Please cite this article in press as: K. Jin, et al., Recent advances in heterogeneous Mn-based electrocatalysts toward biological photo- synthetic Mn 4 Ca cluster, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.041 ARTICLE IN PRESS G Model CATTOD-10531; No. of Pages 10 Catalysis Today xxx (2016) xxx–xxx Contents lists available at ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod Recent advances in heterogeneous Mn-based electrocatalysts toward biological photosynthetic Mn 4 Ca cluster Kyoungsuk Jin 1 , Hongmin Seo 1 , Heonjin Ha 1 , Younghye Kim, Kang Hee Cho, Jung Sug Hong, Ki Tae Nam ∗ Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea a r t i c l e i n f o Article history: Received 5 November 2016 Received in revised form 19 December 2016 Accepted 21 December 2016 Available online xxx Keywords: Water oxidation Mn4Ca cluster Kok cycle Electrocatalyst Mn catalyst a b s t r a c t Outperforming activity of Mn 4 Ca cluster, existing in biological photosynthetic organism have continued to inspire to develop the efficient Mn based water oxidation catalysts. Over the decades, understanding of the key catalytic mechanism and mimicry of the active site of the Mn 4 Ca cluster have been main research interests. This critical issue gives an overview the findings from recent studies on the water oxidizing mechanism and stepwise structural evolution of the Mn 4 Ca cluster during Kok cycle. In addition, recent advances in the development of Mn-based electrocatalysts, particularly those that have the capacity to function at neutral pH, are also presented. © 2016 Elsevier B.V. All rights reserved. 1. Introduction The discovery and development of alternative energy sources is an urgent issue to stem the modern energy crisis. Among the var- ious candidates, hydrogen gas is regarded as the most promising energy resource due to its high energy density and environmen- tally friendly nature. To date, hydrogen energy has been mainly produced by the burning of hydrocarbon fuels at high temperature and pressure. However, such gas reforming processes have high unit costs of production and also generate carbon dioxide and other pollutants, which are detrimental to the environment. For sustain- able industrial applications, water electrolysis is a good alternative because of the requirement for only aqueous solution and a sim- ple electrolyzer, and because the generated products, oxygen and hydrogen gas, can be separated to obtain high-purity hydrogen as an energy source [1]. Electrochemical water splitting consists of two half-cell reac- tions. At the anode, water oxidation results in the production of oxygen molecules, and concomitantly, proton reduction at the cathode generates hydrogen gas. To oxidize water molecules, 4 ∗ Corresponding author. E-mail address: nkitae@snu.ac.kr (K.T. Nam). 1 These authors contributed equally. electrons are required to generate 1 oxygen molecule and 4 protons must be removed. Thermodynamically, 1.23 V are needed to initi- ate the water-splitting process; however, the slow reaction kinetics associated with anodic water oxidation results in large overpo- tential values (5–600 mV). Therefore, the development of efficient oxygen evolution catalyst to reduce the overpotential for electro- chemical water splitting is needed for the practical application of water electrolysis towards future energy production [2,3]. An efficient water-oxidizing catalyst is found in the biological systems of algae, plants, and cyanobacteria. The cubical Mn 4 Ca cluster of biological photosystem II (PS II) oxidizes water with extremely high efficiency and durability. The oxygen evolving reaction mediated by the Mn 4 Ca cluster proceeds at markedly lower overpotential (: 160 mV) and higher turnover frequency (∼2*10 3 s −1 per Mn atom) [4,5] than have been achieved to date with synthetic catalysts. In the Mn 4 Ca cluster, four manganese and one calcium atom are asymmetrically structured in a specific coor- dination with amino acids, particularly histidine and aspartic acid, of the surrounding peptides. Each Mn atom undergoes a successive change in oxidation state as part of the Kok cycle. The asymmet- rically distorted geometry of Mn that occurs in Mn 4 Ca clusters is considered to be a key feature underlying their high catalytic activ- ity. Therefore, understanding the detailed structure of the Mn 4 Ca cluster in PS II is fundamental for resolving the catalytic cycle of this efficient water-oxidizing catalyst. To this end, several spectro- http://dx.doi.org/10.1016/j.cattod.2016.12.041 0920-5861/© 2016 Elsevier B.V. All rights reserved.