demonstrated that the nitrogen present in the as-synthesized electrocatalyst from BP is mainly in the form of pyrrolic- and pyridinic-type
nitrogen species. The electrocatalyst comprising higher amounts of pyrrolic-type
nitrogen species exhibited better electrocatalytic performance towards the ORR in
terms of limited current density, half-wave potential, and onset potential. In this
244
Reprinted from the journal
Topics in Current Chemistry (2020) 378:43