Sustainable hydrogen production is an essential prerequisite of a future hydrogen economy. Water electrolysis driven by renewable resource-derived electricity and direct solar-to-hydrogen conversion based on photochemical and photoelectrochemical water splitting are promising pathways for sustainable hydrogen production. All these techniques require, among many things, highly active noble metal-free hydrogen evolution catalysts to make the water splitting process more energy-efficient and economical. In this review, we highlight the recent research efforts toward the synthesis of noble metal-free electrocatalysts, especially at the nanoscale, and their catalytic properties for the hydrogen evolution reaction (HER). We review several important kinds of heterogeneous non-precious metal electrocatalysts, including metal sulfides, metal selenides, metal carbides, metal nitrides, metal phosphides, and heteroatom-doped nanocarbons. In the discussion, emphasis is given to the synthetic methods of these HER electrocatalysts, the strategies of performance improvement, and the structure/composition-catalytic activity relationship. We also summarize some important examples showing that non-Pt HER electrocatalysts could serve as efficient cocatalysts for promoting direct solar-to-hydrogen conversion in both photochemical and photoelectrochemical water splitting systems, when combined with suitable semiconductor photocatalysts.

Noble metal-free hydrogen evolution catalysts for water splitting

The recent advances in electrocatalysis for oxygen reduction reaction (ORR) for proton exchange membrane fuel cells (PEMFCs) are thoroughly reviewed. This comprehensive Review focuses on the low- and non-platinum electrocatalysts including advanced platinum alloys, core–shell structures, palladium-based catalysts, metal oxides and chalcogenides, carbon-based non-noble metal catalysts, and metal-free catalysts. The recent development of ORR electrocatalysts with novel structures and compositions is highlighted. The understandings of the correlation between the activity and the shape, size, composition, and synthesis method are summarized. For the carbon-based materials, their performance and stability in fuel cells and comparisons with those of platinum are documented. The research directions as well as perspectives on the further development of more active and less expensive electrocatalysts are provided.

http://repository.ust.hk/ir/bitstream/1783.1-77094/1/acs.chemrev.5b00462_withlink.pdf

Recent Advances in Electrocatalysts for Oxygen Reduction Reaction

TiO(2) is one of the most studied compounds in materials science. Owing to some outstanding properties it is used for instance in photocatalysis, dye-sensitized solar cells, and biomedical devices. In 1999, first reports showed the feasibility to grow highly ordered arrays of TiO(2) nanotubes by a simple but optimized electrochemical anodization of a titanium metal sheet. This finding stimulated intense research activities that focused on growth, modification, properties, and applications of these one-dimensional nanostructures. This review attempts to cover all these aspects, including underlying principles and key functional features of TiO(2), in a comprehensive way and also indicates potential future directions of the field.

TiO2 nanotubes: synthesis and applications.

This article provides an up-to-date perspective on the use of anion-exchange membranes in fuel cells, electrolysers, redox flow batteries, reverse electrodialysis cells, and bioelectrochemical systems (e.g. microbial fuel cells). The aim is to highlight key concepts, misconceptions, the current state-of-the-art, technological and scientific limitations, and the future challenges (research priorities) related to the use of anion-exchange membranes in these energy technologies. All the references that the authors deemed relevant, and were available on the web by the manuscript submission date (30th April 2014), are included.

/pdf/anion-exchange-membranes-in-electrochemical-energy-systems-46uquenb56.pdf

Anion-exchange membranes in electrochemical energy systems

Dingcai Wu,*,† Fei Xu,† Bin Sun,† Ruowen Fu,† Hongkun He,‡ and Krzysztof Matyjaszewski*,‡ †Materials Science Institute, Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, People's Republic of China ‡Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States

Design and Preparation of Porous Polymers

Platinum-based nanomaterials are the most commonly adopted electrocatalysts for both anode and cathode reactions in polymer electrolyte membrane fuel cells (PEMFCs) fed with hydrogen or low molecular alcohols. However, the scarce world reserves of Pt and its high price increases the total cost of the system and thus limits the feasibility of PEMFCs. Based on this problem, for PEMFCs to have wide practical applications and become commercially viable, the challenging issue of the high catalyst cost resulting from the exclusive adoption of Pt or Pt-based catalysts should be addressed. One of the targets of the scientific community is to reduce the Pt loading in membrane electrode assemblies (MEAs) to ca. 150 μ g c m MEA − 2 , simultaneously maintaining high PEMFCs performances. The present paper aims at providing the state-of-the-art of low Pt and non-Pt electrocatalysts for: (a) H2-O2 PEMFCs, (b) Direct Methanol Fuel Cells (DMFCs) and (c) Direct Ethanol Fuel Cells (DEFCs). The detailed analysis of a big number of recent investigations has shown that the highest mass specific power density (MSPD) value obtained for H2-O2 PEMFCs has far exceeded the 2015 target ( 5 mW μ g Pt total - 1 ) set by the USA department of energy, while a several number of investigations reported values between 1 and 5 mW μgPt−1. However, the highest values measured under DMFCs and DEFCs working conditions are still relatively low and close to 0.15 and 0.05 mW μgPt−1 respectively. Moreover, the last years, promising results have been reported concerning the design, fabrication, characterization, and testing of novel non-Pt (Pt-free) anodes and cathodes for PEMFCs applications.

http://heraclitus.uth.gr/main/sites/default/files/phd_public_uploads/Low%20and%20non-platinum%20electrocatalysts%20for%20PEMFCs%20Current%20status,%20challenges%20and%20prospects.pdf

Low and non-platinum electrocatalysts for PEMFCs: Current status, challenges and prospects

A Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) based cathode intermediate temperature solid oxide fuel cell (IT-SOFC) was fabricated and tested. The effect of carbon dioxide on the performance of BSCF cathode was evaluated at temperatures ranging from 450 to 750 °C. The current density was recorded at a constant discharge of voltage value and the electrochemical impedance spectra (EIS) measurements were carried out in the absence and in the presence of CO2 in the oxidant gas line (cathode). It was found that the presence even of relatively small quantities (0.28–3.07%) of CO2 negatively affects the H2-IT-SOFC performance. It was shown that as the CO2 content in the cathode side increases and as the operation temperature decreases, the fuel cell performance is seriously aggravated up to 550 °C in a reversible way. A further decrease of the operation temperature deteriorates the SOFC performance irreversibly. However, the cell performance can be recovered after treatment at 800 °C in pure oxygen. It was also shown that as the CO2 content increases, the rate of oxygen electrochemical reduction decreases and the corresponding apparent activation energy increases linearly. The EIS results show that the interface resistance increases dramatically after carbon dioxide is added into the oxidant gas line. It is believed that carbon dioxide and temperature, acting in a synergetic way, decrease at least the cathode activity for oxygen reduction. This behaviour could be attributed to the strong carbon dioxide adsorption on the BSCF surface and to the formation of carbonates at temperatures as low as 500 and 450 °C.

Investigation of a Ba0.5Sr0.5Co0.8Fe0.2O3−δ based cathode IT-SOFC: I. The effect of CO2 on the cell performance

Rational design of the microstructure and intrinsic active sites of nitrogen-doped carbon (NC) materials to achieve highly efficient oxygen reduction reaction (ORR) electrocatalysts is extremely important for many renewable energy devices. Herein, we develop a metal–organic framework (MOF) derived metal-free NC material via a simple and low-cost NH3 activation strategy. With NH3 activation, the ORR catalytic performance of the MOF-derived material shows a great promotion. The material outperforms commercial Pt/C catalyst toward ORR catalysis in alkaline media with ∼28 mV higher half-wave potential. This amazing ORR performance might be attributed to its large specific surface area, hierarchical porosity, and full exposure of valid N species (mainly graphitic-N) to the ORR, which result from the facile NH3 activation.

A Facile Activation Strategy for an MOF-Derived Metal-Free Oxygen Reduction Reaction Catalyst: Direct Access to Optimized Pore Structure and Nitrogen Species

In the present investigation, graphite felt (GF) activated by KOH at high temperature is adopted as the electro-Fenton cathode for degrading organic pollutants contained in the wastewater Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and contact angle results show that through KOH activation the raw graphite felt (RGF) could be provided with: (i) higher surface area, (ii) more oxygen-containing functional groups, and (iii) more hydrophilicity Experimental results show that H2O2 accumulation concentration reaches the maximal value at −07 V of applied cathode potential and the apparent rate constant for dimethyl phthalate (DMP) degradation at the AGF-900 (optimized activation conditions: at 900 °C for 1 h) is 0198 min−1, much higher than that (0020 min−1) obtained at the RGF (∼900% of enhancement) In the meanwhile, AGF-900 possesses good stability without significant performance decay even after 20 reaction cycles and desirable universality for degrading other organic contaminants Moreover, the comparison with other commonly used materials confirms the effectiveness of AGF-900 as the electro-Fenton cathode From the obtained results it is shown that KOH activation at high temperature is a desirable way to decorate graphite felt with more functional groups and then make it more applicable as the electro-Fenton cathode to remove organic pollutants in wastewater

Shuqin Song

Papers

Low and non-platinum electrocatalysts for PEMFCs: Current status, challenges and prospects

Investigation of a Ba0.5Sr0.5Co0.8Fe0.2O3−δ based cathode IT-SOFC: I. The effect of CO2 on the cell performance

A Facile Activation Strategy for an MOF-Derived Metal-Free Oxygen Reduction Reaction Catalyst: Direct Access to Optimized Pore Structure and Nitrogen Species

Preparation and characterization of a novel KOH activated graphite felt cathode for the electro-Fenton process

Pd electrocatalyst supported on carbonized TiO2 nanotube for ethanol oxidation